Comprehensive Energy Systems, vol.1a - Energy Fundamentals [1a, 1 ed.] 978-0-12-814925-6

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1.1 Energy Units, Conversions, and Dimensional Analysis İlhami Yıldız and Yu Liu, Dalhousie University, Halifax, NS, Canada r 2018 Elsevier Inc. All rights reserved.

1.1.1 Introduction 1.1.2 Quantities 1.1.2.1 Relationship Between Quantities 1.1.2.2 Base Quantities 1.1.2.3 Derived Quantities 1.1.2.4 Multiples and Submultiples of Quantities 1.1.2.5 Types of Quantity Equations 1.1.3 Dimensional Analysis 1.1.4 Units and Conversions 1.1.4.1 Useful Units in Electricity 1.1.4.1.1 Coulomb 1.1.4.1.2 Volt 1.1.4.1.3 Watt 1.1.4.1.4 Ohm 1.1.5 Rules for Using SI Units 1.1.5.1 Capitalization 1.1.5.2 Use of Plurals 1.1.5.3 Use of Hyphenation and Space 1.1.5.4 Use of Numerals and Periods 1.1.5.5 Use of Symbols for Mathematical Operations 1.1.6 Overall Examples 1.1.7 Concluding Remarks References Relevant Websites

Nomenclature a a A b Btu c c C C cd d da E E f F F g G h h

hp I

Atto Index; acceleration, m s 2; constant, 2897 mm K Unit of electric current, ampere; area, m2 Index British thermal unit Centi Index; speed of light, 3  108 m s 1 Unit of electric charge, coulomb; Celsius Specific heat, J kg 1 K 1 or Btu lbm 1 1F 1 Luminous intensity unit Deci Deka Exa Energy, J or Btu h 1 Femto Fahrenheit Force, N or lbf Gravitational acceleration, m s 2 Giga Hecto Height, m; pump head, m; enthalpy, J kg 1, heat transfer coefficient, W m 2 K 1 or Btu h 1 ft 2 1F 1; Planck’s constant, 6.626  10 34 J s Unit of power in I–P, horsepower Radiant flux density, W m 2 or Btu h 1 ft 2

Comprehensive Energy Systems, Volume 1

2 2 3 3 3 3 4 4 4 6 6 6 6 6 6 6 6 9 9 9 10 22 22 23

J k k kg kWh K l L m m _ m M mol n N p p P P Pa r R rad s sr

doi:10.1016/B978-0-12-809597-3.00101-2

Unit of energy and work, joule Kilo Dimensionless coefficient, thermal conductivity, W m 1 K 1 or Btu h 1 ft 1 1F 1 Unit of mass, kg Unit of energy, kilowatt-hour ( ¼ 3.6 MJ) Unit of thermodynamic temperature Length, m or ft Length Milli Unit of length, m; mass, kg or lbm Mass flow rate, kg s 1 Mega; mass Unit of amount of substance, mol Nano Unit of force, newton Pico Pressure, Pa or lbf Peta Power, W or Btu h 1 Unit of pressure, pascal Radius, m Thermal resistance, m2 K W 1 or h ft2 1F Btu 1 Unit of plane angle, radian Entropy, J g 1 or Btu lbm 1 Unit of solid angle, steradian

1

Energy Units, Conversions, and Dimensional Analysis

2

T therm ton U

w W W x y Y z Z

Width Unit of power, watt Work Distance, m or ft Yocto Yotta Zepto Zetta

n

y r O

Kinematic viscosity, m2 s 1; specific volume, m3 kg 1 or ft3 lbm 1; frequency, cycles s 1 ¼ hertz ¼Hz Pump power, W or hp Density, kg m 3 or lbm ft 3 Unit of electric resistance, ohm

Subscripts bk Break power fl Fluid power ke Kinetic energy

max p pe v

Maximum Pump; constant pressure Potential energy Constant volume

Superscripts 1 Degree

0

Minute (angle) Second (angle)

V V

Tera; time, s; temperature, 1C, 1F, K, or R Unit of heat energy, 105.5 MJ or 100,000 Btu Refrigeration ton, 12,000 Btu h 1 or 3.52 kW Thermal transmittance, W m 2 K 1 or Btu h 1 ft 2 1F 1 Unit of electric potential, potential difference, and electromotive force, volt Volume, m3 or ft3

Greek letters D Difference F Pump power, W or hp Z Conversion efficiency, % l Wavelength, mm m Micro m Dynamic viscosity, Pa s

1.1.1

00

Introduction

When dealing with engineering and scientific relationships, in order to appreciate the magnitudes of physical quantities, it is essential to have a solid grasp of units, and recognize two types of equations, namely, quantity equations and numerical equations. Both types are found in texts and reference books, and the concept of units and quantities is useful in understanding their respective features. In this chapter, we cover the main features of quantities and quantity equations, and provide the most important units and conversions relating to energy. Quantity equations are also called equations between quantities, or physical equations. And, numerical equations are alternatively called measure equations. We also introduce the technique of dimensional analysis, which is used to derive basic physical relationships without performing a full analysis of a system.

1.1.2

Quantities

In 1954, the 10th general conference on weights and measures (CGPM) decided that an international system should be derived from six base units to provide for the measurement of temperature and optical radiation in addition to mechanical and electromagnetic quantities. Six base units recommended at this conference were the meter, kilogram, second, ampere, degree Kelvin (later renamed kelvin), and candela. In 1960, the 11th CGPM named the system the International System of Units, SI from the French name, Le Système International d'Unités [1]. Later, the seventh base unit, the mole, was added in 1971 by the 14th CGPM [2]. SI is the modern form of the metric system, and today is the most widely used measurement system. Therefore, the International System of Quantities (ISQ) is now a system based on seven base quantities: length, mass, time, thermodynamic temperature, electric current, luminous intensity, and amount of substance. Other quantities, such as area, pressure, and electrical resistance are all derived from these base quantities. The ISQ defines quantity as any physical property that can be measured with the SI units [3]. A quantity may also be a physical constant, such as the gas constant, or the Planck’s constant. Several hundred quantities are employed to describe and measure the physical world, and a few of these quantities are listed below [4]: Length Time Mass Force

Viscosity Energy Speed Power

Area Luminance Angle Temperature

Electromotive force Entropy Pressure Momentum

Energy Units, Conversions, and Dimensional Analysis

3

1.1.2.1 Relationship Between Quantities The study of physics to a great extent can be defined as the study of mathematical relationships among various physical properties. Physical quantities are defined, as above, when these properties allow a reasonable mathematical description. The relationship of all other quantities can be established in terms of a few base quantities selected properly, either by definition, by geometry, by physical law, or by a combination of the base quantities. For instance, pressure is a quantity that is related, by definition, to a quantity force divided by a quantity area. Area, on the other hand, is a quantity related, by geometry, to the product of two quantities of length. Moreover, force is a quantity related (by Newton’s second law) to the quantity mass times the quantity acceleration. The relationships between quantities are expressed in the form of quantity equations. We can relate even an isolated quantity, such as temperature to the quantities pressure, volume, and mass. We can further relate the quantities length and time by using the universal constant and the speed of light. Therefore, if we define our concepts correctly, we can relate any quantity to any other quantity. Thus the equation area ¼length  width is a quantity equation, which states that the quantity (area of a rectangle) is equal to the quantity (length) times the quantity (width).

1.1.2.2

Base Quantities

In order to reduce a set of quantity equations, we have to first establish a number of so-called base quantities. Hence, base quantities are called the building blocks upon which we develop the entire structure and relationships of the physical world. As mentioned earlier, the international system of units, or SI, makes use of seven base quantities: mass (kg), length (m), time (s), temperature (K), electric current (A), luminous intensity (cd), and amount of substance (mol). The number of base quantities, as well as their choice, is quite an arbitrary choice; but, generally, we select quantities that are easy to understand and frequently used, and for which accurate and measurable standards can be established.

1.1.2.3

Derived Quantities

As mentioned in the relationship section earlier, using the selected base quantities as building blocks, derived quantities are expressed as those that can be deducted by definition, geometry, or physical law. Some derived quantity examples are area (equals the products of two lengths), velocity (equals length/time), and force (equals mass  acceleration), pressure, power, etc. We also have what are called supplementary units (as a class of derived units), namely, the plane angle (radian ¼rad¼ m m 1) and solid angle (steradian ¼sr ¼ m2 m 2).

1.1.2.4

Multiples and Submultiples of Quantities

Note that the magnitude of a quantity can have an extremely large range. In an effort to handle such a large range, the SI unit system generated 20 prefixes shown in Table 1. Table 1

Multiples and submultiples in SI unit system

Prefix

Symbol

Multiplier

Example

Yotta Zetta Exa Peta Tera Giga Mega Kilo Hecto Deka Deci Centi Milli Micro Nano Pico Femto Atto zepto yocto

Y Z E P T G M k h da d c m m n p f a z y

1024 1021 1018 1015 1012 109 106 103 100 10 10 1 10 2 10 3 10 6 10 9 10 12 10 15 10 18 10 21 10 24

5 2 7 6 5 8 2 3 6 2 3 5 9 5 2 3 6 5 6 8

Ym¼5 yottameters¼5  1024 m Zm¼2 zettameters ¼2  1021 m Em¼7 exameters¼7  1018 m PJ¼ 6 petajoules¼ 6  1015 J TW¼5 terawatts¼5  1012 W GJ¼8 gigajoules¼8  109 J MW¼2 megawatts¼2  106 W km¼3 kilometers¼3  103 m hL¼6 hectoliters¼600 L dam¼2 decameters¼20 m dL¼3 deciliters¼0.3 L cm¼5 centimeters¼0.05 m mV¼9 millivolts¼9  10 3 V mm¼5 micrometers¼5  10 6 m ns¼2 nanoseconds¼ 2  10 9 s pJ¼3 picojoules¼3  10 12 J fm¼6 femtometers ¼6  10 15 m aJ¼5 attojoules¼5  10 18 J zJ¼6 zeptojoules¼6  10 21 J yJ¼8 yoctojoules¼ 8  10 24 J

Energy Units, Conversions, and Dimensional Analysis

4

1.1.2.5

Types of Quantity Equations

The energy of wind, the pressure at the bottom of an air or water column, the weight of an object, and the viscosity of a liquid are all physical quantities of nature. And, whether they are measured or not, these quantities are always there interacting with each other according to fundamental laws. Physicists often express these laws in terms of quantity equations because quantities conform to these laws. Quantity equations possess two important features: first, they show the relationship between quantities, and second, they can be used with any system of units. There are three basic types of quantity equations: 1. Quantity equations developed from the laws of nature; for instance, Newton’s second law of motion F¼ma where F is the magnitude of the force, m is the magnitude of the mass, and a is the magnitude of the acceleration. 2. Quantity equations developed from geometry; for instance, area of a circle A ¼ p r2 where A is the magnitude of the area, p is the coefficient based on the geometry of a circle, and r is the magnitude of the radius. 3. Quantity equations developed from a definition; for instance, definition of pressure p ¼ F=A where p is the magnitude of the pressure, F is the magnitude of the force, and A is the magnitude of the area. Many quantity equations can be developed as a combination of the basic quantity equations given above, and in all cases, we can use any units we want to describe the magnitudes of the relevant physical quantities.

1.1.3

Dimensional Analysis

Dimensional analysis is quite a useful method for deriving an algebraic relationship between different physical quantities, which relies on good physical intuition in choosing the different appropriate physical variables. The idea behind this analysis is that each variable is expressed in terms of its fundamental units of mass M, length L, and time T, etc., raised to some arbitrary index a, b, c, etc. These unknown indices are then determined by equating the indices of like units [5]. One might also choose force, length, and mass as the base dimensions, with associated dimensions F, L, M, which corresponds to a different basis. It may sometimes be useful to choose one or another extended set of dimensional symbols. In electromagnetism, for instance, it may be advantageous to use dimensions of M, L, T, and Q, where Q is used to represent the dimension of electric charge. Another example is that, for instance, in thermodynamics, the base set of dimensions is often extended to include a dimension for temperature, Y. Let’s now perform a simple dimensional analysis to find an expression for the hydrostatic pressure in a fluid. The hydrostatic pressure is dependent on the density r, the gravitational acceleration g, and depth h. Now, let’s assume a general algebraic equation in the form of r ¼ k ra g b hc where k is a coefficient (dimensionless), and a, b, and c are the indices (numbers) to be determined. Now, we can replace each symbol by its fundamental physical unit, and have
a
b M1 L 1 T 2 ¼ ML 3 LT 2 ðLÞc or M1 L 1 T

2

¼ Ma L

3aþbþc

T

2b

M, L, and T are all independent quantities; therefore, we can equate the indices on both sides, and have the following equations 1 ¼ a;

1¼

3a þ b þ c;

2¼

2b

Then we can solve for and find that a¼ b¼c ¼1; consequently, the expression for hydrostatic pressure can be found as p¼k r g h, where the coefficient k cannot be determined from dimensional analysis because it is dimensionless. More dimensional analysis examples are provided in the examples section later.

1.1.4

Units and Conversions

This section, as modified after ASHRAE [6,7], references the Standard for Metric Practice, ASTM Standard E 380-84 [8], as one of the basic standards for SI usage [9–13]. Table 2 provides conversion factors rounded to three or four significant figures for conversion between SI and I–P. And Table 3 provides conversion factors for different physical quantities related to energy further.

Energy Units, Conversions, and Dimensional Analysis

Table 2

5

SI energy related units and conversions

Divide

By

To obtain

Divide

By

To obtain

ha kPa L m3 kJ kJ m 3; J L kJ L 1 W (m K) 1

0.405 100 159 0.159 1.055 37.3 0.279 1.731

acre bar barrel (42 US gal, petroleum)

J J kg 1 W L m3 mL s 1 Ls 1 mL J 1 g mg L 1 g kg 1 kW kW mm

1.36 2.99 0.0226 3.79 0.00379 1.05 0.0631 0.0179 0.0648 17.1 0.143 9.81 0.746 25.4a

ft
lbf (work) ft
lbf lb 1 (specific energy) ft
lbf min 1 (power) gallon (US, 231 in3) gallon gph gpm gpm ton 1 refrigeration grain (1/7000 lb) gr gal 1 gr lb 1 horsepower (boiler) horsepower (550 ft-lbf s 1) inch

To obtain kPa Pa mm m 1 mN m mm2 mL mL s 1 mm3 mm4 ms 1 MJ GJ (y
m2) 1 JL 1 N kN MPa m3 mPa km km km h 1 ms 1 kPa kPa Pa kPa g N mL mN
m gL 1 ng (s
m2
Pa) ng (s
m
Pa) kg m 3 kg m 3 mg kg 1 kPa EJ L m2 mL mL MJ t (tonne); Mg t (tonne); Mg

by 3.38 249 0.833 113 645 16.4 0.273 16,400 416,000 0.278 3.60a 0.0388 2.12 9.81 4.45 6.89 0.001a 133 1.61 1.85 1.61 0.44 0.100a 0.133 9.80 9.80 28.3 0.278 29.6 7.06 7.49 57.4 1.46 16.0 120 1.00a 6.89 1.055 0.946 9.29 15 5 105.5 1.016 0.907

Multiply in of mercury (60F) in of water (60F) in/100 ft, thermal expansion in
lbf (torque or moment) in2 in3 (volume) in3 min 1 (SCIM) in3 (section modulus) in4 (section moment) km h 1 kWh kWh (y ft2) kWh/100 cfm kilopond (kg force) kip (1000 lbf) kip in 2 (ksi) liter micron of mercury (60oF) mile mile/nautical mph mph millibar mm of mercury (601F) mm of water (601F) meter of water ounce (mass, avoirdupois) ounce (force or thrust) ounce (liquid, US) ounce inch (torque or moment) ounce (avoirdupois) per gallon perm (permeance) perm inch (permeability) lb ft 3 (density, r) lb gallon 1 ppm (by mass) psi quad quart (liquid US) square (100 sq ft) tablespoon (approximately) teaspoon (approximately) therm (US) ton, long (2240 lb) ton, short (2000 lb) (Continued )

W (m K) 1 W (m 1C) 1 W kJ m 3 GJ (y m2) 1

0.144

Btu, IT Btu ft 3 Btu gal 1 Btu ft h 1 ft3 1F Btu in (h ft3 1F) 1 (thermal conductivity, k)

0.293 11.4 0.0000114

Btu h 1 Btu ft 2 Btu (y ft2)

To obtain Wm 2 W (m2 K)

by 3.15 5.68

Multiply Btu (h ft2) 1 (overall heat transfer coef., U) (thermal conductance, C) Btu lb 1 Btu (lb 1F) 1 (specific heat, C)

kJ kg 1 kJ (kg K) kJ (kg 1C) m3 J kJ mPa
s

1

1

2.33 4.19

1 1

0.0352 4.19 4.19 1.00a

mm2 s 1 Pa W W COP m mm ms 1 ms 1 kPa kPa m 1 m2 m2 K W 1 m2 1C W 1

1.00a 0.100a 44.0 70.3 0.293 0.3048a 304.8a 0.00508 0.3048a 2.99 0.0981 0.0929

mm2 s 1 L m3 mL S 1 1. L s 1 Ls 1 N
m mL

92 900 28.3 0.0283 7.78 0.472 28.3 1.36 473

kg g N kg m mPa s

0.454 454 4.45 1.49 0.413

1

mPa s gs kg s

0.176

1490

1 1

0.126 0.00756

1

bushel calorie, gram calorie, kilogram; kilocalorie centipoise, viscosity, m (absolute, dynamic) centistokes, kinematic viscosity, n dyne cm 2 EDR hot water (150 Btu h 1) EDR stream (240 Btu h 1) EER ft ft ft min 1, fpm ft s 1, fps ft of water ft of water per 100 ft pipe ft2 ft2 h 1F Btu 1 (thermal resistance, R) ft2 s 1, kinematic viscosity, n ft3 ft3 ft3 h 1, cfh ft3 min 1, cfm ft3 s 1, cfs ft
lbf (torque or moment) pint (liquid, US) pound lb (mass) lb (mass) lbf (force or thrust) lb ft 1 (uniform load) lbm (ft
h) 1 viscosity (absolute, dynamic, m) lbf (ft
s) 1 viscosity (absolute, dynamic, m) lb h 1 lb min 1

1 1

6

Energy Units, Conversions, and Dimensional Analysis

Table 2

Continued

Divide

By

To obtain

Divide

By

To obtain

kW

0.284

Pa mPa s

47.9 47900

kg m 2 To obtain

4.88 by

lb of steam per hour @2121F (1001C) lbf ft 2 lbf
s ft 2 viscosity (absolute, dynamic, m) lb ft 2 Multiply

kW Pa Wm 2 m m2 m3 To obtain

3.52 133 10.8 0.9144a 0.836 0.765 by

ton, refrigeration (12,000 Btu h torr (1 mm Hg@01C) watt per square foot yd yd2 yd3 Multiply

1

)

a

Conversion factor is exact. Abbreviation: COP, coefficient of performance; EDR, equivalent direct radiation; EER, energy efficiency ratio; SCIM, standard cubic inches per minute.

1.1.4.1

Useful Units in Electricity

1.1.4.1.1

Coulomb

In an electric circuit, the unit of electric charge in SI is the coulomb, and has the symbol C. An ampere, which has the symbol of A, is defined as the amount of charge transported through any cross-section of a conductor in one second by a constant current of one ampere, and is equivalent to the amount of charge on about 6,241,510,000,000,000,000 electrons.

1.1.4.1.2

Volt

In an electric circuit, the unit of electric potential, potential difference, and electromotive force in SI is the volt and has the symbol V. If and when we consider our house wiring as plumbing, volts can then be considered as a measure of the water pressure. One volt is the potential difference between two points on a conductor when the current flowing is one ampere and the power dissipated between the points is one watt. The volt is a derived unit, and in terms of base units it can be expressed as follows:

Volt ¼ watt=ampere ¼ m2 kg = s3 A

1.1.4.1.3

Watt

In an electric circuit, one watt (joules per second) is a current of one ampere at a pressure of one volt. In terms of base units,
Watt ¼ J s 1 ¼ m2 kg s 3

1.1.4.1.4

Ohm

In an electric circuit, the unit of electrical resistance (a derived unit) in SI is called an ohm and has the symbol of O. One ohm is defined as the electrical resistance between two points on a conductor when a constant potential difference of one volt, applied to these points, produces in the conductor a current of one ampere. Ohm is a derived unit, and in terms of base units it can be expressed as follows:

Ohm ðOÞ ¼ volt=ampere ¼ m2 kg = s3 A 2

1.1.5 1.1.5.1

Rules for Using SI Units Capitalization

The names of units start with a lowercase letter when writing the units out except for in a title or the beginning of a sentence. The only exception is “degree Celsius.” Unless they come from an individual's name (in which case the first letter of the symbol is capitalized), lowercase is used in writing symbols for units. The only exception is L for liter. Symbols for numerical prefixes (multiples and submultiples) are also lowercase, except for those representing multipliers of 106 or more, for instance, mega (M), giga (G), tera (T), peta (P), exa (E), zetta (Z), and yotta (Y). It means that all prefixes are written in lowercase when spelled out. Lowercase units: m, kg, s, mol, etc. Uppercase units: A, K, Hz, Pa, C, etc. Symbols rather than self-styled abbreviations should always be used to represent units. Correct usage: A, s. Incorrect usage: amp sec

1.1.5.2

Use of Plurals

Remember that symbols are never expressed as plural. That is, an “s” is never added to the symbol to denote plural. However, when the names of units are spelled out, they are made plural if the number to which they refer is greater than 1. Fractions, on the other hand, are always written as singular. Plurals are used as required when writing unit names. For example, henries is plural for henry. The following exceptions are noted:

Table 3

Conversion factors

Pressure pascal

dyne cm

1 0.100 98,066 105 133.32 101,325 3386.4 6894.8

10 1 980,665 106 1333.2 1,013,250 33,864 68,948

Mass

Volume

Density

2

kg cm

bar

mm Hg

atm

1.0192  10 5 1.01972  10 6 1 1.01972 0.0013595 1.03323 0.034532 0.07030696

10 5 10 6 0.98066 1 0.0013332 1.01325 0.33864 0.068948

0.00750 0.000750 735.559 750.062 1 760.0 25.400 51.715

9.8692  10 9.8692  10 0.96784 0.98692 0.00131579 1 0.033421 0.068046

kg

lb

1¼ 0.45359¼

2.20462 1

metre3

liter

gal

ft3

in3

1 0.001a 0.0037854 0.028317 1.63871  10

1000 1 3.7854 28.317 0.0163871

264.173 0.264173 1 7.48055 4.329  10

35.315 0.035315 0.13368 1 5.787  10

61,023.74 61.02374 231.0 1728a 1

Watt-sec

joule

calorie

fl lb

Btu

1 4.184a 1.3558 1054.35

1 4.184a 1.3558 1054.35

0.2390 1 0.32405 251.9957

0.73756 3.08596 1 777.65

9.4845  104 3.9683  103 1.2859  103 1

kg m

3

(g L

5

1

)

g cm

1 1000 119.827 16.018463 Specific volume

m3 kg

1

(L g

1 0.001 0.008345 0.0624280 J (g K)

or entropy

1 4.184a 4.184a

1

)

lb gal

1

0.001 1 0.119827 0.016018

0.008345 8.34538 1 0.133680

cm3 g

gal lb

1

1000 1 8.34538 62.4280 cal (g K) 0.2390 1 1

lb ft

Btu lb

1

1

7

4

2.953  10 2.953  10 28.959 29.530 0.03937 29.921 1 2.0360

4 5

1.45038  10 1.45038  10 14.223 14.5038 0.0193368 14.6960 0.491154 1

4 5

3

0.0624280 62.4280 7.48055 1 ft3 lb

119.827 0.119827 1 7.48055 1

6

psi

1

16.018463 0.016018 0.133680 1 1F

0.2390 1.0 1

7

Specific heat

1

3

3

in Hg

Energy Units, Conversions, and Dimensional Analysis

Energy

2

(Continued )

8

Table 3

Continued dyne cm

2

2

kg cm

Entropy

Jg 1 1 4.184a 2.3244

Thermal

W (m K) 418.4 418.4a 1.7296

Viscosity (1 poise¼dyne-sec cm

2

¼0.1 newton-sec m

lbm (ft s)

1

1 0.0671955 115,827 32.17405 6.71955  10 Coefficient of

W (m2 K)

heat transfer

1 1.630 10,000 41,869 5.6783

1

cal g 1 0.2390 1 0.5556

Btu lb 1 0.43021 1.8a 1 1

J (s cm 1C)

1 4.184a 0.017296

atm

1

in Hg

cal (s
cm 1C)

1 4.184a 0.017296

0.2390 1 4.1338  10

1

psi

1

Btu h

ft 1F

57.816 241.91 1

3

2

) Nsm

2

mm Hg

W (cm 1C)

a

conductivity

a

1

bar

2

kg (m s)

1.4882 1 172,369 47.88026 0.1 kcal (h m2 1C) 0.8598 1 8598 36,000 4.8823

International Table Btu, cal, and kcal. Linear temperature difference: 1F or 1R, 1C or K.

1

lbf hr ft

1.4882 1 172,369 47.88026 0.1 1

W (cm2 1C) 1  10 4 1.1630  10 1 4.1869 5.6783  10

2

8.6336  10 5.8014  10 1 2.7778  10 5.8014  10 1

4

4

2

lbf s ft 6

3.1081  10 0.020885 3600 1 2.0885  10

6

4 7

cal (h cm2 1C) 2.388  10 5 2.778  10 5 0.2388 1 1.3562  10 4

1

Btu h 0.1761 0.2048 1761.1 7373.5 1

1

Poise g (cm s) 2

3

ft2 1F

14.8819 10 1,723,689 478.8026 1

1

Energy Units, Conversions, and Dimensional Analysis

Pressure pascal

Energy Units, Conversions, and Dimensional Analysis

9

Singular: lux, hertz, siemens Plural: lux, hertz, Siemens Example 1: Correct and incorrect usages

1.1.5.3

Correct usage

Incorrect usage

5 kg 5 kilograms 5.57 kg 5.57 kilograms 0.57 kilogram

5 kgs 5 kilogram – 5.57 kilogram 0.57 kilograms

Use of Hyphenation and Space

Also remember that a hyphen or a space is not used to separate a prefix from the name of the unit. A space, however, is left between a symbol and the number to which it refers, with the exception of the symbols for degree, minute, and second of angles, and for degree Celsius. In three cases the final vowel in the prefix is omitted: megohm, kilohm, and hectare. Example 2: Correct and incorrect usages

1.1.5.4

Correct usage

Incorrect usage

5 kg 40 450 3000 30C 5 km MJ 5s 5 milliseconds

5kg 40  45 0 30 00 30 C 5km mF 5s 5 milli-seconds

Use of Numerals and Periods

Remember that scientific and technical writing is different from any other writings, such as newspaper, magazine, and other writings. In scientific and technical writing, numerals are used for all numbers expressing physical quantities; however, it is a common practice to write out the numbers from one to nine and use numerals for other numbers in newspapers. In ordinary books and magazines, for instance, whole numbers from one through ninety-nine, and any of these followed by “hundred,” “thousand,” “million,” “billion,” etc., are spelled out. Also, keep in mind that the associated number is written as numerals when the unit is represented by an abbreviation or symbol. Periods are never used after SI symbols unless the symbol is at the end of a sentence.

1.1.5.5

Use of Symbols for Mathematical Operations

Units are represented by symbols, not by their spelled-out names, when the units (SI) are used with symbols for mathematical operations. Notes to remember 1. When writing unit names as a product, always use a space (preferred) or a hyphen. Correct usage: newton meter or newton-meter 2. When expressing a quotient using unit names, always use the word per and not a solidus (/). The solidus or slash mark is reserved for use with symbols. Correct usage: meter per second Incorrect usage: meter/second 3. When writing a unit name that requires a power, use a modifier, such as squared or cubed, after the unit name. For area or volume, the modifier can be placed before the unit name. Correct usage: millimeter squared or square millimeter 4. When denoting a quotient by unit symbols, any of the following are accepted form: Correct usage: m/s or m s 1 In more complicated cases, consider using negative powers or parentheses. For acceleration, use m/s2 or m s For electrical potential, use kg.m2/(s3 A) or kg m2 s 3 A 1 but not kg m2/s3/A.

2

but not m/s/s.

Energy Units, Conversions, and Dimensional Analysis

10

Example 3: Correct and incorrect usages Incorrect usage

Correct usage J kg 1 J kg 1 joules per kilogram N.m newton meter newton-meter

1.1.6

joules kg 1 joules/kilogram newton.meter

Overall Examples

Example 4: Area Find: Show the unit of area in SI, and perform dimensional analysis. Solution: Area equation is a quantity equation arising from geometry; for example, the area equation for a pipe is expressed as follows: AreaðAÞ ¼ p r 2 ¼ pðmÞ2 AreaðAÞ ¼ m2 where A is the magnitude of area in m2, the magnitude of p is 3.14 (dimensionless), and r is the magnitude of radius in m. Or in another example, the area for a rectangle is expressed as follows: AreaðAÞ ¼ w  l ¼ m  m AreaðAÞ ¼ m2 2

where A is the magnitude of area in m , w is the magnitude of width in m, and l is the magnitude of length in m. Let’s now perform a simple dimensional analysis to find an expression for the area. The area is dependent on the dimensionless number p and the radius. So, A ¼ k ra where k is a dimensionless number, and a is the number to be determined. Now, we can replace each symbol by its fundamental physical unit, and have L2 ¼ L a L is an independent quantity; therefore we can equate the indices on both sides, and have the following equation a¼2 Consequently, the expression for the area can be found as A¼ k ra, where the coefficient k cannot be determined from dimensional analysis because it is dimensionless; however, from geometry, we know that k¼ p. Example 5: Volume Find: Show the unit of volume in SI and perform dimensional analysis. Solution: Volume equation is a quantity equation arising from geometry; for example, the volume equation for a pipe is expressed as follows: VolumeðV Þ ¼ pr 2 L
¼ p m2 ðmÞ VolumeðV Þ ¼ m3 where V is the magnitude of volume in m3, the magnitude of p is 3.14 (dimensionless), and r is the magnitude of radius in m. Or in another example, the volume for a rectangular cross-section is expressed as follows: VolumeðV Þ ¼ w  l  h ¼ m  m  m VolumeðV Þ ¼ m3 3

where V is the magnitude of volume in m , w is the magnitude of cross-sectional width in m, l is the magnitude of cross-sectional length in m, and h is the magnitude of height. Let’s now perform a dimensional analysis to find an expression for the volume having a tubular cross-sectional area. The volume is dependent on the dimensionless number p, the radius, and length of the tube. So, V ¼ k r a Lb

Energy Units, Conversions, and Dimensional Analysis

11

where k is a dimensionless number, and a and b are the numbers to be determined. Now, we can replace each symbol by its fundamental physical unit, and have L3 ¼ La Lb L is an independent quantity; we can therefore equate the indices on both sides, and have the following equation aþb¼3 In the earlier example, it was determined that a¼2, so this leaves b ¼ 1. Consequently, the expression for the volume can be found as V¼k ra Lb, where the coefficient k cannot be determined from dimensional analysis because it is dimensionless; however, from geometry, we know that k¼p; therefore, V ¼ k r2 L. Example 6: Volume Find: Determine the unit of volume (m3) in SI for a given volume in I–P system. Solution: Volume unit in I–P system is ft3 and remember that 1 ft¼0.3048 m; then VolumeðV Þ ¼ ft3 ð0:3048 m=1 ftÞ3 VolumeðV Þ ¼ 0:028317 m3 Example 7: Mass Find: Determine the unit of mass (kg) in SI for a given mass in I–P system. Solution: Mass unit in I–P system is lbm and remember that 1 lbm ¼ 0.45359 kg; then MassðmÞ ¼ lbm ð0:45359 kg=1 lbm Þ MassðmÞ ¼ 0:45359 kg Example 8: Force Find: Show that the unit of force is newton (N) in SI, and perform dimensional analysis. Solution: The unit of force in SI, defined as that force, which applied to a mass of 1 kg, gives it an acceleration of 1 m s 1. Newton’s second law of motion, a quantity equation established from the laws of nature, is expressed as: ForceðF Þ ¼ mass  acceleration ¼ m a

¼ ðkgÞ m s 2 ¼ kg m s 2 ForceðF Þ ¼ N where m is the magnitude of mass in kg, a is the magnitude of acceleration in m s 2, and F is the magnitude of force in N. The force is dependent on the mass and the acceleration. Now, let’s assume a general algebraic equation for force in the form of F ¼ ma a b where a and b are the indices (numbers) to be determined. Now, we can replace each symbol by its fundamental physical unit, and have
b M1 L1 T 2 ¼ ðMÞa LT 2 or M1 L1 T

2

¼ Ma Lb T

2b

M, L, and T are all independent quantities; therefore, we can equate the indices on both sides, and have the following equations 1¼a

and

1¼b

Then, we can solve for and find that a¼ b ¼c¼ 1; consequently, the expression for force can be found as F¼ m a. Example 9: Force Find: Show that the unit of force is newton (N) in SI for a given force in I–P system Solution: Force unit in I–P system is lbf and remember that 1 lbm ¼0.45359 kg, 1 ft¼0.3048 m, and gravitational acceleration g is 32.174 lbm s 2; then
Force ðF Þ ¼ 1 lbf ¼ 1 lbm 32:174 ft s 2 ¼ 32:174 lbm fts 2 ¼ 32:174 lbm fts

2





0:45359 kglbm1 0:3048 mft

1




12

Energy Units, Conversions, and Dimensional Analysis ForceðF Þ ¼ 4:45 kg m s

2

¼ 4:45 N

where g is the magnitude of gravitational acceleration, and F is the magnitude of force. Note that in I–P system, an acceleration of 9.80665 m s2 corresponds exactly to 32.174048 ft s2 as shown below: g ¼ 9:80665 m s2 ð100 cm=1 mÞ=ð12 in=1 ftÞ=ð2:54 cm=1 inÞ ¼ 32:174048 ft s2 Example 10: Pressure Find: Show the unit of pressure (pascal) in SI, and perform dimensional analysis. Solution: In solids, we deal with stresses; in liquids and gases, however, we deal with pressure, which is defined as the normal component of force per unit area. Therefore the unit for pressure is a derived unit, and has the symbol Pa (pascal) in SI. One pascal is the pressure resulting from a force of 1 N acting uniformly over an area of 1 m2. So, the pressure equation is a quantity equation established from a definition, which is expressed as follows: PressureðpÞ ¼ force=area ¼ F=A ¼ m g=A

¼ ðkgÞ ms 2 m 2 ¼ kgms 2 m 2 ¼ N m

2

PressureðpÞ ¼ Pa where F is the magnitude of force (the mass times the acceleration) in newton (N), and A is the magnitude of area in m2. The pressure is dependent on the mass, the gravitational acceleration, and the area. Now, let’s assume a general algebraic equation for pressure in the form of p ¼ ma g b Ac where a, b, and c are the indices (numbers) to be determined. Now, we can replace each symbol by its fundamental physical unit, and have
b
c M1 L 1 T 2 ¼ ðMÞa LT 2 L 2 or M1 L 1 T

2

¼ Ma Lb

2c

T

2b

M, L, and T are all independent quantities; therefore, we can equate the indices on both sides, and have the following equations 1 ¼ a;

1¼b

2c;

2¼

2b

Then, we can solve for and find that a¼b ¼ c¼1; consequently, the expression for pressure can be found as p¼(m g)/A. Example 11: Pressure Find: Show that the unit of pressure is pascal (Pa) in SI for a given force in I–P system. Solution: Pressure unit in I–P system is psi (lbf in 2), and remember that 1 lbf ¼ 4.45 kg m s 2 ¼ 4.45 N; 1 in¼ 0.0254 m; then Pressure ¼ force=area ¼ F=A ¼ p

   ¼ ð1 lbf Þ 4:45 kg m s 2 =ð1 lbf Þ = in2 6:452  10 4 m2 in 2
¼ 6894:8ðkgÞ m s 2 m 2 

PressureðpÞ ¼ 6894:8 Pa ¼ 6:89 kPa where F is the magnitude of force and A is the magnitude of area. Example 12: Work Find: Show that the unit of work is joule (J) in SI, and perform dimensional analysis. Solution: The unit of work or energy in SI is joule, which has the symbol, J. This is the work done when the point of application of a force of 1 N is displaced 1 m in the direction of the force. One watt-second is equal to 1 J. The work equation is then a quantity equation established from a definition, which is expressed as follows: Work ¼ force  distance ¼ ðmass  accelerationÞ  distance ¼ W ¼ m a Dx ¼ ðkgÞðm s 2 ÞðmÞ ¼ ðkg m s 2 Þm Work ¼ N m ¼ J where W is the magnitude of work in J, m is the magnitude of mass in kg, and a is the magnitude of acceleration in m s 2, and Dx is the distance traveled in m.

Energy Units, Conversions, and Dimensional Analysis

13

The work is dependent on the mass, the acceleration, and the distance traveled, for instance. Let’s now assume that a general algebraic equation for work is in the form of W ¼ ma ab Dxc where a, b, and c are the indices (numbers) to be determined. Now, we can again replace each symbol by its fundamental physical unit, and have

b M1 L1 T 2 L1 ¼ ðMÞa LT 2 ðLÞc or M1 L2 T

2

¼ ðMÞa Lb T

2b




Lc

M, L, and T are all independent quantities; therefore, we can equate the indices on both sides, and have the following equations 1 ¼ a;

1 ¼ b and 1 ¼ c

Then, we can solve for and find that a¼ b ¼c¼ 1; consequently, the expression for force can be found as W ¼m a Dx. Example 13: Energy Find: Show that the unit of energy is joule (J) in SI, and perform dimensional analysis. Solution: Energy is defined as the ability to perform work, and as expressed earlier, the unit of energy in SI is joule, which has the symbol, J. So, the energy equation is a quantity equation established from a definition, which is expressed as follows: Energy ¼ work ¼ force  distance ¼ ðmass  accelerationÞ  distance ¼ E ¼ m a Dx
¼ ðkgÞ m s 2 ðmÞ
¼ kg m s 2 m Energy ¼ N m ¼ J where m is the magnitude of mass in kg, and a is the magnitude of acceleration in m s 2, and Dx is the distance traveled in m. In this case, the energy is dependent on the mass, the acceleration, and the distance traveled. Let’s now assume that a general algebraic equation for energy is in the form of E ¼ ma ab Dxc where a, b, and c are the indices (numbers) to be determined. Now, we can again replace each symbol by its fundamental physical unit, and have
b
M1 L1 T 2 L1 ¼ ðMÞa LT 2 ðLÞc or M1 L2 T

2

¼ ðMÞa Lb T

2b




Lc

M, L, and T are all independent quantities; therefore, we can equate the indices on both sides, and have the following equations 1 ¼ a;

1 ¼ b and 1 ¼ c

Then, we can solve for and find that a¼ b ¼c¼ 1; consequently, the expression for energy can be found as E ¼m a Dx. Example 14: Power Find: Show that the unit of power is watt (W) in SI, and perform dimensional analysis. Solution: The unit of power in SI is watt and has the symbol, W. Power is defined as the rate at which energy is expended or work done. The watt in thermodynamics is defined as “the power which in 1 s gives rise to energy of one joule.” In mechanical terms, however, a power of one watt can move a mass of 1 kg in 1 s, through a distance of one meter with such force that the kilogram mass’s velocity at the end of the meter will be 1 m s 1 greater than it was at the beginning. In an electric circuit, on the other hand, one watt is a current of one ampere at a pressure of one volt. So, the power equation is a quantity equation established from a definition, which is expressed as follows: Power ¼ work=time; or energy generation=time; or energy consumption=time Since the unit of work or energy is J, then Power ¼ J s

1

¼W¼P¼Et

1

¼ energy=time ¼ m a Dx t

1

where t is the time in seconds. In this case, the power is dependent on the mass, the acceleration, the distance traveled, and the time taken to travel. Let’s now assume that a general algebraic equation for power is in the form of P ¼ F a Dxb t

c

Energy Units, Conversions, and Dimensional Analysis

14

where a, b, and c are the indices (numbers) to be determined. Now, we can replace each symbol by its fundamental physical unit, and have

a M1 L1 T 2 L1 T 1 ¼ M L T 2 ðLÞb T c or M1 L2 T

3

¼ Ma Laþb T

2a c

M, L, and T are all independent quantities; therefore, we can equate the indices on both sides, and have the following equations 1 ¼ a;

2 ¼ a þ b and

3¼

2a

c

Then, we can solve for and find that a¼ b ¼c¼1; consequently, the expression for power can be found as P¼ F Dx t m a Dx t 1.

1

¼

Example 15: Volt Find: Express the unit of volt in terms of base units. Solution: As mentioned earlier, in an electric circuit, the unit of electric potential, potential difference, and electromotive force in SI is volt and has the symbol V. One volt is the potential difference between two points on a conducting wire carrying a constant current of one ampere, and the power dissipated between the points is one watt. The volt is a derived unit, and in terms of base units it can be expressed as follows: E ¼ P=I ¼ electromotive force ¼ power=current Volt ¼ watt=ampere ¼ V ¼ W=A ¼ W A 1
¼ J s 1
A 1 1 ¼ JC ¼ ðN mÞ C 1 ¼A O 

 ¼ A kg m2 s 3 A 2 Volt ¼ kg m2 A

1

s

3

Example 16: Radiant flux density Find: How many units of radiant flux density I (W m 2) in SI are in a given amount of Btu h 1 ft2? Solution: Radiant flux density is defined as the amount of energy received on a unit surface in unit time. Radiant flux density ðW m 2 Þ ¼ ðBtu h 1 ft2 Þ ð1054:35 JBtu 1 Þ ðh=3600 sÞ ðft=0:3048 mÞ2 Radiant f lux density ðW m 2 Þ ¼ 3:15 W m

2

Example 17: Boiler horsepower Find: How many kilowatts of power in SI are in 1 boiler horsepower (bhp) in I–P? Solution: Remember that one boiler horsepower is the energy rate needed to evaporate 34.5 lbm of water at 2121F (1001C) in one hour; therefore, it is equal to 33,475 Btu h 1. A boiler horsepower is approximately 13 times larger than mechanical horsepower (engine output). Also remember that 1 Btu¼1054.35 J, and 1 h ¼3600 s; therefore, Boiler power ¼ 1 bhp ½ð33; 475 Btuh 1 Þ=1 bhpŠ ð1054:35 J=1 BtuÞ ð1 h=3600 sÞ Boiler power ¼ 9804 W Boiler power ¼ 9:8 kW Example 18: Horsepower Find: How many kilowatts of power in SI are in 1 horsepower (hp) in I–P? Solution: The horsepower (hp) is a unit in I–P system, sometimes used to express the rate at which mechanical energy is expended. It was originally defined as 550 foot-pounds per second (ft-lbf s 1). Remember that 1 lbf ¼4.45 kg m s 2 ¼ 4.45 N, and 1 ft ¼0.3048 m; then Power ¼ ð550 ft

lbf s 1 Þ ½ð0:3048 m= 1ftÞ ð4:45 kg m s 2 Þ=1 lbf Š Power ¼ 746 W Power ¼ 0:746 kW

Energy Units, Conversions, and Dimensional Analysis

15

Example 19: Refrigeration ton Find: How many Btu h 1 and kWh of refrigeration is provided by 1 t of refrigeration? Solution: Remember that 1 Btu ¼1054.35 J, 1 h ¼ 3600 s, 1 lbm ice ¼1 lbm of liquid water, latent heat of fusion ¼144 Btu lbm 1, and 1 short ton¼ 2000 lbm. Refrigeration is commonly rated in tons. 1 t of refrigeration is the latent heat of fusion needed to melt 1 short ton (2000 lbm) of ice in 24 h. Therefore
1 refrigeration ton ¼ 2000 lbm ice  144 Btu lbm1 =24 h 1 ref rigeration ton ¼ 12; 000 Btu h 1 refrigeration ton ¼ 12; 000 Btu h

1




1054:35 J Btu

1




1

ð1 h=3600 sÞ ð1 kJ=1000 JÞ

1 ref rigeration ton ¼ 3:52 kW Example 20: Energy Find: Show how many MJ of energy 1kWh is equal to. Solution: Remember that 1 MJ of energy is equal to 106 J of energy, and 1 kWh is equal to 1000 J s 1. Therefore,
1 kWh ¼ 1000 J s 1 ð1 hÞ ð60 min=1 hÞ ð60 s=1 minÞ ¼ 3;600;000 J 1 kWh ¼ ð3;600;000 JÞ = ð1;000;000 J=1 MJÞ 1 kWh ¼ 3:6 MJ Example 21: Energy Find: Show how many MJ and kWh of energy 1 therm (US) has. Solution: The therm is a unit of heat energy equal to 100,000 Btu units. It is the energy equivalent of burning approximately 100 ft3 (2.83 m3) of natural gas. Natural gas meters measure volume rather than energy content; therefore natural gas companies use a therm factor to convert the volume of gas used to its heat equivalent, and thence calculate the actual energy use. Please remember that natural gas with a higher than average concentration of butane, ethane, or propane has a higher therm factor, while impurities lower the therm factor.
1 therm ¼ ð100;000 BtuÞ ð1054:35 J=BtuÞ MJ=106 J ¼ 105:5 MJ 1 therm ¼ ð105:5 MJÞ ð1 kWh=3:6MJÞ ¼ 29:3 kWh Example 22: Potential energy Find: Show that the unit of potential energy is joule (J) in SI. Solution: Potential energy ¼ force  elevation ¼ ðmass  gravitational accelerationÞ  elevation Epe ¼ m g h
¼ ðkgÞ m s
2 ðmÞ 2 ¼ kg m s m Epe ¼ N m ¼ J where m is the magnitude of mass in kg, g is the magnitude of gravitational acceleration in m s 2, and h is the elevation from a datum in m. A dimensional analysis can easily be performed to find an expression for potential energy, as we did for work and energy examples earlier. Example 23: Kinetic energy Find: Show that the unit of kinetic energy is joule (J) in SI. Solution: Kinetic energy ¼ 0:5 mass  speed2




Eke ¼ 0:5 m u2 ¼ ðkgÞ m s ¼ kg m s

2


1 2


m

Eke ¼ N m ¼ J where m is the magnitude of mass in kg, and u is the magnitude of speed in m s 1.

16

Energy Units, Conversions, and Dimensional Analysis

A dimensional analysis can easily be performed to find an expression for kinetic energy, as we did for work and energy earlier. Example 24: Pressure energy Find: Show that the unit of pressure energy is joule (J) in SI. Solution: Pressure energy ¼ ðmass  pressure differenceÞ=density ¼ m DP=r
¼ ðkgÞðPaÞ= kg m
3
1 2 ¼ ðkgÞ kg m
s = kg m 3 2 ¼ kg m s m Pressure energy ¼ N m ¼ J where m is the magnitude of mass in kg, DP is the magnitude of pressure difference between two points in Pa, and r is the magnitude of density in kg m 3. Again, a dimensional analysis can easily be performed to find an expression for pressure energy, as we did for work and energy earlier. Example 25: Kinematic viscosity Find: Show the units of kinematic viscosity n in SI. Solution: The quantity equation for dynamic viscosity is given as follows: n ¼ dynamic viscosity=density ¼ m=r ¼ F t=r 3

¼ ðPa sÞ= kg m




¼ kg m

1

2

s




s= kg m

3




m ¼ m2 s 1 where Pa (pascal) is equal to kg (m s ) , m is the magnitude of dynamic viscosity in Pa s, and r is the magnitude of density in kg m 3. Kinematic viscosity is dependent on the force F, the time t, and the density r. Now, let’s assume a general algebraic equation in the form of 2

1

n ¼ F a t b =rc where a, b, and c are the indices (numbers) to be determined. Now, we can replace each symbol in kinematic viscosity by its fundamental physical unit, and overall, we would have
a
c L2 T 1 ¼ M L T 2 ðT Þb M L 3 where a, b, and c are the indices (numbers) to be determined. Now, we can replace each symbol by its fundamental physical unit, and have M0 L2 T

1

¼ Ma c Laþ3c T

2aþb

M, L, and T are independent quantities; therefore, we can equate the indices on both sides, and have the following equations 0¼a

c; 2 ¼ a þ 3c; and

1¼

2a þ b

Then, we can solve for and find that a¼c ¼½, and b¼ 0, which are the indices (numbers); consequently, the expression for kinematic viscosity can be found as n ¼ F t/r. Example 26: Dimensionless number Find: Show that the Reynolds number Re is a dimensionless number. Solution: The quantity equation for Reynolds number is given as follows: 

  Re ¼ ru D=m ¼ kg m 3 m s 1 ðmÞ = kg m

1 2


 s s ¼ Dimensionless

where r is the magnitude of density of the fluid in kg m 3, u is the magnitude of average velocity of the fluid in m s 1, D is the diameter of the pipe in which the fluid flows in m, and m is the magnitude of dynamic viscosity in Pa.s. Example 27: Pump head Find: Show that the pump head hp unit is m in SI. Solution: hp ¼ ðpumping energyÞ=ðgravitational accelerationÞ ¼ Ep =g 

 

 ¼ J kg 1 = m s 2 ¼ N m kg 1 = m s 2 


¼ kg m s 2 m kg 1 s2 m 1 hp ¼ m

Energy Units, Conversions, and Dimensional Analysis

17

where J is the unit of energy joule in SI ( ¼ force  distance ¼ N m), N is the unit of force newton (¼ kg m s 2), and g is the magnitude of gravitational acceleration 9.81 m s 2. Pump head is dependent on the pumping energy Ep and the gravitational acceleration g. Now, let’s assume a general algebraic equation in the form of: hp ¼ Eap g

b

where a, b, and c are the indices (numbers) to be determined. Now, we can replace each symbol in kinematic viscosity by its fundamental physical unit, and overall, we would have
a
b M0 L1 T 0 ¼ M2 L1 T 2 M 1 L1 T 2 where a and b are the indices (numbers) to be determined. Now, we can replace each symbol by its fundamental physical unit, and have M0 L1 T 0 ¼ Ma La

b

2aþ2b

T

M, L, and T are independent quantities; therefore, we can equate the indices on both sides, and have the following equations 0 ¼ a and 1 ¼ a Then, we can solve for and find that a¼0 and b¼ head can be found as hp ¼Ep/g.

b

1, which are the indices (numbers); consequently, the expression for pump

Example 28: Pump power Find: Show that the pump power y unit is W in SI. Solution: Pump power ðyÞ ¼ Ep ¼ kg s Pump powerðhÞ ¼ J s

1

1




J kg

1




¼W

1

_ is the magnitude of mass flow rate in kg s , and Ep is the magnitude of pumping energy in J kg 1. where m If the pump has a conversion efficiency of Z, then the pump input power ybk can be calculated as follows: Pump input power ¼ pump break power ðybk Þ ¼ pump fluid power=efficiency ¼ yfl =h where yfl is the pump output (fluid) power, and the efficiency Z is in decimals (i.e., 0.6 is used for 60% conversion efficiency). A dimensional analysis can easily be performed to find an expression for pump power, as we did for work and energy, and others earlier. Example 29: Pump fluid power Find: Show that the pump fluid power Ffl unit is W in SI. Solution: _ g hp ¼ kg s Pump fluid power ðFfl Þ ¼ m ¼ kg m s Pump f luid powerðFfl Þ ¼ J s

1


2

ms

1


1

ms

2




ðmÞ

¼Nms

1

¼W

1

_ is the magnitude of mass flow rate in kg s , g is the magnitude of gravitational acceleration in m s 2, and hp is the where m magnitude of pump head in m. Ffl ¼ F a Dxb t

c

where a, b, and c are the indices (numbers) to be determined. Now, we can replace each symbol by its fundamental physical unit, and have

a M1 L1 T 2 L1 T 1 ¼ M L T 2 ðLÞb T c or M1

L2 T

3

¼ Ma Laþb T

2a c

M, L, and T are all independent quantities; therefore, we can equate the indices on both sides, and have the following equations 1 ¼ a; 2 ¼ a þ b and

3¼

2a

c

Then, we can solve for and find that a¼b ¼c¼ 1; consequently, the expression for pump fluid power can be found as _ g hpump . Ffl ¼ F Dx t 1 ¼ m Example 30: Pump break power Find: Show that the pump break power Fbk unit is W in SI.

18

Energy Units, Conversions, and Dimensional Analysis

T 100°C

373.15K

212°F

Celsius temperature

Fahrenheit temperature

273.15K

0°C

255.37K

−17.78°C

0K

−273.15°C

Celsius temperature scale

Thermodynamic temperature

Ice point

32°F 0°F

−459.67°F

Fahrenheit temperature scale

Fig. 1 Relationships between thermodynamic temperature and temperature scales.

Solution: Pump break power ðFbk Þ ¼ O o

¼ kg m s 2 m s 1 1 ¼Nms Pump break powerðUbk Þ ¼ J s

1

¼W

2

where O is the magnitude of torque in kg m s , and o is the magnitude of angular velocity of the shaft in m s 1. A dimensional analysis can easily be performed to find an expression for pump break power, as we did in the previous example. Example 31: Temperature Find: Show what the temperature is in SI for a thermodynamic temperature of 300K. Solution: We know that temperature, a property, is related to hotness or coldness of an object; however, it is difficult to give an exact definition of temperature. The zeroth law of thermodynamics indicates that when two bodies have temperature equality with a third body, then in turn they have equal temperatures with each other. Thermodynamic temperature is, however, defined by the third law of thermodynamics in which the theoretically lowest temperature is the zero point. At this point (absolute zero), the particle constituents of matter have minimal motion and can become no colder. By definition, the temperature in degree Celsius is the difference between the thermodynamic temperature and the thermodynamic temperature of 273.15K. Note that, by definition, a temperature interval of 11C is equal to a temperature interval of 1K, and 01C (a.k.a. the ice point) corresponds to 273.15K (Fig. 1). Then, the temperature in SI (1C) can be expressed as follows: Temperatureð1CÞ ¼ Thermodynamic temperatureðKÞ2273:15 K Temperatureð1CÞ ¼ 26:851C Example 2: Temperature Find: Determine the thermodynamic temperature equivalent of 550oC. Solution: Thermodynamic temperature is expressed as follows: Thermodynamic temperatureðKÞ ¼ Temperatureð1CÞ þ 273:15K ¼ 5501C þ 273:15K Thermodynamic temperatureðKÞ ¼ 823:15K

Energy Units, Conversions, and Dimensional Analysis

19

Note that K was substituted for 1C because both units are identical as expressed before (Fig. 1). Example 33: Temperature Find: Determine the temperature in Fahrenheit for a given temperature of 211C. Solution: Note that ice point and boiling temperatures 0 and 1001C on the Celsius scale correspond to 32 and 2121F, respectively. Therefore the Celsius temperature range of (100–01C)¼ 1001C corresponds exactly to the Fahrenheit temperature range of (212–321F)¼ 1801F (Fig. 1). (212–321F)¼(100–01C) ¼(1801F¼1001C); therefore, 1.01C¼ 1.81F. Furthermore, knowing that 1C¼K, we obtain K¼ 1.81F. Then, the relationship between the Celsius temperature (1C) and Fahrenheit temperature (1F) can be defined as follows: Temperatureð1FÞ ¼ ð1:81F=1CÞð1CÞ þ 321F where 321F again is the freezing temperature for water in Fahrenheit temperature scale, which is equal to 01C in the Celsius scale (Fig. 1). Then, the temperature (1F) in this example can be determined as: Temperatureð1FÞ ¼ ð1:81F=1CÞ ð211CÞ þ 321F Temperatureð1FÞ ¼ 69:81FB 7011F The absolute temperature scale related to the Fahrenheit temperature scale is known as the Rankine scale and is designated R. The relationship between the Rankine and Fahrenheit temperature scales is expressed as follows: Rankine ¼ 1F

459:67

Example 34: Temperature Find: Determine the temperature in SI for a given temperature of 701F. Solution: The relationship between the Celsius temperature and the Fahrenheit temperature is defined as follows:
Temperatureð1CÞ ¼ ð1F2321FÞð5=9Þ 1C 1F 1 where 321F is the freezing temperature for water in the Fahrenheit temperature scale, which is equal to 01C in the Celsius scale. Then, the temperature in SI (1C) can be determined as:
Temperatureð1CÞ ¼ ð701F 321FÞ5=9 1C 1F 1 Temperatureð1CÞ ¼ 21:111CB 2111C Example 35: Density Find: Determine the unit (kg m 3) of density in SI if the unit is given in I–P system. Solution: Remember that the density is defined as the mass m per unit volume V. So, the density equation is a quantity equation established from a definition, which is expressed in I–P system as follows: r ¼ m=V ¼ lbm ft

3

Remember that: 1 ft ¼ 0:3048 m; and 1 lbm ¼ 0:45359 kg   r ¼ ½lbm ð0:45359 kg=1 lbm ފ= ft3 ð0:3048 m=1 ftÞ3 q ¼ 16:0184 kg m

3

Density is dependent on the mass m and the volume V. Now, let’s assume a general algebraic equation in the form of r ¼ ma =V b where a and b are the indices (numbers) to be determined. Now, we can replace each symbol in density by its fundamental physical unit, and overall, we would have
b M L 3 ¼ ðMÞa L 3 L and T are independent quantities; therefore, we can equate the indices on both sides, and have the following equations 1 ¼ a and

3¼

3b

Then, we can solve for and find that a ¼ b¼1, which are the indices (numbers); consequently, the expression for kinematic viscosity can be found as r¼ ma/Vb. Example 36: Specific volume Find: Determine the unit (m3 kg 1) of specific volume in SI if the unit is given in I–P system.

20

Energy Units, Conversions, and Dimensional Analysis

Solution: Remember that the specific volume is defined as the volume V per unit mass m, and is therefore a reciprocal of density. So, the specific volume equation is a quantity equation established from a definition, which is expressed in I–P system as follows: n ¼ 1=r ¼ ft3 lbm1 Remember that: 1 ft ¼ 0:3048 m; and 1 lbm ¼ 0:45359 kg   n ¼ ft3 ð0:3048 m=1 ftÞ3 =½lbm ð0:45359 kg=1 lbm ފ m ¼ 0:062428 m3 kg

1

Now, we can replace each symbol in specific volume by its fundamental physical unit, and overall we would have
M 1 L3 ¼ ðMÞa ðLÞb where a and b are the indices (numbers), which are determined as a ¼3 and b¼ kg 1 (in SI).

1, so specific volume has a dimension of m3

Example 37: Specific heat or entropy Find: Determine the unit (J (kg K) 1) of specific heat in SI if the unit is given in I–P system. Solution: The constant pressure specific heat (Cp) and constant volume specific heat (Cv) are useful functions for particularly gases. Remember that the specific heat is defined as the heat energy required to raise the temperature of a unit mass of substance one degree. So, the specific heat equation is a quantity equation established from a definition, which is expressed in I–P system as follows: C ¼ Btu lbm1 1F Remember that: 1 Btu ¼ 1054:35 J; 1 lbm ¼ 0:45359 kg; K ¼ 1:81F C ¼ ½Btu ð1054:35 J=1 Btuފ=½lbm ð0:45359 kg=1 lbm Þ1FðK=1:81Fފ C ¼ 4184 J=ðkg KÞ So, in SI system, the specific heat is defined as the heat energy (J) required to raise the temperature of 1 kg substance 1K (or 11C). Example 38: Enthalpy Find: Determine the unit (J kg 1) of enthalpy in SI if the unit is given in I–P system. Solution: Remember that the enthalpy is defined as the heat energy available per unit mass of a substance. So, the enthalpy equation is a quantity equation established from a definition, which is expressed in I–P system as follows: h ¼ Btu lbm1 Remember that: 1 Btu ¼ 1054:35 J; and 1 lbm ¼ 0:45359 kg h ¼ ½Btuð1054:35 J=1 Btuފ=½lbm ð0:45359 kg=1 lbm ފ 1

h ¼ 2324:5 J kg

Example 39: Entropy Find: Determine the unit (J g 1) of entropy in SI if the unit is given in I–P system. Solution: Remember that the entropy is defined as a measure of the molecular disorder. The higher the disorder of any system, the greater is its entropy; conversely a higher order status of a system gives a low entropy status. Boltzmann showed that there existed a simple relationship between the entropy of a given system of molecules and its probability (thermodynamic) of occurrence. So, the entropy equation is a quantity equation established form a definition, which has the symbol s, and is expressed in I–P system as follows: s ¼ Btu lbm1 Remember that: 1 Btu ¼ 1054:35 J; and 1 lbm ¼ 0:45359 kg s ¼ ½Btuð1054:35 J=1 Btuފ=½lbm ½ð1000 g=1 kgÞ ð0:45359 kg=1 lbm ފ s ¼ 2:3244 J g

1

Energy Units, Conversions, and Dimensional Analysis

21

Example 40: Thermal conductivity Find: Determine the unit (W (m K) 1) of thermal conductivity in SI if the unit is given in I–P system. Solution: Remember that the thermal conductivity is defined as the amount of heat that passes through a unit area when the temperature difference between the two sides is one degree per unit distance. So, the thermal conductivity equation is a quantity equation established from a definition, which is expressed in I–P system as follows: k ¼ Btu h

1

ft 1F

Remember that: 1 Btu ¼ 1054:35 J; 1 ft ¼ 0:3048 m; K ¼ 1:81F; 1 h ¼ 3600 s k ¼ ½Btu ð1054:35 J=1 Btuފ=½hð3600 s=1 hÞ ft ð0:3048 m=1 ftÞ1F ðK=1:81Fފ 1

k ¼ 1:7296 J s




=ðm KÞ

k ¼ 1:7296 W ðm KÞ– 1 Example 41: Heat transfer coefficient Find: Determine the unit (W (m2 K) 1) of heat transfer coefficient in SI if the unit is given in I–P system. Solution: Remember that the heat transfer coefficient is defined as the amount of heat that is transferred from/to a unit area per unit time when the temperature difference between the surface and the ambient is one degree. So, the heat transfer coefficient equation is a quantity equation established from a definition, which is expressed in I–P system as follows: h ¼ Btu h

1

ft2 1F

Remember that: 1 Btu ¼ 1054:35 J; 1 ft2 ¼ 0:0929 m2 ; K ¼ 1:81F; 1 h ¼ 3600 s 
 h ¼ ½Btuð1054:35 J=1 Btuފ= hð3600 s=1 hÞ ft2 0:0929 m2 =1 ft2 1FðK=1:81FÞ h ¼ 5:68 Wðm2 KÞ– 1 Example 42: Thermal transmittance Find: Determine the unit (W (m2 K) 1) of thermal transmittance in SI if the unit is given in I–P system. Solution: Remember that thermal transmittance is the amount of heat that passes through an entire wall, ceiling, etc., section of a unit area per unit time when the temperature difference between the air on the warm side and the air on the cold side is one degree. So, the thermal transmittance equation is a quantity equation established from a definition, which is expressed in I–P system as follows: U ¼ Btu h

1

ft2 1F

Remember that: 1 Btu ¼ 1054:35 J; 1 ft2 ¼ 0:0929 m2 ; K ¼ 1:81F; 1 h ¼ 3600 s 
 U ¼ ½Btuð1054:35 J=1 Btuފ= hð3600 s=1 hÞ ft2 0:0929 m2 =1 ft2 1FðK=1:81FÞ U ¼ 5:68 Wðm2 KÞ– 1 Example 43: Thermal resistance Find: Determine the unit (m2 K W 1 or m2 1C W 1) of thermal resistance in SI if the unit is given in I–P system. Solution: Remember that thermal resistance is the resistance of one unit area to heat flow through a substance per unit time when the temperature difference between the two sides is one degree, and has the symbol of R. Thermal resistance is an additive quantity; that is, 200 material has twice the R-value of 100 . And it does not include the boundary layer resistances. So, the heat transfer coefficient equation is a quantity equation established from a definition, which is expressed in I–P system as follows: R ¼ h ft2 1F Btu

1

Remember that: 1 Btu ¼ 1054:35 J; 1 ft2 ¼ 0:0929 m2 ; K ¼ 1:81C ¼ 1:81F; 1 h ¼ 3600 s 
 R ¼ hð3600 s=1 hÞ ft2 0:0929 m2 =1 ft2 1FðK=1:81FÞ =½Btuð1054:35 J=1 Btuފ R ¼ 0:176 m2 K W

1

Energy Units, Conversions, and Dimensional Analysis

22

Example 44: Wavelength Find: Calculate the maximum wavelength lmax for the Sun’s surface radiation. Solution: Wien’s displacement law states that the wavelength at which a blackbody emits its maximum amount of radiation is inversely proportional to its absolute temperature in Kelvin, lmax ¼ a/T, where l is in mm, a is 2897 mm K, and T is in K. Assuming an average surface temperature of approximately 6000K, we would have its maximum emission at lmax ¼ 2897 mmK=6000K kmax ¼ 0:48 lm A wavelength of approximately 0.5 mm lies within the visible spectrum. Example 45: Frequency Find: Determine the frequency of a radiational wavelength of 0.5 mm. Solution: Wavelength (l) and frequency (n) are related, and this relationship can formally be expressed through the speed of light c, as c ¼ l n, where c is the speed of light, which has a value of 3  108 m s 1, l is the wavelength (mm), and n is the frequency (cycles s 1, also known as Hz). Then, if one is known, then the other can easily be determined using the speed of light, which is a constant. v ¼ c=l ¼ 3  108 m s 1 =ð0:5 mmÞ ¼ 3  1014 mm s 1 =ð0:5mmÞ ¼ 6  1014 cycles s 1 v ¼ 6  1014 Hz Example 46: Energy content of a single wavelength solar radiation Find: Determine the energy content E, of a solar radiation frequency of 6  1014 Hz (or 0.5 mm wavelength). Solution: Solar radiation consists of photons, which can be considered as packets of energy, and is related to frequency v as E¼ h v, where E is the energy content (J), and h is the Planck constant (6.626  10 34 J s). E¼hv ¼ 6:626  10 E ¼ 3:98  10

1.1.7

19

34



J s 6  1014 cycles s 1

J

Concluding Remarks

This chapter provided energy units, conversions, and dimensional analysis. Base and derived quantities, relationships between quantities, and quantity equations were discussed, also covering the three basic types of quantity equations: the quantity equations developed from the laws of nature, the equations developed from geometry, and the equations developed from a definition. Then, the 20 multiples and submultiples in the SI unit system were presented. An introduction was also presented for dimensional analysis, which is quite a useful method for deriving an algebraic relationship between different physical quantities. And then, the Standard for Metric Practice, ASTM Standard E 380-84, as one of the basic standards, is referenced for SI usage. Moreover, conversion factors for energy related units rounded to three or four significant figures for conversion between SI and I–P were provided, as well as conversion factors for different physical quantities related to energy. And the chapter is concluded with some illustrative examples of unit conversions and dimensional analyses.

References [1] [2] [3] [4] [5] [6] [7] [8] [9]

CGMP. Comptes Rendus de la 11e CGPM (1960); 1961, p. 87. CGMP. Comptes Rendus de la 11e CGPM (1971); 1972, p. 78. BIPM. International vocabulary of metrology – basic and associated terms (VIM). 3rd ed.; 2012. Wildi T. Metric units and conversion charts. A metrication handbook for engineers, technologists, and scientists. New York, NY: IEEE Press; 1995. p. 65–6. Andrews J, Jelley N. Energy science: principles, technologies, and impacts 2007;New York, NY: Oxford University Press Inc.; 2007. p. 13–4. ASHRAE. Handbook of fundamentals. Atlanta, GA: American Society of Heating, Refrigeration and Air Conditioning Engineers, Inc.; 1989. ASHRAE. SI Guide for HVAC & R. 6th ed. Atlanta, GA: American Society of Heating, Refrigeration and Air Conditioning Engineers, Inc.; 1984. Standards for Metric Practice. ASTM Standard E380. Philadelphia, PA: American Society for Testing and Materials; 1993. The International System of Units (SI). National Bureau of Standards Special Publication 330. Superintendent of Documents. Washington, DC: U.S. Government Printing Office; 2001. [10] Metric Practice Guide. CSA Standard CAN 3-Z234-1. Rexdale, ON: Canadian Standards Association; 1973.

Energy Units, Conversions, and Dimensional Analysis

[11] ASME Guide. ASME orientation and guide for use of metric units. New York, NY: American Society of Mechanical Engineers; 1982. [12] ISO. SI units and recommendations for the use of their multiples and of certain other units. Geneva: ISO Standard 1000 International Organization for Standardization; 1992. Available from American National Standards for Metric Institute, New York, NY. [13] HRAI. Supplementary metric practice guide for the heating, ventilating, refrigeration, air conditioning, plumbing and air pollution equipment manufacturing industries. Etobicoke, ON: Heating, Refrigeration and Air Conditioning Institute of Canada.

Relevant Websites https://www.britannica.com/science/International-System-of-Units Brittanica. www.convertunits.com/SI-units.php ConvertUnits.com. https://www.ic.gc.ca/eic/site/mc-mc.nsf/eng/lm00068.html Government of Canada. https://www.nasa.gov/offices/oce/functions/standards/isu.html NASA. http://www.checklist.org.br/d/internationalsystemofunits.pdf NIST, U.S. Department of Commerce. http://www.physics.nist.gov/cuu/Units/ NIST, U.S. Department of Commerce. https://www.physics.info/system-international/ The Physics Hypertextbook.

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1.2 Historical Aspects of Energy İlhami Yıldız and Craig MacEachern, Dalhousie University, Halifax, NS, Canada r 2018 Elsevier Inc. All rights reserved.

1.2.1 1.2.2 1.2.2.1 1.2.2.1.1 1.2.2.1.2 1.2.2.1.3 1.2.2.1.4 1.2.2.2 1.2.2.2.1 1.2.2.2.2 1.2.2.3 1.2.2.3.1 1.2.2.3.2 1.2.2.3.3 1.2.3 1.2.3.1 1.2.3.1.1 1.2.3.1.2 1.2.3.1.3 1.2.3.1.4 1.2.3.2 1.2.3.2.1 1.2.3.2.2 1.2.3.2.3 1.2.3.3 1.2.3.3.1 1.2.3.3.2 1.2.3.3.3 1.2.3.3.4 1.2.3.3.5 1.2.3.4 1.2.3.4.1 1.2.3.4.1.1 1.2.3.4.1.2 1.2.3.4.1.3 1.2.3.4.1.4 1.2.3.4.1.5 1.2.3.4.1.6 1.2.3.4.2 1.2.3.4.3 1.2.3.4.4 1.2.3.4.5 1.2.3.5 1.2.3.5.1 1.2.3.5.2 1.2.3.5.3 1.2.3.5.4 1.2.3.5.5 1.2.3.5.6 1.2.3.5.7 1.2.3.5.8 1.2.3.5.9 1.2.4

24

Introduction Preindustrial Man Fire Fire and early man Gunpowder Metallurgy Steam boilers Animals Agriculture Transportation, hunting, and warfare Early Wind and Hydro Sailboats Windmills Waterwheels The Industrial Revolution Steam Engine Savery pump Newcomen atmospheric engine Watt and Boulton–Watt steam engines Solar reflector steam engine Textiles The spinning jenny The cotton gin The sewing machine Mining and Drilling Natural gas Coal mining Blasting caps and dynamite Oil drilling Standard oil Electricity Discovery of electricity Thales of Miletus William Gilbert Otto von Guericke and Charles François du Fay Pieter van Musschenbroek Benjamin Franklin Luigi Galvani and Alessandro Volta Electromagnetism Fuel cells Incandescent bulb War of the currents Transportation and Mass Production Trains and railroads Internal combustion engine Ethanol Automobiles Interchangeable parts The assembly line Ford’s Model T Hot air balloons Powered airplanes Nonrenewables

Comprehensive Energy Systems, Volume 1

25 25 25 26 26 26 26 27 27 27 27 27 27 28 28 28 28 28 29 29 29 29 29 30 30 30 30 30 31 31 31 31 31 31 31 32 32 32 32 33 33 33 34 34 34 34 35 35 35 35 36 36 36

doi:10.1016/B978-0-12-809597-3.00102-4

Historical Aspects of Energy 1.2.4.1 Fossil Fuels and Conventional Energy Sources 1.2.4.1.1 Coal 1.2.4.1.2 Oil 1.2.4.1.3 Natural gas 1.2.4.1.4 Nuclear fission 1.2.5 Renewables 1.2.5.1 Renewable Energy Sources 1.2.5.1.1 Hydro 1.2.5.1.1.1 Small-scale hydro 1.2.5.1.1.2 Large-scale hydro 1.2.5.1.2 Wind turbines 1.2.5.1.3 Geothermal 1.2.5.1.4 Solar 1.2.5.1.5 Biomass and biofuels 1.2.5.1.6 Tidal 1.2.5.1.7 Pyrolysis 1.2.5.1.8 Heat pumps 1.2.6 Near Future Energy 1.2.6.1 Potential Future Energy Sources 1.2.6.1.1 Space solar power 1.2.6.1.2 Generation IV nuclear fission reactors 1.2.6.1.3 Fusion power 1.2.6.1.4 Artificial photosynthesis and solar fuels 1.2.7 Concluding Remarks References Relevant Websites

1.2.1

25 36 37 37 38 38 39 39 39 40 40 40 40 41 42 43 43 44 44 44 44 44 45 45 45 45 48

Introduction

The story of man’s success and his eventual downfall is one that rests largely on the shoulders of our creative exploitation and reimagining of energy and its uses. Throughout history man has been able to utilize energy in ways other species have been unable to grasp, quickly distinguishing man as the alpha species on planet Earth. Ingenious use of energy has led to increased brain development, granted us the ability to travel great distances, allowed for the manufacturing of a variety of products at tremendous speed, and helped to power the machines that influence everything in life from healthcare to communication to science and research. Despite all that man’s command over energy has given him, the rate at which energy has been exploited has left mankind in a compromising position. Finite resources are rapidly being depleted and carbon emissions continue to cause large-scale environmental issues. Once again it will be up to man to overcome these challenges if the species is to survive and thrive. Renewable energy sources offer one answer to the problem and with increased implementation they may 1 day power the world. This is the history of how man and society evolved alongside energy and how we’ve arrived at the current situation.

1.2.2

Preindustrial Man

Perhaps the most influential change in the development of modern man was his shift toward the use of energy to complete tasks on a scale that was previously impossible. Prior to the first use of fire, man was a simple being, similar to many modern apes in terms of energy use. They ate food and used the calories within the food to perform work. This work generally comprised of attaining more food, protecting oneself from predators, and reproducing. This all changed once man began to use energy sources other than simple, raw food. Fire led to cooked food and protection from predators, the use of animals made agriculture and transportation more efficient; soon sailboats and windmills were taking advantage of wind energy for transportation and milling. Regardless of how energy was being used and which source it came from, there is one overarching theme linking these technologies together. They made what were difficult and tedious processes quicker and easier, allowing man more time to perform other tasks and work toward solutions to more advanced problems. It is undoubtedly this concept that led to man’s rapid development as a species and drastic technological advance in the years following the first use of energy.

1.2.2.1

Fire

Fire can most accurately be described as a chemical reaction that occurs between oxygen, heat, and a fuel source. Fire is not an object but rather the visible oxidation that occurs as a result of rapid combustion. This process is not entirely dissimilar from the

26

Historical Aspects of Energy

rusting of metals or the browning of an apple core. However, the crucial difference between these oxidative processes is the rate at which the reaction takes place. Heat, light, and sound are all the byproducts of this rapid reaction and ones that early man took great advantage of in developing as a species [1].

1.2.2.1.1

Fire and early man

The earliest known exploitation of the natural environment for energy production by humans comes in the form of fire. Some estimates state that man may have developed an ability to control and manipulate fire as early as 1.6 million years ago [2]. Stratigraphic evidence goes on to suggest that as early as 1 million years ago, in situ fires were being used by hominins [2]. Fire offered a variety of advantages to early man including protection from insects and predators, warmth, as well as providing illumination during the night. With that being said, perhaps the most important advantage fire provided early man was the ability to cook food [3]. Estimates suggest that prior to the advent of cooking, early man would require between 5.7 and 6.2 h per day chewing a tough, fibrous diet of plants, and raw meat [4]. This time-consuming process meant that when early man was not hunting and gathering they would be chewing. This time spent chewing required large teeth and jaw muscles similar to what we might observe in modern chimpanzees. With a mastery of cooking and genetic evolution, however, these teeth and jaw muscles began to shrink, leaving more room for the development of early man’s brain [3]. The earliest mainstream use of fire for processes other than cooking comes in the form of “fire-stick farming” [5]. Fire-stick farming was first observed in the Paleolithic and Mesolithic ages as a means of clearing large amounts of land [5]. Land was cleared for a variety of reasons, including clearing ground for permanent or temporary human habitats, regenerating plant-based food sources, facilitating travel, and even warfare [6]. Fire-stick farming had the effect of replacing larger older growth forests with faster growing grasses and perennials, drastically reshaping the landscape of the time. The burning process increased nutrient availability, which resulted in higher plant yields [5]. With the ongoing extinction of megafauna at the time, early man was forced to convert to a more plant-dependent diet, reinforcing the importance of these newfound perennials [6]. This process may be the earliest known use of agricultural practices by man. In modern times slash and burn or swidden agriculture practices continue to be prevalent methods for land clearing in agriculture. This process involves cutting down vegetation in an area and setting it on fire. The idea is that as the plant material burns it releases nutrients into the nearby soil resulting in highly fertile land. This land is then used for a number of agricultural practices until it is deemed no longer acceptable due to soil degradation. Current estimates state that there are over 200 million people who practice swidden agriculture globally [7].

1.2.2.1.2

Gunpowder

The discovery of gunpowder is most commonly attributed to Chinese alchemists during the 9th century AD. The active ingredients for gunpowder were discovered when an alchemist accidentally dropped charcoal into a bowl of potassium nitrate (saltpeter). The combination of the ingredients caused the mixture to deflagrate violently and, thus, gunpowder was born. The first widely used application of gunpowder came in the form of crude flamethrowers developed by the Chinese in the mid-1000s. These weapons held gunpowder in a bamboo or paper tube that was attached to an arrow. The arrows were then fired from a bow with devastating effects [8]. During the same time period a device that in modern times is known as a grenade was also developed. This device was described as a “bursting fire ball,” which also contained small bits of porcelain to cause further destruction. These two designs perfectly harnessed the explosive potential of gunpowder and led to many future inventions including rocketry, cannons, and firearms [9]. From here the knowledge and use of gunpowder spread west, through the Middle East, into Europe and eventually to England where Franciscan monk Roger Bacon took up the task of improving on the existing formula. Bacon experimented with various proportions of each ingredient and was the first to note hazel charcoal as the best variety for gunpowder. Bacon also made the important discovery that gunpowder with higher nitrate content was more explosive [8]. Bacon’s work directly influenced the implementation of the cannon and, in later years, firearms into the English military.

1.2.2.1.3

Metallurgy

The next major advancement in the exploitation of fire was seen in metallurgy. Wall paintings in the Old Empire of Memphis suggest that Ancient Egyptians utilized the intense heat generated by fire to melt and cast pure metals. These paintings go on to suggest that ancient Egyptians also developed blow pipes and bellows to deliver more oxygen to the fires, demonstrating their knowledge and comfort with this chemical reaction [10]. Metallurgical processes were greatly enhanced following the advent of coke. Coke is a coal-based product obtained through the destructive distillation of coal. Destructive distillation is a process, whereby a fuel source is heated to high temperatures in the absence of oxygen. This process has the effect of removing most of the volatile components found in the coal, resulting in a carbon mass known as coke [11]. Coking coal allowed for much larger furnaces and subsequently greater output [10]. Additionally coke produces far less smoke than conventional coal, leading to safer work environments [11]. Today, coke is an essential component in the processing of iron ore. With iron being the primary input in steel and many aluminum alloys, it is difficult to say what modern manufacturing would look like without the advent of coke.

1.2.2.1.4

Steam boilers

In modern times, fire sees extensive use in electricity and heat generation through the use of steam boilers. Oxygen-fed fires are used to boil large quantities of water whose steam in turn drives large turbines, generating electricity. Waste heat from this process

Historical Aspects of Energy

27

can also be captured and used in a variety of heating processes; this is known as cogeneration. These topics will be discussed further in the following sections [12].

1.2.2.2

Animals

The use of animals for agriculture, transportation, and hunting dates back thousands of years. Animals, such as cattle, horses, mules, donkeys, camels, elephants, and dogs have all been used for human benefit throughout this time [13]. By exploiting the energy expenditure of these animals, man developed the ability to perform essential tasks quicker and more efficiently. This led to greater crop yields, faster, and further distances traveled, as well as more fruitful hunts.

1.2.2.2.1

Agriculture

Some of the earliest uses of animal energy in agriculture occurs in the Mediterranean regions of Egypt and Ethiopia around 6000 to 5000 BC [14]. Egyptian wall paintings and papyrus records show the use of ard plows, towed by oxen as a means of tilling fields for the planting of crops. The ard plow is composed of a long wooden beam attached to a yoked pair of oxen at one end and an almost perpendicular metal share at the other. This share would be pulled through the ground by the team of oxen, thereby breaking up the packed soil allowing for easier planting and superior plant growth [13]. An operator would walk behind the plow controlling a pair of handles to ensure the share remained upright and in the soil. The entire process enabled the Egyptians to plow far more land, with far less manpower. Ultimately this meant greater crop yields and more food to feed their growing population.

1.2.2.2.2

Transportation, hunting, and warfare

The use of horses for transportation was first observed around 3500 BC by the Botai people of what is now modern-day Kazakhstan. The Botai utilized domesticated horses to gain a speed advantage when hunting wild horses for meat. Additionally, horses were used as a more efficient means of herding sheep. It is estimated that a man can herd around 200 sheep with a good herding dog, but this number can be increased to 500 if the man is on horseback. Furthermore, it is thought that horses would have been used as a means of quick entry and escape during tribal raids on enemy encampments [15]. As the use of horses spread across Europe and Asia, we begin to see the implementation of a new form of transport, the chariot. The earliest known use of the chariot appears in Mesopotamia around 3000 BC [16]. With that being said, it was not until 1800 BC that the chariot was popularized by the Anatolians, who may have helped in shaping modern transportation [15]. The use of the chariot was essential in warfare as it offered high maneuverability and a platform for ranged attacks. Essential to the chariots success was the use of horses or onagers for pulling the carts [16]. This combination of animal labor and drawn carts for transportation was a concept that extended until the advent of the modern automobile. Dogs were the first animals to be domesticated by man around the end of the last ice age. At this time, human society was still largely a hunting and gathering society, and dogs facilitated in this process [17]. Dogs were primarily employed as a means of tracking wounded prey and delivering the killing blow to potentially dangerous injured animals. This partnership led to greater hunting efficiency and safety resulting in more productive hunts. By using dogs to track wounded animals, man was able to expend less energy and time tracking animals and more time hunting further prey [17].

1.2.2.3 1.2.2.3.1

Early Wind and Hydro Sailboats

The earliest known use of wind energy comes in the form of crude sailboats designed and utilized by the Egyptians between 5000 and 4000 BC. These sailboats comprised of a single sail attached to the mast of what was little more than a hollowed log [18]. These boats helped Egyptians move up and down the Nile River and it has also been suggested that the use of these boats directly impacted the spread of the Naquda culture to Southern Egypt [18]. The impact of the sailboat continued to increase and with this came greater technological advancement. Larger sails and crews became common and by 2000 BC, trade in the Mediterranean was highly dependent on the use of the sailboat [19]. By 500 BC the Phoenicians and Greeks had popularized the trireme, which combined the benefits of human and wind energy for even greater propulsion through the water [20]. By AD 800 the Vikings were readily implementing hydrodynamically optimized boats capable of sailing far faster than previous designs [21]. Sailboats, through continued evolution and design upgrades, become a major part of globalization, world trade, and exploration throughout the next 1200 years. Trade throughout the Mediterranean, English Channel, Baltic Sea, and Indian and Atlantic Oceans would not have been possible without the use of wind-powered sailboats. These boats were also essential in the discovery of the Americas and painting the picture of the globe as a whole.

1.2.2.3.2

Windmills

The earliest known record of windmills dates back to 400 BC when a Hindu book known as the Arthasastra of Kautilya suggests the use of windmills for pumping water [22]. This, however, is the only mention of such windmills in history and is, therefore, difficult to confirm. The first confirmed application of wind turbines comes from Heron of Alexandria who implemented a vertical wind turbine into the design of a pipe organ during the 1st century AD [23]. It has been suggested that Heron’s reversal of conventional fan blades may have led to the eventual implementation of horizontal wind turbines in midmillennium Europe [24]. The first known implementation of vertical axis windmills on a large-scale comes from the Persians around AD 800. The Persians utilized

28

Historical Aspects of Energy

these windmills for the purposes of grinding grain, pounding rice, and for irrigation [25]. These same windmill designs have been found as far east as India suggesting that they were efficient enough to imitate [26]. The first horizontal axis windmills appear in Europe between AD 1100 and 1200. These windmills were primarily implemented for the purposes of grinding grain, pumping water, and in the case of the Netherlands, draining flood plains for expansion. At this point, European engineers already had an advanced knowledge of gearing systems. With this they realized that by utilizing horizontally positioned blades in combination with a horizontal to vertical shaft transmission gearing system they could make their windmills more efficient. This was a concept that was clearly already understood by the Europeans based on their implementation of the Vitruvian waterwheel [21]. This design was popularized in England, Belgium, and Normandy and through its success and efficiency quickly spread to the Netherlands, Germany, and Denmark [27]. By the end of the 19th century these windmills were achieving efficiencies of as high as 5% [21].

1.2.2.3.3

Waterwheels

The first known implementation of horizontal axis waterwheels comes from the Romans between 700 and 600 BC [28]. This waterwheel was known as a noria and consisted of buckets that collected water from a surface water source (usually a stream or river) and poured it into irrigation channels at greater potential [29]. These channels helped to provide water to nearby farmland, drastically increasing yield and productivity in areas that had conventionally relied on only rain for watering. Waterwheels driven by camels and oxen have also been employed in Afghanistan and other Middle Eastern countries for the purposes of irrigation. To this day there are regions in Sudan that continue to employ this method of irrigation [30]. By 100 BC the use of waterwheels for milling begins to gain popularity throughout Greece. Once again this method used buckets of water that filled up with the flow of the river and subsequently rotated the waterwheel. The change in weight caused by the filling and emptying of the buckets caused the wheel to rotate more efficiently. This rotation was transferred from the horizontal axis of the wheel to a vertical shaft, which in turn drove a milling stone. These milling stones were used primarily for grinding wheat and corn into flours for bread making [29]. This milling technique quickly gained popularity and by the end of the 1st century AD, was employed as far east as China [29]. Around AD 300 the Romans modified the design so that the buckets could be placed just below the surface of the water. This greatly improved the efficiency of the design [29]. By AD 1086 there were over 5000 watermills in use throughout mainland England and by AD 1800 this number had surpassed 500,000. Mills at this time were no longer simply being used for grinding corn and grain though. These watermills had been adapted for a variety of processes including powering bellows for iron production, grinding ingredients for paper making, sawing timber, crushing olives for olive oil, and in powering textile factories [29].

1.2.3

The Industrial Revolution

To this day, the Industrial Revolution remains the greatest time period for technological advancement in the history of mankind. Even today, many of the processes that allow for mass production, rapid transportation, and that power our lives can be credited to advancements made during this time in human history. The Industrial Revolution brought mankind into the era of fossil fuels and a world of cheap and easily attainable energy, more abundant than anything previously dreamed of. Steam engines gave a use for these fossil fuels and their variety of applications had drastic effects on production and manufacturing. Liquid fuels allowed homes to be lit at night and for automobiles to begin to pop up and replace traditional, animal-driven forms of transportation. Before long, electricity was making its way into the homes and offices of millions around the world, forever changing the way humans live and work. In combination, all of these advancements led to better qualities of life. Products were cheaper, food was more readily available, and healthcare drastically advanced with new findings and innovations. Regardless of what the effect was, the overarching consensus is that the Industrial Revolution sparked this upturn in human life and has had a greater impact on modern life than any other period in human history.

1.2.3.1 1.2.3.1.1

Steam Engine Savery pump

Perhaps the single most critical invention leading to industrialization was the advent of the steam engine. The first steam engine was built by Thomas Savery in 1698 for the purpose of removing water from mines. In Savery’s words, his machine was “an engine to raise water by fire.” Savery’s pump operated by vaporizing water to generate steam and using this steam to fill a secondary tank. Then by isolating the steam from its source and allowing the steam to condense a vacuum would be created, which would draw water from within the mines. This design worked well but was extremely limited in the depth at which it was effective. The main issue with this design was that it could only draw water at around 80 ft below the surface [31]. With the mines of the time aiming to go deeper and deeper, a better pump needed to be developed.

1.2.3.1.2

Newcomen atmospheric engine

The year 1712 saw the invention of the atmospheric engine by Thomas Newcomen [32]. Newcomen’s engine improved on Savery’s design in that it did not rely on a steam vacuum. Newcomen’s design used a horizontal beam with a pivot in the middle weighted on one side and incorporated with a boiler and piston on the other. The weighted side would drop, driving the piston upward, at this point, steam at near atmospheric pressure would fill the void in the cylinder left by the rising piston. Cool water would then be

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sprayed into the cylinder, quickly condensing the steam and thereby changing the pressure, which would pull the piston back down. This had the effect of raising the weighted side, which through another piston mechanism drove water to the surface. This automated engine drastically improved water removal efficiency and had a far greater operating depth than Savery’s design. Newcomen’s design was so revolutionary that it would be another 63 years before a better design was popularized [31].

1.2.3.1.3

Watt and Boulton–Watt steam engines

In 1776, James Watt was able to improve on Newcomen’s design following a critical observation. Watt noted that the repeated heating and cooling of the cylinder was wasting energy and would lead to the more rapid deterioration of the materials. Based on this observation he developed his own design in which the piston and cylinder remained hot at all times by incorporating an external condenser. By alternating the open and closed phases at the top and bottom of the cylinder, steam is able to enter the cylinder in alternate succession, therefore, driving the piston up and down. As new steam enters from the bottom, exhaust steam exits through the top, as the piston travels upward. The process then repeats itself in the other direction. The exhausted steam makes its way from the cylinder to the condenser where the steam is condensed back into water. The newly condensed water is then pumped to a hot water tank and recirculated back through the boiler. By keeping the piston and cylinder hot at all times, less energy was wasted in reheating and the thermal stress on the material was reduced [31]. In 1782 W modified his engine to incorporate a sun and planet gearing system that drove a flywheel. This flywheel had the advantage of providing smooth and constant output as opposed to the pulsating nature of earlier designs [33]. This crucial design change is what eventually led to steam engines becoming viable for the newly constructed factories that would come to drive the Industrial Revolution. The final major advancements in the steam engine came with the Boulton–Watt double acting engine. The greatest improvement in this design came in the form of a parallel motion mechanism that assured perfect alignment of the piston throughout its cycle. Additionally, this mechanism allowed for work to be generated on the upward stroke, where before the upstroke simply served to reset the piston. Watt was famously noted as saying that he was more proud of this invention than he was of the engine itself [31]. This design also incorporated a governor that could be used to throttle back the engine should less output be required. Both of these design changes greatly improved the viability of these engines for factories and made them more attractive as indoor options [31].

1.2.3.1.4

Solar reflector steam engine

In 1860, French inventor Augustine Mouchot became fascinated with the world of solar energy. Mouchot had read and heard stories of “burning mirrors” capable of lighting fire at far greater speed than any of the conventional methods. Mouchot recognized the potential of such energy and set out to find a more practical use for it. The design Mouchot came up with is not entirely dissimilar from modern-day solar dish concentrating collectors. Mouchot’s design oriented mirrors on a concave disk toward a central absorber tube. This heat generated on the tube was then used to boil water and the steam in turn drove a turbine. While there were some who felt that Mouchot’s invention would mean unlimited free energy the reality was that the energy produced was not on par with the steam engines available at the time. France’s climate simply did not lend itself well to solar energy; however, Mouchot demonstrated that the prospect for implementation in hotter regions was certainly there [34].

1.2.3.2 1.2.3.2.1

Textiles The spinning jenny

In the mid-1700s, spinning was a long and arduous task, performed by individuals known as spinners. Spinners used a spinning wheel to wind a single strand of cotton fiber into a yarn. This process involved manually twisting the yarn and ensuring the yarn remained taut until it was wound onto the spindle. Enter James Hargreaves, a British carpenter and weaver who was attempting to find a way in which to optimize this time-consuming process. Hargreaves had been attempting to utilize multiple spinners at once by holding all of the threads in this left hand, however, he quickly ran into difficulties when it came to twisting the yarn. This was due to the horizontal positioning of the spindles, which Hargreaves ingeniously remarked following the observation of a toppled spinning wheel, which continued to spin and operate. In 1764, Hargreaves used this observation to develop his spinning jenny, which utilized vertically positioned spindles to wind eight cotton threads at once. The position of the spindles allowed for the thread to be twisted automatically as well as ensuring the threads remained taut. This clever design could be operated by a single person, drastically reducing time and energy input. Hargreaves went on to develop a 16-thread version of the jenny and later inventors modified the design to be driven by an external engine. The spinning jenny drastically changed the textile industry by reducing labor demands, while increasing output, and is often considered as the machine that began the Industrial Revolution [35].

1.2.3.2.2

The cotton gin

During the late 1700s, cotton was a valued product, however, not in the same way it is today. The main issue with cotton was that seeds embedded in the cotton fibers required separating, a process that at this time could only be done by hand. American Eli Whitney recognized that if a better method for separating the seeds from the fibers could be developed then cotton could see a global upturn in value. With the American South being one of the major global producers of cotton, any production advantage would be massive for the region. In 1794, Whitney developed and patented a machine he called the cotton gin, which was capable of removing embedded seed from raw cotton fibers. The cotton gin operated by loading raw cotton fibers into a hopper where they

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Historical Aspects of Energy

were grabbed at the bottom by a toothed rotating cylinder, operated by a hand crank. This cylinder pulled the fibers through a fingered fence with holes too small for the seeds to pass through. Fibers were knocked from the cylinder on the other side of the fence and the seeds then fell to the floor. Whitney’s cotton gin took what was a laborious manual process and automated it, significantly reducing the time and manpower requirement for seed separating. Much like the spinning jenny, later designs incorporated engines to drive the cotton gin, further increasing efficiency [36].

1.2.3.2.3

The sewing machine

The year 1846 saw the invention of the textile industry’s greatest advancement in the form of the sewing machine. While off work on disability, watching his wife struggle to make enough money for his family by sewing clothes, Elias Howe conceived the idea for an automated sewing machine. Howe’s machine employed many of the concepts still used in modern sewing machine designs including, a needle with an eye located at the point, an automatic feed, and a shuttle carrying a second thread that formed a lockstitch in combination with the needle. The sewing machine drastically improved upon manual sewing techniques by significantly reducing the time required to do a job. For example, a man’s shirt, which would generally take a skilled seamstress approximately 14 and half hours to sew by hand, could now be completed in a little over an hour. This machine changed the textile industry more than any other advent, as clothing could, for the first time, be mass produced in factories [37].

1.2.3.3 1.2.3.3.1

Mining and Drilling Natural gas

Natural gas is a naturally occurring hydrocarbon comprised largely of methane. Natural gas can also be comprised of a variety of other alkanes including propane, butane, ethane, and pentane. Formation of natural gas occurs as a result of decomposing plant and animal material being exposed to high temperatures and pressures, exerted between layers of sedimentary rock beneath the Earth’s surface. Natural gas is used in a variety of processes though its main implementations are in home heating, cooking, and electricity generation [38]. The first natural gas well was drilled in 1821 by American William Hart near Fredonia, New York. Hart drilled his well 27 feet into the ground in order to reach a shale deposit with abundant natural gas. Hart’s well was used to supply natural gas to the nearby town, which used it to light a number of gas-powered lamps. From New York, natural gas development began to spread westward. By 1880, Ohio, Mississippi, Kentucky, and Illinois all had operational natural gas wells [39]. Further history of natural gas is discussed further in later sections.

1.2.3.3.2

Coal mining

While coal mining had been performed for some years prior, it wasn’t until the early 19th century that the industry truly took off. Industrialization demanded more coal to feed the newly developed steam engine, which was seeing increasingly greater implementation on a global scale. In fact, between the years of 1835 and 1882, the volume of global coal production rose from 36 million tons to 422 million tons. This 1172% increase demonstrates the drastic effect that the steam engine had on society in terms of production, transportation, and job creation. By the year 1900, 95% of global energy was derived from the combustion of coal [40]. The implementation of the Newcomen and Watt steam engines allowed miners to go deeper than ever before by removing large volumes of water that had previously limited the depths that mines could feasibly achieve [31]. This meant that more coal deposits could be accessed at greater depths, directly aiding the coal industry at meeting the ever growing demand. In addition to this, the implementation of rail systems made transporting the coal far easier, once again improving efficiency and increasing supply. Blasting caps and dynamite soon followed, further optimizing the labor-intensive process.

1.2.3.3.3

Blasting caps and dynamite

Prior to 1863, blasting was an extremely dangerous process, which often saw the death of its operators due to the innate volatility of the blasting chemical nitroglycerin [41]. Alfred Nobel, a Swedish chemist and engineer, recognized this problem and set out to find a method of making blasting safer for the operator. Nobel’s first contribution was a detonator, which contained a wooden plug filled with a small amount of black powder inserted into a metal container of liquid nitroglycerin. The explosion of the small amount of black powder would set off the larger nitroglycerin container. The process meant operators had more time and a safer method for igniting the nitroglycerin. Nobel went on to improve the design in 1865 to utilize mercury fulminate instead of black powder. The advantage of the mercury fulminate was that it could be ignited by either moderate heat or shock. The resulting detonator was known as a blasting cap and the concepts of this design are still largely employed in the detonation of modern high explosives [42]. After his invention of the blasting cap, Nobel still was not satisfied with the safety of the inherently volatile nitroglycerin. He set out to find a way to make the explosive more stable without compromising its explosive output. This issue was particularly close to Nobel’s heart as a nitroglycerin factory owned by Nobel blew up in 1864, killing his brother and several other employees. What Nobel found was that by mixing the nitroglycerin with kieselguhr (a porous and siliceous soil), the two would form a moldable paste [41,42]. The paste was much more stable under storage conditions, which made handling and transporting much safer for all involved. Most importantly, however, was that the paste did not lose any of its explosive output compared with the liquid nitroglycerin. The paste was molded into a stick, which Nobel would patent under the name of dynamite in 1867. Nobel went on to develop a more powerful form of dynamite known as blasting gelatin in 1875 as well as ballistite, one of the first smokeless

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powders and an obvious precursor to the widely employed cordite [42]. Nobel amassed a great amount of wealth from his inventions and eventually his fortune was used to fund the creation of the Nobel prizes [41].

1.2.3.3.4

Oil drilling

Prior to the 1850s, petroleum was largely seen as more of a nuisance than a valuable product. While it did see limited use in medicinal products, it was better known for dirtying people’s shoes and causing issues in mines. This all changed with the advent of kerosene in 1852 by Canadian physician Abraham Gesner [43]. Gesner produced his kerosene through the distillation of asphalt known as albertite [44]. Kerosene was cheaper and burned longer than the conventional whale oil of the time. This newly developed product spurred a boom in the petroleum industry and the first to capitalize on the rush was The Seneca Oil Company. Seneca Oil owner George Bissell, who had heard of petroleum contaminating salt wells, commissioned Edwin Drake, to go and survey the area for potential drilling sites. Upon arrival, Drake determined that the site had great potential, stating, “Within 10 min of my arrival upon the ground with Dr. Brewer I had made up my mind that (oil) could be obtained in large quantities by boring as for salt water. I also determined that I should be the one to do it.” Bissell and Drake purchased the land from Brewer and Drake set up their drilling operation [44]. Almost immediately, Drake encountered issues; his archaic drilling equipment only allowed him to attain a drilling depth of 16 feet, far less than the 1000 feet Drake has prepared to go to find oil. In order to overcome this hurdle, Drake developed what would become known as a drive pipe. The drive pipe prevented the wells from falling in on them and allowed Drake to drill an average of three feet per day through bedrock. Unfortunately, due to the delays, Drake had lost the financial support of Seneca Oil and was forced to fund the project out of pocket. This all paid off on August 27, 1859 when at a depth of 69.5 feet, Drake’s drill struck petroleum. Drake’s well was able to produce between 20 and 40 barrels of oil per day [45] at a price of $1.25/gallon [44]. By 1876 the wells had gone dry and Drake, who had failed to patent his design or have a backup plan, was left penniless until Seneca Oil reimbursed some of his efforts in 1880 [45].

1.2.3.3.5

Standard oil

In June 1870, John D. Rockefeller amalgamated a number of his companies to create what would become one of the world’s most powerful and influential organizations in Standard Oil. Standard Oil was a petroleum producing, refining, and transporting corporation based out of Cleveland, OH. At the outset, Standard Oil was just another oil refining corporation; however, Rockefeller quickly realized that if his business was to thrive he would need to gain a competitive advantage. Rockefeller realized that the processes of drilling and refining could be altered very little; however, where he could save the most money was on transportation. Rockefeller bargained with the railroad companies to give him a reduced rate in exchange for his exclusive business and large regular shipments. Based on loans he had accrued, Standard Oil became the largest refinery in Cleveland. Rockefeller knew that the other refineries could not produce shipments as large as his, protecting his investment with the railroad companies. This arrangement with the railroad quickly gave Standard Oil an advantage over its competitors. This advantage was one that Rockefeller was not afraid to enforce either; he regularly bought up market supplies of refining chemicals, oil barrels, and train cars, forcing his competition to either sell out to Standard Oil or go out of business [46]. Rockefeller continued to flex this advantage, becoming one of the world’s first multinational companies and the first true economic monopoly. This monopoly certainly did not go unnoticed and in 1911 the US Supreme Court ordered the dissolution of Standard Oil into seven noncompeting regional companies [47]. Rockefeller would still go on to profit from the dissolution; however, the monopoly had been broken up and a healthy competitive market would soon be restored.

1.2.3.4 1.2.3.4.1

Electricity Discovery of electricity

1.2.3.4.1.1 Thales of Miletus The earliest known discovery of electricity comes in 585 BC in a statement written by Thales of Miletus. Thales recognized the attractive powers of amber rubbed with animal fur as well as iron to lodestone. Thales attributed the objects to having some sort of soul, but what he truly discovered were the first observations of static electricity and magnetism [48]. 1.2.3.4.1.2 William Gilbert Very little occurred in the centuries that followed until Englishman William Gilbert came out with his book in 1600, titled De Magnete. Gilbert was the first to use the terms electricity (from the Greek word electron for amber), electric force, magnetic pole, and electric attraction. Additionally, Gilbert had an even greater contribution to the world of science. In his book, he noted how scientists consistently made claims of research but never displayed proof from an experiment. Gilbert changed this method of conducting research by offering gross detail of his experiments on static electricity so that the results could be reproduced by others if warranted [48]. 1.2.3.4.1.3 Otto von Guericke and Charles François du Fay The year 1663 saw Otto von Guericke make another crucial discovery in the understanding of electricity. Von Guericke built a sulfur sphere with a wooden rod through the middle so that the sphere could be easily rotated. By rubbing the sphere, it was found that the sphere would attract objects, such as chaff and feathers. This phenomenon was not a new discovery; however, von Guericke noted that once the chaff or feather touched the sphere it was immediately repelled. This was the confirmation of

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electricity’s attractive and repulsive nature as noted by Niccolo Cabeo in 1620; however, now it was confirmed that an object's charge is not fixed. In creating his rotating sphere, von Guericke had in fact created the world’s first electrical machine. This machines general design would remain the primary means of electricity generation for years to come. In 1733, Charles Francois du Fay repeated this experiment and concluded that the charge was being transferred from the sphere to the feather in repeated cycles. The feather gained charge every time it contacted the sphere and was repulsed; as it lost charge it fell toward the sphere and the cycle repeated [48]. Du Fay termed these two charges resinous ( ) and vitreous ( þ ). 1.2.3.4.1.4 Pieter van Musschenbroek The first major breakthrough in controlling electricity comes from Pieter van Musschenbroek, a Dutch scientist, who was looking at ways of storing electric charge. Van Musschenbroek built a device that consisted of a glass jar filled with water and wrapped in metal foil on the inside and outside. He called his device the Leyden jar, after the city it was built in. A charge was applied between two electrodes, one inside and one outside the jar, using an electrostatic generator with the idea that this would store the charge. It was thought that the charge was stored in the water; however, we now know that it was in fact the glass that was storing the charge. In order to test if the Leyden jar worked, van Musschenbroek touched the inside and outside of the jar simultaneously, later writing that he “would not repeat the experiment if offered the whole kingdom of France.” The Leyden jar worked quite well and in building it, van Musschenbroek had invented the world’s first practical capacitor [48]. 1.2.3.4.1.5 Benjamin Franklin The man often credited with the discovery of electricity is American Benjamin Franklin. While Franklin’s contributions to the understanding of electricity were substantial it would be difficult to conclude that he in fact discovered electricity. Franklin’s first contribution to the field was that electricity acted as a single fluid, not two separate fluids as described by du Fay. In 1747, Franklin hypothesized that when two objects were rubbed together it was not the positive and negative fluids being transferred between the objects, but rather the absence or excess of a single fluid, transferred between the objects, which determined the positive or negative state of the object. To confirm this, Franklin devised an experiment where persons A and B are standing upon a sheet of wax, and a third person C is standing on the floor. Before anything happens, if there is contact between any of the persons then there will be no shock. Person A then rubs a glass tube, thereby, transferring his electricity into the tube. Person B then touches his knuckle to the tube drawing the electricity originally from person A into his own body. Thanks to the wax sheet on the floor, the electric charge in maintained within their bodies. In this instance we say that person A is negatively charged as he has given up some of his electricity and person B is positively charged as he has taken some electricity on. If person C touches either person A or B he will be shocked due to the potential difference between them. However, if persons A and B touch they will receive a shock greater than that of touching person C due to the greater potential difference between them. After the instance of the shock any contact does not result in another shock. This experiment helped in proving the one electrical fluid theory [48]. Franklin’s second major contribution was his discovery that lightning was indeed the same electricity observed in small-scale electrical experiments. To do this he famously tied a metal key to the end of a kite string and flew it during a thunderstorm. Franklin found he was able to draw sparks from the key confirming the nature of lightning. This led to the design and implementation of lightning rods throughout America and eventually the world. The effect was drastic reduction in lightning related injuries and damage [49]. 1.2.3.4.1.6 Luigi Galvani and Alessandro Volta In 1786, Italian physician and professor Luigi Galvani made what would turn out to be a crucial discovery on how the nervous system operates. Galvani’s lab had an electrical generator and he noted that if the generator was sparking, while he touched a dissected frog’s leg with a metal scalpel that the leg would twitch violently. This led Galvani to the conclusion that animal nervous systems were controlled by electrical impulses. While he was correct in this assumption, Galvani went on to hypothesize that the electrical impulses were generated by the frog itself. He based this assumption on a test he performed, whereby he could cause the frog leg to twitch simply by suspending it in a fluid and touching it with different metals. This hypothesis, however, was not satisfactory for Italian Alessandro Volta, who felt that the electrical impulses could not possibly be generated by the frog itself. What Volta found most fascinating was that the twitch only occurred when two distinctly different metals were used. Additionally, Volta became frustrated by the fact that he could not draw any kind of spark from the frog leg (the true indicator of electric current). Based on these observations, Volta decided that the frog leg itself was irrelevant. He designed a test where he took strips of zinc and brass and placed a salt water soaked piece of cardboard between them. He then stacked a number of these piles on top of one another (as many as 60). What Volta found was that he could easily and repeatedly draw a spark from the pile and that unlike the Leyden jar the device did not need to be recharged. Volta had created the world’s first battery, an invention that would completely change technological innovation [48].

1.2.3.4.2

Electromagnetism

Charles-Augustin de Coulomb was a French physicist who focused the majority of his research efforts on the attractive and repulsive forces of electricity. His most important contribution comes in his formulation of Coulomb’s law. Coulomb’s law describes the relationship between electrically charged particles and the force exerted between them. His law was essential in the birth of the new field of electromagnetism [50]. In 1820, French physicist Andre-Marie Ampere became fascinated with the world of electromagnetism. After observing a demonstration first conducted by Hans Christian Orsted, where a magnetic needle was deflected by an adjacent electric current,

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Ampere began to dedicate his life’s work toward this phenomenon. By expanding on the work performed by Orsted, Ampere was able to determine that two wires carrying an electric current also repelled or attracted one another dependent on the direction in which the current flows. From these results, Ampere was able to develop a number of physical and mathematical laws including Ampere’s law, which states, “The mutual action of two lengths of current-carrying wire is proportional to their lengths and to the intensities of their currents” [51]. Through his work, Ampere had created the field of electromagnetism and the unit for current, the ampere, now bears his name. One of the most influential scientists of the 19th century was English chemist and physicist Michael Faraday. Faraday made various contributions to the world of science but perhaps none are more important than his invention of the electric motor. Expanding on the work of Ampere, Faraday realized that if magnetic force was produced in a circular fashion and a current-carrying wire was suspended within the effective radius of this force, then the effect would be the wire rotating in a constant circle at a constant rate around the emitter of the force. This realization led to Faraday’s construction of the electric motor, a design which has since become mainstream in modern technology [52]. The most profound contribution to the field of electromagnetism comes from Scottish mathematician and physicist James Clerk Maxwell. Maxwell was the first man to adequately and completely explain the relationship between electricity and magnetism. His development of “Maxwell’s Equations” was the first to describe the relationship between electricity and magnetism and how they affect and interact with one another. This fundamental understanding of electromagnetism was essential in the development of everything from cell phones to televisions to deep space telescopes [53].

1.2.3.4.3

Fuel cells

There is to this day still much controversy over who was truly the inventor of the world’s first fuel cell. What is known is that during the years of 1838 and 1839 both Christian Friedrich Schonbein and Sir William Robert Grove created functional fuel cells. Particular to Grove, he showed that by submerging two platinum electrodes into a container filled with sulfuric acid and the other two ends, one each in a container of oxygen and hydrogen that an electric current would flow between the electrodes. Grove called his invention the “gas battery” though what he really created was what we now refer to as a fuel cell [54]. While the design has been optimized, the overall concept of the fuel cell remains largely unchanged. Today, the most common applications of fuel cells are in electric vehicles, cogeneration systems, and backup power supplies.

1.2.3.4.4

Incandescent bulb

The first lightbulb was developed in 1807 by English chemist Sir Humphry Davy. Davy’s bulb was known as an arc lamp, which worked by utilizing a battery to create an electric arc between two charcoal sticks positioned 10 cm apart from one another. This design could not operate for long based on the life of the batteries, however, once practical generators became available in the late 1870s the technology took off [55]. Even still these bulbs had short life spans, used too much energy, and were quite expensive. Then in the late 1870s American inventor Thomas Edison set out to devise a filament for a practical, long-lasting, and inexpensive incandescent bulb. Incandescent bulbs generate light by running a current through a thin piece of metal known as a filament. This current generates heat within the metal, which causes it to glow, producing light. The difficulty was that filaments often broke due to the high heat conditions and Edison realized that if he was to accomplish his goals he would need to develop a better filament. Edison began by testing a variety metals as filaments with varying degrees of success. His most efficient model utilized a platinum filament, however, the bulb still only burned for a few hours. Edison hypothesized that tungsten (the material in modern filaments) would make an excellent filament, however, the tools of the day limited his ability to work with this material. Then Edison had his most important idea on the road to producing the first practical lightbulb. Edison began working with carbonized plant materials. He tested everything from hickory to cedar to bamboo, eventually concluding that carbonized cotton thread would be the best for his bulb. On the first test the carbonized cotton thread filament burned for 15 h, a drastic improvement on prior iterations. While the materials have changed slightly, Edison’s basic design is still quite similar to the modern incandescent bulb [56].

1.2.3.4.5

War of the currents

During the late 1880s the debate over how America would be electrified was in full swing. Alternating (AC) and direct current (DC) were at the forefront of this debate and each was supported by powerful figures in Nikola Tesla and Thomas Edison. Edison, backed by General Electric, had already begun building his DC power plants throughout the Northeast. Edison, a wealthy businessman, made it his point to discredit Tesla’s AC power in an attempt to protect his own investment. Edison put on displays where he electrified animals and subsequently invented the basis for the electric chair. Citing the dangers of AC, however, could only keep DC ahead for so long. DC had its own major issues, the first was that it was difficult to step the voltage up or down safely for use in a variety of devices. Additionally, DC could only be transported around a mile before a supplementary power station was required. Tesla recognized the flaws with DC and in doing so developed his own AC motor in 1888. Tesla, however, was not a wealthy man and as such needed to find supporters for his AC motors. Tesla partnered with George Westinghouse and the two set out to bring AC power to the world. The major breakthrough for Tesla and Westinghouse came in the form of the 1893 Chicago World’s Fair. The fair would be the first to be illuminated by electrical light and an open bid was held to secure the rights to lighting it. Edison and General Electric placed a bid of $554,000 only to be outdone by Westinghouse’s bid of $399,000. This showcase was the breakthrough AC power generation needed. It demonstrated the ease and distance of transmission, the simple ability for step-up and step-down of voltage and most of all that it was cheaper than DC current. While Edison still argued that AC

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was too dangerous, ultimately his technology lost out to Tesla’s AC generation. Further major projects soon followed, including being awarded the contract to harness the power of Niagara Falls to generate electricity for the city of Buffalo, NY. This was yet another display of the seemingly limitless potential of AC power generation [57,58].

1.2.3.5 1.2.3.5.1

Transportation and Mass Production Trains and railroads

The earliest railroads were designed by Germans in the mid-1500s. These primitive railroads utilized wooden rails as runners for horse-drawn sleds. The rails made pulling heavy loads far easier on the horses than utilizing the bumpy and often impassable roads. These wooden rail systems were known as wagonways and were the precursor to modern railways. By the 1770s, iron had replaced wood as the material used in rail construction. Iron was more durable and therefore required less maintenance and replacement. Iron tramways spread throughout Europe, with horses still providing the work needed to transport heavy goods. One of the most influential advancements in rail technology came in 1789, when English engineer William Jessop developed flanged wheels to fit over the preexisting rails. The flanged wheels meant that wagons could grip the tracks better, thereby, limiting the amount of times the wagons fell from the tracks. This design choice was one that was carried over into the development of the first locomotive [59]. The first steam locomotive was designed and built by English mining engineer John Blenkinsop in 1812. Blenkinsop’s locomotives hauled coal from a mine in Middleton, England to the nearby city of Leeds [60]. Blenkinsop’s engines ran on cast iron or wooden rails, which incorporated a third cogged rail in which a ratchet wheel ran [60,61]. This had the aim of helping the locomotive ascend hills without slipping on the rails. The issue with the design was that the ratchet wheel frequently slipped out of the cogged rail resulting in down time and safety hazards. Englishman George Stephenson recognized that a better locomotive design was needed and in doing so he designed and built the Blucher. The Blucher was a steam engine that was able to haul eight fully loaded wagons (30 t) of coal at 4 mph (6 kph). Despite the success of the Blucher, Stephenson was not satisfied with his design. He remarked that there was a lot of energy being wasted by escaping steam in the boiler, and wanted to find a means of more effectively utilizing this steam. Stephenson took his observation and used it to design the locomotive steam blast (blast pipe), which directed exhaust steam up and out of a chimney. This process drastically increased the draft in the engine by drawing in fresh air as the exhaust steam and smoke was expelled. In all, the advent of the steam blast drastically increased engine efficiency [61]. Stephenson continued to make improvements and additions to his steam engine as time went on and he quickly became involved in the construction of the first large-scale railways. After meeting with Edward Pease, a promoter of a new railway from Darlington to Stockton, England, Stephenson was able to convince Pease to allow him to build a steam locomotive for the line. On September 27, 1825, the line opened and Stephenson’s locomotive carried 450 people between the two cities. With that, passenger transport was born and a new means of quick, long distance transportation was now possible. In the years that followed, Stephenson went on to help in the construction of a line between Manchester and Liverpool, England, as well as consulting on a number of other lines throughout England, mainland Europe, and North America [61].

1.2.3.5.2

Internal combustion engine

In 1879, German Karl Benz received the patent for the two-stroke internal combustion engine. Benz had based his design on the significant bodies of work performed by Samuel Morey, William Barmet, Eugenio Barsanti, Felice Matteucci, Jean Joseph Etienne Lenoir, and Nikolaus Otto. What is important to note is that, while Benz may be the individual who gets the credit for the internal combustion engine, many of the concepts for the design were borrowed from those who came before him. Otto in particular felt as though he was the true inventor of the internal combustion engine, however, German courts did not feel his patent covered all incylinder compression engines. After this event, knowledge of in-cylinder combustion engines became universal, allowing Benz to develop his two-stroke and later four-stroke engines, which would become the basis for his automobile [62]. The four-stroke engine requires four distinct actions to generate power. These actions generate power, which in turn can be used to propel the automobile. The first is the intake stroke; as the piston travels downward an air/fuel mixture is let into the cylinder. As the piston begins to travel back upward the air/fuel mixture is compressed; this is known as the compression stroke. Once the piston reaches the top of the stroke a sparkplug ignites the compressed air/fuel mixture driving the piston back downward; this is known as the power stroke. Finally, a valve is opened and the fuel exhaust is pushed out as the piston travels back up. This final stroke is known as the exhaust stroke. It was this crucial development that helped Benz to design the world first commercially available automobile.

1.2.3.5.3

Ethanol

While ethanol (ethyl alcohol) and other first generation biofuels have gained quite a bit of attention in recent years they should be thought of as a revisited technology as opposed to an entirely new one. Ethanol as a fuel was first used by American inventor Samuel Morey in 1826 as part of his work on the internal combustion engine. He combined the ethanol with turpentine to form his fuel mixture and was able to use it in combination with his engine to power a small boat and wagon up the Connecticut River at eight miles per hour [63]. It is thought that Morey would have distilled his ethanol from common grains available to him at the time [64]. In 1860, Nikolaus Otto built on Morey’s work, using ethanol in his own effort to develop the internal combustion engine. Otto developed a carburetor, which heated the ethanol, allowing it to vaporize more easily when the engine was first

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35

started. This eliminated many of the early issues associated with ethanol fuels [63]. In 1862, the US congress imposed a $2 alcohol tax in order to control beverage alcohol. Unfortunately due to some concerns with Moonshiners and others looking to find ways of consuming ethanol fuel, it too was included in the tax [64]. This tax drastically impeded the advancement of the technology until 1906 when the tax was lifted due to concern over foreign oil availability [63]. While this did encourage the production of ethanol for a short period of time, eventually petroleum won out due to its low cost, lack of competition with food crops, and shrinking concerns over global availability.

1.2.3.5.4

Automobiles

The year 1885 saw the invention of the world’s first automobile powered by an internal combustion engine. This automobile was also created by Karl Benz whose invention would change transportation forever. Benz’s design utilized a 1600-cc single-cylinder four-stroke engine to power a three-wheeled automobile at ¾ horsepower. The Motorwagen, as it was known, was able to achieve a top speed of 8 mph [65]. Despite the success of the Motorwagen there were still a number of shortcomings with the design. Panhard and Levassor utilized engine licenses from Daimler to improve drastically on the design of the automobile. These improvements included the addition of a steering wheel, inclined steering column, pneumatic tires, a clutch pedal, and the tube type radiator system [66]. Each of these contributions went on to become staples in automobile design and the concepts of Panhard and Levassor are still employed today. Despite these advancements, automobiles of the time were still produced individually by manual labor. Parts were difficult to find and skilled laborers capable of making the repairs were even more of a challenge to locate. There was a need for greater homogeneity in the automobile industry and a few men recognized this need and set out to solve the problem [66].

1.2.3.5.5

Interchangeable parts

To this day, there is still much debate over who was the true father of replaceable parts, however, the man who receives most of the credit is American Eli Whitney. Whitney was an inventor who was tasked by the American government to produce 10,000 firearms for a potential armed conflict with France. At this point in time, a skilled craftsman would generally oversee the construction of a single firearm from milling to final product. This process was time consuming and energy intensive as the production of a single musket took up to 21 man-days [67]. Whitney realized that having each worker produce a single weapon at a time was not efficient and as such, he set out to devise devices that could create each individual part of the weapon. The development of these machines meant that each gun was the same and that parts could be produced first and assembled later by unskilled workers. This meant cheaper and more efficient labor, which completely changed the philosophy of product production from one of a skilled profession to one that could be done by general laborers. With Whitney’s changes, a single rifle could now be produced in only 9 man-days, a significant improvement on prior practices. This seemingly simple realization led to drastic efficiency improvements that are still in practice today [67]. Whitney’s revelation was not only useful in firearm production, however, and the concepts were quickly picked up by the auto industry as well as other manufacturing facilities.

1.2.3.5.6

The assembly line

As time has gone on, credit for the first assembly line has often gone to Henry Ford in 1914, however, this was not the first implementation of this groundbreaking innovation. Some 13 years earlier a man by the name of Ransom Olds had developed a system that took advantage of interchangeable parts and a factory system of work to mass produce his company’s Curved Dash Oldsmobile. By 1903, however, Olds had sold his company and with the sale the assembly line was forgotten until 1914 [68], when Henry Ford completely revolutionized the assembly line and with it the entire production industry. Ford’s major contribution was that of the moving assembly line. Cars were attached at the chassis and towed along the production line where workers would each make one small addition to the cars as they moved. With each worker making the same addition to different cars over and over again, efficiency was greatly increased. To give an idea of the efficiency increase, the fastest a complete chassis could be produced prior to the assembly line was 12 h and 28 min. Following the advent of the assembly line, workers could now complete a chassis in a mere 5 h and 55 min. Ford made further efficiency changes, which included raising the assembly line to waist height, subdividing the jobs, and increasing the number of workers on the line. At its optimal efficiency the Ford assembly line could produce a completed chassis in as little as 1 h and 33 min. This entire process meant that Ford could sell his automobiles for far less, thereby drastically outselling his competitors and bringing the automobile to the masses [69]. All of this success would not have been possible for Ford without his inclusion of replaceable parts. This inclusion served not only to increase production but made repairs and maintenance far easier.

1.2.3.5.7

Ford’s Model T

Perhaps no means of transportation has had more impact on human society than the Ford Model T. The Model T was the first successfully mass produced automobile, effectively shaping the way everything from clothes to ovens to fast food was produced. The first Model T rolled off production lines on October 1, 1908 and even the most optimistic could not predict the impact of this vehicle. While there were other options available at the time, Ford’s Model T offered a number of unique advantages. The first was the cylinder design, which incorporated four cylinders with detachable cylinder heads. This made maintenance far cheaper and easier to perform. The second major advantage was that the Model T frame was constructed of high strength vanadium alloy steel and it rested higher off the ground than other automobiles of the day. These aspects in combination made the vehicle more robust when traveling on the questionable roads of the time. The Model T was also a capable off road vehicle due to its high clearance and

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stronger frame. Most important to the Model T’s success, however, was its low cost. Henry Ford’s vision was to create an automobile so affordable that any man making a common wage could afford it. His assembly line allowed this to be possible as when the Model T first became publically available it cost a mere $850 [70]. By 1925, efficiency had improved to the point where the Model T was being sold for less than $300. It is estimated that at this time, the Model T made up 40% of all vehicle sales in the United States [71].

1.2.3.5.8

Hot air balloons

The pioneers of hot air balloon technology were Etienne and Joseph Montgolfier. The brothers began their venture into balloonlifted flight after the discovery that paper bags filled with hydrogen would rise until the hydrogen diffused from the bag. This led them to the conclusion that should the gas within the balloon be less dense than the air outside then the balloon will rise. This phenomenon is also known as the buoyant force. The brothers then moved on from hydrogen to heating the air within the balloon. The brothers decided to burn moistened straw and wool and allow the smoke to fill the balloon. The brothers hypothesized that the smoke from this mixture would be lighter than air and as their balloon rose they concluded that this was indeed the case. What the brothers failed to realize, however, was that the true cause of the balloon’s ascent was the heating of the air already within the balloon and was not attributed to the fuel source. As the air within the balloon is heated it becomes less dense than the cold air outside resulting in the ascent of the balloon [72]. The first manned flight took place on November 21, 1783 and from there the technology quickly took off [73]. The aspects of the balloon have changed very little since this time. Better heating elements, improved balloon materials, and different air mixtures have all been employed but the overall concept remains the same.

1.2.3.5.9

Powered airplanes

In 1843, the first concept for powered flight was designed and published by William Samuel Henson. Henson’s “aerial steam carriage” was a monoplane design that incorporated a rectangular wing comprised of an upper and lower skin. The design also incorporated two steam-powered propellers located near the back of the wings. Despite publishing his work and producing a prototype in 1857, Henson was never able to achieve sustained flight with his aerial steam carriage. With that being said, many of the concepts employed in Henson’s design went on to directly influence future efforts at sustained, powered flight [74]. A major breakthrough came in the second half of the 19th century by German Otto Lilienthal. Lilienthal was a true airman who realized that controlling the plane after flight was achieved was as much the issue as actually getting off the ground. Lilienthal began testing glider designs and in doing so became the first man to perform a successful controlled glide. He went on to perform a total of over 2500 glides resulting in a total airtime of around 5 h. Lilienthal was prepared to move toward powered flight in 1896, however, on what would end up being his final glide, he encountered a thermal gust, stalling the plane and causing him to fall from 15 m. The fall broke Lilienthal’s spine causing him to die a day later. Despite his death, the glider designs employed by Lilienthal were perhaps the most important discoveries on the road to powered flight [74]. In 1902, Wilbur and Orville Wright built and successfully tested a glider whose basic design would become the first powered aircraft. The brothers then set out to devise a method of powering the large aircraft. They needed an engine that could generate enough thrust to propel the plane, while having it be lightweight, so as not to compromise the ability for the plane to lift off. They put the challenge out to automobile manufacturers, however, they were turned down by each, saying that their required specifications were unrealistic. The brothers took this as a challenge and designed their own four-cylinder, 12-hp internal combustion engine. The entire engine weighed less than 200 pounds, a serious feat for the time. The brothers also developed propellers after realizing that the shape must be designed as a rotating airfoil. On December 17, 1903, the brothers debuted their attempt at a powered aircraft. The design incorporated many of the same features as their latest glider including twin wings, rudders, and canard elevators. Including the pilot, the entire plane weighed around 750 lbs. That day the brothers were able to perform four successful flights ranging from 100 to 800 feet, the last of which resulted in the plane’s destruction following a strong collision with the ground. Nonetheless, the Wright brothers had successfully demonstrated the viability of powered flight, beginning man’s journey into the air [75].

1.2.4 1.2.4.1

Nonrenewables Fossil Fuels and Conventional Energy Sources

Since the start of the Industrial Revolution, fossil fuels have become the primary energy supplier globally. In 2013, it was reported that 98% of the total transportation fuel demand was provided by petroleum-based fuels [76]. This sector’s demand is expected to grow by as much as 50% by 2030 with the majority of this growth taking place in rapidly industrializing countries, such as China and India [77]. Despite this expected demand increase, global supply is projected to be exhausted in as little as 45 to 100 years at the current rate [78–81]. This leads to a difficult scenario where alternatives to fossil fuels must be found in order to have complete energy security. In addition to this the combustion of fossil fuels has a number of negative environmental effects including the release of harmful byproducts, such as carbon dioxide, nitrogen oxides, sulfur oxides, heavy metals, and volatile organic compounds. Nuclear energy and natural gas provide some relief when it comes to environmental damage, however, they too have issues when it comes to dealing with waste and whether or not they are truly renewable energy sources. While they are certainly

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cleaner forms of energy the verdict is still out as to their sustainability and environmental impact. The bottom line is that globally, there is still a massive demand for fossil fuels and without major changes the world could and is finding itself in a large-scale energy crisis.

1.2.4.1.1

Coal

There are a number of key advancements that led to coal’s widespread success as an energy generator. Technological developments reliant on the resource saw greater production capability than ever before. Coal’s first major implementation came in the form of direct home heating and in the firing of large, industrial steam engines [82]. Adoption of coking practices served to further enhance the uses of coal [83]. Town gas otherwise known as coal gas used in the illumination of homes brought yet another use for the resource, but perhaps its largest boost during the Industrial Revolution came through the development and widespread implementation of mobile steam engines. Driving of mobile steam engines in the rail and ship industries led to a greater sense of globalization and nationalization as people and products were connected like never before. Nonenergy uses, such as its use as a feedstock for the synthesis of organic chemicals came next before coal’s greatest use of all. Electricity generation exploded as centralized plants were built to generate electricity for the masses, with all of it being driven by the seemingly unending supply of coal [82]. For centuries, coal was the primary medium used for attaining vast amounts of quick and relatively easy energy in many of the most developed and powerful European nations. Nowhere was this more so the case than in Great Britain, where the black rock served to drive the majority of the nation’s quickly developing industry. Despite the potential of coal, its adoption was not initially welcomed with open arms. The substance’s sulfuric odor, coupled with is relatively low heat supply, did not make it an attractive fuel source for many industries [82]. There were two major changes that led to coal’s widespread adoption throughout Britain. The first came at the end of the Elizabethan era when the newly crowned James I began using coal in his London palace. Up until this point, Elizabethan nobility had rejected the use of coal for a variety of reasons [84]. The second major event in the advancement of coal usage was the development of reverberating furnaces that served to reflect generated heat in order to attain drastically higher temperatures [82]. As coal’s usage became more and more widespread, Great Britain rapidly saw itself become a global economic power due to its wealth of coal resources. To truly grasp the speed at which coal developed, one need simply to consider the following timeline. Somewhere around 1620, coal surpassed biomass as the primary fuel source in Britain. By the mid 1600s, coal was responsible for nearly two thirds of the nation’s thermal energy. By 1700, this figure had reached 75%, by 1750, it accounted for 90% and by 1800, coal was producing in excess of 98% of Great Britain’s total thermal energy. This trend continued until 1950, where coal usage began to take a downward turn and by 1960, coal usage accounted for only 77% of national thermal energy production. Taking this all into account, the use of coal in Great Britain for thermal energy production was greater than 75% of total production for around 250 years [82,85]. This dependency on a singular resource in largely unseen throughout history and its ingenious implementation undoubtedly served to propel Great Britain to becoming a major global power. While coal is often viewed as the fuel of the Industrial Revolution, the use of coal throughout the 20th century rapidly expanded with increasing populations and the subsequent need for greater power generation. 1882 Saw the opening of the world’s first large-scale electric generating plant at Pearl Street in New York. The plant was designed by Thomas Edison who had the idea of generating power for the world through his design. In fact, by the year 1892, Edison’s system of centralized electric power generation and broad distribution grids had almost entirely replaced New York’s dated gas system, which had been in place for more than half a century [86]. The general design of coal-based power plants has changed very little since Edison’s initial implementation. While improvements on the design have been made, many modern facilities still essentially function as steam engines. Combustion of coal provides heat that is used to boil water in a boiler. The steam from the boiler then makes its way to a turbine, which generates electricity. As the steam cools and condenses back into water it is returned to the boiler, continuing the cycle. By returning the recently condensed hot water to the boiler, the system can operate far more efficiently than using fresh, room temperature water. From the 20th century and on, coal’s use in energy production has steadily declined thanks to the cost and availability of alternate fuel sources along with the emphasis placed on renewable energy sources. With that being said, coal remains a highly relied upon resource primarily in the developing world and for electricity generation.

1.2.4.1.2

Oil

While Edwin Drake’s 1859 oil drilling expedition marked the start of the Oil Age, the industry did not truly take off until 1901. In an area known as Spindletop Hill outside of Beaumont, TX, the largest oil deposit of the time had been discovered. Spindletop Hill was an area known for its large subterranean salt domes [87]. During the formation of salt domes, large amounts of salt are forced toward the surface. As these salt domes continue to move upward they open up pockets for natural gas and oil to collect in. This makes drilling in proximity to salt domes quite profitable should the pockets be located [88]. After a strenuous 9 years of exploratory drilling, these pockets were finally found at Spindletop Hill. While Edwin Drake needed only to drill 69.5 feet into the ground to strike oil, the Spindletop boreholes needed to go far deeper. On January 10, 1901, oil was finally struck at a drilling depth of 1139 feet. Based on this depth the pocket was under large amounts of pressure, pressure that blew a gusher 150 feet into the air at a rate of 100,000 barrels per day. By 1904, 285 additional production wells had been constructed on Spindletop Hill, having the effect of tripling the United States national oil production over the 3-year period. The effect of the rush drove down oil prices, making automobiles far more attractive to the everyday man. With a steady and affordable supply of fuel, Henry Ford’s research into alternative fuel sources proved to be unnecessary, ushering in the age of fossil fuel-based transportation [87].

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As global oil demand continued to grow in the wake of multiple oil-based advancements, particularly in the transportation industry, the need to secure more and more of the resource continued to rise. The Middle East became of particular interest to the Western world when in the 1930s geologists employed by Standard Oil of California discovered vast deposits of oil in Eastern Saudi Arabia [89]. The United States took great interest in these deposits and in 1945 made a play to secure Saudi oil for America. Then-president Franklin D. Roosevelt set up a meeting with Saudi monarch Adb al-Aziz Ibn Saud, to set up a deal that outlined the exchange of Saudi oil for American protection in future conflicts. The deal would set the stage for not only economic prosperity between the countries but also a vast amount of tension in the region [89]. In the years that followed, large oil deposits were also discovered in parts of Iraq and Iran, with these deposits once again garnering US attention and acquisition. These acquisitions led to once US-based companies making the jump to the enormous multinational corporations they are today [90]. Following these acquisitions and along with the Western world’s growing dependence on fossil fuels, protecting these oil resources became one of the United States’ principal economic concerns [91]. While oil continued to flow with only small-scale issues, the region reached a tipping point in the late 1970s. While America had made attempts to keep the peace in the Gulf region, it was to little avail, as revolution, war, and civil unrest plagued the Gulf. American intervention, marketed as peacekeeping, led to disdain between the Middle East and the Western world. Despite this, the United States has managed to retain control over many of the crucial reserves in the region. Tensions continue to this day with conflicts, such as the 2003 US-led Iraq invasion having serious oil-driven undertones [89]. The best estimates for 2016 suggest that global oil and liquid fuel consumption has risen to as high as 96 million barrels per day or 35 billion barrels per year. While production was maintained at 97 million barrels per day, demand is expected to climb in excess of 100 million barrels per day over the next 5 years [92]. The issue that arises is that oil is a finite resource, and as global exploitation of this resource continues to rise, the rate at which it is depleted also accelerates. A model known as “peak oil” suggests that we are headed for or have already reached a maximum oil extraction rate and are in the midst of a terminal decline in production. If this is the case, production will not be able to meet demand and the result will be major economic disruption and shifts in conventional ways of life [93]. As such, there is a call for improved and highly available alternatives to oil if this potential crisis is to be averted.

1.2.4.1.3

Natural gas

Man’s discovery of natural gas long predates his understanding and further predates his practical implementation of the resource. In fact, stories from ancient Greeks, Persians, and Indians all tell of unexplainable fires occurring in selected areas. What we now know is that these were likely small amounts of natural gas, leaking to the surface and ignited by lightning strikes. Further to this is the loose knowledge of the Chinese near 200 BC, who drilled small natural gas wells and used them to harvest the gas for use in boiling salt water for drinking purposes. Past these accounts there is very little in the way of advancements in the exploitation of the resource until the 18th century. Around 1785, coal-generated natural gas was used in Western Europe, to produce a flame for use in streetlights and lighthouses. In 1816, the gas was brought to the United States where it served to light streetlamps in Baltimore, MD. As mentioned in Section 1.2.3.3.1 the first natural gas well was drilled in Fredonia, NY in 1821. The development of the natural gas industry slowed, however, as the drive to acquire crude oil continued to drive prospectors across the United States and Europe. In fact, the discovery of natural gas was often seen as an impediment to oil prospectors as it forced them to stop drilling and allow for the gas to seep naturally into the atmosphere. Throughout the 19th century, natural gas was used almost exclusively for lighting purposes. The gas was also highly localized, as the proper transport equipment was not practical or available. With the advancement of leakproof pipelines among other efficiency improvements in the 1920s, transportation of large amounts of natural gas became practical for the first time. Following World War II, this was only further enhanced as vast distribution pipelines began to pop up for natural gas, where for the first time the product could be delivered directly to homes for use. The use of natural gas saw its greatest uptick during the oil crisis of the 1970s. As global oil shortages plagued highly dependent nations, the importance of natural gas resources was never higher. This led to greater developments in production and distribution of natural gas throughout the Western world [94]. To this day the distribution of natural gas for heating, cooking, and other household functions remains largely unchanged. In the United States today, natural gas consumption accounts for 26% of the national energy consumption. This number is expected to rise to 28% during the next 30 years. While the increase in consumption is not expected to see a strong rise, US production of natural gas is expected to take a dramatic upturn. The United States is projected to become a net exporter of natural gas by 2020. This is largely in part due to the advancements and growth of the hydraulic fracturing industry. Hydraulic fracturing is a process whereby large amounts of water, sand, and chemical additives [95] are pumped into a first vertically, then horizontally drilled borehole. The high pressure of the pumped-in chemical solution causes fractures in the low-permeability rock deposits (often shale) and as the water and chemicals are returned to the surface the sand remains to hold these fractures open. The idea is that within these pockets of low-permeability rock are large amounts of natural gas that cannot escape. By fracturing the rock, the gas is able to escape and can be collected at the surface. The process of fracturing typically only lasts a few days, however, a highly productive well can be operational for multiple decades [96]. Public concern over the safety of the chemical additives within the fracking solution have hindered the growth of extraction somewhat, however, the industry continues to reiterate that they are of no risk to humans either directly or through groundwater contamination.

1.2.4.1.4

Nuclear fission

Development of nuclear power generation is often thought to have transpired in three distinct phases. The first occurred in the 1950s and 1960s, as prototype reactors were developed to assess the viability and safety issues surrounding the new technology.

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The second stage saw mass development of large-scale commercialized plants beginning in the 1970s and running through the 1980s. Finally the 1990s saw the technological advancement of light water reactors. These improvements are generally what are still employed today. The most common types of reactors are pressurized water reactors (PWR), boiling water reactors (BWR), and pressurized heavy water reactors (PHWR). While there are a number of different nuclear power generation designs the method in which power is generated is largely the same. Heavy and nearly unstable elements, such as uranium 235 are impacted with neutrons. These neutrons cause the uranium to fall out of stability and split, releasing energy in the form of radiation. This radiation heats water in the core whose steam is then used to turn a turbine. The condensed water is then cycled back through the system to the core. This controlled nuclear chain reaction is known as nuclear fission [97]. Despite the simplicity and the potential for mass power generation there are still a number of social, environmental, and economic issues that plague the nuclear industry. The first is concern over public safety. Despite the vast amounts of precautions taken in nuclear plants around the world, there are still many who feel that the risk is still too great. Accidents, such as Three Mile Island in 1979, Chernobyl in 1986, and Fukushima in 2011 have all led to public uncertainty over the safety of nuclear energy. Secondly, there are further issues that arise as a result of difficulties in disposing of nuclear waste. Spent nuclear material is highly radioactive and must be handled so as not to cause harm to anyone or anything in the environment. Current practice is to ship spent nuclear material to storage facilities, however, there remain concerns over what would happen should an incident occur or the capacity for storage be reached. Finally, new nuclear facilities require a vast amount of capital investment along with meeting long lists of specific guidelines. This may hinder the future development of nuclear energy in favor of newer clean energy projects or larger fossil fuel-based energy sources [98].

1.2.5

Renewables

1.2.5.1

Renewable Energy Sources

Renewable energy sources represent the world’s best chance at future energy security. With fossil fuels being depleted at a rate far greater than they are being replaced it is inevitable that there will come a time when alternative energy sources will need to take over. This is where renewables find their place. Renewable energy refers to energy that is replenished within a short amount of time. This time period is generally considered to be instantaneous with resources, such as wind, hydro, and solar or, up to a few years, for renewables, such as biomass. What separates them from fossil fuels is that their replenishment time is still far shorter than the millions of years it takes for fossil fuels to replenish themselves. In addition to this, renewable energy sources do not encounter many of the same issues when it comes to their environmental impacts. With that being said, harvesting, processing, and construction can all be energy intensive processes that must be considered upon implementing any renewable energy technology. In addition, many renewable technologies are still far away from where they’d like to be from an efficiency standpoint. This largely comes down to technological development and the added costs associated with systems of increased efficiency. With proper development, cost reductions, and subsidies, the best case scenario is that the world can move from detrimental and unreliable fossil fuels to a world powered by renewable energy sources.

1.2.5.1.1

Hydro

The primary implementation of modern hydro is for hydroelectric power generation. Hydropower represents the most effective means of converting natural potential and kinetic energy into electricity. When considering modern turbines, such as the Francis turbine, efficiencies greater than 95% can easily be achieved. This sort of efficiency is unparalleled among similar, turbine derived, renewable energy sources. The period of modern hydropower truly began with Leonhard Euler, who between 1750 and 1754 explained and developed equations for triangle velocity, conversion of angular momentum, and what would become known as Euler’s pump equation. This new understanding would be paramount in the development of modern hydroelectric systems [99]. The first hydroelectric power station known as the Vulcan Street Station was constructed in 1882 in Appleton, Wisconsin [100,101]. The small plant was used to provide electricity to two paper mills along with a single residence [101]. While the scale was quite small, it certainly had a major impact on future development. The next major advancement in hydropower technology comes in 1889 with the construction of the Willamette Falls Station in Oregon City, OR. The Willamette Falls station is often credited as being the world’s first AC hydroelectric plant. Perhaps what the station is better known for is the part it played in the generation of electricity for North America’s first electrical transmission line. In the summer of 1890, the plant was equipped with a 13-mile transmission line running to Portland, OR. The line transmitted electricity at 4000 V to Portland with acceptably low losses thus ushering in the age of mass power transmission [100]. The final major advancement in hydropower technology came from the Mill Creek Station in Redlands, CA. This station was the first in North America to generate electricity as three phase AC [100]. This led to greater efficiencies in transmission and distribution than previous single-phase designs. As hydroelectric power generation moved to greater and greater scales the structures began to serve a multitude of purposes. Perhaps nowhere is this more apparent than with the Hoover Dam, located on the border between Clark County, Nevada and Mohave County, Arizona, United States. The Hoover Dam served to not only generate electricity but also provide much needed flood control and irrigation waters for the region. Constructed between 1931 and 1935, the dam stood 726 ft high and 660 ft wide, far dwarfing similar dams of the time. The Hoover dam set the precedent for modern, large-scale hydro operations and many of its aspects are still utilized in construction of modern dams [102].

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1.2.5.1.1.1 Small-scale hydro Most small-scale hydro operations are designed as run-of-river (ROR) systems. These systems have the distinct advantage of detaining little to no water, resulting in minimal impact to the river and the surrounding aquatic environment. While there is no set definition as to what exactly quantifies as a small-scale hydropower generator, it can generally be thought of as anything ranging from 2.5 to 25 MW. Hydro turbines function by generating mechanical shaft power as a result of applied hydrodynamic force. The potential power generation is a function of the volumetric flow rate and the pressure head. Energy conversion efficiencies for small-scale hydro tend to be in the range of 60 to 80% [103]. The majority of small-scale hydro operations function by diverting water upstream through a small weir. The weir ensures that continuous inflow to the turbines is maintained in order to combat potential inefficiencies. The water then travels through a forebay where the flow rate is slowed dramatically allowing for settling of suspended solids, which could cause harm to the equipment in large quantities. Larger solids are also filtered from the inflow through the use of a bar screen at this stage. The water then enters a penstock (pressure pipe), which delivers the water to the turbine, generating electricity. The water is then redirected back to the stream with minimal losses and subsequent effect on the river [103]. 1.2.5.1.1.2 Large-scale hydro Similar to small-scale hydro operations, large-scale hydro takes advantage of hydrodynamic forces to spin turbines, which in turn drive generators. The majority of large-scale hydropower operations utilize a dam to increase the potential energy within the system. This water is then released toward the turbines, generating electricity. The major advantage of large-scale hydro is that the inflow rate to the turbines can be altered based on peak electricity demand. This means that the system can be more efficient in its water usage, while minimizing energy storage requirements. Some systems even incorporate backward spinning turbines, which during periods of low demand can use electricity from the grid to raise water from lower reservoirs and rivers to the higher potential of the dam. This helps in combating the issues incurred during peak demand periods [104]. The world’s largest hydroelectric dam is Three Gorges Dam located on the Yangtze River near Sandouping, China. The dam was completed in 2009 and stands 185 m high and 2335 m wide. On average, the water retention height sits at 175 m. The dam is comprised of 32 units, each with generating capacity of 700 MW, for a total capacity of 22.4 GW. With a peak inflow rate of 100,000 m3 per second the dam can produce up to 98.8 TWh per year. From a carbon emissions standpoint, this is equivalent to burning 49 million tonnes of coal, effectively eliminating 100 million tonnes of carbon emissions [105].

1.2.5.1.2

Wind turbines

While the idea of utilizing wind energy is far from a recent one, the utilization of wind for electricity generation certainly is. Near the turn of the 20th century, Danish scientist and instructor Poul la Cour designed and built a setup that incorporated a windmill attached to an electrical generator. While it is debated whether or not this was truly the invention of the wind turbine, what is for certain is that la Cour’s design was far more efficient than others of the time. La Cour’s turbine was especially viable for rural communities and he was adamant in demonstrating the viability of his creation, especially in agricultural operations. La Cour is also credited with the invention of the wind tunnel, which he used to test various rotor designs for their aerodynamic properties. Sadly, as industrialization took off, diesel and steam-powered engines offered cheaper more reliable sources of energy and as such, many opted to move toward the dependable engines. The advancement of wind turbine technology slowed throughout World War I and II, but garnered new attention in the wake of these events. In the mid-1950s a student of la Cour’s, Johannes Juul, designed what would become known as the Gedser turbine. The Gedser turbine incorporated a three-blade design positioned upwind with a stall regulated rotor connected to an asynchronous AC generator. This design is credited as being the forefather of modern wind turbines. Research and development of wind turbines escalated further during the oil scare of the 1970s. Many large corporations poured funds into developing larger turbines to meet the required demand. The result of this and other research projects led to the designs we see today [106]. A wind turbine can be thought of as any device that converts the energy of wind into electricity. It is as important to understand the distinction that these are not windmills, which convert wind energy into mechanical work. While the majority of wind turbines on earth are quite small, ranging from 1 to 10 kW, the majority of electricity generation comes from much larger turbines in the range of 1.5 to 5 MW. These larger turbines are generally oriented in equally large wind farms that can incorporate hundreds of these massive turbines. Europe and North America have been the quickest in adopting this new technology with both having numerous wind farms located throughout their continents [107]. The majority of modern wind turbines are known as horizontal wind axis turbines. These wind turbines are generally comprised of three-angled blades, rotating parallel to the ground, which take advantage of the aerodynamic force of lift to rotate and generate torque on a shaft. This shaft is used to drive a generator, which produces electricity. Unlike other renewable energy sources, such as hydroelectric, wind turbines are only able to produce energy from what wind is immediately available. There is no capacity to store it for later use during peak demand periods. As such it would be inconceivable to have a grid that generated 100% of its electricity from wind power as the output is highly variable. Based on this the best implementation of wind turbines is as a secondary producer, serving to lighten the load on whatever other method of electricity generation is being employed [107].

1.2.5.1.3

Geothermal

Knowledge of geothermal energy dates back thousands of years but it was not until the turn of the 20th century that it was first harnessed for use in electricity generation. It is well known that Julius Caesar and the Romans capitalized on steam vents escaping

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the Earth’s crust in the construction of their baths. The region of Larderello, Italy was known for its wealth of such steam vents, and in 1904 Italian businessman and politician Prince Piero Ginori Conti utilized these vents to generate electricity. Conti’s original system was capable of lighting five lightbulbs at a time in his boric acid production facility. In 1914, after a number of optimizations, the system was connected to a 250-kW turboalternator and installed into the distribution grid of the nearby towns of Pomarance and Volterra. In doing so the system became the first distribution grid powered largely by geothermal energy. With continued improvements, the system reached an astounding yearly production of 136.8 MW per year, just prior to the invasion of Italy by allied forces during the Second World War. Sadly, the plant was destroyed during the invasion, though a more modern plant was constructed in the wake of the war. By 1959 the plant reached a yearly generating capacity of 300 MW. Today, the region plays host to a multitude of geothermal facilities with a generation capacity in excess of 9000 GW h 1 [108]. As geothermal activity varies globally, select locations are better than others for electricity generation. Some of the planet’s largest facilities are located in California, Mexico, Iceland, The Philippines, and Indonesia. Geothermal energy can most simply be defined as the heat energy contained within the Earth. Geothermal energy usage truly took off following the Second World War. The technology was seen as being economically competitive as well as being readily available in areas where other technologies were not feasible. Today, geothermal energy is used in 23 countries for electricity generation and 58 for use in other nonelectrical applications [109]. A geothermal system is comprised of three primary elements: a heat source, a reservoir, and a fluid to transfer the heat. The heat source can either be the Earth’s heat itself, obtained by reaching an appropriate drilling depth or a magmatic intrusion that can be found in some cases at shallower depths. The reservoir is an area of permeable rocks that transfer heat to the fluid as the fluid passes through the voids in the rock. The fluid works in a cycle, being heated underground and bringing that heat to the surface; the cooled fluid then makes its way back underground to be heated once more and the cycle continues [109]. This setup is often seen in homes that are heated by geothermal energy. In addition to heating, slight alterations can be made to the design so that the generated steam drives a turbine attached to a generator. This process can be used to generate renewable electricity at a low cost.

1.2.5.1.4

Solar

Solar energy usage can be broadly categorized into two categories: active and passive. Active solar takes advantage of the photovoltaic (PV) effect to directly convert sunlight into electricity. This effect was first discovered and explained in 1839 by French scientist Edmond Becquerel. The PV effect can be best described as the appearance of an electric voltage between two electrodes as a result of introducing a light into the system. The electrodes can be attached to either a solid or a liquid medium depending on the system [110]. The result of this phenomenon is usable electricity, whose generation is achieved through the use of PV cells, comprised of a semiconductor. The design of PV cells was first created by Bell Laboratories in 1954 led by scientists Daryl Chaplin, Calvin Fuller, and Gerald Pearson. Bell’s original PV cell had a conversion efficiency of 6%, though the design was quickly improved upon to reach efficiencies of up to 10%. These efficiencies were achieved through the use of a silicon semiconductor which, when exposed to sunlight, gave up electrons from their atom [110]. By attaching conductors to the terminals in the cell, these electrons can be captured as an electrical current. Depending on the semiconductor material, the reaction can be anywhere from 7 to 40% efficient [111]. Even today, with modern PV cells, the semiconductor that is most often employed is silicon due to its ability to be worked easily with minimum defects [112]. While silicon might not be the best semiconductor, it is cheaper, more available, and more reliable than its competitors. Passive solar is a method of designing objects and buildings so as to take best advantage of the sun’s heat and light. Wide varieties of passive solar techniques have been used for thousands of years for a variety of purposes. Even today, passive solar has a large effect on many construction and design choices. For example, in the northern hemisphere, houses with more south facing windows can utilize the sun’s energy to provide home heating and eliminate some of the cost of supplemental heating methods. Another example of passive solar would be painting an outdoor water storage tank black to preheat the water and reduce the amount of heat and energy required for producing hot water [111]. While details of the invention of solar thermal energy conversion are sparse, the consensus is that the process was invented in 1615 by French engineer Solomon de Caux. De Caux describes a device he created that was able to pump water by concentrating solar energy to cause a desired expansion of air within the system. This air would then, as a result of the expansion, move water throughout the pumping system. While this was the start of solar thermal energy conversion, it was not until around 1854 that solar thermal energy conversion took the form we know today. It was around this time when Austrian C. Güntner designed a solar thermal collector capable of boiling large amounts of water, which generated steam and drove an engine. Very little is known about Güntner, though it is accepted that his design incorporated a series of long, narrow mirrors that rotated to follow the sun throughout the day. The mirrors reflected the sun’s energy onto a boiler tube located above and parallel to the mirrors. Güntner’s design utilized 18.6 m2 of mirrors and was capable of generating 0.745 kW [113,114]. The next major advancement in solar thermal came from French inventor Augustin Mouchot who worked on his design between 1860 and 1878. Mouchot was the first to use conical mirrors, which helped to better focus the sun’s energy on the boiler. Mouchot’s design further incorporated a blackened copper boiler encased in glass to limit heat losses, while encouraging maximum energy transfer. In 1875 and 1878 Mouchot, with the financial backing of the French government, built two more engines, which operated on the same principles as his previous work. Unfortunately, Mouchot’s designs never reached a point of cost effectiveness and the projects were abandoned by the French government. Around this time, Abel Pifre, a fellow French engineer, demonstrated his take on the solar collector. Pifre’s design utilized parabolic reflectors as opposed to Mouchot’s truncated cones. The result was slightly more efficient energy transfer as Pifre demonstrated at the Paris Exposition in 1878 where he used his device to power a printing press. Almost

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simultaneously, Swedish-American engineer John Ericsson created a device that located the boiler tube in the foci of a reflective parabolic trough [113,114]. Ericsson’s most famous design utilized a 3.3-m-long and 4.9-m-wide trough that was capable of generating steam at 240 kPa and producing around 1.2 kW through the use of a reciprocating engine [114]. From this point, designs continued to flourish and improve efficiencies, which many of the aspects included today capitalizing of the same principles discovered over 100 years ago. Modern solar thermal energy conversion to electricity is generally performed under specific conditions. Solar thermal energy conversion can be performed by either concentrating or non concentrating collectors. With that being said, concentrating collectors are able to achieve far higher thermodynamic efficiencies. Concentrating collectors use mirrors to focus sunlight onto an absorber tube or a central tower collector. Regardless of the collection unit the principle is largely the same. The concentrated sunlight heats a fluid (often synthetic oil) whose heat is then used to boil water. The steam generated from the boiling water is used to turn turbines and subsequently generate electricity [115].

1.2.5.1.5

Biomass and biofuels

Biomass usage for heat and electricity generation is a rapidly developing area with the potential to drastically reduce dependency on fossil fuels. While the sources of biomass energy are highly variable, the technology can best be described as the combustion of biological material for the purpose of capturing and utilizing the generated heat [116]. The heat generated from the combustion is used to boil water, whose steam is then used to turn a turbine and generate electricity. Many systems are designed as cogeneration systems in that the steam that passes through the turbine is then utilized to heat nearby buildings or is recirculated back to the boiler to keep the temperature of the water high [117]. Anaerobic digestion is a means of producing biogas (largely methane) from biomass. The process takes advantage of anaerobic microorganisms to break down and decompose organic material. The byproducts of this digestion are approximately 60–70% methane and 30–40% carbon dioxide. Knowledge of organics’ decomposition into biogas has been known for hundreds of years. During the 17th century, Jan Baptista Van Helmont discovered that flammable gases were a byproduct of decaying organic material. In 1776, Alessandro Volta furthered the understanding by concluding that the amount of flammable gas emitted was directly proportional to the amount of decaying organic matter. Between the years of 1804 and 1808, John Dalton and Humphrey Davy were able to determine that the flammable gas emitted from the decaying organics was methane. In 1868, Antoine Bechamp, a French chemist and biologist cited “the organism” as being primarily responsible for the decomposition of the organic matter and the subsequent release of methane. From this point in history, the process only grew in understanding though the work these men performed solidified understanding of the process [118,119]. The first anaerobic digesters were not used for biogas generation, however; rather they were used in the treatment of solid and liquid wastes. The first of such systems was built in 1881 by Frenchman John Mouras [120]. This development was followed and improved upon in 1895 by Englishman Donald Cameron who famously coined the term “septic tank.” The efficiency of Cameron’s system led to it becoming the primary waste treatment method in the city of Exeter, England and led to its spread throughout England and eventually the rest of the Western world [118]. In Exeter, some of the gas from the constructed septic tanks was collected and used for heating and lighting purposes at the central disposal works, marking the first time in history when anaerobic digestion was used for energy conversion [121]. In the years that followed, heating sources and mechanical mixers were added to the systems in order to optimize the digestive process, though to this day the general mechanism remains largely the same [118]. One of the most common implementations of anaerobic digesters is in agricultural operations. Organic solids are often mixed with agricultural manures in order to improve the digestion process. The resultant methane is either captured or burned off as it is produced. Some larger digesters incorporate a generator, which uses the produced methane to generate electricity, reducing operational and labor costs. Capturing methane in this way not only provides a valuable biofuel but also helps in mitigating its effects as a greenhouse gas. Methane has a heat trapping capacity 21 times higher than that of carbon dioxide, making mismanagement highly detrimental to the environment. When produced in this way, methane has to be thought of as a valuable renewable resource. Use of biofuel dates back hundreds of years to when fats and oils derived from plants and animals were used to improve combustion practices. With that being said, the first large-scale commercial implementation of biofuels comes during the 1700s, when whale oil was the primary fuel used to light homes throughout the world. In modern times, the conception of biofuels is one of transportation fuels. This makes sense given that much of the research and public education on biofuels is targeted as cleaner and greener transportation fuels. What is interesting to note is that the first internal combustion engines designed in 1826 were actually designed to run on biofuels. Further to this, Henry Ford’s 1908 Model T was designed and built to run on ethanol, and Rudolph Diesel built his first engine to run on vegetable oil. However, despite the buy-in from the fathers of the automotive industry, eventually fossil fuels due to their vast availability and competitive pricing were able to outcompete biofuels. More recent times have seen a resurgence in biofuel production, thanks to events, such as the 1973 oil crisis, the 1979 oil crisis, and the 1990 oil price shock resulting from the Gulf War. These events spurred the world’s oil-dependent nations into action by increasing research and development of biofuels to meet the ever growing demand. Today, as oil prices continue to rise, supply continues to be in question and the public demand for cleaner fuel alternatives increases, and the need to develop more efficient and costeffective biofuels has never been more apparent [122]. A first generation biofuel refers to any biofuel (biogas, biodiesel, bioethanol, etc.) produced from a conventional food crop. While yields for many first generation biofuels are quite high, the issue that often arises is the food versus fuel debate. This debate is anchored on the notion that conventional food crops are more valuable as food crops than as biofuel crops. With the global population continuing to rise, the need to feed the ever-growing population continues to be at the forefront of global concern and

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better alternatives need to be found. As first generation biofuels reduce the amount of food produced, their value needs to be evaluated as either a food or fuel crop. Second generation biofuels aim to directly combat the issue of food versus fuel. A second generation biofuel refers to any biofuel produced from the lignocellulosic material of a plant. In using the “nonfood” parts of the plant along with crops not considered as food crops, second generation biofuels do not encounter the same food versus fuel issues as first generation biofuels. In addition to plants, microalgae (third generation biofuels) have also become a prevalent area of interest for the production of biofuels in recent years. A number of algal species contain high amounts of lipids, and can be cultivated in mass with the proper system. This makes them an attractive option as production costs decline and fuel demand increases. Microalgae have the added advantage of not being a competing food crop nor do they compete for the same land resources as second generation biofuels. These aspects in combination help to distinguish microalgae as the most promising feedstock for biofuel conversion. The major issue that arises with many second and third generation biofuels is that the yields are not high enough to justify production. With that being said, more and more crops are being tested with greater yields achieved as technology and research advance this field.

1.2.5.1.6

Tidal

The practice of trapping tidal waters behind a barrier in order to capitalize on the potential energy is far from a modern concept. Evidence suggests that as early as the 10th century tidal waters were being trapped and exploited for turning a corn mill in the region of Basra, Iraq. Through the 12th century this practice spread throughout Europe and by the 1600s there were hundreds of such systems in place globally [123]. What is a modern practice, however, is the exploitation of tidal waters for the generation of electricity. Due to the immense potential and reliability of tidal waters, they make an excellent candidate for the implementation of tidal energy systems. The first of such systems, known as tidal range power, was built on the Rance River near St. Malo, France with construction being completed in 1966. The constructed barrage spanned 750 m across the Rance, containing a tidal basin of 22.5 square kilometers [123]. Tidal range power generation functions by taking advantage of the head difference generated between two bodies of water. The difference is created by separating the two water bodies by means of a wall. As the tide moves in and out the wall is closed to detain water, generating the head difference. Once an optimal difference has been achieved, portholes in the wall are opened, allowing water to make its way back toward the lower potential. With two tidal cycles per day the process can be performed four times. As the water passes through the portholes it is used to turn turbines that in turn generate electricity. The process is then repeated with the tidal cycles [124]. Tidal stream turbines function in practice quite similarly to wind turbines. A dynamic fluid is used to turn a turbine and as a result generate electricity. The difference between them is that for tidal stream turbines the dynamic fluid is water, not wind. These turbines are either fastened to the seafloor or tethered by a cable in areas of elevated tidal activity. As the tide moves in and out, the turbine is turned. Tidal stream turbines take up less space and have a far lower impact than tidal range, however, they do not generate as much electricity. There are currently three employed and conceptualized designs for tidal stream generators: horizontal axis turbines (similar to wind turbines), vertical axis turbines, and hydrodynamic lift force energy devices. Currently only the horizontal axis turbines are employed on any large-scale grid connected designs [125].

1.2.5.1.7

Pyrolysis

Pyrolysis is a process that takes carbon-based matter and places it in high temperature oxygen deprived conditions. The lack of oxygen means that there is no fire in the reaction, however, the carbon matter is degraded into one of three forms. The first is biooil. Bio-oil is produced from the heavier gases generated through the action of pyrolysis. As the heavy gases are returned to room temperature, they condense back into liquids known as bio-oil. The second form is synthesis gas or syngas. Syngas is comprised of the lighter gases generated through pyrolysis that remain gaseous at room temperature. Gases, such as hydrogen and methane are some of the most common byproducts of pyrolysis. The third form is biochar. Biochar consists of the solid remnants of everything that is not degraded during the action of pyrolysis. Biochar is a product similar to charcoal that can be implemented in many of the same ways [126]. While the process of pyrolysis is not an entirely new one the utilization of pyrolysis for energy conversion only took off in the last 40 to 50 years. The first major installation and use of pyrolysis for energy conversion comes from the refuse derived fuel (RDF) pyrolysis plant built by Occidental Research Corporation during the 1970s in San Diego, CA. This plant used biomass as its primary feedstock though it is possible that other forms of refuse (plastics, rubber, etc.) may have been used in the conversion. Sadly, the plant shut down after only a few years, citing lack of economic viability as the reason for the closure. Following the failure of the RDF plant, the late 1970s and early 1980s saw the development of Georgia Tech entrained flow pyrolysis process. This process saw biomass ground to around 1 mm and dried to around 10% moisture content prior to pyrolysis. Biomass is also preheated following the grinding in order to further enhance the energy conversion process. Liquid yields from this process were around 50% of the initial dry weight of the biomass. The next major development came during the late 1980s and into the 1990s when a group of researchers at the University of Waterloo showed that significant yields could be achieved using bench-scale pyrolyzers with a fluidized bed. Upon scale up the process was shown to increase liquid yields to as much as 80% of biomass dry weight. From this point on there were a number of improvements focused on scale and slight efficiency upgrades, which are seen in modern pyrolysis installations [127]. Pyrolysis for waste management is often preferred over incineration due to the fact that it produces far fewer dioxins and furans. In addition, some pyrolysis plants have been shown to operate with mediums having a moisture content as high as 40% [126].

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1.2.5.1.8

Heat pumps

A heat pump operates by transferring heat from a source to a sink through a refrigeration cycle. The refrigeration cycle functions by changing the pressure at certain points within the loop in order to vaporize and condense a refrigerant. This change in state either releases or absorbs heat depending on the direction of the state change. This process has a dual benefit of being able to provide heating in the winter or cooling in the summer simply by reversing the flow of the system [128]. The first heat pumps were actually designed as icemakers and for cooling practices in the food industry. The first practical heat pump was designed and built for just this purpose in 1834 by Jacob Perkins. From this point until 1875, a number of small improvements were made to the system including the implementation of various refrigerants including ammonia, methyl ether, and carbon dioxide. In 1875, the Polytechnic Society of Munich began its comparative analysis of refrigeration technologies, which led to most of the significant advancements with the technology. By the year 1918, ammonia had separated itself as the best refrigerant and globally there were a number of compressor manufacturers. It was at this point when refrigeration for space heating (heat pumps) began to garner attention. This was largely due to the advancements made with electric motors, which made running the compressors more economically viable for heating larger spaces [129]. Heat pumps continued to develop throughout the 20th century though for a variety of reasons never saw truly wide scale implementation. In recent years however, the technology has seen a dramatic increase in installations. Domestic scale heat pumps have seen an upturn in sales for a variety of reasons including improved design, more reliable equipment, energy security concerns, environmental concerns, and the increased price of fossil fuels.

1.2.6

Near Future Energy

1.2.6.1

Potential Future Energy Sources

As humans move into the future, the need for efficient, secure, and renewable energy sources will continue to be at the forefront of innovation. Hydro, wind, solar, biofuels, and nuclear all show promise as viable energy solutions and with continued expansion and efficiency improvements will help to power the world. While these technologies are becoming increasingly common in modern society there are a number of interesting technologies on the near horizon of human energy generation. Space solar, capable of drastically enhancing solar energy yields; nuclear fast reactors, capable of using uranium more efficiently; nuclear fusion, which solves many of the issues associated with conventional nuclear power; and artificial photosynthesis, generating valuable hydrogen fuels all will have significant effects on the energy profile in years to come. While these technologies still have some technological challenges they are ideas that could move the world toward a future free from fossil fuel dependence.

1.2.6.1.1

Space solar power

Space solar power represents an interesting approach to harvesting massive amounts of energy from the solar system’s greatest energy producer, the sun. While it may seem that solar works just fine on earth, placing solar arrays in space offers a number of unique advantages over terrestrial applications. Terrestrial solar arrays have two major disadvantages that can and do greatly hinder their productivity. The first is that they are fixed systems and therefore must adhere to natural day–night cycles. This means that the systems are not collecting nearly as much energy during the periods of low and no light. The second issue is that terrestrial solar array efficiency is highly weather dependent. Clouds, rain, and a variety of other environmental factors can all have a great effect on the performance of these systems. These issues in combination serve to detract from the reliability of such systems. Space solar on the other had would directly deal with these two major issues. Space arrays could be placed in geosynchronous orbit and be illuminated nearly year round. This would drastically increase collection over terrestrial arrays. In addition to this, space arrays would not encounter the same weather related issues and would be receiving stronger sunlight as it is not being scattered by Earth’s atmosphere. Following collection the energy would be transmitted back to Earth via wireless transmission. While space solar does offer a number of attractive advantages, the costs associated with implementing such devices is still quite high. At the moment the technology is being viewed as technically possible but economically improbable. With that being said, there have been and are continuing to be a number of funded projects looking at making space solar a more economically viable option moving into the future [130].

1.2.6.1.2

Generation IV nuclear fission reactors

Generation IV nuclear reactors are proposed nuclear systems that take into account at least one of the following considerations: increased efficiency, generation and capture of process heat to be used in other thermal applications, such as the production of hydrogen, increased safety and waste reduction and handling [131]. Following an open submission process the United States Department of Energy selected six designs to pursue further. These designs were supercritical water-cooled reactor (SCWR), lead fast reactor (LFR), sodium fast reactor (SFR), molten salt reactor (MSR), gas fast reactor (GFR), and the very-high-temperature reactor (VHTR). Each of the proposed systems have two things in common, they have much higher operating temperatures and place more of a burden on reaction materials than traditional systems. This means that the construction material is of utmost importance when designing these new reactors [106]. The names of most of these systems are derived from the material used to cool the reactors. As these temperatures are too high for traditional coolants, materials, such as molten salt, high pressure helium gas, liquid sodium, and lead are all being used to cool the reaction. Despite this there are still a number of issues associated with

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the construction of these reactors. The most pressing of these concerns seems to be void swelling of the construction materials. Void swelling occurs in steels in the range of 325–6501C and can cause creeping and embrittlement of the material if not properly accounted for. Specific alloys are being developed to help in combating these issues; however, they still seem far off from full scale implementation [131]. Despite all the technological challenges associated with Generation IV nuclear reactors they still represent a promising way toward cleaner energy production. By taking into account the aspects of efficiency and safety the hope is that these systems can propel nuclear past its negative public image and into a more energy secure future.

1.2.6.1.3

Fusion power

Fusion power is a process in which hydrogen or hydrogen isotopes, nuclei are collided and fused with one another to form a heavier nucleus or helium. This fusion process releases energy, which can be collected and used to generate electricity. The reaction is not entirely dissimilar from the reactions taking place within the sun. The easiest hydrogen isotopes that are used to perform this reaction are deuterium and tritium, which have a net energy release of 17.6 MeV per reaction. The major challenge that arises in making nuclear fusion possible is the opposing charges of the hydrogen atoms. In order to cause the collision, the atoms within the gas must be moving at high speed. Being that the speed of atoms in a gas is related to temperature the hydrogen must be heated in excess of 2  108 1C [132]. Once the collision occurs, electrons and ions are scattered forming a new state of matter known as plasma. It is controlling this superheated plasma that is causing researchers the greatest headache on the road to fusion power viability [133]. Fusion power has the added benefit of generating very little waste when compared with its nuclear fission counterpart. With all that being said, if fusion power can overcome what challenges it currently faces, it has the ability to provide energy security to the world.

1.2.6.1.4

Artificial photosynthesis and solar fuels

The production of solar fuels is an emerging technology performed through a process known as artificial photosynthesis. In this photoelectrochemical process, water is split into its hydrogen and oxygen components using solar energy to drive the reaction. One of the major challenges of this technology is finding acceptable catalysts that make this reaction possible while keeping costs at a minimum. Si and TiO2 have been looked at in great detail and have had reasonable success. In fact, a solar to fuel conversion efficiency of 0.12% was attained using catalyst loaded with Si and TiO2 nanowires oriented to mimic a tree. This conversion efficiency is comparable to the energy produced from natural photosynthesis [134]. While this process can be performed and the prospect of hydrogen-based fuels is promising there are still a number of technological complications when it comes to shifting toward a hydrogen-powered world. The first issue that arises is that of storage. Hydrogen has an inherently low volumetric energy density. This makes storage a challenge, and finding appropriate materials and storage conditions is at the forefront of hydrogen fuel research [135]. Seemingly the most promising solution to this problem would be to store the hydrogen with abundant resources, such as N2, CO2, or O2. This storage method has the added benefit of making hydrogen fuel more useful for a greater number of applications [136]. In addition to hydrogen’s obvious implementation in fuel cells, it has a number of other potentially beneficial applications that will be important in moving toward an energy secure future. When combined with N2, H2 reacts and can be converted into ammonia for agricultural fertilizers [137]. Methanol, propanol, and ethylene can all be produced for use in a variety of applications [137]. Conversion to hydrogen peroxide can also easily be performed for application in the production of various organic and inorganic compounds [138]. Regardless of the application, hydrogen-based fuels generated from artificial photosynthesis have a ton of promise and are likely not far from large-scale implementation.

1.2.7

Concluding Remarks

As man has, and continues to evolve, it is without a doubt that the creative exploitation of energy has been the single greatest aspect in the development and advancement of modern humans. From the earliest homo species who developed the ability to use and control fire, to the massive advancements in modern renewable technologies, man has repeatedly demonstrated his ability to adapt, innovate, and thrive using the resources available to him. Advancements in energy production and usage have not only improved social and economic aspects of life, but have also served to drive the evolution of man physically. Man has grown alongside energy, and in a world where more and more is demanded, continued growth and innovation will be required to sustain the ever-growing population and its ever growing needs.

References [1] National Fire Protection Association. All about fire. Available from: http://www.nfpa.org/press-room/reporters-guide-to-fire-and-nfpa/all-about-fire [accessed 21.04.16]. [2] Berna F, Goldberg P, Horwitz LK, et al. Microstratigraphic evidence of in situ fire in the Acheulean strata of Wonderwerk Cave, Northern Cape province, South Africa. Proc Natl Acad Sci 2012;109(20):E1215–20. [3] Wrangham RW, Jones JH, Laden G, Pilbeam D, Conklin‐Brittain N. The raw and the stolen. Curr Anthropol 1999;40(5):567–94. [4] Gorman RM. Cooking up bigger brains. Sci Am 2012;22:36–7. [5] Bird RB, Bird DW, Codding BF, Parker CH, Jones JH. The “fire stick farming” hypothesis: Australian Aboriginal foraging strategies, biodiversity, and anthropogenic fire mosaics. Proc Natl Acad Sci 2008;105(39):14796–801. [6] Pausas JG, Keeley JE. A burning story: the role of fire in the history of life. BioScience 2009;59(7):593–601.

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Solar power and application methods. Renew Sustain Energy Rev 2016;57:776–85. Mayer M. Why are solar cells made of silicon. Berkley Energy and Resource Collaborative. Available from: http://berc.berkeley.edu/why-are-solar-cells-made-of-silicon_1/ [accessed 09.06.16]. Pytilinski JT. Solar energy installations for pumping irrigation water. Sol Energy 1978;21(4):255–62. Garg HP. Advances in solar energy technology: volume 2: indu. strial applications of solar energy 2012;Dordrecht: Springer Science & Business Media; 2012. Tian Y, Zhao CY. A review of solar collectors and thermal energy storage in solar thermal applications. Appl Energy 2013;104:538–53. Field CB, Campbell JE, Lobell DB. Biomass energy: the scale of the potential resource. Trends Ecol Evolut 2008;23(2):65–72. Reißner F, Schafer J. Inventors; Siemens Aktiengesellschaft, assignee. Cogeneration power plant and method for operating a cogeneration power plant. United States patent application US 14/430,351, 2013 . 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[136] Yin SF, Xu BQ, Zhou XP, Au CT. A mini-review on ammonia decomposition catalysts for on-site generation of hydrogen for fuel cell applications. Appl Catal A: Gen 2004;277(1):1–9. [137] Yamada Y, Tsung CK, Huang W, et al. Nanocrystal bilayer for tandem catalysis. Nat Chem 2011;3(5):372–6. [138] Disselkamp RS. Can aqueous hydrogen peroxide be used as a stand-alone energy source? Int J Hydrog Energy 2010;35(3):1049–53.

Relevant Websites https://www.britannica.com/event/Industrial-Revolution Britannica Encyclopedia. http://www.britishmuseum.org/research/publications/online_research_catalogues/paper_money/paper_money_of_england__wales/the_industrial_revolution.aspx British Museum. http://www.fas.harvard.edu/~histecon/energyhistory/ Harvard University. http://www.nrcan.gc.ca/energy/renewable-electricity/7295 NRCAN. http://alternativeenergy.procon.org/view.timeline.php?timelineID=000015 ProCon.org – History of Energy Timeline. https://energy.gov/science-innovation/clean-energy USDOE. https://www.eia.gov/energyexplained/?page=renewable_home USEIA. https://www.eia.gov/todayinenergy/detail.php?id=10 USEIA. http://teachersinstitute.yale.edu/curriculum/units/1981/2/81.02.06.x.html Yale-New Haven Teachers Institute.

1.3 Environmental Dimensions of Energy Ibrahim Dincer and Calin Zamfirescu, University of Ontario Institute of Technology, Oshawa, ON, Canada r 2018 Elsevier Inc. All rights reserved.

1.3.1 Introduction 1.3.2 Energy Supply and Demand 1.3.3 Environmental Impacts in Energy Sector 1.3.4 Environmental Impact Mitigation 1.3.5 Exergoenvironmental Analysis 1.3.6 Case Studies 1.3.6.1 Solar-Based Power and Heat Generation With Reduced Environmental Impact 1.3.6.1.1 System description and modeling 1.3.6.1.2 Results 1.3.6.1.3 Conclusions 1.3.6.2 Clean Hydrogen With Copper–Chlorine Thermochemical Cycle 1.3.6.2.1 Analysis and modeling 1.3.6.2.2 Results 1.3.6.2.3 Conclusions 1.3.6.3 Comparative NH3-Fuel Assessment for Railway Transportation in Canada 1.3.6.3.1 Analysis and modeling 1.3.6.3.2 Results 1.3.6.3.3 Conclusions 1.3.6.4 Comparative Life Cycle Assessment of Various NH3 Production Methods 1.3.6.4.1 Results 1.3.6.4.2 Case study conclusions 1.3.7 Future Directions 1.3.8 Closing Remarks References Further Reading Relevant Websites

Abbreviations

ExEI GWP RES

Exergetic environmental impact Global warming potential Reservoir

Greek letters ar Linearized heat transfer coefficient g Climate sensitivity factor, m2 K/W e Emissivity j Nonideality parameter Z Energy efficiency

c v s t ζ

Exergy efficiency Stoichiometry factor Stefan–Boltzmann constant, W/m4 K Residence time, s GHG release parameter

Nomenclature

FW h H HHV i IT0 LHV _ m M MCO2 N

Future worth, $ Molar enthalpy, kJ/kmol Enthalpy, kJ Higher heating value, MJ/kg Discount rate, % Irradiance on normal tilted surface, W/m2 Lower heating value, MJ/kg Mass flow rate, kg/s Molecular mass, kg/kmol GHG mitigation factor, kgCO2 /kWh Number of pay periods

CAC COP EI

Aa AW C Dp E_ _ EI ex _ Ex fc;SO2 F

Criteria air contaminant Coefficient of performance Environmental impact

Solar absorber area, m2 Annual cost, $ Concentration, ppm Depletion factor Energy rate, kW Environmental impact rate Specific exergy, kJ/kg Exergy rate, kW Conversion factor of sulfur Radiative forcing, W/m2

Comprehensive Energy Systems, Volume 1

doi:10.1016/B978-0-12-809597-3.00103-6

50 51 56 66 69 71 71 72 73 78 78 79 83 86 87 88 91 92 94 94 98 99 99 99 100 100

49

50

Environmental Dimensions of Energy

N P P PW Q _ Q r

Engine speed, rpm Pressure, kPa Principal cost, $ Present worth, $ Heat, kJ Heat rate, kW Compression ratio

S_ SA t T TH V _ W

Entropy rate, W/K Salvage value, $ Time, s (or years) Temperature, K Time horizon, years Volume, m3 Work rate, kW

Subscripts 0 a ac C cal ch ct d e EL exh f g i ICE ign in H

Reference state, or initial condition Air Air-conditioning Cold side Calandria tubes Chemical Carbon tax Destroyed, displacement Electricity Electric Exhaust Fuel Hot gases Input Internal combustion engine Ignition Input Hot side

H2 HE HP HR m M N o P ph Q R rec ref S sys TP tot W

Hydrogen Heat engine Heat pump Heat recovery Mass or mean Methane Nitrous oxide Output Products Physical Heat Reactants Recovered Reference value Sulfur System Traction power Total Work

eqv Q

Equivalent Heat

Superscripts ch Chemical

1.3.1

Introduction

An intimate connection does exist between energy and the environment. Anything that moves on Earth requires an energy expenditure to generate power, whereas the whole power ultimately dissipates while doing work (moving), affecting thus the environment. Energy is also required for many heating applications. On the other hand, the natural tendency toward population increase goes hand in hand with the tendency of humans to move faster and farther. There is therefore a continual increase in demand for power by humans. The conventional power generating methods convert energy from fuels, in a manner that has serious implications for the environment. Electric power generation, for example, is one of the most highly polluting sectors. Coal-fired power plants throw enormous quantities of carbon dioxide (CO2), particulate matter (PM), and other pollutants into the atmosphere annually. Therefore actions and policies of governments to implement better environmental measures of energy become crucially important, and a thorough basis of energy policies creation is highly required. In any particular jurisdiction and worldwide, the way of administration of energy resources is extremely significant. The energy supply and demand must be controlled and brought into balance. Thus market regulations are required. The environmental pollution related to any energy conversion process must be monitored. Pollution minimization actions in the energy sector must be employed as well. In this regard, every jurisdiction needs to aim to attain such a balance among energy supply, demand, and the environmental impact (EIs), and hence develop policies and strategies. For the sustainable development of society, ideally, one must relate only to energy resources that cause no EIs. On the other hand, increased energy efficiency reduces the EIs, because, for the same services or products, less resource utilization and pollution is normally associated with increased efficiency. Moreover, energy efficiency leads to a reduction of energy demand and therefore extension of the available resources. Furthermore, the use of energy resources in a rational manner through energy conservation measures leads to the stabilization of the rate of growth of energy consumption, which is beneficial for sustainability and the environment. Cost effectiveness of any energy-related activity has major importance because cost effectiveness means more savings or reduced expenses for the same services provided or products produced. However, cost savings and environmental pollution relate to each

Environmental Dimensions of Energy

51

other, because building a real capital calls for production activity, and any production activity causes certain EI. In this picture, energy security has become another important factor. Better energy security implies development of energy policies and geopolitical strategies that eventually assure equitable access to energy resources and therefore better sustainability and a better environment. In the first part of the chapter, an energy outlook is provided with a focus on EI. In addition, the EI due to energy-related activities is reviewed. Major EIs, such as acid rain, greenhouse gas (GHG) emission, ozone depletion, and soil and water pollution are discussed. Methods to quantify the EI of energy conversion systems, based on exergy and life cycle assessment (LCA), are presented. The chapter also comprises case studies in an effort to provide some illustrative examples.

1.3.2

Energy Supply and Demand

Energy is a conservable physical quantity. However, its value relates to the capacity of doing work, which is not conserved because the real processes are irreversible. Whereas, exergy, defined as the potential of producing work with respect to a reference environment, represents the physical quantity responsible to assess the value of any energy supply. Humans master energy by converting it from one form to another while producing useful work. As such, the chemical energy of fuels is converted into hightemperature heat by combustion, whereas heat can be converted into work with heat engines. Any energy conversion process affects the surrounding environment in many ways. This section reviews the fundamental sources and the resources of energy on Earth; as well, it briefly discusses the energy conversion technologies. Earth is bathed by solar radiation; Earth’s crust is abundant with fossil fuels, nuclear fuel, and geothermal heat; the gravitational force of the Moon–Sun–Earth system generate tides, which represents an energy source ready to be harvested. The energy sources on Earth can be categorized in three folds: fossil fuels, nuclear fuels, and renewable energies. The fundamental energy sources available on Earth have a renewable character, because they cannot be depleted. Since fossil and nuclear fuels are considered as finite resources, the term “energy resource” is used for them. The renewables are denoted as “energy sources” because they cannot be depleted. Solar, geothermal, and tidal are the fundamental renewable energy sources on Earth. Geothermal and tidal energies together dissipate on Earth’s surface at a total rate of 47 TW, which represents c. 28% of the world energy demand. The solar radiation is incident on Earth at a rate of approximately 49.54 PW, which is more than 3000 times larger than the world energy demand. The energy rates available from fundamental energy sources and world power demand are compared in Fig. 1. The most important energy source on Earth is solar energy, which is generated by thermonuclear reactions in the Sun and impacts the Earth’s surface after traveling a distance of approximately 150  109 m. The Sun’s surface has a temperature of about 5800K and emits electromagnetic radiation in a broad spectrum ranging from upper ultraviolet to far infrared. The solar radiation impacts the Earth over a region extending from the Arctic to the Antarctic region with the maximum intensity falling over the Sunbelt region. In the polar regions of Earth, i.e., the Arctic (North Pole) and Antarctic (South Pole) regions, solar radiation is present only for about 6 months per year and at lower intensity. Observe that the planet Earth connects to the surrounding universe by two “heat exchangers,” as shown in Fig. 2: the Sunbelt on the “hot side” and the polar regions on the “cold side.” The high-temperature heat exchanger is a hot surface with temperature TH, heated by the Sun, and covers an area AH extending from the southern to the northern polar circle and having the Equator in the middle. The second boundary is a cold surface of area AL formed by the two polar zones (the South and North Poles) having the temperature TL and cooled due to the radiation heat transfer with the universe. The system operates as a “celestial” heat engine between the temperatures limits TH and TL. The solar and terrestrial radiations are received and rejected at the source and the sink, respectively. The heat flux QH received by this heat engine at the hot surface is proportional to AH (TS4 TH4), where Ts is the surface temperature of the Sun, and TH { TS is the average temperature of the globe, i.e., B300K. Neglecting TH with respect to TS results that QHBAHTS4. 100 49.54 Power (PW)

10 1 0.1 0.015 0.01

0.0044

0.003

0.001 Solar derived Geothermal (beam radiation + hydro + wind + biomass)

Tidal

Energy demand

Fig. 1 Energy rate available from fundamental renewable energy sources compared to world energy demand. Modified after Dincer I, Zamfirescu C. Advanced power generation systems. New York, NY: Elsevier; 2014.

Environmental Dimensions of Energy

Q0 /2,T∞

52

Polar zone TL

Sun

Qs,Ts

TH

Q0 /2,T∞

TL Polar zone

Fig. 2 The Sun–Earth heat engine, which moves things on Earth. Reproduced from Dincer I, Zamfirescu C. Advanced power generation systems. New York, NY: Elsevier; 2014.

Hydroenergy 22.00%

Reflected or scattered energy 34.77%

Biomass energy (photosynthesis) 0.02%

Wind energy 0.21% Beam radiation energy 43.00% Fig. 3 Breakdown of renewable energies derived from solar radiation. Data from Dincer I, Zamfirescu C. Advanced power generation systems. New York, NY: Elsevier; 2014.


 4 Similarly, one can estimate the heat flux QL lost by the Earth at the cold surfaces, which is proportional to AL TL4 T1 , where 4 the universe’s background temperature is T1B3K. Since TL cT1 , it results that QL B AL TL . Therefore the efficiency of the terrestrial heat engine is estimated with ZB

QH QL ¼1 QH

AL TL4 AH TS4

ð1Þ

here, Eq. (1) gives a value of more than 0.99 for efficiency, which means that due to the high temperature of the Sun with respect to the temperature of the Earth’s surface corroborated to the low background temperature of the universe, and one would determine a high efficiency of solar energy conversion into work on the Earth. Nevertheless, the work generated by the Sun–Earth heat engine gets dissipated in various processes. The atmospheric and hydrospheric flows “consume” a significant part of this work. More exactly, winds and air movements in the atmosphere and oceanic currents in the hydrosphere are generated by solar energy. The energy forms derived from solar radiation are: hydroenergy, wind energy, ocean currents and waves, biomass energy, and direct solar radiation energy. In Fig. 3 a breakdown of energy forms on Earth derived from the incident solar radiation is shown. Twenty-two percent of the incoming solar radiation can be retrieved in form of hydroenergy, which is a form of potential energy caused by the elevation of water level. Hydroenergy is due to the water cycle in nature – driven by solar energy – which makes water evaporate from oceans and then produces precipitations at higher altitudes with eventual formations of river basins and lakes. Wind energy is another form of dissipation of solar energy. Wind is mechanical movement of air masses due to a gradient in pressure. The pressure difference between two nearby locations is due to the differential heating generated by solar radiation. The wind energy accounts only 0.21% of the incoming solar radiation from which it is derived.

Environmental Dimensions of Energy

53

A very little portion, only 0.02% of the beam radiation, is used in photosynthesis process performed by green plants around the surface of the globe. Although the photosynthesis process has low efficiency (1–4%), because of its massive production of sucrose, glucose, cellulose and other chemical compounds, it represents a major source of energy and food worldwide. Photosynthetic processes are the processes where biomass is produced. Biomass in the form of wood, crop, grass, straw, sugar cane, manure, dung, charcoal, municipal and domestic waste, paper mill residuals, etc. represent a form of energy covering a calorific value range from 4 to 30 MJ/kg or 2 to 10 GJ/m3 (see Ref. [1] for details). The estimated global power generation potential from hydroenergy is the highest among all forms of energy, namely, c. 30 PW. This figure is calculated based on the 22% hydroenergy breakdown, assuming 80% hydraulic energy conversion. The beam radiation has the second highest potential for power generation, if one assumes that solar-to-power conversion efficiency is 25%; thenceforth, the estimated global power generation from the beam radiation is of c. 18 PW. Furthermore, accounting for capacity factor and wind turbine efficiency, the potential of wind power generation is of 0.22 PW. With an assumed biomass-to-power energy conversion efficiency of 50%, the estimated potential of biomass energy is 0.02 PW. There is also power generation potential from the ocean energy in three forms of conversion, namely ocean thermal, ocean waves, and ocean currents [1]. Solar radiation and its derived forms can be converted into power and heat. Photothermal solar energy conversion, meaning conversion of solar radiation into thermal energy, is achieved using a large palette of solar collectors, covering a range from lowtemperature applications (e.g., water heating) to high-temperature applications (e.g., power generation). Photovoltaic (PV) energy conversion is another pathway to convert solar energy into useful electric power. Ocean energy systems cover a large palette of technology to harvest ocean currents and generate power, or, to capture ocean thermal energy. Wind energy can be harvested by using wind turbines, which are high-efficiency wind-to-electricity converters; however, the capacity factor for wind energy is around 0.3 due to wind fluctuations and intermittency. Energy stored in biomass can be retrieved in the form of heat by biomass combustion. In addition, biomass can be converted to various types of biofuels that are then used in heating and power applications. Certain biomasses recovered from crops or other plants can be converted into alcohols using aerobic fermentation processes. From various biomasses, one can obtain liquid fuels via transesterification, Fischer–Tropsch synthesis or other processes. Biomass such as wood, crops, or other plants can be directly combusted to generate heat. Other synthetic fuels, such as biogas or liquid biofuels, can also be combusted to obtain a source of high-temperature heat. Another form of renewable energy is the one that can be recovered from anthropogenic wastes, including the heat wasted by any anthropic processes. There are many combustible materials that can be recovered from industrial and residential settings, including plastics, paper, wood materials, garbage, etc. Deposited garbage in waste collection sites leads to generation of landfill gas, which is a combustible product. Recovered plastic materials can be converted into liquid fuels by catalytic reforming processes. Other waste materials can be combusted or incinerated, depending on the case, generating a high-temperature heat. Many industrial processes generate waste heat that can be recovered and used for various purposes as a source of energy. Waste materials, such as manure or certain forms of biomass, can be converted into biogas by anaerobic digestion. Other sources of heat derived from renewables are not anthropogenic as they occur naturally in various forms, such as direct solar radiation, ocean heat, and geothermal energy. Fig. 4 shows an inventory of sustainable thermal energy sources. Nuclear energy is included in the figure as it is considered sustainable, provided that no essential environmental pollution occurs at the exploitation phase. Spent nuclear fuel is collected and safely deposited in underground repositories. Upstream mining activity and nuclear plant construction has associated pollution, but this is mitigated by the clean operation during the long-term exploitation phase. Other resources of energy available to humans are those derived from fossil fuels and nuclear fuel. Fossil fuel energy is the most used source of energy worldwide with an 80% share currently of the total energy consumption. Fig. 5 shows the energy consumption shares. Renewable energies represent the second largest consumption. In the third place of consumption is nuclear energy with only 6% of shares worldwide. Very few countries use nuclear energy; however, fossil fuel-fired power plants are available in any country around the world. Among the renewables, biomass is responsible for the biggest share with approximately 11%, followed by hydro and geothermal power with approximately 2.7%, and then by wind and solar. Fig. 6 presents a bar chart comparison of reserves versus production ratio for the conventional fuels including coal, petroleum, natural gas, and uranium. Conventional fossil fuels encompass three types of materials, namely coal, petroleum, and natural gas. In addition, there are several other types of nonconventional fossil fuels that started to be exploited more recently, such as shale oil, oil sands, coal bed gas, etc. Fossil fuels are mainly used in various combustion systems, such as furnaces, gas turbines, internal combustion engine (ICE), etc. Coal is mainly formed of organic substances derived from fossilized plants with embedded mineral inclusions. The primary chemical element in coal constituency is carbon, the content of which is over 70% by weight. Coal has a calorific value of 25–35 MJ/kg depending on the type, which ranges from lignite (lowest calorific value) to anthracite (highest calorific value). The proven resources of coal globally are equivalent to the energy of 23.1  106 PJ and with a resource over production ratio of R/P¼122 years. The R/P ratio is calculated based on the amount of resource, preferably expressed in energy units, and the rate of coal production (or consumption) expressed in energy units per year. Coal is mainly used in major power plants and metallurgical and cement industries. Petroleum is a naturally occurring hydrocarbon (HC)-based material that can mainly be found as liquid. Nonconventional sources of petroleum can be present in solid forms, such as bitumen or oil sands. Alkanes, cycloalkanes, aromatics, paraffin, and naphthalene are the main constituents of petroleum. The total world petroleum reserves include 30% conventional oil, 25% extra

54

Environmental Dimensions of Energy

Sustainable thermal energy

Direct use of heat

Combustion

Waste combustion/ incineration

Direct use of heat

Heat recovery

Biogas

Geothermal heat

Combustion Biofuels

Moderator heat rejection

Ocean heat

Biomass

Direct solar radiation

Nuclear fission

Nuclear fuel

Renewables

Plastics reforming

Landfill gas

Process waste heat

Waste materials

Fig. 4 Sustainable thermal energy sources. Modified from Dincer I, Zamfirescu C. Advanced power generation systems. New York, NY: Elsevier; 2014.

Wind and solar 1%

Nuclear fuel 6% Hydro and geothermal 19%

Renewables 14%

Fossil fuels 80%

Biomass 80%

Fig. 5 Energy consumption shares worldwide. Data from Dincer I, Zamfirescu C. Advanced power generation systems. New York, NY: Elsevier; 2014.

heavy oil, 15% heavy oil, and other petroleum forms, such as bitumen, shale oil, and oil sands for the remaining 30% share. Proven fuel reserves of petroleum are 15.9  106 PJ with an R/P ratio of approximately 50 years. The main consumption of petroleum is in the transportation sector. Natural gas containing methane (CH4) as combustible material occurs naturally in many natural gas fields around the world. Natural gas is used in many industries including for fertilizer (ammonia (NH3), urea) production, and as fuel for heating, cooking,

Environmental Dimensions of Energy

55

140 122 120

R/P (years)

100 80 60

60

50

50

40 20 0 Coal

Petroleum

Natural gas

Uranium

Fig. 6 Proven reserves vs. production (R/P ratio) for conventional fuels. Modified after Dincer I, Zamfirescu C. Advanced power generation systems. New York, NY: Elsevier; 2014.

250 OECD

Non-OECD

Energy demand (EJ)

200

150

100

50

0 Residential

Commercial

Industrial

Transportation

Fig. 7 Predicted energy demand of main activity sectors in 2040. Non-OCED, non-organization for economic cooperation and development. Data from DOE/EIA. International Energy Outlook. U.S. Energy Information Administration. Available from: www.eia.gov/ieo; 2013.

and in some cases for power generation. About 20% of the world energy production is derived from natural gas combustion. Natural reserves are estimated to have an equivalent energy of 7  106 PJ with an R/P ratio of 60 years. Another mined fuel is the nuclear fuel. Conventional nuclear fuel is represented by fissile uranium 235U that naturally occurs in the form of U3O8 ore. From 1 t of ore about 6 kg of fissile uranium can be extracted, which is equivalent to 144 TJ electrical energy (or 40 GWh), whereas 1 t of coal can be used to generate 14,000 times less electricity. Conventional nuclear fuel reserves are estimated to be the equivalent of 1.6  106 PJ with an R/P ratio of 50 years. Also note that essentially many more nonconventional nuclear fuel resources do exist, mainly in the form of thorium, but also of spent fuel material reusable in projected breeder reactors of next-generation nuclear power plants. However, fossil and nuclear fuels must be mined. And every mining activity has an EI. Mining requires extensive operation and specific works. It produces tailings, which create environmental burdens. Fossil fuels are consumed and emissions are emitted to the atmosphere during the mining process, as many types of engine-driven equipment are involved in mining, fuel processing, and fuel distribution systems. Conversion of fossil fuels into power is realized in power plants and engines. A combustion process involved in this conversion leads to atmospheric pollution with GHG, nitrogen oxides (NOx), sulfur dioxide (SO2), and other pollutants. Nuclear fuel is converted into power in steam Rankine power stations. Although there are no direct emissions of combustion gases at a nuclear power station, an EI does still exist. At power stations, lake/ocean water ecosystems are impacted due to the heat dumped from the cooling system of the power plant. Indirect and safety systems of nuclear power stations still emit combustion gases to the atmosphere, although the footprint is much smaller than that of a fossil fuel power station. The increase of population definitely leads to an increase in commodity demand. The most comprehensive way to analyze the commodity demand of the society is through energy. This is due to the fact that energy supply is the basis of the majority of human activities. The energy demand depends on the population density and the economic development of the world region. The world’s main economic regions and their predicted energy demands appreciated for the each major sector of activity are shown in Fig. 7.

Environmental Dimensions of Energy

Liquid fuels consumption (EJ)

56

160 2010

2040

120 80 40 0

Fig. 8 Predicted trend of liquid fuel consumption per sector of activity. Data from DOE/EIA. International Energy Outlook. U.S. Energy Information Administration. Available from: www.eia.gov/ieo; 2013.

Hydrocarbon fuels (including coal)

CO2 VOCs Combustion processes

CO

Emissions to environment

N2O, NOx SOx Power generation system

Electricity and mechanical work

Fig. 9 Illustrating environmental impact (EI) of power generation system through atmospheric effluents. VOCs, volatile organic compounds.

Most of the demand is in the industrial sector of non-organization for economic cooperation and development (non-OECD) countries. The OECD countries have slightly more energy demand in the commercial sector than those in the non-OECD ones. Liquid fuels are the main source of energy for transportation vehicles. Other uses of liquid fuels are in the industrial sector and there is relatively less use in the residential and commercial sectors. The predicted trend of liquid fuel consumption per sector of activity is shown in Fig. 8. It clearly appears that transportation is the highest consumer of liquid fuels. The trends show that liquid fuel demand in the transportation sector will increase in the future.

1.3.3

Environmental Impacts in Energy Sector

It is generally agreed that each and every energy system impacts the environment in certain measures. Over the past few decades, energy-related environmental concerns have expanded from primarily local or regional issues, to the international and global nature of major energy-related environmental problems. Particularly in developing or newly industrialized countries, where energy-consumption growth rates are typically extremely high and environmental management has not yet been fully incorporated into the infrastructure, environmental problems are becoming apparent or already exist. Nevertheless, industrialized countries at present are mainly responsible for air pollution, ozone depletion, and carbon emissions. Many developed and developing countries through several national and international institutes and agencies have started taking actions to reduce (or eliminate) the pollutant emissions and to attain a sustainable supply of energy sources. At the Kyoto Climate Change Conference in December 1997, a list of 15 concrete proposals came out for curbing global GHG emissions. The list includes improving the fuel efficiency of automobiles, introducing solar power facilities, and planting forests to act as “green lungs” in densely populated areas. Fig. 9 shows a generic power production system, which consumes fossil fuels, generates useful work, and expels some pollutants in the environment. There may be pollutant emissions, accidents, hazards, ecosystem degradation through air and water pollution, animal poisoning, GHG emission, carbon monoxide (CO) leakages, stratospheric ozone depletion, and emission of SO2, NOx, volatile organic compounds (VOCs), such as CH4, propane and butane, and PM, and other aerosols. Excessive concentrations of these pollutants and ozone have demonstrated health, welfare, and ecological effects felt locally and sometimes regionally. NOx and VOCs are known to be responsible for photochemical smog, which is produced by a series of complex atmospheric reactions. High levels of NO2 cause brownish haze over cities. Its activation requires light, and increasing sunlight promotes the production of tropospheric ozone and other ingredients of photochemical smog. Air pollutants are emitted from a

Environmental Dimensions of Energy

57

Some clean pathways

Conventional pathway

Crude oil

Natural gas

Solar energy

Wind energy

Pipeline transportation

Pipeline transportation

Photovoltaic power

Wind turbine, power generation

Distillation

Reformation

Electricity transmission

Electricity transmission

Distribution

Hydrogen

Hydrogen from water electrolysis

Hydrogen from water electrolysis

Power generation

Compression and distribution

Compression

Compression

Expelled pollutants

Power generation

Power generation

Power generation

CO2, NOx, VOCs, PM, aerosols

Expelled pure water

Expelled pure water

Expelled pure water

Fig. 10 Schematic representation of conventional and some clean power generation pathways (PM, particulate matter; VOCs, volatile organic compounds).

variety of stationary and mobile fuel consumption sources, and energy-related activities contribute significant quantities of all these pollutants. Regulations on emissions are often used to reduce air pollution, and high chimney stacks are used to alleviate localized air pollution (i.e., transport pollutants elsewhere). Indoor air pollution is also of concern (e.g., CO, CO2, and smoke from stoves and fireplaces; various gaseous oxides of nitrogen and sulfur from furnaces; stray natural gas and heating oil vapors; radon emitted by natural gas burning appliances and the surrounding soil; cigarette smoke; formaldehyde from plywood, foam insulation, and glues). Ventilation even in tightly sealed energy-efficient buildings can eliminate most indoor air quality concerns. Knowledge of indoor pollutant dose–response relationships is still incomplete. The conventional pathway of power generation is schematically illustrated in Fig. 10, in comparison with cleaner pathways. After extraction, refining, and distribution, the fuel is used in oil-fired power plants to generate power. Three cleaner pathways for power generation are illustrated through hydrogen energy, as well. Starting with natural gas as energy source, this is refined and distributed; then, it is reformed at the distribution points and converted to hydrogen. This action can be performed using techniques that sequestrate CO2, when possible. The generated hydrogen, when combusted with air, will only expel water at the power generation stage. Nevertheless, if during the reformation process the GHG and other pollutants’ emissions are not mitigated, then this pathway is responsible for some pollution. However, if renewable sources are used as energy inputs, then pollution is better mitigated. With PV and wind power generation, lesser pollution is experienced. Nevertheless, indirect pollution still manifests through the pollution associated with the construction phase of the solar and wind equipment. In the atmosphere, the pollutants expelled by power generation systems produce various hazards. Fig. 11 shows that aerosols and GHGs cause global warming. Extreme events can be generated by global warming or climate change, such as extensive

58

Environmental Dimensions of Energy

GHG, SO2, NOx, VOCs, PM, CO, etc.

Aerosols

Earth climate Greenhouse gases

Power

Fuel

Stack

Albedo effect

Acids formation

Global warming

Greenhouse effect Acid precipitation

Power plant Acidification of waters and soil

Sea level rise + Precipitations change + Extreme events occurrence

Fig. 11 Illustrating the ecosystem damaging effects of atmospheric pollutants of conventional power plants. GHG, greenhouse gas; PM, particulate matter; VOCs, volatile organic compounds.

precipitations, sea level rise, etc. Acidic gases form acid precipitation, which impacts soil and water and their life systems. GHGs are those chemicals that are released due to the atmosphere by natural and anthropogenic activities. When released, GHGs travel through the atmosphere and reach the upper parts of troposphere. At these levels, GHGs absorb an important part of the infrared radiation emitted by the Earth’s surface, and emit back to the surface. As consequence, the Earth’s surface temperature and the air temperature tend to increase, and this process is called the greenhouse effect. On the other hand, aerosols, such as VOCs, soot, PM, etc. are continuously released to the atmosphere and concentrate at its upper layers. Aerosols contribute to the Earth’s albedo. Due to their presence in the atmosphere, aerosols reflect and scatter back in the extraterrestrial space a part of the incident solar radiation. As a consequence, the Earth temperature tends to decrease. This process can be denoted as the albedo effect. The balance between the greenhouse and albedo effects establishes the Earth’s long-term temperature regime and regulates the Earth’s climate. This mechanism of climate control is a natural process. However, it is noted that since the industrial revolution, the anthropogenic impact on climate became obvious due to accentuated emission of GHGs emitted massively by many activity sectors (energy, transportation, industry), which induced global warming. Emissions of SO2 and NOx, which typically characterize the energy supply sector, have a direct EI due to acidification effect. These gases may participate in the complex set of chemical transformations in the atmosphere, resulting in acid precipitation. Road transportation is also an important source of NOx emissions. Most of the remaining NOx emissions are due to fossil fuel combustion in stationary sources. Countries where the energy-related activities occur widely are likely to be significant contributors to acid precipitation. A major problem with acid rain is that its effects often occur in a different country than its source. There is a large variety of major evidence to show the damages of acid precipitation, including:

• • • • • • •

acidification of lakes, streams, and ground waters toxicity to plants from excessive acid concentration corrosion to exposed structures damage to fish and aquatic life damage to forests and agricultural crops deterioration of buildings and fabrics influence of sulfate aerosols on physical and optical properties of clouds.

Hazardous wastes pose special health and environmental threats, and are mainly generated by the chemicals and metal industries. Nonhazardous wastes, for example, bottom ash from power plants and air-pollution control residues, pose disposal problems regarding space and appropriate containment. The commercial use of some solid wastes as building industry products and transportation surfaces is limited by the size of the market. Hazardous air pollutants are usually emitted in smaller quantities compared to those that are the focus of ambient air quality concerns. Lead is the main hazardous air pollutant, and most of the world’s lead pollution comes from the use of lead-based gasoline additives to increase octane ratings. Lead exposure may cause neurological damage. Since the 1970s, many countries have taken steps to phase out these lead-based additives. Additionally, the number of suspected hazardous pollutants is very large, and knowledge of sources, emissions, and effects is still developing. The concern is both localized, effects where micropollutants are discharged, and regional for the toxic pollutants, for example, cadmium, mercury, and polycyclic aromatic hydrocarbons (PAHs). Many energy-related activities emit hazardous air pollutants, for example, HCs (such as benzene) emitted fugitively from oil and gas extraction and processing industries; HC and dioxin emissions caused by the use and combustion of petrol and diesel oil for transport; small quantities of arsenic, mercury, beryllium, and radionuclides released during the combustion of coal and heavy fuel oil; and mercury, chlorinated dioxin, and furan emissions from municipal waste incinerators. Details of acid formation in the atmosphere and acid deposition on land and seas are illustrated in Fig. 12. Acid precursors produced mainly from the combustion of fossil fuels, especially coal and oil, and the smelting of nonferrous ores can be transported long distances through the atmosphere and deposited in different ecosystems.

Environmental Dimensions of Energy

59

Solar radiation Clouds Acids formation via photoreactions: SO2 + H2O → H2SO4 NOx + H2O → HNO3

Dissolution in atmospheric moisture: H2SO4 → 2H+ + SO42− HNO3 → H+ + NO3−

Wind direction

Fumes including SO2, NOx

Acid precipitation

Fossil fuels combustion at mobile and stationary units Earth surface Dry deposition SO2, NOx

Wet deposition H+, NO3−, SO42−

1–2 km > 100 km Fig. 12 Detail of the mechanism of acidic precipitation. Modified after Dincer I, Zamfirescu C. Advanced power generation systems. New York, NY: Elsevier; 2014.

The majority of SO2 emissions come from fossil fuel power plants, while the majority of NOx is emitted by the transportation sector. Another source of acid precipitation is sour gas treatment, which produces H2S that then reacts to form SO2 when exposed to air. Dry deposition of SO2 and NOx creates opportunity for acid formation at the soil level or in seas. Direct deposition occurs at places 1–2 km away from the emission source. Acid precursors that travel to upper levels of the atmosphere enter in photoinduced reactions with water vapor and form acids, such as sulfuric acid and nitric acid. These acids travel far from the point source pollution and are dissociated in the atmospheric moisture. The dissociated acids fall on the Earth’s surface as precipitation (fog, rain, snow, etc.). Acidification and other types of pollutions caused by energy systems may affect the quality of waters including groundwater, because of its role in the supply of drinking and irrigation water. Efforts are continually being made to control energy-related pollution, for example, geothermal fluids containing toxic chemicals, acid drainage from mines, coal wastes, effluents containing hazardous chemicals from power plants and refineries, and thermal pollution from the discharges of cooling systems of power plants. Significant concerns exist about the quality and quantity of available water resources including groundwater, because of its role in the supply of drinking and irrigation water. Table 1 gives the list of main types of pollutant emissions due to energy systems; it also explains the influence of the pollutant on the environment. There are seven categories of EI, which are defined in LCA methodologies according to the norm ISO 14042 [2]. These impact categories are global warming, acidification, ozone depletion, toxicity, photooxidant formation, eutrophication, and depletion of abiotic resources. The norm ISO 14042 [2] includes methodologies for quantitative assessment for each of the EI categories, using specific indicators. Table 2 lists the EI categories and gives the impact indicators that are used to quantify each category of impact. The impact indicators are defined by convention and are based on some measurable quantities. For example, global warming is quantified by the global warming potential (GWP) expressed as kg of CO2 equivalent of GHGs emitted; the acidification potential is measured in SO2 emitted, expressed in so-called Switzerland equivalent; the ozone depletion is quantified by ozone depletion potential (ODP) expressed as tri-chloro-fluoro-methane equivalent; the toxicity is expressed as 1,4-dichlorobenzene equivalent; photooxidant formation is expressed as kilogram (kg) of ethylene equivalent; eutrophication is expressed as kg PO4 equivalent; and the depletion of abiotic resources is expressed as kg antimony equivalent. The norm ISO 14042 [2] establishes precise determination procedures for all these indicators. Subsequently, the determination of GWP will be detailed because GHG emissions represent the main environmental pollution produced by power generation systems. Water vapor is the most important GHG. However, the water vapor concentration in the

60

Atmospheric pollutants released by power generation systems

Pollutant

Explanations

Greenhouse gases (GHGs)

These are gases that produce greenhouse effect. The main GHGs are carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O). Greenhouse effect is the main cause of global warming Arises mostly from the incomplete combustion of fuels, and poses a health and life risk for animals and humans upon inhalation. Intense emission of CO in open spaces may affect birds, whereas emissions in closed spaces (residences, garages) may cause death of exposed persons It is a corrosive gas, which is hazardous to human health and harmful to the natural environment. It results from combustion of coal and fuel oil, smelting of nonferrous metal ores, oil refining, electricity generation, and pulp and paper milling. It causes respiratory difficulties, damages green plants, and is a precursor of acid precipitation NO and NO2. It is produced by combustion of fuels at high temperatures in all combustion facilities from large- to small-scale (including motor engines, furnaces etc.). The main reason for the tropospheric ozone formation, therefore, the photochemical smog. It can lead to respiratory problems of humans and animals. It can also form acids in high-altitude atmosphere Result from hydrocarbon (HC) combustion in engines. Have harmful effects in the atmosphere; impede formation of stratospheric ozone

Carbon monoxide (CO) Sulfur dioxide (SO2)

Nitrogen oxide (NOx)

Volatile organic compounds (VOCs) Particulate matter (PM)

Chlorofluorocarbons (CFC), hydrochlorofluorocarbons (HCFC), hydrofluorocarbon (HFC)

Particles in the air (fly ash, sea salt, dust, metals, liquid droplets, soot) come from a variety of natural and human-made sources. Particulates are emitted by factories, power plants, vehicles, etc., and are formed in the atmosphere by condensation or chemical transformation of emitted gases. PM causes health and environmental effects including acid precipitation, damage to plant life and human structures, loss of visibility, toxic or mutagenic effects on people, and possibly nonaccidental deaths These are mainly refrigerants or propellants. They can contribute to the destruction of the stratospheric ozone layer and to the greenhouse effect

Environmental Dimensions of Energy

Table 1

Environmental Dimensions of Energy

Table 2

Environmental impact (EI) categories and indicators for impact quantification

Impact category

Explanation

Unit

Climate change

It is due to the emissions of greenhouse gases (GHGs). It is quantified based on global warming potential (GWP) measured in kilogram (kg) CO2 equivalent The impact of NH3, NOx, and SO2 present in precipitations and their contribution to acidification of land and waters. It is expressed in SO2 emitted in Switzerland equivalent Depletion of stratospheric ozone layer, especially due to photoreactions involving chlorine. It is quantified by ozone depletion potential (ODP), which represents the amount of a substance that depletes the ozone layer relative to CFC11 (tri-chloro-fluoro-methane) Refers to toxicity produced in air, fresh water, seawater, or terrestrial. It is quantified with respect to the toxicity of DCB (1,4-dichlorobenzene) and measured in kg DCB equivalent Depends on photochemical ozone creation potential expressed with respect to the reference substance: ethylene. Measured in kg of ethylene equivalent Covers all potential impacts of high levels of nutrients, the most important of which are nitrogen (N) and phosphorus (P). It is measured in kg PO4 equivalent. Abiotic resources include all nonliving resources (coal, oil, iron ore, renewable energies etc.). It is measured in kg antimony equivalent

kg CO2,eqv

Acidification Ozone depletion

Toxicity Photooxidant formation Eutrophication Depletion of abiotic resources

Table 3

61

kg SO2,eqv kg CFC11eqv

kg DCBeqv kg C2H4,eqv kg PO4,eqv kg Sbeqv

Principal greenhouse gases (GHGs) and their GWP

Gas

CO2 Methane (CH4) Nitrous oxide (N2O) CFCl3 CF2Cl2

Absorption spectrum (cm 1)

550–800 950–1650 1200–1350 800–900 875–950

Atmospheric concentration

387 ppm 1750 ppb 314 ppb 251 ppt 538 ppt

Atmospheric lifetime

50–200 years 12 years 120 years 50 years 102 years

Global warming potential (GWP) 20 years

100 years

500 years

1 72 289 6,730 11,000

1 25 298 4,750 10,900

1 7.6 153 1620 5200

Source: Reproduced from IPCC. Climate change 2007: Synthesis report. Intergovernmental panel on climate change. Valencia: IPCC Plenary XXVII; 2007.

atmosphere fluctuates rapidly and water vapor absorbs radiation for a wide infrared spectrum. The principal GHGs emitted in the atmosphere (except the water vapor) are listed in Table 3. Three of the gases shown in the table, namely, CO2, CH4, and nitrogen dioxide, are part of natural cycles of carbon and nitrogen and also part of anthropogenic emissions; therefore, they were already present in the atmosphere prior to the industrial era, for which the year 1750 is considered as the reference. As seen, the concentrations of these gases in the year 1750 were higher than zero. Another two manmade substances are shown in Table 3, which are examples of GHGs, the freons. Here is a nonexhaustive list of other freons, given in decreased order of their influence on greenhouse effect: 1,1,2-trichloro-1,2,2-trifluoroethane (CFC113), chlorodifluoromethane (HCFC22), CFC141b, CFC 142b, 1,1,1trichloroethane (CH3CCl3), carbon tetrachloride (CCl4), 1,1,1,2-tetrafluoroethane (HCFC134a). There are other GHGs generated as a result of human activity, but they are emitted in lower quantities and consequently their actual concentrations in the atmosphere are low. It is certain that atmospheric CO2 levels will continue to increase significantly. The degree to which this occurs depends on the fixture levels of CO2 production and the fraction of that production that remains in the atmosphere. Given plausible projections of CO2 production and a reasonable estimate, which states that half the emitted amount will remain in the atmosphere, it is generally believed that at some time during the middle part of the 21st century, the concentration of CO2 will reach 600 ppm levels in the atmosphere. Anthropogenic activities have induced the greenhouse effect that led to global warming. With greenhouse effect, there are two possibilities:

• •

The anthropogenic activities result in more GHG emissions. Since these gases will be more concentrated in the lower layers of the atmosphere, where human activities occur, the troposphere reradiates more toward the Earth’s surface. Therefore the global temperature increases. The GHG concentrations reduce in the vicinity of the Earth’s surface and concentrate toward the upper layers for some unknown reason. In this case, the troposphere reradiates less toward the Earth’s surface. This means a decrease in the Earth’s temperature.

The radiative forcing is defined as the net change in radiation balance at the tropopause, produced by a specified cause. By convention, the radiative forcing is positive and it induces an increase in planetary temperature, and when negative, it is the other way around. The unit of measure of radiative forcing is the same as the unit of radiation energy rate per square meter of Earth’s

62

Environmental Dimensions of Energy

surface, where the Earth’s surface, by convention, is the area of the sphere having the average radius of the planet. The usual symbol for the radiative force is DF. Typical values of radiative forcing are 0–2 W/m2. It can be derived from what was explained above, i.e., that the change in the concentration of GHGs, aerosols, and atmospheric ozone induces radiative forcing. Depending on the concentration of the gas in the atmosphere, there are three regimes of producing radiative forcing: low, moderate, and high concentration. A lowconcentration regime regards gases present in the atmosphere in parts-per-billion or parts-per-trillion. In this case, the forcing is proportional to the concentration change, DFBDC ¼ C C0. Freons and the tropospheric ozone fall in this category. The proportionality constants are as follows: 0.25 for CFC11, 0.32 for CFC12, and 0.02 for tropospheric O3. Quantifying the effect of a particular atmospheric gas on climate is a multivariable problem. If, for example, the gas absorbs more in infrared spectrum, its greenhouse effect is accentuated, that is, it has been associated with a positive radiative forcing. However, one more parameter is also important, that is, the atmospheric lifetime. It is important to know how long the gas is active in the atmosphere with respect to radiative balance control. In the moderate-concentration regime, since the gas molecules at higher concentration absorb much radiation where the absorption band is the strongest, the absorption for the broader range of the ffi pffiffiffiffiffiffi diminishes in rate. Because of this reason,
pffiffiffispectrum the radiative forcing is proportional to the root of the concentration, DF B C C0 . This is the case for CH4 and nitrous oxide (N2O) molecules, whose concentrations are also in ppb, but their relative effects are higher due to a more active absorption spectrum. According to Ref. [3], the radiative forcing of CH4 and N2O is given by:
pffiffiffiffiffiffiffi pffiffiffiffiffiffiffiffiffi ( CM0 f ðCM ; CN0 Þ þ f ðCM0 ; CN0 Þ DFCH4 ¼ 0:036 CM
pffiffiffiffiffiffiffi pffiffiffiffiffiffiffiffi ð2Þ DFN2 O ¼ 0:12 CN CN0 f ðCM0 ; CN Þ þ f ðCM0 ; CN0 Þ

where CM is the CH4 and CN is the N2O concentration, respectively; and index 0 refers to the initial situation. The effect of CH4 and N2O on radiative forcing overlap; this fact is accounted for by the following overlapping function given by h i ð3Þ f ðCM ; CN Þ ¼ 0:47ln 1 þ 2:01  10 5 ðCM CN Þ0:75 þ 5:31  10 15 CM ðCM CN Þ1:52

The third regime – of highly concentrated gases – corresponds to CO2 only, where because of the high concentration, any further change produces less effect. In this situation, the radiative forcing is proportional to the variation of the natural logarithm of the concentration, which is given by DFCO2 ¼ 5:35 lnðC=C0 Þ

ð4Þ

Fig. 13 shows the radiative forcing produced by various causes. The total radiative forcing represented by the superposition of all individual anthropogenic type components is also indicated. Knowing the total radiative forcing creates the opportunity to 2.5 2.0

ΔF (W/m2)

1.5 1.0 0.5 0.0 −0.5 −1.0

Total antropogenic forcing

Solar irradinace fluctuation

Clouds albedo

Direct aerosol albedo

Albedo from black carbon on snow

Albedo from land use

Stratospheric water vapor from methane

Tropospheric ozone

Stratospheric ozone

Halocarbons

Nitrous oxide

Methane

−2.0

Carbon dioxide

−1.5

Fig. 13 The radiative forcing in 2005 with respect to year 1750. Reproduced from IPCC. Climate change 2007: Synthesis report. Intergovernmental panel on climate change. Valencia: IPCC Plenary XXVII; 2007.

Environmental Dimensions of Energy

63

define an equivalent CO2 concentration that, if present alone in the atmosphere would create similar radiative forcing. By equating ¼ DFtotal DFCOeqv 2

ð5Þ

Ceqv ¼ C0 expðDFtotal =5:35Þ

ð6Þ

one can obtain

where C0 is the initial concentration of CO2 in the atmosphere. Therefore the radiative forcing can be expressed in terms of equivalent CO2 concentration change, Ceqv/C0. To show how useful this is for data interpretation, one can read from Fig. 13 that the radiative forcing due to CO2 emission in the industrial era was on average 1.66 W/m2. The total anthropogenic forcing is 1.6. Therefore the relative change in CO2 concentration is   1:6 exp ¼ 1:35 5:35 The equivalent CO2 for this growth, based on the value of C0 ¼ 280 ppm given in Table 3, is Ceqv ¼ 378 ppm. Note that the CO2 concentration is currently of 387 ppm, thus larger than the equivalent carbon associated to the industrial era. This situation is explained by the fact that the radiative forcing accounts for both positive and negative effects on radiation balance of the planet. The usefulness of radiative forcing or the equivalent carbon comes from the possibility to correlate the forcing with the global temperature change. One asks the question, how much change on the planetary temperature is produced by a given total radiative forcing? In this respect, a so-called climate sensitivity factor g can be introduced as follows: g¼

DTe DFtotal

ð7Þ

where DTe is the corresponding temperature variation that can be caused by DFtotal. An average value of 0.6 m2 K/W can be assumed for the climate sensitivity factor for approximate calculations. It is estimated that the radiative forcing caused by doubling the CO2 concentration with respect to the year 1992 is 4.37 W/m2 and the induced global temperature increase is of 2.5K; thus the probable value for climate sensitivity factor is 0.57 m2 K/W. Nevertheless, there must be a time lag between the moment when a radiative forcing occurs and the future moment when – potentially – the radiative balance is established again and the global temperature reaches a new equilibrium. The time to reach a new equilibrium is long because the thermal inertia of the planet’s surface is high (it is composed of large water bodies, ice, biomass, rocks, and land – all having high specific/latent heats). The net forcing produced during an infinitesimal time interval is given by the product Net forcing ¼ DFCO2 ðt ÞfCO2 ðt Þdt

ð8Þ

where the notation fCO2 ðt Þ emphasizes that the fraction of gas mass existent in the atmosphere is a function of time. One can integrate the former quantity over a time horizon (TH) and obtain the total forcing produced by the respective amount of CO2. If another kind of GHG other than CO2 is considered, its integrated forcing over the TH can be normalized with the integrated forcing of CO2. From this reasoning, the following expression results: R TH DFGHG ðt ÞfGHG ðt Þdt ð9Þ GWP ¼ R0TH 0 DFCO2 ðt ÞfCO2 ðt Þdt

Atmospheric concenration (ppm)

Eq. (9) introduces the GWP, which quantifies the GHG effect of a substance emitted into the atmosphere. Table 3 gives the GWP of the main GHGs for three TH, namely, 20, 100, and 500 years. The evolution of atmospheric concentration of the main GHGs based on historical records is presented in Fig. 13. This figure shows a continuous increase of emissions from the industrial revolution until the present time. Each of these gases has an effect on the radiative forcing proportional with its GWP (Fig. 14). 103

102

CO2 CH4 N2O

101

100

10-1 1750

1800

1850

1900

1950

2000

Year Fig. 14 Evolution of atmospheric concentrations of greenhouse gases (GHGs). Reproduced from IPCC. Climate change 2007: Synthesis report. Intergovernmental panel on climate change. Valencia: IPCC Plenary XXVII; 2007.

64

Environmental Dimensions of Energy

Radiative forcing (W/m2)

1.6 1.4 CO2 CH4 N2O

1.2 1.0 0.8 0.6 0.4 0.2 0.0 1750

1800

1850

1900

1950

2000

Year Fig. 15 Evolution of radiative forcing due to main greenhouse gases (GHGs). Reproduced from IPCC. Climate change 2007: Synthesis report. Intergovernmental panel on climate change. Valencia: IPCC Plenary XXVII; 2007.

14.4 14.3

250 Global temperature Sea level increase with respect to year 1870

T (°C)

14.1 150

14.0 13.9

100 13.8 13.7

Sea level increase (mm)

200

14.2

50

13.6 13.5 1850

1890

1930

1970

0 2010

Year Fig. 16 Recorded temperature and sea level changes during the last 250 years. Reproduced from IPCC. Climate change 2007: Synthesis report. Intergovernmental panel on climate change. Valencia: IPCC Plenary XXVII; 2007.

Fig. 15 presents the evolution of radiative forcing induced by three main GHGs in the last 250 years. As it can be observed, radiative forcing and thus the climate are mostly influenced by CO2 emissions, followed by CH4 and N2O emissions. It can be remarked that there is a correlation between radiative forcing increase due to GHGs emissions and the global temperature and sea level increases. In Fig. 16, recorded data of global temperature and sea level increases with respect to year 1870 are represented. According to satellite data, in the last 35 years, the Arctic sea ice shrunk by about 2.7 times. In the last 100 years, the global surface temperature increased about 0.74K. The sea level as well increased about 0.2 m in the last 150 years. Fig. 17 presents the GHG emissions by sectors of activity and by major GHG type. Human activities since the industrial revolution, especially that related to fossil fuel combustion in industrial, commercial, and residential settings and in the transportation sector, have dangerously impacted the global environment. It is clearly remarked from the figures that the energy supply sector is responsible for the majority of GHG emissions in the atmosphere with a 26% share, followed by industry with 17%. As mentioned above, CO2 emissions have the most important anthropogenic impact on global warming. Among the CO2 sources, Fig. 17(B) shows that combustion of fossil fuels is the most important. An increase in fossil fuel combustion leads to an increase in GHG emissions into the atmosphere, which presents an alarming situation with respect to EIs and therefore global warming. Fig. 18 shows the CO2 emission records and predictions caused by combustion of the three major fossil fuels: liquid petrochemical fuels, natural gas, and coal. All the relevant emissions tend to increase as predicted, with the fact that coal is considered as the responsible agent for the majority of the abovementioned atmospheric pollutions.

Environmental Dimensions of Energy

Residential and commercial, 8%

65

Waste, 3%

Transport, 13%

Nitrous oxide, 8% Methane, 15%

Energy supply, 26%

Carbon dioxide, other, 3% Carbon dioxide, deforestration, biomass, 17%

Agriculture, 14%

Carbon dioxide, fossil fuel, 57%

Industry, 19%

Forestry, 17% (A)

(B)

CO2 emissions (billions metric tons)

Fig. 17 Anthropogenic greenhouse gas (GHG) emissions by sectors (A) and major gas type (B). Reproduced from IPCC. Climate change 2007: Synthesis report. Intergovernmental panel on climate change. Valencia: IPCC Plenary XXVII; 2007.

25 Liquid fuels

Natural gas

Coal

20 15 10 5 0 1990

2010

2020

2030

2040

Fig. 18 Records and predictions of carbon dioxide (CO2) emissions due to liquid fuel, natural gas, and coal combustion. Reproduced from IPCC. Climate change 2007: Synthesis report. Intergovernmental panel on climate change. Valencia: IPCC Plenary XXVII; 2007.

Fig. 19 illustrates the specific GHG emissions of the main classes of vehicles in Canada, given in g CO2 equivalent per megajoules (MJ) with respect to the LHV of the specified fuel. Based on the estimated specific GHG emission and the number of vehicles in operation, the total GHG emissions can be determined. Further contribution of the Canadian transportation sector to the change in the annual CO2 concentration of the Earth’s atmosphere can be calculated. As shown, these emissions lead to a 7.5 mK temperature increase per decade. The global value of carbon emission can be evaluated based on the value of carbon energy’s ability to provide economic growth. After all, this is how improvement is measured and reported in a modern industrial society. The economic value of the carbon emissions and power source is reflected purely in producing financial wealth for every country (such as the national gross domestic product (GDP)) using carbon energy. Energy use is the greatest in developed (rich) nations, and we observe a correlation between the growth in GDP and the growth in carbon energy use. Table 4 shows the typical life cycle emissions for power generation from different energy sources (g/kWh). This relationship also holds true at the global level. Hence, the global growth in GHG concentrations in the atmosphere over the last 30 years (measured as parts-per-million CO2 at Mauna Loa, Hawaii where 1 ppm CO2 is approximately 9.1012 tCO2) is directly and linearly correlated to the GWP (measured in terra dollars, $1012). To reduce the effect of the year-to-year noise in the atmospheric CO2 concentrations, 5-year averages for GWP were plotted against the change in CO2 measured over those 5 years. Rather than plotting ppm values of CO2, the change was converted to billions of tonnes (Gt) of CO2 released based on the 7.9 Gt of CO2 required for 1 ppm increase in the atmosphere accompanied by an equal release being absorbed in the oceans. Therefore 1 ppm was taken to be equivalent to a total of 15.8 Gt of CO2 released. It is reasonable to use the year 1950 as the base year because the CO2 build-up prior to 1950 was small. Black carbon, nitrates, soot, primary sulfates, organic carbon, VOCs, chlorides, ozone, and trace metals generated a negative radiative forcing of at least 0.5 W/m2, which is not in balance with the radiative forcing generated by GHG emissions of

66

Environmental Dimensions of Energy

7.5 ΔT

GHG

5.0 75

2.5 70

ΔTrad (mK/decade)

GHG (g/MJ)

80

0.0

65 Car

Heavy truck

Airplane

Rail

Marine

Total

Fig. 19 Energy specific greenhouse gas (GHG) emissions of vehicles in Canada and their contributions to the Earth’s temperature increase per decade.

Table 4

Typical life cycle emissions for power generation from different energy sources (g/kWh)

Power generation from

Switzerland 2000

Canada 2000

IAEA 2000

France (production only)

Natural gas Coal Solar panels Nuclear Oil Wind Hydro

605 1071 114–189 16 856 36 4

N/A 974 N/A 3–15 778 N/A 15

696 978 97 21 811 36 16–23

500 N/A N/A 0 701 N/A N/A

Source: Reproduced from EIA. US Energy Information Administration. Available from: http://www.eia.doe.gov/; 2008. Abbreviation: IAEA ¼ International Atomic Energy Agency.

approximately þ 1.5 W/m2. The unbalance is expected to increase in the future as a more dramatic increase of GHG emissions is projected. Prediction of GHG emissions in the next 100 years is of major importance in shaping the energy policies of today and promoting sustainable energy pathways. Depending on policy measures, it is possible to stop the increasing trend of GHG emissions at a certain point in time. In such a case, after reaching a maximum, the concentration of GHGs in the atmosphere may decrease.

1.3.4

Environmental Impact Mitigation

Many of jurisdiction throughout the world have EI mitigation programs. In Ontario, Canada, the Provincial Government has proposed a bill called the Green Energy Act, which is part of a plan to lead the North American green economy. This bill intends to boost investments in the renewable sector as well as improve the efficiency and management of energy resources. Another program that is being proposed for Ontario is a feed-in-tariff (FIT) program. This program is used as a method to provide incentives for the decentralized production of energy through renewable energy sources. The program makes the process of becoming a power provider much simpler because it provides standardized rules and contracts. As part of the standards, different pricings for supplied electricity are given based on project types and sizes. It is clear from the pricing standards that the program would like to encourage small-scale solar power technology as it has the highest price of electricity at $0.802/kWh. This price is for rooftop solar PVs installations with a capacity under 10 kW. In Ontario, one can also obtain financial compensation for the installation of clean power systems, which may significantly reduce the capital cost. Carbon taxation is another method that has been used with the intent of limiting CO2 emissions as well as providing economic advantages for clean power technologies. Carbon taxes may have a significant impact on the large-scale production of electricity. The application of $200 tax per tonne of CO2 emission will increase the price of electricity from $0.048/kWh to $0.244/kWh. The $200 per tonne of CO2 tax level is the minimum required to equalize the cost of fossil fuel power production with that of renewable sources. By providing real feasible renewable resource options for individuals to invest in, carbon taxation may become a more effective tool to encourage the use of renewable energy.

Environmental Dimensions of Energy

67

250

CC C

US$2008/tCO2

200

C

150

CC

European Union

100

n

atio

50

Annex B countries of Kyoto protocol

g

era

Av

x e ta

RC RC

0 2010

2020

2030

2040

2050

Year Fig. 20 Carbon taxation and forecast for the European Union and Kyoto Protocol countries. CCC, carbon constraint case; RC, reference case.

Fig. 20 shows two predicted cases for the increase of carbon tax in the European Union as well as countries that are abiding by the Kyoto Protocol. For example, in the European Union, a carbon value of €10/tCO2 is assumed in 2010 that will increase, according to the predictions, to €110/tCO2 by 2050. In this report, the geopolitical context; CO2 emission profile; oil, gas, and coal production profiles; H2-technology development; population growth; predicted energy demand; and other aspects are accounted for to propose three scenarios for energy technology development by 2050. The case designated by “RC” in Fig. 20 represents the reference case in which taxes are applied with a minimal degree of political initiative. This case shows a relatively linear increase in taxation with the value of tax shown as higher for the European Union. The case designated by “CCC” is the carbon constraint case, for which more severe political initiatives are applied toward the taxation of carbon emissions. The “RC” scenario predicts carbon taxes still under $50/tonne of CO2 in terms of the 2008 US dollar. The “CCC” case shows taxes reaching $250/tonne of CO2 in terms of the 2008 US dollar, and the average taxation case is shown with the dashed line. It is likely in the future that Ontario will adopt a carbon tax policy. The Province of British Columbia has already implemented a tax on CO2 emissions whereas Ontario intends to implement one. Pollution mitigation in the energy sector and energy-related activities – such as transportation – can be achieved through the use of green hydrogen. When combusted with air, hydrogen generates heat and steam; therefore, it is environmentally benign. When used as a chemical feedstock, a multitude of products can be produced from hydrogen, including synthetic fuels, such as methanol, and fertilizers, such as NH3 or urea. Emitted CO2 can also be converted into synthetic fuels and fertilizers through hydrogen’s reaction with CO2. Today, various policies are in place to develop methods for green hydrogen production. However, in industry, hydrogen is produced mostly by natural gas reforming, a process that emits more CO2 per gigajoules (GJ) than that of the entire life cycle of gasoline, considering its refining, distribution, and combustion altogether. Typical figures describing GHG emissions at hydrogen production worldwide are given as follows: approximately 10 kg CO2 per kg H2 production from natural gas or oil, 20 kg CO2 per kg H2 produced from coal gasification, and 0.43 kg CO2 emitted per kilowatt-hour (kWh) of generated electricity. Fig. 21 shows the GHG emissions worldwide due to hydrogen production processes. One can infer that, if one uses sustainable energy forms (e.g., nuclear, solar, wind, ocean, geothermal, hydro, biomass, industrial waste heat, etc.) instead of producing electricity from fossil fuels to drive hydrogen electrolysis, at least 11% of CO2 emissions from hydrogen production at a global scale could be mitigated. The percentage can even be larger if a higher efficiency of hydrogen electrolysis could be achieved. At present, this efficiency is not limited by the electrolyzer itself, but rather by the power generation process from sustainable/renewable sources. For example, the conversion efficiency of nuclear power plants is typically 30%, which yields a total heat-to-hydrogen efficiency of an electrolyzer driven by nuclear energy to be as low as 15%. Most advanced solar power systems may reach approximately 25% energy efficiency. The present worldwide production of hydrogen is around 50 million tonnes per year. One major hydrogen application is the nitrogen fixation process represented by NH3 synthesis from nitrogen and hydrogen, where nitrogen is derived from air and hydrogen from water or fossil HCs. NH3 is a major fertilizer worldwide and a majorly traded feedstock. In a subsequent process, NH3 is converted to urea, which is also a major fertilizer. Other major uses of hydrogen include synthesis of amines (R-CH2-NH2)

68

Environmental Dimensions of Energy

Electrolysis 11%

Natural gas 38% Coal 28%

Oil 23%

Fig. 21 Greenhouse gas (GHG) emissions of global hydrogen production.

Methanol, 8%

Metallurgy sector, 4% Oil refining and synfuel, 48%

Chemical process by-products (methanol, metallurgy, etc.), 18%

Oil refining, 24%

Heavy oil upgrading, 23%

Ammonia, 40% 1983

2010

Chemical production (ammonia and other), 35%

Methanol, 6%

Metallurgy sector, 7%

Oil refining and synfuel, 62%

Ammonia, 25% 2025

Fig. 22 Past and forecasted hydrogen utilization in Canada. Reproduced from Dincer I, Zamfirescu C. Advanced power generation systems. New York, NY: Elsevier; 2014.

starting also from NH3 feedstocks; these include amine aniline (R-RH2). Other products are alcohols and methanol, being an important product majorly synthesized from hydrogen and CO2. Further examples include cyclohexanol, hydrogen peroxide, butandiol, and a large palette of pharmaceuticals. Hydrogen is massively used for upgrade of petrochemical fuels. The world dependence on crude oil and petroleum products will not be eliminated in the current century. The crude oil consumption in recent years increased from 11 million barrels per day (mbd) in 1950, to 57 mbd in 1970, to around 85 mbd at present. Crude oil (or petroleum) is a naturally occurring HC-based liquid or solid (e.g., bitumen in oil shale, oil sands), found in underground rock formations. The main HCs included in petroleum are alkanes, cycloalkanes, aromatics, asphaltics, naphthalenes and paraffins. Of the total world crude oil reserves, 30% is conventional oil, 15% heavy oil, and 25% extra heavy oil, with the remaining 30% of petroleum found in the form of bitumen in oil shale and oil sands. In Canada, which is a very active country in hydrogen development, hydrogen is utilized as a feedstock for various chemical processes in industries and refineries. Canada produces more than 3 Mt of hydrogen per year. Presently, there is a significant use of hydrogen in Canada for upgrading heavy oil, particularly in the oil sand sites in Alberta. The growth trends of hydrogen utilization in Canada are reported in Fig. 22. The oil-refining sector and NH3 production in 1983 accounted for 40 and 40%, while at present, it is observed that the oil-refining sector increased its share of utilization to 46%. Based on forecasts, by 2025, the oil-refining share for hydrogen demand will reach 62%. One of the immediate factors to consider in comparing fuel alternatives is their potential to reduce levels of GHG and other emissions hazardous to the environment and health. The carbon–hydrogen ratio of various transportation fuels is shown in Fig. 23. Although biodiesel (B100) has a similar carbon content to petroleum diesel, this content is largely neutral in combustion depending on the feedstock. Compared to other fuels used in combustion applications, NH3 has the highest hydrogen energy density (even higher than pressurized and liquefied hydrogen fuel) based on the current storage methods, contains no carbon, and therefore, has a GWP of zero, and produces only nitrogen and water when combusted.

Environmental Dimensions of Energy

69

1.40 Coal

C/H ratio

1.05

0.70 Diesel 0.35

B100

Gasoline LPG

Ethanol Natural gas (CH4) NH3

0.00

H2

Fig. 23 Carbon/hydrogen ratio of common transportation fuels. Reproduced from Dincer I, Hogerwaard J, Zamfirescu C. Clean rail transportation options. New York, NY: Springer; 2015.

The plot in Fig. 23 shows how a the decarbonization of the world energy supply can be achieved (from the one based mostly on carbon (molecular mass 12) to the one based on hydrogen (molecular mass 2)). This reflects a trend in the direction of “hydrogenation” of the world energy supply, which is an evolution toward worldwide use of hydrogen as an energy carrier. In a hydrogen economy, sustainable energy sources like hydro, wind, solar, biomass, geothermal, ocean thermal, and nuclear are used to extract hydrogen from water (or other materials). Hydrogen is used as an energy carrier (by combustion with air forming water again) and as a chemical required in many crucial processes (manufacturing of plastics, foods, fertilizers, synthetic fuels, metallurgical procedures, etc.). Transportation can be made almost zero-polluting with hydrogen and hydrogen-based synthetic fuels, which are carbon neutral. One path to improve hydrogen production from sustainable sources, whenever one disposes of a heat source at intermediate temperature, is by using thermochemical water splitting cycles. In such kinds of cycles, a series of chemical and possibly electrochemical reactions occur, forming a number of intermediary compounds. Water, heat, and in some cases electricity are provided to the cycle. Within the cycle, all intermediate compounds are recycled, while hydrogen and oxygen are delivered as useful products. Moreover, the cycle rejects heat that has typically high exergy content. The attractiveness of thermochemical water splitting cycles is that little or no electricity is required. As a consequence of avoiding electricity generation, the heat-to-hydrogen production efficiency with thermochemical cycles can exceed that of water electrolysis. A further consequence of this is the possibility of approximately 20% GHG mitigation at the source of hydrogen production.

1.3.5

Exergoenvironmental Analysis

The idea of exergoenvironmental analysis is to expand the typical exergy analysis of energy systems so that the EIs can be quantified. Exergy analysis studies the interaction between an energy source and the environment. An understanding of the relations between exergy and the environment may reveal the underlying fundamental patterns and forces affecting changes in the environment, and help researchers to deal better with environmental damage. When interaction between any thermodynamic system and the environment takes place, exergy is destroyed. The destroyed exergy accounts for the immediate effect of the energy system upon the environment. Therefore the primary objective of exergoenvironmental analysis is the identification and quantification of direct and/or indirect EIs in correlation with exergy destroyed by the system. Another objective is to minimize the EIs. Often, this is done by finding means to increase the energy efficiency of the system. Another approach to EI minimization is the reduction of the polluting effluents. The inventory analysis is the core of the EI analysis as it determines the streams of materials and their impacts on the environment. All flows of matter exchanged with the environment and their interrelationship within the analyzed system must be inventoried. When considering exergy in EI analysis, the inventory analysis phase has to account more carefully for mass and energy flows into, out of, and through all the stages of the life cycle; next, the energy flow is associated with an exergy flow; eventually, EI indicators based on exergy can be developed and studied. The system itself is expanded with two subsystems: upstream – the system construction process, and downstream – effluent-environmental and decommissioning processes. All three subsystems – upstream process, actual system and its process(es), and downstream process – must be supplied with exergy, or otherwise no subsystem can run. For the system itself, the exergy supply in the form of any kind of fuel is referred to as direct exergy. For the upstream and downstream subsystems, the supplied exergy is referred to as indirect exergy. Therefore the total exergy supply to be considered in an extended exergoenvironmental analysis is given as follows: _ supply ¼ Ex _ dirrect þ Ex _ upstream þ Ex _ downstream Ex indirect indirect

ð10Þ

70

Environmental Dimensions of Energy

A similar equation can also be written for the EI, namely, EItotal ¼ EIdirect þEIindirect

ð11Þ

where EI is a general environmental impact indicator; EIdirect represents the downstream environmental impact, that is the environmental impact caused by the system due to its operation; and EIindirect represents the environmental impact associated with system construction. The exergoenvironmental analysis requires that for each exergy stream entering or exiting the system, a compounded EI rate can be assigned. Let us consider a combustion process. In the case of systems that are fueled by a combustion process, the atmospheric pollution through flue gas emissions is very relevant. A typical approach is to consider emissions of the following types of atmospheric pollutants: CO, NOx, SO2, CO2, and CH4. The stream of flue gases injects an exergy into the surrounding atmosphere at a rate that can easily be determined from the exergy analysis. The same flow impacts the environment at a rate that can be determined based on the specific method of life-cycle EI assessment. The ratio of the rate of EI to the rate of exergy can then be determined for any exergy stream, including streams of exergy destruction. Therefore the rate of EI can be mapped to the rate of exergy outputs, including the destruction of exergy, as follows: _ W þEI _ m;out þEI _ d;out _ ¼ EI _ Q þEI EI

ð12Þ

_ Q is the environmental impact rate due to heat release, EI _ W is due to work release, EI _ m;out refers to mass release, and EI _ d is where EI associated to exergy destruction. CO2 emission from fuel combustion can be correlated relatively easily with fuel combustion rate based on general combustion parameters, such as stoichiometry, excess air, etc. More difficult to determine though modeling are the rates of CO and NOx, since detailed kinetics modeling for combustion process would be required. Some simplified models can be used as an alternative, such as those by Lazzaretto and Toffolo [4], which relate the mass flow rate of CO and NOx emissions to the fuel (natural gas) consumption. For CO emissions, the correlation is as follows:
 exp 7800 _ CO m T ð13Þ ¼ 179;000
DP0:5 _f m P2 t P

For NOx emissions, the following equation is given [4]:


 t0:5 exp 71;100 _ NOx m T ¼ 150;000
0:5 _f m P 0:05 t DP P

ð14Þ

In Eqs. (13) and (14) the adiabatic flame temperature is denoted with T, combustion pressure is P and DP is the pressure drop across the combustion chamber, and t is the residence time in the combustion zone (approximately 2 ms). If the fuel contains sulfur, then an emission factor of SO2 can be determined. Typically, diesel fuel contains sulfur. Based on Ref. [5], the emission of SO2 and fuel combustion can be correlated as follows:   _ SO2 m MSO2 ð15Þ ¼ 10 6 CS fc;SO2 _f m MS where fc;SO2 is conversion factor of sulfur in SO2, Cs is the sulfur content of fuel in parts-per-million, and M represents molecular mass. In Eq. (15), the factor fc;SO2 is around 0.98 for ultralow sulfur diesel (ULSD), and the sulfur content of fuel is of the order of 15 ppm. The EI can then be determined based on a weighted average of the emitted pollutants. For exergoenvironmental analysis, the chemical exergy of the pollutant can be used to create a weighted averaged EI factor. If one assumes that the polluting effluents are CO2, NOx, and SO2, chemical exergy allows determination of the total exergy of the pollutant stream as follows: _ polutant ¼ m _ CO2 exch _ NOx exch _ SO2 exch Ex NOx þ m NOx þ m SO2

ð16Þ

where the chemical exergy of pollutants is given in Table 3. The EI factor is, therefore, defined as the ratio between the exergy of polluting stream divided to the mass flow rate of fuel _ f . From Eqs. (7), (8), and (9), the following expression for EI results (Table 5): combusted, m

   _ exp 7800 t0:5 exp 71;100 Expolutant MJ ch ch T T EI ¼ 179;000 ¼
DP0:5 exNOx þ 150;000
DP0:5 xNOx _f kg m P2 t P P 0:05 t P

  MSO2 ð17Þ Sppm fc;SO2 exch SO2 MS In analogy with the definition given by Eq. (17), a dimensionless exergetic EI factor can be derived for any system, obtained by dividing the chemical exergy in emitted pollutant to the exergy input into the system. Therefore the exergetic EI factor is defined as follows: þ10

6

ExEI ¼

_ pollutant Ex _ input Ex

ð18Þ

Environmental Dimensions of Energy

Table 5

71

Chemical exergy of atmospheric pollutants

Pollutant

M (kg/kmol)

exch (MJ/kmol)

exch (MJ/kg)

Carbon dioxide (CO2) Methane (CH4) Nitrogen oxides (NOx) Sulfur dioxide (SO2) Carbon monoxide (CO) Volatile organic compounds (VOCs) Particulate matter (PM)

44 16 38 64 28 44 68

19.60 831.66 72.4 311 275 1233 436

0.445 52 2 5 10 28 6

The EI can also be evaluated based on exergy destruction. Indeed, anything beside the useful output of an energy system of any kind discharges into the environment in a form that is quantifiable by exergy destruction. In a dimensionless form, the exergetic sustainability impact can therefore be defined as the ratio between exergy destruction and exergy input. Based on this definition, the exergetic EI becomes equal with the relative exergy destruction, which is also known as depletion factor, and is given as follows: Dp ¼

_ d Ex _ input Ex

ð19Þ

EI factors can be introduced based on LCA methods. LCA is essentially a cradle-to-grave analysis to investigate EIs of a system or process or product. LCA represents a systematic set of procedures for compiling and examining the inputs and outputs of materials and energy, and the associated EIs, directly attributable to the product or service throughout its life cycle. A life cycle is the set of stages of a product or service system, from the extraction of natural resources to final disposal. LCA is a method used to help engineers, scientists, policy makers, and others to assess and compare energy and material use, emissions and wastes, and EIs for various products or processes. Overall, EI cannot be assessed by examining only operation, but must consider all the life stages from resource extraction to disposal during the lifetime of a product. Within an LCA, mass and energy flows and EIs related to plant construction, utilization, and dismantling stages are all accounted for. LCA is a four-step process, namely, goal and scope definition, inventory analysis, impact assessment, and improvement potential. There have been several assessment methods developed over time to classify and characterize the environmental effects of systems, for example, Eco-indicator 99 or CML 2001. In 2001, a group of scientists under the lead of Center of Environmental Science of Leiden University (CML) proposed a set of impact categories and characterization methods for the impact assessment, which is denoted CML 2001. There are two baseline indicators used in the CML 2001 method: human toxicity and global warming. The Eco-indicator 99 method specifies the EI in terms of numbers or scores. It simplifies the interpretation of LCA by including a weighting method. After weighting, it gives a single score for each of the products or processes, which is calculated based on the relative EI. The score is represented on a point scale (Pt), where a point (Pt) means the annual environmental load (i.e., whole production/consumption undertakings in the economy) of an average citizen. The Eco-indicator 99 defines the “environmental damage” in three broad categories: human health, ecosystem quality, and resources. Human health includes the number and duration of diseases and loss of life years due to permanent deaths caused by environmental degradation. Ecosystem quality includes the impact of species diversity, acidification, ecotoxicity, eutrophication, and land use. The resources category corresponds to the depletion of raw materials and energy resources. It is measured in terms of the surplus energy required in future for the extraction of lower quality of energy and minerals. The agricultural resource depletion is studied under the category of land use.

1.3.6 1.3.6.1

Case Studies Solar-Based Power and Heat Generation With Reduced Environmental Impact

Solar energy is available everywhere on the Earth’s surface. Therefore it is an excellent source for local power and heat generation, which avoids losses at energy transmission. Moreover, this technology is expandable: at a reasonable cost, the installed capacity can be upgraded. Because global warming is a major concern, governments of many countries encourage renewable energy technologies, solar energy being one of them. For each kWh of electricity generated through renewable sources, one saves approximately 0.43 kg of CO2 emissions that otherwise would be released to the atmosphere by fossil fuel power plants, and thus contribute to global warming. The most common technology for solar energy harnessing is the PV. The current state-of-the-art PV systems allow for 8–18% solar-to-electricity conversion with an extremely high capital investment, which makes this technology the most expensive among all other power generation alternatives. The explanation of these high costs relate to the expensive and rare materials used, like gold, silver, copper, gallium, arsenic, and silicon, and to the highly energy-consuming, polluting, and capital-intensive

72

Environmental Dimensions of Energy

manufacturing of semiconductors. This process also requires highly qualified personnel, which introduces further cost. Such materials have a long-term harmful EI. For example, according to Ref. [6], the so-called environmental payback time for toxicity associated with the PV panel manufacturing process is around 55 years. The energy payback time (EPBT), defined as the time of operation of the PV plant needed to produce the energy equivalent for its production (including auxiliary elements, e.g., frames and supports, but excluding manpower and costs associated with highly qualified personal utilization and other commercial operations), currently varies between 5 and 10 years depending on climatic conditions. However, one must remark that most studies reporting on the EPBT assume the PV system operation in standard sky conditions throughout the whole year. This is not the situation encountered in real life, because the number of clear days in a year is considerably less than 365 days. For example, in the work of Tiwari and Joshi [7], 276 clear sky days are reported for New Delhi in 2005 with an EPBT of approximately 14 years. The same system would have the EPBT of approximately 7 years if it operates under standard sky conditions throughout the year. The system tested by Tiwari and Joshi [7] is a PV/T type (PV/thermal) system that uses an airflow to cool the PV panel for improving its solar-to-electric conversion. The cooler the panel is, the better it performs in terms of efficiency. At the same time, the heat transferred to the air stream is not lost, but rather used for some processes, for example, drying crop. That is, the PV/T technology cogenerates power and heat, having thus an added value with respect to a PV system only. Typically, the recovered heat is at low grade (up to approximately 451C), and represents 50–100% of the electrical output. If one assumes that the same heat would be produced by an electrical-driven heat pump (HP) having a coefficient of performance (COP) of 3 (which is a common figure for most air heating HPs). The equivalent electrical power production by the PV/T panel is 25% higher than that of a PV panel; therefore, it mitigates 25% more CO2. The investment cost of a PV/T panel is, however, higher than that of a PV panel because it includes a coolant channel and one or more fans (or pumps). Solar-driven heat engines complement the PV and PV/T technologies and overcome some of their drawbacks, being able to better harvest solar energy and produce power, higher grade heat, and mitigate more CO2 comparatively. Point focus solar concentrators can achieve a concentration ratio of up to 10,000–15,000, which corresponds to a maximum temperature at the receiver of over 1500K. Moreover, cogeneration is better applicable to solar-driven heat engines than PV panels because (1) the temperature of the recovered heat is higher (e.g., 50–1501C or more depending on the implementation), and (2) arrangements can be made to minimize the heat loss to the environment. In a solar-driven heat engine setup, the solar radiation is first collected and concentrated in a small-area spot. In this process, 5–15% of the received energy is lost. Further, the heat obtained from the solar radiation is converted to power with an efficiency of 10–25% based on the energy of incident radiation (depending on the source temperature). The rest of the heat, having a lower grade, is rejected to the heat engine sink and can be recovered in the range of approximately 95% for cogeneration. Thus the cogenerated heat represents about 50–75% from the incident radiation: higher work production efficiency (of the heat engine) results in lower heating energy delivered at the sink. Two scenarios are of practical relevance when a solar-driven heat engine replaces a conventional system to provide power versus heating and power in a residential or commercial/industrial setting:

•

•

If the conventional system uses a HP, assuming a COP of 3, the equivalent solar-to-electricity efficiency increases with approximately 60%/3 ¼ 20%, if cogeneration is used with respect to power generation only. Thus the total solar-to-electricity energy efficiency doubles, i.e., from approximately 20% to approximately 40%. Consequently, the CO2 mitigation of the system with cogeneration doubles as well. If the conventional system uses a gas burner for heating, taking into account the CO2 emitted per kWh electric and CO2 emitted per kWh thermal from gas combustion, it can be shown that 60% more CO2 is mitigated by the cogeneration system.

This simple analysis shows that one may expect to double CO2 mitigation if a solar-driven heat engine is used instead of a PV or a PV/T technology, and this is because of better energy efficiency of the thermal component. Not only is the thermal energy efficiency better, but so is the associated exergy efficiency because of the higher grade of the heat. The design of a solar-driven heat engine is a matter of trade-off between the efficiency of concentrating solar collector and that of the heat engine. Any concentrating solar collector has two components: the solar concentration system and the solar receiver, where the light is transformed into heat at a high temperature. A concentrating solar receiver operates at high efficiency if the temperature at its receiver is low (in this way, the heat losses to the environment are minimized).

1.3.6.1.1

System description and modeling

Here, a power and heating system for residences is introduced, as shown in Fig. 24. The system details were provided in Ref. [8]. It is based on a solar concentrator with solar tracking that delivers high-temperature heat to an NH3–water Rankine cycle heat engine. This heat engine generates useful work and delivers useful heat to a heat sink at a low temperature, THR, which is higher than the environmental temperature T0. The engine has four main components, namely, the solar receiver that plays the role of NH3–water desorber and generates vapors; the positive displacement expander that expands the vapors and delivers shaft work; the resorber, where the vapors are cooled, absorbed in liquid, and condensed though a simultaneous process known as desorption; and a liquid pump circulating the working fluid. One introduces MCO2 – the CO2 mitigation expressed in kg CO2 mitigated per kWh of heat and power cogeneration. If one denotes Mw as the CO2 mitigation due to avoiding electricity generation from fossil fuels (in kgCO2/kWhelectric), with the

Environmental Dimensions of Energy

73

Solar receiver

2

Solar concentrator (mirror)

NH3-H2O pump

3

Inverter

Expander

4

Resorber

Residence

Hot water tank

1 Water pump

Fig. 24 System for solar heating and power generation. Reproduced from Zamfirescu C, Dincer I, Verrelli T, Wagar WR. Residential solar power generation systems for better environment. In: Proceedings of global conference on global warming, Istanbul, Turkey, Paper #805; 2008.

~ denoting the ratio between the CO2 mitigation due to avoidance of conventional CO2-emitting dimensionless parameter M heating technologies, MQ (e.g., gas/coal combustion, electrical HPs) and MW , i.e., ~ ¼ MQ =Mw M

ð20Þ

then MCO2 is given by MCO2 ¼ MW

~ _ HR M _ EþQ W IT0 Aa

ð21Þ

~ can vary from 0.5 at CH4 combustion up to one, if direct electrical heating is used. Several economic models The parameter M were elaborated to determine the profitability of the concentrated solar heat and power systems with heat engines. All considered models use a year-by-year cash flow approach. Various scenarios were considered that include or do not include (depending on the case) electrical sell-back to the grid, carbon taxation, and government assistance to the investment. The models become increasingly more complex as the model is upgraded by considering additional parameters. However, the equations remain the same with the cancellation of some factors in the simpler models. Each model compares the residence assisted by a solar collector to a residence having only standard power (the grid) and heating (gas boiler) equipment. The first equation of the model describes the present value of all cash flows with no consideration of a solar power system and it is PW ¼

N X

n¼0

½ðAW e ðnÞ þ AW ct ÞðPW=FW; i; nފ

ð22Þ

where AWe(n) is the annual total cost of electricity increasing about 5.5% per year, AWct is the constant annual carbon tax, and the P/F factor is applied to bring each set of values in terms of present worth (PW) at a discount rate i of 5% per year for the whole life cycle time N, which was taken as 30 years. A similar equation has also been developed to find the total present value of all costs for the system with the purchase of a solar power and heat generator. This is represented as follows: XN ½ðAW 1 ðnÞ þ AW 2 ðnÞ þ AW 3 ÞðPW=FW; i; nފ ð23Þ PW lifetime ¼ SAðPW=FW; i; NÞ P n¼0 where SA is the salvage value of the system, P is the principal cost of the system, AW1 is the annual cost of consumed electricity, AW2 is the annual cost of consumed natural gas, and AW3 is the sum of all constant cash flows including electrical sell-back profits, carbon taxes, and operation and maintenance costs.

1.3.6.1.2

Results

In Fig. 25, the electrical and the thermal outputs of the system are given; the thermal output is three to four times higher than the electrical output for a given insolation. Fig. 26 shows the thermal and electrical efficiency of the system. The average values are 16 and 78% for electrical and thermal efficiencies, respectively. Fig. 27 reports the system exergy efficiency with and without cogeneration. The added value of cogeneration improves the system effectiveness more than two times. Using the system’s energy efficiency (electrical and thermal), it is possible to estimate the CO2 mitigation for the system with ~ namely, 0.0, 0.5, and 1.0, correspond to the following cases: no and without cogeneration. Three values of the parameter M,

300

1000

250

900

200

800

150

700

100

600

50

500 400 1300

0 500

700

900

QHR (W/m2)

Environmental Dimensions of Energy

WE (W/m2)

74

1100

IT0

Fig. 25 The system thermal and electrical output per aperture area for a range of insolations, IT0. Data from Zamfirescu C, Dincer I, Verrelli T, Wagar WR. Residential solar power generation systems for better environment. In: Proceedings of global conference on global warming, Istanbul, Turkey, Paper #805; 2008.

1 0.8 E H

0.6 0.4 0.2 0 500

700

900

1100

1300

IT0

85%

20%

75%

18%

65%

16%



HR

Fig. 26 Energy efficiency of the system with and without cogeneration. Data from Zamfirescu C, Dincer I, Verrelli T, Wagar WR. Residential solar power generation systems for better environment. In: Proceedings of global conference on global warming, Istanbul, Turkey, Paper #805; 2008.

HR 55%



45% 500

700

900

1100

14%

12% 1300

IT0 (W m−2) Fig. 27 System exergy efficiency with and without cogeneration. Data from Zamfirescu C, Dincer I, Verrelli T, Wagar WR. Residential solar power generation systems for better environment. In: Proceedings of global conference on global warming, Istanbul, Turkey, Paper #805; 2008.

cogeneration, replacing natural gas heating systems, and replacing electrical heaters, respectively. Heating system with coal, wood, ~ ¼ 0:5 and M ~ ¼ 1:0. or electrical HPs can be found in between two extreme cases, M The results presented in Fig. 28 show that solar heat engines with cogeneration mitigate at least three times more CO2 than those without cogeneration. Maximum mitigation is obtained when electrical heaters are replaced with solar heaters. The average CO2 mitigation by solar-driven heat engines for power and heat generation can be estimated from Fig. 6, and it is approximately 0.3 kg CO2/kWh.

Environmental Dimensions of Energy

75

0.5 M=1.0 kgCO2/kWh

0.4 0.3

M=0.5

0.2 M =0.0

0.1 0 500

700

900

1100

1300

IT0 Fig. 28 Mitigation of carbon dioxide (CO2) by solar-driven heat engines. Data from Zamfirescu C, Dincer I, Verrelli T, Wagar WR. Residential solar power generation systems for better environment. In: Proceedings of global conference on global warming, Istanbul, Turkey, Paper #805; 2008.

The economic models are described in the previous section, where they were used to analyze the financial benefits or drawbacks of purchasing a residential solar power system. For the case study presented here, one considers the purchase of a solar-tracking concentrating parabolic dish collector of 9 m2 working on the NH3–water Rankine cycle for an average Ontario household. At every point of the analysis, realistic figures were used, and where estimates were necessary, conservative assumptions were made; therefore, the power system is likely to outperform the economic models. Ontario’s interest rates are conventionally low, and usually stay around 5%; therefore, a 5% discount rate was assumed throughout the analysis. The first step of the analysis was to determine the energy consumption of the average Ontario household. For electrical consumption, the average Ontario home consumes approximately 11 kWh/day. In terms of natural gas consumption, the average annual consumption was used to obtain a daily figure of 53 kWh/day. The corresponding monthly energy bill received by the average Ontario home works out to be $25/month for electricity and $45/month for natural gas. As costs of energy sources increase, the consumer costs of electricity and natural gas will also increase. Other sources under Ontario Hydro predict annual increases in natural gas prices up to 10%. To stay conservative, an annual increase for all energy costs was taken as 5.5%. The amount of electricity and heat that can be saved by purchasing a solar power system is an important part of the analysis. The models consider a conservative solar energy input for a 9 m2 collector and a realistic number of clear sky days at 276. Based upon this input, realistic solar to electrical and heat recovery (HR) efficiencies were considered to provide the system energy output. In Ontario, it is reasonable to consider an average daily solar radiation of 5.5 kWh/day. For 9 m2 of aperture area, this gives a total energy input of 49.5 kWh/day. By applying the system’s expected electrical efficiency of 25%, and considering the number of clear sky days per year, the result for this case was an average electrical production of 9.3 kWh/day. Most of the energy that was not converted to electricity was rejected as waste heat. With a conservative estimate of 50% energy capture in the form of heat and once again considering the effect of overcast days, an average heat production of 18.7 kWh/day was obtained. These figures can also be used to derive the CO2 mitigation as a result of this installation. Based upon the average figure of 0.3 kg CO2/kWh of electricity, the mitigation was 1014 kg CO2/year. Based upon the stoichiometric combustion of natural gas, the mitigation was 1751 kg CO2/year. At this point, it is important to note that due to the effect of averaging the figures, the first model assumed that the household always consumes the full content of power produced by the system. In the second model, the system would sell back a portion of its electrical power to the grid at times of peak power, and the extremely high sell-back price of about $0.80/kWh from the FIT program may be greatly appreciated. Also, the likelihood of carbon taxation in the future for Ontario, demonstrated in the final model, means that the system may save more than the first studies predicted. The last factors needed for consideration in the financial models are the initial cost and salvage value of the system and the operation and maintenance costs of owning the system over its 30-year lifespan. The first model considered an initial cost of the system at $10,000 in three scenarios. With no government help, the cost remained at $10,000. With moderate and high levels of government help, this figure dropped to $7500 and $5000, respectively. The salvage value of the system was taken at $1500, which is very realistic considering that much of the collector may be refurbished and reused. Finally, a flat rate was considered for operation and maintenance at $100/year. Fig. 29 shows the first model applied with no electrical sell back to the grid, no carbon taxation applied, and no government assistance received. In Fig. 29, the continuous line represents the cumulative cost of power and heating of the residence operating with conventional equipment, i.e., connected to the grid and using a gas boiler for heating. The dashed line represents the cumulative cost if an investment in solar heating and power system is made at the moment zero of the analysis. When the two lines intersect, one finds the payback period. Moreover, at the end of the life cycle the difference between the two cumulative costs gives the life cycle savings. The result of the first case presented a total savings of $2256, which was normalized to the present dollar worth. The payback period of this investment was found to be 26 years from the analysis illustrated in Fig. 29. The result of the second version (Fig. 30) with moderate help from the government (25% of the investment) produced a total savings of $4756, normalized to the present dollar worth. The payback period of this investment was shorter (20 years). The result of the third case of the model (Fig. 30) with a high level of help from the government (50% from investment) resulted in a total

76

Environmental Dimensions of Energy

Year 0

5

10

15

20

25

30

Cumulative cost ($2009)

0 Conservative cost estimate with no investment

−5000

Investment in residential solar power system

−10,000 −15,000 −20,000 −25,000 −30,000

Cumulative present worth of cash flow

Fig. 29 Cumulative cost analysis of solar power system with no government assistance. Data from Zamfirescu C, Dincer I, Verrelli T, Wagar WR. Residential solar power generation systems for better environment. In: Proceedings of global conference on global warming, Istanbul, Turkey, Paper #805; 2008.

10,000 8000 6000 4000 2000 0 −2000 0

5

10

15

20

25

30

−4000 −6000 −8000 −10,000

No government assistance Moderate government assistance High level of government assistance

−12,000 Year

Fig. 30 Cash flow savings of solar power systems with varying government assistance. Data from Zamfirescu C, Dincer I, Verrelli T, Wagar WR. Residential solar power generation systems for better environment. In: Proceedings of global conference on global warming, Istanbul, Turkey, Paper #805; 2008.

savings of $7256, normalized to the present dollar worth. The payback period of this investment was even shorter at 14 years. In Fig. 30, the cash flow savings of the first case are also presented, obtained by extracting the “continuous” line from the “dashed” line in Fig. 29. To account for the actual nature of fluctuating solar radiation as well as grid and household demand, we can apply a percentage of the system’s production that goes toward household electrical demands. The remaining household demand will be taken from the grid, and the excess electricity during peak production is sold back to the grid. The assumed proportion was that the household would consume 60% of the produced electricity, and the remaining 40% would be sold to the grid at the elevated price due to the FIT program. The adaptations for this model were built onto the previous model for the case of moderate government assistance and provide a savings of $18,510.74 with a payback period of 6.5 years (see Fig. 31). By considering a value of $30/tonne CO2, the model can be upgraded, based upon the model from Fig. 32, to account for this possibility. Since carbon tax was already worked into the price of electricity, the updated model only considered further taxation on the natural gas. In this scenario, the savings were $21,632.82 with a payback period of 5.6 years. These models show that the proposed system is attractive in the conservative case. With consideration of sizable government assistance, the power system will provide very lucrative savings. Another scenario to consider is the sell back of electricity to the grid and the possibility for a carbon tax in Ontario. Both of these scenarios enhance the benefits of purchasing the solar power system. Lastly, the installation of the proposed solar power system is likely to provide abundant energy savings, as well as CO2 mitigation for an average Ontario household. The results of all three models show that the system has the potential to be quite beneficial, and the application of grid sell back and carbon tax will make the system more attractive. The results of the models were compared in Fig. 33 (savings, left and payback

Environmental Dimensions of Energy

77

Year 0

5

10

15

20

25

30

Cumulative cost ($2009)

0 −5000 −10,000 −15,000 −20,000 Conservative cost estimate with no investment

−25,000

Investment in residential solar power system −30,000 Fig. 31 Cumulative cost analysis of solar power system with grid sell back. Data from Zamfirescu C, Dincer I, Verrelli T, Wagar WR. Residential solar power generation systems for better environment. In: Proceedings of global conference on global warming, Istanbul, Turkey, Paper #805; 2008.

Year 0

5

10

15

20

25

30

0

Cumulative cost ($2009)

−5000 −10,000 −15,000 −20,000 −25,000 Conservative cost estimate with no investment

−30,000

Investment in residential solar power system −35,000 Fig. 32 Cumulative cost analysis of solar power system with grid sell back and carbon tax. Data from Zamfirescu C, Dincer I, Verrelli T, Wagar WR. Residential solar power generation systems for better environment. In: Proceedings of global conference on global warming, Istanbul, Turkey, Paper #805; 2008.

25 Payback period (year)

Lifetime savings

$25,000.00 $20,000.00 $15,000.00 $10,000.00 $5,000.00 $0.00 Base case

40% Sell-back

40% Sell-back and carbon tax

20 15 10 5 0 Base case

40% Sell-back

40% Sell-back and carbon tax

Fig. 33 Comparison of models for total system lifetime savings and payback period. Data from Zamfirescu C, Dincer I, Verrelli T, Wagar WR. Residential solar power generation systems for better environment. In: Proceedings of global conference on global warming, Istanbul, Turkey, Paper #805; 2008.

Environmental Dimensions of Energy

78

23% Savings in carbon tax

34% Savings in electricity

Savings in natural gas 43% Fig. 34 Relative portions of financial savings from solar plant factors. Data from Zamfirescu C, Dincer I, Verrelli T, Wagar WR. Residential solar power generation systems for better environment. In: Proceedings of global conference on global warming, Istanbul, Turkey, Paper #805; 2008.

period, right). In Fig. 33, “Base case” denotes the case with moderate (25%) financial help for investing into the system (see Fig. 30), “40% sell back” represents the “moderate” case plus 40% of electricity produced is sold to the grid, while the rest is used locally (see Fig. 31), and “40% sell back and CO2 tax” represents the previous case plus CO2 taxation applied (see Fig. 32). For the full model including grid sell back and carbon taxation, we can determine how much of the cost savings come from the reduction of electrical consumption, the reduction of natural gas, and the money saved on carbon tax. The findings summarized in Fig. 34 show that the largest part of the savings comes from solar heating.

1.3.6.1.3

Conclusions

In this section, it is shown that solar-driven heat engine systems with power and heat cogeneration are beneficial for the environment. The intervention of governments through investment incentives, FIT programs and carbon tax facilitates the development of this technology and its implementation in society. The financial benefit of operating such systems comes from three sources, namely power generation, heat generation, and CO2 mitigation. The efficiency figures used in the present CO2 emission and cost analyses were based on a novel solar-driven heat engine system working with NH3–water. Due to an excellent match of the temperatures at heat sink and source, and due to the ability to adapt to the fluctuating solar conditions by regulating NH3 concentration, the effectiveness of such heat engine is high. The system shows good heat and power generation capability in comparison with organic Rankine cycles or other heat engine systems. The following efficiency values were taken for the analysis: solar-to-electric energy efficiency of the system from 12% (at low insolation) to 18% (at peak insolation), and the average solar-toheating energy efficiency of 80% (it is reasonable to assume that this parameter has a flat variation and therefore is taken as constant). The specific conclusions can be summarized as follows:

• • • • •

The system mitigates 3–4 times more CO2 with respect to PV, due to cogeneration capability. A financial aid of 50% reduces the payback period to half in the most conservative scenario. If an additional FIT program is in place, the payback period becomes lower than 7 years. And if additional carbon tax programs are applied, the payback period becomes approximately 5 years. The largest part of the life cycle savings (43%) comes from the heating component.

1.3.6.2

Clean Hydrogen With Copper–Chlorine Thermochemical Cycle

In this case study, CO2 mitigation at hydrogen production is studied for the copper–chlorine (Cu–Cl) water splitting method. Three energy sources were compared: a sustainable thermal energy source (such as heat recovered from industrial processes), nuclear heat, and heat derived from a coal-fired furnace. A thermodynamic approach to identify HR and temperature upgrading opportunities was presented, when coupling the Cu–Cl water splitting plant with a sustainable thermal energy source at a specified temperature. In this respect, the heat fluxes in/out of the Cu–Cl cycle were determined. The Cu–Cl cycle had two thermochemical reactors (for CuCl2 hydrolysis and CuO
CuCl2 thermolysis) and one electrochemical cell for hydrogen production by Cu–Cl chlorination with HCl. All intermediate chemical compounds were recycled, while only water was consumed, and oxygen and hydrogen were generated. Thermodynamic models are proposed to determine the ideal limit and achievable level of hydrogen production efficiency, and the CO2 mitigation potential through the Cu–Cl cycle, in comparison to water electrolysis. One of the most studied cycle variants is presented in Table 6 and includes four main steps. Based on past results, the table indicates the amount of thermal and electrical energy required to generate one mole of hydrogen using this cycle. One needs approximately 150 kJ/mol of heat at over 820K, 182 kJ/mol at temperatures over 670K, and 123 kJ at temperatures over 500K and approximately 53 kJ/mol of electrical energy.

Environmental Dimensions of Energy

Table 6

79

Main processes within the copper–chlorine water splitting cycle

Process

Chemical equation

Electrochlorination Dehydration Hydrolysis Thermolysis

E: T: T: T:

2CuCl(aq) þ 2HCl(aq)-H2(g) þ 2CuCl2(aq) CuCl2(aq) þ nH2O(l)-CuCl2
mH2O(s) þ (n m)H2O, n47.5 2CuCl2
nH2O(s) þ H2O(g)-CuO
CuCl2(s) þ 2HCl(g) þ nH2O(g),n¼0 CuO
CuCl2(s)-2CuCl(l) þ 0.5O2(g)

4

T (K)

DH

350 473 650 800

52.3a 122.2 181.8 149.4

a In kJ/mol H2. Notes: E: electrochemical step; T: thermochemical step; DH: reaction enthalpy or Gibbs energy. Source: Reproduced from Naterer GF, Dincer I, Zamfirescu C. Hydrogen production from nuclear energy. New York, NY: Springer; 2013.

TH

Cu−Cl plant WEL

WH2 TC

COP WHP Qtot Tm HR WHR

HE WHE

T0 Fig. 35 Illustrating the linkage of copper–chlorine (Cu–Cl) cycle to sustainable thermal energy source. COP, coefficient of performance. Modified after Naterer GF, Dincer I, Zamfirescu C. Hydrogen production from nuclear energy. New York, NY: Springer; 2013.

1.3.6.2.1

Analysis and modeling

A thermodynamic model for coupling the Cu–Cl cycle to a sustainable heat source is proposed in Fig. 35. The cycle itself is represented as a black-box. The details of the cycle parameters can be found in Naterer et al. [9]. This model comprises four components, namely, the Cu–Cl plant, a HP (COP), and two heat engines having efficiencies ZHE and ZHR, where subscript HR stands for heat recovery. The notation WHE stands for the work produced by the heat engine that operates between the heat source and the environment, while the notation WHR refers to the work obtained by conversion of the heat recovered from the Cu–Cl cycle. The work inputs are the HP WHP and the electrical power WEL. The model assumed four temperature reservoirs (RESs) as follows: ambient temperature RES at T0; sustainable heat source RES at Tm4T0; high-temperature RES that delivers useful heat to the Cu–Cl cycle at TH4Tm; and cooler temperature RES at which the Cu–Cl cycle rejects heat at TC, with TH4TC4T0. If Tm and T0 are specified, one can calculate the COP and efficiency of the system

80

Environmental Dimensions of Energy

components as follows: 9 > > > COP ¼ 1 þ j > TH Tm = ZHE ¼ jð1 T0 =Tm Þ > > > ; ZHR ¼ jð1 T0 =TC Þ > Tm

ð24Þ

where temperatures are in Kelvin and j is a parameter ranging from 0 to 1 that expresses the abatement of the COP/efficiency from the Carnot value. For j ¼0, the COP takes its minimal value of 1, and for j¼1, the COP and the efficiency are maximum and equal to the Carnot limit. The system is designed such that the work generated by the two heat engines balances with the power requirement of the HP plus the electrochemical process of the cycle. Therefore one would have: WHE þ WHR ¼ WHP þ WEL

ð25Þ

The total heat required by the Cu–Cl cycle is given as follows: heat input QH ¼ 458 kJ/mol H2, heat rejected at cold side QC ¼ 294 kJ/mol H2. The energy balance at the level of the sustainable heat RES is written as follows: Qtot ¼

WHE þ QH ZHE

WHP

One notes that the work generated by the two heat engines shown in the system diagram can be written as follows: ) WHP ¼ QH =COP WHR ¼ QC ZHR

ð26Þ

ð27Þ

The energy efficiency of the cycle is formulated as follows: Z ¼ WH2 =Qtot

ð28Þ

where WH2 ¼ HHV H2 . Assume that for production of 1 kg of hydrogen in industry, 10 kg of CO2 are emitted to the atmosphere (this is the case for natural gas reforming, which is the lowest amongst all). For producing 1 kg of oxygen, one can derive from data presented by Iora and Chiesa [10] that 86 g of CO2 are emitted to the atmosphere. Using these figures, at least ζ ¼ 70.7 kg CO2 are released by industry to the atmosphere to produce hydrogen and oxygen equivalent to 1 GJ of energy content. Since ζ is given in kgCO2/GJH2, the following equation

MCO2 ¼ ζZ; kgCO2 =GJsustainable ð29Þ thermal

can be derived for CO2 mitigation, if conventional industrial processes to produce hydrogen and oxygen are replaced by systems, such as that proposed in Fig. 35. A second and a third Cu–Cl cycle-based system was considered for comparison purposes. The second system integrated the Cu–Cl cycle with a coal-fired power plant. The third case was an integration of Cu–Cl cycle with a nuclear power plant. Coal-fired power plants are a mature technology, practically used in every country of the world. Moreover, coal is the most abundant among fossil fuel resources. In the last few decades, significant preoccupation of scientific and technical community has been observed toward improvement of coal-fired power plants for enhancing their efficiency and reduction of their carbon footprint. The integration of a coal-fired power plant with the Cu–Cl cycle helps lower GHG emissions. Instead of generating electricity only, the coal-fired power plant works synergistically with Cu–Cl water splitting plant to ultimately produce electricity and hydrogen with reduced pollution. In brief, the integrated system functions as follows:

• • • •

Heat of flue gases is transferred to the water preheater, boiler, superheater (SH), and reheaters (RHs) of the steam Rankine plant; simultaneously, heat of flue gases is transferred to a low-pressure steam RH that conveys thermal energy to the Cu–Cl plant; a novel coal-fired furnace concept with better temperature profile match between hot and cold stream is introduced. The Cu–Cl plant generates hydrogen and oxygen from water whereas hydrogen is stored in a compressed form and oxygen is used to conduct an oxyfuel combustion process with pulverized coal; in order to fulfill the need of oxidant, additional oxygen is generated from air using an air separation unit (ASU). The steam Rankine cycle generates power, of which a part is supplied to the electrochemical reaction within the Cu–Cl cycle. Since oxyfuel combustion is applied, very limited amount of hydrogen is present in stack gases, which are composed of mainly CO2, steam, and oxygen; the stack gas is therefore partially recirculated and partially cooled to condense water and capture CO2.

A modified furnace concept with pulverized coal is introduced here as the main component of an integrated system that couples a steam Rankine power plant with a Cu–Cl water splitting cycle to the same source of energy. A low-pressure steam RH is placed as the first heat exchanger in the flue gas stream. Coal is pulverized at the lower part of the furnace, where it devolatilizes and ignites in an oxidative atmosphere. The furnace system is shown in Fig. 36. The average temperature of gases at the lower part

Environmental Dimensions of Energy

81

Economizer (preheater) E Boiler (convective section) Superheater and reheaters section

D

C

To aftertreatment F

Low pressure steam from/to Cu−Cl plant

Calandria boiler (radiative section) Pulverizers (burners)

B

Fly ash removal

A

Gas recycling blower

Coal grinder Oxygen blower (from Cu−Cl and ASU) Bottom ash Fig. 36 Pulverized coal furnace concept for integrated copper–chlorine (Cu–Cl) water splitting plant with steam Rankine power plant. Modified after Naterer GF, Dincer I, Zamfirescu C. Hydrogen production from nuclear energy. New York, NY: Springer; 2013.

of the furnace (below point A in the figure) is typically around 650–700K. A calandria boiler system with pipes at the lower half of the furnace is installed. Heat transfer by radiation occurs between the hot gases and the pipes (with boiling water) placed at the channel periphery. In the first part of the combustion process, the volatiles are oxidized, while the temperature the of coal particle increases. Subsequently, carbon oxidizes and the flue gas temperature increases further; this process occurs approximately between locations A and B represented in the figure. At point B, coal particles are completely consumed and the flue gas temperature reaches its maximum value (around 1200K). Between points B and C (see the figure), hot flue gas exchanges heat by convection and radiation with low-pressure steam in a heat exchanger linked to the heat supply circuit for the Cu–Cl cycle. In this heat exchanger, superheated steam is reheated from 800 to 900K. In the path of flue gases, it follows two RHs and the SH of the steam Rankine power plant. At point D, the flue gas temperature reaches about 950K. Further, heat is transferred to the last segment of boiler (states D–E in the figure); note that state E at 820–850K corresponds to the pinch point. It follows the economizer after which the flue gas temperature drops to about 550K. A part of the combustion gases – comprising mainly CO2, steam, and oxygen – is recirculated back to the combustion zone. The other part passes to the after-treatment section, where gases are cleaned, PM is extracted, and by further cooling water is condensed and CO2 captured. Fig. 37 represents the integrated system of a coal-fired power plant with a Cu–Cl cycle for hydrogen production. The heat generated by the combustion process is transferred via multiple heat exchangers to the working fluid of steam Rankine plant and to the Cu–Cl water splitting cycle. A radiative heat transfer exists at the bottom part of the furnace (until the maximum flue gas temperature point, B in Fig. 36) between the hot gas and the walls flanked with calandria pipes where forced convection boiling occurs. This process is represented in the diagram in Fig. 37 by the “Calandria HX.” Further, the hottest flue gases in state B (see Figs. 36 and 37) pass through a low-pressure steam RH and then are diverted to the superheating heat exchanger (SH) and the RH heat exchangers (RH1 and RH2), connected in parallel at the hot stream side. Next, the flue gas is directed to the boiler (D–E) and then to the economizer (E–F). The lowest grade heat recovered at the economizer section (F–G) of the furnace is transferred to the dehydration process within the Cu–Cl cycle. The steam generated in the low-pressure SH is transported to the Cu–Cl cycle for heating purposes; this is a secondary steam circuit. Steam for water splitting purpose is extracted from the

Environmental Dimensions of Energy

82

C RH1

SH 8

9

10

11

LP steam reheater

Steam extraction To Cu−Cl cycle

RH2 12

Superheated steam loop from/to Cu-Cl cycle

13

Flue gas to aftertreatment

IPT

HPT

Calandria drum vessel

LPT

B 7

G

Boiler F

Low grade superheated steam loop from/to Cu−Cl cycle

E

COND

14

D

Economizer

Water addition for splitting

Calandria HX

DHX

Radiative heat transfer from combustion channel 6

3 HPP

LPP 2

5

1

15

4 LPHx

HPHx

Fig. 37 Integrated system of coal-fired power plant with a copper–chlorine (Cu–Cl) water splitting cycle for hydrogen production (only the power plant and heat supply system to the copper–chlorine cycle is represented; bold lines represent flue gas stream flow corresponding to Fig. 36). COND, condenser; DHX, direct contact heat exchanger (and deaerator); HPHx, high-pressure heat exchanger (preheater); HPP, high-pressure pump; HPT, high-pressure turbine; IPT, intermediate-pressure turbine; LPHx, low-pressure heat exchanger (preheater); LPP, low-pressure pump; LPT, low-pressure turbine; RH, reheater; SH, superheater. Modified after Naterer GF, Dincer I, Zamfirescu C. Hydrogen production from nuclear energy. New York, NY: Springer; 2013.

low-pressure turbine (LPT) of the power plant. In order to maintain the working fluid balance within the Rankine plant, fresh water is supplied to the direct contact heat exchanger (DHX) in the same amount as it is extracted as steam from the LPT. As a first step of system analysis, a thermodynamic modeling can be applied, based on several assumptions. The crucial component of the system is the pulverized coal furnace, which generates thermal energy for both the power plant and the water splitting plant. The oxyfuel combustion of coal can be modeled by the following equation: C þ 2nH2 þ lð1 þ nÞO2 -CO2 þ 2nH2 O þ ðl

1Þð1 þ nÞO2

ð30Þ

where v represents a stoichiometric number accounting for the hydrogen content of coal, whereas it is assumed that coal is modeled with sufficient accuracy for the purpose of this study as a substance comprising coal and hydrogen. In Eq. (30), the excess oxidant for combustion is denoted with l. During the combustion process – as a first phase – volatile matter emanates from the pulverized coal particle and ignites, generating heat. The particle temperature increases until a level when carbon combustion initiates and, further, the particle consumes due to oxidation process occurring with a continuous emanation of hot flue gases. During all processes, heat is exchanged via radiation with the colder walls of the furnace at its bottom side, walls that are lined with the calandria’s tubes where water boils in forced convection. Therefore the energy balance of the combustion process is written as follows: HR ¼ HP þ Qign

ð31Þ

where HR,P represents the total enthalpy of reactants and flue gases, respectively, and Qign is the total heat amount transferred by radiation from hot coal particles and hot combustion gases to the calandria’s pipes. The enthalpy of the reactants is expressed in the following manner for one mole of carbon combusted: HR ¼ hC þ 2nhH2 þ lð1

nÞhO2 þ y½hCO2 þ 2nhH2 O þ ðl

1Þð1 þ nÞhO2 Š

ð32Þ

where h represents the molar specific enthalpy and y is the recycling fraction of combustion gases. Note that the y fraction of combustion gases from state is recycled and reinjected back to the combustion chamber. The recycling fraction of gases comprises oxygen that enters in combustion reaction with volatiles and coal; the other part of recycled gases plays the role of a heat transfer medium and an adjusting mechanism of the flue gas temperature. If the recycling fraction is high, the flue gas temperature is lower, and vice versa. The enthalpy of products is expressed as follows: HP ¼ hCO2 þ 2nhH2 O þ ðl

1Þð1 þ nÞhO2

ð33Þ

Environmental Dimensions of Energy

Table 7

83

Assumed parameters for modeling

Molar fraction H:C for coal (2v) Thermal conductivity of gases (kg) Prandtl number (Pr) Emissivity of coal (A) Density of coal (rc) Particle inlet temperature (Ti) Thermal diffusivity of coal particle (a)

2 0.05 W/m K 0.7 0.93 1080 kg/m3 300K 1.9  10 7 m2/s

The enthalpy of the products is calculated for the highest temperature of flue gases (state B), whereas the enthalpy of recycled gases in Eq. (32) is calculated for a lower temperature, corresponding to state E. The heat exchange by radiation during the combustion phase is calculated as follows:
 _ ign ¼ ar Acal T g T cal ð34Þ Q where Acal is the heat transfer area of calandria tubes, T cal represents the average temperature of calandria tubes, T g is the average temperature of hot gases during the ignition period, and ar is the linearized coefficient at heat transfer by radiation defined by
 2 2 ar ¼ se T g þ T cal T g þ T cal ð35Þ

where s is the Stefan–Boltzmann constant and e is the emissivity of the tubes’ surface; note that in Eq. (35) is assumed a shape factor of heat transfer by radiation of unity, which is a reasonable assumption due to the large cross-sectional opening of the furnace at its bottom side. Energy, entropy, exergy, and mass balance equations are written for each component and for the overall assembly. For any _ heat transfer rates component “i” enclosed in a control volume, these equations involve the inlet and outlet mass flow rates (m), _ work production rate (W), _ _ d ), the corresponding _ entropy generation rate (S_ gen ), exergy rate (Ex), exergy destruction rate (Ex (Q), process temperature (T), and the temperature of the reference environment (T0). The following set of equations can be written for each system component: P 8P _ i ¼ om _o im > > P P > > _ ¼ _ _ _ > þ Q E i > i o Eo þ W > _ _ > < P P Q P P Q _ þ S_ gen ¼ o S_ o þ o ð36Þ i Si þ i > T i T o > >   > > P P P > T0 _ > _ þ Ex _ d _ oþW _ iþ > i Ex QK ¼ o Ex : K 1 Tk The main parameters assumed for modeling are summarized in Table 7. The variation of flue gas temperature (Tfg) with oxidant excess ratio (l) is presented in Fig. 36 for three cases of stack gas recycling fraction. Reasonable values of (l) of 5–20 correspond to flue gas of 1100 to 1300K.

1.3.6.2.2

Results

Consider the ideal case of j¼ 1, representing the case when the machines involved in the water splitting system (the HP, and heat engines HE and HR) have a maximum (Carnot) COP and efficiency, respectively. The heat sink temperature was assumed as 201C, which corresponds approximately to the average annual temperature of a lake. The temperature of the sustainable thermal energy source Tm was varied between two limits, namely, 40 and 3501C. The minimum value corresponds to the coupling of the Cu–Cl plant to the moderator temperature of a CANDU power plant. The upper bound corresponds to the case when a high-temperature source from sustainable energy is used to drive the cycle. This can be, for example, a solar concentrator, geothermal, process heat, or biomass combustion. The results are presented in Fig. 38 and show that the upper bound of energy efficiency of the overall process is over 60%, provided that the hot source temperature is at its highest value. For a fair evaluation of the Cu–Cl water splitting technology, its efficiency is compared on the same figure to that of water splitting through electrolysis. In this respect, one assumes the same heat RES at a temperature Tm and the same efficiency of the heat engine ZHE as the case of a Cu–Cl system. In this case, the heat engine is used to produce electricity needed to drive the electrolysis process only. From the heat engine efficiency, and assuming an electrolyzer efficiency of 50%, one obtains Fig. 38. This reveals that the efficiency of the Cu–Cl based water splitting system is ideally 2–2.5 times higher than that of the electrolyzer driven by the same source of sustainable thermal energy. If one now includes in the analysis the parameter j, it yields the plot presented in Fig. 39. This plot shows the variation of hydrogen production efficiency of the Cu–Cl cycle, with idealized constant temperature heat RESs, as a function of the abatement of heat engines/pump from the Carnot efficiency. For all cases, the efficiency of the system based on water electrolysis is 2–2.5 times lower. For 50% abatement, one can expect a hydrogen production efficiency range from 4 to 39% if the temperature of the sustainable energy source varies from 60 to 3501C.

84

Environmental Dimensions of Energy

60%

Cu−Cl cycle



40%

20% Electrolysis 0% 40

90

140

190

240

290

340

Tm (°C) Fig. 38 Ideal hydrogen production efficiency from a sustainable thermal energy source with temperature Tm. Data from Naterer GF, Dincer I, Zamfirescu C. Hydrogen production from nuclear energy. New York, NY: Springer; 2013.

60% Tm=350°C



40%

Tm=300°C Tm=150°C

20%

Tm=60°C

0% 0.25

0.5

0.75

1



kgCO2/GJsustainable, thermal

Fig. 39 Efficiency of the water splitting system in Fig. 35 with the abatement j from the Carnot efficiency. Data from Naterer GF, Dincer I, Zamfirescu C. Hydrogen production from nuclear energy. New York, NY: Springer; 2013.

40 Cu−Cl cycle

20

Electrolysis

0 40

140

240

340

Tm (°C) Fig. 40 Ideal maximum CO2 mitigation of hydrogen and oxygen production by two benign technologies. Data from Naterer GF, Dincer I, Zamfirescu C. Hydrogen production from nuclear energy. New York, NY: Springer; 2013.

Next, one transposes the results from Fig. 38 in terms of CO2 mitigation by Cu–Cl and electrolyzer technologies, respectively. The curves in Fig. 40 illustrate this result. The CO2 mitigation potential ranges from 8 to approximately 45 kg CO2 per GJ of sustainable thermal energy used in the process. This figure is 2.5–3 times higher than that obtained if electrolysis is applied. If other sustainable sources like nuclear, electricity, hydro, wind, or power from biomass combustion are used to produce electricity at highest efficiency and electrolyze the water, the whole input energy-to-hydrogen efficiency is approximately 15%, and hence the CO2 mitigation is approximately 11 kg CO2/GJ. The same value is obtained by the Cu–Cl based system if the thermal energy source is available at 701C, which corresponds to coupling the cycle to CANDU power plants. Regarding the electrolysis approach,

Environmental Dimensions of Energy

Stack gas CO2 (23%), steam, O2

30% CO2

85

Supercritical CO2 (partially captured) 2.8 kg/s

Gas separation unit

Power 3 MW

Recycled water

Fresh water Superheater 22 MW (12 MW)

Reheaters 36 MW (20 MW)

Power to grid 48.4 MW Rankine power plant (37.6 MW)

Boiler 45 MW (22 MW)

Calandria 64 MW (44 MW)

Coal 1 kmol/s 360 MW (270 MW)

Furnace (76 MW)

Economizer 53 MW (21 MW)

Power 14 MW

Steam

Power 16 MW

H2 0.3 kmol/s 85 MW (72 MW)

High temperature reactors 100 MW (65 MW)

Air Cu−Cl plant (19 MW)

Dehydration reactor 25 MW (10 MW)

Air separation unit

Oxygen 0.15 kmol/s Oxygen

Fig. 41 Thermodynamic analysis by energy and exergy of the integrated coal-fired plant for hydrogen and power production. The figures in the parentheses represent exergy. Data from Naterer GF, Dincer I, Zamfirescu C. Hydrogen production from nuclear energy. New York, NY: Springer; 2013.

the heat source must be available to at least 1501C in order to obtain the same mitigation. Assuming the abatement j ¼ 50%, the corresponding CO2 mitigation by the Cu–Cl based system lies in the range of 3–27 kg CO2 =GJsustainable . thermal The results of thermodynamic modeling of the second integrated system (coal-fired power plant þ Cu–Cl cycle) are presented as shown in Fig. 41. As indicated in the figure, the flow rate of coal was assumed 1 kmol/s; in the conditions considered here, it results that the exergy destruction in the furnace is 76 MW and the furnace operates with 71% exergy efficiency. The exergy destruction in the Rankine plant is 37.6 MW, while its energy efficiency is 37% and the exergy efficiency is 68%. The exergy efficiency of the Cu–Cl plant is estimated by 79% provided that 50% of heat is recovered inside the cycle, and the rejected heat is of very low grade. The carbon emission of the integrated system is 84 g CO2 emitted per megawatt (MW) of exergy generated (of which 60% represents hydrogen product and 40% represents electricity to grid). In other terms, the carbon emission can be expressed as 50 g

86

Environmental Dimensions of Energy

0.5

0.66

0.64

0.4

0.62

0.6

SI

MCO2

0.3

0.2 0.58 0.1

0.56

0.54

0 0

0.2

0.4

0.6

0.8

1

f Fig. 42 Variation of carbon emissions relative to coal gasification and sustainability index (SI) with CO2 capture fraction (f) for the integrated coal-fired system for hydrogen production. Data from Naterer GF, Dincer I, Zamfirescu C. Hydrogen production from nuclear energy. New York, NY: Springer; 2013.

CO2 emitted per MW of exergy in hydrogen produced, or 6 kg CO2 emitted per kg of produced hydrogen. This amount can be compared with CO2 emissions at coal gasification. An autothermal coal gasification process uses energy from coal combustion in order to extract hydrogen from water. The process can be described by the following overall chemical equation: C þ 2ð1

nÞH2 O þ nO2 -2ð1

nÞH2 þ CO2

ð37Þ

In Eq. (37) it is assumed – conservatively, for determining the CO2 emissions – that coal can be modeled as carbon. By setting the reaction enthalpy of Eq. (37) to zero and solving for the stoichiometric number n, which must take values between 0 and 0.5, one obtains that n¼0.31, and therefore, 1.38 mol of hydrogen must be produced for one mole of carbon in an autothermal coal gasification process. It follows that CO2 emissions of a coal gasification plant are at least higher than 16 kg CO2 per kg H2. This figure represents a reference value for estimation of the carbon mitigation potential of other coal-fired hydrogen production systems, such as that considered in this chapter. In this respect, the relative carbon emissions can be defined as follows: MCO2 ¼ M=Mref

ð38Þ

where M represents CO2 emission per kg of hydrogen production, and, as stated above, the reference value is Mref ¼ 16 kg CO2 per kg H2. Based on Eq. (38) and the thermodynamic model of the integrated system the graph from Fig. 42 is obtained, where carbon mitigation potential and sustainability index (SI) are plotted against the CO2 capture fraction (f). The SI was introduced in the past paper by Dincer and Rosen [11] as reciprocal of depletion factor, Eq. (39). The CO2 capture fraction is varied from 0 to 1, where 0 represents the “no capture” case, while 1 represents the case when all CO2 is captured from the stack gas. When CO2 is captured there is an energy penalty on net production of electricity, while the production of hydrogen is not influenced. It can be observed from Fig. 42 that SI of the system improves more than 10% when carbon is captured completely. The last result is a comparative assessment of nuclear-based and coal-fired production of hydrogen and coal with an integrated system including Cu–Cl water splitting cycle. EI of nuclear-based hydrogen production with Cu–Cl water splitting cycle has been studied in detail by Ozbilen et al. [12]. According to this past study, it appears that the GWP produced by such a system is of the order of 109 kg CO2, for a plant with 125 t H2 production per day. This estimate was based on a LCA. It results that the upper bound of carbon emissions of the system is slightly below 1 kg CO2 per kg of H2 produced. This estimate considers the indirect emissions related to the nuclear power plant since no direct CO2 pollution is produced at these facilities. In the bar plot from Fig. 43 the emissions of the reference coal gasification plant, the nuclear-based Cu–Cl integrated plant, and the coal-fired integrated plant for hydrogen and power generation are compared. It is remarked that even without carbon capture a coal-fired integrated plant is better from a pollution point of view than a coal gasification system. When carbon capture is applied, the coal-fired integrated system approaches, with respect to pollution mitigation, the nuclear-based system, which definitely is the most economically benign.

1.3.6.2.3

Conclusions

The following specific conclusions can be drawn from this case study. For the system driven by sustainable thermal energy sources, the CO2 mitigation that corresponds to this range of efficiencies is 4–16 kg CO2 =GJsustainable . For the system integrated with the thermal

Environmental Dimensions of Energy

87

20

M (kgCO2/kg H2)

16

12

8

4

0 Coal gasification

Nuclear Cu−Cl

0% capture

50% capture

90% capture

Integrated coal-fired Cu−Cl plant Fig. 43 Emission comparison of two integrated hydrogen production systems based on copper–chlorine (Cu–Cl) water splitting cycle; a reference coal gasification system is included in the plot. Data from Naterer GF, Dincer I, Zamfirescu C. Hydrogen production from nuclear energy. New York, NY: Springer; 2013.

coal-fired power plant, carbon emission is reduced by at least 50% with respect to the reference case of coal gasification when the novel integrated system is used. With respect to a nuclear-based integrated system with Cu–Cl cycle, the coal-fired system emits roughly seven times more GHG; this figure is reduced substantially if carbon capture is applied.

1.3.6.3

Comparative NH3-Fuel Assessment for Railway Transportation in Canada

The transportation sector is the largest contributor to GHG emissions by economic sector in Canada, representing approximately 25% of the national total in 2012. In the case of railway transportation, which is fueled by a combustion process, the atmospheric pollution through flue gas emissions is very relevant. A focus of many studies is to consider emissions of the following types of atmospheric pollutants: CO, NOx, SO2, CO2, and CH4. Many statistical data and analyses are available in the literature for the conventional locomotive systems. Regarding the projected, low-emission locomotive systems and fuel, there is a lack of relevant data in the literature, explained by the fact that the newer systems were not built yet at large-scale; only small prototype lab tests and theoretical analyses are available for those. Rail transportation is well positioned to reduce EIs throughout the Canadian transportation sector, with established services in place for passenger and freight operations. Infrastructure improvements and expansion of passenger and commuter rail transit networks can help to increase the number of users. The benefits of such improvements include reduction of urban transportation congestion due to passenger car traffic – with an associated reduction in fuel combustion emissions – and the fact that use of rail for freight transport over long distances can lower the number of on-road freight trucks. In order to significantly reduce the impact of the transportation sector, two primary issues must be addressed in parallel with current fuel consumption and emission reduction measures:

• •

implementation of sustainable, more environmentally benign energy alternatives to traditional high carbon content fuels; and on-road traffic density, particularly passenger cars in population-dense urban regions.

Alternative fuels and energy resources play a key role in both the short- and long-term sustainable development of transportation. As a carbon-free chemical energy carrier, hydrogen (H2) is widely recognized as the ideal synthetic fuel for sustainable development; however, significant challenges with respect to production methods, transport infrastructure, onboard storage, and safety standards require further development before it can be considered fully practical as a transportation fuel. NH3 is the only carbon-free chemical energy carrier (other than hydrogen) suitable for use as a transportation fuel. Furthermore, NH3 has a high octane rating (110–130), can be thermally cracked to produce H2 fuel using only approximately 12% of the higher heating value (HHV), presents no explosion danger when properly transported and stored, has a well-established production and distribution infrastructure, and has zero GWP. In addition to its attractive qualities as a fuel, NH3 is widely used as a NOx reducing agent for combustion exhaust gases using selective catalytic reduction (SCR), and its capacity as a refrigerant can be applied to recover and further utilize engine heat that would otherwise be lost. In terms of environmental sustainability, NH3 can be produced using either fossil fuels or any renewable energy source, using heat and/or electricity, which allows for evolution of NH3 production methods and technologies in parallel with sustainable development.

Environmental Dimensions of Energy

88

Table 8

Environmental Protection Agency (EPA) locomotive criteria air contaminant (CAC) exhaust emission standards (g/bhp
hr)

Duty-Cycle

Tier

Line-haul

Tier Tier Tier Tier Tier

0 1 2 3 4

Year

Hydrocarbon (HC)

Nitrogen oxides (NOx)

Particulate matter (PM)

Carbon monoxide (CO)

1973–92 1993–2004 2005–11 2012–14 2015 þ

1.00 0.55 0.30 0.30 0.14

9.50 7.40 5.50 5.50 1.30

0.22 0.22 0.10 0.10 0.03

5.00 2.20 1.50 1.50 1.50

Source: Reproduced from US EPA. Air pollution control technology fact sheet. Available from: www.ucsusa.org; www.ucsusa.org/assets; 2012.

Natural gas is the primary feedstock used for producing NH3 in Canada, and worldwide. There are 11 NH3 plants operating in Canada, producing an average of 4–5 million metric tonnes annually per plant. Canadian NH3 plants recover a high percentage of process-generated CO2 (approximately 40%) to produce urea, and have the highest feed-plus-fuel energy (FFE) plant efficiency internationally – consuming an average of 33.8 GJ/tonne NH3 for natural gas plants, compared to the international average of 38.6 GJ/tonne NH3. In North America, GHG emission targets are set for freight, passenger, and commuter operations. Freight GHG target emissions are given per 1000 revenue tonne-kilometer (RTK), with a 2015 target of 15.44 kg CO2eq/1000 RTK. Passenger limits are in terms of passenger-kilometers (PAX∙km), with a 2015 target of 0.11 kg CO2eq/PAX∙km. Both passenger and freight targets are based on a 6% reduction from 2010 emissions. Commuter rail GHGs are given per passenger (PAX) and have a 2015 target of 1.46 kg CO2eq/ PAX; a reduction from 2011 levels stated at 2.19 kg CO2eq/PAX. Current criteria air contaminant (CAC) limits, given in Table 8, are in line with those established by the US EPA, and are given in terms of emission intensity standards using a tiered approach based on the year of manufacture, and specific mode of operation of a locomotive. To meet these targets, strict limitations are imposed on diesel fuel standards, particularly with respect to sulfur content – limited to 15 ppm as of 2012 – and locomotive technologies are being updated or replaced with more efficient models and/or retrofits. In addition to technology improvements and diesel emission reduction initiatives, rail and locomotive industries are continuously investigating and implementing alternative fuels and fueling technologies to reduce harmful emissions, with multiple projects throughout North America and internationally since the mid-1980s, including natural gas, biodiesel, and various diesel hybrid technologies. Given that the primary energy resource for NH3 production is natural gas, it is certainly valid to propose to simply use natural gas instead for transportation fuel application, but this does not take into account several key factors that make NH3 such a valuable resource for sustainable development:

• • •

NH3 is not a fossil fuel, and therefore is not limited in its quantity and availability to fuel reserves, which will eventually be completely depleted; NH3 can be produced entirely from renewable resources, and does not require energy and environmentally taxing extraction processes to be made available; and NH3 does not contain any carbon, and therefore does not produce any CO2, CO, or soot (a primary constituent of PM) during combustion.

It is possible to produce NH3 locally (relative to fueling points) from renewable sources, which can reduce lifecycle CO2 emissions by minimizing the environmental and impact of transporting NH3. Most significantly, we can eliminate (or nearly eliminate) the feedstock-related emissions. Biofuels and biomass offer a sustainable option for NH3 production, for both syngas production and as a fuel for power plants to produce electricity for water electrolysis. Furthermore, biofuels may be produced locally from various agricultural feedstock and waste, and municipal solid waste. The production of NH3 can be a method of storing renewable energies, such as solar or wind energy, which can be intermittent.

1.3.6.3.1

Analysis and modeling

Compared to other fuels used in combustion applications, NH3 has the highest hydrogen energy density – higher even than that of pressurized and liquefied hydrogen fuel, based on current storage methods – contains no carbon, has a GWP of zero, and produces only nitrogen and water when combusted. The results are based on a past study of the authors, presented in Ref. [13]. NH3 is compared to other traditional and alternative fuels in Fig. 44 with density, r (kg/m3), indicated for each fuel based on the fuel mass per storage volume. While the comparatively low volumetric energy density of NH3 relative to traditional HCs presents a challenge to its immediate introduction in passenger vehicles as a direct feed combustion fuel, large transportation vehicles, such as rail and heavy-duty trucks are well equipped to carry the additional fuel weight without significant performance penalties. The basic arrangement of a modern diesel–electric locomotive is shown in Fig. 45, and the sizing details are listed in Table 9. During operation, shaft power produced by the two-stroke ICE directly drives the engine governor, cooling water and oil pumps, radiator fans, and air compressor(s) of the locomotive, as well as electric generator(s) used to power the traction motors and various other cooling subsystems and locomotive controls. An auxiliary generator charges a set of batteries that supply power to the engine starter motor.

Environmental Dimensions of Energy

89

50 Diesel (ULSD)  = 850 kg/m3 Biodiesel (B100),  = 880 kg/m3

40

Gasoline,  = 736 kg/m3

H2 (metal hydrides),  ≈ 25 kg/m3

30

LNG,  = 450 kg/m3

GJ/m3

H2 (liquid),  = 70.8 kg/m3

20 LPG,  = 388 kg/m3

10

NH3,  = 603 kg/m3

CNG,  = 188 kg/m3

H2 (compressed),  ≈ 10 kg/m3

0 0

10

20

30

40

50

60

GJ/tonne Fig. 44 Volumetric (GJ/m3) and gravimetric (GJ/tonne) energy density, and mass storage density (kg/m3) of transportation fuels. CNG, compressed natural gas; LNG, liquefied natural gas; LPG, liquefied petroleum gas; ULSD, ultralow sulfur diesel. Data from Dincer I, Hogerwaard J, Zamfirescu C. Clean rail transportation options. New York, NY: Springer; 2015.

Intake 1 air

TC

C1

T1

C1

Res-1 (Ulsd)

RES-3 (water)

Exhaust 5 gases

22

6

sh

P1

T1 (TC start-up) 7

9

4

2

(Cylinder inj.) 23

P

sh 10

C

8

sh Gov

b 18 Res

Res (Oil)

19 12

P 17

P

16

TM M

TP

SM

el

Batt

P el

15

11

Locomotive controls

21 13

RES (air)

Gen1−3

3 Ice

(Start-up)

P

HX-ac (23)

SM

el

20 el

14 Cooling fans

Fig. 45 Diesel locomotive system. ULSD, ultralow sulfur diesel. Modified from Dincer I, Hogerwaard J, Zamfirescu C. Clean rail transportation options. New York, NY: Springer; 2015.

A turbocharger is used to raise the pressure of air at the intake manifold of the engine; this improves engine performance by recovering heat energy from the exhaust gases to drive the turbine and also by increasing the enthalpy of the intake air. The compressed fresh air is cooled in an aftercooler to raise its density – and, therefore, the mass flow rate – of the air entering the cylinders. A head-end power engine (not shown) – typically a smaller diesel ICE – is used for cooling/heating systems and other

Environmental Dimensions of Energy

90

Table 9

Locomotive prime mover sizing details

Engine model (EMD) Traction horsepower, THP (hp) (W_ TP , kW) Brake mean effective pressure, bmep (kPa) Displacement volume per cylinder, Vd (L) Compression ratio, r Bore (m) Stroke (m) Number of cylinders Fuel tank volume, VRES1 (L)

16V-710G3 4000 [2983] 1069 11.635 16:1 0.23019 0.2794 16 8410

SCR

Exhaust

31

30

EXP 28

27

25′

RES-2 (NH3)

RES-1 (ULSD)

24

22

29 HX-1

25″

P1

P2

el

SM (Start-up)

1

Intake air

25 TC

C1

T1

C1

26

9

4

7 P

HX-ac

23

sh (TC start-up)

6

T1 2

(Cylinder inj.)

RES-3 (water)

5

P sh

(26)

10

C

8

sh Gov

RES (air) Gen

3

TM M

TP

SM

el

b

Ice

18 Res

Res (oil)

19 12

P

P el

17

P

15

11

Locomotive controls

21 13

Batt

16

20 el

14 Cooling fans

Fig. 46 Ammonia (NH3)–diesel fueled locomotive. Data from Naterer GF, Dincer I, Zamfirescu C. Hydrogen production from nuclear energy. New York, NY: Springer; 2013.

“hotel power” operations within passenger-occupied rail cars. Though not addressed in this work, the head-end power ICE is another locomotive application with excellent potential for NH3 fueling, since the engine operates constantly, regardless of the locomotive engine speed. Heat energy in the exhaust and engine cooling systems is addressed as a potential area for locomotive performance improvement by integration with NH3 subsystems. In the proposed designs, additional work production and HR processes are configured and analyzed. NH3 for locomotive fueling and exhaust NOx emission control is integrated in System 2, shown in Fig. 46. The liquid NH3 is stored in RES-2 at 10 bar (1000 kPa). Fueling is supplied as an NH3–ULSD mixture, varying the ratio of fuel-energy input between NH3 and ULSD. Liquid NH3 fuel is pumped by P2 from the storage tank and preheated in HX-1, then sent with the diesel fuel supply to the cylinders for combustion. Exhaust NOx emissions are controlled using SCR. Liquid NH3 is drawn from RES-2, and then throttled to atmospheric pressure in the expansion valve. The resulting liquid–vapor mixture enters the exhaust recovery heat exchanger (HX-1) to be heated to a temperature within the considered range of 250–4501C. NH3 vapor is injected into the exhaust upstream of the SCR catalyst. The mixing streams pass through the catalyst bed, where the reaction takes place.

Environmental Dimensions of Energy

Table 10

91

Dual cycle thermodynamic relationships (k¼1.35)

State points

Process

Equations

1–2

Isentropic compression

2–4

Heat addition at constant volume and constant pressure

4–5

Isentropic expansion

5–1

Heat rejection at constant volume

• • • • • • • •

P2 ¼P1rk T2 ¼ T1r(k 1) _ exh cv ðT3 T2 Þ (constant volume process, V2 ¼ V3) Q_ 2 3 ¼ m _ exh cp ðT4 T3 Þ (constant pressure process, P3 ¼P4) Q_ 3 4 ¼ m Q_ in ¼ Q_
2 3 þ Q_ 3 4 k P5 ¼ P4 1r
1ðk 1Þ T5 ¼ T4 r _ exh cv ðT5 T1 Þ Q_ 5 1 ¼ m

For the complete reaction, the products leave the catalyst as water vapor and nitrogen gas. Some NH3 slip is possible, which can cause formation of ammonium sulfates, and unwanted NH3 in exhaust emissions; the EPA permits an acceptable range of 2–10 ppm NH3 slip. The locomotive prime mover is powered by a large two-stroke compression ignition (CI) diesel-fueled engine. Modern diesel CI engine cycles operate between upper and lower limits of the Otto and Diesel cycles according to the thermodynamic dual cycle. The ideal dual cycle consists of four major processes: isentropic compression, heat addition, isentropic expansion, and constant volume heat rejection. Heat addition occurs in two stages: constant volume and constant pressure. The equations describing the cycle processes are outlined in Table 10 for the ideal case with a constant specific heat ratio, k¼ 1.35. The cycle model is developed based on the known engine geometry and operating characteristics, such as compression ratio (r), cylinder volumes (Vi), rated traction power (TP), and maximum cylinder pressure (Pmax ¼P3). _ f ), _ TP ), to the fuel input (Q The energy efficiency of the locomotive ICE is calculated as the ratio of the useful output (W according to the following equation: ZICE ¼

_ TP W _f Q

ð39Þ

where the fuel energy is determined from the mass flow rate and lower heating value (LHV) of the supplied fuel(s), calculated by X _ f ¼ Zc
_ f LHV f Þi ðm ð40Þ Q Exergy efficiency is defined similarly by the equation as follows: cICE ¼

_ TP W _ f Ex

where the fuel exergy is determined from the physical and chemical exergies of the supplied fuel(s), given as follows: X


_ f¼ _ f
ex ph þ ex ch f i m Ex The energy balance is written as follows: hX i _f þm _ rec þ Q _ ac þ Q _ wjc þ E_ loss _ TP þ W _ net;NH3 þ Q _ i hi Þexh þ W _ a ha DE_ ¼ Q ðm

ð41Þ

ð42Þ

ð43Þ

where the losses term refers to friction and heat transfer energy losses in the ICE. The associated exergy balance is given by hX i _ Q þ Ex _ Q þ Ex _ Q þ Ex _ loss þ Ex _ d _ ¼ Ex _ f þm _ TP þ W _ net;NH3 þ Ex _ i ex i Þexh þ W _ a exa ð44Þ DEx ðm rec ac wjc The heat transfer exergy values are calculated as follows:

 _ Q¼ 1 Ex j

 To _ Qj Tj

ð45Þ

where the ambient temperature is T0 ¼ 298K (251C), and Tj refers to the boundary temperature for heat transfer.

1.3.6.3.2

Results

The locomotive is modeled according to the operating conditions given in Table 11, fueled with ULSD for the baseline case. The brake specific fuel consumption is calculated to be 0.255 kg/kW
h for the lean case (j¼ 0.9), which is in reasonable agreement with other studies for the same locomotive engine. Heat is partially recovered from the exhaust gases leaving the ICE by the turbocharger unit; intake air is compressed then cooled (in the aftercooler) prior to entering the engine intake manifold to increase the total mass flow rate of air into the engine. _ exh ) is that of the gases The energy balance for the diesel-only case is shown in Fig. 47(A), where the exhaust heat energy (Q following the turbocharger HR. The heat loss from the engine is primarily associated with the exhaust gas stream, and the

Environmental Dimensions of Energy

92

Table 11

Locomotive rated operating conditions

Fuel input, Q_ f (kW) Traction power, W_ TP (kW) Engine speed, NICE1 (rpm) Turbocharger pressure boost, prTC (B) Cooling water reservoir (RES) temperature, TRES3 (1C) Engine jacket cooling water outlet temp., T12 (1C) Engine cooling Water jacket cooling, Q_ wjc (kW) Aftercooler, Q_ ac (kW) Maximum cylinder pressure, Pmax (kPa)

9,050 2,983 900 1.25 49 85 1,970 752 10,800

ExdRloss 4%

WTP 33%

ExdRirr.comb 29%

Eloss 7%

ExdRexh 29%

Qac 8%

ExdRac 2%

Qexh 30%

Qwjc 22% (A)

ExdRTC 3%

ExdRwjc 14% (B)

ExdRirr.Int. 18%

Fig. 47 (A) Energy balance and (B) exergy destruction ratios for diesel-fueled locomotive engine. Data from Dincer I, Hogerwaard J, Zamfirescu C. Clean rail transportation options. New York, NY: Springer; 2015.

aftercooler and water jacket cooling processes, accounting for nearly two-thirds of the total energy supply from fuel. To evaluate potential HR for the NH3 systems, it is necessary to consider the quality of the heat source using exergy analysis. Exergy destruction ratios of the primary system losses are shown in Fig. 47(B), with an exergy efficiency of cICE ¼31%. Combustion irreversibility is a major source of exergy loss, and is not considered recoverable. The exhaust gases and cooling water have recoverable heat exergies; however, as seen in Fig. 47, the energy content of the coolant does not have high heat exergy. However, the properties of NH3 allow for low-temperature HR, i.e., cooling applications for the locomotive cab. The brake specific emissions of GHGs and NOx are plotted in Fig. 48. Emission factors are applied to the ULSD fuel supply for GHGs and CACs. The NOx emission (tier) limits are shown in Fig. 48. While the tier values are stated based on averaged fuel consumption for daily operation (from idle to rated operation), the NH3–diesel mixture surpasses the requirement for current Tier 2 (and Tier 3) limits when the fuel blend is approximately 55% NH3 even at rated operation, and with the integration of the SCR unit is below this limit for both of the fueling cases. The emission reduction results are given in Table 12, based on constant total energy input to produce the required TP. The NH3–ULSD case assumes 50% fuel energy supply by each fuel, with the resulting emissions calculated based on the consumed diesel fuel. Energy balance of the NH3–ULSD fueled locomotive is shown in Fig. 49(A), with a calculated energy-utilization efficiency of 34.3%. The exhaust energy is lower than the diesel-only case due to lower exhaust temperature for the NH3–ULSD fuel blend, as well as the HR by NH3 in HX-1. The internal (friction) losses are also higher than the diesel-only case, due to the higher volume flow of fuel than in the diesel case increasing the losses to fluid flow and pumping. The exergy efficiency for the NH3–ULSD case is calculated to be 31.4%, and the exergy destructions are described in Fig. 49(B). There is a higher rate of exergy destroyed in combustion than in the diesel-only case, due to the increased amount of energy required to burn NH3, which has a lower flame speed and higher value for minimum ignition energy than diesel fuel.

1.3.6.3.3

Conclusions

This study demonstrates the potential of NH3 as an alternative combustion fuel, heat transfer fluid, NOx-reducing agent, and working fluid for a locomotive application. From the results of the thermodynamic and environmental analyses with 50% fuel

Environmental Dimensions of Energy

93

20

103

10 102

Tier 2,3

101

BSENOx (g/bhp·hr)

BSEGHG (g/bhp·hr)

CO2 (combustion analysis) CO2eq (EFCO2eq = 3.007 kg/L) CH4 (EFCH4 = 0.00015 kg/L) 100

N2O (EFN2O = 0.00110 kg/L)

10−1

Tier 4 1 ULSD NH3-ULSD ULSD (with SCR)

10−2

NH3-ULSD (with SCR)

10−3 0.05

0.28

0.50

0.73

%Qin-NH3

(A)

0.1 0.05

0.95

0.28

0.50

0.73

0.95

Qin%-NH3

(B)

Fig. 48 Brake specific emissions (BSE) of (A) greenhouse gases (GHGs) and (B) NOx for varying ammonia (NH3) fuel energy fraction. SCR, selective catalytic reduction. Data from Dincer I, Hogerwaard J, Zamfirescu C. Clean rail transportation options. New York, NY: Springer; 2015.

Table 12

_ TP ¼2983 kW) Brake specific emissions (W

Ultralow sulfur diesel (ULSD) 50%-NH3 þ SCR

Units

CO2eq

Hydrocarbon (HC)

Nitrogen oxides (NOx)

Particulate matter (PM)

Carbon monoxide (CO)

Sulfur oxides (SOx)

kg/kW∙hr g/bhp∙hr kg/kW∙hr g/bhp∙hr

0.9051 675 0.453 338

0.000671 0.5006 3.36  10 0.02503

0.0169 12.6 0.00245 1.83

0.000355 0.265 6.93  10 0.0517

0.00212 1.58 1.904  10 0.142

0.000751 0.5603 0.000376 0.2801

5

5

4

ExdRloss 15%

Eloss 14%

ExdRirr.comb 34%

WTP 33%

Qrec, HX−1 1%

ExdRexh 17% Qexh 22% ExdRac 2% Qac 8%

(A)

Qwjc 22%

ExdRTC 3%

ExdRwjc 14% (B)

ExdRirr.Int. 16%

ExdRHX−1 1%

Fig. 49 (A) Energy balance and (B) exergy destruction ratios for NH3–diesel fueled locomotive engine. Data from Dincer I, Hogerwaard J, Zamfirescu C. Clean rail transportation options. New York, NY: Springer; 2015.

energy supplied by NH3, the following concluding remarks are made:

•

HR by NH3 processes improves the energy and exergy utilization efficiencies from the baseline diesel ICE efficiencies of 33.2 and 31% to 34.3 and 31.4% for the NH3–diesel case.

Environmental Dimensions of Energy

94

• •

Significant reduction in combustion-related GHGs are achieved for the NH3–diesel fueled locomotive, decreasing from 0.67 kg/s to 0.33 kg/s for rated operating conditions. CAC emissions for NOx, PM, HC, and CO are below Tier 3 emission limits for the NH3–diesel fuel mixture with SCR.

1.3.6.4

Comparative Life Cycle Assessment of Various NH3 Production Methods

NH3, a common refrigerant, fertilizer, and chemical feedstock, is nowadays proposed as a sustainable energy carrier. NH3 can be produced by extracting nitrogen from air and hydrogen form water, and combining them with the help of any type of energy source, including, especially, the renewables. More than 90% of the world NH3 production currently uses the Haber–Bosch synthesis process. This method combines hydrogen and nitrogen over an iron oxide catalyst under very high pressure. Haber– Bosch synthesis at industrial scale is currently performed with different variations in synthesis pressure, temperature, and catalysts. On the other hand, new technologies, such as thermochemical and solid state synthesis processes are currently emerging with a potential to reduce the cost and improve the efficiency of NH3 production. NH3 is one of the largest-produced industrial chemicals in the world. NH3 production consumes almost 1.2% of total primary energy and contributes 0.93% of GHG emissions. Approximately 1.5 t of CO2 is emitted to the atmosphere during the production of 1 t of NH3. Naphtha, heavy fuel oil, coal, natural gas coke oven gas, and refinery gas are commonly used as feedstock in NH3 production. Natural gas is the primary feedstock used for producing NH3 in Canada, and worldwide. There are 11 NH3 plants operating in Canada, producing an average of 4–5 million metric tonnes annually per plant. Canadian NH3 plants recover a high percentage (around 40%) of process-generated CO2 to produce urea, and are ranked internationally as having the highest FFE plant efficiency, consuming an average of 33.8 GJ natural gas per tonne of NH3 produced. In comparison, the international FFE average is of 38.6 GJ/tonne NH3. Globally, 72% of NH3 is produced using steam reforming of natural gas. Steam CH4 reforming method is currently the least energy intensive technique among all others. In China, coal is intensively used and is generally characterized by its high energy intensities. Natural gas costs are 70–90% of the production cost of NH3. Since NH3 production is based on natural gas in the SMR method, if natural gas prices rise, the production costs for NH3 increase in parallel. The ability to use one fuel in all types of combustion engines, gas turbines, burners, and directly in efficient fuel cells is a significant advantage. Storage and delivery infrastructure would be greatly simplified, far fewer fuel formulations would be required and it would be easier to produce based on a given standard, and refueling stations would be standardized and lower in cost. NH3 is one of a very short list of fuels that can be used in nearly every type of engine and gas burner with only small modifications. Gas burners can be equipped with in line partial reformers to split approximately 5% of the NH3 into hydrogen. This mixture produces a robust, unpolluted burning open flame. One pipeline to a home could provide NH3 to furnaces/boilers, fuel cells, stationary generators, and even vehicles. Due to the very minor reforming enthalpy exhibited by NH3, it can easily be reformed to hydrogen for any application that would require hydrogen. Relatively minor modifications allow efficient use of NH3 as a fuel in diesel engines; high compression ratio spark-ignition engines can produce outstanding efficiencies of over 50% using NH3 fuel. NH3 is also a very suitable fuel for use in solid oxide fuel cell and gas turbines. These medium-temperature fuel cells promise to be low cost, highly efficient, and very robust.

1.3.6.4.1

Results

Four different NH3 production methods were selected for comparative assessment purposes. The results of this case study were detailed in a previously published paper by the authors [14]. Since the most common NH3 synthesis process is Haber–Bosch and the most mature hydrogen production method is electrolysis, electrolysis and Haber–Bosch based NH3 production methods are utilized using various resources. Hydropower is evaluated as a renewable source and yields higher efficiencies. Biomass is also evaluated as a renewable source, and a biomass-fired power plant is utilized in the study. Municipal waste is a significant source for power production especially for large cities. Therefore a municipal waste-based power plant is also used. Finally, nuclear waste heat available in nuclear power plants is an important source of energy, which can be utilized in the electrolysis process to decrease the required amount of electricity. Each selected system is described in detail and relevant efficiency terms are determined. In order to analyze biomass, municipal waste, and hydropower-based NH3 production processes, the following diagram is used as illustrated in Fig. 50. For nuclear-based high-temperature electrolysis-based NH3 production method, the schematic diagram shown in Fig. 51 is utilized. Electricity can be produced by burning municipal solid waste as a fuel. Therefore in this system, the required electricity was taken from a municipal waste incineration power plant. Waste-specific air and water emissions from incineration, and auxiliary material consumption for flue gas cleaning were included in the LCA. Short-term emissions to river water, long-term emissions to ground water from slag compartment (from bottom slag) and residual material landfill (from solidified fly ashes and scrubber sludge), and process energy demands for municipal waste incineration plant were taken into account. Share of carbon in the biogenic waste was around 60.4%. Share of iron in the metallic/recyclable waste was around 60%. The waste used in the calculations contained 21% paper, 8% mixed cardboard, 15% plastics, 3% laminated materials, 2% laminated packaging (e.g., tetra bricks), 3% combined goods; 3% glass, 2% textiles, 8% minerals, 9% natural products, 22% compostable material, 2.65% inert metals, 1% volatile metals, 0.0065% batteries, and 0.35% electronic goods. The LHV of the solid waste fuel was 11.74 MJ/kg and thermal efficiency was taken as 25%. The generated electricity was used to power the electrolyzer, cryogenic air separation plant, and Haber–Bosch process as illustrated in Fig. 50. Using commercial electrolyzer and cryogenic air separation, NH3 was synthesized in the Haber–Bosch plant.

Environmental Dimensions of Energy

95

Air

Resource (biomass/municipal waste/hydro)

Power plant Electricity

Electricity

Water

Electrolysis

Cryogenic air separatioin

O2

H2

Ammonia synthesis

O2

N2

NH3 Ammonia storage Fig. 50 Schematic representation of ammonia (NH3) production via hydropower/municipal waste/biomass-based electrolysis and Haber–Bosch process. Modified after Dincer I, Hogerwaard J, Zamfirescu C. Clean rail transportation options. New York, NY: Springer; 2015.

Air Uranium

Nuclear power plant Electricity Electricity

Water Nuclear heat

High temperature electrolysis

H2

Cryogenic air separatioin

O2

Ammonia synthesis

O2

N2

NH3 Ammonia storage Fig. 51 Schematic representation of ammonia (NH3) production via nuclear high-temperature electrolysis and Haber–Bosch process. Modified after Bicer Y, Dincer I, Zamfirescu C, Vezina G, Raso F. Comparative life cycle assessment of various ammonia production methods. J Clean Prod 2016;135:1379–95.

96

Environmental Dimensions of Energy

The nuclear-based system consisted of a nuclear power plant, high-temperature electrolyzer, cryogenic ASU, and a Haber–Bosch synthesis plant as shown in Fig. 51. The required electricity was utilized from a nuclear power plant and the required heat for hightemperature electrolysis was supplied from nuclear waste heat. Air was separated into nitrogen and oxygen using cryogenic methods. The LCA included fuel elements, chemicals, and diesel requirements as well as the relevant transport requirements. Water use for cooling was also accounted for. Considered radioactive waste streams were spent fuel to reprocessing and conditioning; operational low active waste for conditioning in the intermediate repository; and contaminated waste from dismantling. Nonradioactive wastes were taken into account as well. The diesel requirements for the yearly test of diesel emergency generators were accounted for. The transport requirements were calculated with the standard distances for chemical and diesel requirements and specific distances for fuel recharge and radioactive waste. In the biomass-based system, electricity generation was accomplished by gasification of biomass followed by a gas turbine. Biomass was produced within the boundaries of this system and was thus not shown as a fuel input. Fuel and material extraction, biomass production, biomass gasification power plant, and transportation were all included in the LCA. Infrastructure requirements for biomass production, biomass gasification, and biomass electricity generation were also included. The net power cycle efficiencies that can be achieved were between 23 and 34%. For the specific biomass-fired power plant, the energy efficiency was taken as 33%. The hydropower-based system included shares of electricity produced by of run-of-river and RES hydropower plants. Electricity production shares were determined on annual average and on the level of net production. The average energy efficiency of the plant was taken as 92%. The generated electricity was transmitted to electrolyzer, cryogenic air separation plant, and Haber–Bosch process. For LCA, SimaPro 7 software was used to generate the results presented herein. The LCA was performed using the CML 2001 and Eco-indicator 99 methods. The impact categories considered for the CML 2001 method were global warming and human toxicity. The impact categories considered for Eco-indicator 99 method were human health, ecosystem quality, and resources. Based on the inputs and outputs of the system, energy and exergy efficiency terms were determined, and thermodynamic calculations were conducted using engineering equation solver (EES) software. The following assumptions were made in the assessment:

• • • • • • • •

NH3 end product was evaluated as liquid at 341C and 101.3 kPa. Nitrogen production through cryogenic ASU was already defined in SimaPro 7 and directly utilized in the study. The inputs used for SimaPro 7 calculations were feedstock, energy or electricity, and emissions. The processes for NH3 production contained production of hydrogen and nitrogen separately. The mass balance was used to identify the amount of hydrogen and nitrogen required for one kg of NH3 production. The fugitive emissions were considered negligible. The system operated at steady state. Kinetic and potential energy changes throughout the processes were neglected.

The ambient environment had a temperature of T0 ¼ 251C and a pressure of P0 ¼100 kPa. The GWP was highest for the biomass-based NH3 production method followed by nuclear high-temperature based NH3 production as shown in Fig. 52. The values were 0.85 kg CO2-eq and 0.84 kg CO2-eq per kg of NH3, respectively. The municipal waste-based NH3 production had the lowest GWP of 0.34 kg CO2-eq per kg of NH3. The impact on human health due to human toxicity was maximum for the NH3 production from nuclear hightemperature method corresponding to 0.95 kg 1,4-DB-eq per kg of NH3. This is due to the high volume release of nuclear waste pollutants and GHGs by nuclear systems. NH3 from the municipal waste-based method yielded the lowest human toxicity value as seen in Fig. 53.

Ammonia from municipal waste electrolysis Ammonia from hydropower electrolysis

Ammonia from nuclear high temperature electrolysis Ammonia from biomass electrolysis

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

GHG emissions (kg CO2 eq) Fig. 52 Lifecycle greenhouse gas (GHG) emissions of ammonia (NH3) production pathways obtained by CML 2001 method. Data from Bicer Y, Dincer I, Zamfirescu C, Vezina G, Raso F. Comparative life cycle assessment of various ammonia production methods. J Clean Prod 2016;135:1379–95.

Environmental Dimensions of Energy

97

Ammonia from municipal waste electrolysis Ammonia from Biomass Electrolysis

Ammonia from nuclear high temperature electrolysis

Ammonia from hydropower electrolysis

0

0.2

0.4

0.6

0.8

1

Human toxicity 500a (kg 1,4-DB eq)

0.007 0.006 0.005 0.004 0.003 0.002 0.001 Ammonia from municipal waste electrolysis

Ammonia from coal gasification

Ammonia from biomass electrolysis

Ammonia from hydropower electrolysis

0 Ammonia from nuclear high temperature electrolysis

Abiotic depletion (kg Sb eq)

Fig. 53 Human toxicity values of various ammonia (NH3) production methods according to CML 2001. Data from Bicer Y, Dincer I, Zamfirescu C, Vezina G, Raso F. Comparative life cycle assessment of various ammonia production methods. J Clean Prod 2016;135:1379–95.

Fig. 54 Abiotic depletion values of various ammonia (NH3) production methods according to Eco-Indicator 99. Data from Bicer Y, Dincer I, Zamfirescu C, Vezina G, Raso F. Comparative life cycle assessment of various ammonia production methods. J Clean Prod 2016;135:1379–95.

Abiotic resources are natural resources including energy resources, such as iron ore and crude oil, which are considered as nonliving. The abiotic depletion was highest for nuclear high-temperature based NH3 production method and followed by hydropower-based method as is illustrated in Fig. 54. This is due to the fact that uranium is the primary source of energy and feed source; as well, it indicates the large consumption of uranium for unit mass of NH3 produced. The Eco-indicator 99 method considers multiple categories, such as ecotoxicity, acidification, and mineral damage to determine the damage impact to human health, damage to ecosystem, and damage to resources. The emissions are combined as a single score at the highest level. Damage assessments are relatively calculated based on the highest damage factor.

Environmental Dimensions of Energy

98

Relative damage assessment (%)

100

80

60

40

20

0 Human health

Ecosystem quality

Resources

Ammonia from hydropower electrolysis Ammonia from municipal waste electrolysis Ammonia from nuclear high temperature electrolysis Ammonia from biomass electrolysis Fig. 55 Damage assessment of ammonia (NH3) production methods according to Eco-Indicator 99. Data from Bicer Y, Dincer I, Zamfirescu C, Vezina G, Raso F. Comparative life cycle assessment of various ammonia production methods. J Clean Prod 2016;135:1379–95.

100 Human health

Ecosystem quality

Resources

90 80

32.2

6.1

70 Rank (mPt)

11.9

60

4.7

50 40 69.7

30

57.3

12.3 20 10

2.5

10.8 1.4

17.3

13.2

Ammonia from hydropower electrolysis

Ammonia from municipal waste electrolysis

0 Ammonia from nuclear high temperature electrolysis

Ammonia from biomass electrolysis

Fig. 56 Comparison of ammonia (NH3) production methods according to Eco-Indicator 99. Data from Bicer Y, Dincer I, Zamfirescu C, Vezina G, Raso F. Comparative life cycle assessment of various ammonia production methods. J Clean Prod 2016;135:1379–95.

As Fig. 55 indicates, NH3 production from the biomass-based method had the highest damage to human health and ecosystem quality, corresponding to 100%. Municipal waste-based NH3 production had the lowest impact on human health, ecosystem quality, and resources. The single score of NH3 production from the nuclear high-temperature method corresponds to 94.1 mPt, which was the maximum value of all methods. It was followed by the biomass-based NH3 production method, which was equal to 87.7 mPt for one kg of NH3 production as illustrated in Fig. 56. The lowest single score was calculated for the municipal wastebased NH3 production method, corresponding to 25.3 mPt, which can be considered as the most environmentally benign method.

1.3.6.4.2

Case study conclusions

A LCA of the selected NH3 production methods was conducted. Different resource-based NH3 production methods were thermodynamically analyzed and the energy and exergy efficiency values were comparatively assessed. It was concluded that LCA is an

Environmental Dimensions of Energy

99

important and reliable tool to study NH3 production methods as it covers the period from cradle to grave. The following concluding remarks were noted in this study:

• • • • • • •

NH3 production from biomass- and municipal waste-based water electrolysis provides a credible alternative for distributed NH3 production facilities and can enhance local fertilizer production capacity. Municipal waste-based NH3 production has the lowest abiotic depletion, global warming, and human toxicity values among all other methods, and as such can be evaluated as the most environmentally benign method. Hydropower-based NH3 production method gives highest SI and has the highest energy and exergy efficiencies. Nuclear high-temperature based NH3 production yields highest energetic and exergetic performance after hydropower-based NH3 production method. The energy efficiencies of hydropower-, nuclear-, biomass-, and municipal waste-based NH3 production methods are calculated as 42.7, 23.8, 15.4, and 11.7%, respectively. The exergy efficiencies of hydropower-, nuclear-, biomass-, and municipal waste-based NH3 production methods are yielded as 46.4, 20.4, 15.5, and 10.3%, respectively. The SI values of the hydropower-, nuclear-, biomass-, and municipal waste-based NH3 production methods are obtained as 1.866, 1.257, 1.183, and 1.115, respectively.

Renewable sources, with their improved efficiency, can reduce the overall environmental footprint and can replace the current fossil fuel-based, centralized NH3 production facilities.

1.3.7

Future Directions

Humans must act with more caution when they put in place and use energy conversion technologies. After two centuries of industrialization important lessons are learned, and one of most important learnings is that systematic development of energy system must thoroughly consider the environmental impacts. Therefore, development of meaningful tools to assess the complex environmental impact of energy system is terribly needed. There has been remarkable progress in this respect made by various researchers, especially by introducing exergy-related methods in life cycle assessment and exergoenvironmental analysis. Future directions are expected to include the development of compounded indicators for environmental impact assessment based of multiple criteria, development of better analysis method of the environmental impact in energy and transportation sectors, strengthen the link between environmental impact assessment, economic assessment, policy making and planing in any energy related developments. More impotently, there is a need to develop integrated models, methodologies, approaches, tools, parameters, indicators, indexes, metrics, etc. where the energetic, environmental, economic, social, educational, technical, ethical, and sustainability dimensions are incorporated to make comprehensive assessment tools.

1.3.8

Closing Remarks

In this chapter, the EIs of energy-related activities were discussed. Understanding the energy supply and demand is an important starting point for the analysis of global and regional impacts of any energy-related activity. The EIs of energy exploitation from mined resources and from renewable sources were also reviewed in the chapter. One major impact factor is due to the emission of GHGs, which influences the radiative energy balance at the Earth’s surface. Relevant pollutant emissions were revised from energyrelated sectors of activity. Development of sound policies in energy-related sectors becomes highly important. The development of cleaner energy solutions can be well encouraged by applying carbon taxation policies. Reductions in carbon emissions can be achieved if carbonfree synthetic fuels are used as an alternative to the current petrochemical fuels. Hydrogen will play a major role in this respect, helping to reduce the EIs caused by pollutant effluents. Exergoenvironmental analysis is a useful tool for the assessment of EIs of energy systems and processes. Basically, any exergycarrying stream (including the exergy destroyed) impacts the environment. Therefore exergy-based EI factors can be formulated, such as depletion factor or exergetic impact factor.

References [1] Dincer I, Zamfirescu C. Advanced power generation systems. New York, NY: Elsevier; 2014. [2] ISO International Standard 14042E. Environmental management – life cycle assessment life cycle impact assessment. Geneva: International Organisation for Standardisation (ISO); 2000. [3] IPCC. Climate change 2007: Synthesis report. Intergovernmental panel on climate change. Valencia: IPCC Plenary XXVII; 2007. [4] Lazzaretto A, Toffolo A. Energy, economy and environment as objectives in multi-criterion optimization of thermal systems design. Energy 2004;29:1139–57. [5] EIA. U.S. Energy Information Administration. Available from: http://www.eia.doe.gov/; 2008. [6] Jungbluts N, Bauer C, Dones R, Frischknecht R. Life cycle assessment for emerging technologies: case studies for photovoltaics and wind power. Int J Life Cycle Assess 2005;10:24–34. [7] Tiwari A, Joshi AS. Exergy analysis of the hybrid-PV/T air collector. In: World renewable energy conference IX and exhibition, Florence, Italy; 2006.

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[8] Zamfirescu C, Dincer I, Verrelli T, Wagar WR. Residential solar power generation systems for better environment. In: Proceedings of global conference on global warming, Istanbul, Turkey, Paper #805; 2008. [9] Naterer GF, Dincer I, Zamfirescu C. Hydrogen production from nuclear energy. New York, NY: Springer; 2013. [10] Iora P, Chiesa P. High efficiency process for the production of pure oxygen based on solid oxide fuel cell–solid oxide electrolyzer technology. J Power Sources 2009;190:408–16. [11] Dincer I, Rosen M. Thermodynamic aspects of renewables and sustainable development. Renew Sustain Energy Rev 2005;9:169–89. [12] Ozbilen A, Dincer I, Rosen MA. Environmental evaluation of hydrogen production via thermochemical water splitting using the Cu–Cl cycle: A parametric study. Int J Hydrog Energy 2011;36:9514–28. [13] Dincer I, Hogerwaard J, Zamfirescu C. Clean rail transportation options. New York, NY: Springer; 2015. [14] Bicer Y, Dincer I, Zamfirescu C, Vezina G, Raso F. Comparative life cycle assessment of various ammonia production methods. J Clean Prod 2016;135:1379–95.

Further Reading Afgan NH, Carvalho MG. Sustainable assessment method for energy systems: indicators, criteria and decision making procedure. Boston, NY: Kluwer Academic Publishers; 2000. Boyle G, editor. Renewable energy: power for a sustainable future, 3rd ed. Oxford: Oxford University Press; 2012. Cleveland JC, editor. Encyclopedia of energy 6. Amsterdam: Elsevier; 2004. Diesendorf M. Sustainable energy solutions for climate change. New York, NY: Taylor and Francis; 2014. Dincer I, Zamfirescu C. Sustainable energy systems and applications. New York, NY: Springer; 2012. Dincer I, Zamfirescu C. Sustainable hydrogen production. New York, NY: Elsevier; 2016. Grasman SE. Hydrogen energy and vehicle systems. New York, NY: CRC Press; 2013. Kreith F. Principles of sustainable energy systems. 2nd ed. Boca Raton, FL: CRC Press; 2013. Sieniutycz S. Energy optimization in process systems and fuel cells. 2nd ed. New York, NY: Elsevier; 2013. Sieniutycz S. Thermodynamic approaches in energy systems. New York, NY: Elsevier; 2016. Sovacool BK. Contesting the future of nuclear power. New Jersey, NJ: World Scientific; 2011. Tester JW, Drake EM, Driscoll MJ, Golay MW, Peters WA. Sustainable energy: choosing among options. 2nd ed. Cambridge, MA: MIT Press; 2012.

Relevant Websites https://www.bp.com/ BP. https://www.ec.gc.ca/?lang=En Government of Canada. https://www.google.ca/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8&ved=0ahUKEwjFqp2Ri_XTAhVmhlQKHbMhBUQQFggsMAA&url=https%3A%2F%2Fwww. ipcc.ch%2Fpdf%2Fassessment-report%2Far4%2Fsyr%2Far4_syr_full_report.pdf&usg=AFQjCNF1vMh4ycFe-SFefLEJe9E9PodSEQ&sig2=RZQRsETtxbkU7YTbIqjJXQ Intergovernmental Panel on Climate Change. https://www.iso.org/standard/37456.html International Organization for Standardization. hhttp://webbook.nist.gov/chemistry/ National Institute of Standards and Technology. http://www.statcan.gc.ca/eng/start Statistics Canada. http://databank.worldbank.org/data/home.aspx The World Bank. http://www.eia.doe.gov/ U.S. Energy Information Administration.

1.4 Sustainability Dimensions of Energy Ibrahim Dincer and Calin Zamfirescu, University of Ontario Institute of Technology, Oshawa, ON, Canada r 2018 Elsevier Inc. All rights reserved.

1.4.1 1.4.2 1.4.3 1.4.4 1.4.5 1.4.6 1.4.7 1.4.7.1

Introduction Sustainability and Energy Policy Sustainability Indicators Exergy-Based Sustainability Assessment of Energy Systems Sustainability Assessment Based on Exergetic Lifecycle Analysis Clean Energy Solutions for Better Sustainability Case Studies Exergosustainability Assessment of a Concentrated Photovoltaic-Thermal System for Residential Cogeneration Thermodynamic analysis Results Exergosustainability Assessment of High-Temperature Steam Photoelectrolysis Plant Thermodynamic analysis Results Exergosustainability Assessment of a Heat Pump Dryer System description Results Future Directions Closing Remarks

1.4.7.1.1 1.4.7.1.2 1.4.7.2 1.4.7.2.1 1.4.7.2.2 1.4.7.3 1.4.7.3.1 1.4.7.3.2 1.4.8 1.4.9 References Further Reading Relevent Websites

Abbreviations ASI COP EcI EI EPV ExCDR ExIE ExSI FF GF IP

Aggregated sustainability indicator Coefficient of performance Eco-indicator Environmental impact Energy product value Construction exergy expenditure Exergetic investment efficiency Exergetic sustainability index Filling factor Greenization factor Improvement potential

Nomenclature A AM Bi c C CC CIEx CM CP Cex cp Dp e

Area, m2 Amount of construction material, t Biot number Speed of light constant, m/s Cost Capital cost, $ Capital internment effectiveness Cost of construction materials, $ Cost of pollution, $ Exergy-specific price, $/kWh Specific heat, J/kg K Depletion number Universal electric charge, C

Comprehensive Energy Systems, Volume 1

LT NSI PBP PP QP RC RPC SEI SF SI SRW

Lifetime, years Normalized sustainability indicator Payback period, years Performance parameter Quality parameter Resource consumption Removal pollution cost Sustainability efficiency indicator Scaling factor Sustainability index Specific reversible work

E_ Ex _ Ex EEx fo&m h I IC Ind J kB L LExS m

Energy rate, kW Exergy, kJ Exergy rate Embodied exergy, GJ/t Operation and maintenant factor Planck constant Irradiance, W/m2nm Investment cost, $ Sustainability indicator Photocurrent density, A/m2 Boltzmann constant Length, m Levelized exergy cost savings, $/GJ Mass, kg

doi:10.1016/B978-0-12-809597-3.00104-8

102 105 111 119 122 130 133 133 135 137 138 139 141 142 142 144 150 150 151 151 151

101

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tyear U V w W _ W W

Number of hours in a year, h Utility Voltage, V Weighting factor Work, kJ Work rate, kW Moisture content, kg H2O/kg

Greek letters f Dimensionless moisture content F Quantum efficiency Z Energy efficiency

c s s T

Exergy efficiency Standard deviation Stefan–Boltzmann constant, W/m2K4 Transmittance

Subscripts 0 a abs act b d ex g gt hp in m oc

opt ORC out ph pr PV pw r ref rev tot th w

Optical Organic Rankine cycle Output Photocurrent Pollutant removal Photovoltaic Pollutant waste Reflector Reference Reversible Total Thermal Water

comb sep

Combined Separated

n n_ Q _ Q

Diode nonideality factor Molar flow rate, mol/s Quantity Heat rate, kW Specific emission, kg/t Specific exergy cost, $/GJ Time, s Temperature, K

SE SExC t T

Reference state Aperture Absorbed Actual Blackbody Destroyed Exergetic Band gap Gas turbine Heat pump Input Mass transfer Open circuit

Superscripts ch Chemical

1.4.1

Introduction

World population continues to grow and energy consumption is increasing. These are the facts documented based on historical observations. The world energy consumption in 2012 was 13,834 Mtoe, it grew to approx. 14,817 Mtoe in 2016 and is expected to escalate dramatically with continuing consumption and population increase trends [1]. There is no demonstrable prospect that concludes that the world population and energy consumption will stop growing. Arguably, energy resources will be depleted and humans will strive to harvest them. This again cannot be demonstrated. As time passed, more and more diverse resources were discovered, according to the records. It is clear that future society will be more structured, complex, and faster. It is also clear that future will consume more fuel and foods than the present, while moving faster and spending more power. Thence, energy will be consumed at a higher rate than that of today. Facilitating access to water and energy becomes crucial; they are the needs of any individual, group, or the world. The real challenge of the humankind is to evolve in a sustainable fashion. Modern society requires many types of services to maintain a good standard of living, having electricity, hot water, space heating, air conditioning, fuels, various chemicals, and materials, etc. Traditional methods of producing these commodities are primarily driven by combustion of fossil fuels, which is a major contributor to air pollution. The global population and its demand for these services are rapidly increasing, so the rising use of fossil fuels is having a major impact on climate change. We live in a period characterized by accentuated transition efforts from fossil fuel-based economy toward a projected economy based on sustainable energy. The development of global society passed through the fossil fuel era, when most primary energy was obtained by combustion of coal, petroleum, and natural gas. At present, there is a larger panoply of primary energy sources including nuclear power and hydroelectricity; in addition, other forms of renewable energy gain terrain, like wind, solar, biomass combustion, and biofuels for transportation. It is projected that the development of a hydrogen-fuel infrastructure for transportation will be needed. In the near future, the combustion of fossil fuels will be performed in a cleaner fashion such that zero-emission power generation will be achieved at an

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attractive cost. Hydrogen is a clean energy carrier because it can be generated by low or zero-carbon sources (e.g., nuclear, water, biomass, solar). Subsequently hydrogen can be converted to electricity, synthetic fuels, or heat with little or no environmental pollution. Additionally, hydrogen is an important chemical that enters the constituency of many materials used in industry and society: plastics, foods, pharmaceuticals, fertilizers, metallic materials, construction materials, etc. In cleaner generation of hydrogen, less pollution is generated from the production of such hydrogen-containing other materials. Therefore, hydrogen itself can be viewed as a critical commodity for future society, which can replace or at least complement the fossil fuels (oil, coal, natural gas) that are extensively used at present. In this section, we will follow the historical evolution of the concept of “hydrogen as clean energy carrier,” emphasizing the main issues and achievements to date. Consider the history of primary energy that society uses for supplying its needs. There are three types of energies to be considered, namely, the energy of coal, fossil hydrocarbons (petroleum, oil shale, oil sands, etc. and natural gas), and that of renewable/sustainable sources that are “zero-carbon” sources. The renewable/sustainable energies are those that do not release greenhouse gas (GHG) emissions upon their use (although some GHGs are emitted indirectly – e.g., due to the construction of an energy distribution infrastructure). Typical examples of “zero-carbon” energies are biomass, hydroelectric, wind energy, solar, nuclear, and others. The International Environmental Agency describes renewable energy as the energy derived from natural processes using continually replenished sources [1]. The authors compiled data from existing literature to develop the chart shown in Fig. 1 representing the share of “zero-carbon,” coal, and fossil fuel energy use worldwide since 1800 until the present time. The chart is extrapolated until 2100 based on a predictive model described in Ref. [2]. Before the first industrial revolution (1800), the world energy supply comprised only “zero-carbon” sources: wood, wind, hydro, and animal power. Starting with the industrial revolution and the introduction of the steam engine, the consumption of coal increased sharply. One can speak about a “mechanization” period of around 100 years during which Western civilizations implemented an industrial society. There was remarkable technical and scientific progress in this period, especially in England and France. By 1900, another source of energy started proliferating in society, namely, petroleum. Furthermore, the advances in electric power generation and the electrical grid propelled a second industrial revolution around 100 years ago, which was called the “electrification” period. With the help of electricity, process devices like pumps, compressors, conveyers, mixers, etc. became more effective, which encouraged the sharp development of the process industry. Germany is noted in the beginning of the 20th century for important inventions in the chemical and process industry. One of the most important processes ever invented is the Haber–Bosch ammonia synthesis process (1913). Manufacturing of ammonia by this process became a major user of hydrogen in the world. Further evolution of the process led to synthesis of urea at an industrial scale, which is a major fertilizer produced from ammonia and carbon dioxide. In the Ruhr region of Germany, a high development of coal mining and metallurgy occurred at the beginning of the 20th century. Hydrogen was highly needed by the metallurgical sector. Consequently, by 1938, a hydrogen pipeline of 240 km was installed to deliver 250  106 Nm3/year of hydrogen obtained from coal. Presently in Europe, there are about 1500 km of hydrogen pipelines, while in the United States, there are about 900 km of hydrogen pipelines. The geopolitical context in the first half of the 20th century encouraged the development of engines and fuels for transportation. Gasoline gained in proportion due to its high octane number, its high energy density, and cheap process of extraction from

100 Coal Fossil hydrocarbons Zero carbon

Energy share (%)

80

60 Future 40

20

0 1800

1850 Mechanization

1900

1950 Year Electrification

2000

2050 Hydrogenation

Fig. 1 Shares of world energy sources since the start of the first industrial revolution, and prospects to 2100.

2100

100

12

80

10

60

8

40

6

20

0 1800

Meq (kg/kmol) Carbon (thick line) Carbon-free 1850

1900

1950 Year

2000

2050

Meq (kg/kmol)

Sustainability Dimensions of Energy

Energy share (%)

104

4

2 2100

Fig. 2 Illustration of the historical trend of energy decarbonization.

crude petroleum. Currently, the road transportation sector uses mostly gasoline and diesel and the marine transport industry uses heavy petroleum or diesel. Hydrogen has found uses in space transport as liquefied rocket fuel. The intellectual activity and the acquired technological and scientific knowledge created the start of a new 100-year era, which will follow the “mechanization” and “electrification” eras. This has been often cited as the “hydrogenation” of society. According to predictions suggested as shown in Fig. 1, during the next 100 years, the “zero-carbon” energy sources will increase in share and a “hydrogen civilization” will emerge. The rise of a hydrogen civilization is induced by preemptive actions founded on scientific analyses regarding the negative impact of GHG emissions on climate and the depletion of fossil fuel resources. The chart in Fig. 2 shows how a basic analysis which predicts the decarbonization of the world energy supply. The plot is made under the assumption that the average carbon content of fossil hydrocarbons (mainly natural gas and petroleum) is around 1:3, while 2:3 is the hydrogen content (the carbon to hydrogen ratio for methane is 1:4, while for gasoline it is B1:2). Therefore the carbon content of the global energy supply is obtained from Fig. 1 by addition of the coal curve to one-third of the fossil hydrocarbon curve. The noncarbon share of the energy supply is obtained by adding 2/3 of the fossil hydrocarbon curve with the zero carbon curve. The plot shows how carbon-free sources of energy decreased from 1800 to approx. 1950; and again increased and are predicted to further increase. Accounting for molecular mass of carbon (12 kg/kmol) and hydrogen (2 kg/kmol) the averaged molecular mass of the fuel sources (carbon-based + carbon-free) is determined. As shown in the figure, before 1800 the molecular mass of the equivalent fuel sources has been 2 kg/kmol, illustrating that only negligible fossil carbon sources of energy were used before the industrial revolution. This equivalent molecular mass increased to a peak of around 11 kg/kmol in the first half of 20th century, illustrating that the world depended mostly on coal at that time. That has been the prominent era of steam engine. The curve climbed down, being today at a figure of approx. 6 kmol/kg; this number shows a mix of fuel sources, which includes mostly petroleum liquids. The prediction to 2100 shows that the equivalent molecular mass of world fuel is around 3 kg/kmol, that is, very close to the hydrogen mass of 2 kg/kmol, an almost fully decarbonized energy supply. The curve (Meq) shows an interesting feature: it sharply rises during the coal-dependent “mechanization era,” decreases with fluctuations during the “electrification” era, and steadily decreases during the future “hydrogenation” era. This downward character toward energy decarbonization is a result of societal will demonstrated during the last 40 years. Sustainable development is the natural way of development. It is so because to be sustainable is the only way in which anything persists: sustainable can be translated as “persistent in time.” The quest is how to guide and influence the sustainable development of the world. Energy sustainability means harmonious and rational development at three levels: power generation, power distribution, and power utilization (consumption and destruction). Among other aspects, sustainability implies facilitating the flows of knowledge (know-how), which became crucially important; see also Ref. [31]. At the beginning of the 21st century, a new branch of science emerged, namely, sustainability science, having an interdisciplinary character involving the following main disciplines: physics, chemistry, biology, medicine, social and economic sciences, and engineering. One of the goal of sustainability science is to model the complex interactions between society, economy, and environment, also accounting for resource depletion. One crucial aspect is the provision of theoretical foundations and tools for sustainability assessment. Science became more and more the dominant driving force influencing the development of society. Due to science and the establishment of the information era, a global self-consciousness is developed – the so-called noosphere – that made humans aware of possible catastrophic scenarios related to the potentially unsustainable development of technology. National and international institutions were created in the last quarter of the 20th century to promote sustainable development of society. As such, the World Commission on Environment and Development acted within the United Nations for a period of

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5 years in mid-1980s and defined formally the concept of sustainable development and showed that societal development and the environment are interrelated for any foreseeable time horizon. Sustainable development has been defined by the United Nations as “development that meets the needs of the present without compromising the ability of future generations to meet their own needs.” Due to its wide character, spanning from environment to society, sustainability is difficult to assess based on quantifiable indicators. Some efforts to categorize the tools for sustainability assessment are reported by Ness et al. [3]. In the past, attempts were made to formulate empirical indicators for sustainability assessment, such as “environmental sustainability index,” “wellbeing index,” “human development index,” etc. More recently, product-related assessment methods were proposed based on product material flow analysis, lifecycle assessment, and thermodynamic analysis. Among those, the sustainability, environmental and energy assessment based on exergy analysis is noted for furthering the goal of more efficient resource use with reduced waste rejection into the environment. Many factors contribute to achieving sustainable development. One of the most important is the requirement for a supply of energy resources that is fully sustainable. Increased efficiency is also considered critically important. Kyoto Protocol, in 1997, envisaging to control GHG concentrations in the atmosphere, is intimately related to the goal of sustainable development achievement. Availability and utilization of energy resources are highly related to the level of sustainable development. Sustainable energy supplies are required for sustainable development. These type of resources must be available for a reasonably foreseeable future at an affordable cost and with reasonable access, without causing negative societal and environmental impacts (EIs). Therefore better design of energy systems must be developed with increased efficiency and reduced fossil fuel consumption such that (1) less primary energy resources are used, and (2) reduced pollutants, such as GHG (i.e., GHGs, such as CO2, CH4), acid gases (SO2, NOx responsible for acid precipitation), and chlorofluorocarbons (CFCs, responsible for destroying the stratospheric ozone) are emitted. Considering the significance of energy conversion, power generation, distribution, and utilization in the global economy, any improvement of designs toward efficient and environmentally benign drying technology will help the sustainable development of society. In this respect, exergy analysis plays a major role, as reported in [4], because it connects sustainability with energy and the environment. In this chapter, the sustainability aspects of energy systems are discussed. There are various aspects to be analyzed in this respect. We will discuss in this chapter the theoretical assessment tools and indices for sustainable development and their impacts in shaping energy policies and strategies, which ultimately influence the progress of society toward better sustainability. The sustainability concept and its assessment methods are introduced with a focus on exergy-based assessment. Some case studies are provided as well.

1.4.2

Sustainability and Energy Policy

Sustainability refers to the feature of a system to be sustainable. If we refer to human-made (engineered) systems, a system to be sustainable must be conceived such that it does not permanently damage the environment, and it does not consume excessive natural resources, rather it operates effectively for a sufficiently long period of time without jeopardizing the ability of future generations to meet their needs of natural resources and clean environment. To be truly sustainable, an energy system must meet the following criteria: (1) minimal or no negative environmental or social impact; (2) no natural resource depletion; (3) able to supply the current and future population’s energy demand; (4) equitable and efficient; (5) air, land, and water protection; (6) little or no net carbon or other GHG emissions; and (7) safety today without burdening future generations. There are of course several other criteria to consider, such as abundance, local availability, cost effectiveness, reliability, safety, and environmental friendliness. The march toward a global sustainable society is importantly influenced by policy and decision makers. Fig. 3 shows how policy and decision makers influence the way in which the world’s future is shaped toward sustainability provided that appropriate theoretical tools for sustainability assessment are made available and green technologies are developed. Behind any policy and

Assessment tools (exergy, sustainability index, etc.)

Present: transition period from fossil fuel economy

Policy and decision makers

Technological development

Fig. 3 The role of assessment tools and indicators in shaping the future sustainable society.

Future: sustainable energy society

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decision, there must be a rationale support, which constrains the decision spectrum. Apart from various ideologies that do exist, the status of technological development and the existent theoretical assessment and prediction tools play a major role in influencing policy making and high-level decisions. When shaping sustainable energy policies and strategies, it is important to understand the benefit of clean energy technologies at one hand, and the effects of fossil fuel-based systems on the other. The kinds of energy technologies with zero or minimum negative EI (e.g., through associated pollution) – that is, more environmentally benign and more sustainable – are in general named green energy. Considering the benefits of green energy, sustainability of green energy supply and progress is assumed to be a key element in the interactions between nature and society. An essential increase in the scale and pace of policy instruments and their effectiveness is required to change course toward a sustainable path. It is important to possess theoretical tools to quantitatively assess green energy development and:

• • • • • • • •

help elaborate rationale strategies and policies; help understand main concepts and issues about green energy use and sustainability aspects; develop relations between green energy use and sustainability development; encourage the strategic use and conservation of the green energy sources; provide the methods for energy security, implementation and development; increase the motivation on the implementation of green energy strategies for better energy supply; give an idea to reduce the negative EIs by considering the possible green energy strategies; and form a scientific platform to discuss the possible green energy strategies for sectoral use.

The need of policy is justified by the fact that free markets must be enforced to meet the needs of vulnerable groups, reduce environmental pollution, and ensure the energy security. At upper level, strategies are needed; where a strategy represents a plan or method toward a goal (in our discussion “the goal” means progress toward a global society based on sustainable energy). Once a strategy is established, the policy, which represents the course of actions to implement the strategy, must be elaborated. Any policy will specify “policy instruments,” which represent specific measures taken to implement a policy. Examples of policy instruments are:

• • • • •

imposing efficiency standards; setting public procurement policies; imposing appliance labeling norms; obligation to buy or supply energy from renewable sources; and supporting research and development in demonstration projects.

Some examples of promising policies are given here as follows: phasing out subsidies for fossil fuel-based energy, restructuring the energy sector, supporting energy sector innovation, promoting energy efficiency, financing rural energy, providing decentralized options, and improving access to modern and efficient cooking fuel. Some other rule of thumb aspects that influence elaborations of green energy strategies and policies can be summarized as follows:

• • • • • •

Competition among sustainable energy technologies may be hampered by market distortions that give advantages to certain players. Improving energy efficiency requires less investment than does new generation. Financing large energy projects discourages smaller renewable energy projects. Commercialization of green energy technologies is not occurring quickly enough to meet the challenges of sustainability. For significant sustainable progress, a critical mass of public support is needed. It is important to liberalize energy markets in order to offer an opportunity to change.

The sustainable development is at the confluence of energy resources sustainability, environmental sustainability, economic sustainability, and social sustainability as shown in Fig. 4. Green energy resources and technologies are a key component of sustainable development for three main reasons. Firstly, they generally cause less EI than those of other energy sources. The variety of green energy resources provides a flexible array of options for their use. Secondly, they cannot be depleted. If used carefully in appropriate applications, green energy resources can provide a reliable and sustainable supply of energy almost indefinitely. Thirdly, they favor system decentralization and local solutions that are somewhat independent of the national network, thus enhancing the flexibility of the system and providing economic benefits to small isolated populations. Also, the small scale of the equipment often reduces the time required from initial design to operation, providing greater adaptability in responding to unpredictable growth and/or changes in energy demand. The major considerations involved in the development of green energy technologies are illustrated in Fig. 5 and include social and EIs, commercialization, technical aspects, and economic factors. Apart from these considerations, one can identify a number of parameters (factors) that are important in establishing green energy strategies and policies. They include public information, environmental education, innovation stimulation, promotion of technologies, financing, and very important elaboratory evaluations tools and techniques. Green energy technologies are expected to play a key role in sustainable energy scenarios for the future. The foremost factor that will determine the specific role of green energy and technologies will likely be energy demand. Therefore, in order to compensate

Sustainability Dimensions of Energy

Social sustainability

107

Environmental sustainability

Sustainable development

Energy and resources sustainability

Economic sustainability

Fig. 4 Interdependence of the factors affecting sustainable development. Modified after Dincer I, Rosen MA. Thermodynamic aspects of renewable and sustainable development. Renew Sustainable Energy Rev 2005;9:169–89.

Increase of green energy and technology utilization

Commercialization Research and development, information technology, incentives, training, education, communication, etc.

Social and environmental impact Social benefits, global peace, environmental impact, higher living standard, clean air and environment, etc Technical aspects Availability, grid connection, technology level and use, technological innovations, advanced technologies, etc.

Economic factors Investments, generation costs, lower operation cost, lower cost energy recovery, lower cost transportation, externalities, etc.

Fig. 5 Considerations involved in development of green energy technologies. Modified after Dincer I, Rosen MA. Thermodynamic aspects of renewable and sustainable development. Renew Sustainable Energy Rev 2005;9:169–89.

the energy requirement, it will be possible to produce green energy from renewable energy sources, such as hydraulic, solar, wind, geothermal, wave, biomass, etc. Green energy technologies are largely shaped by broad and powerful trends that have their roots in basic human needs. Wastes (convertible to useful energy forms through, e.g., waste-to-energy incineration facilities) and biomass fuels are also usually viewed as sustainable/green energy sources. In conjunction with this, the increasing world population requires the definition and successful implementation of green energy technologies. Briefly, the important parameters and their interrelations as outlined in Fig. 6 are definitely required to carry out the best green energy program and select the most appropriate green energy technology/technologies for sustainable development. The green energy and technologies can be utilized for many applications as shown in Fig. 7. Thus it can be said that green energy and technologies are expected to play a key role in sustainable energy scenarios for the future. The foremost factor that will determine the specific role of green energy and technologies will likely be energy demand. Therefore, in order to compensate the energy requirement, it will be possible to produce green energy from renewable energy sources, such as hydraulic, solar, wind, geothermal, wave, biomass, etc. If so, the green energy and technologies can be utilized for many applications as shown in the figure. Thus it can be said that green energy and technologies, which are abundantly available, can help to:

• • • •

provide a more environmentally benign and more sustainable future; increase energy security; facilitate or necessitate the development of new, clean technologies; reduce air, water, and soil pollution and the loss of forests;

108

Sustainability Dimensions of Energy

Successful green energy program

Public and government channels

Agencies, training, facilities

Professional support Media support Public support

Energy utilization Environmental impact

Sustainable green energy program

Public information/ awarness

Communication

Environmental education and training

Public

Essential factors for green energy technologies

Innovative green energy strategies

Promoting green energy resources

Effective sustainable green energy program

Short and long term policies for implementation

Public relations, training, counseling

Environmentally benign green energy programs

Financing

Evaluation tools and techniques

Implementation of green energy systems and technologies locally and globally for sustainable development

Monitor each step and evaluate the data findings obtained

Fig. 6 Main factors influencing sustainable energy strategies and policies. Modified after Midilli A, Dincer I, Ay M. Green energy strategies for sustainable development. Energy Policy 2006;34:3623–33.

• •

reduce energy-related illnesses and deaths; and reduce or stop conflicts among countries regarding energy reserves, etc.

Therefore green energy and related technologies are needed to ensure global stability by reducing the harmful effects of fossilbased energy consumption. Thus the importance of green energy in reducing the world’s problems and achieving a sustainable energy system should be emphasized considering the sustainable green energy strategies, and a transition to green energy economy, should be encouraged, and developed countries, in particular, should increase investments in green energy and technologies. In order to develop and publicize the sustainable green energy technologies in a developed or less developed country, the following important green energy strategies should be taken into consideration:

• • • • • • •

industrial and technological support for transition to green energy technologies; control of the projection and analysis of green energy sources; governmental and public support for green energy economy; production, consumption, distribution, conversion, management, and marketing of green energy; research, development, and application of sustainable green energy technologies; availability, productivity, and reliability of green energy and technologies; and design and fabrication of green energy-based environmental and ecological applications.

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109

Application areas of green energy and technologies • fuel cells • gas turbines • hydrogen plants • heating • cooling • cooking • air conditioning • pumping, etc.,

• ammonia synthesis • fertilizer production • oil distillation • petrochemical production • metallurgical applications • ni and Fe production • energy storage • flammable mixtures • electronic industry • glass and fiber production • nuclear reactors • power generation systems

Applications for power generation

Vehicle applications

Domestic applications

Industrial applications

Navigation applications

Aeronautics application

• fuel cells • IC engines • combustion • efficiency improvement • defense industry • transport • power generation • ship engines • defense • communication • transportation • tourism • pollution control • energy storage • gas turbines • jet engines • defense industry • rockets • antimissile • space industry • energy storage

Fig. 7 Applications of green energy technologies. Modified after Midilli A, Dincer I, Ay M. Green energy strategies for sustainable development. Energy Policy 2006;34:3623–33.

Energy conservation is vital for sustainable development, and for the best benefit of present and future generations, should be implemented by all possible means. A secure supply of energy resources is generally agreed to be a necessity but not a sufficient requirement for development within a society. Furthermore, sustainable development demands a sustainable supply of energy resources that, in the long term, is readily and sustainably available at a reasonable cost, and can be utilized for all required tasks without causing negative societal impacts. Supplies of such energy resources as fossil fuels (coal, oil, and natural gas) and uranium are generally acknowledged to be finite; other energy sources, such as sunlight, wind, and falling water, are generally considered renewable and therefore sustainable over a relatively long period of time. Here, we look at the renewable energy resources and energy conservation. While not all renewable energy resources are inherently clean, there is such a diversity of choices that a shift to renewables carried out in the context of sustainable development could provide a far cleaner system than would be feasible by tightening controls on conventional energy. Furthermore, by being naturally site-specific, they favor power system decentralization and locally applicable solutions more or less independent of the national network. It enables citizens to perceive positive and negative externalities of energy consumption. Consequently, a small scale of equipment often makes the time required from initial design to operation short, providing greater adaptability in responding to unpredictable growth and/or changes in energy demand. The exploitation of renewable energy resources and technologies is a key component of sustainable development. There are three significant reasons for this as follows:

• • •

They have much less EI compared to other sources of energy because there is no energy source with zero EI. There are such a variety of choices available in practice that a shift to renewables could provide a far cleaner energy system than would be feasible by tightening controls on conventional energy. Renewable energy resources cannot be depleted unlike fossil fuel and uranium resources. If used wisely in appropriate and efficient applications, they can provide reliable and sustainable supply energy almost indefinitely. In contrast, fossil fuel and uranium resources are finite and can be diminished by extraction and consumption. They favor power system decentralization and locally applicable solutions more or less independent of the national network, thus enhancing the flexibility of the system and the economic power supply to small isolated settlements. That is why many different renewable energy technologies are potentially available for use in urban areas.

Taking the aforementioned reasons into consideration, the relations between energy conservation and sustainability are finally presented as in Fig. 8. As suggested in the figure, energy resources and their utilization are intimately linked to sustainable development. For societies to attain or try to attain sustainable development, much effort must be devoted not only to discovering sustainable energy resources, but also to increasing the energy efficiencies of processes utilizing these resources. Under these

110

Sustainability Dimensions of Energy

Energy sustainability

Sustainability program

• availability of energy • energy management • energy production and consumption • energy conservation and distribution • productivity of energy

• sustainable energy strategies • innovative energy strategies • sustainable energy programs • short-long term energy policies • programs for clean energy

Energy conservation and sustainability

Economic sustainability

Environmental sustainability

• availability of energy • energy management • energy production and consumption • energy conservation and distribution • productivity of energy

• political support • reliability/knowledge investments • environmental planning • marketing innovations • environmental control mechanisms

Fig. 8 Linkages between energy conservation and sustainable development.

circumstances, increasing the efficiency of energy-utilizing devices is important. Due to increased awareness of the benefits of efficiency improvements, many institutes and agencies have started working along these lines. Many energy conservation and efficiency improvement programs have been and are being developed to reduce present levels of energy consumption. To implement these programs in a beneficial manner, an understanding is required of the patterns of “energy carrier” consumption, for example, the type of energy carrier used, factors that influence consumption, and types of end-uses. Environmental concerns are an important factor in sustainable development. For a variety of reasons, activities that continually degrade the environment are not sustainable over time, for example, the cumulative impact on the environment of such activities often leads over time to a variety of health, ecological, and other problems. A large portion of the EIs in a society are associated with its utilization of energy resources. Ideally, a society seeking sustainable development utilizes only energy resources that cause no EI (e.g., that release no emissions to the environment). However, since all energy resources lead to some kind of EI, it is reasonable to suggest that some (not all) of the concerns regarding the limitations imposed on sustainable development by environmental emissions and their negative impacts can be in part overcome through increased energy efficiency. Clearly, a strong relation exists between energy efficiency and EIs because, for the same services or products, less resource utilization and pollution is normally associated with increased energy efficiency. The share of energy R&D expenditures going into energy conservation grew greatly since 1976, from 5.1% to 40.1% in 1990 and 68.5% in 2002. This indicates that within energy research and development, research on energy conservation is increasing in importance. When R&D expenditures on energy conservation are compared with expenditures for research leading to protection of the environment in the 2000s, the largest share was spent on environment research. In fact, it is not easy to interpret the current trends in R&D expenditures because energy conservation is now part of every discipline from engineering to economics. A marked trend was observed since the mid-1970s, in that expenditures for energy conservation research grew significantly both in absolute terms and as share of total energy R&D. They also grew more rapidly than environmental protection research, surpassing it in the early 1980s. Therefore if R&D expenditures reflect long-term concern, there seems to be relatively more importance attached to energy conservation as compared to environmental protection. In addition to the general trends discussed above, consider the industrial sector and how it has tackled energy conservation. The private sector clearly has an important role to play in providing finance that could be used for energy efficiency investments. In fact, governments can adjust their spending priorities in aid plans and through official support provided to their exporters, however, they can only indirectly influence the vast potential pool of private sector finance. Many of the most important measures to attract foreign investors include reforming macroeconomic policy frameworks, reforming energy market structures and pricing, banking reform, debt recovery programs, strengthening the commercial and legal framework for investment, and setting up judicial institutions and enforcement mechanisms. These are difficult tasks that often involve lengthy political processes. Thus Fig. 9 presents a series of important factors that can contribute to improving energy conservation in the real life. Although there are a large number of practical solutions to environmental problems, three potential solutions are given priority as follows:

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111

Improvement factors of energy conservation

Understanding energy conservation concepts

Understanding key energy-related concepts

Current energy usage

Energy-related operation and maintenance control

Assessment of energy conservation opportunities

Goal and scope definition, energy management program, funding opportunities, consumer-consultant relations, local and industrial energy improvements.

Assessment of energy conservation opportunities, cost-effectiveness of energy utilization, simple-payback of energy conservation, energy conservation measurements, energy saving potentials, and individual energy conservation.

Energy consumption and sources, energy costs, energy recovery possibilities, energy saving, energy production capacity, energy conversation, and energy-related industries.

Energy conservation in maintenance and operations, energy saving and cost effectiveness, staff education, energy education cost, energy recovery units and techniques, and application of energy conservation measurements.

Energy systems and equipments survey, current energy use analysis, recognition of energy saving in local and industrial applications, encourage of energy saving, social and technical investment for energy conservation, and completion of cost/benefit energy conservation.

Fig. 9 Improvement factors of energy conservation for better sustainability.

• • •

energy conservation technologies (efficient energy utilization); renewable energy technologies; and cleaner technologies.

Among these technologies, we pay special attention to energy conservation technologies and their practical aspects and EIs. Each of these technologies is of great importance and requires a careful treatment and program development. Considering the above priorities to environmental solutions, the relevant technologies are summarized in Fig. 10.

1.4.3

Sustainability Indicators

The sustainability indicators are constructed based on the drivers-pressures-state-impact-responses (DPSIR) model of the society, as shown in Fig. 11. In this model, the drivers are the developments in society, economy, and the environment. These developments (or changes) exert pressures on the sustainability, and as a consequence, the sustainability state changes in some direction. This eventually leads to foreseeable impact on sustainable development: it improves or degrades the sustainability. The society can respond to this change by taking actions to reverse the impacts (see the double arrow in the figure). In addition, the society can send feedback to drivers (i.e., to impose educated changes in the society, economy, and environment). The responses can also act directly on the pressures and on the state of sustainability by applying adequate measures if possible. The responses must be effective, because their effectiveness affects directly the sustainable development. Two simplified DPSIR models exist, which apply to sustainability assessment, namely, drivers-state-responses (DSR) or pressure-state-response (PSR). Any conceivable sustainability indicator has some degree of subjectivity because it is not actually possible to objectively determine and fully understand the interrelationships between social change, environment, energy, and economic development. The independent formulation of figures of merit for energy systems and formulation of environmental, economic and social indicators is more facile. The

112

Sustainability Dimensions of Energy

Environmental information services Energy and environment management centers

Research and development centers for environmental problems Potential solutions to environmental problems

Energy conservation technologies

Renewable energy technologies

Clean environment technologies

Pollution control technologies

Green buildings Energy efficiency technologies

Greenhouse gas reduction technologies

Waste and soil management technologies

Recycling technologies

Fig. 10 Linkages between possible environmental and energy conservation technologies.

Drivers

Pressures

State

Impact

Responses

Fig. 11 Drivers-pressures-state-impact-responses (DPSIR) model for sustainability assessment.

United Nations developed more than 130 indicators for sustainability assessment of which 30% are social indicators, 17% are economic indicators, 41% are environmental indicators, and the remaining 12% represent institutional indicators. Approximately 33% of sustainability indicators are of the driving force (drivers) kind; 39% are state indicators, and 18% are response-kind indicators. Table 1 gives the significant indicators that can be used to elaborate aggregate sustainability indexes (SIs). In principle, the indexes can be elaborated according to the purpose of the analysis. Some specific aspects on elaboration and adoption of a holistic sustainability assessment methods are discussed as follows:

• • • • • •

Sustainability assessment should integrate ecological conditions assessment of human and life systems habitat with economic development, social wellbeing, and with equity and disparity of current population and future generations. Obtain a consensus on adoption of a time horizon sufficiently long to be in accordance with ecosystem time scale and anticipated society time scale. Sustainability indicators and indices must be able to link sustainability categories to society goals. Sustainability assessment should integrate the following category of models: economic models, stress–response models, multiple capital models, social–economic–environmental models, and human–ecosystem wellbeing models. Definition of a reference environment (or state) is required for determining the sustainability change. Monetary assessment of environmental damage should be included in sustainability assessment.

Fig. 12 represents a generic model of an energy system, which is useful for sustainability assessment. The energy system is a generic one. It may be a power plant where chemical energy of a fuel is converted to electric power. Other energy conversions can be nuclear-to-power, wind-to-power, hydraulic pressure-to-hydropower, solar-to-power, and so on. Beside energy conversion for power generation, the energy system can also include power distribution. There are two categories of inputs into the energy system, as shown in the model from the figure: the resources and the process enablers. The process enablers are of three kinds: the human operators, the energy system (including all machines and technical

Table 1

Categories and types of indicators influencing the sustainability assessment

Kind

Drivers indicators

State indicators

Responses indicators

Category Social

Economic

Environmental

unemployment rate population growth rate adult literacy rate motor fuel consumption per capita loss rate due to natural disasters per capita GDP investment share of GDP annual energy Consumption natural resource consumption (RC) capital goods imports water consumption per capita generation of wastes ozone-depleting potential (ODP) substances emission emission of greenhouse gas (GHG), SO2, NOx energy use in agriculture

• • • • • • • • • • • • • • • • • • • •

poverty gap index income inequality index school life expectancy house price per income floor area per person proven mineral reserves fossil fuel reserves lifetime (LT) of energy reserves share of renewable energy manufacturing added value ground water reserves monthly rainfall index desertification rate pollutants concentration acute poisoning number of scientists per capita number of engineers per capita internet access per capita telephone line access other information channels

Source: Reproduced from UN. Indicators of sustainable development: guidelines and methodology. 3rd ed. New York, NY: United Nations.

• • • • • • • • • • • • • • • • • • • •

gross domestic product (GDP) on education childhood immunization infrastructure expenditure per capita health expenditure per capita hazardous chemicals in foods environmental protection expenditure funding rate on sustainable development amount of funds on sustainability percentage of new funds on sustainability funding grants on technology wastewater treatment expenditures natural resource management air pollution mitigation expenditures waste management expenditures number of restricted chemicals policy on sustainable development environment protection programs GDP share of R&D expenditures number of R&D personnel per capita ratification/implementation of agreements

Sustainability Dimensions of Energy

Institutional

• • • • • • • • • • • • • • •

113

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Process enablers: Operators • safety aspects • health impact • education aspects Energy system related • system type/capacity • life cycle • construction related emissions

Input resources: Construction materials and labour • metallic materials • concrete, wood • advanced materials

Environment • temperature • pressure • composition

Energy system

Oxidant (if applies) • humidity • temperature • flow rate Energy input • work (or electric power) • heat

Process outputs: Exergy • electric power • process heat • synthetic fuels • materials Recyclable or disposable streams • rejected heat • emissions • expelled water/moisture

Wastes: Pollutants emissions • GHGs • NOx and SO2 • other pollutants Energy wastes • heat wasted • waters heating due to heat rejection at power generation Material wastes • wasted moisture • wasted process material • indirect garbage • landfill wastes resulting from energy system scrapping

Fig. 12 Sustainability assessment model of energy systems based on material and energy balances. GHG, greenhouse gas. Modified after Linke B, Das J, Lam M, Ly C. Sustainability indicators for finishing operations based on process performance and part quality. Procedia CIRP 2014;14:564–9.

components), and the surrounding environment (characterized by certain temperature, pressure, and composition). With respect to the human operators, the sustainability indicators must mainly quantify safety, health, and educational aspects. Related to the energy system, the sustainability indicators must consider the lifecycle and the specific environmental emissions and impact related to the system construction. The input resources are of three kinds: construction materials and labor for system fabrication; oxidant (if applicable), for example, combustion air; and energy (power). For each of the inputs relevant indicators, quantities, and qualities can be determined. For example, the construction materials are characterized by type, specific texture, shape, and dimensions. The energy input is required in the form of work (power) and heat (heating rate). The outputs are the actual process outputs and the process wastes. The main output of an energy system is typically the generated power. Three types of wastes can be considered: environmental pollutants, energy wastes, and material wastes. The pollutant emissions are due mainly to combustion processes directly or indirectly associated with the respective energy conversion system. Also, the emission of landfill gas due to drying system scrapping at the end of the lifetime (LT) is a pollutant waste. The

Sustainability Dimensions of Energy

115

Technological knowhow and advancements

Present: Fossil fuel-based economy in transition

Policy and decision making

Sustainability assessment indicators

Planned future: sustainable energy society

Energy and exergy efficiency, sustainability index, greenization factor, other performance indicators

Fig. 13 The role of sustainability assessment indicators in shaping policies for a planned sustainable energy society.

energy wastes can be observed in the form of heat rejected into the environment. This heat can be rejected directly by heat transfer at the system boundary. Another way of energy waste is through warm water released in lakes, rivers, or seas where power plants are installed. It is demonstrated that the cooling water for power plant condensers affect the aquatic environment by changing the local temperature, which often negatively impacts the life systems. Noise is another way of energy waste with EI. The material wastes include also landfill material generated at product scrapping phase. Apart from various ideologies that do exist, the status of technological development and the existent theoretical assessment and prediction tools play a major role in influencing policy making and high-level decisions. Some examples of energy policies and strategies include encouraging the expansion of energy- and exergy-efficient systems, expanding the use of renewable energy, and widening clean fossil fuel combustion technologies. Behind any policy and decision there must be a rationale support, which constrains the decision spectrum. Fig. 13 illustrates how sustainable development strategies can be shaped with the help of assessment of and advancement in technology. The implications of the limited nature of energy and other resources have led in part to significant efforts in energy-utilization efficiency improvement, and resource recycling and reuse. For example, refuse and other solid wastes are now often used to supplement fuel supplies. Recycling (resource recovery) extends the LTs of many of natural resources, and is often profitable and usually beneficial environmentally. Energy efficiency and conservation can postpone shortages of energy resources, reduce environmental damage, and provide economic benefits. The efficient use of energy is of particular importance to developing countries, as it can forestall the need for very large capital investments. Enormous potential exists through improvements in energy efficiency and conservation for decreasing the world’s total energy consumption, and thereby the effects of energy consumption on the environment. Often such improvements require a myriad of small changes in consumption patterns. Increased energy efficiency reduces energy-related EIs, such as those listed in the Chapter Environmental Dimensions of Energy (00103) (e.g., environmental damage due to the process of extracting energy resources from the ground, and the competition for water between hydropower and such other uses as agriculture and recreational activities, and associated damage to water quality). In addition, improved efficiency enhances the reliability of future energy supplies, and improves the longevity of energy supplies. The potential for energy efficiency is significant during both energy production and consumption (e.g., the 30% of oil in a reservoir that is extracted from onshore wells could be improved upon using secondary recovery techniques, such as water flooding, and thermal stimulation). The following measures are essential for reducing the EI of energy systems:

• • •

Increasing the efficiency of power generation systems. Energy conservation measures. Use of more environmentally benign energy sources.

When shaping sustainable energy policies and strategies, it is important to understand the benefit of clean energy technologies on one hand, and the effects of fossil fuel-based systems on the other. The kinds of energy technologies with zero or minimum negative EI (e.g., through associated pollution), that is, which are more environmentally benign and more sustainable, are in general named renewable energy. Considering the benefits of renewable energy, sustainability of renewable energy supply and progress is assumed to be a key element in the interactions between nature and society. An essential increase in the scale and pace of policy instruments and their effectiveness is required to change course toward a sustainable path. It is important to possess theoretical tools to quantitatively assess renewable energy development, and:

• • •

help elaborate rationale strategies and policies, help understand main concepts and issues about renewable energy use and sustainability aspects, develop relations between renewable energy use and sustainability development,

116

• • • • •

Sustainability Dimensions of Energy

encourage the strategic use and conservation of the renewable energy sources, provide the methods for energy security, implementation and development, increase the motivation on the implementation of renewable energy strategies for better energy supply, give an idea to reduce the negative EIs by considering the possible renewable energy strategies, and form a scientific platform to discuss the possible renewable energy strategies for sectoral use.

The need of policy is justified by the fact that free markets must be enforced to meet the needs of vulnerable groups, reduce environmental pollution, and ensure the energy security. Upper level strategies are needed, where a strategy represents a plan or method toward a goal (in our discussion “the goal” means progress toward a global society based on sustainable energy). Once a strategy is established, the policy, which represents the course of actions to implement the strategy, must be elaborated. Any policy will specify “policy instruments,” which represent specific measures taken to implement a policy. Examples of policy instruments are:

• • • • •

Imposing efficiency standards. Setting public procurement policies. Imposing appliance labeling norms. Obligation to buy or supply energy from renewable sources. Supporting research and development in demonstration projects.

One of the most important aspects is thorough evaluation of the costs of reducing CO2 emissions. From a developing-country perspective, the discussion of costs and benefits has to take into account the need for policies promoting rapid economic growth. Achieving such a balance between economic development and emissions abatement requires the adoption of domestic policies aimed at improving the efficiency of energy use and facilitating fuel switching, and the implementation of international policies enabling easier access to advanced technologies and external resources. It is expected that some countries will offer certain tax reductions for those businesses that promote renewable energy technologies, especially because these technologies are characterized by low or zero CO2 emission. Some possible solutions include cleaning fossil fuels before combustion; burning them more cleanly, for example, using fluidized bed technology for coal; using renewable energies; switching to hydrogen economy; implementing thermal energy storage technologies; promoting efficient public transport; using more fuel-efficient vehicles; etc. An example of development of a sustainability assessment framework is given here from Ontario Hydro in 1997 (currently Ontario Power Generation). According to Ref. [6], there are five categories for sustainability assessment established by Ontario Hydro, namely: (1) energy and resource use efficiency, (2) environmental integrity, (3) renewable energy, (4) financial integrity, and (5) social integrity. For the first category of energy and resource use efficiency the following indicators are considered in a compounded manner:

• • • • •

power consumption and transmission losses as a percentage of sales fuel conversion efficiency water withdrawals fuels and commodities consumption internal energy recovery and savings

For the environmental integrity category, the monitored emission rates are used for all activities including design, development, construction, commissioning, operation, decommissioning, and material management. Here are the considered individual sustainability indicators for this category:

• • • • • • • • •

GHG emissions ozone-depleting substances emissions acid gas releases waste management indicator radioactivity levels of generated wastes hazardous wastes emissions reportable spills compliance violations environmental expenditures Regarding the specific indicators for renewable energy use, the following indicators are considered:

• • •

energy share generated from renewable sources energy generated from wind power energy generated from solar power

The consistent generation of cash flow is considered by Ontario Hydro with the help of the financial integrity category for sustainability assessment. Here, the following specific indicators are considered:

• •

net income interest coverage

Sustainability Dimensions of Energy

• •

117

debt ratio total cost of the energy unit

The interaction of Ontario Hydro with the communities and with its own employees determines the social integrity category for sustainability assessment. These specific indicators are selected such that innovation and greater employee involvement in sustainability is encouraged; thence, the indicators are as follows:

• • • • • • •

employee accident severity corporate citizenship program employee productivity payments in lieu of taxes number of public fatalities aboriginal grievances severity of environmental complaints and their number

The Ontario Hydro corporate developed two composite indices that assess sustainability on long-term targets and are based on the above five categories of specific indices. These are (1) resource use efficiency composite indicator focusing on inputs (e.g., fuels, water) and (2) environmental performance indicator focused on outputs (pollutants and wastes emissions). The composite indicators are constructed such that they take positive values. The reference value is set to zero at the level of year 1995. The value of 100 was predicted to be reached by the composite indicator in year 2000, while in year 2002 it reached the value of 190. This case study shows how a corporation can assess sustainability internally, which helps adapt its strategy to achieve its goals. In this respect, an adoption of a clear definition of the development vision is crucial. It is noteworthy that the sustainability must be related to both the quantity and the quality. Some sustainability indices are introduced next based on past works of Midilli et al. [5] and Dincer and Zamfirescu [7]. Those are defined as follows:

• • • • •

Ecological footprint (EF) analysis is an accounting tool enabling the estimation of resource consumption (RC) and waste assimilation requirements of a defined human population or economy in terms of corresponding productive land use. Sustainable process index (SPI) is a means of measuring the sustainability of a process producing goods. The unit of measure is square meter (m2) of land. It is calculated from the total land area required to provide raw materials, process energy (solar derived), infrastructure (including energy generation production facilities), and waste disposal. Sectorial impact ratio (Rsi) is based on the provided financial support of public, private, and media sectors for transition to green energy-based technologies, and depends on the total green energy financial budget as a reference parameter. Technological impact ratio (Rti): this parameter quantifies the provided financial support for research and development, security, and analysis of green energy-based technologies. This parameter depends on the total green energy financial budget as a reference parameter. Practical application impact ratio (Rpai): this parameter quantifies the provided financial support for design, production, conversion, marketing, distribution, management, and consumption of green fuel from green energy sources and also depends on the total green energy financial budget.

Another largely used sustainability indicator based on mass and energy balances is the eco-efficiency indicator representing the ratio between value in outputs and the EI. If one denotes with eco-indicator (EcI), and with energy product value (EPV), and with EI associated to the EPV, then the EcI is expressed mathematically as follows: EcI ¼

EPV EI

ð1Þ

Linke et al. [8] introduced an assessment indicator for sustainability denoted as the “sustainability efficiency indicator” (SEI) defined by the ratio between a selected performance parameter (PP) and quality parameter (QP) for a specified RC; this has the mathematical formula as follows: SEI ¼

PP QP
RC

ð2Þ

The SEI indicator can be extended for drying systems in various ways, such as in the form of SEs per final moisture and energy input, where the PP is the SEs, the QP is the final moisture content, and the RC is the energy input. Sustainability indicators of various types can be aggregated into one single figure of merit for sustainability, denoted as an aggregated sustainability indicator (ASI). The aggregated indicators for sustainability can be formed by various possible methods of normalization, ranking, weighting, and scaling. In order to be possible to aggregate the individual indicators, these have to be normalized first. The normalization can be done based on a reference sustainability indicator. Various methods of normalization were proposed in past studies, such as in Refs. [9–11]. Denote Indref an arbitrary selected reference sustainability indicator. This indicator can be the maximum of the dimensionless indicators, defined as follows: Indref ¼ max fIndi ji ¼ 1; 2; …; ng where n is the number of dimensionless sustainability indicators Indi to be aggregated.

ð3Þ

118

Sustainability Dimensions of Energy

Accordingly, the normalization can be done based on the reference value as follows: NSIi ¼

Indi Indref

ð4Þ

The sustainability indicator can be also normalized based on the relative alleviation from the average, as follows: NSIi ¼

Indi Ind Indi

ð5Þ

where Ind represents the average of dimensionless indicators. Instead of the average value, a target value of the indicator can be used in Eq. (5). A variation for the normalization given by Eq. (5) is as follows, where the weighted average is used instead of the arithmetic average as follows: NSIi ¼

Indi

Ind s si

ð6Þ

where si represents the standard average of Indi. The normalization can be also done by scaling as follows: NSIi ¼

Indi minfIndi jiA f1; ngg max fIndi jiA f1; ngg minfIndi jiA f1; ngg

ð7Þ

Based on the normalized sustainability indicator (NSI) various aggregation possibilities exist such that an aggregated (or compounded) sustainability indicator can be obtained. A typical approach is that of a weighted average, determined as follows: P wi NSIi ASI ¼ P ð8Þ wi where ASI is the aggregated sustainability index and wi are the weighting factors, which can be chosen or determined based on various considerations; in principle, the weighting factors can be determined as follows: NSIi wi ¼ P NSIi

ð9Þ

The use of weighting factor injects an element of subjectivity on the ASI. Based on [11] the weighting factors can be defined according to trade-offs between different criteria, each criteria being represented by a normalized indicator. The tradeoffs are generally established by the decision maker and therefore the vision of decision maker influences the process. Three types of decision maker archetypes can be envisioned, namely, the individualist, the egalitarian, and the hierarchist. The individualist decision maker is not interested on the inter- and intragenerational equity in relation to sustainability. She/he will favor criteria related to production performance to the detriment of criteria related to environmental benefit. The egalitarian decision maker is concerned with the inter- and intragenerational equity, therefore he/she adopts a wider view with attention to the long-term effects and EI. The moderate approach is that of the hierarchist decision maker who will compromise and negotiate a balanced approach on sustainability. Making energy systems more sustainable is the synonym of increasing their performance and efficiency, while enhancing the share of renewable energy supply. Greenization effort is a key solution to achieve sustainable energy systems by trying to make them more efficient, more environmentally benign, and more cost effective. An ASI for energy systems will then account for the relative improvement toward greenization with respect to a baseline case. Therefore a greenization factor (GF) has been proposed in Ref. [12]. This factor has been related to the ASI as shown in [13]: GF ¼

ASI

ASIref ASI

ð10Þ

In Eq. (10), the following inequality is assumed: ASI4ASIref. Therefore a GF of zero signifies no improvement with respect to the baseline system. When ASI becomes significantly larger than ASIref, the GF defined according to Eq. (10) tends to be 1. Fig. 14 shows the greenization process for improved sustainability of energy systems. The trend must be a reduction of dependence on polluting conventional sources and an increase of the ability to convert renewable energy sources. In the same time, the efficiency of the energy system must increase in time. Furthermore, due to an increased use of green energy sources and an increased system efficiency, the environmental pollution can be reduced and minimized for the same net output. This requires an ultimate modification and retrofitting of the existing energy systems. Increasing the energy system efficiency is another key for a greener system. For steam power plants, this can be achieved by optimizing the operating parameters and the components of the system. This would include better turbine designs with higher efficiencies, and heat exchangers with higher effectiveness and lower temperature differences, and the selection of the boiler. Integrated energy systems for multigeneration purposes are a promising approach for achieving greener and sustainable energy systems through increasing the operating efficiency of the system. The technology changes of integrated power generation systems have evolved so fast during the last decades. Cogeneration, mainly combined heat and power (CHP), extended for trigeneration by producing cooling as a useful commodity of the system.

Sustainability Dimensions of Energy

Greener

Green Conventional (polluting) energy sources

119

Green + conventional energy mix

Green energy sources

Less fossil fuel-based energy Useful energy Reference energy system

Greenized, more efficient energy system

Less polluting effluents

Polluting effluents

Polluted environment

Useful energy

Less polluted environment

Green, highly efficient energy system

Useful energy

No or miror polluting effluents

Minimally polluted environment

Fig. 14 Greenization of energy systems for increased sustainability.

Further commodities have been recently considered for multigeneration energy systems. Besides electric power, heating, and cooling, other useful outputs include fresh water, domestic hot water, hydrogen, syngas, ammonia, and other commodities. For such systems, it is expected to perform at higher efficiencies and lower EI, which results in greener systems. These systems can be further greenized when considering renewable energy sources to fuel the system.

1.4.4

Exergy-Based Sustainability Assessment of Energy Systems

Exergy analysis offers a basis for sustainability assessment because exergy is a measure of abatement of the system subjected to the analysis from the environment. Exergy analysis is of major importance in assessment of sustainability, because exergy-based efficiency of systems and processes represent a true measure of imperfections. It also indicates the possible ways to improve the energy systems and to design better ones. Destruction of exergy must be reduced as much as possible. Assessment of the exergy destruction offers the opportunity to quantify the EI and the sustainability of any energy system Ness et al. [3] categorizes exergy as one of the emerging methods for sustainability assessment. A precursor of the exergy-based sustainability assessment is regional and sectorial exergy analysis. Wall [14,15] presented exergy analysis for Japan and the United States, respectively. Utlu and Hepbasli [16] presented a sectorial exergy assessment for Turkey. Sectorial exergy assessment of transportation sector is comparatively studied for six countries in Ref. [7]. In a sectorial exergy assessment, a geopolitical region or the whole world is analyzed based on energy and exergy method accounting for the involved mass, energy, and exergy balances. In this analysis the thermodynamic system can be a sector of activity, such as industrial, commercial, transportation, agricultural, utility, etc. Fig. 15 shows a model for sectorial exergy assessment. The exergy efficiency of a sector will then be defined as the ratio between the exergy delivered to the user and the exergy input. One of the most important sectors is the electric utility, which is responsible for power generation and distribution. The electric utility sector does not include the electricity produced by industrial establishments for their own use. The electric utility sector also includes electrical power generated by desalination plants. Desalination plants produce electricity as a byproduct with a high efficiency, that is, about 46% as compared to the average efficiency of a power station, which is 28%. Countries with desert regions, such as Saudi Arabia, use water desalination to produce drinkable water. In Saudi Arabia, the overall efficiency of the utility sector, calculated by Ref. [17], is 31.75%. In Ref. [7] the averaged world exergy efficiency of the electric utility sector is determined to be approximately 25%. Sectoral exergy assessment has been expanded to cover environment and sustainable development by Ref. [18]. It is shown that exergy is at the confluence of energy, environment, and sustainable development. The need to understand the linkages between exergy and energy, and the EI, has become increasingly significant. Less exergy destruction implicitly leads to reduced EI. Further, as energy policies increasingly play an important role in addressing sustainability issues and a broad range of local, regional, and global environmental concerns, policy makers also need to appreciate the exergy concept and its

120

Sustainability Dimensions of Energy

Energy loss/ exergy destroyed

Energy, exery inputs (primary energy sources/ associated exergy)

Thermodynamic macrosystem (activity sector, geopolitical region, country, etc.) Useful energy/ exergy

Fig. 15 Thermodynamic analysis at macroscale based on energy and exergy.

ties to these concerns. The environmental impact assessment (EIA) methods for energy systems can be classified in four categories:

• • • •

Environmental tools, including impact assessment, and EFs. Thermodynamic tools; performance indicators; energy, exergy, and material flux analyses. Sustainability tools, including lifecycle assessment, SPI, exergetic sustainability index (ExSI). exergetic improvement potential (IP), GF, etc. Risk assessment.

EIA is an environmental tool used in assessing the potential EI of a proposed activity. The derived information can assist in making a decision on whether or not the proposed activity will pose any adverse EIs. The EIA process assesses the level of impacts and provides recommendations to minimize such impacts on the environment. Risk assessment can estimate the likelihood of potential impacts and the degree of uncertainty in both the impact and the likelihood it will occur. Once management has been informed about the level of risk involved in an activity, the decision of whether such a risk is acceptable or not can be subsequently made. Bejan [19] studied the application of entropy generation minimization principles to formulate energy policy aspects. The primary purpose of Bejan’s work was threefold: (1) to employ the art of entropy generation minimization at a level as yet unexplored; (2) in the process, to present a unified framework with a sound theoretical basis for making and analyzing energy policy proposals; and (3) to demonstrate the potential benefits of a dialog between different disciplines, academic or professional, by presenting the result of an ongoing one between an “accountant” and an “engineer.” Further developments led to the use of exergy destruction within energy systems as a quantifier of EI. Therefore exergy-based methods can be developed to help policy making. The relative magnitude of exergy destruction within a system or process with respect to energy input suggests the meaning of a resource depletion factor. Connelly and Koshland [20] proposed such a factor denoted by them as “depletion number” and defined as follows: Dp ¼

Exd Exin

ð11Þ

Assume that the analyzed system is from the domain of industrial ecology, that is, it comprises a number of combined technologies and industrial fluxes that operate as a whole. In this case, the exergy efficiency of the integrated system can be evaluated on the base of depletion number of each independent component. In an abstract manner, the exergy efficiency of ðcombÞ industrial ecology systems is illustrated in Fig. 16. For combined technologies, the depletion number Dp is lower than that for ðsepÞ separate technologies Dp , which is expressed by ¼ DðsepÞ p

_ comb _ comb Ex Ex p1 p2 ð1Þ ð2Þ DP þ comb DP comb _ p2 _ comb _ p1 þ Ex þ Ex Ex p2

_ comb Ex p1

ð12Þ

_ comb and Ex _ comb are the rates of output exergy flows for products 1 and 2, respectively. where Ex p1 p2 The application of exergy method to an industrial ecology analysis can be done by calculating the exergy flows of every stream of matter and energy and associating depletion numbers to every independent technology or process. Further, the depletion

Sustainability Dimensions of Energy

Exergy rate of output streams

Exergy rate of output streams

Exp(1)

Exp(2)

121

Exergy rate of output streams Exp(comb)

Combined technology Technology 1 Depletion number

Technology 2 Depletion number

Technology 1 Depletion number

Dp(1)

Dp(2)

Dp(1)

Exergy rate of input streams

Exergy rate of input streams

(1) ExIn

ExIn(2)

(A)

Technology 2 Depletion number Dp(comb)

Dp(2)

Exergy rate of input streams Exin(comb)

(B)

(C)

Fig. 16 Depletion number of separate and combined technologies.

number of the separate and combined technologies is calculated and compared in order to quantify the benefit of technology integration from both an energetic and ecologic point of view. The depletion factor is related to the exergy efficiency of the system as follows: c¼1

Dp

ð13Þ

Let us consider an energy source, such as a fuel. When the fuel is used in an engine its internal energy is converted into work. The maximum energy conversion potential equals the chemical exergy of the fuel. Better converter is the same as higher exergy efficiency or smaller depletion number, according to Eq. (13). A SI can be introduced as the reciprocal of the depletion number. Such a SI can be determined for any type of energy system. Mathematically, the definition of SI is written as follows: SI ¼ 1=Dp Another criterion to assess a renewable energy system is the IP defined as follows:   Exout IP ¼ Dp 1 Exin

ð14Þ

ð15Þ

In addition, the GF introduced previously according to Eq. (10) can be expressed alternatively using exergy destruction as follows: GF ¼

Exd;ref Exd Exd;ref

ð16Þ

where Exd is the lifecycle exergy destruction of the studied system and Exd,ref is the exergy destruction of the reference system. Both the system and the reference system process generate the same amount of product. But, the energy, exergy, environmental, and cost parameters of the two systems differ. Observe from Eq. (16) that if Exd-0 then GF-1, meaning that the system tends to be greener. If xd ¼ Exd,ref, no greenization effect is observed, both the reference and the studied system having similar EI. The GF can be also defined based on an EI indicator. If one denotes EI, then the GF for an energy system becomes: GF ¼

EIref EI EIref

ð17Þ

In the above equation, the EI must be specified for two cases: the reference system and the greenized system. Depending on the specific problem analyzed, various types of EI factors may be formulated. For many energy systems, specific GHG emissions can be used as EI factor. Fig. 17 shows the representation of a generic energy system for sustainability analysis. The scope of the analysis is the entire lifespan of the system. The energy from the source is converted to a useful form, such as power or exergy-carrying synthetic fuel. The produced exergy corresponds to stream 5 in the figure. A part of this stream is then used to maintain the system in operation for the LT span and to construct a newer system, ready for operation at the end of the lifespan of the old system. The net exergy

122

Sustainability Dimensions of Energy

System boundary for exergosustainability assessment Source exergy input 1

Overall system for the lifecycle

5

7

Exergy output to user

Newly constructed system

6

System construction process (exergy is destroyed) Exergy destroyed at system construction Exergy destroyed at system operation

Materials

Fig. 17 Representation of an energy system for exergosustainability assessment.

Sustainable development

Environmental impact

Efficiency Exergy Energy

Environment Energy and material balances

Fig. 18 Representation of the exergy at the confluence of energy, environment, and sustainable development. Reproduced from Dincer I, Zamfirescu C. Sustainable hydrogen production. New York, NY: Elsevier; 2016.

given to the used equals to the difference between stream 5 and stream 6. If the exergy output in 7 is positive (if there is output) then an ExSI can be defined as follows: ExSI ¼

Ex7 Ex5

ð18Þ

Other approaches based on the second law of thermodynamics led to proposal of sustainability indicators, such as renewability or resources, toxicity of generated emissions, input of used materials, recoverability of the products at the end of their use, and technological efficiency [21].

1.4.5

Sustainability Assessment Based on Exergetic Lifecycle Analysis

Exergy, EI, and economic factors are well interrelated with sustainability. Besides, these energy and material balances and system’s efficiency are important. Fig. 18 shows a representation of the intimate connection between exergy and sustainability and other factors. As shown by Dincer [22], exergy efficiency can be correlated with SI and the EI index. It is also correlated with the lifecycle sustainability index (LCSI). Here, the difference between SI and LCSI consists of the fact that SI refers to the system utilization

Sustainability Dimensions of Energy

123

phase only, while LCSI considers the system lifecycle including the construction and scrapping phases. An exergetic lifecycle analysis can be conducted to determine the lifecycle exergy destruction and from here to assess the system sustainability. By using exergy destruction as a measure to quantify environmental effects associated with emissions and resource depletion, the EIA is made based on purely physical principles. An understanding of the relations between exergy and the environment may reveal the underlying fundamental patterns and forces affecting changes in the environment, and help researchers to deal better with environmental damage. The discharged wastes in the environment and their impacts can be quantified by accounting for exergy destructions. It is known that the exergy destroyed (and wasted) by the system is of two kinds:

• •

internal exergy destruction, which represents the lost opportunity to perform work; the EI and rejected wastes due to all upstream processes (e.g., power generation) can be related to the internal exergy destruction; lost exergy, or exergy destruction at system interaction with its surroundings, which is related to the discharged wastes by the process itself. In principle, all discharged wastes by the system can be recovered to use their exergy and reduce the EI; however, this action may be very expensive and generally is not undertaken in practice, except when it is justified economically or enforced by sustainability policies.

The economic factor is also an important one to consider in sustainability. Economy is related to wellbeing, and it connects to value and utility. In the real world, the economic factor has one of the most important influences in selecting any technical design. Ultimately, a business case must be presented in terms of investment cost (IC) and profitability, and this guides the system development. It is worth noting that the cost is a very volatile factor; therefore, any technical-economic analysis is not absolute, but subjected to regional and temporal economic constraints. In economics, the theory of value attempts to explain the correlation between value and price of traded goods and services. In today’s economy, the trades are normally made by paying a price in money as a standardized currency for payments of goods, services, and debts. The price must reflect the value of the trade, which is related to costs and profitability. The theory of value offers an ideological basis for quantification of the benefit from a traded good or service. This helps assigning a price to a value. Three theories of value received much attention in the last century. The first is the power theory of value, which states that political power and the economy (which is constrained by laws of trade) are so highly interlaced that prices are established based on an internal hierarchy of values of the society rather than on a production and demand balance. The second is the labor theory of value, which states that the value is determined by the labor developed to produce the good or the service including the labor spent to accumulate any required capital for the production process. The third is the utility theory of value, which quantifies the value of tradable goods and services based on their utility. There is no direct way of measuring the utility as a representation of the preferences for trading various services or goods because this depends on subjective factors of human individuals, such as wishes and wants. However, the utility can be observed indirectly though the price that is established by trading activity. The price is determined by the balance between marginal utility and marginal cost. Here, the term marginal means an infinitesimal change. Let us assume that a quantity Q of products are traded; if one denotes U a quantified utility of the product, then the utility marginality is the derivative dU/dQ; also, if the production cost is C, then the marginal cost would be dC/dQ. The price according to the utility theory of value is spontaneously established such that dU dC ¼ dQ dQ

ð19Þ

Eq. (19) explains why gold price is much higher than that of water. The marginality of gold cost (the change in cost for an infinitesimal change of quantity) is obviously much higher than the marginality of water cost. The utility of water is much higher than the utility of gold, but the scarcity of gold is high, whereas the availability of water is high. This means that the marginality of water utility is much smaller than the marginality of gold utility, and therefore the marginalities of the costs of water and gold must be on the same relationship. As Georgescu-Roegen [22] points out, if a theory quantifies value through a conservable quantity then it fails. For example, if when applying the theory of labor one assumes that any type of labor can be valued through the amount of mechanical work (or energy) deployed to do it, then, this leads to misappropriations (which are in fact common) because energy does conserve and value does not. Value degrades or augments. Another example is that if, when applying the theory of utility, one assumes that the utility is expressed in terms of mass (amounts, quantities) of a precious metal (gold), then this is a fallacy because although finite, mass is conserved (we do not consider here any nuclear reactions). The true values that humans and also any other living organisms appreciate are the sources of low entropy or high exergy. These quantities do not conserve. When used, a low entropy source is converted in a high entropy waste by living organisms, which in the meantime perform their activity. Equivalently, one says that the high exergy sources are degraded by the Earth systems (living species, natural cycles, etc.): exergy degrades from the source to waste. The net exergy absorbed by the Earth, consequently, is gradually destroyed, but during this destruction, it manages to drive the Earth’s water, wind, and other natural systems, as well as life on Earth. Because it does not conserve, a source of low entropy can be used only once and never reused. The same stands for exergy: once destroyed it cannot be reused. Exergy is also related to the surrounding environment as it accounts for its temperature, pressure, and species concentration. Therefore due to these attributes, exergy can be used in establishing a theory of value. In fact, exergy represents the part of energy that is useful to society and therefore it has economic value. Furthermore, once the economic value of exergy is expressed in terms of currency, then it can effectively be used for exergoeconomic and exergoenvironmental analyses. Various methods can be approached to price exergy for analyses purpose. It is

124

Sustainability Dimensions of Energy

important to determine sound methods to set the prices and the costs in relation to exergy content. This in fact requires formulation of a theory of costs based on exergy. It has been suggested that when analyzing a thermal system it is reasonable to distribute costs in relation to outputs and accumulations of exergy. With regards to the prices of physical resources (fuels, materials), these also must be set in a tight relation with the resource exergy content, such as to foster resource saving and effective technology. Let us do a simple attempt to quantify exergy in terms of monetary currency. In doing this, one considers the main fuels in a society. The energy content is taken as the lower heating value (LHV) of the fuel and the exergy content is the chemical exergy of each fuel. Table 2 gives the specific energy and exergy content of fuels considered in this brief analysis. The price per unit of mass of each fuel is also given. When the price is divided to chemical exergy, then the exergy-specific price Cex is obtained. Table 2 is constructed for Canada; however, the methodology presented subsequently is general. In our approach, a country or region must be considered first. Then, the primary energy sources are inventoried. For Canada, the following primary energy sources can be considered: coals, refined natural gas, natural gas liquids, crude oil derivate, hydro, nuclear, and biomass derivate (here, wind and solar are neglected, as they are not highly represented). Further, the method for exergy price estimation goes as follows:

• • • • • •

• • •

Cost of each fuel type is obtained from the market and expressed in dollars per kilogram. For the case of hydro and nuclear, this step is skipped. Based on the available statistics, the consumed energy fraction (CEF) for each type of fuel is determined. In Table 2 the CEF is obtained from the previous work by Dincer and Zamfirescu [7], chapter 17. The CEF represents the fraction of specified primary energy source from the total energy consumed from primary sources. The LHV and specific chemical exergy for categories of fuels are averaged. For example, the mean LHV for natural gas liquids is an average of the LHVs for the LPG, methanol, and ethanol. The specific price of fuel is averaged for each fuel category. For example, the mean fuel price Cf for fuels obtained from crude oil (crude oil derivate) is the average of prices of gasoline, diesel, kerosene, and fuel oil. The quality factor for each category of fuel is determined as indicated in the table, that is, by the ratio between averaged specific chemical exergy and LHV. The quality factor for hydropower and nuclear energy is 1. The exergetic price Cex for each fuel category is determined as shown in the table, by the ratio between the averaged fuel cost and specific chemical exergy. The exergetic price of hydropower results from the specific price of electric power divided by 0.8, as it is fair to assume that the exergy efficiency of the hydro power plant is 80%. The exergetic price of nuclear energy is determined from the price of electric power divided by 0.31 based on the fact that, as shown in Dincer and Zamfirescu [23], the exergy efficiency of CANDU power plants is 31.3%, in average. The cost of electric power in Canada is taken at an average cost of ¢11/kWh. P The consumed exergy factor for each fuel category is calculated with CExFi ¼ CEFigi/ (CEFigi), where i is an index representing each type of the fuel. These represent weighting factors. P The averaged price of exergy results as a weighted average, Cex ¼ CExF i  Cex;i . The exergy price for Canada, based on the exergy of the primary resources, is 8.4 ¢/kWh or 23.3 $/GJ. This compares well with the average electricity price of ¢11/kWh.

Once a price of exergy is determined, further models can be created to establish a costing scheme for other items of interest in an exergosustainability analysis. Nonenergetic costs, such as labor, material supply, environment remediation expenditure, incidental expenditures, etc. can be priced using exergy content as a basis for cost accounting. The economic value of system outputs can be also allocated based on exergy. The practical connection between exergy price, wasted exergy, and EI can be discovered by correlating recorded emission data at the level of a region or globally with the chemical exergy of emitted pollutants. Here, some exergy destruction versus emission data correlation is presented for Ontario. The general aspects of environmental policy in Ontario can be described as follows:

• •

•

in Ontario, the Environmental Protection Act by the Ministry of Environment exists, giving the legislation on environmental quality and air pollution limits, which are conceived such that human health and the ecosystem are not endangered; the potential of a substance to impact the environment is evaluated using a set of 10 parameters: ○ transport ○ persistence ○ bioaccumulation ○ acute lethality ○ sublethal effects on mammals ○ sublethal effects on plants ○ sublethal effects on nonmammalian animals ○ teratogenicity ○ mutagenicity/genotoxicity ○ carcinogenicity an aggregated indicator is determined based on the 10 impact parameters (above), referred as the point of impingement (PoI), which is determined based on the best known available pollution control technology;

Table 2

Calculation table for exergy price based on primary energy

Fuel

Coal Refined natural gas Natural gas liquids

Crude oil derivate

Hydro Nuclear Biomass derivate

Whole tree Wood pellets Wood chips Pine wood Sawdust Straw Rice straw Waste paper Biogas

ex ch

24.0 50.7 46.0 19.9 28.8 43.5 42.8 43.1 40.1 N/A N/A 19.7 14.6 10.0 18.9 8.0 14.5 14.1 17.7 22.5 ch g ¼ ex

15.0 52.4 54.9 22.4 29.5 47.7 44.2 49.1 41.1 N/A N/A 22.1 18.5 11.0 24.8 8.5 16.5 15.9 20.1 23.2 Cex ¼

LHV

  MJ kg

Cf ch ex

Cf

  $ kg

0.15 0.18 1.43 0.47 0.88 1.74 1.78 0.94 1.03 N/A N/A 0.2 0.2 0.2 0.5 0.2 0.3 0.8 0.1 2.0

CEF(%)

Calculated item     ex ch MJ LHV MJ kg kg

8.3 30.8 2.9

24.0 50.7 31.6

15.0 52.4 35.6

45.1

42.4

7.4 5.0 0.5

N/A N/A 15.6

g CExF ¼ PCEF ðCEF gÞ

Source: Reproduced from Dincer I, Zamfirescu C. Drying penomena: theory and applications. New York: Wiley; 2016. Abbreviation: N/A ¼ not applicable.

 

g

Cex

0.15 0.18 0.93

0.62 1.03 1.13

45.5

1.37

N/A N/A 17.8

N/A N/A 0.05

Cf

$ kg

CEF  g

CExF(%)

CExF  Cex

10.0 3.43 26.1

0.052 0.318 0.033

5.1 31.3 3.3

0.51 1.07 0.85

1.07

30.1

0.484

47.7

14.3

1 1 1.14

34.7 89.6 2.8

0.074 0.050 0.005

7.3 4.9 0.4

0.16 6.6 0.14

$ GJ

Averaged price of exergy: Averaged price of electricity:




8.4 ¢/kWh 11.0 ¢/kWh

$ GJ




Sustainability Dimensions of Energy

Equations used:

LPG Methanol Ethanol Gasoline Diesel Kerosene Fuel oil

Data input   LVH MJ kg

125

126

•

• • • •

Sustainability Dimensions of Energy

the methodology denoted removal pollution costs (RPCs) is applied to correlate the exergy of the waste stream with the cost of removing pollutants from the waste stream prior to discharge into the surroundings. The cost for waste emissions is evaluated as the total fuel cost per unit fuel exergy multiplied by the chemical exergy per unit fuel exergy, and divided by the exergy efficiency of the pollution removal process; in Canada, the environmental pollution costs (EPCs) are estimated based on qualitative and quantitative evaluations of the pollution cost to the society for compensation and correction of the environmental damage and to prevent harmful discharges into the environment. Table 3 gives the EPCs of Ontario pollutants; the average composition of volatile organic compound (VOC) emissions in Ontario are approximated as given in Table 4; the average composition of particulate matter (PM) emissions in Ontario are given in Table 5; the fuel cost for three types of fossil fuels, namely, coal, no. 6 fuel oil, and natural gas are in average value of CN$ 2013 as follows:

Table 3

Estimations of exergetic cost of atmospheric pollutants for Ontario

Pollutant

Environmental pollution cost (EPC) ($/kg)

M (kg/ kmol)

Molar environmental pollution cost (MEPC) ($/kmol)

exch (MJ/kmol)

exPC ($/MJ)

Point of impingement (PoI) (mg/m3air)

CO2 CH4 NOx SO2 CO Volatile organic compounds (VOCs) Particulate matter (PM)

0.0402 1.2998 3.8944 3.551 5.5074 0.603 5.5074

44 16 38 64 28 44 68

1.77 20.80 148.18 227.26 154.21 26.53 374.50

19.60 831.66 72.4 310.99 275.00 1233.00 436.00

0.090 0.025 2.047 0.731 0.561 0.021 0.860

56,764 N/A 500 830 6000 N/A N/A

Sources: Reproduced from Carpenter S. The environmental cost of energy in Canada. In: Sustainable energy choices for the 90’s. Proceedings of the 16th annual conference of the solar energy society of Canada, Halifax, NS; 1990. p. 337–42; De Gouw JA, Warneke C, Stohl A, et al. Volatile organic compounds composition of merged and aged forest fire plumes from Alaska and western Canada. J Geophys Res 2006;111:D10303; Pellizzari ED, Clayton CA, Rodes CE, et al. Particulate matter and manganese exposures in Toronto, Canada. Atmos Environ 1999;33:721–34, Ontario Regulation 419/05. Note: Costs are in 2013$ based on the Canadian consumer price index.

Table 4

Approximated average volatile organic compound (VOC) composition and characteristics in Ontario

VOCs

Formula

M (kg/kmol)

y (kmol/kmol)

exch (MJ/kmol)

Point of impingement (PoI) (mg/m3air)

Methanol Acetonitrile Acetaldehyde Acetone Acetic acid Butanone Toluene

CH3OH CH3CN CH3CHO CH3COCH CH3COOH C2H5COCH3 C6H5CH3

32 41 44 58 60 72 92

0.020 0.823 0.001 0.111 0.025 0.015 0.005

612 1169 1063 1636 780 2755 3771

12,000 180 500 48,000 2,500 250 2,000

Source: Reproduced from De Gouw JA, Warneke C, Stohl A, et al. Volatile organic compounds composition of merged and aged forest fire plumes from Alaska and western Canada. J Geophys Res 2006;111:D10303, Ontario Regulation 419/05.

Table 5

Approximated average particulate matter (PM) composition and characteristics in Ontario

Particulate matter (PM)

Formula

M (kg/kmol)

y (kmol/kmol)

exch (MJ/kmol)

Point of impingement (PoI) (mg/m3air)

Lead Cadmium Nickel Chromium Copper Manganese Vanadium Aluminum Calcium Magnesium

Pb Cd Ni Cr Cu Mn V Al Ca Mg

207 112 59 52 63 55 51 27 40 24

0.053 0.097 0.185 0.210 0.173 3.115E-7 0.214 0.006 0.054 0.008

249.2 298.4 242.6 584.4 132.6 487.7 721.3 795.7 729.1 626.9

10 5 5 5 100 7.5 5 26 14 60

Source: Reproduced from Pellizzari ED, Clayton CA, Rodes CE, et al. Particulate matter and manganese exposures in Toronto, Canada. Atmos Environ 1999;33:721–34, Ontario Regulation 419/05 (2013).

127

Sustainability Dimensions of Energy

•

○ average coal: $1.411/GJ LHV ○ average no. 6 fuel oil: $1.864/GJ LHV ○ average natural gas: $3.899/GJ LHV Table 6 gives the EPC and RPC for the main fossil fuels in Ontario.

A simplified method to estimate the cost of pollutant removal from the waste stream is based on the exergy efficiency of pollutant removal. According to Rosen and Dincer [27], this exergy efficiency is in the range of 1%–5%. Therefore once the exergy destroyed due to pollutant discharge is known, the required exergy to remove the pollutant from the waste stream can be calculated. Furthermore, the average price of exergy can be estimated for any geopolitical region; for Canada, it is approximated as Cex ¼ 8:4 ¢/kWh ¼ 2.3 ¢/MJ. When the exergy required to remove the pollutants is multiplied by exergy price, the removal pollutant cost RPC is obtained. Therefore one has: RPC ¼ Cex cpr Exd;pw

ð20Þ

where Cex is the exergy price, cpr is the exergy efficiency of pollutant removal from the waste stream, Exd,pw is the exergy destroyed due to pollutant waste in the environment. Assuming an average exergy efficiency of pollutant removal from waste stream of 3%, the RPC can be estimated as 0.7 $/GJ. The pollution removal cost of power generation can be roughly estimated based on statistical data that allow for the estimation of the exergy destructions. Table 7 gives the rough estimate of RPC associated with Canadian power generation. The cost of pollution (CP) associated with the construction of power generation facilities, reparations, and maintenance is not included. As given, the total RPC for power generation is B1870 mill $ and the overall exergy efficiency of the power generation sector is 51%. The RPC becomes $2.5/MWh generated power. Lifecycle pollutant emission from power generation technologies is determined in Table 8 based on multiple literature data sources [24–26]. The amounts of atmospheric pollutants are given with respect to the gigajoule of source exergy. Using the data from Table 7, weighted average pollutant emissions are obtained for the Canadian power generation mix. The averages are given in kilogram of pollutant per megawatthour of power generated; in order to convert from source exergy basis to generated power basis, the average Canadian exergy efficiency of power generation is used. Then, the exergy-based EPC for power generation is calculated for each pollutant in dollars per megawatthour; the Canadian average of EPCex is $17.8/MWh. Therefore the cost of pollutant removal from the waste stream is much lower to society than the cost of pollutant emission. Materials used for system construction bring associated embodied energy (EE) and pollution. Table 9 gives the EE, specific pollution, and exergetic pollution cost with various construction materials. The EE represents the amount of energy spent to produce one ton of material. The specific emission (SE) represents the mass of pollution (here kilogram CO2 equivalent emitted in Table 6

Environmental pollution costs (EPC) and removal pollution costs (RPC) for fossil fuels in Ontario

Pollutant

EPC ($/GJfuel

CO2 CH4 NOx CO SO2

exergy)

RPC ($/GJfuel

exergy)

Coal

No. 6 fuel oil

Natural gas

Coal

No. 6 fuel oil

Natural gas

5.2662 5.1724 0.0469 2.1038 0.4958

5.7352 1.1524 0.03082 0.03082 0.402

7.7184 0 0.03082 0.3484 0

3.35 0.8174 0.938 7.5442 2.5594

2.7604 0.1608 0.469 0.0938 1.5544

1.7822 0 0.3886 0.4556 0

Source: Reproduced from Rosen MA, Dincer I. Exergy analysis of waste emissions. Int J Energy Res 1999;23:1153–63. Note: Monetary values are in CN$ 2013.

Table 7

Pollution removal cost for power generation in Canada

Primary energy

Exergy input (PJ)

Power output (PJ)

Exergy destruction (PJ)

Removal pollution cost (RPC) (mill $)

c (%)

Nuclear Hydro Coal Fuel oil Natural gas Natural gas liquids Diesel fuel Biomass Secondary sources Overall

1073 1698 630 62.5 254 33.6 8.3 66 1629 5454

339 1358 378 23 112 8 3.2 19 543 2783

734 340 252 39.5 142 25.6 5.1 47 1086 2671

514 238 176 28 99 20 4 33 760 1870

32 80 60 37 44 24 38 29 33 51

Source: Reproduced from Dincer I, Zamfirescu C. Sustainable energy systems and applications. New York, NY: Springer; 2011 [Chapter 17].

128

Sustainability Dimensions of Energy

Table 8

Lifecycle emissions into the atmosphere for power generation technologies (kg/GJ)

Technology

Kilogram per gigajoule fuel exergy CO2

CH4

NOx

SO2

CO

Coal fired power plants Fuel oil fired power plants Natural gas fired power plants Photovoltaic (PV) power generation Wind power generation Hydro power Nuclear power generation

274 176 112 44 33 9 48

73 47 30 12 9 2 13

Kilogram pollutant per megawatthour power EPCex ($ per megawatthour power)

CO2 42 1.7

CH4 11 14.7

0.180 0.115 0.073 0.045 0.033 0.006 0.032 Canada NOx 0.029 0.1

0.400 1.374 0.200 0.876 0.150 0.558 0.090 0.038 0.070 0.028 0.015 0.044 0.065 0.241 averages SO2 CO 0.060 0.194 0.2 1.1

Volatile organic compound (VOCs)

Particulate matter (PM)

0.251 0.160 0.102 0.003 0.003 0.008 0.044

8.463E-6 5.439E-6 2.557E-6 1.336E-6 1.024E-6 0.268E-6 1.488E-6

VOCs 0.035 0.02

PM 1.288E-6 7.1E-6

Total ($/MWh) 17.8

Sources: Reproduced from Carpenter S. The environmental cost of energy in Canada. In: Sustainable energy choices for the 90’s. Proceedings of the 16th annual conference of the solar energy society of Canada, Halifax, NS; 1990. p. 337–42; De Gouw JA, Warneke C, Stohl A, et al. Volatile organic compounds composition of merged and aged forest fire plumes from Alaska and western Canada. J Geophys Res 2006;111:D10303; Pellizzari ED, Clayton CA, Rodes CE, et al. Particulate matter and manganese exposures in Toronto, Canada. Atmos Environ 1999;33:721–34; Rosen MA, Dincer I. Exergy analysis of waste emissions. Int J Energy Res 1999;23:1153–63. Abbreviation: EPCex ¼ exergetic environmental pollution cost.

Table 9 Embodied energy (EE), specific emission (SE) and exergetic pollution cost of construction materials (CM) Material

EE (GJ/t)

SE (kg CO2/GJ)

EPCex ($2013/GJ)

Concrete Iron Steel Stainless steel Aluminum Copper Fiberglass

1.4 23.5 34.4 53 201.4 131 13

24 11 11 62 10 57 62

70.5 29.3 29.3 29.3 29.9 60.1 66.0

Source: Reproduced from Rosen MA, Dincer I. Exergy analysis of waste emissions. Int J Energy Res 1999;23:1153–63. Abbreviation: EPCex ¼ exergetic environmental pollution cost; SE ¼ specific greenhouse gas (GHG) emissions.

Table 10 Embodied exergy (EEx) and the environmental pollution costs (EPCs) Material

EEx (GJ/t)

EPC ($/t)

Concrete Iron Steel Stainless steel Aluminum Copper Fiberglass

1.3 21.1 31 47.7 181.3 117.9 11.7

102 687 1009 1553 6023 7873 858

the atmosphere) per gigajoule of energy used in the fabrication process of the material. Concrete, copper, and fiberglass have the highest EPC among the listed materials. The highest EE is due to aluminum fabrication, which as it is known, requires an energyintensive electrochemical process. Using an averaged quality factor, the embodied exergy (EEx) can be obtained from the EE in the construction materials. Table 10 gives the EEx and EPCs for the respective construction materials. Using the concepts introduced here, the exergy destruction can be utilized to determine the EI of a system in various manners as follows:

• •

The removal of pollution cost can be approximated by multiplying the exergy destruction with the exergy price estimated for a specific region or country. The rate of exergy input into the system can be used to determine the system physical size.

Sustainability Dimensions of Energy

• • • • • • • •

129

From the system physical size, the mass amounts of materials required for system construction are determined. The amount of each construction material correlates with embedded energy required for its extraction and with GHG emissions and EPC (see Table 9). The lifecycle total exergy input into the system is equal to the exergy required for system construction, system operation, and system salvage. Based on the lifecycle, total exergy input, and the exergy efficiency of power and heat generation system, the exergy destruction can be determined at power and heat generation. The emissions and EPC due to lifecycle total exergy supply are determined based on the exergy destruction and exergetic EPC of the power and heat generation subsystem. The exergy destruction of the system itself allows for determination of pollutant wastes and EPC for operation during the entire LT. If a percent of materials recycling is provided, then the wasted energy and emissions of scrapped system can be determined. The total pollutant emissions and EPC result from the summation of the terms associated to power and heat generation from primary sources, system manufacturing, system operation, and system scrapping.

Scaling factors (SFs) are important for predicting the cost of a scaled-up system in correlation with its size (or production capacity). Typically, for many process equipment and chemical plants, the SF is of the order of 0.6. Note that the system size and capacity can be correlated to the produced exergy or consumed exergy. This means that the amount of construction materials required to build an energy system can be approximated based on a scaling law correlated to the input exergy. Therefore the amount of construction materials (AM) required to build the system can be modeled as follows: _ 0:6 AM ¼ SF Ex in

ð21Þ

_ in is the exergy input. where SF is the scaling factor and Ex The SF depends on the type of the equipment (or plant) and the type of the material. EEx and the amount of materials can be used to estimate the cost of materials as mapped on an exergy value. The following formula can be used to estimate cost of construction materials (CM): CM ¼ EEx
AM
SExC

ð22Þ

where EEx is the embodied exergy in one ton of materials (cf. Table 10) and specific exergy cost (SExC), which cf. Table 2, which is 8.4¢/kWh or 23.3$/GJ. In addition, the cost associated with the environmental pollution can be specified for each material with the help of the indicator called EPC given in dollar per ton of pollutant. Table 10 gives the EPC associated to the fabrication of some materials. Accordingly, the CP due to the use of a specified construction material can be determined as follows: CP ¼ AM
EPC

ð23Þ

The total cost of material and total CP are added to determine the capital cost (CC) in a fair way, which accounts for the sustainability aspect, as follows: CC ¼ CM þ CP

ð24Þ

Assume that the money to cover CC is covered by capital bonds (CBs) offered by the government due to policy on alternative energy and invested capital (IC) sourced both from private funds and government. Denote r the fraction of CB from total CC. Then the IC becomes: IC ¼ ð1

r ÞCC

ð25Þ

The operation and maintenance cost can be modeled as a factor (fo&m) of the cost associated with exergy destruction. Denote with payback period (PBP) in years. Then, the total O&M cost for the duration of PBP is given as follows: _ d SExC fo& m PBP Co& m ¼ 3600tyear Ex

ð26Þ

The total cost spent to have and to run the system during the PBP, meaning the sum of IC plus the cost of operation and maintenance, is Ctot ¼ IC þ Co& m

ð27Þ

Assume that the energy system produces the amount of exergy Exprod during the whole PBP. The value of the produced exergy is then equal to ExprodSExC. If the total cost is smaller than the value of the produced exergy, namely, CtotoExprodSExC, then the difference of ExprodSExC Ctot represents exergy cost savings. Therefore one can define levelized exergy cost savings (LExS) as follows: LExS ¼ SExC

Ctot Exprod

ð28Þ

where LExS is measured in dollars per gigajoule. In Eq. (28) the LExS is a parameter that accounts for EI due to system construction (through EEx and EPC associated with materials fabrication) and also accounts for the costs due to operation and maintenance (through the factor fo&m, which can incorporate the CP). Therefore LExS as defined is a parameter that quantifies the sustainability. If LExS is positive, then the system may be sustainable; however, if LExS calculated with Eq. (28) results negative, then the system can be considered as nonsustainable.

130

Sustainability Dimensions of Energy

In order to estimate the extent to which the system is sustainable, the system model shown in Fig. 17 will be used. We base the sustainability assessment on the exergy method; therefore this is denoted as an exergosustainability assessment, and it is part of the exergetic lifecycle assessment. The exergy balance equation must be written for the whole LT of the system. As a rule of thumb, the LT is three times longer than PBP. If there is net exergy output in state 2 (Fig. 17), then the system is sustainable because in that case, the system is able to produce sufficient exergy to construct itself and pay for its emissions. Otherwise, if net exergy in #5 is zero or negative, the system is not sustainable. As can be deduced from the figure, the net exergy amount Ex7 can be approximated as follows: Ex7 ¼ Ex5

ð29Þ

Ex6

The term Ex6 in Eq. (29) represents the exergy amount embedded in construction materials plus the exergy required to remove the pollutants emitted during the construction process from the atmosphere. This exergy can be related to the CC with the help of the SExC. One obtains the following: Ex6 ¼

CC SExC

ð30Þ

An ExSI can be defined showing the following ratio of exergies ExSI ¼

Ex7 Ex5

ð31Þ

The ExSI is subunitary. For a system to be sustainable, the ExSI must be positive and close to 1. The exergy amount delivered to the user results from combining Eqs. (29) and (31) as follows: _ 5 Ex7 ¼ 3600LT tyear Ex

CC SExC

ð32Þ

Therefore the exergy SI becomes: ExSI ¼ 1

1.4.6

CC _ 5 SExC 3600LT tyear Ex

ð33Þ

Clean Energy Solutions for Better Sustainability

In this section, clean energy solutions to achieve better sustainability are discussed. The possible sustainable energy solutions include mainly renewables and hydrogen, which form the basis for clean energy systems. Those clean energy solutions are revised here comparatively and ranked according to their outputs. In this respect, the work of Dincer and Acar [28,29] is taken as reference. The ranking of the energy sources is based on technical, economical, and environmental criteria, which are then compounded to obtain aggregated indicators for ranking. Clean energy systems have the potential to do the following: (1) reduce emissions by taking advantage of renewable and cleaner sources; (2) lower energy input requirements; (3) increase system efficiencies by expanding useful outputs (i.e., multigeneration); and (4) reduce emissions and waste by recovering energy. In Refs. [28,29], the renewable energies are qualified as sustainable energy based on dispatchability, geographical diversity, predictability, and control criteria. Indicators were used to quantify the sustainability of renewable energies based on those four criteria. Each indicator has been set for a scale from 1 to 10. Then an aggregated indicator has been calculated as the average of particular indicators. The results are shown in Fig. 19 in the form of a chart. The dispatchability indicator is ranked at maximum value (rank 10) when the power generator can be loaded from zero to full capacity without significant delay. Geographical diversity indicator quantifies the degree to which siting of the technology may mitigate variability and improve predictability, without substantial need for additional network. The technology with 100% mitigation potential has a rank 10 assigned for the geographical diversity. The predictability indicator quantifies the accuracy to which plant output can be predicted at relevant time scales. The control indicator shows technology capability of active control and response during normal situations (steady state, dynamic) and during network fault situations. In terms of dispatchability, biomass and geothermal have the highest performance, while ocean and wind have the lowest dispatchability. Wind has the highest geographical diversity; on the other hand, it has very low predictability. In terms of control, biomass, geothermal, and hydropower provide better performance. Overall, biomass and geothermal are closest to the ideal case, and wind shows the poorest performance. Electric power production from renewable energy is of significant interest in today’s world, both in centralized and decentralized systems. In Dincer and Acar [28], a comparative assessment table is compiled giving the annual generation (TWh), capacity factor, mitigation potential (GtCO2), energy requirements (in kilowatt-hour thermal per kilowatt-hour electric), specific GHG emissions (gCO2/kWh), and production cost (US¢/kWh) (Table 11). NSIs were then calculated based on each parameter. For the parameters that needed to be maximized, i.e., annual generation, capacity factor, and mitigation potential, Eq. (7) multiplied by factor 10 is used to determine the normalized sustainability potential. For parameters that need to be minimized, i.e., energy requirements, specific GHG emissions, and production cost, the

Sustainability Dimensions of Energy

131

Normalized sustainability indicator

10

8 Biomass Geothermal Hydropower

6

Ocean Solar 4

Wind

l tro on C

ilit y ab ic t ed Pr

G di eog ve r rs ap ity hi c

D

is

pa

tc

ha

bi lit y

2

Fig. 19 Normalized sustainability indicators (NSIs) for various power generation technologies.

Table 11

Normalized sustainability indicators (NSIs) for renewable energies

Energy source

NSIAG

NSICF

NSIMP

NSIER

NSIGHG

NSIPC

Coal Oil Gas Nuclear fusion Biomass Geothermal Hydro (large scale) Hydro (small scale) Ocean Solar (photovoltaic (PV)) Solar (CSP) Wind

10 1 5 4 0 0 4 0 0 0 0 0

9 8 6 10 6 9 4 5 2 0 2 1

0 0 0 4 2 5 5 3 6 2 2 10

1 9 8 0 10 0 0 0 1 7 1 0

0 9 6 2 0 0 2 0 1 1 0 0

10 0 0 0 0 1 1 3 6 10 6 0

Source: Reproduced from Dincer I, Acar C. A review of clean energy solutions for better sustainability. Int J Energy Res 2015;39:585–606. Abbreviations: AG, annual generation; CF, capacity factor; CSP, concentrated solar power; ER, energy requirements; GHG, specific greenhouse gas emissions; MP, mitigation potential; PC, production cost.

following equation is used to determine the normalized indicator: NSIi ¼ 10

max fIndi jiA f1; ngg Indi max fIndi jiA f1; ngg minfIndi jiA f1; ngg

ð34Þ

The obtained NSIs are given in Table 10. These results show that large-scale hydro and nuclear options have the highest annual generation, and solar concentrated solar power (CSP) has the lowest. In terms of capacity factor, nuclear and geothermal give the closest to ideal case results, while solar PV has the poorest performance. Mitigation potentials show that wind gives the ideal results, and biomass has the least among the selected options. Geothermal and hydro have the ideal energy requirements, and biomass has the poorest performance. Hydro has the lowest emissions, while solar technologies have the highest. When it comes to production costs, nuclear, wind, and biomass have the best performance, while ocean and solar technologies have the highest production costs per kWh electricity (Table 12). The aggregation of the NSIs has been done in Dincer and Acar [28] by averaging. The aggregated indicator is used to rank the sustainable technology. The bar chart shown in Fig. 20 compares the ASI for the renewable energies. Nuclear has the highest ranking compared with renewables because it is already seen as a mature technology. In 2012, nuclear contributed 10.9% of the total global electricity generation, while this number is 21.2% for all renewables combined. Nuclear also has a capacity factor of 86%, which is among the highest of all technologies and a competitive-levelized cost between 4 and 7 US¢/kWh. Wind is the second strongest option with an annual growth rate around 34%. Wind technology is simple, and it is mature in developed countries. Although wind energy is a small industry, it is competitive. Hydropower contribution percentage to overall

132

Table 12

Sustainability Dimensions of Energy

Normalized sustainability indicators (NSIs) for nonatmospheric emissions of energy sources

Energy

Land use

Water consumption

Quality of discharge

Ground contamination

Biodiversity

Coal Gas Nuclear Biomass Geothermal Hydro (with storage) Hydro (run of river) Ocean Solar (photovoltaic (PV)) Wind

High 0 Moderate 3.3 Moderate 3.3 Low to high 3.3 Low 6.6 High 0 Low 6.6 Low 6.6 Low to high 3.3 Moderate 3.3

High 0 Low 6.6 High 0 Moderate 3.3 Zero 10 Moderate3.3 Low 6.6 Zero 10 Zero to low 8.3 Zero 10

Moderate to high 1.6 Zero to high 5 High 0 Moderate 3.3 Low 6.6 Moderate 3.3 Zero 10 Zero 10 Low to high 3.3 Zero 10

Low to high 3.3 Low 6.6 High 0 Low 6.6 Zero 10 Moderate 3.3 Zero 10 Zero 10 Zero 10 Low 6.6

High 0 Low 6.6 Moderate to high 1.6 High 0 Low 6.6 Moderate 3.3 Low 6.6 Low 6.6 Zero 10 Low 6.6

Source: Reproduced from Dincer I, Acar C. A review of clean energy solutions for better sustainability. Int J Energy Res 2015;39:585–606.

8

7.06 6.49

6.57

6.44

6

5.4

ASI

4.17 4

3.14

2.66

2.3

2

d W

SP r(

C

in

)

) r( So

la

la So

O

ce

PV

an

) le ca ls al

ro yd H

H

yd

ro

(s

G

(la

m

rg

eo

e

th

sc

er

al

m

e)

al

s as om Bi

N

uc

le

ar

fu

si

on

0

Fig. 20 Aggregated sustainability indicators (ASIs) for nuclear and renewable energies. CSP, concentrated solar power; PV, photovoltaic.

renewable electricity generation is expected to decrease as geothermal, solar, wind, biomass, and ocean electricity generation technologies evolve. Various EIs can be considered when assessing the sustainability of energy technologies. Beside the atmospheric pollution, the following EIs are relevant: land use, solid waste and ground contamination, biodiversity, water consumption, and quality of discharge. Table 12 gives a sustainability assessment of energy technologies ranked with normalized indicators, which quantify nonatmospheric pollution. The following ranks are assigned to the values of the indicators: zero (10), low (3.3), medium (6.6), and high (0). When compared with the other options presented in the table, solar (PV and thermoelectric) has the lowest nonair impact. However, the water quality/discharge issue should be addressed. Coal has the highest EI, which is expected. In regard to nuclear power, radioactive waste and contamination appear to be major concerns as they need careful treatment and handling. Another concern may be high water consumption in nuclear power plants. Land use of hydropower and adverse impact of biomass on biodiversity should also be addressed in order to make them more sustainable. The NSIs for nonatmospheric emissions of energy sources are aggregated by averaging. The aggregation results are shown in Fig. 21, where the energy technologies are ranked. Ocean, geothermal, and wind result as the best sustainable technology according to nonatmospheric pollution-based indicators. The worst options in this respect are coal and nuclear energy. Besides power generation, the other major use of energy is for heating and cooling applications. The demand of heating and cooling worldwide is significant. Conventional heating uses fossil fuel combustion. However, many new applications relate to renewable energies for heating and cooling. Residual heat from industry is a beneficial source for nonconventional heating and cooling applications. Renewable energy systems for heating and cooling use solar, geothermal, and biomass as sources. Ocean thermal energy can potentially contribute to dispatching heating and cooling demands. Heat pumps, district heating, bathing/ swimming, pond heating, drying, refrigeration, HVAC, and industrial heat requirements are some of the current methods of heating/cooling use. The cooling can also be produced by renewable energy-based absorption cooling. The renewable heating and cooling technologies were ranked for sustainability assessment according to their typical capacity (MW), IC, capacity factor, and system LT. Normalized indicators were obtained for these ranking criteria. The ASI for those technologies is determined as an average of the NSIs. The results are shown in Fig. 22. The ideal ranking is 10. It shows that geothermal district heating, geothermal pond-based heating, and biomass steam turbine cooling heating and power are the best

Sustainability Dimensions of Energy

133

7.96

8

7.3

6.98 5.62

6 4

3.3 2.64

2

0.98

0.98

(ru n

W

in

d

(P V)

So l

ar

riv er ) O ce an

e)

of

al

H yd ro

eo

H yd ro

G

(w ith

th e

st or ag

rm

as s m

ar Bi o

N uc le

G

C oa

as

0

l

Non-atmospheric pollution ASI

8.64 7.96

Fig. 21 Nonatmospheric pollution-based aggregated sustainability indicators (ASIs) for energy sources. PV, photovoltaic.

Solar domestic hot water Geothermal heat pumps Geothermal ponds Geothermal greenhouse heating Geothermal district heating Geothermal building heating Biomass anaerobic digestion CHP Biomass steam turbine CHP Biomass municipal solid waste CHP Biomass domestic heating 0

1

2

3

4

5

6

7

8

ASI Fig. 22 Aggregated sustainability indicator (ASI) for renewable heating technologies. CHP, combined heat and power.

sustainable technologies. Overall, the averaged ASI for biomass sources is the best with 5.2/10 ranking, followed closely by geothermal with 4.9/10 and solar heating with 2.3/10.

1.4.7 1.4.7.1

Case Studies Exergosustainability Assessment of a Concentrated Photovoltaic-Thermal System for Residential Cogeneration

In this case study, a novel concentrated photovoltaic (PV) thermal system that combined PV power generation with a special Rankine engine is to be assessed. Fig. 23 shows a system description. The system has several compound parabolic concentrators (CPCs) of through type installed on the southward face of a residence. The CPC concentrates light both on a vapor generator and small-area PV module. The vapor generators produce high-pressure cyclohexane vapors in a calandria (thermosiphon) loop that is part of an organic Rankine cycle (ORC). The system generates power through concentrated PV and the ORC jointly, and heat for water heating through the ORC condenser. The following emerging energy technologies are integrated in this system: concentrated PV with CPC as hybridized with an organic vapor generator, cyclohexane ORC with thermo-mechanical solar energy storage and cogeneration. The hybridized CPC system is detailed in Fig. 24. It captures sunlight under a half acceptance angle (yc ¼ 60 degree) and an aperture Aa. All light captured at this angle will reach the receiver surface of aperture Ar and never escape back. Part of the light is absorbed directly by a copper tube coated in black. The tube is placed in a glass shell, which is vacuumed. Inside the tube, saturated cyclohexane vapors are generated in form of bubbles. A part of the light falls on the PV modules installed at the back. The modules are covered with low-band reflection coating.

134

Sustainability Dimensions of Energy

Southward roof 2 Compund parabolic concentrator

3 Vapor accumulator

11

1

4

5

Thermo-syphon loop 6

Regenerator 7 11

Condenser Hot water tank (cogeneration) 9 10 8

Fig. 23 Concentrated photovoltaic (PV) thermal system for residential cogeneration.

Direct radiation

Aa

Concentrated PV module covered with low band reflecting coating c

Black-coated copper tube with vacuumed glass shell (vapor generator)

APV/2

Fig. 24 Hybridized compound parabolic concentrator (CPC) for concentrated photovoltaic (PV) and vapor generation (cross-sectional cut).

The PV coating is of dielectric type and reflects all light with wavelength longer than 900 nm. This radiation is eventually absorbed by the vapor generator (mainly) after repeated reflection on the CPC, as it cannot escape back through the aperture. The vapor accumulator is well insulated thermally as it keeps (all day including overnight) hot vapors at 1201C under pressure. The

Sustainability Dimensions of Energy

Table 13

135

Materials amount correlation and environmental parameters

Material

Scaling factor (SF)

Concrete Iron Steel Stainless steel Aluminum Copper Fiberglass

1E-5 1.3E-4 6.6E-4 2.6E-4 4.5E-4 1.5E-4 3E-5

Specific emission (SE) (kgpollutant/tmaterial) SECO2

SECH4

SENOx

SESO2

SECO

SEVOC

SEPM

2.1E þ 1 5.8E-1 3.9E-1 1.4E þ 0 6.1E-2 5.4E-1 5.9E þ 0

5.3E þ 0 1.5E-1 1.0E-1 3.8E-1 1.6E-2 1.4E-1 1.5E þ 0

1.4E-2 4.0E-4 2.7E-4 1.0E-3 4.2E-5 3.7E-4 4.1E-3

2.8E-2 8.1E-4 5.5E-4 2.0E-3 8.8E-5 7.5E-4 8.2E-3

9.2E-3 2.7E-4 1.8E-4 6.7E-4 2.8E-5 2.5E-4 2.7E-3

1.7E-2 4.7E-4 3.2E-4 1.2E-3 5.1E-5 4.5E-4 4.9E-3

6.4E-7 1.8E-8 1.2E-8 4.4E-8 1.9E-9 1.7E-8 1.8E-7

Abbreviations: PM, particulate matter; VOC, volatile organic compound.

pressurized vapor in #3 generates work by passing through an expander. A part of the expanded vapors are extracted at an intermediate pressure in #5, cooled to 581C as in #7 and condensed in a coil, in #7–8 immersed in a water tank that stores water at approximately 451C for service (kitchen, bathroom). A part of the expanded vapors is extracted to low pressure in #4, and cooled at 301C in #6 and then condensed at a condenser temperature of B201C. The condenser rejects the heat in a ground-coil, buried at a depth of B2 m, such that the temperature remains constant throughout the year. Assumptions: The following assumptions are made for this case study:

•

•

•

•

•

_ 1 For determination of exergy input Ex ○ total area concentrator exposed to Aa ¼ 100 m2 ○ total area of PV arrays APV ¼ 1 m2 ○ the total annual irradiance is of Elight ¼ 1.8 MWh/m2 ○ the total number of sufficient sunshine is tyear ¼ 4000 h per year ○ the sunlight spectrum can be assimilated to that of a blackbody at Tsun ¼6000K ○ the reference temperature is taken as T0 ¼ 298K For calculating the photocurrent ○ the transmittance of the PV coating T l is zero for lo200 nm and for l4900 nm ○ the average quantum efficiency of the PV cell is Fe ¼ 0.8 ○ the optical efficiency of concentrator is Zopt ¼ 0.8 ○ the band gap temperature Tg ¼ 11,000K ○ the diode nonideality factor ni ¼ 1.5 ○ the temperature of the PV cell is TPV ¼ 901C ○ the resistance of PV array is Rs ¼1E-5 O For calculation of the ORC ○ thermal efficiency of solar concentrator Zth ¼0.7 ○ energy efficiency of the ORC ZORC ¼ 0.25 ○ the temperature of heated water T4 ¼501C ○ supply temperature of water Tw ¼ 151C ○ amount of hot water produced per day mw ¼ 2000 kg ○ specific heat of water is cp ¼4285 J/kgK For the economic analysis ○ SExC ¼$23/GJ ○ fraction of CBs from total CC, r ¼0.3 ○ the operation and maintenance fraction fo&m ¼ 1E-9 ○ the PBP ¼ 5 years For the exergosustainability analysis ○ the system LT ¼ 20 years

Table 13 gives the SFs for the materials used by the system, as well as the specific pollution due to fabrication processes using various materials.

1.4.7.1.1

Thermodynamic analysis

Here, the thermodynamic analysis aims ultimately to quantify the irreversibilities, that is, to determine the exergy destruction. Exergy balance equation is applied for the overall system, which is described as shown in Fig. 25. The only input is the exergy from the sunlight, while the outputs delivered are the power (produced by the ORC engine and PV) plus the exergy associated to the heating of water. The exergy balance equation for the overall system states that exergy input is equal to the exergy delivered to the

136

Sustainability Dimensions of Energy

PV-power Sunlight exergy input 1

ORC-power

Overall system

Heating

2 3

Exergy to 5 the user

4

Exergy destroyed

Reference environment at T0

Fig. 25 Description of exergy balance for the overall energy system. ORC, organic Rankine cycle; PV, photovoltaic.

Table 14

Parameters related to the photovoltaic (PV) array modeling

Parameter

Equation

Saturation current density

 J0 ¼ ð1:5E 9Þexp   J voc ¼ ni ln 1 þ Jph0

Dimensionless open circuit voltage Open circuit voltage

Tg TPV



Voc ¼(kBTPV)/(e)voc

  Rs Jph APV 1 Voc R1 E A IPV ¼ Zoptic t AlightsTr 4 0 T l Il;b dl year a ð sun Þ

Filling factor (FF)

FF ¼

The concentrated light irradiance incident on the PV surface

voc lnðvoc 0:72Þ voc þ1

user plus exergy destroyed. The exergy balance is: _ 1 ¼ Ex _ 5 þ Ex _ d Ex

ð33Þ

_ 5¼W _ 2þW _ 3 þ Ex _ 4 Ex

ð34Þ

where the exergy delivered to the user is

_ 3 the power generated by ORC, and Ex _ 4 is the exergy associated to the water heating _ 2 being the power generated by PV, W with W process. _ 1 , is: Therefore, the exergy rate of the light falling on the heliostat mirrors, input Ex   T0 _ 1 ¼ Elight 1 Ex ð35Þ Aa Tsun tyear _ 2 one accounts for the fact that when the concentrated sunlight of partial spectrum falls on the For the calculation of W photocathode a photonic current is generated as follows: Z 1 e Elight Ar
Fe Jph ¼ Zoptic lT l Il;b dl ð36Þ 4 hc tyear Aa sTsun 0

4 where sTsun approximates the irradiance of sun, Fe is the average quantum efficiency, T l is the transmittance of the PV coating, Zoptic is the optical efficiency of the concentrator, and Il,b is the spectral irradiance of blackbody at temperature Tsun. The power developed by the PV array is given by

_ 2 ¼ FF V oc Jph W

ð37Þ

Table 14 gives the quantities that must be calculated for the PV array. Note that kB is Boltzmann constant kB ¼ 1.381E-23 J/K and e is the universal electric charge e¼ 1.602E-19 C. Furthermore, one defined the thermal efficiency of solar _ a and light energy on the absorber E_ light;abs . This efficiency is given as concentrator as being the ratio between heat absorbed Q follows: Zth ¼

_ abs Q _Elight;abs

ð38Þ

Moreover, the energy efficiency of the ORC is defined as the ratio between the network generated and heat absorbed by vapor _ abs . Mathematically, one writes: generator Q ZORC ¼

_3 W _ abs Q

ð39Þ

The energy balance for the photonic radiation on the solar concentrator states that the light energy rate passing through the aperture must be equal to the light energy rate absorbed on the PV array plus the light energy rate on the thermal absorber (that is

Sustainability Dimensions of Energy

137

the vapor generator); this statement is expressed as follows: Elight Aa ¼ IPV APV þ E_ light;a tyear

ð40Þ

Based on the thermal efficiency of the solar concentrator the above equation becomes: _ abs Elight Q ¼ Aa Zth tyear

ð41Þ

IPV APV

Furthermore, based on the ORC efficiency, the above equation can be manipulated to determine the power generated by the ORC engine, as follows:   _ 3 ¼ ZORC Zth Elight Aa IPV APV W ð42Þ tyear The exergy associated with the water heating delivered to the user is given as follows:   _ 4 ¼ mw cp ðT4 Tw Þ 1 T0 Ex T4 24  3600 From Eqs. (24), (29), and (30), the total exergy delivered to the user becomes:    _ 5 ¼ voc lnðvoc 0:72Þ 1 eRs Jph APV kB TPV voc Jph þ ZORC Zth Elight Aa Ex tyear kB TPV voc e voc þ 1 þ

1.4.7.1.2

 mw cp ðT4 Tw Þ 1 24  3600

T0 T4

ð43Þ

IPV APV





ð44Þ

Results

A simple engineering equation solver (EES) code is created calculate the integrals required to determine the values of Jph, voc, and IPV for the PV array. The code is listed as given in Table 15. The main results with the code are the following: Jph ¼ 7949 A/m2, voc ¼27.21, and IPV ¼ 21,976 W/m2. The following results are obtained based on the assumed data for the case study analyses presented above. The light exergy input becomes   T0 _ 1 ¼ Elight 1 Aa ¼ 42;765 W Ex Tsun tyear Total exergy output is calculated as follows: _ 5 ¼ voc Ex

 lnðvoc 0:72Þ 1 voc þ 1 þ

  Elight eRs Jph APV kB TPV voc Jph þ ZORC Zth Aa tyear e kB TPV voc

 mw cp ðT4 Tw Þ 1 24  3600

IPV APV



 T0 ¼ 9506 W T4

The total exergy destroyed results as follows: _ 1 _ d ¼ Ex Ex

_ 5 ¼ 33;259 W Ex

_ 1 ¼ 42;765 W as given in Table 16. Also given the assumptions r ¼0.3, fo&m ¼1E-9, The pollutant emissions are calculated for Ex PBP ¼5 years, the total cost CC corresponding to the PBP can be determined with Eq. (26) in which the IC is obtained from Table 15

Engineering equation solver (EES) code to calculate the photovoltaic (PV) array parameters

$units J K //Given E_light¼ 1.8E6 "Wh/m2" t_year¼4000"h" T_sun¼ 6000 A_a¼100"m2" A_PV¼1 T_g¼11000 n_i¼1.5 T_PV¼90 þ T_zero#

R_s¼1E-5 eta_optic¼0.8 Phi¼0.8 Tau¼0.9integr¼integral(lambda/1e6*Phi*Tau*Eb(T_sun,lambda),lambda,0.2,0.9) J_ph¼eta_field*e#/h#/c#*A_a/A_PV*E_light/t_year/sigma#/T_sun̂4*integr J_0¼(1.5E9)*exp(-T_g/T_PV) v.oc¼n_i*ln(1 þ J_ph/J_0) I_PV¼integral(Phi*Tau*Eb(T_sun,lambda),lambda,0.2,0.9)*E_light/t_year/sigma#/T_sun̂4*A_a/A_PV

138

Table 16

Sustainability Dimensions of Energy

The results regarding the atmospheric pollutants

Material

Amount AM (t)

Concrete Iron Steel Stainless steel Aluminum Copper Fiberglass Total (kg)

1.20E-02 1.56E-01 7.93E-01 3.12E-01 5.41E-01 1.80E-01 3.60E-02

Pollutant emissions, Epollutant (kgpollutant) ¼AM  SEpollutant ECO2

ECH4

ENOx

ESO2

ECO

EVOC

EPM

2.5E-01 9.1E-02 3.1E-01 4.4E-01 3.3E-02 9.7E-02 2.1E-01 1.4E þ 00

6.4E-02 2.3E-02 7.9E-02 1.2E-01 8.6E-03 2.5E-02 5.4E-02 3.7E-01

1.7E-04 6.2E-05 2.1E-04 3.1E-04 2.3E-05 6.7E-05 1.5E-04 9.9E-04

3.4E-04 1.3E-04 4.4E-04 6.2E-04 4.8E-05 1.4E-04 3.0E-04 2.0E-03

1.1E-04 4.2E-05 1.4E-04 2.1E-04 1.5E-05 4.5E-05 9.7E-05 6.6E-04

2.0E-04 7.3E-05 2.5E-04 3.7E-04 2.8E-05 8.1E-05 1.8E-04 1.2E-03

7.7E-09 2.8E-09 9.5E-09 1.4E-08 1.0E-09 3.1E-09 6.5E-09 4.4E-08

Abbreviations: AM, amount of construction material; PM, particulate matter; VOC, volatile organic compound.

Eqs. (21–24) and the cost of operation and maintenance is obtained from Eq. (25). Thence, for the levelized exergy savings by using the system becomes:   _ d SExC fo& m PBP ð1 r ÞCC þ 3600tyear Ex $ LExS ¼ SExC 109 ¼ 4:36 _ 5 GJ 3600PBP tyear Ex Given system LT ¼20 years, the ExSI is written as ExSI ¼ 1

CC ¼ 0:76 _ 5 SExC 3600LT tyear Ex

Therefore the system is sustainable since only 100% 76% ¼ 24% of produced exergy can be used to build the system itself and to compensate pollution, whereas the remaining 76% is delivered to the user. In order to encourage the use of renewable energy a policy plan can be established as follows: _ d . This will 1. To encourage research and development by providing focused grants aiming at reducing exergy destruction Ex lead to an increase of savings on exergy and therefore to better sale of the system. Also, the ExSI will improve. Assume that this measure aims at 5% reduction of exergy destruction. Then the expected effect of this measure is written as _ d -0:95Ex _ d. Ex 2. To improve the system’s economic performance by increasing the CBs (which applies a higher fraction r representing the ratio between CBs and CC). This will again increase the exergy savings obtained with the system. Assume that one increases the CBs with 10%; then r-1.1r. 3. To apply higher carbon tax to the coal, petroleum, and natural gas power generators such that the SExC increases in the jurisdiction. This will lead to an improved ExSI and better savings with the system. Consider the application of carbon tax to induce SExC in the jurisdiction higher with 5%, then SExC-1.05SExC. With the considered policy measured, the new values for LExS and ExSI become, respectively: LExS ¼ 1:05SExC

109

ð1

ExSI ¼ 1

  _ d fo& m PBP 1:1r ÞCC þ 3600tyear 0:95Ex $ ¼ 6:34 _ 5 GJ 3600PBPtyear Ex CC ¼ 0:78 _ 5 1:05SExC 3600LT tyear Ex

In conclusion, the policy measures are necessary to encourage the system sales since the exergy savings as well as the ExSI increase.

1.4.7.2

Exergosustainability Assessment of High-Temperature Steam Photoelectrolysis Plant

In this case study, a solar hydrogen production system is considered in which a heliostat field concentrates the sunlight on the surface of a high-temperature photoelectrochemical cell for steam electrolysis, having a light-exposed photocathode. Liquid water is pumped atop of the tower and H2 and O2 gases are generated. The solar radiation is concentrated on photoelectrochemical cells; those are assembled in modules to form arrays. The cell has a transparent glass with a concave shape to increase the strength as the gas inside is at B20 atm. A mixture of 90% steam and 10% H2 is fed, and, due to the photoelectrochemical reaction occurring there, a mixture of 90% H2 and 10% steam is extracted at approximately 10001C. The permeable photocathode has a rough surface (3D, “volumetric” configuration) and it is doped with cheap metallic electrocatalysts and CuO, Cu2O semiconductors that operate as photosensitizers (excite electrons into the conduction band when photons are absorbed).

Sustainability Dimensions of Energy

139

Assumptions: _ 1: The following assumptions are made for determination of Ex ○ total reflecting area heliostat mirrors Ar=915E3 m2 ○ total aperture area Aa=725 m2 ○ the total annual irradiance is of Elight=2.0 MWh/m2 ○ the total number of sufficient sunshine hours is tyear=4000 h per year ○ the sunlight spectrum can be assimilated to that of a blackbody at Tsun=6000K ○ the reference temperature is taken as T0=298K ● The following data for calculating the photocurrent are used: ○ the heliostats have an aluminum reflective surface. The spectral reflectance of aluminum is approximated as follows 8 > < 0:92 for λo700 nm Rλ ¼ 0:88 for λo700 nm and λo900 nm > : 0:98 for λ4900 nm

•

● ●

● ●

○ the spectral quantum efficiency Fe,λ=0.9 for λ≤580 nm and Fe,λ=0.6 for λ≤1033 nm and Fe,λ=0 for λ41033 nm. ○ the field efficiency is given as ηfield=0.7 The following data are assumed for calculation of the exergy content of produced hydrogen: ○ the chemical exergy of hydrogen exch H2 ¼ 236 MJ/kmol The following data are assumed for economic analysis: ○ the SExC=$23/GJ ○ fraction of CBs from total CC, r=0.3 ○ the operation and maintenance fraction fom=1E-9 ○ the PBP=20 years For the sustainability analysis, the following data are assumed: ○ The system LT=40 years The assumed values of the SF and the SEs for the case study are given in Table 17.

1.4.7.2.1

Thermodynamic analysis

Thermodynamic analysis aims to determine the exergy destruction. Exergy balance equation is applied for the overall system is described as shown in Fig. 26. The inputs are the sunlight and the water; and the only useful output is hydrogen (carrying its chemical exergy). The exergy balance determines the exergy destroyed, which accounts for any irreversibility plus the release of oxygen in the atmosphere (as a waste from the process). Table 17

Materials amount correlation and environmental parameters

Material

Concrete Iron Steel Stainless steel Aluminum Copper Fiberglass

SF

SE (kgpollutant/tmaterial)

1381 24 236 118 2 5 1

SECO2

SECH4

SENOx

SESO2

SECO

SEVOC

SEPM

2.1E þ 1 5.8E-1 3.9E-1 1.4E þ 0 6.1E-2 5.4E-1 5.9E þ 0

5.3E þ 0 1.5E-1 1.0E-1 3.8E-1 1.6E-2 1.4E-1 1.5E þ 0

1.4E-2 4.0E-4 2.7E-4 1.0E-3 4.2E-5 3.7E-4 4.1E-3

2.8E-2 8.1E-4 5.5E-4 2.0E-3 8.8E-5 7.5E-4 8.2E-3

9.2E-3 2.7E-4 1.8E-4 6.7E-4 2.8E-5 2.5E-4 2.7E-3

1.7E-2 4.7E-4 3.2E-4 1.2E-3 5.1E-5 4.5E-4 4.9E-3

6.4E-7 1.8E-8 1.2E-8 4.4E-8 1.9E-9 1.7E-8 1.8E-7

Abbreviations: PM, particulate matter; SE, specific emission SF, scaling factors; VOC, volatile organic compound.

Sunlight exergy input

Hydrogen exergy output

1 Overall system

3

Water exergy input 2

Reference environment at T0 Fig. 26 Description of exergy balance for the overall hydrogen production system.

Exergy destroyed

140

Sustainability Dimensions of Energy

The exergy balance equation for the overall system states that exergy input is equal to the exergy output plus exergy destroyed. The exergy input is: _ 3 þ Ex _ d _ in ¼ Ex Ex

ð45Þ

_ d is the destroyed exergy rate and Ex _ in is the rate of input exergy, which is given as follows: where Ex _ 1 þ Ex _ 2 D Ex _ 1 _ in ¼ Ex Ex

ð46Þ

_ 2 ) is negligible with respect to the exergy rate of light input: (Ex _ 1 ). Therefore the In Eq. (46) the exergy rate carried by water (Ex exergy balance equation becomes: _ 3 þ Ex _ d _ 1 ¼ Ex Ex

ð47Þ

_ 1 , is: Furthermore, the exergy rate of the light falling on the heliostat mirrors, input Ex   T0 _ 1 ¼ Elight 1 Ar Ex tyear Tsun

ð48Þ

When the concentrated sunlight falls on the photocathode a photonic current is generated. The photonic current density is given by: Z 1 e Elight Ar
lFe;l Rl Il;b dl ð49Þ Jph ¼ Zfield 4 hc tyear Aa sTsun 0

4 where sTsun approximates the irradiance of sun, Fe,l is the spectral quantum efficiency, Rl is the reflectance of heliostat mirrors, Zfield is the efficiency of the heliostat field, Il,b is the spectral irradiance of blackbody at temperature Tsun. The molar flow rate of produced hydrogen results from the photonic current:

n_ H2 ¼

Jph Aa zF

ð50Þ

where z¼2, F¼96,486,700 C/kmol (Faraday’s constant). The exergy rate carried by the generated hydrogen becomes: _ 3 ¼ n_ H2 exch ¼ Jph Aa exch Ex H2 H2 zF

ð51Þ

where exch H2 ¼ 118 MJ/kmol is the chemical exergy of hydrogen. The total hydrogen produced in kilogram for the PBP results as follows: mH2 ¼ 7200tyear

Jph Aa PBP zF

ð52Þ

Fig. 27 shows the system representation for sustainability analysis, as based on the exergetic lifecycle assessment. If there is net exergy output on hydrogen in state 2, then the system may be sustainable because in that case, the system is able to produce sufficient exergy to construct itself and pay for its emissions. Otherwise, if net exergy in #5 is zero or negative, the system is not sustainable. As deduced from the figure, the net exergy amount Ex5 can be approximated with: Ex5 ¼ Ex3

ð53Þ

Ex4

The term Ex4 represents the exergy amount embedded in construction materials plus the exergy required to remove the pollutants emitted during the construction process from the atmosphere. This exergy can be related to the CC with the help of the SExC. One has:

Source exergy input

1

Ex4 ¼

CC SExC

3

5

Overall system for the lifecycle

ð54Þ

Exergy output to user

Newly constructed system

4

System construction process (exergy is destroyed)

Exergy destroyed at system operation

Exergy destroyed at system construction Materials

Fig. 27 System model for exergosustainability analysis of H2 production system.

141

Sustainability Dimensions of Energy

Therefore the net exergy amount delivered in #5 becomes _ 3 Ex5 ¼ 3600LT tyear Ex

CC SExC

ð55Þ

The ExSI can be defined in a similar way as suggested previously in Eq. (30). Here, using the current notations, the ExSI is written as follows: ExSI ¼

1.4.7.2.2

Ex5 ¼1 Ex3

CC _ 5 SExC 3600LT tyear Ex

ð56Þ

Results

The calculations of the integral of Eq. (49) for the photocurrent density requires the elaboration of a simple code. The sequence of code given in Table 18 can be used to calculate the photonic current density. This code is written in EES and required conversion from microns to m for the wavelength l. Using the code, the following value is obtained, namely, Jph ¼75,061 A/m2. The following results are obtained based on the assumptions for the case study analyses presented above. The light exergy input becomes     T0 2E6 298 _ 1 ¼ Elight 1 1 Ex Ar ¼ 915E3 ¼ 434;777;500 W tyear Tsun 4000 6000 Given Jph ¼ 75,061 A/m2, one calculates the exergy carried by hydrogen as follows: _ 3 ¼ Jph Aa exch ¼ 75; 061  725  118E6 ¼ 66;552;888 W Ex H2 zF 2  96; 486; 700 The total exergy destroyed becomes: _ 1 _ d ¼ Ex Ex

_ 3 ¼ 434; 77; 500 Ex

30;521;184 ¼ 368;224;612 W

_ 1 ¼ 434;777;500 W as given in Table 19. The CC calculations are given in Table 20. Denote The pollutants are calculated for Ex LH2P as the levelized hydrogen price for selling. The following economic balance states that the revenues from hydrogen selling during the PBP balance the total cost: mH2 LH2 P ¼ Ctot Table 18

Engineering equation solver (EES) code to calculate the photocurrent density

$units K J A_r¼915E3 A_a¼725 E_light¼2E6 t_year ¼4000 Int_a ¼0.92*0.6*integral(Lbd_3a/1e6*Eb(T_sun,Lbd_3a),Lbd_3a,0.2,0.58) Int_b ¼0.92*0.6*integral(Lbd_3b/1e6*Eb(T_sun,Lbd_3b),Lbd_3b,0.58,0.7) Int_c ¼0.88*0.3*integral(Lbd_3c/1e6*Eb(T_sun,Lbd_3c),Lbd_3c,0.7,0.9) Int_d ¼0.98*0.3*integral(Lbd_3d/1e6*Eb(T_sun,Lbd_3d),Lbd_3d,0.7,0.9) J_ph¼(Int_a þ Int_b þ Int_c þ Int_d)*eta_field*e#/h#/c#/sigma#/T_sun̂4*A_r/A_a*E_light/t_year

Table 19

The results regarding the atmospheric pollutants for H2 production system

Material

Concrete Iron Steel Stainless steel Aluminum Copper Fiberglass Total (kg)

Amount AM (t)

210,452,416 3,657,392 35,964,352 17,982,176 304,783 761,957 152,391

Pollutant emissions, Epollutant (kgpollutant) ¼AM  SEpollutant ECO2

ECH4

ENOx

ESO2

ECO

EVOC

EPM

4.3E þ 9 2.1E þ 6 1.4E þ 7 2.6E þ 7 1.9E þ 4 4.1E þ 5 9.0E þ 5 4.4E þ 9

1.1E þ 9 5.5E þ 5 3.7E þ 6 6.7E þ 6 4.9E þ 3 1.1E þ 5 2.3E þ 5 1.1E þ 9

3.0E þ 6 1.5E þ 3 9.7E þ 3 1.8E þ 4 1.3E þ 1 2.8E þ 2 6.2E þ 2 3.0E þ 6

6.0E þ 6 2.9E þ 3 2.0E þ 4 3.6E þ 4 2.7E þ 1 5.8E þ 2 1.3E þ 3 6.1E þ 6

1.9E þ 6 9.7E þ 2 6.5E þ 3 1.2E þ 4 8.6E þ 0 1.9E þ 2 4.2E þ 2 2.0E þ 6

3.6E þ 6 1.7E þ 3 1.2E þ 4 2.1E þ 4 1.5E þ 1 3.4E þ 2 7.4E þ 2 3.6E þ 6

1.3E þ 2 6.6E-2 4.4E-1 7.9E-1 5.7E-4 1.3E-2 2.7E-2 1.4E þ 2

Abbreviations: AM, amount of construction material; SE, specific emission; PM, particulate matter; VOC, volatile organic compound.

142

Sustainability Dimensions of Energy

Table 20

Capital cost (CC) calculations for H2 production system

Material

CM ($)

CP ($)

Total ($)

Concrete Iron Steel Stainless steel Aluminum Copper Fiberglass Total

6,292,527 1,774,932 25,642,583 19,728,245 1,270,913 2,066,198 41,009 56,816,407

21,466,146 2,512,628 36,288,031 27,926,319 1,835,706 5,998,884 130,752 96,158,467

27,758,674 4,287,560 61,930,614 47,654,565 3,106,619 8,065,082 171,760 CC¼152,974,874

Abbreviations: CM, cost of construction material; CP, cost of pollution.

The total cost Ctot corresponding to the PBP can be determined with Eq. (26) in which the IC is obtained from Eqs. (21–24) and the cost of operation and maintenance is obtained from Eq. (25). Based on the above equations and the assumptions made and the estimated CC the levelized hydrogen price LH2 P is determined as follows: LH2 P ¼

ð1

_ d SExC fo& m PBP r ÞCC þ 3600tyear Ex 7200

Jph Aa zF tyear PBP

¼ 1:31

$ kg

Given the system LT ¼ 40 years, the ExSI becomes: ExSI ¼ 1

CC ¼ 0:73 _ 3 SExC 3600LT tyear Ex

In order to encourage the use of hydrogen, a policy plan can be established as follows: _ d . This will lead to a 1. Encourage research and development by providing focused grants aiming at reducing exergy destruction, Ex decrease of levelized hydrogen price and therefore to better revenue from sales. Also, the ExSI will improve. _ d _ d -0:95Ex Assumption: Encourage research aiming at 5% reduction of exergy destruction, then Ex 2. Try to improve the system’s economic performance by increasing the CBs (which applies a higher fraction r representing the ratio between CBs and CC). This will again increase the exergy savings obtained with the system. Assumption: Increase the CBs with 10%, then r-1.1r 3. Apply higher carbon tax to the coal, petroleum, and natural gas power generators such that the SExC increases in the jurisdiction. This will lead to an improved ExSI and better revenues with the system. Assumption: Apply carbon tax to induce SExC in the jurisdiction higher with 5%, then SExC-1.05SExC Due to the considered policy measures, the new values for the levelized hydrogen price for sale LH2P and the ExSI become: LH2 P ¼

ð1

_ d fo& m PBP 1:1r ÞCC þ 3600tyear 0:95Ex J A 7200 phzF a tyear PBP

ExSI ¼ 1

¼ 1:22

$ kg

CC ¼ 0:74 _ 3 1:05SExC 3600LT tyear Ex

In conclusion, based on the system assessment, the system is sustainable since only 27% of produced exergy is used to build itself and compensate pollution. However, for better economic success a system improvement may be beneficial. Once policy measures are applied by encouraging research through grants, increase in investment CBs, and application of carbon taxation to power generators, the levelized hydrogen price becomes compatible with the price of hydrogen produced by conventional methods and the ExSI increases to 0.83.

1.4.7.3

Exergosustainability Assessment of a Heat Pump Dryer

In this case study, an industrial Douglas fir wood chips drying process is considered as the reference system for exergy sustainability assessment. The reference system is taken from the previous work of Coskun et al. [30]. The reference system uses combustion gases for drying. An improved system is considered in which the combustor is replaced with a heat pump that recirculates air as drying agent, whereas the air has similar parameters as the flue gases. It is assumed that the improved system is connected to the Ontario regional grid.

1.4.7.3.1

System description

The reference industrial drying system uses a directly heated rotary kiln with an average capacity of 93 t/h moist material. The kiln is supplied with preheated wood chips, which after being dried are separated from the flue gases in cyclones. The system also

Sustainability Dimensions of Energy

143

includes a local gas turbine power plant with cogeneration. The core process, which is the drying process in the rotary kiln, is described in Fig. 28. In the reference system configuration, no humid air is recycled; therefore, all material and exergy in state 4 is wasted. The overall reference drying system configuration is described for a lifecycle exergosustainability assessment in Fig. 29, whereas the state point parameters are given in Table 21. The following remarks are made about the reference drying system: ● State point parameters 9, 11, 12, 14, and 15 were calculated based on balance equations under the following assumptions; the calculated values are shown with italic font in the table ○ mixing processes are ideal (no exergy destruction); ○ there is negligible heat loss in the combustor; and ○ the energy efficiency of the gas turbine is 20% (including the generator losses). ● The reversible work rate of 893 kW is assumed for the drying process, as in Ref. [30]. ● Because no chemical reactions occur, the chemical exergy of wood is not considered; in addition, the industrial process is conducted to process wood as construction material and not firewood. ● The balance of plant require auxiliary power of 670 kW.

1 2

4 Rotary kiln dryer

3

State Description 1 Drying air 2 Moist material 3 Power input 4 Humid air 5 Dry product 6 Destroyed exergy (heat losses) 7 Destroyed exergy (internal irreversibility)

5 6 7

T (K) 739 288 N/A 403 363 309 N/A

P (kPa) 101.325 101.325 N/A 101.325 101.325 N/A N/A

 (g/kg) 72 N/A N/A 217 N/A N/A N/A

(kg/kg) N/A 0.84 N/A N/A 0.006 N/A N/A

m (kg/s) 81.53 14.09 N/A 81.53 14.09 N/A N/A

E (kW) 90,385 1322 765 88,610 3479 383 N/A

Fig. 28 Wood chips drying process in an industrial rotary kiln dryer.

22

Fuel sources

21

Fuel processing

23

Wastes/ pollution

20

Wastes/ pollution

17 Exergy resources

Power generation (grid)

16

18

19 Reference system

Materials sources 24

Improved system

25

Materials processing and system manufacture 26 Wastes/ pollution

27 System scrapping and materials recycling

28

Wastes/ pollution

Fig. 29 Model for lifecycle operations for drying system manufacture, scrapping, and fuel production.

Ex (kW) 20,256 0 765 11,964 323 10 8724

144

Sustainability Dimensions of Energy

Table 21 State 8 9 10 11 12 13 14 15

State points description and parameters for the reference drying system Description

T (K)

P (kPa)

o (g/kg)

Air leak in Flue gas Flue gas Air input Fuel (nat. gas) Flue gas Fuel (nat. gas) Air input

298 749 803 298 298 533 298 298

101.325 100 100 101.325 101.325 101.325 1200 101.325

7.2 74 60.6 7.2 N/A 123.1 N/A 7.2

ṁ (kg/s)

Ė(kW)

Ėx (kW)

2.23 79.3 62.4 61.1 1.26 16.9 0.31 16.47

691 89,694 71,834 2,651 69,183 17,890 17,176 714

0 20,256 7,722 0 65,410 12,534 16,239 0

● The site consumes 2 MW of power for housekeeping and other industrial uses. ● The fuel for gas turbine and combustor is natural gas.

1.4.7.3.2

Results

We now calculate the energy efficiencies and the exergy destroyed by each component and the overall system. The exergy destroyed by the rotary kiln unit is the sum of the exergy wasted as humid air (stream 4); the exergy of the warm, discharged wood; the exergy wasted as heat loss (stream 6); and the internal irreversibility (stream 7). The exergy input into the kiln is the sum of exergies for drying air, moist material, and power. Therefore from Fig. 28 one has: _ in;kiln ¼ 20;256 þ 0 þ 765 ¼ 21;021 kW _ d;kiln ¼ 11;964 þ 323 þ 10 þ 8724 ¼ Ex Ex The exergy efficiency of the process is determined based on given reversible work as follows: _ rev 893 W ¼ ¼ 0:0425 _Exin;kiln 21;021

ckiln ¼

The exergy destroyed in the combustor can be calculated based on the data from Table 21 and it is: _ 11 þ Ex _ 12 _ d;comb ¼ Ex Ex

_ 10 ¼ 57;687 kW Ex

The exergy destroyed by the gas turbine is determined as follows: _ 14 þ Ex _ 15 _ d;gt ¼ Ex Ex

_ 13 ¼ 270:2 kW Ex

The total exergy destruction becomes: _ d;gt þ Ex _ d;comb þ Ex _ d;kiln ¼ 78;979 kW _ d ¼ Ex Ex whereas the total exergy input is _ 12 þEx _ 14 ¼ 81;649 kW _ in ¼ Ex Ex In order to operate the plant consumes 1.57 kg/s natural gas. The annual operation hours are assumed 260 h. The system LT is 25 years. The total amount of natural gas consumed is 36,738 t or 1.91 PJ of exergy. The overall plant produces two useful outputs: the reversible work for drying and generated power for other process (2 MW); therefore the exergy efficiency becomes: c¼

893 þ 2000 ¼ 0:0354 81; 649

The SI of the process is calculated based on Eqs. (12) and (13) as follows: SI ¼

1 1

c

¼

1

1 ¼ 1:03 0:0354

The system sustainability is low as indicated by a low value of the SI (a value close to 1). There should be room for improvement. A more profound analysis must therefore be made, considering the whole lifecycle of the system. We will start this analysis by an attempt to estimate the sizes of system components and from here determine the amounts of materials needed. First, the drying time must be determined. We take the moisture diffusivity for Douglas fir as D ¼32.4 E-10 m2/s. Assume the wood chips have a slab form with an average half width L¼ 0.0002 m. Assume that the equilibrium moisture content is 10% of the final moisture content (this is a reasonable assumption for practical systems). Therefore, W e ¼ 0:1W 5 ¼ 0:0006 kg/kg, and the final dimensionless moisture content becomes: ff ¼

W6 W2

We 0:006 0:0006 ¼ 0:0064 ¼ We 0:84 0:0006

Sustainability Dimensions of Energy

145

Under the assumption that Bim4100 and using a transient drying model for the slab, the following value is obtained for the drying time:   pL2 4 ln Ff ¼ 45s tdry ¼ 4D p Assume that inside the kiln the particles move at the periphery such that the particles’ velocity makes an angle of p/6 with air velocity. The dimensions of the kiln can then be found such that the gas flow regime is turbulent. We approximated that if Uair ¼ 20 m/s, then the required kiln diameter to accommodate the 82 kg/s of air flow rate (as given in the table shown in Fig. 28) is 3.3 m and Reynolds number is 890,000. Furthermore, the wood velocity component in the axial direction becomes Uwood ¼ 0.36 m/s. Therefore the length of the dryer is estimated as Lkiln ¼ Uwood tdry ¼ 17 m If the kiln is made of stainless steel of 8 mm thickness, then the mass of steel is mssteel ¼ pdkilnLkiln ¼10.7 t. A supporting structure of carbon steel will be required, say 80% lighter; mcsteel ¼ 3.4 t. A concrete platform must be casted below the kiln with an approximate size of Lkiln  dkiln  0.3m-mconcreteD40 t. The combustor generates B62 kg/s hot gases at 803K, which is equivalent to 141 m3/s. With a combustion residence time of 5 ms (practical value) and a combustion zone of 0.5 m, the combustor can be approximated as a cylinder with 1.3 m diameter and 5 m height (length), placed vertically; therefore the required metal mass (carbon steel) is 15 t including the supports. The concrete required for the foundation is 3 t. The gas turbine is made of stainless steel with aluminum blades and has a connected generator that comprises mainly copper and iron. The flow rate of air at gas turbine suction is 15 m3/s. The estimated materials are 13 t stainless steel, 2 t carbon steel, 0.5 t aluminum, 2 t iron, 4 t copper for the generator, and 9 t concrete. The construction materials data is summarized in Table 22 where the embedded energy, SEs, and EPC are given. In order to perform the lifecycle assessment, the model presented in Fig. 29 is elaborated. Primary exergy resources are consumed and destroyed to generate the power required for the essential operations during its LT. A part of the consumed power is used for fuel processing (stream 17) in which the stream 22 of refined natural gas is produced and distributed to supply the reference drying system for the LT. Another part of electrical grid exergy is consumed for materials processing and system manufacture, stream 18; and, another part for system scrapping and materials recycling, stream 19. In all processes, wastes and pollution are emitted to the environment. Assume that 10% of scrapped materials are recycled. Therefore the embedded energy for materials processing and manufacture can be approximated with 90% of 2689 GJ given in Table 22; this is 2420 GJ. The SEs will be of 106.7 t GHG with an EPC of $88,194. The exergy consumed to refine and distribute the natural gas represents a small fraction of the fuel exergy. Based on data from lifecycle assessment presented in Dincer et al. [10] for the system lifecycle, the exergy used to extract, refine, and distribute the fuel is estimated to 9950 GJ as given in Table 23. The total exergy consumed for fuel processing, materials extraction, manufacturing, and system scrapping becomes 9950 þ 2420 þ 135¼ 12,505 GJ. Based on the averaged exergy efficiency of the Canadian grid, the consumed exergy resource is 12; 505/0.51 ¼ 24,520 GJ (stream 16 in Table 23). Based on calculations shown in Table 8, the EPC for the Canadian grid is 17.8 $/MWh or 4.9 $/GJ; thence, the EPC associated to power production for materials, manufacturing, fuel processing, and system scrapping is 4.9  24,520 ¼ $120,146. The amount of dried wood chips is 14.9 kg/s  (25  260  3600)s ¼ 3.487E5 t, which are to be valorized on the market. The total expenditure in the fuel is 1.91E6 GJ t  (3.889  52/50) $/GJ ¼ $3.68E6, where the fraction 52/50 is the ratio of chemical exergy to the LHV of natural gas. The total exergy destruction during system operation is 78.979 MW  (25  260) h ¼513.3 GWh ¼ 1.85E6 GJ. The total EPC for the LT is, according to data given in Tables 23, $15.825E6 of which 2% (totaling $325,672) represents the pollution costs associated with materials extraction, manufacturing, system scrapping, and recycling, fuel processing, and distribution; note that the exergy destruction associated with this EPC is 18.3 GWh ¼ 0.07E6 GJ. Therefore the total LT exergy destruction of the reference drying system is Exd,ref ¼1.92E6 GJ. The total exergy input to sustain the LT operation and the system construction and scrapping is from Table 23: Exlc,in ¼24,250 þ 1.91E6 ¼ 1.934E6 GJ. The total Table 22

Pollution parameters of construction materials for drying system

Material

m (t)

EE (GJ)

SE (kg GHG)

EPCex ($2013)

Concrete Iron Carbon steel Stainless steel Aluminum Copper Total

52 2 20 23.7 0.5 4

73 47 688 1256 101 524 2689

1,752 517 7,568 77,827 1,010 29,868 118,542

5,146 1,377 20,158 36,800 3,020 31,492 97,993

Abbreviations: EE, embodied energy; EPC, environmental pollution cost; GHG, greenhouse gas; SE, specific emission.

146

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Table 23

Embedded exergy and environmental pollution costs (EPCs) for reference drying system lifecycle

Stream

Description

Ex (GJ)

EPC ($)

16 17 18 19 20 21 22 23 24 25 26 27 28

Exergy extracted from primary sources for power generation Exergy consumed for fuel processing Exergy consumed for materials processing and system manufacture Exergy consumed for system scrapping and materials recycling Environmental waste stream for grid power generation for lifetime (LT) Primary fuel sources extracted (natural gas) Refined natural gas distributed to consumption point Waste emissions of pollutants due to fuel processing Materials extracted (ores) for system manufacturing Embedded energy in the constructed system Polluting wastes due materials processing and system manufacture Recycled materials Wastes and pollution due to system scrapping and recycling

24,520 9550 2420 135 N/A 1.91E6 1.91E6 N/A N/A 2420 N/A 134 N/A

N/A N/A N/A N/A 120,146 N/A 15.5E6 77,332 N/A N/A 88,194 N/A 40,000

reversible work is Wlc,rev ¼ 873 kW  (25  260) h¼20,428 GJ. The sustainability of the reference drying system can be assessed based on the following parameters: ● Thermodynamic parameters – i.e., ○ Total lifecycle exergy destruction, Exd,ref ¼1.92E6 GJ. ○ Lifecycle exergy efficiency is clc ¼Wlc,rev/Exlc,in ¼ 0.020428/1.934 ¼ 0.01. ○ Specific reversible work (SRW) ¼ Wlc,rev/Exd,ref ¼ 0.020428/1.92 ¼ 0.011. ● Exergoeconomic parameter – i.e., ○ Specific exergetic capital investment (ExCI) defined as the ratio of exergy invested in system construction Exinv ¼ 9950 þ 2420 þ 135 ¼ 12,505 GJ and the amount of reversible work needed to dry the product ExCI ¼ Exinv/Wlc,rev ¼ 12,505/20,428 ¼0.612. ○ Exergetic investment efficiency (ExIE) defined here as the ratio between exergy expenditure for investment and exergy destroyed, ExIE ¼ Exinv/Exd,ref ¼ 12,505/1.92E6 ¼ 0.0065. ○ Capital investment effectiveness defined here by the ratio between exergy input into the drying system for operation during the LT and the exergy consumed for materials extraction and system manufacturing, capital internment effectiveness (CIEx) ¼ 1.91E6/36,490 ¼52.2. ● EI parameter – i.e., ○ Lifecycle EPClc ¼ $15.825E6. ○ EPC for system construction and scrapping EPCcs ¼$128,194. ○ Construction exergy expenditure (ExCDR) to lifecycle exergy destruction ratio ExCDR¼ 0.0081. The sustainability can be assessed based on an ASI. Here, we formulate the ASI based on exergy destruction using thermodynamic, economic, and environmental assessment parameters. In this respect, one notes that the social benefit of the system is proportional to the sum of the reversible work, the exergy investment in the system construction, and the exergy expenditure equivalent to EPC for system construction and scrapping. The benefit of the investment in the reversible work is in fact that the dry product has market and societal value. The benefit exergy expenditure for system construction is in fact the drying system as a good, capable for production. The benefit on exergy investment to compensate for the environmental pollution at system construction and scrapping represents a general societal good consisting of a better environment. The sum of these three terms must be compared with the lifecycle exergy destruction in order to form the ASI. Therefore the equation for ASI becomes: ASI ¼ SRW þ ExIE þ ExCDR

ð57Þ

where SRW is the specific reversible work, ExIE is the exergetic investment efficiency, ExCDR construction exergy expenditure to lifecycle exergy destruction ratio. For the reference system, the ASI becomes ASIref ¼ 0:011 þ 0:0065 þ 0:0081 ¼ 0:0256 This aggregate index must be compared to that for the improved system. The reference system can be improvement with respect to sustainability provided that more sustainable energy sources are used as input and in addition the system irreversibilities are reduced by providing a better design. In the reference system, the energy source is from natural gas. If instead of combusting natural gas, a heat pump is used, supplied from the Ontario regional grid, then the associated EI will be substantially reduced because in Ontario the grid power is relatively clean, as in the energy mix much hydro and nuclear do exist. Also, when a heat

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pump is used, no drying agent is expelled to the environment; therefore, heat is regenerated internally and less energy supply is required. This explains why a heat pump dryer will have a higher exergy efficiency than that of the reference dryer. We assume that the drying process as described in Fig. 28 remains the same. Therefore, the same reversible work is required, _ rev ¼ 839 kW, for the same production rate of 14.09 kg/s dry product. The exergy input required to drive the process must be the W _ in;kiln ¼ 21; 021 kW. same as for the reference case, namely, Ex The heat pump concept is relatively simple and it allows for moisture extraction from humid air prior to heating. Fig. 30 shows the heat pump system for drying air recirculation and moisture removal. In state 4, a flow rate of 81.53 kg/s of humid air with 403K and 217 g/kg humidity ratio enters the heat pump system and it is split in two fractions, #8 and #13. Once expanded to a vacuum pressure, water condensates both in states 9 and 14. However, water from state 9 is gravitationally separated whereas the moisture from state 14 continues to flow together with the air stream. Since water is separated under vacuum, a pump is used to pressurize it to atmospheric pressure in 11. A transcritical carbon dioxide heat pump heats the drying air from state 15 to state 16 where it is recompressed to atmospheric pressure and delivered to the required parameters in state 1: temperature 739K and humidity ratio of 7.2 g/kg. The carbon dioxide heat pump uses the surrounding medium (e.g., a lake) to draw heat at reference temperature T0 ¼ 298K and evaporate the working fluid from state 17 (low vapor quality two-phase mixture) to state 18 (saturated vapor). The vapor is superheated to state 19 using internal heat regeneration, and further compressed so that it is able to deliver heat by heat transfer to the drying air in processes 20–21. Typical mass, energy, and exergy balances are written for each component from Fig. 30 and solved to determine the state points. The heat pump system state points descriptions and thermodynamic parameters are given in Table 24. The net required work input for the heat pump system can be calculated as follows: _ net;in ¼ E_ 1 W

E_ 16 þ E_ 11

E_ 10 þ E_ 20

E_ 18 þ E_ 9

E_ 8 þ E_ 14

E_ 13 ¼ 12;866 kW

The heat input for the heat pump is _ in ¼ E_ 18 Q

E_ 17 ¼ 30;763 kW

The exergy of the heat input is zero because the system boundary is set at T0. The heat output neglecting heat losses is _ out ¼ Q _ in þ Wnet;in ¼ 43;629 kW Q The coefficient of performance (COP) of the heat pump becomes: COP ¼

_ out 43;629 Q ¼ 3:39 ¼ _ 12;866 W net;in

Drying air output 1

13 4 Humid air input

14 8 16

15

20

12 9

11

Transcritical carbon dioxide heat pump

Legend Drying air Removed water Carbon dioxide Work Heat Fig. 30 Heat pump for improved wood chips drying system.

21

10

22 17

19

18

148

Table 24

Sustainability Dimensions of Energy

State descriptions and thermodynamic parameters for the heat pump system

State

Description

T (K)

P (kPa)

o (g/kg)

x (kg/kg)

1 4 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

Drying air (output) Humid air (input) Humid air Saturated humid air Liquid water (vacuum) Liquid water Dry air Humid air Saturated humid air Cold drying air Preheated drying air Two-phase vapor-liquid CO2 Saturated CO2 vapor Superheated CO2 vapor Hot supercritical CO2 Cooled supercritical CO2 Subcooled CO2 liquid

739 403 403 321.7 321.7 321.7 321.7 403 321.7 321.7 594.2 283 283 526.3 614.2 381.7 297.9

101.325 101.325 101.325 44.32 44.32 101.325 44.32 101.325 44.32 44.32 44.32 4485 4485 4485 10,133 10,133 10,133

72 217 217 217 N/A N/A 0 217 217 72 72 N/A N/A N/A N/A N/A N/A

N/A N/A N/A N/A 1 1 N/A N/A N/A N/A N/A 0.019 1 N/A N/A N/A N/A

E_ (kW) 59,359 59,231 39,578 33,296 30,635 30,636 2,661 19.653 16,533 19,194 45,014 25,628 7,731 18,729 26,651 831.3 25,628

Because the heat is delivered at temperature T1 ¼739K, the exergetic COP becomes:  
_ out 1 T0 Q 43;629 1 298 T1 739 COP ex ¼ ¼ ¼ 2:0 _ net;in 12;866 W The exergy balance equation allows for determination of the exergy destruction, which is   _ out 1 T0 þ Ex _ d;hp ¼ 3707 kW _ net;in ¼ Q _ d;hp -Ex _ 1þW Ex T1 The total exergy destruction of the drying process coupled to the heat pump is equal to the sum of exergy destruction by the kiln and the exergy destruction by the heat pump: _ d;hp þ Ex _ d;kiln ¼ 3707 þ 21;021 ¼ 24;728 kW _ d ¼ Ex Ex The total power required for the drying process is equal to the power consumed by the blowers and kiln rotation system plus the power consumed by the heat pump: _ act ¼ 765 þ 12;866 ¼ 13;631 kW W The exergy efficiency of the dryer becomes: c¼

_ rev 839 W ¼ ¼ 0:061 _ 13;631 W act

The efficiency is improved 1.7 times with respect to the exergy efficiency of the reference system. Furthermore, the SI of the improved system becomes: SI ¼

1 1

c

¼

1

1 ¼ 1:065 0:061

Taking into account the additional 2 MW required onsite, the total power consumed by the grid by the improved system is _ in ¼ 15;866 kW. For a lifecycle of 25 years and 260 h of annual operation the total electric energy consumed from the grid W becomes Win ¼ 371,265 GJ. To this energy, the amount of energy required for system construction and scrapping must be added. The improved system no longer has the combustor and the gas turbine. However, it is fair to assume for a rough estimation that the amount of materials, such as copper, aluminum, and stainless steel, used for the gas turbine, combustor, and the electric power generator are now used to construct the heat pump, which itself includes similar components as for the reference system: electrical motors, compressors, and turbines. Therefore Table 22 remains unchanged for the improved system. However, in Table 24 the streams 17, 21, 22, and 23 do not exist because only electrical power is demanded by the improved system. The lifecycle operations of the improved system are described as shown in Fig. 29. Table 25 gives the embedded exergy and EPCs for improved drying system. The sustainability assessment parameters are calculated for the improved system in a similar manner as for the reference system. The sustainability for the two systems are compared as shown by the results given in Table 26. The total exergy destruction for the LT is given by the sum of the exergy destruction by the system itself and the exergy destruction at power generation, which is 373,828(1 0.51) þ 24,728¼ 207,904 GJ, where 0.51 is the exergy efficiency of the grid. The exergy input for the lifecycle is given

Sustainability Dimensions of Energy

Table 25

Embedded exergy and environmental pollution costs (EPCs) for the improved drying system lifecycle

Stream

Description

Ex (GJ)

EPC ($)

16 17 18 19 20 24 25 26 27 28

Exergy extracted from primary sources for power generation Electric energy consumed by the system for operation (for lifetime (LF)) Exergy consumed for materials processing and system manufacture Exergy consumed for system scrapping and materials recycling Environmental waste stream for grid power generation for LF Materials extracted (ores) for system manufacturing Embedded energy in the system constructed and delivered Polluting wastes due to materials processing and system manufacture Recycled materials Wastes and pollution due to system scrapping and recycling

373,820 371,265 2420 135 N/A N/A 2420 N/A 134 N/A

N/A N/A N/A N/A 1.848E6 N/A N/A 88,194 N/A 40,000

Table 26

149

Sustainability comparison of reference and improved dryer system

Parameter type

Parameter

Thermodynamic

Exd;ref clc SRW ExCI ExIE CIEx EPClt EPCcs ExCDR ASI GFEx;d GFASI

Exergoeconomic

Environmental

Sustainability

Drying system Reference

Improved

1.92E6 GJ 0.01 0.011 0.612 0.0065 52.2 $15.825E6 $128,194 0.0081 0.0256 0 0

0.21E6 0.055 0.098 0.612 0.060 145 $1.98E6 $128,194 0.065 0.223 0.89

Abbreviations: ASI, aggregated sustainability indicator; CIEx, capital internment effectiveness; EPC, environmental pollution cost; ExCDR, construction exergy expenditure; ExCI, specific exergetic capital investment; ExIE, exergetic investment efficiency; GF, greenization factor; SRW, specific reversible work.

in Table 25, Exlc,in ¼ 373,820 GJ. The total reversible work amount is previously calculated as Wlc,rev ¼ 20,428 GJ. Thence, the lifecycle exergy efficiency becomes clc ¼ 0.055 showing 5 times improvement with respect to the reference case. The SRW becomes SRW ¼ Wlc,rev/Exd,ref ¼ 20,428/207,904 ¼ 0.098. The specific exergetic capital investment remains unchanged because the investment and the reversible work are not affected by the system change, and the same investment has been assumed for both systems. Also, the EPC for system construction does not change. However, the ExIE changes to ExIE ¼ Exinv/ Exd,ref ¼ 12,505/207,904 ¼ 0.060. Finally, the ASI for the improved system is determined by Eq. (47) and becomes: ASI ¼ 0:098 þ 0:060 þ 0:065 ¼ 0:223 Further comparison of the improved and reference system is done using the GF. Using Eq. (15) the exergy destruction based GF becomes: GF ex ¼

Exd;ref Exd 1:92 0:21 ¼ ¼ 0:89 Exd;ref 1:92

If the ASI is used for the GF, then from Eq. (10) one obtains: GF ASI ¼

ASI

ASIref 0:223 0:0256 ¼ ¼ 0:88 ASI 0:223

In this case study, the exergosustainability assessment of a drying system is demonstrated. Although the analysis is approximated, it clearly demonstrates the main steps to follow in order to assess the system. The scope of the analysis is extended to the entire LT, which considers three phases: materials extraction and system construction (including fuels processing), system operation, and system scrapping and materials recycling. For all phases, the embedded exergy, the EPC, and exergoeconomic costs can be determined such that eventually the overall sustainability can be assessed by an aggregated index. Furthermore, if the system is improved or another system is comparatively assessed for sustainability, the GF can be used, which is a simple method to quantify the system improvement toward greenization. The example shows that application of heat pumps, when clean grid power is locally available, is beneficial.

150

1.4.8

Sustainability Dimensions of Energy

Future Directions

New ways of thinking at energy sustainability emerged in recent years. Bejan [31] brings to attention that sustainability requires to sustain the flow of useful energy (or exergy) throughout the inhabited by humans. The problem has wide implications because this global flow of exergy must be controlled wisely by human such that it maintains propel all basic and more elaborated activity. For example, a proper distribution of water – a basic need – in arid areas implies a proper distribution of exergy resources and power generation. Distribution of knowledge requires information transfer through communication network, which are also related to the way in which exergy and power generation is distributed over the globe. Sustainability of energy/exergy supply is such an important. The application of Constructal law – see Bejan [31] – in sustainability shows real promises toward discovering better designs (configurations) in power generation and use, transportation, technology, knowledge, wealth, and government. For instance in Bejan [31] [Figure 1.2] the energy use is shown in a linear dependence in a double-logarithmic plot. This demonstrates that the world speeds up toward more energy consumption, anot less. There is struggle in this, because as time passes more energy resources are involved in humans sustainability, and these resources must be wisely administrated. The answer from the Constructal law view point is that hierarchy is emergent in generating, distributing, and consuming power globally. The new aspect consists of realizing that sustainability of energy sources is rooted in sustainability of humans species. Sustainability emerges though better designs, while the concepts of design, better and sustainability are revealed as belonging to physics. Novel development in sustainability of energy systems led to consideration of “smart energy systems,” a term coined in Dincer and Acar [32]. An energy system that uses technologies and resources that are adequate, affordable, clean, and reliable is denoted as “smart energy system.” These systems can be developed and assessed for efficiency, environmental performance, energy, and material resources. Generation of multiple and diverse products is possible with smart energy systems supplied from a single energy sources. The multigeneration is shown to be beneficial for decreasing the environmental impact per unit of product and increasing the efficiency, thus enhancing the sustainability. It is important to portray the “energy to sustainability” for future in a mathematical form as “3S þ 2S ¼ Sustainability” as illustrated in Fig. 31. The figure shows that the energy picture covers 3S approach, including source-system-service, meaning that a source is always needed to provide energy input to a system which will produce services (useful commodities, such as electricity, heat, cooling, hot water, drying air, hydrogen, ammonia, fuels, and fresh water). This, of course, makes the 3S, and 2S will be added to this, as illustrated in Fig. 31, since there is a need for energy storage between the source and system, depending on the source and system, especially for renewables (such as solar energy: which is available during daytime and not available at night. So, the storage becomes an essential need), and between system and source due to fact that the production becomes more than the demand at most of the times. So, there is a need to do storage to offset the mismatch between demand and supply. This overall makes the equation 3S þ 2S what really brings sustainability. So, it is extremely important to dwell on these concepts and apply correctly as well as address correctly.

1.4.9

Closing Remarks

In this chapter, sustainability aspects of energy are reviewed with a focus on development of assessment tools and exergy methods for sustainability. The role of sustainability indicators on sustainability assessment and policy making is described. There is a large interdependence of factors affecting sustainability of energy systems and technologies, such as environmental, economic, social factors. The linkage of sustainability with energy conservation is also presented. The DPSIR model is very relevant, as shown, for elaborating sustainability indicators focused on varied aspects. Normalization of sustainability indicators can be made based on reference indicators, such as the maximum or average values or based on the alleviation from standard average of indicator variance. Once normalized indicators are determined, those can be aggregated to form a single SI used to rank and assess the sustainability of any specific energy technology. There are various

Source

Service

System

Storage

Storage

Sustainability Fig. 31 Dimensions of sustainability under 3S (Source–System–Service) with 2S (Storage) options.

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151

ways to consider weighting factors to aggregate the normalized indicators. The weighting factors can be defined according to trade-offs between different criteria, each criteria being represented by a normalized indicator. The trade-offs are generally established by the decision maker and therefore the vision of decision maker influences the process. Three types of decision maker archetypes can be envisioned, namely, the individualist, the egalitarian, and the hierarchist. An ASI for energy systems will then account for the relative improvement toward greenization with respect to a baseline case. Therefore a GF has been proposed in the literature. Exergosustainability assessment of energy systems considers the entire lifespan of the system and investigates the flows of exergy. It is then possible to define an ExSI that accounts for system construction and for alleviation of any environmental pollution. SFs and pollution cost become important in the analysis. Three exergosustainability case studies are presented to illustrate the method.

References [1] IEA. International Energy Agency technical report. Key world energy statistics. Available from: http://www.iea.org/publications/freepublications/publication/KeyWorld2014.pdf; 2014. [2] Bareto L, Makihira A, Riahi K. The hydrogen economy in the 21st century: a sustainable development scenario. Int J Hydrogen Energy 2009;28:267–84. [3] Ness B, Urbel-Piirsalu E, Anderberg S, Olsson L. Categorising tools for sustainability assessment. Ecol Econ 2007;60:498–508. [4] Kanoglu M, Dincer I, Cengel YA. Exergy for better environment and sustainability. Environ Dev Sustain 2009;11:971–88. [5] Midilli A, Dincer I, Ay M. Green energy strategies for sustainable development. Energy Policy 2006;34:3623–33. [6] Hardi P, Zdan TJ. Assessing sustainable development: principles in practice. Winnipeg, MB: The International Institute for Sustainable Development; 1997. [7] Dincer I, Zamfirescu C. Sustainable energy systems and applications. New York, NY: Springer; 2011. [8] Linke B, Das J, Lam M, Ly C. Sustainability indicators for finishing operations based on process performance and part quality. Procedia CIRP 2014;14: 564–9. [9] Singh RK, Murty HR, Gupta SK, Dikshit AK. An overview of sustainability assessment methodologies. Ecol Indic 2009;9:189–212. [10] Dincer I, Rosen MA, Zamfirescu C. Economic and environmental comparison of conventional and alternative vehicle options. In: Pistoia G, editor. Electric and hybrid vehicles. New York, NY: Elsevier; 2010. [11] Hacatoglu K, Dincer I, Rosen MA. A new model to assess the environmental impact and sustainability of energy systems. J. Clearer Prod 2015;103:211–8. [12] Dincer I, Zamfirescu C. Potential options to greenize energy systems. Energy 2012;46:5–15. [13] Dincer I, Zamfirescu C. Sustainable hydrogen production. New York, NY: Elsevier; 2016. [14] Wall G. Exergy conversion in Japanese society. Energy 1990;15:435–44. [15] Wall G.. Exergy use in the Swedish society 1994. In: TAIES’97 Thermodynamic analysis and improvement of energy systems conference, Beijing, China; 1997. [16] Utlu Z, Hepbasli A. A review and assessment of the energy utilization efficiency in the Turkish industrial sector using energy and exergy analysis method. Renew Sustainable Energy Rev 2007;11(7):1438–59. [17] Dincer I, Rosen MA. Exergy: energy, environment and sustainable development. Oxford: Elsevier; 2013. [18] Rosen MA, Dincer I. Exergy as the confluence of energy, environment and sustainable development. Exergy, Int J 2001;1:3–13. [19] Bejan M. Energy policy, in entropy generation through heat and fluid flow. New York, NY: Wiley; 1994. [20] Connelly L, Koshland CP. Two aspects of consumption: using an exergy-based measure of degradation to advance the theory and implementation of industrial ecology. Resour Conserv Recycl 1997;19:199–217. [21] Delwuf J, Van Lagenhove H. Integtaing industrial ecology principles into a set of environmental sustainability indicators for technology assessment. Resour Conserv Recycl 2005;43:419–32. [22] Dincer I. Exergy as a tool for sustainable drying systems. Sustain Cities Soc 2011;1:91–6. [23] Dincer I, Zamfirescu C. Advanced power generation systems. New York, NY: Elsevier; 2014. [24] Carpenter S. The environmental cost of energy in Canada. In: Sustainable energy choices for the 90’s. Proceedings of the 16th annual conference of the solar energy society of Canada, Halifax, NS; 1990. p. 337–342. [25] De Gouw JA, Warneke C, Stohl A, et al. Volatile organic compounds composition of merged and aged forest fire plumes from Alaska and western Canada. J Geophys Res 2006;111:D10303. [26] Pellizzari ED, Clayton CA, Rodes CE, et al. Particulate matter and manganese exposures in Toronto, Canada. Atmos Environ 1999;33:721–34. [27] Rosen MA, Dincer I. Exergy analysis of waste emissions. Int J Energy Res 1999;23:1153–63. [28] Dincer I, Acar C. A review of clean energy solutions for better sustainability. Int J Energy Res 2015;39:585–606. [29] Dincer I, Acar C. Review and evaluation of hydrogen production methods for better sustainability. Int J Hydrog Energy 2015;40:11094–111. [30] Coskun C, Bayraktar M, Oktay Z, Dincer I. Energy and exergy analyses of an industrial wood chips drying process. Int J Low-Carbon Technol 2009;4:224–9. [31] Bejan A. The physics of life: Evolution of everything. St. Martin’s Press; 2016. [32] Dincer I, Acar C. Smart energy systems for better sustainability. Applied Energy 2017;194:225–35.

Further Reading Dincer I, Zamfirescu C. 2016. Drying phenomena. Chichester: Wiley; 2016. Georgescu-Roegen N. 1986. The entropy law and the economic process. East Econ J 1986;12:3–25.

Relevent Websites https://energy.gov/management/spo/sustainability-performance-office Office of Management. http://ontario-sea.org/?gclid=CjwKEAjwr_rIBRDJzq-Z-LC_2HgSJADoL57HsFAKGqK7VMZrkEiwccY4q_97DkE8DP5Q4ChtcsTnAxoC9Yvw_wcB Ontario Sustainable Energy Association.

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https://www.siemens.com/global/en/home/company/topic-areas/sustainable-energy.html Siemens. https://www.asme.org/engineering-topics/articles/sustainability/the-ammonia-economy The American Society of Mechanical Engineers. http://www.exergoecology.com/ The Exergoecology Portal. http://cbey.yale.edu/programs-research/defining-sustainability-indicators-and-metrics Yale Center for Business and the Environment.

1.5 Thermodynamic Aspects of Energy Ibrahim Dincer and Calin Zamfirescu, University of Ontario Institute of Technology, Oshawa, ON, Canada r 2018 Elsevier Inc. All rights reserved.

1.5.1 Introduction 1.5.2 Basic Concepts in Thermodynamics 1.5.3 Laws of Thermodynamics 1.5.3.1 Zeroth Law of Thermodynamics 1.5.3.2 First Law of Thermodynamics 1.5.3.3 Second Law of Thermodynamics 1.5.3.4 Constructal Law 1.5.4 Equations of State 1.5.4.1 States of Aggregation 1.5.4.2 Ideal Gas Theory 1.5.4.3 Real Gases 1.5.5 Thermodynamic Equilibrium 1.5.6 Exergy 1.5.7 Thermodynamic Analysis Through Energy and Exergy 1.5.7.1 Mass Balance Equation 1.5.7.2 Energy Balance Equation 1.5.7.3 Entropy Balance Equation 1.5.7.4 Exergy Balance Equation 1.5.7.5 Formulations for System Efficiency 1.5.8 Thermodynamic Cycles 1.5.8.1 Totally Reversible Cycles 1.5.8.2 Otto and Diesel Power Cycles 1.5.8.3 Brayton Cycles 1.5.8.4 Vapor Power Cycles 1.5.9 Future Directions 1.5.10 Closing Remarks References Further Reading Relevant Websites

Nomenclature a a a A A b BWR c c C COP DOF e e_ ex E E_ ER Ex

acceleration (m/s2) intermolecular attraction parameter (Pa) chemical activity area (m2) Helmholtz free energy (J) molecular size parameter (m3/mol) back work ratio speed of light in vacuum (m/s) mass fraction specific heat (J/kg K) coefficient of performance degree of freedom specific energy (J/kg) specific work rate (W/kg) specific exergy (J/kg) energy (J) energy rate (W) expansion ratio exergy (J)

Comprehensive Energy Systems, Volume 1

_ Ex f F F g g G G h h htr H I Iv k kB Keq KE l ℒ

doi:10.1016/B978-0-12-809597-3.00105-X

155 158 167 167 168 170 172 175 176 177 181 183 186 189 190 190 191 192 193 193 195 198 200 205 209 210 210 210 211

exergy rate (W) activity coefficient force (N) Faraday constant (C/mol) gravitational acceleration (m/s2) specific Gibbs free energy (J/kg) gravitational force (N) Gibbs free energy (J) specific enthalpy (J/kg) Planck constant (J s) heat transfer coefficient (W/m2 K) enthalpy (J) electric current (A) luminous intensity (cd) parameter in PRSV equation Boltzmann constant (J/K) equilibrium constant kinetic energy (J) length (m) global external size (m)

153

154

Thermodynamic Aspects of Energy

S_ Sv SSP t T u U v u V V_ V w w_ W _ W x X y z Z

entropy rate (W/K) svelteness specific size parameter time (s) temperature (K) specific internal energy (J/kg) internal energy (J) specific volume (m3/kg) molar specific volume (m3/mol) volume (m3) volumetric flow rate (m3/s) global internal size (m3) mass specific work (J/kg) mass specific work rate (W/kg) work (J) power (W) vapor quality mass fraction mole fraction elevation (m) compressibility factor

Greek letters a angle (rad) a dimensionless parameter in Peng–Robinson equation g adiabatic exponent κ isentropic exponent Z efficiency m chemical potential (J)

n y c ξ s u o

stoichiometric factor specific energy of flowing matter (J/kg) exergy efficiency extend of reaction Stefan–Boltzmann constant (W/m2 K4) speed (m/s) acentric factor

Subscripts 1 0 A b c C ch conc cons CV eq d deliv f g gen H i i in ke l

L loss mix nf out p pe ph r R rev si sh so surr sys tot u v vg t tot

local or low temperature lost mixture nonflow output constant pressure potential physical or preheating reduced repulsion reversible sink superheating source surroundings system total useful constant volume or vapor vapor generator total total

m _ m M n N NA p P PE PR q q_ Q _ Q r R R ℜ rc rv s S

mass (kg) mass flow rate (kg/s) molecular mass (kg/kmol) amount of substance (mol) number of molecules number of Avogadro (molecules/mol) thermodynamic probability pressure (Pa) potential energy (J) pressure ratio mass specific heat (J/kg) mass specific heat rate (W/kg) heat (J) heat rate (W) pressure ratio gas constant (J/kg K) universal gas constant (J/kmol K) performance parameter cut-off ratio volume ratio specific entropy (J/kg K) entropy (J/K)

bulk reference state attraction boundary or boiling critical state Carnot chemical concentration consumed control volume equilibrium destroyed delivered flow or final generated generated high temperature general index initial input kinetic liquid

Thermodynamic Aspects of Energy

Superscripts F average value ( )

1.5.1

ch f

155

chemical formation

Introduction

It is important to point out that the situation of energy and thermodynamics can be likened to the example of chicken and egg, which represents a situation where it really becomes impossible to definitely state which of these two things existed first and which one caused the other. Therefore, energy and thermodynamics are intrinsically related. In this section, the connections between energy and thermodynamics are reviewed with a brief historical perspective. Thermodynamics is essentially considered the subject of energy as a physical quantity in the most comprehensive manner. Furthermore, the science of thermodynamics appeared and developed due to the need of humans to master energy and power, technologically. It is a fact that the occurrence of the energy concept as well as the formation and development of thermodynamics science have been stimulated by the practical need of designing, developing, and evaluating heat engines and power, assessing their effectiveness, and measuring their efficiency. The concept of energy evolved during the ages. The fact that the etymology of the word energy comes from the Greek word energia put forward by Aristotle (approximately 350 BC) with the initial meaning of activity is uncontroversial. It appears that the word itself, energia, was actually invented by Aristotle out of the noun ergon, which means action, labor, or work. This Greek noun originated from the Indo-European precursory wergon, which derived in Old English as weorc and in Modern English changed to work. Aristotle describes energia as the property of any perceived entity of “acting something out” toward a goal or enactment. He regarded energia as a property that cannot be converted and transferred as it cannot be detached from material objects. Many currents of thought such as rationalism, empiricism, various doctrines seeking for spiritual truth and other philosophical currents largely adopted and adapted for their use the Aristotle’s concept of energia. Due to empiricism, which is a thought emphasizing the sensory experience as prime source of knowledge, energy evolved gradually toward today’s concept as defined in physics. Modern physics has been developed on the empiricism foundation, that is, on identifying and observing phenomena. When the observed things happen in the same way an innumerable number of times, then a phenomenon is identified. Concise description of a phenomenon in the form of a compact statement represents a law of physics. There are as many laws of physics as there are many phenomena to summarize. The law of energy conservation is one of them. This law is one of the two main legs of classical thermodynamics. The law of energy conservation was developed first in mechanics. According to Crease [1], conservation of kinetic energy has been stated by Thomas Young in his 1807 lecture on “Collisions.” At that time, heat has been perceived according the then widely accepted caloric theory of heat, as the manifestation of a hypothesized caloric fluid. Putting theory forward is another arm of science (here physics) beside the empirical observations to identify phenomena. Theory is predictive as it aims to explain phenomena. When its predictions are contradicted by observations, the theory fails and opportunity arises to formulate a better theory. After the fallacy of the caloric theory, further development in thought, including contributions of Rumford, Carnot, Clausius, and William Thompson, led to the complete formulation in 1851 of the dynamic theory of heat. Later, this theory became known as thermodynamics, which recognizes heat as a form of energy transfer. Thermodynamics is a branch of physics founded on two main laws, known as the laws of thermodynamics, out of which the law of energy conservation is known as the first law of thermodynamics (FLT). The second law of thermodynamics (SLT) summarizes the process of conversion of heat into work, stating that heat flows spontaneously “one-way,” i.e., from higher to lower temperature. The laws of thermodynamics cannot be derived from any other principles. As such, they are “first principles” of physics. Other laws of physics, which are not first principles, can be derived from first principles. The power of physics relies on its laws, and especially on its first principles. This is how physics has been thought as documented, for example, in the 1912 textbook by Chahart and Chute, First Principles of Physics [2], where it is stated that “applications of physical principles is constantly changing but the principles remain the same.” Crease [1] comments that energy as a conservable quantity transferable in the form of heat and work of all forms (hydro, wind, elastic, mechanical, electrical, magnetic, gravitational, etc.) has been widely promoted due to the book by William Thompson and Peter Trait, A Treatise of Natural Philosophy (1968). Nevertheless, in the last part of the 18th century the pioneers of steam engine technology and thermodynamics science coined the definition of energy in physics as the capacity of doing work. This capacity is independent of the objects upon which it does, or from which receives, work; it is a capacity that can be transferred and converted. Therefore, energy (from physics) is a concept quite different than the energia of Aristotle. Bejan [3] points out that the extreme generality of thermodynamics laws is owed to the way in which the concept of thermodynamic system is defined: as a black box having neither specified shape nor internal structure. Thermodynamics recognizes that energy and matter can flow in or out of the system and thus a global balance or imbalance of flows can be formulated. Besides being transferred together with matter, energy can be also transferred in other two forms, which do not imply matter transfer across a boundary: one organized, named work; and one disorganized (sometimes called chaotic), called heat. The energy manifested as heat is connected to temperature through entropy, a physical quantity introduced by Clausius when formulating the SLT in the form of an imbalance (namely, SgenZ0)). Thermodynamics, as formulated in its classical terms, relates to the concepts of equilibrium and of nonequilibrium. At equilibrium, nothing flows. No change can be manifested inside the system that is at equilibrium. The state of equilibrium is also known

156

Thermodynamic Aspects of Energy

Fluid mechanics

Entropy generation through heat and fluid flow

Heat transfer

Thermodynamics Fig. 1 Thermodynamic optimization with EGM at the confluence of three disciplines. Reproduced from Bejan A. Entropy generation through hean and fluid flow. New York, NY: Wiley;1982.

in thermodynamics as dead state. A system at dead state cannot have evolution, it has a pattern within it but has no organization (since organization is related to flowing, while through a system at dead state nothing flows). As revised in Bejan [3], the classical thermodynamics theory is able to predict the dead state that any closed system can possibly achieve in certain conditions. In this prediction, the second law is useful as it indicates the temporal direction – the time arrow – of the evolution toward the dead state. By invoking the first and second laws of thermodynamics together, the dead state parameters of any closed thermodynamic system can be predicted. If the closed system is an isolated system, which means that the system’s boundary does not allow for any work, mass, and heat transfer across it, then the system necessarily evolves in time toward the dead state, while it conserves its internal energy. For the isolated system, entropy necessarily reaches a maximum at the dead state. Thermodynamics has been extensively used as a powerful tool for design and optimization of energy systems and many other engineered systems as well. Design practices with thermodynamics tools evolved toward establishing of two main methodologies known as first law analysis and second law analysis. Furthermore, energy engineering benefited much from the minimum entropy generation methodology for design [4]. This method requires a simultaneous consideration of thermodynamics (both laws) and heat transfer while predicting the parameters of a better design with reduced irreversibility (or minimized generated entropy). Energy systems optimization seeking for an end design has been an effervescent body of research after 1980 when thermal sciences were established at the confluence of thermodynamics and heat transfer under the larger umbrella called thermodynamics optimization. One of the major formulations of thermodynamic optimization problems is in the form of entropy generation minimization (EGM) through heat and fluid flow, which is an interdisciplinary field invoking thermodynamics, heat transfer, and fluid mechanics. Fig. 1 depicts the EGM interdisciplinary character. In the engineering of energy systems there has always been a strong push toward making better devices, processes, and systems. The problem is wide, expanding toward the concept of sustainability. The common consensus to tackle this problem is that we need sustainable energy solutions the cover the following six key pillars, stated also in Ref. [5], namely, (1) better efficiency, (2) better cost-effectiveness, (3) better resources use, (4) better design and analysis, (5) better energy security, and (6) better environment. At the same time, one needs to clearly define what better means in thermodynamics/physics terms. Moreover, sustainable solutions require considering the environment as part of the problem. This is why exergy analysis may have a key role in creation of sustainable energy systems. Exergy intrinsically accounts for the environment in which a system works, and at large, it accounts for the Earth’s environment with its subsystems such as the atmosphere, hydrosphere, and lithosphere. Exergy expresses the maximum work that a system can develop while it is allowed to evolve toward reaching equilibrium with an unambiguously defined reference environment (also called reference state). This work will be produced if the system evolves reversibly until it reaches a full thermodynamic equilibrium with the reference state. In exergy analysis the reference environment is assumed to be a thermodynamic system at dead state. Moreover, the reference environment is viewed as a large reservoir, the parameters of which never change. Exergy analysis allows for determination of the maximum reversible work, which is the work generated or consumed for the system to reach the dead state at the parameters of the reference environment. When reversible work is compared with actual work the system irreversibilities or exergy destructions can be determined. Determination of exergy destructions (of various kinds and of various subprocesses or subsystems) is the main fruit of the exergy analysis. The method is extensively applied for optimization of energy systems as an alternative and equivalent method to EGM. Exergy analysis has been found very useful for energy systems design and optimization accounting for environmental impact and sustainability. Thus Fig. 2 shows exergy at the confluence of three domains: energy, environment, and sustainable development. Exergy analysis developed in multiple ad-hoc design methodologies such as exergy destruction minimization, exergy efficiency maximization, exergy cost accounting, exergy–cost–energy–mass method, specific exergy cost method, exergetic life cycle assessment, exergoenvironmental analysis, and exergosustainability assessment [6]. Exergy is vital to the moving of both animate and inanimate systems on Earth. We learn from Georgescu-Roegen [7] that lowentropy sources are converted in high-entropy wastes by living organisms so that they live. In the same line (and equivalently), one

Thermodynamic Aspects of Energy

Energy

157

Environment Exergy

Sustainable development Fig. 2 Exergy analysis as a key tool at the confluence of energy, environment, and sustainable development quests. Adapted from Dincer I, Rosen MA. Exergy: energy, environment and sustainable development. New York, NY: Elsevier; 2013.

Closed thermodynamic system 0.025% Incident solar radiation

TBG = 3K rev = 1–3/6000=99.95%

North pole

TBG = 3K

TS = 6000K W 100% 99.95% Sunbelt region

All earth subsystems acting as brakes 99.95%

South pole 0.025%

TBG = 3K

Surroundings: Background radiation

Work consumption for: Precipitation, wind, ocean currents, climate control, life systems, carboncycle,water cycle, nitrogen cycle, sulfur cycle etc + associated irreversibilities

Fig. 3 Engine and brake earth destroys all received exergy while energy is conserved. Adapted from Dincer I, Zamfirescu C. Advanced power generation systems. New York, NY: Elsevier; 2014.

can say that high-exergy sources (fuels, foods, winds, sunlight, etc.) are degraded by earth systems (animate and inanimate and manmade) while they move on the landscape. The energy is conserved but the ability to move (or change) is destroyed as illustrated in Fig. 3. More importantly is that while exergy is destroyed by the systems observable on Earth, those move (or flow) through the background and doing so, they develop flow configuration. Bejan introduced the idea that any flowing system that is free to move is a live system [1,8]. Therefore the thermodynamic concept of live state has been proposed by Bejan [8] as an opposite to the dead state; “the live-state system is not in equilibrium with its environment.” Not being in equilibrium, the live-state system moves and configures as it has flow and is endowed with freedom to change and therefore with evolution. This demarche of introducing the new life-state concept in thermodynamics is legitimate as it facilitates focus on those abundant nonequilibrium systems. A system in live-state is a finite-size flow system endowed with freedom for a finite period of time, equal to its lifespan. The livestate system persists for as much time as the system is able to evolve freely toward greater access to flow. While evolving, the livestate system organizes as it constructs configuration. Here, in the ability of live-state system of constructing configurations that evolve toward better access of flow over the landscape, is the essence of the Constructal Law formulated by Bejan in 1996 [9]: “for a finite-size flow system to persist in time (to live), it must evolve in such a way that it provides easier access to the imposed (global) currents that flow through it.” Once it stops flowing, the live system disappears while other live systems form on the landscape. Thus the evolution process never ends as newer life cycles manifest with rhythms and configurations. An emerging body of science evolved in the last 20 years from thermodynamics and constructal law, which is known today as constructal thermodynamics. The evolution of the related body of knowledge is illustrated in Fig. 4, which presents the constructal

158

Thermodynamic Aspects of Energy

1851

1982

1996

Thermodynamics

Thermal sciences

Constructal thermodynamics

Heat transfer Caloric theory

Mechanics

Evolution Fig. 4 The emergence of constructal thermodynamics. Adapted from Bejan A. The physics of life: the evolution of everything. New York, NY: St. Martin’s Press; 2016.

thermodynamics at the confluence of heat, work, and evolution (design). Constructal thermodynamics is beneficial for predicting, understanding, designing energy systems and any other systems (natural and manmade) since the constructal law governs how the live systems emerge and persist. It defines the concept of better in physics terms: better is the live-state system that secures more freedom to morph and evolves freely. If the live system morphs in freedom it becomes better since it flows easier. The time arrows postulated by the constructal law points toward the more organized and hierarchical design, which spreads its flow better over the territory. The future designs will be better than the present ones. The time arrow for sustainable energy systems points toward evolution such that the newer design produces less waste and more useful output. In the first part of this chapter the main quantities and concepts used in thermodynamics are introduced. Ideal gas theory, the laws of thermodynamics, and the fundamental thermodynamic relationships are then presented. The chapter proceeds by introducing equations of state for real fluids. The thermodynamics of solutions and strong electrolytes are also presented as those are important for modern energy systems such as molten alkali electrolysis or electrochemical fuels synthesis or fuel cells. Thermodynamics of chemical and electrochemical reactions and chemical equilibrium is briefly introduced. A focus of the chapter is on energy systems analysis through energy and exergy methods; therefore, exergy is introduced rigorously, as well as the balance equations for mass, energy, entropy, and exergy. The last part of the chapter is devoted to exergy analysis of relevant energy systems and conversion methods such as conventional power plants, fuel cells, solar and other renewable energy conversion systems, biomass, and biofuels.

1.5.2

Basic Concepts in Thermodynamics

In natural sciences one appeals to fundamental principles and physical laws to construct theories that are useful tools to predict and understand physics. Theories must be validated by empirical evidence, i.e., experiment. Therefore, one needs to relate to quantities that are measurable by instruments, which are known as “physical quantities.” Physical quantities are of two types, namely extensive and intensive. An extensive physical quantity is the sum of individual quantities in all constituents of a given system (here system means generically any defined and confined region of universe). Intensive quantities are independent of the extent of the system. Examples of extensive quantities are mass, volume, electric charge, and energy. Some relevant intensive quantities in thermodynamics are temperature, pressure, and specific volume. Seven physical quantities – known as fundamental physical quantities – have been chosen to represent the International System of Units (ISU) [10]. These are length, mass, time, electric current, thermodynamic temperature, amount of substance (molar), and luminous intensity (measured in candela). The fundamental quantities cannot be derived from any other quantities using constitutive relationships. Any physical quantity that is not fundamental can be derived from the fundamental quantities. Therefore, the name used for physical quantities that are not fundamental is derived physical quantity. Each of the physical quantities has an associated unit, forming thus the ISU. The units of measure of any derived physical quantity can be determined based on the units of fundamental quantities and the constitutive relationships according to the dimensional analysis method. In this respect the dimensional symbols for the fundamental quantity must be used to express the dimension for the derived quantities. The SI gives the standard definitions, symbols, and units for fundamental quantities. The fundamental physical quantities of the ISU (SI) are given in Table 1. The definitions of the fundamental SI units and their symbols are given in Table 2. Among those fundamental quantities, temperature is the most important one for thermodynamics. Temperature is the physical quantity that represents a measure of the average kinetic energy per degree of freedom of the molecular constituents of a substance. In physics any substance consists of molecular constituents having a degree of freedom in correspondence to the state of aggregation. Beside temperature, volume and pressure are very important in thermodynamics. Volume and pressure are both derived physical quantities. Volume is a physical quantity representing the three-dimensional extent of a system. This is an extensive property, having the dimension equal to length to the power 3, noted L3, and having as unit of measure the cubic meter, m3. The

Thermodynamic Aspects of Energy

Table 1

159

Definitions and standard symbols for the fundamental physical quantities

Quantity

Symbol

Definition

Length Mass Time Electric current Thermodynamic temperature

l m t I T

Amount of substance

n

Luminous intensity

IV

Geometric distance between two points in space. Quantitative measurement of inertia that is the resistance to acceleration. A measurable period of the progress of observable or nonobservable events. Rate of flow of electric charge. A measure of kinetic energy stored in a substance that has a minimum of zero (no kinetic “internal” energy). The number of unambiguously specified entities of a substance such as electrons, atoms, molecules, etc. Luminous flux of a light source per direction and solid angle, for a standard model of human eye sensitivity.

Source: Bureau Interational des Poids et Mesures. The International System of Units (SI). 8th ed. Paris: Bureau Interational des Poids et Mesures; 2006.

Table 2

Definition of the fundamental units according to the International System of Units (ISU)

Quantity

Dimensional symbol

Unit

Symbol

Definition

Length

L

meter

m

Mass

M

kilogram

kg

Time

T

second

s

Electric current

I

Ampere

A

Thermodynamic temperature

Θ

Kelvin

K

Amount of substance

N

mole

mol

Luminous intensity

J

candela

cd

The length of the path traveled by light in vacuum during a time interval of 1/299,792,458 of a second. The weight of the International Prototype of the kilogram made in platinum-iridium alloy. Duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom. Constant current that, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 m apart in vacuum, would produce between these conductors a force equal to 2  10 7 N/m of length. The fraction 1/273.16 of the thermodynamic temperature of the triple point of water (having the isotopic composition defined exactly by the following amount of substance ratios: 0.00015576 mol of 2H per mole of 1H, 0.0003799 mol of 17O per mole of 16O, and 0.0020052 mol of 18O per mole of 16O). The amount of substance of a system that contains as many elementary entities as there are atoms in 0.012 kg of carbon 12. The luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 THz and that has a radiant intensity in that direction of 1/683 W/sr.

Source: Bureau Interational des Poids et Mesures. The International System of Units (SI). 8th ed. Paris: Bureau Interational des Poids et Mesures; 2006.

common symbol for the volume is V, therefore one writes volume as follows: V ¼ l3 ðm3 Þ

ð1Þ

where the unit of measure is written in parentheses. Volume is an extensive quantity, however, if one divides volume to the mass or the amount of substance enclosed inside the volume one obtains the specific volume, which is an intensive quantity. In dimensional analysis notation one writes that the dimension of the specific volume is given by L3M 3, where L is the dimension for length and M is the dimension for mass. The specific volume is thus defined as follows: v¼

V ðm3 =kgÞ m

ð2Þ

where m is the mass of substance. If the specific volume is expressed with respect to the amount of substance (in moles) then the molar specific volume can be defined as follows:

160

Thermodynamic Aspects of Energy

v¼

V ðm3 =molÞ n

ð3Þ

The dimension of the molar specific volume is therefore L3N 1. In thermodynamics the unit mole (mol) is commonly used and defined as a certain amount of substance containing all the components. The mole is the amount of substance of a system that contains as many elementary entities as there are atoms in 0.012 kg of carbon 12. The number of these atoms is known as the number of Avogadro, NA ¼ 6.023  1023 molecules/mol. The related equations that correlate the number of moles, the number of Avogadro, and the mass of substance comprised in one mole of substance – also known as molecular mass – are given as follows: n¼

N m ¼ NA M

ð4Þ

where N represents the number of molecules (or atoms) comprised in a batch of substance. The definition of pressure in physics is related to the definition of force. Force represents a measure of action or interaction between systems of nature. It is generally understood that bulk matter (or mater with rest mass) and fields (photons, gravity, etc.) constitute the substance of the universe. Forces do exist in any region as they manifest their presence in the form of force fields (e.g., electromagnetic or gravitational fields). The electromagnetic field is said to propagate (act) through photons moving at the speed of light. Photons have mass, but no rest; which means that photons have no rest mass. Being matter with no rest, photons do propagate in space regions characterized by absolute vacuum. Gravitational fields also propagate through vacuum. The forces manifest also within bulk matter; this matter is conceptualized in physics as a collection of particles having rest mass. As known, the particles possessing rest mass include quarks and leptons, which are able to combine and form protons, neutrons, and electrons. Furthermore, protons and neutrons combine to form nuclei, having a positive electric charge. Nuclei combine with electrons of negative electric charge to form atoms, which thus become neutral with respect to the electric charge. In total there are 116 known kinds of atom corresponding to 116 chemical elements from the periodic table of elements. The force acting between two systems can produce acceleration, meaning a change of velocity. If an object changes its velocity, then it necessarily behaves so because it is under the action of a force. Here we define velocity as a vector having the system speed as magnitude. The direction of velocity vector is the same as the direction of displacement vector. Since speed is defined as the displacement per unit of time one notes that the dimension for speed is LT 1 with the SI unit of m/s and the definition relationship u¼

dl ðm=sÞ dt

ð5Þ

Acceleration represents the change in time of velocity. The magnitude of acceleration is equal to the derivative of speed with respect to time and whence will have the dimension of LT 2 and the SI unit of m/s2; the definition of acceleration is written therefore as follows: a¼

du d2 l ¼ 2 ðm=s2 Þ dt dt

ð6Þ

Now since mass and acceleration were formally introduced, the physical quantity force is defined as a vector quantity having the same direction as the acceleration that it produces and the magnitude equal to the product of mass and acceleration. The dimension for force is MLT 2, meaning that force is measured in SI in kg m/s2, a unit that is shorthanded as Newton (symbol N); thence 1 N ¼1 kg m/s2. The constitutive equation defining force is written as follows: F ¼ ma ðNÞ

ð7Þ

Pressure is defined now as a scalar quantity equal to the ratio between a force and the normal area over which force is exerted uniformly. The dimension of pressure is equal to the dimension of force divided to the dimension of area, that is: ML 1T 2 having the SI unit of kg/s2m, which is the same as N/m2 and shorthanded as Pa (Pascal). One says that 1 Pa represents the force of 1 N (Newton) exerted on 1 m2 surface. The relationship defining pressure is given as follows: P¼

F ðN=m2 Þ A

ð8Þ

The atmosphere that surrounds the Earth can be considered a reservoir of low-pressure air. Its weight exerts a pressure that varies with temperature, humidity, and altitude. Atmospheric pressure also varies from time to time at a single location, because of the movement of weather patterns. The standard value of the atmospheric pressure (or the pressure of standard atmosphere) is 101,325 Pa or 760 mmHg. Atmospheric pressure is often measured with an instrument called a barometer; thence, the name of barometric pressure. While these changes in barometric pressure are usually less than 12.5 mm of mercury, they need to be taken into account when precise measurements are required. The force action resulting on a displacement of matter is quantified by a physical quantity referred to as work (also stated as “the work of a force”). Consider a situation as in Fig. 5 where a force acts on a system obliquely at angle a and produces a displacement l. Then the dot product of force and displacement represent the work of the force. Provided that the force and displacement have the same direction (a ¼0) the work becomes W ¼ Fl. In the example from Fig. 5, the work of gravity force becomes: W ¼ Gz¼ mgz. Being defined as a dot product, work is a scalar. If there is no displacement, then there is no work. If there is no force, then there is no work produced. Work has dimension of force multiplied to length, which is ML2T 2 with the unit kg m2/s2 or N m

Thermodynamic Aspects of Energy

Initial position

161

→

F

→ →

W = F . l = Fl cos()

z

→

G, gravitational force W = Gz = mgz z=0 z Final position

 →

l

→

Initial position

Final position

Figure 5 Defining the physical quantity work.

(Newton-meter); this unit is known as Joule, J, where 1 J¼ 1 N m. Generally, work is given by W ¼ FlcosðaÞ

ð9Þ

where a is the angle between the direction of the force and that of the displacement. Carnot [11] introduced the most fundamental concept in thermodynamics, which is that of the thermodynamic system. According to its definition, a thermodynamic system is a part of the universe under consideration, delimited for the purpose of analysis by a real or imaginary boundary. The rest of the universe is separated from the thermodynamic system under consideration by the well-defined boundary. The rest of the universe is denoted as the surroundings of the thermodynamic system. Following this definition, a thermodynamic system can be anything, for example, a human body, a room, a building, a power plant, a heat exchanger, a piston, a terrestrial atmosphere, a planet, the solar system, etc. Since it occupies space, any thermodynamic system imaginable must be characterized by a volume. The boundary of the system can be permeable to interactions with the surroundings, or, on the other hand, the boundary can fully isolate the system with respect to its surroundings. When a thermodynamic system is fully isolated with respect to its surroundings, such that it does not interact with anything from “outside,” the thermodynamic system is denoted as an isolated system. An isolated system is a purely mental exercise. There is no such a thing in the universe since all space is filled with fields (at least gravity). Nevertheless, an isolated system is a good approximation for many types of analyses. For instance, a relatively small body in outer space is in a state of imponderability (gravity is there but can be neglected); if there is no interaction from its surroundings, that becomes an isolated system. When gravity is not of interest for the analysis, even if it is manifest, one can conceptualize certain thermodynamic systems on Earth’s surface as isolated systems. The notions of equilibrium and quasistatic process will be briefly introduced, subsequently. Consider an isolated thermodynamic system. General experience will lead us to assume that after a sufficiently long time all the movement of internal constituents within the system will cease. There is no “push” or “pull” with the “outside.” The system will achieve a dead state, or internal equilibrium in which no change can be triggered within it. If the system is at equilibrium, then no change occurs inside the system. Now let us assume that the isolated system did not yet achieve the equilibrium state but it is close to that state. In the vicinity of the equilibrium, the internal movement must be very slow. Movements reduce progressively as the equilibrium approaches. If the system is sufficiently close to the state of equilibrium, the changes within the system are so soft, almost imperceptible such that each moment can be approximated as being in internal equilibrium. The process performed by the system when it passes from one to the other approximated equilibrium state is denoted as a quasistatic process. The word quasistatic means “almost statically,” suggesting that the changes within the system are so small that the system is perceived in a “static” state – no changes. Let us review now the possible interactions that can occur between a thermodynamic system and its surroundings. Interaction between an thermodynamic system and its surroundings can occur by mass (matter) transfer, work transfer, and heat transfer. Matter transfer can occur through any open pores or ports on the system boundary. Matter can go in or can go out, or neither go in nor go out. A thermodynamic system with a boundary impermeable to mass transfer is denoted as a closed system. Still, a closed system has a boundary that permits work and heat transfer. If a thermodynamic system can interact with the surroundings by mass transfer, by work transfer, and by heat transfer, then the system is denoted as open system or control volume (CV). Fig. 6 illustrates how a thermodynamic system can interact with its surroundings through work transfer. The system under consideration is a piston and cylinder mechanism. In that system, a gas is enclosed inside the cylinder. The gas is not able to escape out of the cylinder. The cylinder and piton mechanism is well sealed, as it does not allow matter penetration from the surroundings. Thenceforth, the gas represents a closed thermodynamic system. Now, let us assume that under the influence of internal pressure P the piston is displaced an infinitesimal amount dl. The gas force that acted on the piston is related to gas pressure and piston area, F¼ PA. Since the piston is part of the system boundary, we denote the piston work as boundary work. The geometry of the system boundary is not important; it can be planar as in Fig. 5 or have any other geometry. What is important is that the boundary must be free to move. The boundary work for an infinitesimal boundary displacement becomes: dWb ¼ F dl ¼ P A dl  P dV

ð10Þ

Now let us assume that this closed system (the gas) expands inside the cylinder from a volume V1 to a larger volume V2 following a quasistatic path 1-2 indicated in Fig. 5 with the continuous line. Since the process is quasistatic, the pressure varies with volume according to a continuous function P(V). In this situation, the boundary work exerted toward the surroundings is

162

Thermodynamic Aspects of Energy

P

1

Path of the process

Wnet 2 dW = PdV A

dV

V

P

Fig. 6 Illustrating work transfer through the thermodynamic system boundary.

Cross sectional duct area, A

Local flow velocity, L

V, m

Duct Average uniform flow velocity, 

Control volume of infinitesimal length dz

Fig. 7 Cross-sectional control volume (CV) of infinitesimal length through an open duct.

given by the following integral: Wb 

Z

V2

P dV

V1

ð11Þ

Eq. (11) defines boundary work for any quasistatic process of a thermodynamic system. The path along which the process occurs is important. For the other path, a different P(V) dependence occurs therefore the integral from Eq. (11) takes another value. The value will be in any situation equal to the area below the curve representing the quasistatic process. In particular, if the gas system discussed contracts quasistatically according to the path 2-1 indicated in Fig. 5 with a dashed line, then the work at return has a smaller magnitude than the work at the push 1-2. The work 2-1 is exerted reversely with respect to 1-2, namely form the surrounding toward the system. Thenceforth, the area enclosed between the two pathways 1-2 and 2-1, the hatched area in Fig. 5, represents the net work exchanged quasistatically by the system with its surroundings during a cyclical process. The closed pathway 1-2 (upper) 2-1 (lower) is denoted as a thermodynamic cycle. Let us now analyze an open system (CV) and introduce the notions of flow rate and flow work. A very particular CV is an open duct as that shown in Fig. 7. If there is a flow of a fluid in the fact, then the velocity of the flow will have a distribution in crosssection such that at the walls velocity is essentially zero. An average flow velocity can be determined for the cross-section when the integral of the local velocity uL is taken over the area, as follows: u

1 ∬A uL dA A

ð12Þ

Thermodynamic Aspects of Energy

163

Flow plug pushed inside CV

. P m

m m P P V V

Control volume (CV) mCV

A l Flow plug to be pushed inside CV Boundary of CV Fig. 8 Introducing the concept of flow work.

The volumetric flow rate through the cross-section is thenceforth defined based on average velocity u and the area of the crosssectional surface A as follows: V_  u A

ð13Þ

If the specific volume v of the fluid is uniform throughout the flow cross-section then the mass flow rate is defined as follows: _  m

V_ v

ð14Þ

Let us now consider a CV as sketched in Fig. 8. A pocket of fluid of mass m is represented in the figure. A flow work is a measure of the action of pushing (or pulling) a volume of fluid inside (or from) a CV. This represents a flow-displacement work. Considering an infinitesimal element of the slice-like pocket of fluid represented in the figure, the flow work is according to work definition equal to: dWf ¼ dðF lÞ ¼ dðP A lÞ  dðP V Þ

ð15Þ

Integrating the above equality over the entire cross-sectional area one obtains that the flow work equals pressure multiplied to volume, namely: Wf  ∬A dðP V Þ ¼ P V

ð16Þ

_ f ¼ PV. _ Furthermore, a specific flow work can be defined as V is From Eq. (15) the flow work rate can be expressed with W replaced with v, namely wf  P v

ð17Þ

Many devices such as turbines, compressors, pumps, etc. are open systems that perform or consume work. Both flow work and boundary work are exchanged when such devices operate. The work generated or consumed by an open system (CV) is denoted as useful work and represents the difference between boundary work and flow work, given as follows: Wu ¼ Wb

ð18Þ

Wf

In differential form for the specific useful work one has dwu ¼ P dv

dðP vÞ ¼

ð19Þ

v dP

therefore the useful work is given by the integral: Wu ¼

Z

ð20Þ

v dP

Consider now an isolated thermodynamic system of mass m. Assume that a net force F pulls upon a spot of the system boundary. Since the system is isolated, the boundary is not deformed (no boundary work is produced). However, the effect of the net force F is acceleration of the entire system and its displacement along the line of action of the force. The force, mass, and acceleration of the thermodynamic system are thenceforth related through the Newton’s law F ¼ ma. Furthermore, under a uniform acceleration the system velocity with respect to an inertial frame of reference can be written as follows: u2f ¼ u2i þ 2al, where l is the displacement and vi,f are the initial and final speeds. Accordingly, the work of the force F can be expressed as follows: W ¼ Fl ¼ mal ¼ m

u2f

u2i 2

¼

1 2 mu 2 f

1 2 mu 2 i

ð21Þ

164

Thermodynamic Aspects of Energy

The above equation can be rewritten as follows: 1 2 1 mu þ Win ¼ mu2f 2 i 2

ð22Þ

where the subscript “in” in Win is added as a reminder that the thermodynamic system received work due to the action of force F. An equivalent situation as above is to consider that the system is initially at a high velocity vi and under the influence of a friction force it reduces the speed to uf oui . For such a situation the following equation writes: 1 2 1 mu ¼ Wout þ mu2f 2 i 2

ð23Þ

where the subscript “out” in Wout reminds that the thermodynamic system does work against its surroundings. Set vf to zero in Eq. (23). The maximum work generated or in other words the capacity of doing work of the assumed system while decelerating from its initial velocity to zero speed is thenceforth obtained as follows: KEi ¼

1 2 mu 2 i

ð24Þ

where the notation KE signifies kinetic energy and index i refers to initial state of velocity ui . Eq. (24) defines the kinetic energy of a system, which represents the capacity of doing work of that system due to its speed. Kinetic energy requires knowledge of velocity. Consequently, kinetic energy is defined relative to an inertial frame of reference. Based on Eqs. (22) and (23) the work generated or consumed by the system due to deceleration or acceleration, respectively, can be compactly written as follows: W ¼ KEf

KEi

ð25Þ

where f and i stand for final and initial state, respectively. In Eq. (25) if the work is positive then the process is an acceleration, that is, the system receives kinetic energy. Moreover, if the work is negative then the process will be a deceleration, thence the system outputs work. Let us analyze now the capacity of doing work of the conservative type of force fields. A force field that has the property that, when matter is displaced the change in the associated potential energy does not depend on the path taken, but only on the initial and final position of the displacement, is denoted as conservative. Any conservative force field has an associated potential energy. There are many types of potential energies, each having associated a type of force field:

• • • • • • •

Gravitational Elastic energy, produced by elastic forces Magnetic energy, produced by magnetic forces Electrostatic energy, produced by Coulomb forces Chemical potential energy, produced by Coulomb forces while chemical reactions occur. During chemical reactions the atoms and electrons rearrange releasing or absorbing energy at the same time; this energy is of electrical type in nature and manifests among nuclei, electrons, and molecules Thermal potential energy, which is a consequence of kinetic energy of molecules and the potential energy due to their relative position Nuclear energy, which is caused by various nuclear forces (e.g., weak, strong)

In Fig. 5 the work of gravity force on the Earth’s surface is approximated with W ¼mgz, where z is the elevation with respect to a datum. This is a good approximation since that gravity acceleration g on the Earth’s surface is approximately constant. Thenceforth, the capacity of doing work of the gravity force field is expressed as follows: PE ¼ mgz

ð26Þ

where PE stands for potential gravitational energy. Note that a datum must be defined a priori, or otherwise no potential energy can be introduced. The work deployed while the system moves from an initial elevation zi to a final elevation zf is given as follows: W ¼ PEf

PEi

ð27Þ

where the work is received by the system if positive (PEf4PEi) or given by the system if negative (PEfoPEi). When the work is positive, then the system movement is against the gravity force, whereas if the work is outputted then the system is accelerated. Assume that a system (material point) is at a zero speed at elevation z with respect to a datum on the Earth’s surface. Once it starts moving, the system accelerates and the speed increases. Therefore, the system gains kinetic energy. The infinitesimal change of the kinetic energy in the gravitational field is therefore:  2   mu du dðKEÞ ¼ d ¼ mudv ¼ m ðudt Þ ¼ mgdz ¼ dð PEÞ ð28Þ 2 dt

Thermodynamic Aspects of Energy

165

Q in = hA (T∞ − T(t)) At t = 0,T = Ti At t > 0,T = T(t) T∞ Fig. 9 Heat transfer through a system boundary.

In Eq. (28) the negative sign at mgdz is necessary to account for the fact that if speed increases the elevation decreases, that is, dv and dz have opposite signs. The equalities chain from Eq. (28) is general and teaches that d(KE þ PE) ¼0 or E ¼ KE þ PE ¼ const

ð29Þ

where E stands for energy. Eq. (29) suggests that energy never destroys, but rather converts from one form to another form. Here in our example, the total mechanical energy defined as the sum of kinetic and gravitational potential energy remains constant. Kinetic energy converts to potential energy and vice versa. This is a particular form of the principle of energy conservation, which will be introduced comprehensively with the FLT in a later section of this chapter. Assume that a thermodynamic system is in a state 1 having the total mechanical energy E1. A process happens such that after a while the system reaches a state 2 having a different total mechanical energy E2. Based on the above considerations one can write: E1 ¼ E2 þ W12

ð30Þ

where W12 is the work transferred in or out of the system; if E2oE1, then work is produced by the system, that is, the system acts upon its surroundings; reversely, if E2oE1, then work is given to the system from its surroundings. Work is a form of energy transfer through a system boundary. If work is transferred, the energy of the system changes. Another form of energy transfer between a thermodynamic system and its surroundings is through heat. Heat transfer is a different mechanism of energy exchange related to kinetic energy of the molecules/atoms. Heat amount is denoted with Q and is measured with the same units as work and energy, Joule. Fig. 9 shows schematically a heat transfer process through the boundary of a thermodynamic system. Assume that a system is immersed in a fluid (gas or liquid) and the system has a distinct boundary. At an initial moment, the temperature of the system is T¼ Ti whereas the temperature of the surroundings is T1aTi. In the above scenario, a heat transfer process will happen through the system boundary, transferring energy in the form of heat from the region with high temperature to the region with low temperature. The mechanism of heat transfer for the considered case is through convection, which is the heat transfer mode that takes place within a fluid by mixing one portion of the fluid with another. The rate of heat transfer through convection is proportional to the heat transfer area and the temperature difference across the system boundary; T1–Ti as in the figure. The heat transfer rate is written mathematically as follows: _ ¼ htr AðT1 Q

Ti Þ

ð31Þ

_ indicates an energy transfer from the surroundings toward the system if T14Ti or vice versa, The heat transfer process at rate Q an energy leak from the system into the surroundings for T1oTi. The effect of heat transfer is a change in energy of the system. A balance equation analogue with Eq. (30) holds, in which the mechanism of energy transfer is heat instead of work; one has: E1 ¼ E2 þ Q12

ð32Þ

_ from the initial moment t1 to final moment t2. where Q12 is the integral of Q From Eq. (32) one gets that if the system loses heat it also loses energy and vice versa. In many situations a heat transfer process leads to a change of system temperature in accordance with the change of system energy. The amount of heat transfer is proportional to the temperature difference (final T2 – initial T1) and the mass of the system. If a specific heat can be defined for the thermodynamic system then the energy amount transferred as heat becomes: Q12 ¼ mCp ðT2

T1 Þ

ð33Þ

Energy of a thermodynamic system is an extensive quantity that depends on the extent of the system. However, a specific energy can be introduced that is an intensive physical quantity defined as the energy of a system divided to its mass, as follows: e¼

E m

ð34Þ

where one assumes that the specific energy of the system is uniform throughout the system. Energy can be transferred across the thermodynamic system boundary through matter transfer. If one denotes m as the mass transferred across the boundary, then the following balance exists between the initial and final moments: E1 þ me ¼ E2 where m is the mass of matter added to the system and e is the specific energy of that matter.

ð35Þ

166

Thermodynamic Aspects of Energy

Since energy can be transferred through mass, work, and heat transfer, the general open thermodynamic system or CV can be represented as shown in Fig. 10. Provided that the thermodynamic system cannot exchange matter with the exterior, the name for it will be a closed thermodynamic system. Heat and work transfer are possible for a closed system but no mass transfer as suggested in Fig. 11. Furthermore, if energy transfer is allowed only in the form of work transfer then the closed system is also denoted to as adiabatic system, represented as shown in Fig. 12. Finally, a thermodynamic system, which is closed and cannot exchange energy in form of heat and work with the surroundings, is denoted as an isolated system, represented as shown in Fig. 13.

Surroundings

Mass exchange port

Energy exchange by work Energy exchange by heat

Open system System boundary

Surroundings Fig. 10 Representation of an open thermodynamic system.

Surroundings

Impermeable wall

Energy exchange by work

Energy exchange by heat

Closed system (no mass exchange with surroundings)

System boundary Surroundings Fig. 11 Representation of a closed thermodynamic system.

Surroundings

Impermeable wall

Energy exchange by work

Energy exchange by

Adiabatic system (no mass and heat exchange with surroundings)

heat System boundary Surroundings Fig. 12 Representation of an adiabatic thermodynamic system.

Thermodynamic Aspects of Energy

167

Impermeable and insulated wall

Surroundings No energy and no mass exchange Isolated system

System boundary Fig. 13 Representation of an isolated thermodynamic system.

System under consideration E Plateau A

B C

D Fig. 14 Illustrating various types of equilibrium.

1.5.3

Laws of Thermodynamics

In classical thermodynamics the systems and processes are assumed at thermodynamic equilibrium and quasistatic. The equilibrium state can be either stable or unstable; it can be either neutral or metastable. Fig. 14 shows a representation of equilibrium of a sphere in the gravitational field. The ball is in a neutral equilibrium at the plateau A. Here any position is a stable equilibrium position. If the ball quasistatically moves on the plateau, it still remains at equilibrium: once the action that displaces the ball stops, the ball does not move anymore. That is, in a neutral equilibrium any neighboring state is a stable equilibrium state. In any stable equilibrium state the system can stay an indefinite amount of time. If the ball is lifted at point B, then a metastable equilibrium is achieved. In the metastable state the system can stay a finite amount of time. The metastability has a finite lifetime. The system rests on that state but spontaneously will leave it for a more preferred one, which is one of lower energy. Any little push – quasistatic – will make the system move toward a more stable equilibrium state. Here, a small push from B moves the system in state C, which is a transitional state. This is unstable and therefore the system falls in the stable equilibrium at D. In a stable equilibrium state the system will always return after a small push. If the push is strong enough, then the system eventually departs from equilibrium and may reach an unstable equilibrium state as in E. The unstable equilibrium state has a short lifetime, as the system will move abruptly toward a state of more stable equilibrium. A thermodynamic system is in thermodynamic equilibrium if nothing moves within it (inside its boundary). An insulated thermodynamic system is said to be in thermodynamic equilibrium when no mass, heat, work, chemical energy, etc. are exchanged between any parts within the system. If there is anything that moves inside, the system is at nonequilibrium. Two thermodynamic systems are said to be in mechanical equilibrium if they cannot exchange energy in the form of work after they are put in a perfect mechanical contact. Two thermodynamic systems are said to be in thermal equilibrium if they cannot exchange heat after they are put in perfect thermal contact through their boundaries; or in other words, they have the same temperature. Two thermodynamic systems are in chemical equilibrium if they have the same chemical composition which does not change in time. A system at internal equilibrium has a uniform pressure, temperature, and chemical potential (or chemical composition) throughout its volume.

1.5.3.1

Zeroth Law of Thermodynamics

The zeroth law of thermodynamics is a statement about thermodynamic equilibrium expressed as follows: “if two thermodynamic systems are in thermal equilibrium with a third, they are also in thermodynamic equilibrium with each other.”

168

Thermodynamic Aspects of Energy

Eout = ΣQout + ΣWout

Ein = ΣQ in + ΣWin Closed system

Ein

Eout

System boundary

Surroundings Ein = ΔEsys + Eout

⎛ ⎞ Total energy ⎜ ⎟ ⎝ entering the system ⎠

=

⎛ Change in total enrgy ⎞ ⎜ ⎟ of the system ⎝ ⎠

+

⎛ ⎜ ⎝

total energy leaving the system

⎞ ⎟ ⎠

Fig. 15 The first law of thermodynamics (FLT) written for a closed system.

1.5.3.2

First Law of Thermodynamics

The FLT is a postulate of the energy conservation principle: “energy can be neither created nor destroyed.” The FLT can be phrased as “you can’t get something from nothing.” If one denotes E the energy (in KJ) and DEsys the change of energy of the system, then the FLT for a closed system undergoing any kind of process is written in the manner illustrated in Fig. 15. There are two mathematical forms for FLT namely on an amount basis or on a rate basis. These mathematical formulations are indicated as follows: Ein

Eout ¼ DEsys ; on amount basis; and E_ in

E_ out ¼ dEsys =dt; on rate basis

ð36Þ

For the closed system assumed in Fig. 15 the change in system energy can be correlated with the specific energy and system mass as follows: DEsys ¼ m ðe2

e1 Þ

ð37Þ

where index 1 represents the initial state and index 2 the final state and e is the specific total energy of the system comprising internal energy, kinetic energy, and potential energy, and expressed as follows: 1 e ¼ u þ u2 þ g z 2

ð38Þ

where u represents the so-called internal energy of the system. Internal energy represents a summation of many microscopic forms of energy including vibrational, chemical, electrical, magnetic, surface, and thermal. The internal energy is proportional to the average force that molecules exert on the system boundary. Internal energy is an extensive quantity that plays the role of a state function. According to the thermodynamic definition a state function is a property of a system that depends only on current state parameters. When a change occurs, the state function is not influenced by the process in which the transformation is performed. Variation of internal energy can be expressed based on the specific internal energy, which is defined by the relationship u¼

U m

ð39Þ

where m is the system mass; as inferred from the definition, the specific internal energy is an intensive property. Using the specific internal energy the following expression for a system’s internal energy is obtained for a closed system DU ¼ m ðu2

• •

u1 Þ

ð40Þ

In classical thermodynamics there is a sign convention for work and heat transfer, which is the following: P P The heat is positive when given to the system, that is, Q ¼ Qin Qout is positive when there is net heat provided to the system. P P The net work, W ¼ Wout Win is positive when work is generated by the system. Using the sign convention, the FLT for closed systems becomes: Q

W ¼ DEsys or q

w ¼ Desys

ð41Þ

where Q ¼m q and W ¼ m w. A newer formulation of FLT avoids the sign convention. In this respect the energy entering the system is expressed as P P P P Qin þ Win , for the closed system. Furthermore, the energy leaving the system is Eout ¼ Qout þ Wout . ConseEin ¼ quently, the FLT or Ein ¼ DEsys þ Eout becomes X X X X Qin þ Win ¼ DEsys þ ¼ Qout þ Wout ð42Þ

Thermodynamic Aspects of Energy

169

FLT can be written in differential form as follows: de ¼ dq

dw

ð43Þ

Taking in account that for a closed system dw¼ P dv, it results that de ¼ dq

P dv

ð44Þ

If one assumes that there is no kinetic and potential energy change, the FLT for closed system becomes du ¼ dq

P dv

ð45Þ

If the system is a CV, then the energy term will comprise the additional term of flow work. In this case the total specific energy of a flowing matter is given as follows: y ¼ h þ 0:5 u2 þ g z

ð46Þ

h ¼ u þ Pv

ð47Þ

where h is a state function denoted as specific enthalpy and defined as follows:

Using enthalpy formulation, the FLT for a CV that has neither velocity nor elevation is dðh PvÞ ¼ dq Pdv, which can be expanded to the following equation: dh ¼ Pdv vdP ¼ dq Pdv. Thus, the FLT for CVs (open systems) is as follows: dh ¼ dq þ v dP

The SLT for CV, using the sign convention for heat and work, is formulated mathematically in rate form as follows: X X d ð m eÞ _ þ _ þ _ yþ _ y¼W Q m m dt out in

ð48Þ

ð49Þ

An important consequence of the FLT is that the internal energy change resulting from any process is independent of the thermodynamic path followed by the system during thermodynamic transformations. In turn, the rate at which the internal energy content of the system changes is dependent only on the rates at which heat is added and work is done. Internal energy is generally dependent on temperature and specific volume u¼ u(T, v). Thence, the total derivative of internal energy is as follows:     ∂u ∂u dT þ dv ð50Þ du ¼ ∂T v ∂T T Eq. (50) is an opportunity to introduce specific heat at constant volume Cv, which represents the amount of heat required to increase the temperature of a constant volume system with 1K, as is observed from the above equation, du¼ dq¼ Cv dT. The specific heat is equal to the change in internal energy with temperature at constant volume   ∂u ð51Þ Cv  ∂T v From Eqs. (45), (50) and (51) it results that dq ¼

∂u ∂T





v

dT ¼ Cv dT

ð52Þ

Furthermore, from Eqs. (45) and (50) it results that P

  ∂u ∂u T

ð53Þ

With Eqs. (50), (51), and (53) combined, the total derivative of internal energy becomes du ¼ Cv dT

P dv

ð54Þ

Specific enthalpy is a function of temperature and pressure, therefore h ¼ h(T, P). The total differential form of the specific enthalpy becomes     ∂h ∂h dT þ dP ð55Þ dh ¼ ∂T P ∂P T Eq. (55) gives the opportunity to introduce the specific heat at constant pressure Cp, which represents the amount of heat required to increase the temperature of a system evolving at constant pressure with 1K. The specific heat is the change of enthalpy with temperature at constant pressure defined according to:   ∂h ð56Þ Cp  ∂T P

170

Thermodynamic Aspects of Energy

In addition it is observed from the above equation that for an isobaric process the heat exchange is governed by the equation dh¼ dq ¼Cp dT, and also the following thermodynamic identity is noted:   ∂h ð57Þ v ∂P T Thenceforth, the total derivative of specific enthalpy becomes dh ¼ Cp dT þ vdP

1.5.3.3

ð58Þ

Second Law of Thermodynamics

The SLT refers to the concept of reversible and irreversible processes. A reversible process is a quasistatic process. One says that a thermodynamic process is reversible if during a transformation both the thermodynamic system and its surroundings can be returned to their initial states. All infinitesimal changes through which the process evolves must be reversible, such that the overall process is reversible. A process performed by a thermodynamic system is irreversible if it cannot return to the initial state because of dissipation that occurs (irreversibly) by friction, heat rejection, electrical, chemical, mechanical losses, etc. In any real application there is dissipation; therefore, reversibility is only an idealization that is important from a theoretical point of view. Reversible processes can be characterized as follows:

• • •

externally reversible – a process with no associated irreversibilities outside the system boundary. internally reversible – a process with no irreversibilities within the boundary of the system during the process. totally reversible – a process with no irreversibilities within the system and surroundings.

The classical statements of SLT, which say in essence that heat cannot be completely converted into work although the opposite is possible (work can be completely converted into work), are the Kelvin–Planck statement and Clausius statement, which are equivalent. Kelvin–Planck statement It is impossible to construct a device, operating in a cycle (e.g., heat engine), that accomplishes only the extraction of heat energy from some source and its complete conversion to work. This simply shows the impossibility of having a heat engine with a thermal efficiency of 100%. Clausius statement It is impossible to construct a device, operating in a cycle (e.g., refrigerator and heat pump), that transfers heat from the lowtemperature side (cooler) to the high-temperature side (hotter). The Clausius inequality provides a mathematical statement of the SLT namely I dQ r0 ð59Þ T where the circular integral indicates that the process must be cyclical. At the limit when the inequality becomes zero then the processes are reversible (ideal situation). A useful mathematical artifice is to attribute to the integral from Eq. (59) a new physical quantity, namely entropy. Entropy is an extensive property. In fact the integral is denoted as generated entropy: I dQ ð60Þ Sgen ¼ T Generated entropy of a system during a process is a superposition of entropy change of the thermodynamic system DSsys and the entropy change of the surroundings DSsurr. Any real process must have generated positive entropy; the following cases may thus occur: (1) Sgen40, real, irreversible process; (2) Sgen ¼ 0, ideal, reversible process; (3) Sgeno0, impossible process. Therefore, the mathematical formulation of the SLT becomes Sgen ¼ DSsys þ DSsurr ¼  0

ð61Þ

Since for a reversible process Sgen ¼ 0 it results from Eq. (60) that entropy change of the system is the opposite of the entropy change of the surroundings   Q ð62Þ DSrev ¼ DSsurr ¼ T rev For an irreversible process the following inequality must be true since the generated entropy is always positive:   Q DSsys 4DSsurr ¼ T surr

ð63Þ

Although the change in entropy of the system and its surroundings may individually increase, decrease, or remain constant, the total entropy change (the sum of entropy change of the system and the surroundings or the total entropy generation) cannot be

Thermodynamic Aspects of Energy

less than zero for any process. Note that entropy change along a process 1-2 is defined based on the integral Z 2 dQ S1 2 ¼ T 1

171

ð64Þ

therefore by differentiation one obtains dQ ¼ T dS

ð65Þ

Entropy quantifies the molecular random motion within a thermodynamic system and is related to the thermodynamic probability (p) of possible microscopic states as indicated by Boltzmann equation S ¼ kB ln p

ð66Þ

The engineering usefulness of SLT consists of the possibility to define and predict the performance limits of systems. As it is known, a heat engine is a device operating based on a specific thermodynamic cycle, which eventually generates useful work. In the reverse, a heat pump consumes work in order to increase the temperature of a working fluid and generate heating. A refrigerator is similar to a heat engine, except that it generates cooling. The environment plays the role of a large thermal energy reservoir of a specified temperature. A thermal energy reservoir is a thermodynamic system with relatively large thermal energy capacity that can absorb or deliver large quantities of heat without observable changes to its temperature. A heat source represents a thermal reservoir capable of providing thermal energy to other systems. A heat sink represents a thermal reservoir capable of absorbing heat from other systems. The environment can play both the role of heat source or heat sink, depending on the application. A heat engine operates cyclically by transferring heat from a heat source to a heat sink. While receiving more heat from the source (QH) and rejecting less to the sink (QC), a heat engine can generate work (W). As stated by the FLT energy is conserved, thus QH ¼ QC þ W. A typical “black box” representation of a heat engine is presented in Fig. 16(A). According to the SLT the work generated must be strictly smaller than the heat input, WoQH. The thermal efficiency of a heat engine – also known as energy efficiency – is defined as the net work generated by the total heat input. Using notations from Fig. 16(A), energy efficiency of a heat engine is expressed (by definition) with Z¼

W ¼1 QH

QC QH

ð67Þ

If a thermodynamic cycle operates as refrigerator or heat pump, then its performance can be assessed by the coefficient of performance (COP) defined as useful heat generated per work consumed. As observed in Fig. 16(B), the energy balance equation (EBE) for a heat pump is written as QC þ W ¼ QH, according to SLT QHZW (this means that work can be integrally converted in heat). Based on its definition, the COP is COP ¼

QH QH ¼ W QL þ W

ð68Þ

The Carnot cycle is a fundamental model in thermodynamics representing a heat engine (or heat pump) that operates between a heat source and a heat sink, both of them being at constant temperature. This cycle is a conceptual (theoretical) cycle and was proposed by Sadi Carnot in 1824. The cycle comprises fully reversible processes, namely two adiabatic and two isothermal processes. The efficiency of Carnot cycle is independent on working fluid that performs cyclically the processes. Based on the definition of Carnot cycle it results that s2 ¼ s3 and s4 ¼ s1 (for heat engine). The heat transferred at source and sink are QH ¼ TH(s3 ¼ s4) ¼ TH(s2 ¼ s1) and QL ¼ TL(s2 ¼s1). Therefore, energy efficiency of the Carnot cycle is Z¼1

TL TH

ð69Þ

Heat source (TH)

Heat sink (TH)

QH

QH W

W

Heat engine

QL

FLT: QH = W + QL SLT: W ≤ QH

QC

Heat sink (TL) (A) Fig. 16 Conceptual representation of a heat engine (A) and heat pump (B).

Heat engine

Heat source (TL) (B)

172

Thermodynamic Aspects of Energy

and COP of reversed Carnot cycle is COP ¼

TH ðTH TL Þ

ð70Þ

Carnot efficiency expressed by Eqs. (69) and (70) is a useful criterion to assess practical heat engines, refrigerators, heat pumps, or other energy conversion systems with respect to the idealized case of reversible heat engine. Accordingly, energy efficiency (Z) and COP of a reversible thermodynamic cycle (Carnot) is the highest possible and any actual (irreversible) cycle has smaller efficiency (Zrev4Zirrev) and (COPrev4COPirrev). Carnot cycle is useful to develop the absolute temperature scale, which is a scale independent of the properties of substances that are used for measurement (e.g., mercury, alcohol, water, etc). As it results from Eqs. (67)–(70) for reversible heat engines one has   QH TH ¼ ð71Þ QL rev TL where index “rev” signifies “reversible processes.” The absolute temperature scale is developed by assigning a temperature value to the thermodynamic state of triple point of pure water; this value is T0 ¼273.16K. Thus, if a heat engine operates between T and T0 then the temperature T can be expressed in function of reference temperature T0 as T ¼ T0

Q Q0

ð72Þ

Therefore, absolute temperature can be determined by measurement of heat transfer at source and sink of a reversible heat engine. At the lowest limit, temperature is 0K (zero Kelvin), namely, when the reversible engine operates between reference temperature T0 at source and zero absolute at sink. In this case, according to Eq. (71) the heat delivered to the sink is nil and the heat absorbed from the source is completely transformed in work because in this extreme – an idealized case – Carnot efficiency must be 1 (limit never attainable). At 0K the entropy of a nonvibrating system such as crystals is zero.

1.5.3.4

Constructal Law

Classical thermodynamics is concerned with black box systems without specified internal configuration. It teaches about quasistatic processes that change the thermodynamic state from an equilibrium state to another equilibrium state. Natural systems have configuration, they are free to evolve, they appear (give birth, become alive) and disappear (die, reach dead state), they have finite size and finite lifetime. Thermodynamics require conceptual tools and extension to be able to predict systems at nonequilibrium. In the last 20 years a new body of thermodynamics emerged that identifies systems at nonequilibrium as live systems. A nonequilibrium system is a thermodynamic system denoted to be in a live state. The live state opposes the dead state, which characterizes a system with internal thermodynamic equilibrium. A live state system is free to flow, to move. Following Bejan [3] one recognizes that a live system organizes, constructs itself, creates configuration, design, pattern, rhythm, and sound. We already reviewed in the Introduction that a new first principle of physics was formulated by Bejan [9] in 1996, which strengthens thermodynamics to cover all phenomena of design occurrence and evolution that characterize nonequilibrium systems. According to Bejan [3] “constructal law commands that the changes in configuration must occur in a particular direction in time, toward designs that allow currents to flow more easily.” Here is an example illustrating the occurrence of life state and the evolution of a thermodynamic system at nonequilibrium. Shown in Fig. 17 is a region on Earth’s surface where a cloud is formed and charged with electricity. The charging process can be thought of as quasistatic, evolving toward a metastable equilibrium state. Negative charges at cloud bottom attract positive charges on Earth’s surface. Once the lightning initiates, the thermodynamic system will become a live system, a system with internal nonequilibrium that is animated with flows. Electrons will flow and open pathways to discharge on the ground. Constructal law commands that the flow configuration changes over time such that greater access to flow is obtained. The charges flow faster and faster, over larger area, constructing a more elaborated flow pattern. This is the constructal time arrow revealed by the change toward higher access to currents over the finite space where the system exists. At the beginning the flow is small, but later is large and more complex. Much later, the live system decays as its freedom will be reduced, the constraints upon it increased and the push diminished. The live system of the lightning has a finite lifetime. Once it decays, conditions occur for other flows to manifest. Constructal law proposes to look holistically to the phenomena of flow formation, growing, evolution, and decay that manifest over a finite space region. It considers a wider view in the analysis, regardless if the flow is of energy, matter, or any immaterial flow such as information. If a flow (of any kind) occurs anywhere, then its evolution rate first will increase and later will decrease according to a bellshaped curve, Fig. 18. Flow evolution requires a first phase of invasion over the territory followed by a consolidation phase of its pathways, structure, and configuration. At a point in time evolution slows down and eventually stops while the flow cannot morph freely. Constructal law states that the flow to persist in time (to evolve, to live) must be free to morph such that its currents will flow faster. When the flow becomes unable to morph, it ceases to exist; there will be a decay phase. The bell-like curve of evolution rate corresponds to an S-shaped (or logistic) curve of evolution, Fig. 19. A live state is born from a dead state when there is a push

Thermodynamic Aspects of Energy

_ _ _ _ _ _ _ _

Thermodynamic system Constructal law of evolution

++ + ++ + ++ ++ + + +++ + + + ++ ++ __ _ _ _ _ _ _ _ _ _ _ __ _ _ _ _ _ _ _

_ _ _

_ _ _ __ _ _ __ __

l cta u w r t o ns arr Come ti

C tim ons e truc ar ta ro l w

++++++++ Live state Second law tim e arrow

++++++++++++++++++ Earthsurface Dead state

Dead state Fig. 17 Illustrating the concept of system thermodynamic at live state and the time arrows.

Evolution rate tion

truc

s Con

Others will continue

on

ati

lid so

n

Co

Decay Live state

Invasion Dead state

Dead state Time

Fig. 18 Evolution rate as predicted by constructal law.

Evolution

Dead state ow

r

e

d

law

Se

C on

st

ru

ct

al t

im

e

ar

ro

w

Live state

n co

ar

tim

Dead state Fig. 19 The S-shape of flow configuration evolution as predicted by constructal law.

Time

173

174

Thermodynamic Aspects of Energy

to flow. The live state evolves and later, when evolution slows down, the live state decays to death (flow disappears). Constructal law is analytically formulated as an extension to thermodynamics in Ref. [12]. Constructal law treats thermodynamic systems at internal nonequilibrium as flow systems. Indeed, any system having internal nonequilibrium must possess flows. The nonequilibrium system, or the system at live state, or the flow system (all similar names for the same thing) has identity. Recall that in classical thermodynamics, a system at equilibrium is identified with state parameters such temperature, pressure, volume. In Constructal extension to thermodynamics of a nonequilibrium system, the live system is identified by new parameters as introduced in Bejan, see Ref. [13], namely:

• • •

Global external size, i.e., the length scale of the flow system, denoted with L Global internal size, i.e., the total volume of flow “ducts,” denoted with V The live system performance parameter(s), for example, global flow resistance or other parameters, generically denoted with ℜ

The live system has two length scales defining its configuration (structure, architecture, drawing, geometry). The ratio of external to internal length scale defines a dimensionless parameter of live system configuration, introduced in Ref. [12] as svelteness, namely: L ffiffiffiffi Sv ¼ p 3 V

ð73Þ

Fixed L and V-ℜ must improve while Sv remains unchanged

ð74Þ

If live system internal size V and external size L are constrained the system will survive if it is free to change in time such that its performance ℜ improves. All flow configurations along the evolution pathway are called nonequilibrium flow structures since they are changed in time toward a better one. Eventually, the constrained live system will achieve a state of equilibrium flow structures out of which it cannot evolve as far the internal size constraint is not relaxed. At a state of equilibrium flow structures the freedom to morph is the highest that can be achieved under constraints and the performance is the same for any of the equilibrium structures. The live system evolution under constant global external size L is analogue as evolution at constant V, namely in both cases the system tends to increase its performance in time. This situation corresponds to the survival of the fittest. Therefore, one describes the survival of the fittest as follows:

When the flow system has a constrained global external size L and a constant global performance ℜ the system must evolve such that its svelteness Sv increases. Svelteness quantifies compactness. The flow structure occupies smaller fraction of the available space when the system is more compact. This survival, by maximization of the use of the available space, is written as follows: Fixed L and ℜ- Sv must increase ði:e:; V decreasesÞ

ð75Þ

When the live system has a constrained global performance ℜ and constrained global internal size V it persists only if it evolves toward a greater svelteness Sv (this means larger global external flow scale L). This statement of the constructal law is written compactly as follows: Fixed V and ℜ- Sv must increase ði:e:; L increasesÞ

ð76Þ

A appealing application of constructal thermodynamics ideas is in energy system design. The live system features in this case a flow of knowledge (or know-how). The knowledge evolves such that better access to its flow is observed over time. Engines evolved clearly toward better performance as ruled by constructal law in the form of Eq. (73). Furthermore, recent advances reported in Ref. [14] showed the benefit of increasing the energy system’s global external size L by increasing the number of generated outputs. This strategy is meaningful when the system performance with single output cannot be reasonably improved. Therefore, multigeneration systems are governed by constructal law as expressed by Eq. (74). The number of generated outputs in a multigeneration system will be a measure of its freedom to morph. Exergy destruction is the performance parameter. The global internal size V can be measured in terms of investment cost on system hardware. Fig. 20 shows knowledge evolution of energy systems design with multigeneration. Once such a trend is revealed, evolution and technological progress can be fast forwarded. For any fixed cost and fixed number of generated outputs (e.g., two outputs: heat and power) an optimum system design can be found that minimizes the exergy destruction for that case. The optima of designs can be then observed at fixed cost or at fixed exergy destruction as in the figure. A locus (or frontier) of ideal system configurations can be predicted in that way. Fig. 21 gives an actual example taken from Ref. [15] that proposed and analyzed a multigeneration energy system constructed around a micro gas turbine. In addition, the system integrates a dual-pressure heat recovery steam generator, an organic Rankine cycle (ORC), an absorption chiller, an ejector refrigeration cycle, a domestic water heater, and a proton exchange membrane water electrolyzer. Fig. 21 shows the Pareto front of the optimized solutions in terms of exergy efficiency and total cost rate of generated products. A multiobjective optimization search code has been used as detailed in the original work [15]. Four particular points, A, B, C, and D, are indicated on the Pareto front. These are particular optimization solutions, as follows:

• • •

Point A corresponds to a minimum cost rate; this is the cheapest system among the best solutions. Points B and C represent a compromise between cost and performance (here exergy efficiency). Point D shows the system with maximum exergy efficiency but also with the higher cost.

Number of generated outputs (freedom to morph, compexity)

Thermodynamic Aspects of Energy

rgy

on

cti

tru

s de

175

Optima at constant cost (or global internal size)

e Ex

Optima at constant exergy destruction

(o

r g Har lob dw al are int c er os na t ls ize

Ideal configurations locus

)

Fig. 20 Predicted evolution of energy systems toward multigeneration. Adapted from Adapted from Dincer I, Zamfirescu C. Advanced power generation systems. New York, NY: Elsevier; 2014.

68 D

Pa

re

66

 (%)

C

to

fro

nt

Ideal system

System with the highest exergy efficiency

64 B

62 System with the lowest cost A 60 590

600

610

620

Ctot ($/h), proportional to global inetrnal size Fig. 21 Two-objective optimization of a multigeneration system. Adapted from Ahmadi P, Dincer I, Rosen MA. Thermodynamic modelling and multi-objective evolutionary-based optimization of a new multigeneration enery system. Energy Convers and Manag 2013;76:282–300.

Additional results of a similar kind referring to design generation of multigeneration energy systems are reviewed in Ref. [14] for thermally driven energy systems [16]; integrated biomass-fueled systems [17]; solar-based coal gasification [18]; solar-based multigeneration with hydrogen production [19]; solar-based trigeneration of power, heating, and desalination [19]; multigeneration of power, hot water, and fuel from biomass and coal [19]; power, heating, and cooling trigeneration with natural gas and biomass-fueled solid oxide fuel cell integrated with ORC and absorption cooling [20]; hybrid power, hot water, and cooling trigeneration with internal-reforming tubular solid oxide fuel cell integrated with heat recovery vapor generator and ORC, parabolic solar-through concentrator and Li-Br absorption chiller [21]; ammonia water-based trigeneration for waste heat recovery [22]; and gas turbine trigeneration with absorption cooling, heating, and power [23].

1.5.4

Equations of State

The equations of state describe the thermodynamic behavior of bulk matter (which is formed by groups of atoms, molecules, and clusters of them) generally in terms of temperature, pressure, and specific volume. Other state variables such as specific internal energy, specific entropy, and specific volume can be also used to formulate equations of state. In this section the states of aggregation of matter are introduced and thereafter equations of state are discussed for ideal gas, real fluids, solids, ideal and real solutions, and electrolytes.

176

Thermodynamic Aspects of Energy

1.5.4.1

States of Aggregation

There are four forms of aggregation of substances, denoted also as phases or states, namely, solid, liquid, gas, and plasma. There are also some intermediate or transition phases such as supercritical fluid, etc. Each of the properties of a substance in a given state has only one definite value, regardless of how the substance reaches the state. Temperature, pressure, and specific volume represent a set of thermodynamic properties that define completely the thermodynamic state and the state of aggregation of a substance. The thermodynamic state of a system can be modified via various interactions, among which heat transfer is one. Heat can be added or removed from a system. When sufficient heat is added or removed at a certain condition, most substances undergo a state change. For pure substances the temperature remains constant until the state change is complete. This can be from solid to liquid, liquid to vapor, or vice versa. The solid state of aggregation is represented by atoms arranged in specific patterns. Strong interaction forces occur among the atoms forming a solid state of aggregation. The pattern in which the atoms are arranged is very regular in substances with crystalline structure such as water, graphite, diamond, metals, and salts. Some solid materials have irregular pattern such as amorphous glass. Fig. 22 shows the crystalline structure of graphite. Liquid and gases are generically denoted as fluids. They display smaller interatomic (intermolecular) interactions than solids do. Liquid is an incompressible fluid whereas a gas is a compressible fluid. Liquid state has a definite volume but no fixed shape. A gas does not have a definite volume but rather occupies the space as confined; it also does not have a shape. A representation of solid, liquid, and vapor phases of a pure substance is qualitatively exhibited also on a temperature–volume (T–v) diagram in Fig. 23. In this diagram “T” is the triple point of the pure substance. The triple point represents that thermodynamic state where solid, liquid, and vapor can coexist. For example, the triple point of water occurs at 273.16K, 6.117 mbar and specific volume is 1.091 dm3/kg for ice, 1 dm3/kg for liquid water, and 206 m3/kg for vapor. Below the triple point isobar there is no liquid phase. A sublimation or desublimation process occurs that represents phase transition between solid and vapor.

Crystalline graphite

Amorphous solid

Fig. 22 Graphite pattern as an example of crystalline solid state compared with the irregular pattern of an amorphous solid.

I

T

Supercritical isobars

G

D

Critical point isobar F

Vapor quality lines 0.05 0.1 0.2

H Liquid saturation line

Vapor saturation line 0.5

Normal boiling point isobar, 101.3 kPa

C

B E A T

Triple point isobar

v Fig. 23 Temperature–volume diagram of a pure substance.

Thermodynamic Aspects of Energy

Critical isotherm

P

Liquid region

177

Supercritical fluid region

Solidification line

Solid region

Critical isobar Boiling line

Critical point

Vapor region Triple point

Sublimation line

T Fig. 24 Pressure vs. temperature diagram of a substance.

Between triple point isobar and critical point isobar three phases do exist: solid, liquid, and vapor. In addition there are defined thermodynamic regions of subcooled liquid, two-phase and superheated vapor. Subcooled liquid regions exist between the critical isobar and liquid saturation line (see Fig. 23). The two-phase region is delimited by liquid saturation line at the left, vapor saturation line at the right, and triple point isobar at the bottom. Superheated vapor exists above the vapor saturation line and below the critical isobar. At temperatures higher than the temperature of critical point and above the critical isobar there is a thermodynamic region denoted as the “supercritical fluid region,” where the substance is neither liquid nor gas, but has some common properties with gases and with liquid. On the diagram in Fig. 23, the constant vapor quality lines and state points A to I are indicated. These state points are representative for various processes as follows:

• • • • • •

A–B–C–D: Constant pressure process. A–B: Represents the process where water is heated from the initial temperature to the saturation temperature (liquid) at constant pressure. At point B, the water is a fully saturated liquid with a quality x ¼0, but no water vapor has formed. B–C: Represents a constant-temperature vaporization process in which there is only phase change from a saturated liquid to a saturated vapor. As this process proceeds, the vapor quality varies from 0 to 100%. Within this zone, the water is a mixture of liquid and vapor. At point C we have a completely saturated vapor and the quality is 100%. C–D: Represents the constant-pressure process in which the saturated water vapor is superheated with increasing temperature. E–F–G: Represents a nonconstant-temperature vaporization process. In this constant-pressure heating process, Point F is called the critical point where the saturated-liquid and saturated-vapor states are identical. The thermodynamic properties at this point are called critical thermodynamic properties, for example, critical temperature, critical pressure, and critical specific volume. H–I: Represents a constant-pressure heating process in which there is no change from one phase to another (only one is present). However, there is a continuous change in density during this process.

The pressure versus temperature diagram is also an important tool that shows phase transitions of any substance. Fig. 24 qualitatively shows the P–T diagram of a pure substance. There are four regions delimited in the diagrams: solid, vapor, liquid, and supercritical fluid. The phase transition lines are sublimation, solidification, boiling, critical isotherm, and critical isobar; the last two lines are represented only for supercritical region (at pressure and temperature higher than critical). Another important state diagram is the pressure versus volume diagram for pure substances. Fig. 25 shows the pressure–volume diagram. In this diagram the triple point isotherm and the normal boiling point isotherm can be observed. A number of fundamental physical constants are very important for substance characterization. Examples are the universal gas constant, Boltzmann constant, Faraday constant, and elementary electric charge. In addition, some standard parameters such as standard atmospheric pressure and temperature, standard molar volume, and solar constant are very important for thermodynamic analysis. Table 3 presents fundamental physical constants and standard parameters. In the table are indicated the constant name, its symbol, the value and units, and a brief definition.

1.5.4.2

Ideal Gas Theory

Ideal gas theory is very important for analysis of processes because in most of the situations moisture content is extracted in the form of water vapor, which behaves as an ideal gas. An ideal gas can be described in terms of three parameters: the volume that it

178

Thermodynamic Aspects of Energy

Critical point isotherm Supercritical isotherm Saturated vapor line

P

Critical point

Saturated liquid line

Normal boiling point isotherm

Vapor quality lines 0.5 0.2 0.1 0.05 Triple point isotherm v Fig. 25 The pressure–volume diagram of a pure substance.

Table 3

Fundamental constants and standard parameters of substance

Constant/parameter

Value

Definition

Speed of light in vacuum

c ¼299,792,458 m/s

Elementary charge Faraday’s constant Gravitational acceleration

e ¼1.60218  10 F ¼96,485 C/mol g¼ 9.80665 m/s2

Planck’s constant

h¼ 6.626  10

Boltzmann constant

kB ¼1.3806  10

Number of Avogadro

NA ¼6.023  1026 molecules/kmol

Standard atmospheric pressure

P0 ¼101.325 kPa

Universal gas constant

ℛ ¼8.314 J/mol K ℛ ¼Pv/T s ¼5.670373  10

Maximum speed at which matter and information can be transported in the known cosmos. Electrical charge carried by a single proton. Electric charge of one mole of electrons. Standard gravitational acceleration represents the gravitational force (G) per unit of mass. g ¼G/m. Indicates the magnitude of energy of a quanta which expresses the proportionality between frequency of a photon and its energy according to E¼h v. Represents a measure of kinetic energy of one molecule of ideal gas. Ratio of constituent entities of a bulk substance to the amount of substance. NA ¼N/n. Pressure of the terrestrial atmosphere at the level of sea in standard conditions. Represents a measure of kinetic energy of one mole of an ideal gas at molecular level. Constant in Stefan–Boltzmann law expressing the proportionality between fourth power of temperature and blackbody’s emissive power.

Stefan–Boltzmann constant

37

19

C

kJ s

23

8

J/K. kB ¼ℛ/NA

W/m2 K4

occupies, the pressure that it exerts, and its temperature. According to the definition, ideal gas represents a special state of mater which can be delimited by a system boundary. It is assumed that:

• • • •

All particles have rest mass (m40; the particles are not photons). The number of particles with respect to the system volume is small. Total volume of particles is negligible with respect to system volume. Collisions of particles with each other is much less probable than the collisions with system boundary.

The practical advantage of treating real gases as ideal (in certain conditions) is a simple equation of state with only one constant. The ideal gas equation of state can be written in the following form: P V ¼ m R T or P v ¼ R T or P v ¼ R T

ð77Þ

Thermodynamic Aspects of Energy

179

where P is the pressure in Pa, V is the gas volume in m3, m is mass of gas in kg, T is gas temperature in K, R is known as the gas constant and is given in J/kgK, v is mass specific volume in m3/kg, u is molar specific volume in m3/kmol, and R is the universal gas constant of 8.134 J/mol K. Observe that the gas constant is specific to each particular gas and depends on the universal gas constant and the molecular mass (M) of the gas according to R¼

R M

ð78Þ

Eq. (76) is known as “the thermal equation of state” of the ideal gas because it expresses the relationship between pressure, specific volume, and temperature. It is possible to express the ideal gas equation in terms of internal energy, specific volume, and temperature. In this case the equation of state is called the caloric equation of state. In particular, for ideal gas only, the internal energy depends on temperature only. The caloric equation of state for a monoatomic ideal gas is given as follows: u ¼ 1:5 R T

ð79Þ

where u is the molar specific internal energy. Since h¼ u þ P v it results that the enthalpy of monoatomic ideal gas is given by h ¼ 2:5 R T

ð80Þ

Combining the two caloric equations of state from Eqs. (79) and (80), the known Robert Meyer equation for ideal gas can be derived as follows: Cp ¼ Cv þ R

ð81Þ

It can be remarked that for ideal gas the internal energy is a function of temperature only. Therefore, specific heat for ideal gas is Cv ¼1.5 R and CP ¼2.5 R. The ratio of specific heat at constant pressure and constant volume is known as the adiabatic exponent, namely g¼

CP Cv

ð82Þ

The adiabatic exponent has the following values for ideal gas: monoatomic gas 1.4 and 5/3 ¼ 1.67 for diatomic gas. There are some special cases if one of P, v, or T is constant. At a fixed temperature, the volume of a given quantity of ideal gas varies inversely with the pressure exerted on it (in some books this is called Boyle’s law), describing a volume change as follows: P1 V1 ¼ P2 V2

ð83Þ

where the subscripts refer to the initial and final states. Eq. (83) is employed by analysts in a variety of situations: when selecting an air compressor, for calculating the consumption of compressed air in reciprocating air cylinders, and for determining the length of time required for storing air. If the process is at constant pressure or at constant volume then Charles’s law applies: V1 V2 P1 P2 ¼ and ¼ T1 T2 T1 T2

ð84Þ

If the number of moles of an ideal gas does not change in an enclosed volume, then the combined ideal equation of state is given as follows: P1 V1 P2 V2 ¼ T1 T2

ð85Þ

The basic processes with ideal gas are known as follows: isothermal, isochoric, isobaric, isentropic, and polytropic. Table 4 gives the description and relevant equation for each process. The isentropic process is similar to the polytropic one, however, with an exponent n equal to the adiabatic exponent. Ideal gas air isochoric, isobaric, isothermal, and isentropic (i.e., polytropic with n¼ g ¼ 1.4) processes are shown in Fig. 26. The specific entropy change of an ideal gas with constant specific heats is given by the following equations, depending on the type of process (at constant pressure or at constant volume): Table 4

Simple thermodynamic processes and corresponding equations for ideal gas model

Process

Definition

Equation

Work expression

Isothermal Isochoric

T¼const. v ¼const.

P1 v1 ¼ P2 v2 P1 P2 T1 ¼ T2

w1 w1

Isobaric Polytropic

P ¼const.

General

P, v, T vary constant mass

n

Pv ¼const.

v1 v2 T1 ¼ T2  n P2 V1 P1 ¼ V2 P1 v1 P2 v2 T1 ¼ T2

¼

 n=ðn T2 T1

1Þ

w1 w1

2 2

¼ P1 v2 ln ðv2 =v1 Þ ¼0

¼ P ðv2 v1 Þ 1 ¼ P1 v1 Þ 2 n 1 ðP2 v2 R2 w1 2 ¼ 1 Pdv 2

180

Thermodynamic Aspects of Energy

450 400

Air as ideal gas 3

1

P (kPa)

350

1 → 2: Isochoric expansion 1 → 3: Isobaric heating 1 → 4: Isothermal expansion 1 → 5: Polytropic expansion (n = 1.4)

300 250 200 150 100 0.4

5

2 0.6

0.8

1.0

4

1.2 1.4 v (m3/kg)

1.6

1.8

2.0

2.2

Fig. 26 Ideal gas processes represented in P–v diagram.

A (area) Elastic collision →



Molecules l Fig. 27 Cube-shaped thermodynamic system enclosing the ideal gas at internal thermodynamic equilibrium at temperature T and pressure P.

s2

s1 ¼ Cv0 ln

    T2 v2 þ R ln and s2 T1 v1

s1 ¼ Cv0 ln

    T2 P2 þ R ln T1 P1

ð86Þ

Assume a thermodynamic system of arbitrary shape enclosing an ideal gas. For simplicity let this shape be a cube as shown in Fig. 27. Based on momentum conservation law, the force exerted by one particle on the wall, during collision, is given as follows: F¼

mu

mð uÞ m u2 ¼ l Dt

ð87Þ

One also assumes that there is a uniform distribution of particle collisions for the three Cartesian directions; thus only 1/3 of particles exert force on a wall. Therefore the pressure expression is determined by divided the force with wall area A, as follows: P¼

1 u2 Nm 1 ¼ Nm ¼ r u2 lA 3 3V 3

ð88Þ

where N is the number of particles, each having a mass m, density is denoted with r¼ NM/V, and V ¼ lA is the volume of the thermodynamic system. The kinetic energy of a single gas particle can be expressed based on the average particle velocity; in this respect, Eq. (88) is solved for v2 and one obtains KE ¼

1 3P V m u2 ¼ 2 2 N

ð89Þ

Thermodynamic Aspects of Energy

181

The degree of freedom of monoatomic gas molecules is DOF ¼ 3, because there are only three possible translation movements along Cartesian axes. According to its thermodynamic definition, temperature (T) is a measure of the average kinetic energy of molecules per degree of freedom. The quantitative relation between temperature and kinetic energy of one single molecule is according to KE 1  kB T DOF 2

ð90Þ

where kB is the Boltzmann constant defined in Table 3. Solving Eq. (90) for T results in the thermodynamic expression for temperature as follows: T

2 KE kB DOF

ð91Þ

From Eqs. (89) and (91) the following expression is obtained for temperature: T¼

PV PV PV Pv ¼ ¼ ¼ kB N kB ðN=NA ÞNA kB n NA kB NA

ð92Þ

where v is the molar specific volume and n is the amount of substance (number of moles). The following thermodynamic definition of temperature is further obtained: Pv  P vR kB NA

ð93Þ

R ¼ kB N A

ð94Þ

T where R is the universal gas constant given as follows:

In many practical situations mixtures of real gases can be approximated as mixtures of ideal gases. There are two ideal gas models for gas mixtures: the Dalton model and Amagat model. For both models it is assumed that each gas is unaffected by the presence of other gases. The Dalton model assumes that the mixture is at constant temperature and volume whereas the Amagat volume considers the case when temperature and pressure are constant. Table 5 gives a comparison between models of Dalton and Amagat for ideal gas mixtures. The equations relating the thermodynamic parameters of the component gases with the parameters of the mixture are given in Table 6.

1.5.4.3

Real Gases

Ideal gas theory fails at predicting the thermodynamic states of real gases in dense phase, that is, next to the saturation line and critical point. More elaborated equations of state are therefore required. Those are the equations of state for real fluids, which are also able to predict the saturation line and liquid state parameters. In this respect, the compressibility factor (Z) is introduced to measure the deviation of a real substance from the ideal-gas equation of state. The compressibility factor is defined by the following relation: Table 5

Dalton and Amagat models for ideal gas mixtures

Definition

Dalton model

Amagat model

Assumptions

T and V are constant Ptot ¼ P1 þ P2 þ ⋯ þ PN Pi V ¼ ni R T P Ptot V ¼ ð n ÞR T

T and P are constant Vtot ¼ V1 þ V2 þ ⋯ þ VN PVi ¼ ni P RT PVtot ¼ ð n ÞR T

Equations for the components Equation for the mixture

Table 6

Relevant parameters of ideal gas mixtures

Parameter

Equation

Total mass of a mixture of N components Total number of moles of a mixture of N components Mass fraction for each component Mole fraction for each component

P m mtot ¼ P i ni ntot ¼ ci ¼ mi =mtot    ¼ VVtoti yi ¼ nntoti ¼ PPtoti Dalton model Amagat model P P ðn M Þ ðyi Mi Þ Mmix ¼ mntottot ¼ ntoti i ¼ P Umix ¼ ðn U Þ P i i ðni Hi Þ Hmix ¼ P Smix ¼ ðni Si Þ P S2 S1 ¼ R ðni ln yi Þ

Molecular weight of the mixture Internal energy of the mixture Enthalpy of the mixture Entropy of the mixture Entropy difference for the mixture

182

Thermodynamic Aspects of Energy Pv RT

Z

ð95Þ

where specific volume is expressed on a mass basis. The order of magnitude is about 0.2 for many fluids. For accurate thermodynamic calculations compressibility charts can be used, which express compressibility factor as a function of pressure and temperature. In this way, an equation of state obtained based on compressibility factor is the following: Pv ¼ Z R T

ð96Þ

where the compressibility factor is a function of pressure and temperature. According to the so-called principle of corresponding states compressibility factor has a quantitative similarity for all gases when it is plotted against reduced pressure and reduced temperature. The reduced pressure is defined by the actual pressure divided by the pressure of the critical point P¼

P Pc

ð97Þ

where subscript c refers to critical properties and subscript r to reduced properties. Analogously, the reduced temperature is defined by Tr ¼

T Tc

ð98Þ

The compressibility charts showing the dependence of compressibility factor on reduced pressure and temperature can be obtained from accurate P, v, T data for fluids. These data are obtained primarily based on measurements. Accurate equations of state exist for many fluids; these equations are normally fitted to the experimental data to maximize the prediction accuracy. A generalized compressibility chart Z¼ f(Pr, Tr) is presented in Fig. 28. As seen in the figure, at all temperatures Z tends to 1 as Pr tends to 0. This means that the behavior of the actual gas closely approaches ideal gas behavior, as the pressure approaches zero. There are also several equations of state for accurately representing the P–v–T behavior of a gas over the entire superheated vapor region, i.e., the van der Waals equation and its subsequent improvements, the Benedict–Webb–Rubin, the Redlich and Kwong, and Soave equations. However, some of these equations of state are complicated, due to the number of empirical constants they contain, and are more conveniently used with computer software to obtain results. The most basic equation of state is that of van der Waals, which is able to predict the vapor and liquid saturation line and a qualitatively correct fluid behavior in the vicinity of the critical point. This equation is described in Table 7. The Peng–Robinson equation of state is an improvement on Soave equation of state, which predicts better in the vicinity of the critical point. Similarly to the precursory semiempirical equations such as van der Waals, the equation by Peng–Robinson models the pressure in a gas (in Pa) as an effect of two additive terms, the repulsion pressure PR ¼

RT u b

ð99Þ

and the attraction pressure (negative term) 1

Tr = 1.5 0.8 Tr = 1.4 Tr = 1.3

Z

0.6

Tr = 1.2 0.4 Tr = 1.1 Tr = 1

0.2 0

1

2

3

4

5

6

7

Pr Fig. 28 Generalized compressibility chart averaged for water, oxygen, nitrogen, carbon dioxide, carbon monoxide, methane, ethane, propane, nbutane, isopentane, cyclohexane, n-heptane.

Thermodynamic Aspects of Energy

Table 7

183

Description of the van der Waals equation of state

Item

Equation

Reduced pressure, temperature, and specific volume Reduced internal energy

Pr ¼

Thermal equation of state

Pr ¼ 8 Tr =ð3 vr ur ¼ 4 R Tr

ur ¼

Caloric equation of state

PA ¼

a g ðu; bÞ

P Pc

; Tr ¼

T Tc

u

ðPc vc Þ

; vr ¼ 1Þ

v vc

3=vr2

ð100Þ

where g(u, b) is a function of the molar volume u. In Eq. (99) the parameter b is related to the size of the molecule and temperature, b(T), whereas in Eq. (100) the parameter a is a measure of intermolecular attraction forces, which depends on temperature, a(T), with temperature measured in K. The function is specified in the Peng–Robinson equation of state as follows: g ðu; bÞ ¼ vðv þ bÞ þ bðv

bÞ

ð101Þ

From Eqs. (99)–(101) the equation of state by Peng–Robinson can be expressed as follows: P¼

RT u b

With the following notations:

aðT Þ vðv þ bÞ þ bðv

aP bP and B ¼ RT R2 T 2 the equation can be expressed in the following implicit form:    Z3 ð1 BÞZ 2 þ A 3B2 2B Z AB

ð102Þ

bÞ

ð103Þ

A¼

B2

 B3 ¼ 0

ð104Þ

where the compressibility factor Z defined as in Eq. (95). The critical compressibility factor of Peng–Robinson fluid is Zc ¼ 0.307. Peng and Robinson [24] correlate the parameters a(T) as follows: aðT Þ ¼ aðTc ÞaðTr ; oÞ

ð105Þ

where a(Tr, o) is a dimensionless parameter that depends on the reduced temperature and the acentric factor. At critical temperature a(Tr, o)¼ 1. At any other temperature, the parameter a has been determined by imposing thermodynamic equilibrium between saturated vapor and saturated liquid for pure substances. Doing so, a correlation has been regressed for a, given as follows:  pffiffiffiffiffi pffiffiffi a¼1þk 1 Tr ð106Þ

where k is correlated with the acentric factor o, which is a property characteristic to any fluid. Strjeck and Vera [25] correlated the parameter k for improved accuracy in the vicinity of liquid–vapor saturation with temperature and the acentric factor as follows:  pffiffiffiffiffi k ¼ k0 þ k1 1 þ Tr ð0:7 Tr Þ ð107Þ with

k0 ¼ 0:378893 þ 1:489753o

0:1731848o2 þ 0:0196554o3

ð108Þ

and parameter k1 being a constant that is given for any pure fluid. The parameter k1 can be approximated to zero for water and alcohols at supercritical temperature; it can be also approximated to zero for any other fluid at reduced temperatures above 0.7.

1.5.5

Thermodynamic Equilibrium

The concept of equilibrium has been previously described in Sections 2 and 3. Here we will invoke the first and second laws of thermodynamics simultaneously to analyze the conditions required for a closed system to reach equilibrium. Equivalently, the results of this analysis apply for a closed system that departs from equilibrium. There are in total five distinct cases to be considered, depending on the process constraints imposed upon the closed system. The first situation is when the closed system is constrained to fixed volume and fixed entropy together. The system is free to displace from equilibrium by changing other parameters than entropy and volume, for example, species concentration. In this situation the principle of minimum internal energy applies, which states that if a closed system evolves at constant entropy and volume then the internal energy of the system is maximum when the system reaches thermodynamic equilibrium. Mathematically, the following relationships are valid:

184

Thermodynamic Aspects of Energy 

∂U ∂Xi



S;V

¼ 0 and



∂2 U ∂Xi2



S;V

40; i ¼ 1…n

ð109Þ

where U(S, V, Xi), i ¼1…n is the internal energy of a homogeneous closed thermodynamic system with n þ 2 degrees of freedom and Xi are mass fractions of each system component. The second case reflects the minimum enthalpy principle, which states that a closed thermodynamic system that is restricted to evolve at constant entropy and pressure has a minimum enthalpy at thermodynamic equilibrium. Mathematically, the principle is expressed as follows:    2  ∂H ∂ U 40; i ¼ 1…n ð110Þ ¼ 0 and ∂Xi S;P ∂Xi2 S;P The third case refers to a closed system restricted to evolving at constant temperature and volume when the principle of minimum Helmholtz energy A applies. The Helmholtz free energy of a thermodynamic system is defined as follows: A¼U

TS

ð111Þ

The principle of minimum Helmholtz energy states that a closed thermodynamic system approaching equilibrium at constant temperature and volume tends to a state of minimum Helmholtz free energy. Mathematically this principle translates as follows:  2    ∂A ∂ A 40; i ¼ 1…n ð112Þ ¼ 0 and ∂Xi T;V ∂Xi2 T;V The fourth case is when the closed system is restricted to fixed temperature and pressure, in which case the principle of minimum Gibbs free energy holds. A closed system approaching equilibrium at constant temperature and pressure tends to a state of minimum Gibbs free energy. Mathematically one can write for this case:    2  ∂G ∂ G ¼ 0 and 40; i ¼ 1…n ð113Þ ∂Xi T;P ∂Xi2 T;P In addition, a fifth case does exist when the closed system is isolated (it has an impermeable boundary). The principle of maximum entropy holds in this case, which states that an isolated system with constrained constant internal energy will tend to a state of maximum entropy while approaching equilibrium. Mathematically, this principle is written as follows:    2  ∂S ∂ S ¼ 0 and o0; i ¼ 1…n ð114Þ ∂Xi U ∂Xi2 U where S¼S(U, Xi), i ¼ 1…n. Consider a mixture of chemical species. The infinitesimal Gibbs energy variation caused by a differential change in species concentrations, dni , can be calculated using chemical potentials mi , defined for the component “i” of the system according to the equation mi ¼

∂G ; i ¼ 1…n ∂ni

ð115Þ

Thenceforth, dG ¼

X

mi dni

ð116Þ

If a mixture of gases obeys the ideal gas law, then the following equation expresses the variation of chemical potential with pressure   Pi mi ðPi Þ mi ðP0 Þ ¼ RT ln ð117Þ P0 where P0 is a reference pressure while Pi is the partial pressure of component “i.” A similar equation does exist for ideal solutions, where partial pressure is replaced by molar concentration, namely:   ci mi ðci Þ mi ðc0 Þ ¼ RT ln c0

ð118Þ

where c0 represents the concentration at reference conditions. For real chemical systems, in order to account for nonideal conditions and still keep the same form for the relationship describing the variation of chemical potential, the thermodynamic activity “ai” is defined by: mi ðai Þ

mi ðai ¼ 1Þ ¼ RT ln ðai Þ

ð119Þ

where the chemical potential for ai ¼ 1 must be defined and it is known as the standard state thermodynamic potential, denoted by m0i ¼ mi ðai ¼ 1Þ. The thermodynamic activity has the unit of measure similar to the unit of concentration. Thermodynamic activity is often expressed through a dimensionless activity coefficient f that quantifies the abatement from ideal conditions of a component

Thermodynamic Aspects of Energy

185

(species) of a chemical system through the equation ai ¼ ci fi

ð120Þ

According to the definitions for chemical potential and thermodynamic activity given in Eqs. (116), (118), and (119), the Gibbs energy of a chemical reaction has the general expression given as follows: Y  X  DG ¼ ni m0i þ RT ln ðcf Þni i ð121Þ P  0 ni mi can where ni is the stoichiometric number that is positive for the products (P) and negative for the reactants (R), the term be expressed as the sum of products minus the sum of reactants, where reactants are the chemical species that are consumed, and the products represent the species that are generated. Therefore, one has: X  X 0  X 0  ni mi ni mi ¼ DG0 ð122Þ ni m0i ¼ P

R

where DG0 is the standard Gibbs free energy of the reaction. From Eqs. (120) and (121) one obtains the following:   DG ¼ DG0 þ RT ln Keq

where

Keq ¼

ð123Þ

Y ðcf Þni i

ð124Þ

is known as the equilibrium constant of the reaction. A potential diagram is useful to describe chemical equilibrium, and even more generally, thermodynamic equilibrium. Similar to the case represented previously as shown in Fig. 15, a potential surface (pathway) of any process can be described as shown in Fig. 29. In any general chemical reaction there must be two potential surfaces, one corresponding to reactants and another for products. Moving along the reaction coordinate, the potential energy of the system decreases and reaches a minimum in #2. An energy barrier 2-3 of magnitude DGact has to be overcome in order for the system to move on the product’s potential surface, toward the right. If an electric field is applied by polarization of the electrode (anode for the case shown in the figure), the whole potential surface characterizing the thermodynamic system of the reaction products displaces at lower energy with a magnitude proportional to the polarization potential, namely FDEact, where DEact represents the activation overpotential. Consequently, the activation energy of the reaction under the influence of polarization decreases as it can evolve along the path 1 2 3 4. The following possibilities do exist regarding the spontaneity and equilibrium state of a chemically reacting system, as given in Table 8. The reaction can proceed spontaneously (S) forward toward consumption of the reactants and generation of products. The reaction can be nonspontaneous (NS) and therefore does not proceed in a forward direction. The reaction is at equilibrium €, therefore the backward and forward processes have the same rate and there will be no net change.

Reactants Potential surface 3 Products 3

1

ΔGact

ΔGact

e.g., H2O(l )

e.g., O2(g) + 4H+ + 4e−

2

G 4

FEact Reaction coordinate Fig. 29 Potential surfaces and stable equilibrium states for a general chemical reaction.

4

Polarized potential surface

186

Thermodynamic Aspects of Energy

Table 8

Criteria for spontaneity and equilibrium of chemical reactions

Spontaneity

DG

DStot

DH

DS

T

DG

Spontaneity

DH

DS

T

DG

Spontaneity

Spontaneous (S) Nonspontaneous (NS) Equilibrium (E)

o0 40 0

40 o0 0

o0 o0 o0

40 o0 o0

any low high

o0 o0 40

S S NS

40 40 40

o0 o0 40

any low high

40 40 o0

NS NS S

Those criteria (S, NS, E) can be categorized based on reaction enthalpy (DH) and entropy (DS). Those parameters allow for calculation of the reaction Gibbs free energy, DG ¼ DH TDS and further of the total entropy change of the reaction (the thermodynamic system at constant T and P) and its surroundings, as follows: DStot ¼

DG=T

ð125Þ

In general both the enthalpy and entropy of reactants and products increase with the temperature. Consequently, the temperature variation of reaction enthalpy and entropy are very slow. Therefore, in most cases, temperature is the key parameter that influences the reaction equilibrium. As given in Table 8, if a system is at equilibrium, then its Gibbs free energy is zero. From Eq. (125) the following approximate estimation of the temperature at equilibrium can be derived, by imposing DG¼ 0, namely: Teq D

DG DS

ð126Þ

The equilibrium constant is defined based on Gibbs free energy of the reaction in standard conditions, DG0, as follows:   DG0 Keq ¼ exp ð127Þ RT Furthermore, the equilibrium constant can help determine the reaction mixture composition at the equilibrium as follows:

• • •

If equilibrium constant is Keq4103, then the equilibrium mixture comprises mainly reaction products. If equilibrium constant is Keqo10 3, then the equilibrium mixture comprises mainly reactants. If equilibrium constant is 1034Keq410 3, then the equilibrium mixture comprises both reactants and products.

According to Le Châtelier’s principle, if a reaction mixture at equilibrium is placed upon a stress the system reacts in the direction that relieves the stress. For a reaction that reduces the moles of gas, this means that:

• •

An increase of pressure produced by shrinking the volume moves the reaction direction toward reducing the moles of gas. An expansion of volume that produces a decrease of pressure will move the reaction toward an increase of the mole of gas.

Similarly to the reaction quotient, the notion of extent of reaction – denoted as ξ – is very useful for calculating the equilibrium conditions. The extent of reaction is a parameter between 0 and some positive value that depends on the reaction stoichiometry. When the reaction quotient is smaller than 1, the extent of reaction is near zero, and the total free energy of reaction mixture decreases when the reaction proceeds spontaneously in the forward direction. The total Gibbs energy of the reaction mixture can be defined as the sum of moles of chemical species “i” multiplied by the respective molar specific Gibbs free energy at constant temperature and pressure. Therefore, the extent of a reaction is defined implicitly as follows:   ∂G DG ¼ ð128Þ ∂ξ T;P where Gi is the molar specific free energy of species i (reactant of product) and ni is the number of moles of species “i.” Fig. 30 shows a plot of the total Gibbs free energy of the reaction mixture versus the extent of reaction. At equilibrium, one must have dG¼ 0 with a minimum as suggested in the figure. Therefore, Gibbs free energy minimization will determine the reaction mixture composition at the equilibrium, in terms of the number of moles ni. A reaction achieves full conversion when all the reactants are consumed and products generated in a stoichiometric amount. This is not the case with actual reaction for several reasons. When a reaction has an equilibrium constant around 1 there will not be a full conversion of the reactants into products because both reactants and products will be present in the reaction mixture. In addition, two or more reactions can occur simultaneously therefore creating undesired products. Also several types of losses occur with purification and separation of the products and the presence of impurities. The theoretical yield is the amount of product that can be generated by a reaction according to the stoichiometry. The actual yield will be smaller than the theoretical yield. The percent yield is defined as the ratio of actual to theoretical yield.

1.5.6

Exergy

Exergy represents the maximum work that can be produced by a thermodynamic system when it comes into equilibrium with its surrounding environment. This statement assumes that at an initial state there is a thermodynamic system that is not in

Total Gibbs free energy of reaction mixture

Thermodynamic Aspects of Energy

Reaction spontaneity

=0 only reactants

187

Reaction spontaneity

=1 only products

 Extent of reaction

Fig. 30 Gibbs free energy variation vs. the extent of reaction.

equilibrium with the environment. In addition it is assumed that – at least potentially – mechanisms of energy (and mass) transfer between the system and the environment must exist, such that eventually the system can evolve and such equilibrium condition will eventually occur. The system must at least exchange work with the environment. Another remark is that exergy by definition assumes the existence of a reference environment. The system under analysis will interact only with that environment. Exergy analysis is a method appertaining to engineering thermodynamics and can be used to determine the alleviation of manmade and natural systems from the ideal case. Here, by ideal system one understands a reversible system. In many practical problems, the reference environment is assumed to be the Earth’s atmosphere, characterized by its average temperature and pressure; often standard pressure and temperature are used for reference environment: P0 ¼101.325 kPa, T0 ¼ 298.15K. In some class of the problems when reacting systems are present, the chemical potential of the reference environment must be specified. In such cases, thermodynamic equilibrium will refer to all possible interactions; one can say that a system is in thermodynamic equilibrium with the environment if it has the same temperature with it (thermal equilibrium), the same pressure with it (mechanical equilibrium), and the same chemical potential with it. Therefore, exergy includes at least two components; one is thermomechanical and one is chemical. Exergy cannot be conserved. Any real process destroys exergy as, similarly, generates entropy. Exergy is destroyed and entropy is generated due to irreversibilities. According to Dincer and Rosen [6] the exergy of a closed (nonflow) thermodynamic system comprises four terms, namely physical (or thermomechanical), chemical, kinetic, and potential. In brief, total exergy of a nonflow system is Exnf ¼ Exph þ Exch þ Exke þ Expe

ð129Þ

The exergy of a flowing stream of matter Exf represents the sum of the nonflow exergy and the exergy associated with the flow work of the stream (P–P0)V, therefore Exf ¼ Exnf þ ðP

P0 ÞV

ð130Þ

The physical exergy for a nonflow system is defined by Exph ¼ ðU

U0 Þ þ P0 ðV

V0 Þ

T0 ðS

S0 Þ

ð131Þ

where U is internal energy, V volume and S entropy of closed system that is in nonequilibrium with the environment, T0 is the reference temperature of the surroundings environment, and index 0 refer to the values of the parameters when the system is in thermomechanical equilibrium with the environment. The kinetic and potential exergies of the system equal the kinetic and potential energy respectively, which are given by known formulas, namely: Exke ¼

1 m u2 and Expe ¼ m g ðz 2

z0 Þ

ð132Þ

where m is the system mass, and u is its (macroscopic) velocity, z is the system elevation, and z0 is a reference elevation of the environment (e.g., ground level). Consider a system that is in thermomechanical equilibrium with the reference environment (it has the same temperature and pressure as the environment – T0, P0), but it is not in chemical equilibrium with the reference environment because it has other chemical composition. Chemical exergy represents the maximum work that can be extracted during a process when the system composition changes to that of the environment. There are two main components of chemical exergy: (1) exergy due to chemical reaction, (2) exergy due

188

Thermodynamic Aspects of Energy

Table 9

Standard chemical exergy of some elements B

C

Ca

Cl2

Cu

F2

Fe

H2

I2

K

Mg

ex , kJ/mol

628.1

410.27

729.1

123.7

132.6

505.8

374.3

236.12

175.7

336.7

626.9

Element

Mo

N2

Na

Ni

O2

P

Pb

Pt

Pu

Si

Ti

731.3

0.67

336.7

242.6

3.92

861.3

249.2

141.2

1100

855.0

907.2

Element ch

ch

ex , kJ/mol

Source: Rivero R, Grafias M. Standard chemical exergy of elements updated. Energy 2006;31:3310–26.

to concentration difference. When a chemical compound is allowed to interact with the environment, chemical reactions may occur, involving unstable species. Eventually, more stable species are formed and further reaction is not possible. If a substance is not present in the atmosphere then the reference for zero chemical exergy is the most stable state of that substance in seawater. There have been tables of chemical exergy of elements developed in past literature data. A recent source for tabulated data of standard chemical exergy of elements is Rivero and Grafias [26]. Table 9 tabulates the chemical exergies of some of the most encountered chemical elements in industrial processes. Standard chemical exergy of elements is useful for calculation of chemical exergy of chemical compounds provided that their Gibbs energy of formation is known. Moreover, if system compounds have other concentration or other phase as that corresponding to the environment then various processes such as dilution or concentration may occur until there is no difference in concentration between system components and the environment. P  The chemical exergy depends on the difference between chemical of system components ni m0i being in ther-  0potential  momechanical equilibrium but not in chemical equilibrium with it mi  , and the chemical potential of system components m00 i P . Therefore, the chemical exergy of the system is when they are brought in chemical equilibrium with the environment, ni m00 i defined as X   Exch ¼ ð133Þ ni m0i m00 i

Let us analyze the chemical exergy due to concentration difference between the system and the surrounding environment. Let us assume the thermodynamic system at state 1 in nonequilibrium with the environment. If mass transfer is permitted with the environment a dilution process occurs until the moment when the system components are fully diluted and there is no concentration gradient; this state is denoted with 2. The maximum work extractable from process 1-2 represents the exergy due to concentration difference and is given by DExch conc ¼ Ex1

Ex2 ¼ ðU1

U2 Þ þ ðP1 V1

P2 V2 Þ

T0 ðS1

S2 Þ ¼ T0 ðS2

S1 Þ

ð134Þ

Here, one accounts that the process of diffusion is isothermal and one assumes that the gases involved are ideal gas U2 ¼ U1 and P2V2 ¼P1V1. Furthermore, according to the FLT T dS¼dU þ P dV, therefore for an isothermal process of ideal gas for which dU ¼ 0 and d(P V) ¼ 0, one has TdS ¼ d(PV)–vdP, or TdS¼ –vdP. Consequently, the chemical exergy due to difference in concentration of the gas component i having molar fraction yi is given as follows: Exch conc;i ¼

R T0 ln ðyi Þ

ð135Þ

Rivero and Grafias [26] give a general equation for chemical exergy calculation of any chemical compound that can be derived in a similar manner as illustrated above for water chemical exergy. In order to determine the chemical exergy of a compound it is required to know its standard specific Gibbs free energy of formation, Dfg0. Then, using Dfg0 and the standard exergy of the elements, the following formula must be used to determine the chemical exergy of the compound as X   ex ch ¼ Df g 0 þ ð136Þ n ex ch element element

where v is the stoichiometric factor representing the number of moles of element per one mole of chemical compound. Because the intensive properties (pressure, temperature, chemical potential) of the natural environment vary temporally and spatially, the natural environment is far from a thermodynamic equilibrium. Thenceforth, being departed from equilibrium the natural environment possesses work potential; that is, it has nonzero exergy. However, in most of the cases the environment changes slow as chemical reactions are not activated due to a reduced rate of the involved transport processes. Fossil fuel supply reserves or forests do not burn spontaneously since no activation energy is provided. Therefore, a compromise can be made between the theoretical requirements of the reference environment and the actual behavior of the natural environment. That is, a standard reference model can be adopted that is useful for exergy and environmental impact calculations. The reference environment is in stable equilibrium, acting as an infinite sink and source for heat and materials, experiencing only internally reversible processes with unaltered intensive states. Natural environment models attempt to simulate realistically subsystems of the natural environment. Some relevant chemical components for the reference environment are water (H2O), gypsum (CaSO4
2H2O), and limestone (CaCO3). The stable configurations of C, O, and N, respectively, may be taken to be those of CO2, O2, and N2 as they exist in air saturated with liquid water at T0 and P0 (the temperature and pressure for the reference environment). Hydrogen reaches the equilibrium with the

Thermodynamic Aspects of Energy

Table 10

189

Reference environment described in Rivero and Grafias

Atmosphere

Hydrosphere

Lithosphere

E: y: E: y: E: x: E: x: E: y: E: y: E: y: E: y: E: y: E: y: E: y:

Ar CO2 9.13E 3 3.37E 4 Kr N2 9.87E 7 0.7634 B(OH)3 HScO24 3.87E 8 3.42E 4 Na þ MoO24 1.08E 7 0.4739 AgCl Al2SiO5 1E 9 2.07E 3 CuCO3 K2Cr2O7 1.35E-6 5.89E 6 GeO2 Gd(OH)3 9.21E 8 9.49E 8 Mg3Si4O10(OH)2 8.67E 4 PtO2 Pr(OH)3 1.57E 7 1.76E 11 Sm(OH)3 SiO2 0.407 1.08E 7 Tm(OH)3 Tl2O4 1.49E 9 7.59E 9

D2O 3.37E 6 Ne 1.76E 5 BiO þ 9.92E 11 HPO24 4.86E 7 Au 1.36E 9 Dy(OH)3 4.88E 8 HfO2 1.15E 7 MnO2 2.3E 5 PuO2 8.4E 20 SnO2 4.61E 7 UO3
H2O 1.48E 8

H2O 2.17E 2 O2 0.2054 Br 8.73E 4 Rb þ 1.46E 6 BaSO4 4.2E 6 Er(OH)3 4.61E 8 HgCl2 5.42E 10 Nb2O3 1.49E 7 RaSO4 2.98E 14 SrCO3 2.91E 5

He 4.89E 6 Xe 8.81E 8 Cl 0.5658 SO24 1.24E 2 Be2SiO4 2.1E 7 Eu(OH)3 2.14E 8 Ho(OH)3 1.95E 8 Nb(OH)3 5.15E 7 Re2O7 3.66E 12 Ta2O5 7.45E 9 V2O5 1.83E 6

Cs þ lO3 2.34E 9 5.23E 7 WO24 SeO24 1.18E 9 5.64E 10 CaCO3 CdCO3 1.4E 4 1.22E 8 CaF2
3Ca3(PO4)2 2.24E 4 In2O3 IrO2 2.95E 9 3.59E 12 NiO OsO4 1.76E 6 3.39E 13 Rh2O3 RuO2 3.29E 12 6.78E 13 TeO2 Tb(OH)3 1.71E 8 9.48E 12 Yb(OH)3 Y(OH)3 1E 6 4.61E 8

Kþ 1.04E

2

CeO2 1.17E 6 Fe2O3 6.78E 3 La(OH)3 5.96E 7 PbCO3 1.04E 7 Sb2O5 1.08E 10 ThO2 2.71E 7 ZnCO3 7.45E 6

Li þ 2.54E

5

CoFe2O4 22.85E 7 Ga2O3 2.89E 7 Lu(OH)3 7.86E 9 PdO 6.37E 11 Sc2O3 3.73E 7 TiO2 1.63E 4 ZrSiO4 2.44E 5

Note: x is the mass fraction in hydrosphere; y is the molar fraction used for atmosphere and lithosphere. Source: Rivero R, Grafias M. Standard chemical exergy of elements updated. Energy 2006;31:3310–26.

environment after reaction with oxygen and forming liquid water at T0 and P0. Calcium reaches thermodynamic equilibrium with the environment after reacting with either CO2 or sulfur and forming CaSO4
2H2O and CaCO3 at T0 and P0. Equilibrium and constrained-equilibrium models were also formulated in which all the materials present in the atmosphere, oceans, and a layer of the crust of the Earth are pooled together and an equilibrium composition is calculated for a given temperature. The selection of the thickness of crust considered is subjective and is intended to include all materials accessible to thermal processes. Thicknesses varying from 1 to 1000 m, and a temperature of 251C were considered. Exergy values obtained using these environments are significantly dependent upon the thickness of crust considered, and represent the absolute maximum amount of work obtainable from a material. Assuming that the environment is a large reservoir that when a particular substance interacts with it, the following type of processes occur after sufficiently long time: (1) mechanical equilibration (the substance pressure will equal to that of the surroundings), (2) thermal equilibrium (meaning that the substance temperature becomes equal to that of the environment), (3) chemical reaction equilibrium (meaning that the substance enters in a series of chemical spontaneous chemical reactions with the environment such that it decomposes and eventually forms only chemical species present in the environment), (4) concentration equilibrium (the chemical species resulting from substance reaction with the environment dilute or concentrate such that they reach the concentration in the environment). The processes (1) and (2) refer to the thermomechanical equilibrium. The departure of the substance stream from the temperature and pressure of the environment is a measure of the thermomechanical exergy. Moreover, the processes (3) and (4) represent chemical equilibrium that is associated with chemical exergy. Rivero and Grafias [26] proposed a model for the standard environment considering that the relative humidity in the atmosphere is 70%, carbon dioxide concentration is 345 ppm in volume, and the salinity of seawater is 35%. The list of stable components in the atmosphere and their concentration in the reference environment is given in Table 10. The chemical elements in the atmosphere for this model are Ar, C, D2, H2, He, Kr, N2, Ne, O2, and Xe. The molar fraction of the species in the atmosphere is related to the standard chemical exergy under the assumption of ideal gas behavior as follows:   ex ch y ¼ exp ð137Þ RT0 The standard reference model for the environment is taken as a base for calculating the chemical exergy of the chemical elements for which an extract is given in Table 9. Once the exergy of chemical elements is known, the chemical exergy of any chemical compound can be determined based on Eq. (134).

1.5.7

Thermodynamic Analysis Through Energy and Exergy

Thermodynamic analysis is generally based on four types of balance equations, which will be presented here in detail. These are: mass balance equation, EBE, entropy balance equation (EnBE), and exergy balance equation (ExBE). Thermodynamic analysis using balance equations is documented in detail in Dincer and Rosen [6]. Here a brief introduction on this method is presented.

190

Thermodynamic Aspects of Energy

1.5.7.1

Mass Balance Equation

The effect of mass addition or extraction on the energy balance of CV is proportional with the mass flow rate, defined as the amount of mass flowing through a cross-section of a flow stream per unit of time. For a CV – according to the conservation of mass principle – the net mass transferred to the system is equal to the net change in mass within the system plus the net mass leaving the system. P _ in enter the system while a number of streams of total mass flow Assume that a number of streams with total mass flow rate m P _ out leave the system such as shown in Fig. 31. Consequently, the mass of the CV will change with differential amount rate m dmcv. The mass balance equation for a general CV can be written for nonsteady state system as follows (Fig. 32): MBE:

X

_ in ¼ m

X

_ out þ m

dmcv dt

ð138Þ

and for a steady flow system as follows: MBEsteady flow :

1.5.7.2

X

_ in ¼ m

X

ð139Þ

_ out m

Energy Balance Equation

The EBE is an expression of the FLT with a sign convention relaxed. Therefore the variation of system energy between states 1 and 2 is    

1 1 DEsys ¼ mDesys ¼ m u2 þ u22 þ gz2 u1 þ u21 þ gz1 ð140Þ 2 2 For a closed system the EBE is written with the help of the total specific energy of a nonflowing thermodynamic system e ¼u þ 0.5u2 þ gz, namely: EBEClosed System :

X

q_ in þ

X

w_ in ¼

X

q_ out þ

X

w_ out þ

d_e dt

mout,1

Control volume (CV) mCV

min

∑min = ∑mout +

dmcv dt

mout,2 Boundary of CV Fig. 31 Illustrative sketch for mass balance equation.

Surroundings

System boundary

∑S in System dSsys dt

∑Sout

Sgen and

∑S in + Sgen = ∑Sout +

Total entropy entering

+

Generated entropy

=

dSsys dt

Total entropy exiting

+

Fig. 32 Explanatory sketch for the entropy balance equation (EnBE) – a statement of SLT.

Entropy change of the system

ð141Þ

Thermodynamic Aspects of Energy

191

The EBE for CVs must account for the existence of flow work and boundary work and for the rate of change of total energy [d (me)/dt]; thence it can be formulated as follows:

X X X X X X _ out þ _ in þ _ out þ dðm eÞ _ in ¼ _ þ _ þ my Q Q W W ð142Þ EBEOpen System : my dt sys out in where y is the total energy of a flowing matter, which represents the sum of internal energy, flow work, kinetic energy, and potential energy defined by 1 1 y ¼ u þ P v þ u2 þ g z ¼ h þ u2 þ g z 2 2

ð143Þ

In a steady flow system, mass flow rate, pressure, temperature, etc. do not change in time, thence the integration of the _ _ dt between initial state 1 and a later state 2 of the open system is _ hÞdt, dQ ¼ Qdt, following equations dðm hÞ ¼ ðm and dW ¼ W straightforward. In a steady flow regime, the EBE can be written in rate form: X X _ out þ W _ in þ W _ out þ _ in þ _ hÞ ¼ m _ 2 e2 þ Q _ hÞ ðm ðm ð144Þ _ 1 e1 þ Q EBESteady Flow :m out

in

1.5.7.3

Entropy Balance Equation

The SLT can be expressed in form of an EnBE that states for a thermodynamic system, entropy input plus generated entropy is equal to entropy output plus change of entropy within the system. In other words, the EnBE postulates that the entropy change of a thermodynamic system is equal to entropy generated within the system plus the net entropy transferred to the system across its boundary (that is, the entropy entering minus the entropy leaving). Entropy can be transferred outside of the system as heat, but it cannot be transferred as work. Fig. 32 illustrates schematically the EnBE, which is written mathematically according to: EnBE:

X

S_ in þ S_ gen ¼

X

dSsys S_ out þ dt

ð145Þ R 2 dQ

The entropy transferred across the system boundary or along a process 1-2 is S1 2 ¼ 1 T . The general EnBE takes special form for closed systems. For a closed system there is no mass transfer at the system boundary. Therefore, entropy can be transferred only by heat. If the closed system is also adiabatic, then there is neither entropy transfer due to mass nor due to heat transfer, henceforth S_ sys ¼ S_ gen . If the system is closed but is not adiabatic, then the EnBE becomes Z _  XZ dQ _ dSsys X dQ þ EnBEclosed system : þ S_ gen ¼ ð146Þ dt T T out in The EnBE for an open system (CV) has the following expression in rate form: XZ d Q XZ dQ _ X _ X dS _ _ þ S_ gen ¼ CV þ ms ms EnBEcv : þ þ dt T T out out in in

ð147Þ

The EnBE for a steady flow through a CV must account for the fact that there are no temporal variations of parameters; thence the mass enclosed in the CV and specific entropy of the CV remains constant in time; consequently: XZ dQ XZ dQ _ X _ X _ _ þ S_ gen ¼ EnBEsteady state : ms ð148Þ ms þ þ T T out out in in _ out ¼ m _ in ¼ m _ applies the EnBE simplifies to In the case when m XZ dQ XZ dQ _ _ EnBE: þ mðsin sout Þ þ S_ gen ¼ T T out in

ð149Þ

In the case that the process is adiabatic, there is no heat transfer across the system boundary, therefore, the EnBE simplifies to EnBE:m sin þ S_ gen ¼ m sout . The generated entropy is the sum of entropy change of the system and of its surroundings. There are three relevant cases that can be assumed at heat transfer across the system boundary for determination entropy generation. Consider a thermodynamic system that has a diabatic boundary. As illustrated in Table 11 the EnBE for this system is given by the difference between Q/T0 and Q/Tsys. It is assumed in this case that there is no wall with finite thickness at the system boundary. Therefore, temperature profile has a sharp change. A more accurate assumption is to assume the existence of a wall at the boundary. In this case there will be a variation of temperature across the wall. In the Case 2 represented in Table 11 the entropy generation has to be calculated by integration accounting of temperature profile. In the third case, in addition to a wall, one considers the existence of boundary layers at the inner and outer sides of the wall. Therefore, the entropy generation will be the highest in assumption Case 3.

Thermodynamic Aspects of Energy

Entropy transfer across a wall boundary

Tsys

Case 2: Wall DT considered

Diabatic system boundary

Sgen

Q

Tsys

Q T0

Sgen;2 4 TQ0

Q Tsys

Wall

Tsys

T0

T0

Boundary layer

Q

Q

Sgen

Q/Tsys

Q/T0

Q/Tsys Sgen;1 ¼

Wall

Tsys Diabatic system boundary

T0 Tsys

Case 3: Wall and boundary layer

Tsys

Q Tsys

Q/T0

Case 1: No wall effect considered

Q/Tsys

Table 11

Q/T0

192

Sgen

Sgen;3 4Sgen;2 4 TQ0

Q Tsys

Modified from Cengel Y, Boles M. Thermodynamics: an engineering approach. New York, NY: McGraw-Hill; 2014.

Surroundings

System boundary

∑Exin

∑Exd System Exsys and Exd

∑Exin =

Total entropy entering

=

dExsys dt

Exergy change of system

+ ∑Exin + ∑Exd

+

Total exergy leaving

+

Total exergy destroyed

Fig. 33 Explanatory sketch for the exergy balance equation (ExBE).

1.5.7.4

Exergy Balance Equation

The ExBE introduces the term exergy destroyed, which represents the maximum work potential that cannot be recovered for useful purpose due to irreversibilities. For a reversible system, there is no exergy destruction since all work generated by the system can be made useful. The exergy destruction and entropy generation are related by the expression Exd ¼T0DSgen, where T0 is the reference temperature. If Exd40 then the process is irreversible; if Exd ¼ 0 then the process is reversible; if Exdo0 the process is impossible. The total exergy entering a thermodynamic system must be balanced by the total exergy leaving the system plus the change of exergy content of the system plus the exergy destruction. Fig. 33 shows an explanatory sketch for the ExBE. Exergy can be transferred to or from a system by three means: work, heat, and mass. Therefore, the ExBE can be expressed generally in rate form as  

 

X T0 _ dEx X _ T0 _ dVCV _ _ þm _ jþ 1 _ jþ 1 Q ¼ Q P0 þ Exd ExBE: ð150Þ þ W Wþm T T dt dt out in where the total specific exergy is defined with j ¼ ðh

h0 Þ þ T0 ðs

1 s0 Þ þ u2 þ g ðz 2

z0 Þ þ exch

ð151Þ

Exergy transfer between the system and surroundings can be done by work, mass transfer, and heat transfer. The exergy due to work transfer (ExW) is by definition equal to the work: ExW ¼ W. However, if the system impinges against a moving boundary then the exergy must be diminished accordingly, thence ExW ¼W–P0(V–V0). The exergy associated to mass transfer (Exm) is Exm ¼mj. The exergy due to heat transfer can be expressed based on Carnot factor according to  Z ExQ ¼ system 1 boundary

 T0 dQ T

ð152Þ

Thermodynamic Aspects of Energy

193

For a thermodynamic system at steady state the ExBE simplifies to:   X  

X T0 _ T0 _ _ d _ þm _ þm _ jþ 1 _ jþ 1 Q ¼ Q þ Ex ð153Þ W ExBEsteady : W state T T out in P P P P _ in _ out and assumes that there is no exergy destroyed, the _ ¼ _ in and Q _ ¼ _ out Q Q W W If Eq. (151) denotes W reversible work can be obtained as follows:  X T0 _ _ rev ¼ m _ ðj1 j2 Þ þ Q ð154Þ 1 W T

1.5.7.5

Formulations for System Efficiency

The term efficiency originates mainly from thermodynamics, when the attempt of assessing the heat conversion into work led to its initial formulation as the “work generated per total heat energy input.” However, efficiency as assessment criterion can be applied widely for any systems and processes. A general efficiency expression of a system – as a measure of its performance and effectiveness – is represented by the ratio of useful output per required input. Here it is recognized an efficiency criterion based on the FLT, also called energy efficiency. If the system is an energy system then its input and output must be forms of energy. Therefore, for an energy system, the energy efficiency is written as Z¼

E_ deliv ¼1 E_ cons

E_ loss E_ cons

ð155Þ

Any source of energy stream is characterized by an associated exergy. By analogy with energy efficiency, the exergy efficiency is defined as the ratio between exergy associated to the useful output and the exergy associated to the consumed input, namely: c¼

_ deliv Ex ¼1 _ cons Ex

_ d Ex _Excons

ð156Þ

In Table 12, the efficiency formulations for the main devices used in process engineering are given. Turbine is the first device analyzed in the table. A high enthalpy flow enters the turbine; work is produced, and a lower enthalpy flow exits the turbine. The turbine efficiency quantifies various losses such as the isentropic losses, the heat losses from the turbine shell, and the friction losses. Isentropic efficiency is one of the most used assessment parameters for turbines. Isentropic efficiency (Zs) is defined by the ratio of actual power generated to the power generated during an isentropic expansion. For an isentropic expansion there is no entropy generation; the turbine operation is reversible. Therefore, isentropic efficiency is a relative measure of alleviation from thermodynamic ideality. The expansion process 1-2 is the actual process, while the process 1-2s is the reversible process (isentropic). Exergy efficiency of a turbine is defined as the ratio of generated power and rate of exergy consumed. A compressor is a device used to increase the pressure of a fluid under the expense of work consumption. Compressors are typically assessed by the isentropic efficiency which, for the case of compressors, is the ratio of isentropic work and actual work. Pumps are organs used to increase the pressure of liquids on the expense of work input. The liquid is incompressible and _ s¼m _ vðP2 P1 Þ. Hydraulic turbines are devices therefore, the power required for pumping the liquid for a reversible process is W that generate work from potential energy of a liquid. Nozzles and diffusers are adiabatic devices used to accelerate or decelerate a fluid, respectively. The exergy efficiency of a nozzle is nil because they produce no work although the expanded flow has work potential. A heat exchanger is a device that facilitates heat transfer between two fluids without mixing. It is known that heat exchangers are assessed by their effectiveness parameter, which represents the ratio between the actual amount of heat transfer and the maximum amount of heat possible to transfer. For a heat exchanger the exergy source is derived from the hot fluid that during the process reduces its exergy. The exergy of the cold fluid represents the delivered exergy as useful product of a heat exchanger. Regarding the energy exchange, ideally if there are no energy losses, all energy from the hot fluid is transferred to the cold fluid. However, some losses are unavoidable in practical systems; therefore one can define an energy efficiency of heat exchangers as the ratio between energy delivered and energy consumed. Regarding the exergy efficiency of a heat exchanger, this is given by exergy retrieved from the cold fluid divided by exergy provided by the hot fluid. Many practical devices are used to mix streams. Mixing chambers accept multiple stream inputs and have one single output. Mixers can be isothermal or one can mix a hot fluid with a cold fluid, etc. A combustion chamber or a reaction chamber can be modeled from a thermodynamic point of view as a mixing device, whereas mixing is accompanied by chemical reaction. Very similar to mixers are stream separators. In this case, an input stream is separated in two (or more) different output streams.

1.5.8

Thermodynamic Cycles

Thermodynamic cycles are looped processes used to convert thermal energy into work or work into thermal energy. A thermodynamic cycle is connected to two temperature reservoirs: a heat source and a heat sink. The cycle can be run forward to produce work or reversely to transfer heat from lower to higher temperature reservoirs. The cycle is run by a gas or vapor/liquid system that

Thermodynamic Aspects of Energy

194

Table 12

Energy and exergy efficiency of some important devices for power generation

Device

Equations

1. Turbine

T

1

1: m, h1 W

2s

2: m, h2

2 s

Balance equations: _ 2¼m _ MBE:m_ 1 ¼ m _ 1 h1 ¼ W_ þ m _ 2 h2 EBE:m _ 1 s1 þ S_ gen ¼ m_ 2 s2 EnBE:m ExBE:m_ ½ðh1 h2 Þ T0 ðs1 Efficiency equations: _ _ Z ¼ WW_ ¼ mm_ ððhh11 hh2s2 ÞÞ s

c¼ 2. Compressor

T

2s

1: m, h1

2

W 2: m, h2

1

s

2: m, h2

1: m, h1

v2 = v1 = v

4. Hydraulic turbine

W 2: m, h2 v2 = v1 = v

W_ rev E_ xcons

1: m, h1

Nozzle

¼

_ ðh1 h2 Þ m _ ½h1 h2 T0 ðs1 s2 ފ m

¼

s1 ފ þ E_ xd

1

_ ðh2s h1 Þ m _ ½h1 h2 T0 ðs1 s2 ފ m

Balance equations: _ 2¼m _ MBE:m_ 1 ¼ m _ 1 h1 þ W_ ¼ m _ 2 h2 EBE:m _ 1 s1 þ S_ gen ¼ m_ 2 s2 EnBE:m ExBE:W_ ¼ m_ ½h2 h1 T0 ðs2 Efficiency equations: _ _ ðP2 P1 Þ Z ¼ W_W s ¼ mv _ ðh2 h1 Þ m W_ rev E_ xcons

s1 ފ þ E_ xd

2: m, h2

¼

_ ½h2 h1 T0 ðs2 s1 ފ m _ ðh2 h1 Þ m

Balance equations: _ 2¼m _ MBE:m_ 1 ¼ m _ 1 h1 ¼ W_ þ m _ 2 h2 EBE:m _ 1 s1 þ S_ gen ¼ m_ 2 s2 EnBE:m ExBE:m_ ½ðh1 h2 Þ T0 ðs1 s2 ފ ¼ W_ þ E_ xd Efficiency equations: _ _ ðh1 h2 Þ m Z ¼ WW_ ¼ mv _ ðP1 P2 Þ s _ W_ c¼ _ ¼ m_ ½h mhðh1 T hð2sÞ s ފ E xcons

5. Diffuser and nozzle

Diffuser

W_ Ex1 Ex2

2

c¼

1: m, h1

¼

cons

W

1: m, h1

W_ E_ xcons

Balance equations: _ 2¼m _ MBE:m_ 1 ¼ m _ 1 h1 þ W_ ¼ m _ 2 h2 EBE:m _ 1 s1 þ S_ gen ¼ m_ 2 s2 EnBE:m ExBE:W_ ¼ m_ ½h2 h1 T0 ðs2 Efficiency equations: _ Z ¼ W_ s W_ ¼ mm_ ððhh2s hh1ÞÞ c¼

3. Pump

s2 ފ ¼ W_ þ E_ xd

1

2

0

Balance equation: _ 2¼m _ MBE:m_ 1 ¼ m _ 1 h1 ¼ m _ 2 h2 EBE:m _ 1 s1 þ S_ gen ¼ m_ 2 s2 EnBE:m ExBE:m_ ½ðh1 h2 Þ T0 ðs1 Efficiency equations: Z ¼ hh11 hh2s2 and c ¼ 0

1

2

s2 ފ ¼ E_ xd

2: m, h2

(Continued )

Thermodynamic Aspects of Energy

Table 12

195

Continued

Device

Equations

6. Heat exchanger

1c: mc h1c

1h: mh h1h

2c: mch2c

2h: mh h2h

Balance equations: _ 2h ¼ m_ h and m_ 1c ¼ m_ 2c ¼ m_ c MBE:m_ 1h ¼ m _ h ðh1h h2h Þ ¼ m _ c ðh2c h1c Þ EBE:m _ c ðh2c h1c Þ þ m_ h ðh2h h1h Þ EnBE:S_ gen ¼ m ExBE:m_ h ½ðh1h h2h Þ T0 ðs1h s2h ފþ m_ c ½ðh1c h2c Þ T0 ðs1c s2c ފ¼ E_ xd Efficiency equations: _ p DT Þ _ ðmC cold e ¼ Q_ cold ¼ mC Q max ð _ p Þmin ðT1h T2h Þ _ ðE_ x2c E_ x1c Þcold Z ¼ Q_ cold ; c ¼ _ Q hot ðE x1h E_ x2h Þhot c¼

Balance equations: _3 MBE:m_ 1 þ m_ 2 ¼ m _ 1 h1 þ m _ 2 h2 þ E_ in ¼ m _ 3 h3 ; assume adiabatic EBE:m _ 1 s1 þ m _ 2 s2 þ S_ gen ¼ m _ 3 s3 ; assume adiabatic EnBE:m _ 2 ex2 þ E_ xin ¼ m _ 3 ex3 þ E_ xd ExBE:m_ 1 ex1 þ m Efficiency equations:

7. Mixing chamber or chemical reactor

3: m3, h3

Ein and Exin

1: m1, h1

_ cold ½h2c h1c T0 ðs2c s1c ފ m _ hot ½h2h h1h T0 ðs2h s1h ފ m

Z¼

m_ 3 h3 _ 1 h1 þm _ 2 h2 þE_ in m

c¼

m_ 3 ex3 _ 1 ex1 þm_ 2 ex2 þE_ xin m

2: m2, h2

8. Separation device

3: m3, h3

Ein, Exin and Qxin

Balance equations: _2þm _3 MBE:m_ 1 ¼ m _ 3 h3 EBE: m_ 1 h1 þ Q_ in þ E_ in ¼ m_ 2 h2 þ m _ _ 1 s1 þ QT in þS_ gen ¼m_ 2 s2 þ m _ 3 s3 EnBE:m in _ 2 ex2 þ m _ 3 ex3 þ E_ xd ExBE:m_ 1 ex1 þ Q_ in 1 TTin0 þ E_ xin ¼ m Efficiency equations: Z¼ c¼

1: m1, h1

m_ 2 h2 þm_ 3 h3 _ 1 h1 þQ_ in þE_ in m _ 2 ex2 þm m  _ 3 ex3

_ 1 ex1 þQ_ in 1 m

2: m2, h2

T0 Tin

þE_ xin

is assumed to follow quasistatical processes (reversible). There are three types of totally reversible thermodynamic cycles, these being Carnot, Stirling, and Ericsson. There are three main types of internally reversible but externally irreversible thermodynamic cycles, namely Otto, Diesel, and Brayton.

1.5.8.1

Totally Reversible Cycles

The very condition for a cycle to be totally reversible is that its heat addition and rejection processes be isothermal. If the heat addition and rejection processes are not isothermal then the cycle must be externally irreversible because a finite temperature difference must occur between a thermal reservoir at fixed temperature and the working fluid. Carnot, Stirling, and Ericsson cycles include two isothermal heat addition processes. The other two processes (from a total of four) are adiabatic for Carnot cycle, isochoric for Stirling cycle, and isobaric for Ericsson cycle. The efficiency of any totally reversible heat engine (e.g., Carnot, Stirling, Ericsson) connected to a heat source of temperature TH and to a heat sink of temperature TL is given by the Carnot factor Ztot;rev ¼ 1

TL TH

ð157Þ

The P – v diagram of Carnot cycle is given in Fig. 34. This cycle can be realizable by a heat engine working with an ideal gas and comprising a nonadiabatic isothermal compressor connected to the heat sink followed by an adiabatic-isentropic compressor, followed by a nonadiabatic isothermal turbine connected to the heat source, followed by an adiabatic–isentropic turbine.

196

Thermodynamic Aspects of Energy

3.5 Carnot power cycle (air as ideal ags)

Isothermal heat addition (TH, QH) 3.0

3

1 → 2: Isothermal compression 2 → 3: Adiabatic compression 2 → 3: Isothermal expansion 3 → 4: Adiabatic expansion

P (bar)

2.5 4 2.0

1.5

2

1.0 Isothermal heat rejection (TL,QL) 0.5 0.3

0.4

0.5

0.6

0.7

0.8

1 0.9

v (m3/kg) Fig. 34 Carnot cycle diagram with ideal gas working fluid.

TH TL P

Heat addition

1

Work output

T = TH = const v = const. regeneration

Move TH TL

4 2 TH TL

v = const. regeneration

Move Work input

Heat rejection

3

V

TH TL Fig. 35 P–V diagram of the ideal Stirling cycle.

Regarding the Stirling cycle, this can be performed in a double effect piston and cylinder set-up, which contains a regenerator porous matrix placed inside as shown schematically in Fig. 35. The working medium can be an ideal gas. Obviously, the processes of heat transfer, regeneration, and work exchange must be reversible. The descriptions of the processes are given below, as follows:

• •

•

Isothermal expansion with heat addition, process 1–2: The expansion space is heated externally, and the gas undergoes isothermal expansion. As the left piston moves leftwards it generates useful work output. Isochoric cooling with heat regeneration, process 2–3: Both pistons are moved slowly rightwards at the same pace so that the enclosed volume remains constant. The gas passes through the porous matrix of the regenerator and reaches into the other side. This process is assumed frictionless (reversible). During the process a heat transfer occurs between the gas and the regenerator matrix. At the beginning of the process the gas is at temperature TH and at the end of it the temperature of the gas reaches TLoTH. Isothermal compression and heat rejection, process 3–4: Active work is given to the piston from the right from the outside so that this moves and compresses the gas. At the same time the cylinder is in thermal contact with the heat sink. Therefore, it removes continuously heat from the gas to the heat sink such that the compression process remains isothermal. The work input required by process 3–4 is smaller than the work generated by process 1–2 such that the net generated work is positive.

Thermodynamic Aspects of Energy

•

197

Isochoric heating with heat regeneration, process 4–1: Both pistons displace leftwards keeping a constant space between them such that the enclosed volume remains constant. During the process, heat is transferred from the matrix to the gas such that at the end of the process the gas increases its temperature to TH.

The practical implementation of an ideal Stirling engine is impossible because there is much unavoidable irreversibility, especially of heat transfer type under finite difference temperature. For example, heat must be transferred from the regenerator matrix to the gas under infinitesimal temperature difference such that at the end of the process the matrix is at the initial gas temperature and the gas is at the initial matrix temperature. This process is practically impossible. Although it is in principle possible to provide heat to the Stirling engine by internal combustion, doing such will induce very high irreversibilities to the regeneration process because combustion is fast while regeneration must be slow. Therefore, practical Stirling engines are implemented either with external combustion or using heat sources such as concentrated solar radiation or waste heat recovery. Whence the cylinders of the Stirling engine are heated and cooled by external sources, which need additional temperature differences and also some thermal response time. Despite these disadvantages the Stirling engine has been used successfully with low-temperature heat source using helium or air as the working fluid. However, Stirling engine design is a most complicated task because the dynamic behavior of the engine mechanism and performance of the heat exchangers highly influence the efficient operation of the engine. The Ericsson cycle comprises of two isothermal and two isobaric processes. The processes are described with the P–V diagram as shown in Fig. 36. The Ericsson heat engine would consist of a compressor, an expander, and one heat exchanger with the role of regenerator (the configuration looks similar to that of a closed-loop Brayton cycle; the difference is that the compression and expansion process for the Ericsson cycle are not adiabatic but rather are accompanied by heat transfer such that instead of being isentropic, these processes are isothermal). The cycle processes are (see the figure):

• • • •

Isothermal expansion and heat addition, process 1–2: In this process the working fluid is heated by an external heat source and expansion occurs simultaneously with the addition of heat at constant temperature. During this process useful work output is obtained. Isobaric heat removal with regeneration, process 2–3: The working fluid passes through the regenerator, where its temperature reduces at constant pressure while simultaneously the regenerator transfers the heat to the process 4–1. Isothermal compression with heat removal, process 3–4: In this process the working fluid is compressed with constant temperature maintained due to a continuous heat rejection process. The work input required for compression is covered by a part of the work generated by the expansion. Isobaric heat absorption from the regenerator, process 4–1: The compressed working fluid flows through the regenerator where heat is transferred and the temperature increases. For a reversible process the regenerator effectiveness is 1.

The difficulty of implementing an ideal Ericsson cycle arises from the irreversible nature of the isothermal compression, isothermal expansion and regeneration processes. Maintaining constant temperature during the compression imposes an extremely slow process and an “infinitely” large heat transfer area of the cylinder such that the temperature differences are minimized. The same issues stand for the expander. Regarding the regenerator due to the finite temperature differences required for a heat transfer to occur, the effectiveness cannot be 1. A regenerator with effectiveness of 1 requires an infinite heat transfer surface, which at least leads to finite pressure drop along the flow, whence generation of irreversibility.

P = const. regeneration

P

Heat addition

Expander

4

1

Compressor

4

Heat rejection

1

Work output

Q 2

Work input

T = TH = const T = TL = const 3

3

P = const. regeneration

2

V

Fig. 36 P–V diagram of an ideal Ericsson cycle.

198

Thermodynamic Aspects of Energy

1.5.8.2

Otto and Diesel Power Cycles

The fundamental thermodynamic cycles that are internally reversible are Brayton, Otto, and Diesel. All these cycles are a good choice for internal combustion engines and thus for engine-driven power generators. The Brayton cycle has been extensively analyzed in the previous two sections. This cycle is better suited for large-scale power generation is combined cycle power plants. In this section the Otto and Diesel cycles are analyzed in detail. These cycles are the most common for engine-driven generators, which consist of an internal combustion engine installed on the same chassis with an electrical generator and a power regulation block. Applications of engine-driven power generators can be both stationary and portable (mobile). The Otto cycle is the ideal cycle for spark ignition engines. This cycle was established in the 1870s after the successful demonstration of the four-stroke spark-ignition engine by Nikolaus Otto. The cycle and the processes are presented in Fig. 37 where the piston–cylinder operation and the P–v diagram is illustrated. The cycle consists of two isentropic and two isochoric processes of a gas. The numerical example taken in the diagram is for a volume ratio rv ¼v1/v2 ¼8 and a pressure ratio r¼ P3/ P1 ¼90. As it will be shown subsequently, for these conditions the energy efficiency with real air as working fluid is 47.3% while the exergy efficiency is 51.9% and Carnot factor is 0.911. In thermodynamic state 1 there is a gas enclosed in a cylinder in thermal equilibrium with the low-temperature reservoir at T1 ¼T0. The processes are given as follows:

• • • •

Isentropic compression, process 1–2: The piston is moved leftward and compresses the gas while at the same time the cycle receives work from the outside. The process is isentropic and adiabatic. Isochoric heat addition, process 2–3: The piston remains in fixed position while heat is added to the working fluid. In internal combustion engine this heat is due to a combustion process (with spark ignition). The temperature and pressure of the gas increase considerably during the isochoric heat addition process. Isentropic expansion, process 3–4: The piston moves down (see the figure) and produces a motor stroke that generates usable work. The expansion continues until the volume reaches the maximum stroke, when v4 ¼ v1. Isochoric heat removal, process 4–1: The piston remains in fixed position while heat is removed from the gas by placing the gas in thermal contact with the heat sink. In practical applications with internal combustion engine, during process 4–1 the heat is removed by expelling the gas to the atmosphere while allowing fresh air in.

The Otto cycle is externally irreversible because its heat addition and removal processes are not isothermal. Therefore, the efficiency of this cycle is lower than that given by the Carnot factor. Under the standard air assumption an analytical expression for the cycle efficiency can be derived. During the isochoric processes there is no work exchange with the surroundings, therefore one must have Qin ¼ Cv ðT3

T2 Þ and Qout ¼ Cv ðT4

T1 Þ

ð158Þ

Using the above equation and the isentrope equations T2 ¼ rvg 1 T1 and T3 ¼ rvg 1 T4 , the efficiency of the internally reversible Otto cycle is Z¼1

Qout ¼1 Qin

T4 T3

T1 ¼1 T2

rv1

g

ð159Þ

× 106 20

Real air assumption 10

3

P (Pa)

Heat addition  = const

Work output Expansion s = const

2 1

4

Compression s = const

Work input

0.1 0.05 0

Heat rejection P = const

5

10

Fig. 37 Ideal Otto cycle under real air assumption for rv ¼ 8, r¼90.

15  (m3/kmol)

1

20

25

Thermodynamic Aspects of Energy

199

The exergy efficiency of the cycle results from an ExBE, which can be written as follows:  _ in 1 ExBE: Q

T0 Tso



 _ out 1 _ in ¼ Q þW

T0 T0



_ out þW

ð160Þ

From the above balance equation in which it is assumed that the sink temperature is the same as the reference temperature T0 the following equation for the exergy efficiency results:

c¼

_ net _ Z W W  net  ¼ ¼ T0 _ in T0 1 Ex _ Qn 1 Tso Tso

ð161Þ

In Fig. 38 the variation of energy and exergy efficiency of the Otto cycle against the volume ratio is given for three values of pressure ratios. The typical range of volume ratio during compression process for spark ignition engines is of approximately 7–11. In this range the energy efficiency of the ideal Otto cycle is of 40–50%. The exergy efficiency depends on an additional parameter beside rv which is the pressure ratio between the maximum and minimum pressures in the cycle. Observe that the exergy efficiency for rv ¼ 7…11 and r ¼70…120 spans from 44% till approximately 68%; the exergy efficiency of the internally reversible Otto cycle gives an indication of the magnitude of exergy destructions due to the heat transfer at source and sink side; the exergy destruction is therefore of the order of 32% to 66% from exergy input. Diesel cycle is used in many applications of industrial scale power generation where large engine-generator groups can be installed. The engines that are based on the Diesel cycle are denoted as compression-ignition internal combustion engines. This thermodynamic cycle and the compression-ignition engine were proposed in Germany by Rudolf Diesel around 1895. In this type of engine the ignition of the combustion process is obtained by compressing the air up to a temperature superior to the autoignition of the fuel. In the moment when the air reaches sufficiently high temperature then pressurized liquid fuel is injected into the cylinder and the ignition process occurs instantly. Because air temperature must be sufficiently high (over 800K) the volume ratio (also called compression ratio) for the Diesel cycle must be in the typical range of rv ¼12–24. Since no spark is given to initiate combustion process in a compression ignition engine the duration of the combustion is relatively slow. Consequently, the pressure can be kept almost steady provided that the injection occurs at top dead center (TDC – the upmost position of the piston). Since the volume increases during the combustion, the temperature of the gases will follow this increase. Fig. 39 presents an ideal Diesel cycle. It comprises the following processes:

• • • •

Isentropic compression, process 1–2: The gas entrapped in the cylinder is compressed adiabatically by moving the piston leftward. Work consumption is required. Isobaric heat addition, process 2–3: During the heat addition process the pressure is maintained constant by increasing the cylinder volume. The temperature of the gas increases due to the heat addition process. Isentropic expansion, process 3–4: This is the motor stroke when the piston produces useful work. The expansion stroke stops when v4 ¼v1. Isochoric heat removal, process 4–1: Heat is rejected while the cylinder volume remains constant. This process is similar to that described for the Otto cycle above.

  for r = 70

0.8

 for r = 90  for r = 120

 and 

0.6

0.4

0.2

Typical range 0 2

4

6

8

10 rv

Fig. 38 Efficiency variation with rv for ideal Otto cycle under standard air assumption.

12

14

16

200

Thermodynamic Aspects of Energy

Heat addition P = const

× 106 20

Real air assumption 10 2

3

Work output

P (Pa)

Expansion s = const 1 4 Compression s = const

Heat rejection v = const

Work input

0.1 0.05 0

5

10

1

15  (m3/kmol)

20

25

Fig. 39 Ideal Diesel cycle under real air assumption for rv ¼18, rc ¼3.

0.8

 and 

0.6

0.4

Typical range  for rc = 2  for rc = 4  for rc = 2  for rc = 4

0.2

Standard air assumption 0

0

5

10

15 r

20

25

30

Fig. 40 Efficiency variation with rv for ideal Diesel cycle under standard air assumption.

It is easy to demonstrate that the efficiency of standard air Diesel cycle is given by the following equation: Z¼1

1 g rg

1

rcg rc

1 1

ð162Þ

where rc ¼ V3/V2 is denoted as cut-off ratio. Fig. 40 illustrates the variation of energy and exergy efficiency of the Diesel cycle for a range of typical compression ratios (rv). As seen, the ideal cycle energy efficiency can reach values of over 60% while the exergy efficiency approaches 80%.

1.5.8.3

Brayton Cycles

Combustion turbine power plants started to be developed commercially in the 1930s based on simple Brayton cycle of open type, which includes a turbocompressor a combustion chamber and gas turbine. Combustion turbine appears to be the most successful technology for the conversion of chemical exergy of gaseous and liquid fuels into electric power. A large palette of fuels can be used, starting with natural gas, fuel oil, coal gas, etc. Further technological developments led to commercialization of advanced combustion turbine power generators with enhanced efficiency due to use of regenerator, gas reheater, compressor intercooler, and turbine blade cooling, which allows for higher operating temperature. With respect to steam Rankine power plants, combustion

Thermodynamic Aspects of Energy

201

turbines offer essentially smaller start-up time (anyhow, below 1 min), which recommends their use for peak load compensation in regional power grids. The efficiency of combustion turbine power plants reach remarkable values of over 40% in conditions when the expelled gases are still at elevated temperature (over 625K). Henceforth, conventional power generation systems that couple a combustion turbine with a Rankine cycle were developed starting in the 1950s. These are known as combined cycles and their efficiency of power generation surpasses 55%. In an actual combustion turbine the working fluid is represented by pressurized combustion gases that expand and generate power. In order to make fuel utilization efficient, the excess air provided to combustion chamber can be in the range of minimum 4 up to over 50. Henceforth, air is well present in combustion gases, and consequently, approximating the properties of combustion gases with that of air is generally accepted. When the working fluid is modeled as ideal gas air with specific heats corresponding to 298K (Cp ¼1.005 kJ/kgK, g¼ 1.4), then the combustion turbine cycle is denoted with “air-standard Brayton cycle.” If all internal processes are reversible, then the cycle is called “ideal, standard-air Brayton cycle.” We will study next the ideal Brayton cycles of various configurations aiming to determine the influence of the main parameters on the upper limit of efficiency according to energy and exergy. Therefore, for this section analysis, the working fluid is standard air and the compressors and turbines and heat exchangers operated with no internal irreversibility. The most basic Brayton cycle comprises four processes, namely isentropic compression, isobaric heat addition, isentropic expansion, and isobaric heat rejection. The mechanical devices that perform these processes are the compressor, heater, turbine, and cooler indicated schematically in the plant diagram from Fig. 41. The fact that in the Brayton cycle the heat addition and rejection processes are isobaric rather than isothermal means that this cycle is externally irreversible. Recall that only the cycles that have isothermal heat addition and rejection (e.g., Carnot, Stirling, and Ericsson) can be externally reversible. Another remark is that the Brayton cycle can be implemented both as internal combustion engine system or as external combustion system, depending on the manner in which the isobaric heat transfer occurs at the heat source side. The overwhelming majority of combustion turbine power plants operate based on open Brayton cycle: 1-2-3-4 in Fig. 41. Due to the direct connection with the atmosphere, the intake pressure and gas expelling pressure are equal to the atmospheric pressure, which represents a limitation of the turbine’s working conditions. This limitation can be overcome if the cycle is closed using a heat exchanger having the role of a cooler. Henceforth, the turbine can be made to discharge in vacuum, which means that more work is generated and an enhanced efficiency is obtained. The turbine discharge pressure can be adjusted indirectly by varying the heat sink temperature. Closed cycle Brayton cycle (1-2-3-4-1 in the figure) must operate with an inert working fluid, either dry air or helium, etc. The heat input can be derived from a combustion process but is applied externally – by heat transfer – to the heater process 2-3. The combustion gases never mix with the working fluid in this case, and the Brayton cycle–based system becomes a socalled external combustion engine. It is important to note that the open and closed Brayton cycles are equivalent from a thermodynamic point of view. Namely, in the open cycle the atmosphere plays the role of cooler, where the isobaric cooling process 4-1 occurs, whereas in a closed cycle the same process is conducted in an engineered heat exchanger. The T–s diagram of an ideal air-standard Brayton cycle of basic configuration is shown in Fig. 42. The EBE and ExBE can be written for each of the cycle processes. Assume that the surrounding atmosphere is at standard conditions defined by T0, P0, which define the reference state for exergy calculations. The EBE, EnBE, and ExBE for each process are given in Table 13. The ideal cycle is

Qin Tso 2

Compressor

Legend Qin – input heat (e.g., combustion)

3

Heat exchanger or combustion chamber

Qout – heat rejected Tso – source temperature Tsi – sink temperature Turbine

Ideal processes Generator 1 – 2: Isentropic compression 2 – 3: Isobaric heat addition 3 – 4: Isentropic expansion 4 – 1: Heat rejection

Power shaft

1

Heat exchanger

Tsi Qout Fig. 41 Schematics of a simple Brayton cycle-based power plant.

4

Type of the cycle Open: 1 – 2 – 3 – 4 Closed: 1 – 2 – 3 – 4 – 1 Note: Thermodynamically open and closed cycles are equivalent

202

Thermodynamic Aspects of Energy

1300

3

Working fluid: standard air (perfect gas assumption) 1080

P = 10 P0 T (K)

860

4 640

2 420

P = P0 = 1.01325 bar 1

200 5.6

5.8

6.0

6.2

6.4

6.6

s (kJ/kg K) Fig. 42 Representation of a basic air-standard Brayton cycle in T–s diagram for r¼10 and T1/T3 ¼0.24. Table 13

Energy (EBE), entropy (EnBE), and exergy balance equations (ExBE) for the ideal basic Brayton cycle

Process

MBE

EBE

EnBE

ExBE

Compression (1-2)

m_ 1 ¼ m_ 2

h1 þ wc ¼ h2

Heat addition (2-3)

m_ 2 ¼ m_ 3

h2 þ qso ¼ h3

s1 þ sg;c ¼ s2 sg;c ¼ 0 s2 þ qTso3 þ sg;so ¼ s3

ex1 þ wc ¼ ex2 þ exd;c exd;c ¼ 0   ex2 þ qso 1 TT03 ¼ ex3 þ exd;so

Expansion (3-4)

m_ 3 ¼ m_ 4

h3 ¼ h4 þ wt

Heat removal

m_ 4 ¼ m_ 1

h4 ¼ h1 þ qsi

s3 þ sg;t ¼ s4 sg;t ¼ 0 s4 þ sg;si ¼ s1 þ qTsi1

N/A

wc þ qso ¼ wt þ qsi

ex3 ¼ ex4 þ wt þ exd;t exd;t ¼ 0   ex4 ¼ ex1 þ qsi 1 TT01 þ exd;si   wc þ qso 1 TT03 ¼ wt   þqsi 1 TT01 þ exd

Overall

qso T3

þ sg ¼

qsi T1

internally reversible, which means that the compression and expansion processes evolve isentropically (no entropy is generated). However, the heat transfer processes at heat sink and heat source are assumed internally reversible but externally irreversible, which means that the system boundary at the source is set to T3 (highest temperature within the cycle) and the sink temperature is set to T1 (lowest temperature within the cycle). Consequently, there will be entropy generation and exergy destruction within the cycle. The net power generated by the cycle is represented by the difference wnet ¼ wt ¼wc. Also, the net generated power can be expressed based on back work ratio (BWR), a parameter introduced also for the Rankine cycle. The BWR represents the ratio between the work consumed by the compressor(s) and the work generated by the turbine(s). Mathematically it is written as follows: ð163Þ

BWR ¼ wc =wt

Therefore one has wnet ¼wt(1 BWR). Another way of expressing the net power output results from the overall energy balance given in Table 13: wnet ¼ qso qsi. Furthermore, the energy efficiency of power generation (Z) is represented by the ratio of net power production and the rate of heat input; this can be mathematically written as the ratio of mass specific quantities as follows: Z¼1

qsi qso qsi Z  ¼ ; furthermore c1 ¼ 4Z qso 1 TT03 qso 1 TT03

ð164Þ

Note that in the above equation the exergy efficiency (c1) of the cycle has been introduced as the ratio of useful work output to the exergy input at the heat source. An alternative way of defining the exergy efficiency is with respect to the exergy received by the cycle, which is represented by the exergy input at source (exin) and the exergy output at sink (exout). From ExBEs given in Table 13 one has wnet ex   d   ¼1 c2 ¼ ð165Þ T0 exin ex out qso 1 qsi 1 T0 T3

T1

Thermodynamic Aspects of Energy

203

There is a third way of expressing the exergy efficiency of the cycle, namely as the ratio of its actual energy efficiency and the efficiency of a reversible heat engine operating between T1 and T3 heat reservoirs; henceforth c3 ¼

Z 1

T1 T3

¼

1 1

qsi qso T1 T3

ð166Þ

One remarks from the above three exergy equations that if T1 ¼T0 then c1 ¼ c2 ¼ c3. This identity is anyhow valid for open-type Brayton cycle, for which it is natural to assume that the reference temperature is that of the surrounding air. For special cases with closed Brayton cycles, the temperature T1 may be different than T0, henceforth c1ac2ac3. For the standard-air Brayton cycle, the perfect gas equation of state can be applied to further derive analytical expressions for cycle efficiency. The perfect gas equation of state assumes constant specific heats (independent of temperature). As mentioned above, for standard air, the specific heats are estimated for 298.15K. Using the general equation Dh¼ cpDT and the EBE, the energy efficiency becomes     T4 1 cp ðT4 T1 Þ T1 T1   ¼1 ð167Þ Z¼1 T3 cp ðT3 T2 Þ T2 1 T2 Another general equation for perfect gas describes the interrelation between pressure and temperature along an isentropic process 1-2 according to T1 P1κ ¼ T2 P1κ , with κ ¼ ðg

1Þ=g

ð168Þ

and g the specific heat ratio. Since processes 1-2 and 3-4 are isentropic and evolve between the same pressures, P1 ¼ P4 and P3 ¼ P2, one has that  κ  κ T3 P3 P2 T2 T4 T3 ¼ ¼ ð169Þ ¼ ¼ T4 P4 P1 T1 T1 T2 Therefore, the energy efficiency of standard-air Brayton cycle becomes Z¼1

r κ ; with pressure ratio defined by r ¼

P2 41 P1

ð170Þ

From the above equation it results that the efficiency of the air-standard Brayton cycle depends only on pressure ratio. In other words, it does not depend directly on the cycle’s highest and lowest temperature (T3 and T1). However, due to the cycle configuration, some restrictions must be imposed on sink and source of temperature for the cycle to function. Henceforth, the temperature of the heat source (T3) must be greater than the temperature at the compressor discharge (T2). Since T2 ¼ r K T1 it comes out that, for the cycle to function, one must have T3 4r κ T1

ð171Þ

Differently as for the energy efficiency, the magnitude of heat sink temperature influences the cycle’s exergy efficiency because the exergy input depends on this temperature. The exergy efficiency of the cycle decreases when the source temperature (T3) becomes higher and the pressure ratio is fixed because Z will not vary and the Carnot factor grows. According to the efficiency relation, the cycle efficiency increases monotonically with pressure ratio, r. Also, with pressure ratio increase, the temperature at the turbine entrance increases. Because of materials-related issues both the pressure and temperature at turbine inlet must be limited to maximum values. Typically, the pressure at gas turbine inlet is limited to 20–30 bar. The maximum temperature is limited to about 1200–1500K. These constructive limitations justify the cycle design for a maximum acceptable heat source temperature. If his strategy is taken, some design parameters trade-offs can be found, as discussed next. As it results from Fig. 43 when the temperatures at heat source (T3) and heat sink (T1) are restricted to fixed values the net generated work has a maximum for certain pressure ratio. It is observed that for small pressure ratios the specific work generated by the turbine is relatively small with respect to the work consumed by the compressor. In another extreme situation when pressure ratio is chosen at high value, the work generated by the compressor is relatively high as compared to the magnitude of the work generated by the turbine. Henceforth, in these two extreme situations (low- and high-pressure ratios) the net generated work represented by the difference between wt and wc is small. However, for intermediate pressure ratios the net generated work is high and reaches a maximum at certain pressure ratios. In Fig. 43 it is clearly observed that the net, mass-specific work output wnet has a maximum for pressure ratio roptD14. This is very relevant for power plant design because if the mass-specific work is high, then the power plant is more compact. In order to assess the system compactness for an imposed net power output one can introduce a system SSP defined as the reciprocal of the net mass-specific work output, namely SSP ¼

1 ðkg=kJÞ wnet

ð172Þ

The SSP is plotted in Fig. 44 against the pressure ratio. On the same plot are superimposed the energy and exergy efficiency curves for the cycle. As observed, the minimum of SSP (which corresponds to the most compact system) corresponds to the “knee” of efficiency curves. If the pressure ratio is higher than ropt then the growing rate of efficiency decreases fast and, from the economic

204

Thermodynamic Aspects of Energy

900 wnet 800

wc

700

wt

w (kJ/kg)

600 500 400 300 200 100 0 10

0

20

30 r

40

50

60

Fig. 43 Mass specific work variation with pressure ratio for the basic standard-air Brayton cycle when heat sink and source temperatures are restricted to 298K and 1200K, respectively.

× 10−3 6.50

0.9 SSP

6.00



5.50

0.7 0.6

5.00 0.5 4.50

1 and 

SSP (kg/kJ)

0.8

1

0.4 4.00

0.3

3.50 3.00

0.2 0

10

20

30 r

40

50

0.1 60

Fig. 44 Minimization of the specific size parameter (SSP) of the system for the standard-air Brayton cycle with sink/source temperatures restricted to 298K and 1200K, respectively.

point of view, it does not make sense to enhance the efficiency because the plant size and the equipment cost will increase substantially. The efficiency of the Brayton cycle can be improved if some modifications to the basic configuration are applied, such that the external irreversibility due to the finite temperature difference at sink and source is reduced. The improvement can be obtained in three ways:

• •

•

By regeneration, in which case the air is preheated after the compression using the heat recovered from the hot gas expelled by the turbine. Henceforth, the heat input is reduced for similar net output work and the efficiency increases. By reheating, in which case 2 (or more) subsequent turbines are used with interstage reheating. Typically in an actual combustion turbine only a high-pressure and a low-pressure turbine are installed on the same shaft with a secondary combustion chamber inserted in between the stages such that the working gas is reheated prior to the inlet into the lower stage turbine. The effect of reheating is represented by a substantial increase of net output work, which leads to the increase of cycle efficiency. By intercooling, in which case the air compression is done in stages with interstage cooling and heat rejection in the environment. The intercooling leads to a decrease of the compression work, thus the back work ratio decreases and the overall cycle efficiency increases.

Thermodynamic Aspects of Energy 1.5.8.4

205

Vapor Power Cycles

It is of general knowledge that steam power emerged as a technology to generate motive force in the 18th century. Steam Rankine cycle is the most used type of thermodynamic cycle for power generation worldwide. Another version of Rankine cycle is the ORC, a technology that has been developed up to commercial level over the last few decades and is used nowadays in some specific applications, especially in renewable energy systems. In a first phase of their development (18th and 19th centuries), steam power stations operated based on a noncondensing Rankine cycle using reciprocating steam engine (also known as steam piston). The motive force generated by noncondensing steam engine fueled with wood or coal was extensively used in the 19th century in many industries including textile, mining, agriculture (for irrigation and crop processing), pumping stations, all sorts of mills, and for transport vehicles such as rail locomotives and ships. A major progress on steam power generation was marked by the invention of the steam turbine by the end of the 1800s. Since then, steam turbines have become a major prime mover and are responsible today for more than 80% of electrical power generated worldwide. The operation of steam power plants is based on Rankine cycles that use steam as a working fluid. The most basic vapor power cycle is the simple ideal Rankine cycle. This thermodynamic cycle can be executed with an ideal machine comprising four components: an ideal pump that operates isentropically and adiabatically (no heat transfer, but only work input), an ideal heat exchanger system that plays the role of vapor generator, a turbine able to operate isentropically and adiabatically (no heat transfer but only work output), and a condenser with no pressure losses and infinite heat transfer surface (or equivalently infinite heat transfer coefficient or zero temperature difference between working fluid and heat sink). The vapor generator consists of a preheating section, boiling section, and vapor superheating section; it operates without pressure losses and without any temperature difference between the heat source and the working fluid (or equivalently it has an infinite heat transfer surface or infinite heat transfer coefficient). When working fluid is steam – which is the case for any conventional type of steam power plant – the system is denoted as a “simple ideal steam Rankine cycle.” Note that when an organic fluid or other type of working fluid instead of steam is used the allure of the T–s diagram might be quite different, depending on the type of the fluid. The basic configuration (four components) of Rankine cycle and the simple ideal Rankine cycle T–s diagram are presented in Fig. 45. The cycle comprises four processes: (1) isentropic pressurization of the working fluid in saturated liquid state, process 1-2; (2) isobaric vapor generation, process 2-3 (with subprocesses of heating 2-3a, boiling 3a-3b, and superheating 3b–3); (3) isentropic expansion of the superheated working fluid; and (4) a condensation process of the working fluid until it reaches saturated liquid state, process 4–1.

Qsg = Qph + Qb + Qsh Qb

Qph

3b

3a Preheating

Qsh

Boiling

Superheating 3

T

3 2

Wt

Wp

3a 3b

Turbine

Pump 2 4

1

s

1 4 Condenser Qc

Fig. 45 Basic steam Rankine power plant: (a) power plant schematics and (b) ideal Rankine.

206

Thermodynamic Aspects of Energy

The simple ideal Rankine cycle is not a totally reversible cycle because the heat addition process at the source side is not isothermal. Therefore, the cycle as shown in Fig. 45 has external irreversibility due to heat transfer at the source side. The heat transfer irreversibilities in this case are determined by the temperature difference T3–T, where T–3 represents the temperature of the working fluid, which increases along the process 2-3a-3b-3. Since it is externally reversible an exergy efficiency smaller than 1 must be assigned to the cycle. The thermodynamic analysis can be pursued according to the first and second laws. The mass, energy, entropy, and exergy balances for each of the cycle components and the overall balances for the cycle are given in Table 14. For the case of the simple ideal Rankine cycle there are no exergy destructions in condenser, pump, and turbine. The irreversibilities in the vapor generator are estimated according to the temperature of heat source (Figs. 44 and 45). Let us denote the temperature of the heat source Tso. When the simple ideal Rankine cycle is analyzed, the heat source is considered at a constant temperature that is equal to the highest temperature in the cycle. In other words, the vapor generator has an “infinite surface for heat transfer,” which ensures that the working fluid reaches the heat source temperature level in the thermodynamic state #3 (see the cycle in Fig. 44); T3 ¼Tso. On the other hand the condensation temperature must be equal ideally with the sink temperature; T1 ¼ T4 ¼ Tsi. In addition, the sink temperature is ideally the same as the reference temperature; Tsi ¼ T0. In these ideal assumptions the general balance equations given in Table 14 take a simplified form. For example, the mass flow rate is the same for all system state _ ¼m _1 ¼m _ 2 ¼ ⋯. Furthermore, there is no entropy generation and exergy points, henceforth it can be denoted generally with m destruction in condenser, pump, and turbine. For the vapor generator the integrals in EnBE and ExBE can be solved directly since Tso ¼ const. For the overall cycle the net power is: _t _ net ¼ W W

_p W

ð173Þ

and the total heat input is written as _ vg ¼ Q _ sh þ Q _bþQ _ sh ¼ m _ ðh3 Q

h2 Þ

ð174Þ

The energy efficiency of the cycle can be defined in terms of net output work by the total heat input at source side (vapor generator), namely Z¼

_p _t W _ net W W ¼ _ _ Qvg Qin

ð175Þ

The exergy efficiency expresses the ratio between the net exergy delivered (net output work) and the actual exergy consumed. In the Rankine cycle the exergy consumed is the same as the exergy delivered by the heat source. Therefore, the exergy efficiency is given by c¼

_p _t W _ net W W ¼ _Exin _Exso

ð176Þ

Here, the exergy rate at the source is generally expressed as indicated in Table 14. For the simple ideal Rankine cycle, the exergy rate at the source is given in Table 15, which also gives the balance equations for the overall cycle and its components. Accordingly, the exergy efficiency when the heat source is of constant temperature is: c¼ 

1

_ net W  T0 Tso

_ so Q

¼

Z ZC

ð175Þ

where ZC is the Carnot factor of the constant temperature heat source. The specific exergy of the flow represents a thermomechanical exergy calculated based on specific enthalpy and entropy of the working fluid at a given thermodynamic state. According to its definition, the exergy is calculated with ex ¼ h

h0

T0 ðs

s0 Þ

ð176Þ

Here, index 0 represents the reference state which is assumed to be that of water at T0 ¼ 251C and P0 ¼101.325 kPa. The variation of specific exergy Dex for any process is calculated with: _ m _ ¼ exout Dex ¼ DEx

ex in

ð177Þ

_ is a reference mass flow rate and subscripts “out” and “in” refer to outlet and inlet conditions, respectively. where m For each process within the simple ideal Rankine cycle either heat or work is exchanged between the working fluid and the surroundings. Processes that exchange work with the surroundings are the pumping and the expansion; the specific work is determined with: _m _ ¼ hout w¼W

hin

ð178Þ

Here, w is the mass specific work. Similarly, the heat exchange processes that occur in preheater, boiler, superheater, and condenser are described by the following equation that expresses the mass specific heat flux: _m _ ¼ hout q¼Q

hin

ð179Þ

Balance equations for Rankine cycle of basic configuration Comp

Pump (1–2)

_ 1¼m _2 MBE: m EBE: m_ 1 h1 þ W_ p ¼ m_ 2 h2 _ 1 s1 þ S_ g ¼ m _ 2 s2 EnBE: m _ 1 ex1 þ W_ p ¼ m _ 2 ex2 þ E_ xd ExBE: m

Condenser (4–1)

_ 4¼m _1 MBE: m _ 1 h1 þ Q_ c EBE: m_ 4 h4 ¼ m R dQ_ _ 4 s4 þ S_ g ¼ m_ 1 s1 þ 41 c EnBE: m  Tsi  R1 T0 _ 4 ex4 ¼ m_ 1 ex1 þ 4 1 dQ_ c þ E_ xd ExBE: m Tsi

Overall cycle

MBE: there is no MBE because no mass crosses the boundary of the overall system.

EBE:W_ p þ Q_ sh þ Q_ b þ Q_ sh ¼ Q_ c þ W_ t ; R 3b _ R 3a dQ_ R3 _ R1 _ EnBE: 2 Tsoph þ 3a dTQsob þ 3b dTQsosh þS_ g ¼ 4 dTQcc R 1 ExBE:W_ p þ E_ xso ¼ W_ t þ 4 1 TTsi0 dQ_ c þ E_ xd

where E_ xso ¼

R 3a  1 2

T0 Tso

 dQ_

ph

þ

R 3b  3a 1

T0 Tso



dQ_ b þ

_ 3a ¼ m _ 3b MBE: m _ 3a h3a þ Q_ b ¼ m _ EBE: m R 3b dQ_ b 3b h3b _ _ 3b s3b _ 3a s3a þ 3a m EnBE: m T  þ S g ¼ R 3b so T0 _ _ 3b ex3b þ E_ xd dQ_ b ¼ m ExBE: m 3a ex3a þ 3a 1 Tso

Vapor generator (2–3)

_ 3¼m _4 MBE: m _ 4 h4 þ W_ t EBE: m_ 3 h3 ¼ m _ 3 s3 þ S_ g ¼ m _ 4 s4 EnBE: m _ 3 ex3 ¼ m_ 4 ex4 þ W_ t þ E_ xd ExBE: m

_ 2¼m _ 3a MBE: m _ 2 h2 þ Q_ ph ¼ m _ 3a h3a EBE: m R _ _ 2 s2 þ 23a dTQph þ S_ g¼ m_ 3a s3a EnBE: m R 3a so ExBE: m_ 2 ex2 þ 2 1 TT0 dQ_ ph ¼ m_ 3a ex3a þ E_ xd

R3  3b 1

T0 Tso



so

_ 3b ¼ m_ 3 MBE: m _ 3b h3b þ Q_ sh ¼ m _ EBE: m R 3 dQ_ sh 3 h3_ _ 3b s3b þ 3b EnBE: m m_ 3 s3 T þ S g ¼ R 3 so T0 _ 3 ex3 þ E_ xd _ dQ_ sh ¼ m ExBE: m 3b ex3b þ 3b 1 Tso

dQ_ sh

Note: EBE, energy balance equation; EnBE, entropy balance equation; ExBE, exergy balance equation; MBE, mass balance equation; Tso, temperature at heat source (K); Tsi, temperature at heat sink (K).

Thermodynamic Aspects of Energy

Turbine (3–4)

Balance equations Vapor generator (2–3)

Balance equations

Vapor generator (2–3)

Comp

Vapor generator (2–3)

Table 14

207

208

Balance equations for the simple ideal Rankine cycle

Turbine (3–4)

MBE: m_ 3 ¼ m_ 4 ¼ m_ _ ðh4 −h3 Þ EBE: W_ t ¼ m EnBE: S_ g ¼ 0 ExBE: E_ xd ¼ 0

Condenser (4–1)

MBE: m_ 4 ¼ m_ 1 ¼ m_ _ ðh4 −h1 Þ EBE: Q_ c ¼ m EnBE: S_ g ¼ 0 ExBE: E_ xd ¼ 0

Overall cycle

_ ðh3 −h2 Þ EBE: W_ p þ Q_ sg ¼ Q_ c þ W_ t ; where Q_ sg ¼ Q_ sh þ Q_ b þ Q_ sh ¼ m _ Q_ EnBE: S_ g ¼ QT0c þ Tsosg   ExBE: E_ xd ¼ W_ p −W_ t þ E_ xso ; where E_ xso ¼ 1− T0 Q_ so Tso

Preheating (2–3a)

MBE: m_ 1 ¼ m_ 2 ¼ m_ EBE: W_ p ¼ m_ ðh2 −h1 Þ EnBE: S_ g ¼ 0 ExBE: E_ xd ¼ 0

Balance equations MBE: m_ 2 ¼ m_ 3a ¼ m_ EBE: Q_ ph ¼ m_ 3a ðh3a −h2 Þ Q_ EnBE: S_ g ¼ m_ ðs3a −s2 Þ− Tsoph   _ ExBE: E xd ¼ m_ ðex2 −ex3a Þ þ 1− T0 Q_ ph

Boiling (3a–3b)

Pump (1–2)

Comp

MBE: m_ 3a ¼ m_ 3b ¼ m_ _ ðh3b −h3a Þ EBE: Q_ b ¼ m _ EnBE: S_ g ¼ m_ ðs3b −s3a Þ− TQsob   ExBE: E_ xd ¼ m_ ðex3a −ex3b Þ þ 1− TTso0 Q_ b

Superheating (3b–3)

Balance equations

Vapor generator (2–3)

Comp

MBE: m_ 3b ¼ m_ 3 ¼ m_ EBE: Q_ sh ¼ m_ ðh3 −h3b Þ Q_ EnBE: S_ g ¼ m_ ðs3 −s3b Þ− Tsoph  _ ExBE: E xd ¼ ðex3b −ex3 Þ þ 1− T0 Q_ sh

Tso

Tso

Thermodynamic Aspects of Energy

Table 15

Thermodynamic Aspects of Energy

Table 16

209

Summary of important parameters for Rankine cycle

Quantity

Definition

Unit

Heat input Net output work

Q_ in ¼ Q_ ph þ Q_ b þ Q_ sh W_ net ¼ W_ t W_ p

kJ/kg kJ/kg %

_

Z ¼ W_ net Q in _ deliv ¼ W_ net Ex

Energy efficiency, Eq. (68) Exergy delivered Exergy efficiency, Eq. (69) Carnot factor Exergy efficiency (ideal maximum) Back work ratio

c¼

_ Ex deliv _ cons Ex

c¼

W_ net W_ rev

ZC ¼ 1

BWR ¼

Pressure ratio

PR ¼

Expansion ratio

ER ¼

P2 P1 v4 v3

T0 T3

¼ W_ p W_ t

Z ZC

kJ/kg % % % % – –

Note that if heat or work is transferred out of the cycle then the sign is negative, whereas if heat or work is received by the working fluid the amount has a positive sign. Thus work of the pump is negative, and that of the turbine is positive. Heat transfer at the preheater, boiler, and superheater is positive, whereas at the condenser it is negative. In thermodynamic state #2 the working fluid must have the same specific entropy as in state #1 and the same pressure as in state #3. In thermodynamic state #3a the liquid is saturated at the specified boiling temperature Tb. In state #3b there is a saturated vapor at temperature Tb. State #3 is superheated vapor with temperature equal to Tb þ DTsh. State #4 represents the expanded working fluid at pressure equal to P1 and the same specific entropy as in state #3. The vapor quality x (required for state #4) is related to specific enthalpy at the state point based on equation: h ¼ ð1

xÞhl þ xhv

ð180Þ

where h1 and hv are the specific enthalpies of saturated liquid and vapor, respectively, at the pressure corresponding to the thermodynamic state. Some important parameters are defined in Table 16. The back work ratio (BWR) represents the ratio of the work required to turn the pump and the work delivered by the turbine. As observed, in the Rankine cycle the back work ratio is extremely low; below one percent. This is very favorable for high efficiency of power generation. The work necessary to pressurize liquid (pump work) is negligible with respect to the work generated by expansion of gas (turbine work). In this respect, it is an advantage of the Rankine cycle to operate with a working fluid that changes the phase from subcooled liquid to superheated vapor. Other types of thermodynamic cycles that operate with gas will have a BWR higher than 30%. The pressure ratio (PR) quantifies the ratio between high and low pressure of the Rankine cycle (see Table 16). In steam Rankine cycle the value of PR is quite high due to the fact that steam expands in a vacuum. Hence, it is necessary for a turbine with multiple expansion stages to make the actual expansion process more efficient. Moreover, the parameter ER (expansion ratio) compares the vapor specific volume at turbine exit to the volume at turbine exit (see Table 16). In Rankine cycle the volume of expanded flow is more than 100 times larger than steam volume at turbine inlet. The expansion ratio is a crucial parameter for turbine design and also quantifies the measure in which the size of low-pressure steam pipes at turbine outlet must be larger than high-pressure steam pipes at turbine inlet.

1.5.9

Future Directions

Thermodynamics and the energy concept have been shaped and developed together since the industrial revolution and inception of the steam engine. The need of design fundamentals for heat engine technology and mastering energy has been a strong push toward development of thermodynamics. The systemic approach of thermodynamics constructed around the idea of black-box thermodynamic systems proved to be powerful and useful for understanding the universe and helping humans to develop technology. Nevertheless, the theory is incomplete and thermodynamics certainly evolves. Nature is not made of black boxes but rather live systems, which evolve and decay to leave the place to other movers on Earth’s surface and the universe. Humans evolved toward the establishment of a human-plus-machine realm, in which technology and engineering design play a major role. Therefore, recognizing systems at thermodynamic nonequilibrium as major players became important. Expansion of thermodynamics in this respect is brought by constructal law, which also brings up the idea of live state in thermodynamics, as an opposite of dead state. The new extension makes thermodynamics more complete, as it is required by the current trends and thought development. This new direction will be more prominent in the near future as it promotes technology development toward vascularized flow structure, and hierarchical systems with multiscales. Exergy will continue to play a major role as an important tool required for achieving better performance by reducing

Thermodynamic Aspects of Energy

210

irreversibility. Multigeneration is an intriguing concept, which integrates itself into the new trend in thermodynamics, energy, and technology.

1.5.10

Closing Remarks

In this chapter, thermodynamics and energy are reviewed, emphasizing the intimate connection between the science of thermodynamics, engineering as a tool for technology development, and the human need to better master energy. A brief historical perspective is given in the introduction of this chapter to illustrate the evolution of energy concept, growth of thermodynamics science, and today’s trends. Basic concepts of thermodynamics are introduced in a summarized fashion. The laws of thermodynamics are presented and the new extensions with constructal law introduced. The concept of live state and its usefulness are explained. Equations of state of ideal gas and real fluids are presented. Some insights on thermodynamic equilibrium are given. A major focus of the chapter is on exergy and exergy analysis. In the final part of the chapter, an overview of thermodynamic cycles is provided. Some future directions of energy and thermodynamics are mentioned as well.

References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26]

Crease RP. Energy in the history of philosophy and science. In: Editor E, editor. Enciclopedia of energy. vol. 2. New York, NY: Elsevier; 2004. p. 417–21. Chahart HS, Chute HN. First principles of physics. Boston, MA: Allyn and Bacon; 1912. Bejan A. Constructal thermodynamics. Int J Heat Technol 2016;34:S1–8. Bejan A. Entropy generation minimization: the new thermodynamics of finite size devices and finite time processes. J Appl Phys 1996;79:1191–218. Dincer I, Zamfirescu C. Sustainable energy systems and applications. 2nd ed. New York, NY: Springer; 2011. Dincer I, Rosen MA. Exergy: energy, environment and sustainable development. New York, NY: Elsevier; 2013. Georgescu-Roegen N. The entropy law and the economic processes. Eastern Eur J 1986;12:3–25. Bejan A. The physics of life: the evolution of everything. New York, NY: St. Martin’s Press; 2016. Bejan A. Constructal-theory network of conducting paths for cooling a heat generating volume. Int J Heat Mass Transf 1997;40:799–816. Bureau Interational des Poids et Mesures. The International System of Units (SI). 8th ed. Paris: Bureau Interational des Poids et Mesures; 2006. Carnot S. Réflexions sur la Puissance Motrice du Feu et sur les Machines propres à Développer cette Puissance. Paris: Bachelier; 1824. Bejan A, Lorente S. The constructal law and the thermodynamics of flow systems with configuration. Int J Heat Mass Transf 2004;47:3204–14. Bejan A. Advanced engineering thermodynamics. 4th ed. New York, NY: Wiley; 2016. Dincer I, Zamfirescu C. Advanced power generation systems. New York, NY: Elsevier; 2014. Ahmadi P, Dincer I, Rosen MA. Thermodynamic modelling and multi-objective evolutionary-based optimization of a new multigeneration enery system. Energy Convers Manag 2013;76:282–300. Zamfirescu C, Dincer I. Renewable-energy-based multigeneration systems. Int J Energy Res 2012;36:1403–15. Ahmadi P, Dincer I, Rosen MA. Development and assessment of an integrated biomass-based multi-generation energy system. Energy 2013;56:155–66. Ozturk M, Dincer I. Thermodynamic assessment of an integrated solar power tower and coal gasification system for multi-generation purpose. Energy Convers Manag 2013;76:1061–72. Ozturk M, Dincer I. Thermodynamic analysis of a solar-based multi-generation system with hydrogen production. Appl Therm Eng 2013;51:1235–44. Al-Sulaiman FA, Dincer I, Hamdullahpur F. Energy analysis of a trigeneration plant based on solide oxide fuel cell and organic Rankine cycle. Int J Hydrog Energy 2010;35:5104–13. Ozcan H, Dincer I. Thermodynamic analysis of an integrated SOFC, solar ORC and absorption chiller for tri-generation applications. Fuel Cells 2013;12:781–93. Khaliq A, Kumar R, Dincer I. Performance analysis of an industrial waste heat-based trigeneration system. Int J Energy Res 2009;33:737–44. Khaliq A, Choudhary K, Dincer I. Exergy analysis of a gas turbine trigeneration system using the Brayton refrigeration cycle for inlet air cooling. Proc IMechE Part A: J Power Energy 2009;224:449–61. Peng D-Y, Robinson DB. A new two-constant equation of state. Ind Eng Chem Fundam 1976;15:59–64. Stryjek R, Vera JH. PRSV: and improved Peng–Robinson equation of state for pure components and mixtures. Can J Chem Eng 1986;64:323–33. Rivero R, Grafias M. Standard chemical exergy of elements updated. Energy 2006;31:3310–26.

Further Reading Bejan A, Dincer I, Lorente S, Reis AH, Miguel AF. Porous media in modern technologies: energy, electronics, biomedical and environmental engineering. New York: Springer Verlag; 2004. p. 396. Cetinkaya E. Experimental investigation and modeling of integrated tri-generation systems [Ph.D thesis]. Oshawa, ON: University of Ontario Institute of Technology; 2013. Dincer I. Refrigeration systems and applications. third ed. London: John Wiley & Sons, Ltd.; 2017. p. 727. Dincer I, Hamut HS, Javani N. Thermal management of electric vehicles. London: John Wiley & Sons, Ltd.; 2017. p. 457. Dincer I, Hogerwaard J, Zamfirescu C. Clean rail transportation options. New York: Springer Verlag; 2015. p. 223. Dincer I, Joshi A. Solar-based hydrogen production systems. New York: Springer Verlag; 2013. p. 141. Dincer I, Ratlamwala T. Integrated absorption refrigeration systems: comparative energy and exergy analyses. New York: Springer Verlag; 2016. p. 270. Dincer I, Rosen MA. Thermal energy storage systems and applications. London: John Wiley & Sons, Ltd.; 2002. p. 580. Dincer I, Rosen MA. Exergy. first ed. Oxford: Elsevier Science, Ltd.; 2007. p. 454. Dincer I, Rosen MA. Thermal energy storage systems and applications. second ed. London: John Wiley & Sons, Ltd.; 2011. p. 600. Dincer I, Rosen MA. Exergy analysis of heating, refrigerating and air conditioning. Oxford: Elsevier Science, Ltd.; 2015. p. 388. Dincer I, Rosen MA, Ahmadi P. Optimization of energy systems. London: John Wiley & Sons, Ltd.; 2017. p. 453. Dincer I, Zamfirescu C. Drying phenomena: analyses and applications. London: John Wiley & Sons, Ltd.; 2016. p. 482. Dincer I, Zamfirescu C. Sustainable hydrogen production. Oxford: Elsevier Science, Ltd; 2016. p. 479.

Thermodynamic Aspects of Energy

Relevant Websites https://constructal.org/ Constructal Blog. http://ca.wiley.com/WileyCDA/WileyTitle/productCd-ER.html John Wiley and Sons, Inc. http://www.nrel.gov/ National Renewable Energy Laboratory. http://exergoecology.com/ The Exergoecology Portal.

211

1.6 Exergy Ibrahim Dincer, University of Ontario Institute of Technology, Oshawa, ON, Canada r 2018 Elsevier Inc. All rights reserved.

1.6.1 Introduction 1.6.1.1 A New Concept: Analysis, Improvement, Design, and Assessment 1.6.2 Thermodynamics Laws 1.6.2.1 Zeroth Law of Thermodynamics 1.6.2.2 First Law of Thermodynamics 1.6.2.3 Second Law of Thermodynamics 1.6.3 Energy versus Exergy 1.6.4 Types of Exergy 1.6.4.1 Reference Environment 1.6.4.2 Energy and Exergy Efficiencies 1.6.4.3 A Simple Procedure for Energy and Exergy Analyses 1.6.5 Thermodynamic Systems 1.6.5.1 Closed Systems 1.6.5.1.1 Rigid tank 1.6.5.1.2 Piston cylinder mechanism 1.6.5.2 Open Systems 1.6.5.2.1 Nozzles 1.6.5.2.2 Diffusers 1.6.5.2.3 Fans 1.6.5.2.4 Pumps 1.6.5.2.5 Turbines 1.6.5.2.6 Compressors 1.6.5.2.7 Heat exchangers 1.6.5.2.8 Mixing chambers 1.6.5.2.9 Expansion valve 1.6.5.3 Some Thermodynamic Cycles 1.6.5.3.1 Steam Rankine cycle 1.6.5.3.2 Refrigerators 1.6.5.3.3 Heat pumps 1.6.6 Case Study 1.6.6.1 Systems Description 1.6.6.1.1 Biomass-based integrated system 1.6.6.1.2 Geothermal-based integrated system 1.6.6.2 Analysis 1.6.6.2.1 Biomass integrated with steam cycle 1.6.6.2.2 Geothermal single flash integrated with steam cycle 1.6.6.3 Results and Discussion 1.6.6.3.1 Biomass integrated with steam cycle 1.6.6.3.2 Geothermal single flash integrated with steam cycle 1.6.7 Future Directions 1.6.8 Concluding Remarks Acknowledgment References Further Reading Relevant Websites

Nomenclature A e E

212

area (m2) specific energy (kJ/kg) energy; total solar energy reaching solar pond; wind energy (kW)

ex _ Ex Ex ExQ h

213 214 215 215 215 216 218 219 220 221 222 222 222 222 224 226 227 228 229 229 231 233 235 238 239 240 240 243 248 250 250 250 250 252 252 257 259 259 261 262 263 264 264 264 264

specific exergy (kJ/kg) exergy rate (kW) exergy (kJ) exergy associated with heat (kJ) specific enthalpy (kJ/kg)

Comprehensive Energy Systems, Volume 1

doi:10.1016/B978-0-12-809597-3.00106-1

Exergy

x y

specific entropy (kJ/kg K) entropy (kJ) entropy generation (kJ) entropy change (kJ) temperature (K) specific internal energy (kJ/kg) internal energy; heat loss from pond surface to air (kJ) specific volume (m3/kg) volume (m3) velocity (m/s) specific work (kW/kg) work rate (kW) quality mass ratio

Greek Letters Z efficiency m chemical potential Q entropy creation

r t c

density exergetic temperature factor exergy efficiency

Subscripts a avg cc comp con d dest en ex f g gen he

kin ov p ph pot Q s st t th turb 1–11 0

kinetic overall pump physical potential heat ideal steam turbine thermal turbine state numbers reference environment state

H ke KE m m_ P pe PE Pr q Q Q_

enthalpy (kJ) specific kinetic energy (kJ/kg) kinetic energy (kJ) mass (kg) mass flow rate (kg/s) pressure (kPa) specific potential energy (kJ/kg) potential energy (kJ) reduced pressure (kPa) specific heat transfer (kJ/kg) heat (kJ) heat transfer rate (kW) heat transfer into system across region r on system boundary (kJ) pressure ratio

Qr rp

1.6.1

actual average combustion chamber compressor condenser destruction destruction energy exergy flash biomass gas generation heat exchanger

s S Sgen DS T u U

213

v V V w _ W

Introduction

Energy has always been the most critical issue for humanity, who used wood as the first source of energy, followed by coal, oil, and natural gas. It has historically been the source and cause of conflicts, wars, and peace. Since the industrial revolution, energy competition has been even stiffer. It has become more apparent that humanity needs more efficient, more cost effective, more environmentally benign and more sustainable options and solutions. Such a requirement has been the main motivation behind going beyond traditional methods of analyses and techniques. Traditionally, the first law of thermodynamics, which is recognized as the conservation law, has been the only tool comprehensively used in design, analysis, and assessment of thermodynamic systems. It is now crystal clear to everyone that the first law of thermodynamics is insufficient and incapable of addressing practical systems with irreversibilities (or losses and inefficiencies). That is why the second law of thermodynamics has been brought into the picture to account for irreversibilities or destructions through entropy and exergy. Exergy has distinguished itself to be a primary tool under the second law of thermodynamics. Generally, thermodynamics is defined by many as the science of energy and entropy referring to the first and second laws of thermodynamics, respectively. Here, thermodynamics is redefined as the science of energy and exergy, which makes it more convenient to cover both laws of thermodynamics, making both energy and exergy quantities in the same unit consistently, and reaching out to key efficiencies through energy and exergy efficiencies. This way, the concepts dwell on the right pillars for practical applications. Exergy efficiency becomes important for practical systems and applications because it is a true measure of system performance and indicates how much the actual performance deviates from the ideal performance.

214

Exergy

Exergy analysis is now recognized as a thermodynamic analysis technique based primarily on the second law of thermodynamics and appears to be the only tool for assessing and comparing processes and systems rationally and meaningfully. Consequently, exergy analysis can assist in improving and optimizing designs and analyses. Two key features of exergy analysis become more attractive, i.e., (1) it determines the true locations, types, and magnitudes of irreversibilities, losses, destructions, and inefficiencies; and (2) it identifies the potential for a system to be more efficient (more details and examples are provided in Ref. [1]). This particular contribution aims to introduce exergy in basic terms, explaining what it can do under AIDA (which refers to analysis, improvement, design, and assessment) as a new concept, highlighting its importance through examples, stating its contents through balance equations, discussing its linkages to the environment and sustainable development, and explaining the differences between exergy and energy (e.g., between the first law of thermodynamics and second law of thermodynamics). Therefore, the fundamental principles of thermodynamics starting with these two laws of thermodynamics are explained, while focusing on the fundamental aspects of exergy analysis, which are further treated and connected to practical systems and applications. Furthermore, it is focused on the differences between the two main concepts of thermodynamics, energy and exergy, which is then followed by explanations of the main categories of the thermodynamic systems. In this regard, case studies are presented to involve the applications of exergy analysis to integrated energy systems, including biomass, geothermal, and steam power systems. The operating conditions and state properties, such as temperature, pressure, and flow rate, are varied to investigate its effects on the energy and exergy efficiencies of the overall systems studied. Also, exergy destruction rates for individual components of an integrated system are studied and presented to determine the magnitudes and possibilities for performance improvement. Finally, it dwells on some future directions of exergy and its use through enhanced exergy methods and methodologies.

1.6.1.1

A New Concept: Analysis, Improvement, Design, and Assessment

When we deal with processes, systems, and applications, there are four key items, namely AIDA, which are needed to be addressed. In conjunction with this, author introduce a new AIDA concept to cover these four items. AIDA is a female name and originally comes from Arabic [2]. It has many meanings such as being happy as a distinguishing feature, and being a helper to collaborator. Since these four letters make this acronym suitable to dwell on exergy and its role, author is inclined to take the meaning of helper, and make a connection to exergy. Exergy is then introduced as a true thermodynamic tool for AIDA [3] as clearly illustrated in Fig. 1. Exergy analysis permits many of the shortcomings of energy analysis to be overcome. Exergy analysis, stemming from the second law of thermodynamics, is useful in identifying the causes, locations, and magnitudes of process inefficiencies. The exergy associated with an energy quantity is a quantitative assessment of its usefulness or quality. Moreover, exergy analysis acknowledges that although energy cannot be created or destroyed, it can be degraded in quality, eventually reaching a state in which it is in complete equilibrium with the surroundings and hence of no further use for performing tasks. Recently, exergy has been a prime tool under thermodynamics, and its use has been extended to economy under exergoeconomics (or thermoeconomics) by including cost accounting, environment under exergoenvironomics (or exergoenvironmental analysis) by including environmental impact accounting (assessment), and exergosustainability by including sustainability accounting (assessment).

Analysis

Assessment

Exergy

Design

Fig. 1 Exergy for analysis, improvement, design, and assessment (AIDA).

Improvement

Exergy

215

B

A

C

Fig. 2 Schematic presentation of the zeroth law of thermodynamics.

1.6.2

Thermodynamics Laws

As stated earlier in the chapter, thermodynamics is defined as the science of energy and exergy based on the prime justification presented. The word energy gives the feeling of the ability to cause changes. The word thermodynamics was essentially derived from Greek words therme, which means heat or thermal energy, and dunamis, which means power. The origin of the word thermodynamics is very descriptive of most of the current energy production processes where the thermal energy is converted to work or electrical energy. There are three thermodynamics laws that the science and the analysis methodology of thermodynamics is derived from. The thermodynamics laws are presented based on their number – zeroth, first, and second – not based on the historical dates at which they were introduced.

1.6.2.1

Zeroth Law of Thermodynamics

The zeroth law of thermodynamics states that if there are two bodies (A and B) that are in thermal equilibrium with another body (third body as C), as illustrated in Fig. 2, then these two bodies are also in thermal equilibrium with each another. Since such a conclusion may seem trivial and easy to reach, some may feel that it is not worth being one of the basic laws of thermodynamics. However, since the conclusion drawn from the zeroth law of thermodynamics cannot be concluded using the other two basic laws of thermodynamics, and it validates the temperature measurement, the zeroth law of thermodynamics was first formulated by RH Fowler in 1931. In reality, the zeroth law was formulated more than half a century after the first and the second laws of thermodynamics were formulated. However, since the zeroth law should have been formulated before the first and the second laws of thermodynamics, it was given the name zeroth law.

1.6.2.2

First Law of Thermodynamics

One of the main and fundamentally guiding laws of life is the conservation of energy principle, which means that energy is neither created nor destroyed, but just changes forms, for example, heat to work in thermal power plants and work to heat in refrigerators and heat pumps. It also confirms that the total amount of energy is always conserved and remains constant with no generation or destruction. Fig. 3 illustrates a nice example of the rolling rock to show the variation of total energy content in every step. Therefore, this example on the conservation of energy principle presents the change of energy kinetically and potentially throughout the process while having the total energy remain constant. As shown in Fig. 3, when the rock was on top of the hill and not moving, the energy that rock possessed was entirely potential energy at 10 units, making the total energy content 10 units only. Then, when the rock started to roll down the hill, the potential energy started to transform to kinetic energy with the assumption that there are no frictional losses including air and ground frictions. Through the rolling process, the total energy, which is in this case the sum of the potential energy and the kinetic energy, remained constant throughout the process. Since the reference point to the potential energy was the bottom of the hill, then the entire energy the rock has at the bottom of the hill is kinetic energy. Another example of the conservation of energy principle is when a person is eating more than the amount of energy s/he needs to do the activities that s/he does during the day. Then, the total energy in is higher than the energy out, which means that excess part of the energy entering the body is stored in the body. Storing energy in the human body can take many forms, but all of them will result in an increase in the human body mass (which leads to weight gain, maybe ending up with obesity). From this example, we can conclude that the energy conservation principle was applied, and no energy was created or destroyed. The change in the energy content of a system is equal to the difference between the energy getting into the system minus the energy getting out of the system. The first law of thermodynamics is the expression of the principle of energy conservation, and it considers energy as a thermodynamic property. From the statement of the first law of thermodynamics, we can analyze the energy interactions of an

216

Exergy

PE = 10 KE = 0

8 PE = 2 KE =

P E = 6 KE = 4 PE KE = 0 =1 0

P KE E = = 0 10

PE = 2 KE = 8

Fig. 3 Illustration of energy conservation concept through a rolling rock with a total of 10 units of energy.

energy device or an entire power plant since the first law of thermodynamics keeps track of the energy interactions and makes sure that no energy is created or destroyed, and the energy is just being converted from one form to another. The first law of thermodynamics helps in measuring the performance of energy devices and energy systems through the definition of what is referred to either the first law efficiency or the energy efficiency. However, the first law of thermodynamics has the disadvantage of not considering the quality of the energy it deals with, but rather only with its quantity. For example, if we consider the upper part of the water in the ocean, which is usually at the ambient temperature. Based on the first law of thermodynamics, it has a massive amount of energy; however, this massive amount of energy is useless since it is at the ambient temperature. It is because of this disadvantage that the first law has, that the second law of thermodynamics is necessary to account for irreversibilities, losses, inefficiencies, and destructions, respectively.

1.6.2.3

Second Law of Thermodynamics

Going back to the previous example that explains what the first law of thermodynamics lacks in terms of presenting the work potential or maximum amount of energy or available energy of a system or a medium. The key weakness of the first law of thermodynamics is that is does not account for irreversibilities, losses, inefficiencies, and destructions. Since all practical systems are irreversible and end up with irreversibilities, losses, inefficiencies, and destructions, there is a strong need to go beyond this law. This can only be done by considering the second law of thermodynamics as the prime tool. If one further elaborates on the issues, the following example is helpful. The first law of thermodynamics presents that the energy in the upper part of the water in the ocean has more energy than a combustion product that results from a complete combustion of 1 kg of natural gas. When presented in this way the disadvantages of the first law of thermodynamics appear clearly. To tackle this shortage in the first law of thermodynamics, the second law of thermodynamics needs to be included. This second law of thermodynamics considers both quantity and quality of energy, and it hence states that actual processes occur in the direction of decreasing quality of energy. Following the example that has been stated twice regarding the upper part of the water in the ocean since it is at the ambient temperature and pressure then its quality is quantified and it is zero since it is already at ambient conditions, which are considered dead state (or reference state). One more example may be presented to illustrate the direction of thermal energy (heat) from the high-temperature side to the low-temperature surroundings. For example, if a hot piece of pizza, as shown in Fig. 4, is left in the room, the pizza will cool down with time by losing its heat up until it reaches the surrounding temperature. It basically reaches the dead state temperature. However, if the cold piece of pizza remains in the room, its temperature will not increase since the energy travels in the direction of reduction of quality. Similar to the first law of thermodynamics, the second law of thermodynamics introduces a new thermodynamic property, which is called exergy. Many dwell on entropy, rather than exergy, under the shelter of the second law of thermodynamics, which may not suitable to make comparison between these two laws, or a comparison with energy. One cannot compare entropy with energy, but can compare energy and exergy since both properties have the same units and can compare quantities and qualities. One can go to energy efficiency via energy analysis while exergy is used to go to exergy efficiency. Therefore, exergy and energy concepts should be considered for comparison purposes for analysis and assessment. Of course, this does not deny the role of entropy, but instead emphasizes the limitation of entropy use.

Exergy

217

Losing thermal energy to the surrounding since energy travel in the direction of decreasing energy quality

Fig. 4 Presentation of the second law of thermodynamics in that the energy travels in the direction of decreasing the quality.

Second system Energy conversion percentage (energy efficiency) = 31% produces energy that 50% is electrical energy and the remaining is thermal energy at 80 °C

First system Energy conversion percentage (energy efficiency) = 30% produces only electrical energy

Losses

(A)

Losses

(B)

Fig. 5 Illustration of two energy conversion cases: (A) electricity-only generation with an efficiency of 30% and (B) cogeneration with electricity and heat at an efficiency of 31%.

An example for that is the two systems shown in Fig. 5. The system in Fig. 5(A) produces only electrical energy with an energy efficiency (energy conversion percentage) of 30%. The second system in Fig. 5(B) produces energy in which 50% of it is electrical energy and the remaining is thermal energy at a temperature of 801C, and has an energy conversion percentage of 31%. Based on the first law of thermodynamics only, a decision maker may consider building the second system and discarding the other. However this is a mistake since if the second law of thermodynamics is considered then the first system will have a higher performance, because the quality of electrical energy is much higher than that of thermal energy. It means that the quality of the energy produced by the cogeneration system is lower than that of the energy produced by the other system, and after considering the exergy point of view the first system should be built.

Exergy

218

1.6.3

Energy versus Exergy

Energy comes in many forms, ranging from kinetic to potential, from chemical to electrical, from fission to fusion, and from nuclear to magnetic. Thermodynamics plays a key role in the analysis of processes, systems, and devices in which energy transfers and energy transformations occur. The implications of thermodynamics are far-reaching and applications span the range of the human enterprise. Throughout our technological history, our ability to harness energy and use it for society's needs has improved. The industrial revolution was fueled by the discovery of how to exploit energy in a large scale and how to convert heat into work. Nature allows the conversion of work completely into heat, but heat cannot be entirely converted into work, and doing so requires a device (e.g., a cyclic engine). Engines attempt to optimize the conversion of heat to work. Note that most of our daily activities involve energy transfer and energy change. The human body is a familiar example of a biological system in which the chemical energy of food or body fat is transformed into other forms of energy such as heat and work. Engineering applications of energy processes are wide ranging and include power plants to generate electricity, engines to run automobiles and aircraft, refrigeration and air conditioning systems, etc. Many examples of such systems are discussed here. In a hydroelectric power system, the potential energy of water is converted into mechanical energy through the use of a hydraulic turbine. The mechanical energy is then converted into electric energy by an electric generator coupled to the shaft of the turbine. In a steam power-generating plant, chemical or nuclear energy is converted into thermal energy in a boiler or a reactor. The energy is imparted to water, which vaporizes into steam. The energy of the steam is used to drive a steam turbine, and the resulting mechanical energy is used to drive a generator to produce electric power. The steam leaving the turbine is then condensed, and the condensate is pumped back to the boiler to complete the cycle. Breeder reactors use uranium-235 as a fuel source and can produce some more fuel in the process. A solar power plant uses solar concentrators (parabolic or flat mirrors) to heat a working fluid in a receiver located on a tower, where a heated fluid expands in a turbogenerator as in a conventional power plant. In a spark-ignition internal combustion engine, chemical energy of fuel is converted into mechanical work. An air–fuel mixture is compressed and combustion is initiated by a spark device. The expansion of the combustion gases pushes against a piston, which results in the rotation of a crankshaft. Gas-turbine engines, commonly used for aircraft propulsion, convert the chemical energy of fuel into thermal energy that is used to run a gas turbine. The turbine is directly coupled to a compressor that supplies the air required for combustion. The exhaust gases, upon expanding in a nozzle, create thrust. For power generation, the turbine is coupled to an electric generator and drives both the compressor and the generator. In a liquid-fuel rocket, a fuel and an oxidizer are combined, and combustion gases expand in a nozzle creating a propulsive force (thrust) to propel the rocket. A typical nuclear rocket propulsion engine offers a higher specific impulse when compared to chemical rockets. A fuel cell converts chemical energy into electric energy directly making use of an ion exchange membrane. When a fuel such as hydrogen is ionized, it flows from the anode through the membrane toward the cathode. The released electrons at the anode flow through an external load. In a magnetohydrodynamic generator, electricity is produced by moving a high-temperature plasma through a magnetic field. A refrigeration system utilizes work supplied by an electric motor to transfer heat from a refrigerated space. Low-temperature boiling fluids such as ammonia and refrigerant-134a absorb thermal energy as they vaporize in the evaporator causing a cooling effect in the region being cooled. These are only some of the numerous engineering applications. Thermodynamics is relevant to a much wider range of processes and applications not only in engineering, but also in science. A good understanding of this topic is required to improve the design and performance of energy-transfer systems. There are many more examples and their elaborations are available elsewhere [1]. The exergy of a system is defined as the maximum shaft work that can be done by the composite of the system and a specified reference environment. The reference environment is assumed to be infinite, in equilibrium, and to enclose all other systems. Typically, the environment is specified by stating its temperature, pressure, and chemical composition. Exergy is not simply a thermodynamic property, but rather is a property of both a system and the reference environment. The term exergy comes from the Greek words ex and ergon, meaning from and work, respectively. The exergy of a system can be increased if exergy is input to it (i.e., work is done on it). The following are some terms found in the literature that are equivalent or nearly equivalent to exergy: available energy, essergy, utilizable energy, available energy, and availability. Exergy has the characteristic that it is conserved only when all processes occurring in a system and the environment are reversible. Exergy is destroyed whenever an irreversible process occurs. When an exergy analysis is performed on a plant such as a power station, a processing plant, or a refrigeration facility, the thermodynamic imperfections can be quantified as exergy destructions, which represent losses in energy quality or usefulness (e.g., wasted shaft work or wasted potential for the production of shaft work). Like energy, exergy can be transferred or transported across the boundary of a system. For each type of energy transfer or transport, there is a corresponding exergy transfer or transport. Exergy analysis takes into account the different thermodynamic values of different energy forms and quantities, e.g., work and heat. The exergy transfer associated with shaft work is equal to the shaft work. The exergy transfer associated with heat transfer, however, depends on the temperature at which it occurs in relation to the temperature of the environment. Some important characteristics of exergy are briefly described and illustrated as follows: [1]:

• •

A system in complete equilibrium with its environment does not have any exergy. No difference appears in temperature, pressure, concentration, etc. so there is no driving force for any process. The exergy of a system increases the more it deviates from the environment. For instance, a specified quantity of hot water has higher exergy content during the winter than on a hot summer day. A block of ice carries little exergy in winter while it can have significant exergy in summer.

Exergy

• • •

219

When energy loses its quality, exergy is destroyed. Exergy is the part of energy that is useful and therefore has economic value and is worth managing carefully. Exergy by definition depends not just on the state of a system or flow, but also on the state of the environment. Exergy efficiency is defined as a measure of approach to ideality (or reversibility). This is not necessarily true for energy efficiencies, which are often misleading.

After introducing both energy and exergy quantities, we can now look at the balance equations of the particular thermodynamic quantities. In this regard, a general balance for any quantity in a system may be written as: Inputs

Outputs ¼ Accumulation

ð1Þ

Here, input and output refer, respectively, to quantities entering and exiting through system boundaries. Generation and consumption refer respectively to quantities produced and consumed within the system. The generation term becomes an input term while the consumption is an output term. Accumulation refers to buildup (either positive or negative) of the quantity within the system. The versions of the general balance above may be written for mass, energy, entropy, and exergy. Mass and energy, being subject to conservation laws (neglecting nuclear reactions), can neither be generated nor consumed. Consequently, the general balance equation, as given in Eq. (1), for each of these quantities are written as Mass input Energy input

Mass output ¼ Mass accumulation Energy output ¼ Energy accumulation

ðEntropy input þ Entropy generationÞ Exergy input

Entropy output ¼ Entropy accumulation

ðExergy output þ Exergy consumptionÞ ¼ Exergy accumulation

ð2Þ ð3Þ ð4Þ ð5Þ

Here, entropy is created during a process due to irreversibilities, but cannot be consumed. However, exergy is consumed during the process due to destructions and losses as to be taken into consideration. These balances describe what is happening in a system between two instants of time. For a complete cyclic process, where the initial and final states of the system are identical, the accumulation terms in all balance equations are zero.

1.6.4

Types of Exergy

After presenting the general balance equations, it is important to consider the types of exergy for various types of thermodynamic systems. Two types of systems are normally considered: open (flow) and closed (nonflow). In general, open systems have mass, heat, and work interactions, and closed systems have heat and work interactions, but no mass exchanges. Mass flow into, heat transfer into, and work transfer out of a system are considered to be positive. Mathematical formulations of the principles of mass and energy conservation and entropy nonconservation can be written for any system, following the general physical interpretations in Eqs. (2)–(5). Nonflow exergy: The exergy Exnonflow of a closed system having a mass of m, or the nonflow exergy, is defined as Exnonflow ¼ Ex nonflow;ph þ Ex0 þ Ex kin þ Ex pot

ð6Þ

where Expot ¼ PE Exkin ¼ KE P Exo ¼ i ðmio mioo ÞNi Exnonflow;ph ¼ ðU U0 Þ þ P0 ðV

V0 Þ

T0 ðS

S0 Þ

where the system has a temperature T, pressure P, chemical potential mi for species i, entropy S, energy E, volume V, and number of moles Ni of species i. The system is within a conceptual environment in an equilibrium state with intensive properties To, Po, and mioo. The quantity mio denotes the value of m at the environmental state (i.e., at To and Po). The terms on the right side of Eq. (6) represent, respectively, physical, chemical, kinetic, and potential components of the nonflow exergy of the system. Exergy is a property of the system and conceptual environment, combining the extensive properties of the system with the intensive properties of the environment. Physical nonflow exergy is the maximum work obtainable from a system as it is brought to the environmental state (i.e., to thermal and mechanical equilibrium with the environment), and chemical nonflow exergy is the maximum work obtainable from a system as it is brought from the environmental state to the dead state (i.e., to complete equilibrium with the environment). Flow exergy. The exergy of a flowing stream of matter Exflow is the sum of nonflow exergy and the exergy associated with the flow work of the stream (with reference to Po), i.e., Exflow ¼ Ex nonflow þ ðP

P0 ÞV

ð7Þ

or Exflow is expressed in terms of physical, chemical, kinetic, and potential components as Exflow ¼ Ex flow;ph þ Ex0 þ Exkin þ Ex pot

ð8Þ

220

Exergy

where Ex o ¼

X

ðmio

mioo ÞNi

i

Ex flow;ph ¼ ðH

H0 Þ

T0 ðS

S0 Þ

Thermal Exergy: Consider a control mass, initially at the dead state, being heated or cooled at constant volume in an interaction with some other system. The heat transfer experienced by the control mass is Q. The flow of exergy associated with the heat transfer Q is denoted by Ex, and is written as Z f ð1 T 0 =T ÞδQ ð9Þ ExQ ¼ i

where δQ is an incremental heat transfer, and the integral is from the initial state (i) to the final state (f). This “thermal exergy” is the minimum work required by the combined system of the control mass and the environment in bringing the control mass to the final state from the dead state. If the temperature T of the control mass is constant, the thermal exergy transfer associated with a heat transfer is written as Ex Q ¼ ð1

T0 =T ÞQ ¼ tQ

ð10Þ

where t is called the “exergetic temperature factor.” Exergy of work. The exergy associated with shaft work ExW is by definition Wx . The exergy transfer associated with work done by a system due to volume change is the net usable work due to the volume change, and is denoted by WNET. Exergy of electricity. As for shaft work, the exergy associated with electricity is equal to the energy. Exergy destruction. For a process occurring in a system, the difference between the total exergy flows into and out of the system, less the exergy accumulation in the system, is the exergy consumption (destruction) Exd, expressible as Exd ¼ T0 Sgen

ð11Þ

which points out that exergy destruction is proportional to entropy creation, and is known as the Gouy–Stodola relation.

1.6.4.1

Reference Environment

Exergy is evaluated with respect to a reference environment, so the intensive properties of the reference environment partly determine the exergy of a stream or system. The reference environment is a condition where there is stable equilibrium, with all parts at rest relative to one another. No chemical reactions can occur between the environmental components. The reference environment acts as an infinite system, and is a sink and source for heat and materials. It experiences only internally reversible processes in which its intensive state remains unaltered (i.e., its temperature To, pressure Po, and the chemical potentials mioo for each of the i components present remain constant). The exergy of the reference environment is zero. The exergy of a stream or system is zero when it is in equilibrium with the reference environment. There are multiple reference environment models, and some of these models are listed and described as follows: (a) Natural environment subsystem models. One of the important models for the reference environment modeling is the natural environment subsystem type. These types of models simulate in a realistic manner the subsystems of the natural environment. Baehr and Schmidt [4] proposed a model that consists of saturated moist air and liquid water in phase equilibrium. Their model was later modified by Refs. [5,6], and their modification allowed sulfur-containing material to be analyzed. The following model presents the pressure and the temperature of the reference environment as shown in Table 1 as 251C and 1 atm, respectively; other properties of the model are that water (H2O), gypsum (CaSO4
2H2O), and limestone (CaCO3) condense at 251C and 1 atm. It is important to know that the stable configurations of C, O, and N are taken to be CO2, O2, and N2 as they are naturally available in the air. (b) Reference substance models. In this particular model, as a common approach, a “reference substance” is selected and assigned zero exergy for every chemical element. One such model in which the reference substances were selected as the most valueless substances found in abundance in the natural environment was proposed by Szargut [7]. The criterion for selecting such reference substances is consistent with the notion of simulating the natural environment, but is primarily economic in nature, and is vague and arbitrary with respect to the selection of reference substances. Part of this environment is the composition of moist air, including N2, O2, CO2, H2O, and the noble gases; gypsum (for sulfur); and limestone (for calcium). Another model in this class, in which reference substances are selected arbitrarily, was proposed by Sussman [8]. This model is not quite similar to the natural environment. Consequently absolute exergies evaluated with this model do not relate to the natural environment, and cannot be used rationally to evaluate efficiencies. Since exergy-consumption values are independent of the choice of reference substances, they can be rationally used in analyses. (c) Equilibrium models. The equilibrium model groups all the materials that are present in the atmosphere, oceans, and a layer of the crust of the earth in a pool together, and an equilibrium composition is calculated for a given temperature a proposed by Ahrendts [9]. However, in the equilibrium model the selection of the thickness of crust considered may be subjective, where the intention here is to include all materials accessible to technical processes. Ahrendts [9] considered in the equilibrium model the

Exergy

Table 1

221

A reference environment model

Temperature Pressure

To ¼298.15K Po ¼ 1 atm

Composition

(i) Atmospheric air saturated with H2O at To and Po, with the following composition: Air constituents Mole fraction 0.7567 N2 0.2035 O2 0.0303 H2O Ar 0.0091 0.0003 CO2 0.0001 H2 (ii) The following condensed phases at To and Po: Water (H2O) Limestone (CaCO3) Gypsum (CaSO4
2H2O)

Source: Adapted from Gaggioli RA, Petit PJ. Use the second law first. Chemtech 1977;7:496–506.

thicknesses varying from 1 to 1000 m, and a temperature of 251C. For the range of thicknesses that Ahrendts [9] considered, the model differed significantly from the natural environment model. The exergy values that were obtained by using these environments are significantly dependent on the thickness of the crust considered and represent the absolute maximum amount of work obtainable from a material. However, a technical process to obtain a work from a material is currently not available, which means that the Ahrendts [9] equilibrium model does not give meaningful exergy values for an actual process so that we can apply them to the analysis of these actual processes. (d) Constrained-equilibrium models. Ahrendts [9] modified his equilibrium environment model by proposing a new version of it. In his new proposed model the calculation of an equilibrium composition excludes the possibility of the formation of HNO3 and its compounds, meaning that all chemical reactions based on this new proposed model are formed in constrained equilibrium, and all other reactions are in unconstrained equilibrium. When a thickness of crust of 1 m and a temperature of 251C were used, the model appears to be similar to the natural environment. (e) Process-dependent models. A model that contains only components that participate in the process being examined in a stable equilibrium composition at the temperature and total pressure of the natural environment was proposed by Bosnjakovic [10]. His particular model appears to be dependent primarily upon the process examined, and is not general. Note that the exergies evaluated for a specific process-dependent model are relevant only to the process; they cannot rationally be compared with exergies evaluated for other process-dependent models.

1.6.4.2

Energy and Exergy Efficiencies

Efficiency has always been an important consideration in decision-making regarding resource utilization and performance assessment of systems and applications. Efficiencies are often evaluated as ratios of energy quantities, and are often used to assess and compare various systems. Power plants, heaters, refrigerators, and thermal storages, for example, are often compared based on energy efficiencies or energy-based measures of merit. There are two key efficiencies as energy efficiency, based on energy analysis (under the first law of thermodynamics) and exergy efficiency (under the second law of thermodynamics). However, energy efficiencies are often misleading in that they do not always provide a measure of how nearly the performance of a system approaches ideality. Further, the thermodynamic losses that occur within a system (i.e., those factors that cause performance to deviate from ideality) often are not accurately identified and assessed with energy analysis. The results of energy analysis can indicate the main inefficiencies to be within the wrong sections of the system, and a state of technological efficiency different than actually exists. This requires a realistic efficiency to be defined. This can only be done by exergy efficiency since exergy analysis permits many of the shortcomings of energy analysis to be overcome. It, by stemming from the second law of thermodynamics, is useful in identifying the causes, locations, and magnitudes of process inefficiencies. Efficiencies determined using ratios of exergy do provide a measure of an approach to an ideal. Exergy efficiencies are often more intuitively rational than energy efficiencies because efficiencies between 0% and 100% are always obtained. Measures that can be greater than 100% when energy is considered, such as coefficient of performance, normally are between 0% and 100% when exergy is considered. Energy (Zen) and exergy (Zex) efficiencies are often written for steady-state processes occurring in systems as

Zen ¼

Useful energy output Total energy input

ð12Þ

Exergy

222

Zex ¼

Useful exergy output Total exergy input

ð13Þ

It is finally more important that exergy efficiencies often give more illuminating insights into process performance than energy efficiencies because (1) they weigh energy flows according to their exergy contents, and (2) they separate inefficiencies into those associated with effluent losses and those due to irreversibilities. In general, exergy efficiencies ultimately provide a true measure of potential for improvement.

1.6.4.3

A Simple Procedure for Energy and Exergy Analyses

In energy and exergy analyses of any process, system, or application, there is a need to follow a kind of unified procedure for a common and understandable solution. This way the solution can easily be checked by the people in the area. It also helps to benchmark the approach or methodology. In this regard, a simple procedure for performing energy and exergy analyses involves the following steps (see Ref. [1] for details): • Subdivide the process under consideration into as many sections as desired, depending on the depth of detail and understanding desired from the analysis. • Perform conventional mass and energy balances on the process, and determine all basic quantities (e.g., work, heat) and properties (e.g., temperature, pressure). • Based on the nature of the process, the acceptable degree of analysis complexity and accuracy, and the questions for which answers are sought, select a reference environment model. • Evaluate energy and exergy values, relative to the selected reference environment model. • Perform exergy balances, including the determination of exergy consumptions. • Select efficiency definitions, depending on the measures of merit desired, and evaluate values for the efficiencies. • Interpret the results, and draw appropriate conclusions and recommendations, relating to such issues as design changes, retrofit plant modifications, etc.

1.6.5

Thermodynamic Systems

The following section of the chapter breaks down the energy systems into two main categories, which are closed systems and open systems. Then each category of thermodynamic system is broken into different types of thermodynamic devices, and each of these devices is analyzed using both main concepts of thermodynamics, energy and exergy.

1.6.5.1

Closed Systems

A thermodynamic system is referred to as a closed energy system if the material of the system exchanges energy with the surrounding in only two forms, thermal energy and work, and the mass of the system remains constant (no mass entered the system or left the system). Since the mass of the system remains constant, the closed systems are usually referred to as fixed mass systems. Any system that these conditions applies to is considered as a closed energy system, for example, the piece of pizza shown in Fig. 4 is not considered as a closed energy system, because while the pizza is getting colder and colder the pizza loses mass to the surroundings in the moisture that it loses (losing H2O). The energy and exergy analyses of this type of system is demonstrated in detail in the following section through the explanation of the rigid tank system, and the piston cylinder system.

1.6.5.1.1

Rigid tank

A rigid tank is considered as a category of closed energy system, since the mass of the system in the case of a rigid tank remains constant without having a mass entering or leaving the system. A rigid tank is referred to as any energy system that has rigid walls and no mass entering or leaving the system; an example is a room with zero ventilation, highly insulated walls, and fan that is running, as shown in Fig. 6. The fan introduces work energy to the system; since the walls are insulated (adiabatic), then the work introduced into the mass of the closed system increases the internal energy of the closed system. When the internal energy increases, the room air temperature increases, as shown in Fig. 6. This may seems like a contradictory conclusion since we usually use a fan to cool the room, and that is the case if there is a ventilation, and now the job of the fan is to keep circulating the air and the temperature of the air will feel like a lower temperature. The balance equations applied to a rigid tank system, which can also be a zero ventilated room as shown in Fig. 6, are written based on the rigid tank system shown in Fig. 7. Mass balance equation ðMBEÞ : m1 ¼ m2 ¼ constant

ð14Þ

Energy balance equation ðEBEÞ : m1 u1 þ Qin þ Win ¼ m2 u2 þ Qout þ Wout

ð15Þ

Entropy balance equation ðEnBEÞ : m1 s1 þ Qin =Ts;in þ Sgen ¼ m2 s2 þ Qout =Ts;out

ð16Þ

Exergy balance equation ðExBEÞ : m1 ex1 þ Qin ð1

To =Ts;in Þ þ Win ¼ m2 ex2 þ þQout ð1

To =Ts;out Þ þ Wout þ Exd

ð17Þ

Exergy

223

Fig. 6 Illustration of a closed energy system with an insulated room containing a fan, with no airflow crossing the boundary.

Boundary of the system Qin Wout

Mass, m state 1 to state 2 Q out

Win

Fig. 7 A rigid tank system, which is one of the two categories of closed energy systems.

Here, subscript o denotes properties at the reference environment conditions, subscript d refers to destruction, temperature subscript s refers to the temperature of the surface where heat transfer took place. Q is the thermal energy, ex is the specific exergy of both moving and stationary mass; however, for a moving mass and a stationary mass different equations are used to calculate the specific exergy. The specific exergy for a stationary mass is calculated as follows: ex ¼ ðu

uo Þ

To ðs

so Þ

ð18Þ

Here, the reference environment temperature should be in Kelvin or Rankine (based on the units of the remaining variables in the equation). However, for a moving mass, the specific exergy is calculated using a different expression than Eq. (18) and that will be presented while discussing the open energy systems and devices. Example 1: Consider a rigid tank filled with helium that is heated by a heat source at a temperature of 1001C. The rigid tank has a volume of 1 m3 and contains 1 kg of real gas helium. Calculate (a) both pressure and temperature of the helium in the rigid tank after 10 kJ of heat is transferred to the system while knowing that the system initially was at ambient temperature of 251C, and (b) the exergy destruction of the process of heating the helium in the rigid tank. Solution: The first step in solving an energy system or device in the point view of thermodynamics is writing the balance equations, and they are as follows: MBE : m1 ¼ m2 ¼ m ¼ constant EBE : m1 u1 þ Qin ¼ m2 u2 EnBE : m1 s1 þ Qin =Ts;in þ Sgen ¼ m2 s2 ExBE : m1 ex1 þ Qin ð1

To =Ts;in Þ ¼ m2 ex2 þ Ex d

224

Exergy

(a) The second step is to find the material in the tank properties and then calculate the missing or required information by the help of the balance equations. At a temperature and pressure of 251C and 1 atm the enthalpy, entropy, and exergy of the helium is as follows (note that the properties were found using the Engineering Equation Solver (EES) software): uo ¼ 934:2 kJ=kg so ¼ 27:98 kJ=ðkgKÞ ex o ¼ 0 kJ=kg The properties for state 1 of the helium in the rigid tank, which are based on the initial temperature and the specific volume of the rigid tank: u1 ¼ 934:2 kJ=kg s1 ¼ 24:21 kJ=ðkgKÞ ex 1 ¼ ðu1

uo Þ

To ðs1

so Þ ¼ 1123 kJ=kg

Then by substituting the values in the energy balance equation and with the help of the mass balance equation, and then by substituting the result of the mass balance equation into the energy balance equation, one can find the result as follows: m  u1 þ Qin ¼ m  u2 1  934:3 þ 10 ¼ 1  u2 u2 ¼ 944:3 kJ=kg Then since the tank is rigid, which means that the volume of the tank will not change plus the mass of the helium in the tank will remain constant, this leads to having a constant specific volume throughout the heating process. v1 ¼ v2 ¼ V=m ¼ 1 m3 =1 kg ¼ 1 m3 =kg From the two independent properties v2 and u2 the final pressure and temperature of the rigid tank are: T2 ¼ 28:211C and P2 ¼ 627:9 kPa (b) The exergy destruction is calculated by using the exergy balance equation. 1  1123 þ 10ð1

298:15=398:15Þ ¼ 1  1123 þ Ex d

Exd ¼ 4:962 kJ

1.6.5.1.2

Piston cylinder mechanism

The mechanical unit that is associated with one of the most frequently encountered types of mechanical work in practice (moving boundary work) is the piston cylinder device. During the operation of the piston cylinder mechanism, the inner surface of the piston moves backward and forward producing work that is referred to as boundary work or moving boundary work, while others refer to it as P  dV work for reasons that will be discussed later in the following subsection. An example of the applications of the piston cylinder mechanism is the internal combustion engine in most automotive vehicles, and an example of such device is shown in Fig. 8. The balance equations are written for the piston cylinder mechanism as follows. Fig. 9 shows a schematic diagram of a piston cylinder device:

ExBE : ðQin ð1

MBE : m1 ¼ m2 ¼ constant

ð19Þ

EBE : m1 u1 þ Qin ¼ m2 u2 þ Wout

ð20Þ

EnBE : ðQin =Ts Þ þ Sgen þ m1 s1 ¼ m2 s2

ð21Þ

ðTo =Ts ÞÞÞ

ðWout

Po ðV2

V1 ÞÞ

Ex d ¼ Ex2

Ex 1

ð22Þ

where u is internal energy, s is entropy, Ts is source temperature, T0 is the dead state (environment) temperature, Sgen is entropy generation, P0 is the dead state pressure, V is volume. Note that the exergy of a closed system is either positive or zero, and never becomes negative. Example 2: Consider a piston cylinder device, as shown in Fig. 9, which initially contains air at a pressure of 400 kPa. The air in the piston cylinder device receives 50 kJ of work per each kg of air in the device through the rotating paddle. The air in the device receives thermal energy (heat) coming from a source with a temperature of 1001C. The piston cylinder device operates at a constant temperature approximately at 171C. The air volume in the device increased to three times its original volume. Calculate (a) the boundary work produced by the mechanism and (b) the heat input from the source to the system, (c) the entropy generation, and (d) the exergy destruction.

Exergy

225

Spark generator Fuel

Exhausts

Combustion chamber

Fig. 8 Illustration of a piston cylinder mechanism as a closed system, which is commonly used in the internal combustion engines of vehicles.

Moving boundary

Wout

From initial state 1 to final state 2

Qin

Win = 50 kJ/kg Fig. 9 A schematic diagram of a piston cylinder mechanism as a closed system involving heat input Qin and boundary work output Wout,b.

• • •

Assumptions: Here are some assumptions: Air trapped inside the piston cylinder device is treated as an ideal gas. The changes in the kinetic and potential energies are neglected. The specific heat is assumed to be constant throughout the process. Solution: Analyzing the piston cylinder device using the balance equations, they are written as follows: MBE : m1 ¼ m2 ¼ constant EBE : m1 u1 þ Qin þ Win;paddle ¼ m2 u2 þ Wout;b EnBE : m1 s1 þ Qin =Ts þ Sgen ¼ m2 s2 ExBE : m1 ex1 þ ð1 ðTo =Ts ÞÞQin þ Win;paddle ¼ m2 ex2 þ Wout;b þ Exd

226

Exergy

(a) Boundary work. Since the temperature of the air inside the device remains constant through the process and the pressure remains constant due to the piston motion and production of boundary work then m1u1 ¼ m2u2, which means that the EBE reduces to qin þ win;paddle ¼ wout;b For the boundary work produced by the system, since it is operating at a constant temperature then the following equation is used to calculate the boundary work: wout;b ¼ RTlnðv2 =v1 Þ ¼ 0:287 kJ=ðkgKÞ  290 K  lnð3Þ ¼ 91:4 kJ=kg (b) The heat (thermal energy) added to the system is calculated as follows. Then substituting the boundary work in the EBE: qin þ 50 kJ=kg ¼ 91:4 kJ=kg qin ¼ 91:4 50 ¼ 41:4 kJ=kg (c) The entropy generation is calculated based on the entropy balance equation written at the beginning of the solution as the first step in solving energy systems. From the entropy balance equation: m1 s1 þ Qin =Ts þ Sgen ¼ m2 s2 By dividing the above equation by the mass of the system, which is constant since the system is a closed energy system, the division result is as follows: s1 þ qin =Ts þ sgen ¼ s2 5:274 þ 41:4=ð100 þ 273:15Þ þ sgen ¼ 6:417 sgen ¼ 1:032kJ=ðkg KÞ (d) The exergy destruction is calculated as follows. Based on the relationship between the exergy destruction and the entropy generation, it is utilized as follows: exd ¼ To sgen ¼ 298:15  1:032 ¼ 307:6 kJ=kg

1.6.5.2

Open Systems

This type of thermodynamic system differs from the closed system with a unique difference that there is mass flow crossing the system boundary. It is also important to look at the energy dimensions. When a thermodynamic system, such as an open system, exchanges energy with the surroundings not only through work and heat interactions but also with a third type of energy interactions, this third type is called flow energy. An open energy system is a system where mass of the system can change or remain constant, however, mass can enter, leave or enter and leave the controlled volume of the system. Since the mass can change in this type of energy system, then to define the system we need to choose the boundary of the system to contain a certain volume and that volume remains constant throughout the energy interactions. An example of an open energy system is the water tank shown in Fig. 10, where mass is entering the tank from an opening at the top and mass is leaving the tank from the bottom while the tank is heated to produce hot water. In the system shown in Fig. 10, since the system has the ability to receive in or deliver out energy in the form of mass, then it is an open system and the controlled volume is shown by the dashed line, which contains the inner volume of the tank. The balance equation applied to any open system such as the general open system in Fig. 11 is written as follows: _ out MBE : m _ in ¼ m

ð23Þ

_ in ¼ m _ out _ out hout þ W EBE : m _ in hin þ Q

ð24Þ

_ in =Ts Þ þ m _ in sin þ S_ gen ¼ m _ out sout EnBE : ðQ

ð25Þ

_ in ð1 ExBE : Q

_ out þ Ex _ d _ in exin ¼ m _ out ex out þ W ðTo =Ts ÞÞ þ m

ð26Þ

where specific exergy of a flowing fluid (i.e., flow exergy) is given by ex ¼ ðh

ho Þ

To ðs

so Þ

ð27Þ

Here the kinetic and the potential energy changes are neglected. Most control volumes encountered in practice such as turbines, compressors, heat exchangers, pipes, and ducts operate steadily, and thus they experience no changes in their mass contents as well as their volumes. In terms of exergy for an open steady-flow system, the rate of exergy entering a steady-flow system in all forms

Exergy

227

Mass entering the tank

Heater

Water tank

Mass leaving the tank

Fig. 10 Illustration of a water tank unit as an open system where there is mass flow in and mass flow out in addition to the heat input through a heat exchanger.

Boundary of the system

m in Q in Wout mout

Fig. 11 A schematic diagram of an open energy system where there is mass flow crossing the boundary in addition to heat and works transfers.

(heat, work, mass transfer) must be equal to the amount of exergy leaving plus the exergy destroyed. The following subsection of the chapter having different open energy systems with steady-flow operation modes are discusses and analyses systems through both laws of thermodynamics because most of the power plants’ and other real systems’ behaviors approximate a steady-flow device. The open energy systems or devices that are considered in this subsection are primarily nozzle, diffuser, fan, pump, compressor, turbine, mixing chamber, expansion valve, and heat exchanger.

1.6.5.2.1

Nozzles

Nozzles are open-type thermodynamic systems that convert the forms of energy between kinetic energy and pressure energy. Nozzles are typically recognized reversed diffusers, and so, their goal is the exact opposite to that of diffusers. A nozzle is considered an adiabatic device since the heat transfer that takes place while accelerating the fluid is negligible. Therefore, the isentropic (i.e., reversible and adiabatic) process serves as a suitable model for nozzles. These devices usually do not introduce any extra energy such as heat or work and they also do not extract work or heat out of the fluids that pass through them. What they do is just change the forms of energy that the fluid possesses and usually they are designed to be adiabatic. The use of the balance equations to assess the performance of nozzles and diffusers is presented through the coming examples. Example 3: Consider that an adiabatic nozzle, as shown in Fig. 12, has an accelerating steam entering with a pressure of 600 kPa, at a temperature of 500K and with a velocity of 120 m/s. The adiabatic nozzle has a cross-sectional area ratio of 2:1, which will result in an exit velocity of 380 m/s. Calculate (a) the exit temperature, (b) the exit pressure, and (c) the exergy efficiency of the process.

228

Exergy

V2 = 380 m/s

P1 = 600 kPa T1 = 500 K V1 = 120 m/s

Adiabatic nozzle Fig. 12 Schematic diagram of the adiabatic nozzle in Example 3.

Solution: The first step in solving a thermodynamics device is to write the four balance equations as follows: _ 2 ¼ constant MBE : m _1¼m     _ 2 h2 þ ðV22 Þ=2000 EBE : m _ 1 h1 þ ðV12 Þ=2000 ¼ m _ 2 s2 EnBE : m _ 1 s1 þ S_ gen ¼ m     V12 V2 _ d _ 2 ex 2 þ 2 ¼m þ Ex ExBE : m _ 1 ex 1 þ 2000 2000 (a) Exit temperature. The enthalpy of the air at the inlet of the nozzle is equal to 503.2 kJ/kg, and by using the EBE the enthalpy of air at the exit of the nozzle is as follows:     503:2 þ 1202 =2000 ¼ h2 þ 3802 =2000

h2 ¼ 438:0 kJ=kg(which has a corresponding temperature of 437K). (b) To calculate the exit pressure and since the air is considered as an ideal gas then the ideal equation for the first and the second stages of air through the nozzle are derived from the MBE as follows: _1 ¼m _ 2 -ðA1 V1 Þ=v1 ¼ ðA2 V2 Þ=v2 -ðA1 V1 Þ=ðRT1 =P1 Þ ¼ ðA2 V2 Þ=ðRT2 =P2 Þ m P2 ¼ ðA1 T2 V1 Þ=ðA2 T1 V2 Þ  P1 ¼ ð2Þ  ð437  120Þ=ð500  380Þ  600 ¼ 331 kPa (c) The exergy efficiency is calculated as follows: Zex:Nz ¼

_ recovered Ex ¼ _ expended h1 Ex

ðV22 Þ=2 ðV12 Þ=2 ¼ h2 To ðs1 s2 Þ þ V12 =2 503:2

3802 =2000 1202 =2000 ¼ 79:7% 438:0 298ð5:709 5:742Þ  1202 =2000

In summary, the results show that a nozzle with entering cross-sectional area twice the exiting cross-sectional area was able to increase the velocity of more than three times the inlet velocity.

1.6.5.2.2

Diffusers

Diffusers are considered steady-flow devices that increase the pressure of fluids by reducing their kinetic energy or in other words reducing the fluid moving velocity. These devices usually do not introduce any extra energy such as heat or work and they also do not extract work or heat out of the fluids that pass through them. What they do is just change forms of energy that the fluid possesses and usually they are designed to be adiabatic. The use of the balance equations to assess the performance of nozzles and diffusers is presented through the coming examples. Example 4: Consider that a gas flow of oxygen (O2) with a velocity of 270 m/s enters an adiabatic diffuser as shown in Fig. 13, with a pressure of 60 kPa and a temperature of 71C, and exits the diffuser at a pressure of 85 kPa and a temperature of 271C. Calculate (a) the exit velocity of the fluid, (b) the ratio of the inlet area to the exit area of the nozzle, and (c) the isentropic and exergy efficiencies of the process. Solution: The first step in solving a thermodynamics device is to write the four balance equations as listed below: _ 2 ¼ constant MBE : m _1¼m     _ 2 h2 þ ðV22 Þ=2000 EBE : m _ 1 h1 þ ðV12 Þ=2000 ¼ m _ 2 s2 EnBE : m _ 1 s1 þ S_ gen ¼ m     V12 V2 _ d _ 2 ex 2 þ 2 ¼m þ Ex ExBE : m _ 1 ex 1 þ 2000 2000

Exergy

P1 = 60 kPa T1 = 7°C V1 = 270 m/s

229

T2 = 27°C P2 = 85 kPa

Adiabatic diffuser Fig. 13 Schematic diagram of the adiabatic diffuser in Example 4.

(a) From the EBE the velocity at the exit of the diffuser is calculated as follows:       8956 þ 2702 =2000 ¼ 8939 þ V22 =2000 -V2 ¼ 190:8 m=s

(b) From the mass balance equation the ratio of the inlet area of the diffuser to the exit area of the diffuser is calculated as follows: _ 2 ¼ ðA1 V1 Þ=v1 ¼ ðA2 V2 Þ=v2 _1 ¼m m RA ¼ A1 =A2 ðV1 RA Þ=v1 ¼ V2 =v2 -RA ¼ ðv1 V2 Þ=ðv2 V1 Þ ¼ 0:935 (c) The isentropic and the exergy efficiencies are calculated as follows: The isentropic kinetic energy is calculated by recalculating part a in the example but with the second enthalpy being the isentropic enthalpy. Zisen;Df ¼

2 V2;isen KEexit;isen ¼ 53:8% ¼ 2 V2;actual KEexit;actual

_ recovered Þ=ðEx _ expended Þ ¼ ðh2 Zex:Nz ¼ ðEx

1.6.5.2.3

h1

298  ðs2

s1 ÞÞ=ðV12 =2000Þ ¼ 72:6%

Fans

Fans receive shaft work (mechanical work) to operate and transfer it to the fluid as a mechanical energy. Fans are considered open thermodynamic systems or devices, since there is mass flow coming in and leaving out their control volume. The analysis of these devices based on the first and the second law of thermodynamics is shown in the following example. The fan receives a mechanical work as shown in Fig. 14 and increases the velocity of air entering the fan from one side. Example 5: 20 W of electrical power is given to a fan to increase the velocity of 1.0 kg/s of air from zero to 6.3 m/s as shown in Fig. 15. Determine if that is possible, and if it is not possible calculate the required power to increase the velocity of air from zero to 6.3 m/s. Solution: One may be able to begin the solution with the balance equations as follows: _ out ¼ m _ MBE : m _ in ¼ m  2    2  _ in ¼ m _ in Vin _ out hout þ m _ out Vout =2000 þ W =2000 EBE : m _ in hin þ m _ out sout EnBE : m _ in sin þ S_ gen ¼ m ExBE : By substituting the MBE into the EBE and since the statement of the question did not mention any change in temperature and pressure of the air since all the energy entering the system is converted into kinetic energy then the enthalpies cancel each other:  2  =2000 1  ð02 =2000Þ þ 0:020 ¼ 1  Vout Vout ¼ 6:3 m=s

Since the final velocity of air is equal to what the statement of the question claimed then the fan can deliver 1.0 kg/s of air at a velocity of 6.3 m/s from stationary air.

1.6.5.2.4

Pumps

Pumps are considered mechanical devices that receive shaft work to operate and transfer the energy that they received in the form of work to the fluid as a mechanical energy. Since the mass can enter and leave their control volume of the pumps, the pumps

230

Exergy

Win

Fan Fig. 14 A schematic diagram of a mechanical device, the fan, where it receives mechanical shaft work; and as a result, it increases the velocity of the fluid.

1 kg/s of air

Vf = 6.3 m/s

20 W

Stagnant air

Fan Fig. 15 A schematic diagram of the fan for Example 5.

are then considered and analyzed as open thermodynamic systems. The analysis of such a device based on the first and the second law of thermodynamics is shown in the following example. The pump revives the shaft work and then by the help of the mechanical design of the pump, the supplied liquid is pumped to a higher pressure using the supplied shaft work as shown in Fig. 16. Example 6: The pressure of water at an ambient condition is increased to a high pressure of 900 kPa at the exit of a pump. The temperature of the water entering the pump is 151C, the water enters the pump through an opening of 1 cm in diameter, and the exit is 1.5 cm in diameter. If the mass flow rate of the water is 0.5 kg/s, calculate the velocity of the water at the inlet and the exit of the pump (Fig. 17).

Exergy

231

Discharge Pump

Win

In flow

Fig. 16 A schematic diagram of a pump, where the supplied liquid (in flow) is pressurized and discharged (discharge) using the supplied shaft work (W_ in Þ:

P2 = 900 kPa Pump

Win

P1 = 101 kPa T1 = 15°C Mass flow rate = 0.5 kg/s

Fig. 17 A pump that increases the pressure of water at an ambient condition is increased to a higher pressure of 900 kPa at the exit of the pump.

Solution: It is first necessary to write four balance equations as follows: _ out ¼ m _ MBE : m _ in ¼ m  2   2   _ in ¼ m _ in Vin =2000 þ W _ out hout þ m _ out Vout =2000 EBE : m _ in hin þ m _ out sout EnBE : m _ in sin þ S_ gen ¼ m  2    2   _ in ¼ m _ d _ in Vin =2000 þ W _ out exout þ m _ out Vout ExBE : m _ in ex in þ m =2000 þ Ex

From the mass balance equation, the velocity is calculated at the inlet when the specific volume is equal to 0.001001 m3/kg as follows: _  Vin ¼ m

vin 0:001001 ¼ 0:5  ¼ 6:37 m=s Ain p  0:012 =4 Ain Vin rin ¼ Aout Vout rout

However since water is considered as an incompressible substance then the previous equation reduces to the following: Ain Vin ¼ Aout Vout Vout ¼

1.6.5.2.5

    Ain Vin Din 2 0:01 2 ¼ 6:37  ¼ Vin ¼ 2:83 m=s Aout Dout 0:015

Turbines

Turbines are recognized as work-producing devices that are commonly employed in many power-generating systems and applications, ranging from steam Rankine cycles, which use steam turbines to hydro power plants, which use a hydro turbine. There are also closed or open types of air-standard Brayton cycles where gas turbines are utilized. Furthermore, there are some types of organic Rankine cycles where expanders are utilized as work-producing devices. The type of work produced by these turbines and expanders are essentially shaft work, unlike the type of boundary movement work produced by reciprocating type units. As the fluid is flowing through the turbine, the high-pressure and -temperature fluid passes around the blades of the turbine. The blades of the turbine have a cross-sectional profile similar to those of airplanes (airfoil profile). The high-pressure and -temperature fluid will flow around the blades, which will produce different pressures between the top and the bottom of the blade, which will have a net force acting in the direction from high-pressure side to the low-pressure side. The net force on each blade will rotate the hub of the blades, and that is how mechanical work is produced. The energy that will rotate the hub of the blades is extracted from the

232

Exergy

State 1 Steam turbine

Wout

State 2

Fig. 18 A schematic diagram of a turbine, showing the type of energy produced.

State 1

T1 = 625°C P1 = 12 MPa

Steam turbine

Wout

x2 = 0.95 P2 = 10 kPa State 2

Fig. 19 Schematic diagram of the steam turbine in Example 6.

high-pressure and -temperature fluid, which will result in reduction of the pressure and temperature of the flowing fluid. The turbines are considered open thermodynamic systems, since the mass can enter and leave the control volume of the turbines. Often turbines are treated as steady-flow devices, and they are analyzed based on that. The performance of the turbines is measured through both energy and exergy efficiencies, as illustrated in the following example. A schematic diagram of a steam turbine is shown in Fig. 18. Example 6: Consider an adiabatic steam turbine, as shown in Fig. 19, with the following inlet and exit states: P1 ¼ 12,000 kPa, T1 ¼6251C, P2 ¼ 10 kPa, x2 ¼0.95. Taking the dead-state temperature of steam as saturated liquid at 251C, determine the isentropic efficiency and exergy efficiency of the steam turbine. Solution: The balance equations are then written as follows: _2 MBE : m _1 ¼m _ out _ 2 h2 þ W EBE : m _ 1 h1 ¼ m _ 2 s2 EnBE : m _ 1 s1 þ S_ gen ¼ m _ out þ Ex _ d _ 2 ex 2 þ W ExBE : m _ 1 ex 1 ¼ m

Exergy

233

Using the EBE to calculate the specific power produced by the steam turbine as follows: h1 ¼ h2 þ wout wout ¼ 1206 kJ=kg Then using the ExBE the exergy destructed through the expansion process is calculated as follows: ex1 ¼ ex2 þ wout þ exd ex d ¼ 268:2 kJ=kg Then by using the exergy efficiency definitions these are the resulting efficiencies: Zisen;Tr ¼ 0:708 ¼ 70:8% Zex;Tr ¼

1.6.5.2.6

wout ¼ 0:742 ¼ 74:2% ex in

Compressors

Compressors are recognized as work-consuming devices and are commonly employed in many cycles and systems, such as gas turbine systems, refrigeration systems, heat pumps, etc. A compressor is used to increase the pressure of a gas, which means that it has a similar job as pumps and fans. A power (essentially work) is supplied to the compressor from an external source by a connected rotating shaft, which means that compressors are work-consuming devices as shown in Fig. 20. The thermal energy losses in the form of heat rejection through the walls of the compressor are neglected and usually compressors are assumed to be adiabatic. Since compressors have a constant volume and mass can enter and leave that constant or controlled volume then compressors are treated as open energy systems and they are analyzed based on that. The process of analyzing such devices is presented in detail in the next example. Example 7: Consider an air compressor, as shown in Fig. 21, that compresses air entering it at an atmospheric pressure and a temperature of 280 K, and the air exits the compressor at a pressure and a temperature of 600 kPa and 400 K. The air mass flow rate of the air entering the compressor is around 1 kg/s, and the compressor loses heat through its walls by an amount of 40% of the total work rate running the compressor. Calculate (a) the work consumed by the compressor and (b) both energy and exergy efficiencies of the compressor. Assumptions: Here, there are a few assumptions made as follows: • The changes in the kinetic and potential energies and exergies of the flowing air are neglected. • The air is treated as ideal gas.

High pressure gas

Win

Compressor

Low pressure gas

Fig. 20 A schematic diagram of the compressor receiving work from an external source to convert that work into increase in the pressure of a gas.

234

Exergy

P2 = 600 kPa T2 = 400 K

Win

Compressor

P1 = 101 kPa T1 = 280 K

Fig. 21 A schematic diagram of Example 7 compressor.

Table 2

The properties of the inlet and the exit streams

State point

P (kPa)

T (1C)

h (kJ/kg)

s (kJ/kgK)

ex (kJ/kg)

Reference state 1 2

101.3 100 600

25 7.0 127

298.6 280.5 401.4

5.696 5.637 5.482

NA 0.5579 166.6

Note: The reference environment temperature and pressure are taken as 251C and 1 atm.

Solution: The first step in solving this example is to write the balance equations as follows: _2 MBE : m _1¼m _ out _ in ¼ m _ 2 h2 þ Q EBE : m _ 1 h1 þ W _ _ 2 s2 EnBE : m _ 1 s1 þ Sgen ¼ m _ in ¼ m _ d þ Ex _ _ _ 2 ex2 þ Ex ExBE : m _ 1 ex 1 þ W

Qout

(a) The power consumed during the compression process needs to be calculated in this section. Using the EBE to calculate the power consumed by the compressor: _ out þ m _ in ¼ Q _ 2 h2 W

_ 1 h1 m

Here, the properties of the stream entering and leaving the compressor are obtained from air properties tables or using the EES, which contains a database of properties of most fluids through a large range of temperatures and pressures; the state properties are tabulated in Table 2. The work input needed for the compressor is calculated as _ in ¼ 0:4  W _ in þ m _ 2 h2 W _ in þ 1  401:4 _ in ¼ 0:4  W W

_ 1 h1 m 1  280:5

_ in ¼ 201:5 kW W (b) The energy and exergy efficiencies of the compressor are then calculated as follows:

Zen ¼

Zex ¼

ðm2 h2 m1 h1 Þ 1  ð401:4 280:5Þ ¼ ¼ 0:6 ¼ 60:0% _ out 201:5 W

ðm2 ex2 m1 ex 1 Þ 1  ð166:6 ð 0:5579ÞÞ ¼ ¼ 0:8295 ¼ 83:0% _ out 201:5 W

Exergy 1.6.5.2.7

235

Heat exchangers

In a closed-type heat exchanger, two fluid streams exchange heat without getting mixed. There are, of course, numerous types of heat exchangers: plate, finned, cross-flow, and shell and tube. One of the most commonly used heat exchangers is the shell and tube type heat exchanger, where the high-pressure fluid flows through the pipes that are surrounded by the lower pressure fluid. Heat exchangers are usually given different names that reflect the purpose of exchanging the heat between two fluids. Steam generators are treated as heat exchangers that exchange the heat between hot gases and water to produce high-pressure steam. The closed feedwater heater is employed as a heat exchanger that exchanges the heat between two different water streams in a Rankine cycle or a steam power plant. As a further example, boilers used in the Rankine cycles are treated as a heat exchanger, which has a similar function as the steam generator; however, the steam that exits the boiler usually is saturated vapor. The thermodynamic analyses and performance assessments of heat exchangers based on energy and exergy approaches are presented in the coming examples. Here, the heat exchangers are presented through two main cases in energy systems, such as steam power plant, water heating, cooling space using chilled water, plus others. These two cases are presented in the next examples, such that the first case is considered in Examples 8 and 9, and the second case is presented in Example 10. Example 8: Consider a closed-type heat exchanger, as shown in Fig. 22, where water at a pressure of 100 kPa and flowing at a mass flow rate of 1.0 kg/s is heated from a temperature of 301C to 801C by means of the heat input supplied from a heat source at a temperature of 1201C. Calculate (a) the amount of heat required to heat the water and (b) the energy and exergy efficiencies of the heating process and hence heat exchanger, respectively. Solution: One can always write the balance equation for both the energy system as a whole or for each component or a device, and they are written as follows: _ out ¼ m _ water MBE : m _ in ¼ m _ in ¼ m _ out hout EBE : m _ in hin þ Q _ in =Ts þ S_ gen ¼ m _ out sout EnBE : m _ in sin þ Q   _ in 1 To ¼ m _ d _ out exout þ Ex ExBE : m _ in ex in þ Q Ts The properties of water at the inlet and the outlet conditions are presented in Table 3. (a) Substituting the water properties at different streams in the EBE and the required thermal energy (heat) is then calculated as follows: _ in ¼ ð1  335:0Þ ð1  125:8Þ þ Q _ in ¼ 209:2 kW Q (b) The energy and exergy efficiencies of the heating process are calculated using the efficiency equations, which are derived from the energy and the exergy balance equations.

P1 = 100 kPa T1 = 80°C

P1 = 100 kPa T1 = 30°C Qin Fig. 22 A schematic diagram of the heat exchange process presented in Example 7.

236

Table 3

Exergy

The properties of the inlet and exit streams

State point

P (kPa)

T (1C)

h (kJ/kg)

s (kJ/kgK)

ex (kJ/kg)

Reference state In Out

101.3 101.3 101.3

25 30 80

104.8 125.8 335.0

0.367 0.437 1.075

0.000 0.1734 18.94

P1 = 100 kPa T1 = 99°C

P1 = 100 kPa T1 = 35°C Qout

Fig. 23 A schematic diagram of the heat exchange process discussed in Example 8.

The energy efficiency equation derived from the EBE is obtained as follows: _ out hout Zen ¼ ðm Zen ¼ ð1:0  ð335:0

_ in _ in hin Þ=Q m

125:8ÞÞ=209:2 ¼ 1 ¼ 100%

As we can see the energy efficiency is 100%, which will give no indication if it was compared to a similar process, and that is why considering the quality of the energy will give us another important measure for the performance of the energy processes and devices. The exergy efficiency equation derived from the ExBE is written as follows: _ _ _ out ex out m _ in ex in Þ=Ex Zex ¼ ðm Qin   298 _ _ ¼ 1 Ex  209:2 ¼ 50:55 kW Qin 120þ273:15

Zex ¼ ð1:0  ð18:94

0:1734ÞÞ=ð50:55Þ ¼ 0:371 ¼ 37:1%

Example 9: This time consider a closed-type heat exchanger, as shown in Fig. 23, where water at an ambient pressure of 100 kPa and a temperature of 991C is used to maintain the temperature of a building at 301C; the water exits the building heating system at a temperature of 351C. Calculate (a) the amount of heat extracted from hot water for the building, and (b) both energy and exergy efficiencies of the heat exchanging process. Solution: It is always necessary to write the balance equations for the entire system and/or its components. Here, the balance equations are written for the heat exchanger shown in Fig. 23 as follows: _ out ¼ m _ water MBE : m _ in ¼ m _ out _ out hout þ Q EBE : m _ in hin ¼ m _ out =Ts _ out sout þ Q EnBE : m _ in sin þ S_ gen ¼ m  _ out 1 _ dþQ _ out ex out þ Ex ExBE : m _ in exin ¼ m

To Ts



The properties of water at the inlet and the outlet conditions are presented in Table 4.

Exergy

Table 4

237

The properties of the incoming and the exiting streams

State point

P (kPa)

T (1C)

h (kJ/kg)

s (kJ/kgK)

ex (kJ/kg)

Reference state In Out

101.3 101.3 101.3

25 99 35

104.8 414.8 146.7

0.367 1.296 0.505

0.000 33.13 0.686

Heat exchanger

4 T4=200°C

3 T4=20°C

2 T2=300°C P2=150 kPa

1 T1=400°C P1=150 kPa

Fig. 24 A schematic diagram of the heat exchanger used to produce steam.

(a) Substituting the water properties at different streams in the EBE, amount of heat required for the building is then calculated as follows: _ out ð1  414:8Þ ¼ ð1  146:7Þ þ Q _ out ¼ 268:2 kW Q (b) The energy and exergy efficiencies of the heating process are calculated using the efficiency equations, which are derived through energy and exergy balance equations. The energy efficiency equation is derived from the EBE as follows: _ out =ðm _ in hin Zen ¼ Q Zen ¼ 268:2=ð1:0  ð414:8

_ out hout Þ m 146:7ÞÞ ¼ 1 ¼ 100%

As one can see the energy efficiency is 100%, which does not represent what is actually happening thermodynamically and does not account for irreversibilities or exergy destructions. This impractical situation makes it a requirement to go one step ahead to include exergy analysis and exergy efficiency assessment to reflect the reality. In this regard, the exergy efficiency equation is derived from the ExBE as follows: _ _ =ðm _ in exin m _ out ex out Þ Zex ¼ Ex Qout     T 298 o _ out ¼ 1 _ _ ¼ 1  268:2 ¼ 4:424 kW Ex Q Qout Ts 30 þ 273:15 Zex ¼ ð4:424Þ=ð33:13 0:686Þ ¼ 0:136 ¼ 13:6% Example 10: The hot exhaust gases leaving a gas turbine at 4001C and 150 kPa at a rate of 0.8 kg/s are to be used to produce saturated steam at 2001C in an insulated heat exchanger as shown in Fig. 24. Water at an ambient temperature of 201C enters the heat exchanger and the exhaust gases leave the heat exchanger at 3501C. Determine the rate of steam production, the rate of exergy destruction in the heat exchanger, and the exergy efficiency of the heat exchanger.

238

Exergy

Solution: It is firstly necessary to write the thermodynamic balance equations for mass, energy, entropy, and exergy as follows: _2 MBE : m _1 ¼m _3 ¼m _4 m _ 3 h3 ¼ m _ 2 h2 þ m _ 4 h4 EBE : m _ 1 h1 þ m _ 3 s3 þ S_ gen ¼ m _ 2 s2 þ m _ 4 s4 EnBE : m _ 1 s1 þ m _ d _ 3 ex 3 ¼ m _ 2 ex2 þ m _ 4 ex 4 þ Ex ExBE : m _ 1 ex1 þ m

The properties of water are obtained from the thermodynamic steam tables to be T3 ¼ 201C; liquid-h3 ¼ 83:91 kJ=kg; s3 ¼ 0:29649 kJ=kgK T4 ¼ 2001C; saturated vapor-h4 ¼ 2792:0 kJ=kg; s4 ¼ 6:4302 kJ=kgK An energy balance on the heat exchanger gives the rate of steam production: _ a h1 þ m _ w h3 ¼ m _ a h2 þ m _ w h4 m _ a cp ðT1 m

_ w ðh4 T2 Þ ¼ m

ð0:8 kg=sÞð1:063 kJ=kg1CÞð400

h3 Þ

_ w ð2792:0 350Þ1C ¼ m

83:91ÞkJ=kg

_ w ¼ 0:01570 kg=s m The specific exergy changes of air and water streams as they flow in the heat exchanger are Dex a ¼ cp ðT2 Dex w ¼ ðh4

h3 Þ

T1 Þ

To ðs2

To ðs4

s1 Þ ¼ ð1:063 kJ=ðkg KÞÞð 50KÞ

s3 Þ ¼ ð2792:0

83:91Þ kJ=kg

ð293K  0:08206 kJ=ðkg KÞÞ ¼

ð293K  ð6:4302

29:106 kJ=kg

0:29649Þ kJ=ðkg KÞ ¼ 910:913 kJ=kg

The exergy destruction is determined from an exergy balance as _ a ex1 þ m _ w ex3 Þ ðm

_ a ex2 þ m _ w ex4 Þ ðm

_ d¼0 Ex

Rearranging and substituting, _ d¼m _ a Dex a þ m _ w Dexw ¼ ð0:8 kg=s  29:106 kJ=kgÞ þ ð0:01570 kg=s  910:913 kJ=kgÞ ¼ 8:98 kW Ex The exergy efficiency for a heat exchanger may be defined as the exergy increase of the cold fluid divided by the exergy decrease of the hot fluid. That is, Zex;HX ¼

1.6.5.2.8

_ w Dex w m ð0:01570 kg=s  910:913 kJ=kgÞ ¼ 0:614 ¼ 61:4% ¼ _ a Dex a m ð0:8 kg=s  29:106 kJ=kgÞ

Mixing chambers

Mixing chambers are open-type thermodynamic (heat exchanging) systems, which are used to mix more than one stream of fluids. There are many names for the mixing chamber and these names usually refer to the specific functions that they perform, such as open feedwater heater. The open feedwater heater is a mixing chamber that mixes at least two streams of water at different temperatures, for example in a power plant to increase the quality of energy of one of the streams that entered the mixing chamber. Mixing chambers are direct contact heat exchangers where instead of just exchanging thermal energy in the heat exchanger, they are mixed together. A schematic diagram of a mixing chamber mixing two fluids (1 and 2) to produce a mixture of these two fluids is shown in Fig. 25. Note that a mixing chamber can even mix more than two fluid streams. However, the two streams that appear in Fig. 25 are just an example for this kind of process. Analyzing and measuring the performance of such systems is done through the application of the first and the second laws of thermodynamics; this is presented in the next example. Example 11: Open feedwater heater Consider a real open feedwater heater, as shown in Fig. 26, as taken from a single reheat regenerative cycle for power production associated with a thermal power plant that receives water from the low-pressure turbine with an enthalpy of 2601.3 kJ/ kg and mass flow rate of 46.6 kg/s. The mixing chamber mixes that water with another stream with an enthalpy of 222.3 kJ/kg and mass flow rate 1103.5 kg/s. Calculate the enthalpy of the exiting water.

Exergy

239

Fluid 1

The resulting mix of fluid 1 and fluid 2

Fluid 2 Fig. 25 A schematic diagram of mixing two fluids 1 and 2 to produce a resulting mixture of the two fluids.

Open feed water heater h = 222.3 kJ/kg Mass flow rate = 1103.5 kg/s

h = 2601.3 kJ/kg Mass flow rate = 46.6 kg/s

Fig. 26 A schematic diagram of the mixing chamber discussed in Example 10.

Solution: Let us write all four balance equations for mass, energy, entropy, and exergy as follows: _2 ¼m _3 MBE : m _1þm _ 2 h2 ¼ m _ 3 h3 EBE : m _ 1 h1 þ m _ 2 s2 þ S_ gen ¼ m _ 3 s3 EnBE : m _ 1 s1 þ m _ d _ 2 ex2 ¼ m _ 3 ex 3 þ Ex ExBE : m _ 1 ex 1 þ m Here, 1 and 2 are for the two streams entering the open feedwater heater (mixing chamber), and 3 is the stream exiting the mixing chamber. Based on the MBE, the mass of the water stream exiting the mixing chamber while following the assumption made earlier in the derivation of the mass balance equation with no leaks in the system is calculated as follows: 44:6 þ 1103:5 ¼ 1148:1 kg=s ð46:6  2601:3Þ þ ð1103:5  222:3Þ ¼ 1148:1  h3 h3 ¼ 12217621=38270 ¼ 319:2 kJ=kg

1.6.5.2.9

Expansion valve

Expansion (throttling) valve is recognized as a device that reduces the pressure of a flowing fluid without producing or consuming energy. One of the applications of the expansion valves is in refrigeration cycles, where the liquid refrigerant is throttled by the use of the expansion valve. The expansion valve reduces the pressure of the liquid. It may also result in converting part of the liquid refrigerant into vapor, however, since there is no energy entering the fluid or leaving it and the formation of the vapor requires

240

Exergy

State 1 at T1 and P1

State 2 at T2 and P2

Fig. 27 A schematic diagram of the expansion valve.

thermal energy. The source of this thermal energy comes from the liquid phase of the fluid, which will result in reducing the temperature of the refrigerant. Example 12: Consider a refrigeration system using a throttling (expansion) valve for propane as shown in Fig. 27. Here, the propane enters the pipe at a pressure of 11 bar and a temperature of 101C. The expansion valve reduces the pressure of the propane to 3 bar. Calculate the exit temperature of propane and the exergy efficiency of the throttling process. Solution: It is necessary to write the mass, energy, entropy, and exergy balance equations for this expansion valve: _ 2 ¼ constant MBE : m _1¼m _ 2 ðh2 Þ EBE : m _ 1 ðh1 Þ ¼ m _ _ 2 s2 EnBE : m _ 1 s1 þ Sgen ¼ m _ d _ 2 ex2 þ Ex ExBE : m _ 1 ex 1 ¼ m (a) Using the EBE one can calculate the enthalpy at the exit of the throttling device: h1 ¼ h2 ¼ 225:7 kJ=kg And then from the pressure and the enthalpy of the propane, the quality of the propane is calculated at the exiting propane with a quality of 0.154, and the final temperature is the saturation temperature of propane at 3 bar and hence it becomes 14.181C. (b) The exergy efficiency of the throttling process is calculated as follows: Zex;ThV ¼

1.6.5.3

_ out _ 2 114 Ex Ex ¼ 96% ¼ ¼ _Exin _ 1 118:8 Ex

Some Thermodynamic Cycles

In the previous section, the focus was on the units or components or devices as thermodynamic systems. Here, the focus is now on power, refrigeration, and heat pump cycles. Each of these cycles is treated as a thermodynamic system as a whole. As a common practice for thermodynamic analysis, one can write the balance equations for mass, energy, entropy, and exergy for the entire system and take each component of the entire cycle to write all four of these balance equations separately. Some of these components may be treated as closed and some may be open systems. The key point is that the four balance equations are written accordingly by considering all inputs and outputs as they cross the system boundary and that it is necessary to consider entropy generation absolutely as an input term and exergy destruction as an output term. Of course, these will then balance mass, energy, entropy, and exergy contents and flows accordingly. In order to illustrate the analyses and performance assessments of these three actual cycles, namely Rankine cycle, refrigerator, and heat pump, there are three examples presented in the following sections.

1.6.5.3.1

Steam Rankine cycle

Steam Rankine cycle is known as one of the main power-generating cycles, which consists of four key devices, namely a boiler (heat exchanger), a steam turbine, a condenser (heat exchanger), and a pump, as illustrated in Fig. 28. This cycle needs heat input for the boiler either by burning fossil fuels, such as oil, coal, and natural gas, or by obtaining the necessary heat from renewable energy sources, such as solar, geothermal, and biomass. There may be other options to obtain the heat from nuclear reactors and various industrial processes to drive such cycles for power production. Let us look at the operating principle of such a steam Rankine cycle: saturated liquid water is pumped through the boiler where it receives the heat to reach the desired temperature to be superheated vapor before moving into the steam turbine to get expanded for mechanical work production, and the steam, after losing its energy with reduced pressure, flows into a condenser where it is cooled down to change its phase to saturated water at the condenser temperature before getting into the pump where it is again pumped through the boiler. This is a closed cycle, and the mass flow rate of steam/water remains constant. The turbine work is used to rotate the shaft of the electric generator to produce electricity. Example 13 is presented to show how to thermodynamically analyze such an actual steam Rankine cycle for power generation through energy and exergy approaches. Example 13: In an actual Rankine cycle, as shown in Fig. 29, with an inlet pressure to the turbine of 7 MPa and a temperature of 5001C, the turbine exit pressure is 100 kPa. The isentropic efficiency of the turbine is 94%. The pressure losses in the pump,

Exergy

241

Qin

Boiler

Steam turbine

Pump Coupling

Wout

Win

Condenser

Qout Fig. 28 A schematic diagram of a simple Rankine cycle, showing the correct location of the components and the correct direction of energy and mass flows.

condenser, and boiler are considered negligible. If the temperature of the water leaving the condenser is 501C and the mass flow rate is 20 kg/s of water in the cycle, calculate (a) heat input rate to the boiler, (b) the net work rate produced, and (c) the energy and exergy efficiencies of the cycle. Solution: Writing all mass, energy, entropy, and exergy balance equations is the first step in calculating the vapor fraction and the exergy efficiency of the throttling process. For condenser: _ out MBE : m _ in ¼ m _ cond _ out hout þ Q EBE : m _ in hin ¼ m _ _ cond =Tcond _ out sout þ Q EnBE : m _ in sin þ Sgen;cond ¼ m _ _ _ d;cond _ out ex out þ Ex ExBE : m _ in ex in ¼ m þ Ex Qcond

For pump: _ out MBE : m _ in ¼ m _ p¼m _ out hout EBE : m _ in hin þ W _ out sout EnBE : m _ in sin þ S_ gen;p ¼ m _ p¼m _ d;p _ out exout þ Ex ExBE : m _ in exin þ W For boiler: _ out MBE : m _ in ¼ m _ boiler ¼ m _ out hout EBE : m _ in hin þ Q _ _ boiler =Tboiler ¼ m _ out sout EnBE : m _ in sin þ Sgen;boiler þ Q _ _ _ d;boiler _ out ex out þ Ex ExBE : m _ in exin þ Ex ¼m Qboiler

242

Exergy

Qin

P = 7.0 MPa T = 500°C Boiler

Steam turbine

Pump

Isentropic efficiency is 94%

Coupling

Win Wout Condenser P = 100 kPa

T = 50°C Mass flow rate 20 kg/s

Qout Fig. 29 A schematic diagram of the actual Rankine cycle discussed in Example 13.

For steam turbine: _ out MBE : m _ in ¼ m _ st _ out hout þ W EBE : m _ in hin ¼ m _ _ out sout EnBE : m _ in sin þ Sgen;st ¼ m _ d;st þ W _ st _ out exout þ Ex ExBE : m _ in exin ¼ m (a) Note that the balance equations listed above can be solved by using EES. Here, the pump work is shaft work and defined as vDP where v is the specific volume of the incoming saturated water and DP is the pressure difference between outlet and inlet pressures. This equation is used as follows to find the specific pump work:   wp ¼ vin;p Pout;p Pin;p ¼ 0:001012  ð7000 100Þ ¼ 6:984 kJ=kg Then : wp ¼ hout;p

hin;p -6:984 ¼ hout;p

209:4

hout;p ¼ 216:4 kJ=kg qboiler ¼ hout;boiler

hin;boiler ¼ 3411

216:4 ¼ 3194 kJ=kg

_ boiler ¼ m _  qboiler ¼ 20  3194 ¼ 63; 883 kW Q (b) In the cycle there is a pump that consumes work rate, and there is a steam turbine that produces mechanical work rate. Both are needed to find the net work output, which will be defined as turbine work minus pump work as written below: wst;is ¼ hin;st

hout;st;is ¼ 3422

2466 ¼ 944:5

wst;a ¼ wst;is =Zis ¼ 944:5=0:94 ¼ 1005 kJ=kg

Exergy

243

Qout

TH Condenser

QH Win Compressor

Expansion valve

Refrigerator

R Win QL TL

Boiler Qin

(A)

(B)

Fig. 30 Illustrations of (A) a schematic diagram of a simple mechanical vapor-compression refrigeration cycle and (B) directions of energy flows.

wnet ¼ wst;a wp ¼ 1005 6:984 ¼ 944:5 kJ=kg _ net ¼ m _  wnet ¼ 20  944:5 ¼ 19; 957 kW W (c) Both energy and the exergy efficiency equations for the overall steam Rankine cycle are calculated as follows: _ boiler ¼ 19; 957=63; 883 ¼ 31:24% _ net =Q Zen;RC ¼ W _ net =Ex _ _ Zex;RC ¼ W Qboiler ¼ 19; 957=42; 076 ¼ 47:43% Note that the exergy efficiency calculated above is higher than the corresponding energy efficiency (the so-called thermal efficiency) due to the fact that the exergy input is much less than the corresponding energy input, which results in a higher efficiency with the same actual work output.

1.6.5.3.2

Refrigerators

From the second law of thermodynamics one knows that energy transfer takes place in the direction of reduction of energy quality. Since the quality of thermal energy (so-called heat) is specified by the temperature at which the thermal energy transfer appears to be through heat from high temperature to low temperature mediums. In the case of refrigeration, it is necessary to increase the energy quality of the stream from low temperature to high temperature, which can only be done without violating the second law of thermodynamics. This clearly indicates that it is not possible to produce a cooling effect without supplying external energy input in the form of work for vapor-compression refrigeration systems and in the form of heat for absorption refrigeration systems. A basic vaporcompression refrigeration system is schematically illustrated in Fig. 30, which consists of four main components, namely evaporator, compressor, condenser, and expansion valve (may als be called throttling valve). Let us look at the operating principle of such a vaporcompression refrigeration cycle: saturated or slightly superheated refrigerant enters a compressor where it is compressed to be superheated refrigerant vapor at high temperature and high pressure, and it then enters a condenser to be cooled down to the desired condensing temperature at a constant pressure to change its phase from superheated vapor to saturated refrigerant or slightly subcooled refrigerant before moving into an expansion valve to get isenthalpic process, and then into an evaporator where the necessary cooling effect is provided by the refrigerant by absorbing the heat. The refrigerant, after absorbing the heat, becomes again saturated vapor or slightly superheated vapor before moving into the compressor. This is a closed cycle, and the mass flow rate of refrigerant remains constant throughout. The evaporator is used as an ultimate unit to produce cooling effect in a room or a cold store or a fridge. Example 14 is presented to illustrate how to thermodynamically analyze such an actual mechanical vapor-compression refrigeration cycle for cooling purposes through energy and exergy approaches. It is important to note here that the performance of

244

Exergy

TH = 25°C Qout = 2600 kJ/h

Condenser Win Compressor

Expansion valve

Boiler Qin = 1500 kJ/h TL = −15°C Fig. 31 Graphical illustration of the actual refrigeration system considered in Example 14.

refrigeration cannot be defined in the same way as for a steam Rankine cycle since the efficiency becomes more than 100%, which is practically impossible, and nonsense. This was a key motivation to find a criterion to measure the performance of the cycle, which is known as the coefficient of performance. This is defined as the ratio of evaporator cooling load divided by the compressor work input. Example 14: A refrigeration cycle, as shown in Fig. 31, is used to keep a food department at 151C in an environment at 251C. The total heat gain from the food compartment is estimated to be 1500 kJ/h, and the heat rejection in the condenser is 2600 kJ/h. Determine (a) the power input to the compressor in kW, (b) the COP of the refrigerator, and (c) the minimum power input to the compressor if a reversible refrigerator was used. Solution: (a) The power input is determined from an energy balance on the refrigeration cycle: _H _ in ¼ Q W

_ L ¼ 2600 Q

 1500 ¼ 1100 kJ=h

 1 kW ¼ 0:306 kW 3600 kJ=h

(b) The COP of the refrigerator is COPen;R ¼

_L Q ð1500=3600Þ kW ¼ 1:36 ¼ _ 0:306 kW W in

(c) The maximum (ideal) COP of the cycle, based on the Carnot approach, and the corresponding minimum power input are COPen;R;rev ¼

_ min ¼ W

1 QH =QL

1

¼

1 TH =TL

1

¼

258 ¼ 6:45 298 258

QL 1500=3600 ¼ 0:065 kW ¼ COPen;R;rev 6:45

Exergy

245

TH = 22°C Q out

T3 = 46.3°C Saturated liquid

2

3 Condenser

Isentropic efficiency 80% Expansion valve

Win

Compressor

1 P1 = 140 kPa Volume flow rate = 375 L/min

4

Boiler Qin TL = −10°C Fig. 32 Graphical illustration of an actual refrigeration unit considered in Example 15.

Example 15: A refrigeration system, as illustrated in Fig. 32, using R-134a as the refrigerant is employed to keep a space at 101C by rejecting heat to ambient air at 221C. R-134a enters the compressor at 140 kPa at a flow rate of 375 l/min as a saturated vapor. The isentropic efficiency of the compressor is 80%. The refrigerant then leaves the condenser at 46.31C as a saturated liquid. Determine (a) the rate of cooling provided by the evaporator, (b) the cycle COP, (c) the exergy destruction in each component of the cycle, (d) the second-law efficiency of the cycle and exergy efficiency, and (e) the total exergy destruction in the cycle. Solution: The first step in solving this actual refrigeration problem is to write the necessary four balance equations for each component of the cycle. For compressor: _2 MBE : m _1¼m _ comp ¼ m _ 2 h2 EBE : m _ 1 h1 þ W _ 2 s2 EnBE : m _ 1 s1 þ S_ gen;1-2 ¼ m _ comp ¼ m _ 2 ex 2 þ E_ d;1-2 ExBE : m _ 1 ex1 þ W For condenser:

_3 MBE : m _2¼m _ cond _ 3 h3 þ Q EBE : m _ 2 h2 ¼ m _ cond Q Ts þ E_ d;2-3

_ 3 s3 þ EnBE : m _ 2 s2 þ S_ gen;2-3 ¼ m _ _ _ 3 ex3 þ Ex ExBE : m _ 2 ex 2 ¼ m Qcond

246

Exergy

For expansion valve: _4 MBE : m _3¼m _ 4 h4 EBE : m _ 3 h3 ¼ m _ 4 s4 EnBE : m _ 3 s3 þ S_ gen;3-4 ¼ m _ 4 ex4 þ E_ d;3-4 ExBE : m _ 3 ex 3 ¼ m For evaporator: _1 MBE : m _4 ¼m _ boiler ¼ m _ 1 h1 EBE : m _ 4 h4 þ Q _ Q _ 1 s1 EnBE : m _ 4 s4 þ S_ gen;4-1 þ boiler ¼ m Ts _ _ _ 1 ex 1 þ E_ d;4-1 ExBE : m _ 4 ex 4 þ Ex Qboiler ¼ m The overall cycle balance equation MBE : m ¼ constant _ boiler þ W _ cond _ comp ¼ Q EBE : Q _ boiler =Tboiler þ S_ gen;3-4 ¼ Q _ cond =Tcond EnBE : Q _ _ _ ExBE : Ex _ þ W comp ¼ Exd;ov QL

(a) The properties of R-134a are (R-134a tables) P1 ¼ 140 kPa x1 ¼ 1

) h1 ¼ 239:17 kJ=kg s1 ¼ 0:9446 kJ=kg K v1 ¼ 0:1402 m3 =kg

P3 ¼ Psat@46:31C ¼ 1200 kPa ) P2 ¼ 1200 kPa s2 ¼ s1 ¼ 0:9446 kJ=kg K P3 ¼ 1200 kPa x3 ¼ 0

)

h2s ¼ 284:09 kJ=kg

h3 ¼ 117:77 kJ=kg s3 ¼ 0:4244 kJ=kg K

h4 ¼ h3 ¼ 117:77 kJ=kg ) P4 ¼ 140 kPa s ¼ 0:4674 kJ=kg K h4 ¼ 117:77 kJ=kg 4 h2s h1 h2 h1 284:09 239:17 -h2 ¼ 295:32 kJ=kg 0:80 ¼ h2 239:17

ZC ¼

) P2 ¼ 1200 kPa s ¼ 0:9783 kJ=kg K h2 ¼ 295:32 kJ=kg 2 The mass flow rate of the refrigerant is _ ¼ m

V_ 1 ð0:375=60Þ m3 =s ¼ 0:04458 kg=s ¼ v1 0:1402 m3 =kg

The refrigeration load, the rate of heat rejected, and the power input are _ L ¼ mðh _ 1 Q _ H ¼ mðh _ 2 Q _ ¼ mðh _ 2 W

h4 Þ ¼ ð0:04458 kg=sÞð239:17 h3 Þ ¼ ð0:04458 kg=sÞð295:32 h1 Þ ¼ ð0:04458 kg=sÞð295:32

117:77Þ kJ=kg ¼ 5:41 kW 117:77Þ kJ=kg ¼ 7:92 kW 239:17Þ kJ=kg ¼ 2:50 kW

Exergy

247

(b) The COP of the cycle, which is known as energy-based COP or energetic COP, is COPen;R ¼

_1 Q 5:41 ¼ ¼ 2:16 _ in 2:50 W

(c) Noting that the dead-state temperature is T0 ¼ TH ¼ 295K, the exergy destruction in each component of the cycle is determined as follows: Compressor: S_ gen;1

2

_ 2 ¼ mðs

s1 Þ ¼ ð0:04458 kg=sÞð0:9783

0:9446Þ kJ=kgK ¼ 0:001502 kW=K

_ d;1-2 ¼ To S_ gen;1-2 ¼ 295  0:001502 ¼ 0:4432 kW Ex Condenser: S_ gen;2

3

_ 3 ¼ mðs

s2 Þ þ

_H Q TH

7:92 kW ¼ 0:002138 kW=K 295 K _ d;2-3 ¼ To S_ gen;2-3 ¼ 295  0:002138 ¼ 0:6308 kW Ex

¼ ð0:04458 kg=sÞð0:4244

0:9783Þ kJ=kgK þ

Expansion valve: S_ gen;3

4

_ 4 ¼ mðs

s3 Þ ¼ ð0:04458 kg=sÞð0:4674

0:4244ÞkJ=kgK ¼ 0:001916 kW=K

_ d;3-4 ¼ To S_ gen;3-4 ¼ 295  0:001916 ¼ 0:5651 kW Ex Evaporator: S_ gen;4

1

_ 1 ¼ mðs

s4 Þ

_L Q TL

5:41 kW ¼ 0:0006964 kW=K 263K _Exd;4-1 ¼ To S_ gen;4-1 ¼ 295  0:0006964 ¼ 0:2054 kW

¼ ð0:04458 kg=sÞð0:9446

0:4674Þ kJ=kg K

(d) The exergy of the heat transferred from the low-temperature medium is     _ To 1 ¼ 5:41  295 1 ¼ 0:3163 kW _ _ ¼Q Ex QL TL 263 This is also the minimum power input for the cycle. The exergy-based COP or exergetic COP is calculated as _ _ W _ ¼ COPex;R ¼ Ex QL

0:3163 ¼ 0:263 2:503

The second-law efficiency for the refrigeration cycle is also determined from COPex;R ¼ COPen;R =COPen;R;rev where COPen;R;rev ¼

TL TH

TL

¼

10 þ 273 ¼ 8:22 ð22 ð 10ÞÞ

Substituting, COPex;R ¼

COPen;R 2:16 ¼ 0:263 ¼ COPen;R;rev 8:22

The results are identical as expected. (e) The total exergy destruction in the cycle is the difference between the exergy supplied (power input) and the exergy recovered (the exergy of the heat transferred from the low-temperature medium): _ d;ov ¼ W _ Ex

_ _ ¼ 2:503 Ex QL

0:3163 ¼ 1:845 kW

The total exergy destruction can also be determined by adding exergy destructions in each component: _ d;ov ¼ Ex _ d;1-2 þ Ex _ d;2-3 þ Ex _ d;3-4 þ Ex _ d;1-4 ¼ 0:4432 þ 0:6308 þ 0:5651 þ 0:2054 ¼ 1:845 kW Ex

248

Exergy

Qout

TH Condenser

QH Win Compressor

Expansion valve

Heat pump

HP Win QL TL

Boiler Qin

(A)

(B)

Fig. 33 (A) A schematic diagram of the simple heat pump showing the main components. (B) The directions of energy flows.

The results are calculated to be identical as expected. The exergy input to the cycle is equal to the actual work input, which is 2.503 kW. The same cooling load could have been accomplished by only 26.3% of this power (0.3163 kW) if a reversible system were used. The difference between the two is the exergy destructed in the cycle (1.845 kW). It can be shown that increasing the evaporating temperature and decreasing the condensing temperature would also decrease the exergy destructions in these components.

1.6.5.3.3

Heat pumps

Heat pumps are similar to refrigeration systems with a little difference in that the heat pump is employed to provide heating through the condenser while a refrigerator provides a cooling effect through the evaporator. Heat pumps have four components (namely expansion valve, evaporator, compressor, and condenser) the same as refrigerators, as illustrated in Fig. 33. There are commonly used heat pumps used for heating in the winter and in a reversed mode for cooling in the summer as shown in Fig. 34. The operating principle is same as for refrigeration systems. The heat pump shown in Fig. 33 is the single stage and mechanical heat pump system, which means that the presentation of the heat pump is for the simplest heat pump design. However, there are many other types of numerous designs of heat pumps, for example the cascaded heat pump in which two conventional heat pumps are connected. These two pumps are connected in such a way that the condenser of the bottoming cycle rejects heat to the boiler of the upper cycle. The reason behind the cascaded heat pump is to avoid having excessively large pressure difference between the condenser and the evaporator of the heat pump. An excessively large pressure difference will result in high work requirement on the compressor where a single compressor cannot achieve such a large pressure increase. These are sometimes practically impossible. That is why the concept of multistage compression was introduced, which may bring high capital costs plus high operational costs. However, cascading heat pumps help reduce the operational cost and thus will result in savings. Another type of heat pump system is shown in Fig. 34, where the same system can operate in two modes, refrigeration and heat pumping. The critical aspect is to switch the pipe connection between the exit and the inlet of the compressor as shown in Fig. 34. Example 16: Consider the refrigeration system given in Example 15, this time for heating purposes, reversed to serve as a heat pump. Of course, the condenser heat will be used for heating. All data and information given in Example 15 will be used here. Calculate (a) the condenser heat load and (b) both energetic and exergetic COPs. Solution: The balance equations for mass, energy, entropy, and exergy are written the same as given in Example 15 for each component of the system. Here, the balance equations for the condenser and the overall system are repeated as follows:

Exergy

Win

249

Win

Compressor

Compressor

Outdoor side

Outdoor side Indoor side

Indoor side Qin

Qout

Qin

Qout Boiler Condenser

Boiler

Condenser

Expansion valve

Expansion valve

(A)

(B)

Fig. 34 A schematic diagram of a system that can be switched between a heat pump operating for (A) heating mode and (B) refrigeration mode.

For condenser: _3 MBE : m _2¼m _ cond _ 3 h3 þ Q EBE : m _ 2 h2 ¼ m _ cond Q Ts þ E_ d;2-3

_ 3 s3 þ EnBE : m _ 2 s2 þ S_ gen;2-3 ¼ m _ _ _ 3 ex3 þ Ex ExBE : m _ 2 ex 2 ¼ m Qcond The overall cycle balance equation

MBE : m ¼ constant _ cond _ boiler þ W _ comp ¼ Q EBE : Q _ boiler =Tboiler þ S_ gen;3-4 ¼ Q _ cond =Tcond EnBE : Q

_ comp ¼ Ex _ d;ov _ _ þW ExBE : Ex QL The properties of R-134a are taken from the thermodynamic tables for R-134a: P1 ¼ 140 kPa x1 ¼ 1

) h1 ¼ 239:17 kJ s1 ¼ 0:9446 kJ=kg K v1 ¼ 0:1402 m3 =kg

P3 ¼ Psat@46:31C ¼ 1200 kPa ) P2 ¼ 1200 kPa h2s ¼ 284:09 kJ=kg s2 ¼ s1 ¼ 0:9446 kJ=kg K P3 ¼ 1200 kPa x3 ¼ 0

)

h3 ¼ 117:77 kJ=kg s3 ¼ 0:4244 kJ=kg K

h4 ¼ h3 ¼ 117:77 kJ=kg ) P4 ¼ 140 kPa s ¼ 0:4674 kJ=kg K h4 ¼ 117:77 kJ=kg 4 ZC ¼ 0:80 ¼

h2s h2

h1 h1

284:09 239:17 -h2 ¼ 295:32 kJ=kg h2 239:17

250

Exergy ) P2 ¼ 1200 kPa s ¼ 0:9783 kJ=kg K h2 ¼ 295:32 kJ=kg 2

The mass flow rate of the refrigerant is _ ¼ m

ð0:375=60Þ m3 =s V_ 1 ¼ 0:04458 kg=s ¼ v1 0:1402 m3 =kg

(a) The condenser heat load is calculated, along with the evaporator load and compressor work rate, as follows: _ H ¼ mðh _ 2 Q

h3 Þ ¼ ð0:04458 kg=sÞð295:32

117:77ÞkJ=kg ¼ 7:92 kW

_ L ¼ mðh _ 1 Q

h4 Þ ¼ ð0:04458 kg=sÞð239:17

117:77ÞkJ=kg ¼ 5:41kW

_ ¼ mðh _ 2 W

h1 Þ ¼ ð0:04458 kg=sÞð295:32

239:17ÞkJ=kg ¼ 2:50 kW

(b) Both energetic and exergetic COPs are calculated as follows:

COPen;HP ¼

_H Q 7:92 ¼ ¼ 3:17 _ in 2:50 W

In Example 15, T0 ¼TH ¼ 295K was considered. This makes the thermal exergy rate for the condenser zero as follows:     _ 1 To ¼ 7:92  1 295 ¼ 0 kW _ _ ¼Q Ex QH TH 295

which means it reaches the dead state and brings no potential work or exergy to do something useful. If we consider a condenser temperature of 351C (308K), it then becomes     _ 1 To ¼ 7:92  1 295 ¼ 0:33 kW _ _ ¼Q Ex QH TH 308 Based on this result the exergetic COP becomes _ _ W _ ¼ COPex;HP ¼ Ex QH

1.6.6

0:33 ¼ 0:132 2:50

Case Study

In this case study, energy and exergy analyses of two integrated systems are presented to show how to analyze these two systems and evaluate their performances through energy and exergy efficiencies along with some parametric studies to show how the energy and exergy efficiencies change by changing the operating conditions and state properties. The first system considered is a biomass-based gas turbine cycle integrated with steam Rankine cycle, and the second system studied is a geothermal flash system integrated with steam Rankine cycle. These two systems are studied in order to show the importance of renewable/alternative energy sources in future power generation and the importance of exergy analysis during the designing stage of the system.

1.6.6.1 1.6.6.1.1

Systems Description Biomass-based integrated system

In this integrated system biomass is used as a fuel in the combustion chamber. The integrated system studied is shown in Fig. 35. Air is compressed from state 1 to state 2 using a compressor whose power is supplied by the turbine. The compressed air at state 2 enters the combustion chamber where biomass is combusted with compressed air to leave at state 3 as exhaust gases. Exhaust gases enter the gas turbine where they expand to produce power. The exhaust gases leaving the gas turbine at state 4 are passed through the shell and tube heat exchanger where they release heat to the water entering the heat exchanger at state 7. After releasing heat in the heat exchanger, the exhaust gases are released to the environment at state 5. The heat gained by the water entering the heat exchanger leaves at state 8 as a saturated vapor. The saturated vapor at state 8 enters the turbine where it expands to produce power. After expanding, the saturated mixture leaves at state 9 to enter the heat exchanger where it releases heat to the air, which is used as a cooling medium. After releasing heat condenser water enters the pump at state 6. The overall net power produced is used for later purposes.

1.6.6.1.2

Geothermal-based integrated system

In this system, geothermal is used as an energy source. The integrated system studied is shown in Fig. 36. The geothermal water at state 1 is expanded in the expansion valve in order to leave at state 2 as a saturated mixture. Saturated mixture at 2 enters the separation chamber (flash chamber) where saturated vapor is extracted at state 3 and saturated water is extracted at state 6.

Exergy

251

Biomass Compressed air

Combustion chamber 2

3

Compressor

Gas turbine

Biomass cycle

Wout

Win

4

1

Air

Exhaust gasses

Exhausts 5

7

8 Steam generator Steam turbine

Pump

Steam cycle

Condenser 6

9

Fig. 35 Schematic of biomass cycle integrated with steam cycle.

Saturated vapor at state 3 enters the turbine where it expands in order to produce power and leaves at state 4. Steam at state 4 enters the condenser to release heat to air, which is used as cooling medium. After releasing heat, steam from state 4 leaves at state 5 where it combines with steam coming at state 7 to be reinjected to the geothermal well. Saturated water leaving at state 6 is passed through the shell and tube heat exchanger where it releases heat to the water coming at state 11. The heated water then enters the turbine at state 8, where it expands to produce power. After expanding, the vapor at state 9 enters the condenser in order to release heat to the air, which is used as cooling medium, and leaves at state 10. Water leaving at state 10 enters the pump where its pressure is pumped to the pressure at state 11. The overall net power produced by the integrated system is used for later purposes.

Exergy

252

3 Steam turbine

Separator

Wout 4 8

Condenser

2 Steam turbine Expansion valve 6 9

7 Heater

1

From production well

5

Wout

To reinjection well

11

Condenser Pump

To reinjection well

Mixer Win 10

Streams 7 and 6 are mixed and the product stream is 8

Fig. 36 Schematic of geothermal cycle integrated with steam cycle.

1.6.6.2 1.6.6.2.1

Analysis Biomass integrated with steam cycle

The first system studied consists of a gas cycle using biomass in a combustion chamber integrated with steam cycle for power production. The system studied is shown in Fig. 37. The topping cycle is a biomass gas-turbine cycle that has a pressure ratio of 8. Air enters the compressor (state 1) at 300K and the turbine (state 3) at 1300K. The isentropic efficiencies of the compressors, pumps, and turbines are 85%. The bottoming cycle is a simple Rankine cycle operating between the pressure limits of 7 MPa and 5 kPa. The steam is heated to a temperature of 5001C in a heat exchanger by the heat supplied from the exhaust gases of a biomass combustion system. The exhaust gases leave the heat exchanger (state 5) at 450K. The ambient temperature and pressure are at 271C and 100 kPa. All the components are considered adiabatic. Biomass cycle is modeled on the basis of air-standard theorem. Air is treated as an ideal gas. Temperature and pressure losses in the pipes and connections are neglected. It is assumed that 20% of the power is lost due to the parasitic losses. Energy and Exergy Analyses: The pressure at state 2 is calculated as Pr2 ¼ rp  Pr1

ð28Þ

where Pr2 is the reduced pressure at state 2, rp is the pressure ratio, and Pr1 is the reduced pressure at state 1. The pressure at state 4 is calculated as Pr4 ¼

1  Pr3 rp

where Pr4 is the reduced pressure at state 4, rp is the pressure ratio, and Pr3 is the reduced pressure at state 3. First each component of the system is analyzed by writing the balance equations as follows:

ð29Þ

Exergy

Specific exergy destruction (kJ/kg)

2500

2271

2000

1500 1153 1000

500 202.8 225.1

178.1 145.6 195.5 14.58 12.05

1.218

0

co

co

e

s

cy

cl

as om

as om

Components

Bi

Bi

om

as

s

Bi

Bi

om

as

s

cy

cy

cl

e

cl

e

co

m

pr es so r( m s m p id cy re bu ea cl s s e s l) t or io co n (a m c c ha bu tu m st al io be Bi ) n om r ch ( i de as am al s Bi be ) cy om r( cl e a as ct tu s ua rb cy in l) cl e e (id tu ea rb in l) St e ea (a m ct cy ua cl l) e St tu ea r bi m ne cy cl e H pu ea St m te ea p xc m h an cy cl ge e r co nd O en ve se ra r ll sy st em

0

Fig. 37 Specific exergy destruction in each component and overall system.

Air compressor: _2 MBE : m _1 ¼m _ comp ¼ m _ 2 h2 EBE : m _ 1 h1 þ W _ 2 s2 EnBE : m _ 1 s1 þ S_ gen;comp ¼ m _ comp ¼ m _ d;comp _ 2 ex 2 þ Ex ExBE : m _ 1 ex1 þ W Combustion chamber: _ bio ¼ m _3 MBE : m _2þm _ _ 3 h3 EBE : m _ 2 h2 þ mbio LHV bio ¼ m _ _ bio sbio ¼ m _ 3 s3 EnBE : m _ 2 s2 þ Sgen;CC þ m _ d;CC _ bio exbio ¼ m _ 3 ex 3 þ Ex ExBE : m _ 2 ex2 þ m Gas turbine: _4 MBE : m _3 ¼m _ GT _ 4 h4 þ W EBE : m _ 3 h3 ¼ m _ 4 s4 EnBE : m _ 3 s3 þ S_ gen;GT ¼ m _ GT þ Ex _ d;GT _ 4 ex 4 þ W ExBE : m _ 3 ex 3 ¼ m Heat exchanger:

_7 MBE : m _4þm _5þm _8 m _ 7 h7 ¼ m _ 5 h5 þ m _ 8 h8 EBE : m _ 4 h4 þ m _ 7 s7 þ S_ gen;HX ¼ m _ 5 s5 þ m _ 8 s8 EnBE : m _ 4 s4 þ m _ d;HX _ 7 ex 7 ¼ m _ 5 ex 5 þ m _ 8 ex 8 þ Ex ExBE : m _ 4 ex 4 þ m

253

254

Exergy

Steam turbine: _9 MBE : m _8 ¼m _ ST _ 9 h9 þ W EBE : m _ 8 h8 ¼ m _ 9 s9 EnBE : m _ 8 s8 þ S_ gen;ST ¼ m _ ST þ Ex _ d;ST _ 9 ex 9 þ W ExBE : m _ 8 ex8 ¼ m Condenser : _ a:in MBE : m _9þm _ a;out _6þm m _ a:in ha;in ¼ m _ 6 h6 þ m _ a;out ha;out EBE : m _ 9 h9 þ m _ a:in sa;in þ S_ gen;C ¼ m _ 6 s6 þ m _ a;out sa;out EnBE : m _ 9 s9 þ m _ d;C _ a:in ex a;in ¼ m _ 6 ex 6 þ m _ a;out exa;out þ Ex ExBE : m _ 9 ex9 þ m Pump: _7 MBE : m _6¼m _ pump ¼ m _ 7 h7 EBE : m _ 6 h6 þ W _ 7 s7 EnBE : m _ 6 s6 þ S_ gen;pump ¼ m _ pump ¼ m _ d;pump _ 7 ex 7 þ Ex ExBE : m _ 6 ex 6 þ W Overall system: _ bio ¼ m _5 MBE : m _1þm _ pump ¼ m _ GT þ W _ ST _ comp þ W _ 5 h5 þ W _ bio LHV bio þ W EBE : m _ 1 h1 þ m _ bio sbio þ S_ gen;ov ¼ m _ 5 s5 EnBE : m _ 1 s1 þ m _ comp þ W _ pump ¼ m _ GT þ W _ ST þ Ex _ d;ov _ bio exbio þ W _ 5 ex5 þ W ExBE : m _ 1 ex1 þ m From the balance equations we can rearrange them to calculate the required work, thermal energy, etc. The ideal specific work needed by the compressor of the biomass cycle to compress air from state 1 to state 2 is calculated as wcomp;g;s ¼ hs;2

ð30Þ

h1

where wcomp,g,s is the ideal specific work needed by the compressor of the biomass cycle, hs,2 is the ideal specific enthalpy at state 2, and h1 is the specific enthalpy at state 1. The ideal specific power produced by the turbine of the biomass cycle is defined as wturb;g;s ¼ h3

ð31Þ

hs;4

where wturb,g,s is the ideal specific work produced by the turbine of the biomass cycle, h3 is the specific enthalpy at state 3, and hs,4 is the ideal specific enthalpy at state 4. The ideal specific heat input to the biomass cycle is found using qin;g;s ¼ h3

ð32Þ

hs;2

where qin,g,s is the ideal specific heat input to the combustion chamber of the biomass cycle, h3 is the specific enthalpy at state 3, and hs,2 is the ideal specific enthalpy at state 2. The ideal parasitic loss is calculated as wparasitic;g;s ¼ 0:2  ðwturb;g;s

wcomp;g;s Þ

ð33Þ

where wparasitic,g,s is the ideal specific parasitic loss of the biomass cycle. The ideal specific net power produced by the biomass cycle is defined as wnet;g;s ¼ wturb;g;s

wcomp;g;s

wparasitic;g;s

ð34Þ

where wnet,g,s is the ideal net specific power produced by the biomass cycle. The specific exergy at state 1 is found using ex 1 ¼ ðh1

h0 Þ

T0  ðs1

s0 Þ

ð35Þ

where ex1 represents specific exergy at state 1, h0 represents specific enthalpy at ambient state, s1 is specific entropy at state 1, and s0 is specific entropy at ambient state. The same formulation is used to calculate specific exergy at each state. The ideal specific thermal exergy of the biomass cycle is found using  T0 ð36Þ ex th;g;s ¼ 1  qin;g;s Tavg;g;s

Exergy

255

ðT3 þTs;2 Þ and exth,g,s is the ideal specific thermal exergy of the biomass cycle, and Tavg,g,s is the ideal average where Tavg;g;s ¼ 2 temperature. The ideal energy and exergy efficiencies of the biomass cycle are calculated using wnet;g;s ð37Þ Zen;g;s ¼ qin;g;s Zex;g;s ¼

wnet;g;s ex th;g;s

ð38Þ

The actual specific work needed by the compressor of the biomass cycle to compress air from state 1 to state 2 is calculated as wcomp;g;a ¼

hs;2

h1

ð39Þ

Z

where wcomp,g,a is the actual specific work needed by the compressor of the biomass cycle, hs,2 is the ideal specific enthalpy at state 2, h1 is the specific enthalpy at state 1, and Z is the isentropic efficiency, which is 85%. The actual specific power produced by the turbine of the biomass cycle is defined as wturb;g;a ¼ Z  ðh3

hs;4 Þ

ð40Þ

where wturb,g,a is the actual specific work produced by the turbine of the biomass cycle, h3 is the specific enthalpy at state 3, hs,4 is the ideal specific enthalpy at state 4, and Z is the isentropic efficiency, which is 85%. The actual specific heat input to the biomass cycle is found using qin;g;a ¼ h3

ð41Þ

h2

where, qin,g,a is the actual specific heat input to the combustion chamber of the biomass cycle, h3 is the specific enthalpy at state 3, and h2 is the actual specific enthalpy at state 2. The actual parasitic loss is calculated as   wparasitic;g;a ¼ 0:2  wturb;g;a wcomp;g;a ð42Þ where wparasitic,g,a is the actual specific parasitic loss of the biomass cycle. The actual specific net power produced by the biomass cycle is defined as wnet;g;a ¼ wturb;g;a

wcomp;g;a

wparasitic;g;a

ð43Þ

where wnet,g,a is the actual net specific power produced by the biomass cycle. The actual specific thermal exergy of the biomass cycle is found using  T0  qin;g;a ð44Þ exth;g;a ¼ 1 Tavg;g;a ðT3 þTa;2 Þ where Tavg;g;a ¼ and exth,g,a is the actual specific thermal exergy of the biomass cycle, and Tavg,g,a is the actual average 2 temperature. The actual energy and exergy efficiencies of the biomass cycle are calculated using wnet;g;a Zen;g;a ¼ ð45Þ qin;g;a Zex;g;s ¼

wnet;g;a ex th;g;a

The specific power consumed by the pump of the steam cycle is found using  P7 P6 wp ¼ v6 Z

ð46Þ

ð47Þ

where wp is specific power consumed by the pump, v6 is specific volume at state 6, and Z is the isentropic efficiency of the pump. The mass ratio “y” is defined as mass flow rate of the steam divided by mass flow rate of the biomass gas and is calculated using y¼

h4 h8

h5 h7

ð48Þ

The specific power produced by turbine of the steam cycle is defined as wturb;st ¼ ðh8

h9 Þ

ð49Þ

where wturb,st is the specific work produced by the turbine of the steam cycle, h8 is the specific enthalpy at state 8, and h9 is the specific enthalpy at state 9. The specific heat input to the steam cycle is found using qin;st ¼ h8

h7

ð50Þ

where qin,st is the specific heat input to the steam cycle, h8 is the specific enthalpy at state 8, and h7 is the specific enthalpy at state 7.

256

Exergy

The specific heat ouput fron the condenser of the steam cycle is found using qcon;st ¼ h9

ð51Þ

h6

where qcon,st represents specific heat output from the condenser of the steam cycle. The parasitic loss is calculated as wparasitic;st ¼ 0:2  ðwturb;st

wp Þ

ð52Þ

wparasitic;st

ð53Þ

where wparasitic,st is the specific parasitic loss of the steam cycle. The specific net power produced by the steam cycle is defined as wnet;st ¼ wturb;st

wp

where wnet,g,a is the actual net specific power produced by the biomass cycle. The specific input thermal exergy of the steam cycle is found using  T0 ex th;in;st ¼ 1  qin;st Tavg;st

ð54Þ

8Þ where Tavg;st ¼ ðT7 þT and exth,in,st is the specific input thermal exergy of the steam cycle, and Tavg,st is the average temperature. 2 The specific condenser thermal exergy of the steam cycle is found using  T0 ex th;con;st ¼ 1 ð55Þ  qcon;st Tavg;con

6Þ where Tavg;con ¼ ðT9 þT and exth,con,st is the specific condenser thermal exergy of the steam cycle, and Tavg,con is the average 2 temperature. The energy and exergy efficiencies of the steam cycle are calculated using wnet;st Zen;st ¼ ð56Þ qin;st

Zex;g;s ¼

wnet;st ex th;st

ð57Þ

The net overall power output of the integrated system is found using wnet;ov ¼ y  wnet;st

wnet;g;a

The ideal specific exergy destruction in the compressor is found using ex dest;comp;g;s ¼ ex 1

exs;2 þ wcomp;g;s

ð58Þ

where exdest,comp,g,s represents ideal specific exergy destruction in the compressor. The actual specific exergy destruction in the compressor is found using ex dest;comp;g;a ¼ ex 1

ex 2 þ wcomp;g;a

ð59Þ

where exdest,comp,g,a represents actual specific exergy destruction in the compressor. The ideal specific exergy destruction in the combustion chamber is found using ex dest;cc;g;s ¼ ex s;2

ex 3 þ exth;g;s

ð60Þ

where exdest,cc,g,s represents ideal specific exergy destruction in the combustion chamber. The actual specific exergy destruction in the combustion chamber is found using ex dest;cc;g;a ¼ ex 2

ex3 þ exth;g;a

ð61Þ

where exdest,cc,g,a represents actual specific exergy destruction in the combustion chamber. The ideal specific exergy destruction in the turbine of the biomass cycle is found using ex dest;turb;g;s ¼ ex 3

exs;4

wturb;g;s

ð62Þ

where exdest,turb,g,s represents ideal specific exergy destruction in the turbine of the biomass cycle. The actual specific exergy destruction in the turbine of the biomass cycle is found using ex dest;turb;g;a ¼ ex 3

ex 4

wturb;g;a

ð63Þ

where exdest,turb,g,a represents actual specific exergy destruction in the turbine of the biomass cycle. The specific exergy destruction in the turbine of the steam cycle is found using ex dest;turb;st ¼ ex 8

ex 9

wturb;st

ð64Þ

where exdest,turb,st represents specific exergy destruction in the turbine of the steam cycle. The specific exergy destruction in the pump of the steam cycle is found using ex dest;p;st ¼ ex 6

ex7 þ wp

where exdest,p,st represents specific exergy destruction in the pump of the steam cycle.

ð65Þ

Exergy

257

The specific exergy destruction in the heat exchanger is found using exdest;he ¼ ex 4 þ ex7

ex5

ð66Þ

ex 8

where exdest,he represents specific exergy destruction in the heat exchanger. The specific exergy destruction in the condenser of the steam cycle is found using ex dest;con;st ¼ ex 9

ex 6

ð67Þ

exth;con;st

where exdest,con,st represents specific exergy destruction in the condenser of the steam cycle. The overall specific exergy destruction is found using ex dest;ov ¼ ex1 þ ex th;g;a þ wp þ wcomp;g;a

ex5

wturb;g;a

wturb;st

exth;in;st

ð68Þ

where exdest,ov represents overall specific exergy destruction. The overall energy and exergy efficiencies are then defined as

1.6.6.2.2

Zen;ov ¼

wnet;ov qin;g;a

ð69Þ

Zex;ov ¼

wnet;ov exth;g;a

ð70Þ

Geothermal single flash integrated with steam cycle

The second system studied here consists of a geothermal single flash integrated with steam cycle for power production. The system studied is shown in Fig. 36. The enthalpy of water as it enters to the system (state 1) is 2255 kJ/kg. The associated mass flow rate is determined to be 100 kg/s at state 1. The fluid at state 8 is saturated water vapor. The ambient temperature and pressure are at 251C and 100 kPa. The turbine and pump efficiencies are 0.85. The mass flow rate of the steam cycle is taken to be 6 kg/s. All the components are considered adiabatic. Temperature and pressure losses in the pipes and connections are neglected. It is assumed that 20% of the power is lost due to the parasitic losses. Energy and exergy analyses: The specific power consumed by pump of the steam cycle is found using  P11 P10 wp ¼ v10 ð71Þ Z where wp is specific power consumed by the pump, v10 is specific volume at state 6, and Z is the isentropic efficiency of the pump. First each component of the system is analyzed by writing the balance equation as follows: Expansion valve: _2 MBE : m _1¼m _ 2 h2 EBE : m _ 1 h1 ¼ m _ 2 s2 EnBE : m _ 1 s1 þ S_ gen;exp ¼ m _ d;exp _ 2 ex2 þ Ex ExBE : m _ 1 ex 1 ¼ m Flash chamber: _3þm _6 MBE : m _2 ¼m _ 3 h3 þ m _ 6 h6 EBE : m _ 2 h2 ¼ m _ 3 s3 þ m _ 6 s6 EnBE : m _ 2 s2 þ S_ gen;FL ¼ m _ d;FL _ 3 ex 3 þ m _ 6 ex 6 þ Ex ExBE : m _ 2 ex 2 ¼ m Steam turbine (in the geothermal cycle): _4 MBE : m _3¼m _ t:f _ 4 h4 þ W EBE : m _ 3 h3 ¼ m _ 4 s4 EnBE : m _ 3 s3 þ S_ gen;t;f ¼ m _ t;f þ Ex _ d;t;f _ 4 ex4 þ W ExBE : m _ 3 ex3 ¼ m Condenser (in the geothermal cycle): _ a;in ¼ m _5þm _ a;out MBE : m _4þm _ a;in ha;in ¼ m _ 5 h5 þ m _ a;out ha;out EBE : m _ 4 h4 þ m _ _ a;in sa;in þ Sgen;con;f ¼ m _ 5 s5 þ m _ a;out sa;out EnBE : m _ 4 s4 þ m _ d;con;f _ a;in ex a;in ¼ m _ 5 ex 5 þ m _ a;out exa;out þ Ex ExBE : m _ 4 ex 4 þ m

258

Exergy

Heat exchanger: _ 11 ¼ m _7þm _8 MBE : m _6þm _ 11 h11 ¼ m _ 7 h7 þ m _ 8 h8 EBE : m _ 6 h6 þ m _ 11 s11 þ S_ gen;HX ¼ m _ 7 s7 þ m _ 8 s8 EnBE : m _ 6 s6 þ m _ d;HX _ 11 ex 11 ¼ m _ 7 ex7 þ m _ 8 ex 8 þ Ex ExBE : m _ 6 ex 6 þ m Steam turbine (in the steam cycle): _9 MBE : m _8 ¼m _ t;st þ m _ 9 h9 EBE : m _ 8 h8 ¼ W _ 9 s9 EnBE : m _ 8 s8 þ S_ gen;t;st ¼ m _ t;st þ m _ d;t;st _ 9 ex 9 þ Ex ExBE : m _ 8 ex 8 ¼ W Condenser (in the steam cycle): _ a;in MBE : m _9þm _ a;out _ 10 þ m m _ a;in ha;in ¼ m _ 10 h10 þ m _ a;out ha;out EBE : m _ 9 h9 þ m _ a;in sa;in þ S_ gen;con;t ¼ m _ 10 s10 þ m _ a;out sa;out EnBE : m _ 9 s9 þ m _ d;con;t _ a;in ex a;in ¼ m _ 10 ex10 þ m _ a;out ex a;out þ Ex ExBE : m _ 9 ex9 þ m Pump: _ 11 MBE : m _ 10 ¼ m _ p ¼m _ 11 h11 EBE : m _ 10 h10 þ W _ 11 s11 EnBE : m _ 10 s10 þ S_ gen;p ¼ m _ p¼m _ d;p _ 11 ex11 þ Ex ExBE : m _ 10 ex 10 þ W Overall system: _5þm _7 MBE : m _1¼m _ p ¼m _ t;f þ W _ t;st _ 5 h5 þ m _ 7 h7 þ W EBE : m _ 1 h1 þ W _ 5 s5 þ m _ 7 s7 EnBE : m _ 1 s1 þ S_ gen;ov ¼ m _ p¼m _ t;f þ W _ t;st þ Ex _ d;ov _ 5 ex 5 þ m _ 7 ex 7 þ W ExBE : m _ 1 ex1 þ W From the balance equations, we can rearrange them to calculate the required work, thermal energy, and others. The mass flow rates at state 3 and state 6 are found using _2 ¼m _3þm _6 m

ð72Þ

_ 2 x2 ¼ m _ 3 x3 þ m _ 6 x6 m

ð73Þ

_ 2 is the mass flow rate at state 2, m _ 3 is mass flow rate at state 3, m _ 6 is mass flow rate at state 6, x2 is quality at state 2, x3 is where m quality at state 3, and x6 is quality at state 6. The condenser load of the flash power plant is found using _ con;f ¼ m _ 4 h4 Q

_ 5 h5 m

ð74Þ

_ 10 h10 m

ð75Þ

_ con;f represents condenser load of the flash power plant. where Q The condenser load of the steam power plant is found using _ con;st ¼ m _ 9 h9 Q

_ con;st represents condenser load of the steam power plant. where Q The power produced by the turbine of the flash power plant is calculated using _ t;f ¼ m _ 3 h3 W

_ 4 h4 m

ð76Þ

_ t;f represents power produced by the turbine of the flash power plant. where W The power produced by the turbine of the steam power plant is calculated using _ t;st ¼ m _ 8 h8 W

_ 9 h9 m

ð77Þ

_ t;st represents power produced by the turbine of the steam power plant. where W The power consumed by the pump is found using _ p¼m _ 11 h11 W _ p represents power consumed by the pump. where W

_ 10 h10 m

ð78Þ

Exergy

259

The overall parasitic loss is calculated as _ parasitic;ov ¼ 0:2  ðW _ t;st þ W _ t;f W

_ pÞ W

ð79Þ

_ parasitic;ov is the parasitic loss of the steam cycle. where W The net power produced by the overall system is found using _ t;st þ W _ t;f _ net;ov ¼ W W

_p W

_ parasitic;ov W

ð80Þ

_ net;ov is the net power produced by the overall system. where W The thermal exergy rate of the condenser of the flash power plant is found using  T0 _ con;f ex_ con;th;f ¼ 1 Q Tavg;con;f

ð81Þ

5Þ where Tavg;con;f ¼ ðT4 þT and ex_ con;th;f is the thermal exergy rate of the condenser of the flash power plant, and Tavg;con;f is the average 2 temperature of the condenser of the flash power plant. The thermal exergy rate of the condenser of the steam power plant is found using  T0 _ con;st ex_ con;th;st ¼ 1 ð82Þ Q Tavg;con;st 10 Þ where Tavg;con;st ¼ ðT9 þT and ex_ con;th;st is the thermal exergy rate of the condenser of the steam power plant, and T avg;con;st is the 2 average temperature of the condenser of the steam power plant. The exergy destruction rate in the flash chamber is found using

ex_ dest;flash chamber ¼ ex_ 2

ex_ 3

ex_ 6

ð83Þ

_ 2 ½ðh2 h0 Þ T0 ðs2 s0 ފ and ex_ dest;flash chamber represents exergy destruction rate in the flash chamber, ex_ 2 represents where ex_ 2 ¼ m exergy rate at state 2, ex_ 3 represents exergy rate at state 3, and ex_ 6 represents exergy rate at state 6. The rate of exergy at each state is found using the same formula as mentioned above. The exergy destruction rate in the turbine of the flash power plant is found using ex_ dest;t;f ¼ ex_ 3

ex_ 4

_ t;f W

ð84Þ

where ex_ dest;t;f represents the exergy destruction rate in the turbine of the flash power plant. The exergy destruction rate in the condenser of the flash power plant is found using ex_ dest;con;f ¼ ex_ 4

ex_ 5

ex_ con;th;f

ð85Þ

where ex_ dest;con;f represents exergy destruction rate in the condenser of the flash power plant. The exergy destruction rate in the turbine of the steam power plant is found using ex_ dest;t;st ¼ ex_ 8

ex_ 9

_ t;st W

ð86Þ

where ex_ dest;t;st represents exergy destruction rate in the turbine of the steam power plant. The exergy destruction rate in the condenser of the steam power plant is found using ex_ dest;con;st ¼ ex_ 9

ex_ 10

ex_ con;th;st

ð87Þ

where ex_ dest;con;st represents exergy destruction rate in the condenser of the steam power plant. The exergy destruction rate in the pump is calculated using ex_ dest;p ¼ ex_ 10

_p ex_ 11 þ W

ð88Þ

where ex_ dest;p represents exergy destruction rate in the pump. The overall exergy destruction rate in the integrated system is defined as _p ex_ dest;ov ¼ ex_ 1 þ W

_ t;f W

ex_ 12

_ t;st W

ex_ con;th;f

ex_ con;th;st

ð89Þ

The overall energy and exergy efficiencies are the defined as Zen;ov ¼

_ net;ov W _ 1 h1 m _ 12 h12 Þ ðm

ð90Þ

_ net;ov W ðex_ 1 ex_ 12 Þ

ð91Þ

Zex;ov ¼

1.6.6.3 1.6.6.3.1

Results and Discussion Biomass integrated with steam cycle

Energy and exergy analyses are carried out for each component of the integrated system. Conducting exergy analysis is very important in order to see how the system will perform in real life. The results obtained are shown in Tables 5 and 6.

260

Exergy

Table 5

Thermodynamic properties at each state point

State points

Specific exergy (kJ/kg)

Specific enthalpy (kJ/kg)

Reduced pressure (kPa)

Pressure (kPa)

Specific entropy (kJ/kg K)

Temperature (K)

Specific volume (m3/kg)

0 1 2s 2a 3 4s 4a 5 6 7 8 9

N/A 12.42 53.9 74.67 615 186.3 244.8 15.86 0.3484 7.404 1389 56.35

N/A 300.4 544.7 587.8 1396 789.4 880.4 452.1 137.8 146 3411 2274

N/A 1.386 11.09 11.09 330.9 41.36 41.36 5.775 N/A N/A N/A N/A

100 N/A N/A N/A N/A N/A N/A N/A 5 7000 7000 5

N/A 1.702 2.299 2.374 3.273 2.676 2.785 2.116 0.4762 0.4803 6.8 7.455

298 300 540 582 1300 770 853 450 306 306.5 773 306

N/A N/A N/A N/A N/A N/A N/A N/A 0.001005 N/A N/A N/A

Table 6

Results of the integrated system

Components

Specific energy (kJ/kg)

Specific thermal exergy (kJ/kg)

Specific exergy destruction Energy efficiency Exergy efficiency (kJ/kg) (%) (%)

Biomass cycle compressor (ideal) Biomass cycle compressor (actual) Biomass cycle combustion chamber (ideal) Biomass cycle combustion chamber (actual) Biomass cycle turbine (ideal) Biomass cycle turbine (actual) Steam cycle turbine Steam cycle pump Heat exchanger Steam cycle condenser Steam cycle net Biomass cycle net (ideal) Biomass cycle net (actual) Biomass cycle (ideal) Biomass cycle (actual) Steam cycle Overall system

244.2 287.9 851.4

N/A N/A 575.6

202.8 225.1 14.58

N/A N/A N/A

N/A N/A N/A

808.3

552.4

12.05

N/A

N/A

606.7 515.7 1137 8.273 3265 2136 903.4 290 182.7 N/A N/A N/A 301.2

N/A N/A N/A N/A 1462 56 N/A N/A N/A N/A N/A N/A N/A

178.1 145.6 195.5 1.218 1153 0 N/A N/A N/A N/A N/A N/A 2271

N/A N/A N/A N/A N/A N/A N/A N/A N/A 34.06 22.6 27.67 37.26

N/A N/A N/A N/A N/A N/A N/A N/A N/A 50.38 33.08 61.77 54.53

The results shown in Table 6 indicate that system performance varies from energy analysis to exergy analyses. The results show that for the given ambient condition the exergetic efficiencies of the biomass cycle, steam cycle, and overall system are higher than energetic efficiencies of the respective systems. It is observed that for the ideal case of the biomass cycle when isentropic efficiencies of the compressor and turbine are taken to be 100%, energetic and exergetic efficiencies are 34.06% and 50.38%, respectively. However, for the actual case of the biomass cycle when isentropic efficiencies of the compressor and turbine are taken to be 85%, the energetic and exergetic efficiencies are found to be 22.6% and 33.08%, respectively. However, for the steam cycle, energetic and exergetic efficiencies are found to be 27.67% and 61.77%, respectively. Moreover, for the overall system energetic and exergetic efficiencies turn out to be 37.26% and 54.53%, respectively. These results show that efficiencies vary from energy analysis to exergy analysis. In this case, exergy efficiencies are obtained to be higher than energy efficiencies because of the ambient conditions chosen. It is not true to generalize that exergy efficiencies are always lower than energy efficiencies. As it can be seen in this case, exergy efficiencies can be higher than energy efficiencies. Fig. 37 displays specific exergy destruction in each component of the system studied and in the overall system. It can be seen that the maximum specific exergy is destructed in the heat exchanger. This shows that the heat exchanger can be modified in order to reduce the exergy destruction in it so that more power can be generated. Fig. 38 helps visualize how overall energy and exergy efficiencies vary with variation in ambient temperature. It is observed that the overall energy efficiency doesn’t vary with rise in ambient temperature but the overall exergy efficiency increases from 53.9% to 55.6% with rise in ambient temperature.

40

56

38

55.5

36

55

34

54.5

32

54

30 290

294

298

302

306

261

ex,ov (%)

en,ov (%)

Exergy

53.5 310

Ambient temperature (K) Fig. 38 Variation in overall energy and exergy efficiencies with rise in ambient temperature.

Table 7

Thermodynamic properties at each state point

State points

Exergy rate (kW)

Specific enthalpy (kJ/kg)

Mass flow rate (kg/s)

Pressure (kPa)

Specific entropy (kJ/kg K)

Temperature (K)

Quality

Specific volume (m3/kg)

0 1 2 3 4 5 6 7 8 9 10 11 12

N/A 81,305 61,749 58,862 10,068 6.438 2881 17.49 4164 827.1 16.84 19.36 8.53

104.8 2255 2255 2768 2210 108.1 719.2 108.9 2741 2263 191.7 192.2 108.3

N/A 100 100 74.94 74.94 74.94 25.06 25.06 6 6 6 6 100

100 8243 791.5 791.5 10 10 791.5 791.5 420 10 10 420 10

0.3669 4.852 5.508 6.666 6.976 0.378 2.042 0.3783 6.88 7.144 0.6489 0.6491 0.3787

298.2 570.3 443.2 443.1 318.9 298.9 443.1 299 418.5 318.9 318.9 319 299

N/A 0.65 0.7494 1 0.8436 N/A 0 N/A 1 0.8661 0 N/A N/A

N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A 0.00101 N/A N/A

1.6.6.3.2

Geothermal single flash integrated with steam cycle

Energy and exergy analyses are carried out for each component of the integrated system. Conducting exergy analysis is very important in order to study how the system will perform in real life. The results obtained from this study are tabulated in Tables 7 and 8. The results obtained showcase that system performances differ a lot from energy analysis to exergy analysis. It is also important to conduct exergy analysis as it shows where most of the exergy is being destructed. It can be seen in Table 8 that the most amount of exergy is being destructed in the turbine of the geothermal cycle followed by the condenser of the geothermal cycle. Conducting exergy analysis shows that more can be produced from the same geothermal turbine if the exergy destruction is minimized. Finding where most of the energy is lost is not possible by conducting only energy analysis; it is exergy analysis that pinpoints where the maximum amount of exergy is being lost. Also, the overall energy and exergy efficiencies obtained are 16.67% and 44.3%, respectively. This exergy analysis is conducted for a specific ambient condition and it is important to mention that exergetic parameters vary with variation in ambient conditions. Fig. 39 shows how much exergy destruction rate takes place in each component of the system and in the overall system. It can be seen that the maximum exergy destruction rate takes place in the turbine of the geothermal cycle. This is a sign that if one needs to improve the performance of the overall system, the performance of the geothermal cycle turbine should get first preference in component improvement strategy. The variation in the overall energetic and exergetic efficiencies with rise in ambient temperature is plotted in Fig. 40. It can be seen that the overall energetic efficiency remains constant and is not affected by the increase in ambient temperature. However, overall exergetic efficiency increases from 42.1% to 47.1% with an increase in ambient temperature.

262

Exergy

Table 8

Results of the integrated system Energy rate (kW)

Thermal exergy rate (kW)

Exergy destruction rate (kW)

Energy efficiency (%)

Exergy efficiency (%)

Geothermal cycle condenser Geothermal cycle turbine Steam cycle condenser Steam cycle turbine Steam cycle pump Heat exchanger Flash chamber Overall system

157,498 41,879 12,430 2864 2.924 N/A N/A 35,792

5501 N/A 810.2 N/A N/A N/A N/A N/A

4,561 6,915 0 473 0.41 1,281 5.763 30,246

N/A N/A N/A N/A N/A N/A N/A 16.67

N/A N/A N/A N/A N/A N/A N/A 44.3

35,000

30,246

30,000 25,000 20,000 15,000 10,000

6915

4561

5000

473

0

1281

0.41

5.763

m

r ls

ys

te

be ve ra l O

h as Fl

te xc ea H

ch

ha

am

ng

m pu e cl cy

St

ea

m

cy m ea St

er

p

e rb tu e cl

co e cl cy m

St

ea

in

se nd

tu e cl cy

al m er th

eo G

en

rb

in

se en nd co e cl cy al m er th G

eo

r

e

0

r

Exergy destruction rate (kW)

Components

Components

Fig. 39 Exergy destruction in each component and overall system.

1.6.7

Future Directions

As stated previously, exergy comes from the second law of thermodynamics and serves as a critical tool for design, analysis, assessment, and improvement of energy systems. Although its importance and role are now recognized by many, this is still not enough. It is more crucial to implement exergy approaches, methods, techniques, technologies, metrics, indexes, parameters, ranks, criteria, etc. in every sector of economy, including: • In the residential sector, especially through low-exergy applications by using renewable energy sources and process and waste heats for power, heating, cooling, fuel, and fresh water requirements; • In the industrial sector, especially through exergetically efficient and effective integrated energy systems for multigeneration applications by using all possible energy sources in an environmentally benign and sustainable manner; • In the transportation sector, especially through the use of clean fuels and exergetically efficient and effective powering options; • In the utility sector, especially through the use of clean fuels and exergetically efficient and effective integrated energy systems for multigeneration applications by using all possible energy sources in an environmentally benign and sustainable manner; • In the agricultural sector, especially through exergetically efficient and effective integrated energy systems for multigeneration applications by using all possible energy sources in an environmentally benign and sustainable manner; and • In the commercial and public sectors, especially through low-exergy applications by using renewable energy sources and process and waste heats for power, heating, cooling, fuel, and fresh water requirements. It is also necessary to enhance the applicable exergy methods after integrating with some other methods, such as life cycle assessment, environmental impact assessment, industrial ecology, sustainable development and cost assessment, etc., as illustrated in Fig. 41, which will refer to integrated exergy solutions. It is also necessary to extend the use of exergy to other disciplines more aggressively, including engineering (ranging from industrial engineering to bioengineering areas) and nonengineering disciplines (ranging from biosciences to medical sciences). This way humanity will be able to make a difference in the way it produces, transports, transforms, converts, utilizes, manages, and controls energy. This coming era will be an exergy era, where quality will be more important than quantity.

50

50

40

48

30

46

20

44

10

42

0 290

294

298 302 Ambient temperature (K)

306

263

ex,ov (%)

en,ov (%)

Exergy

40 310

Fig. 40 Variation in overall energy and exergy efficiencies with rise in ambient temperature.

Exergy + Cost accounting + Environmental impact + Sustainability + LCA + Industrial ecology Exergy + Cost accounting + Environmental impact + Sustainability + LCA Exergy + Cost accounting + Environmental impact + Sustainability Exergy + Cost accounting + Environmental impact Exergy + Cost accounting Exergy

Fig. 41 An illustration of integrated exergy tools.

1.6.8

Concluding Remarks

In this chapter, the role of exergy is discussed and its use for a new concept of AIDA. A general exergy analysis methodology is presented along with both energy and exergy efficiencies. Two illustrative examples, studying two integrated systems, are presented, which involve energy and exergy analyses, and energy and exergy efficiency assessments of two environmentally benign integrated power-generating sources. Exergy destruction rates for the components of the system considered are investigated for possible improvement, and some parametric studies are also performed to rank these components accordingly. Furthermore, it is obtained that when ambient temperature is varied, the energy efficiency of the overall system in both case studies remains constant, whereas exergy efficiency of the overall system in both case studies varies vastly. The chapter presents that energy analysis itself is not sufficient enough to determine performance of any system and exergy analysis should always be considered to project a more appropriate performance of a system. It is thus concluded that in the first case study, the heat exchanger destructs the highest amount of exergy (1153 kJ/kg), and in second case study, the geothermal cycle turbine destructs the maximum amount of exergy (6915 kW).

264

Exergy

Acknowledgment The author acknowledges the support provided by the Natural Sciences and Engineering Research Council of Canada, and The Turkish Academy of Sciences.

References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]

Dincer I, Rosen MA. Exergy: energy, environment and sustainable development. 2nd ed. Oxford: Elsevier; 2013. Wikipedia. Aida. [Online] 2011. Available from: http://en.wikipedia.org/wiki/Aida. Dincer I, Ratlamwala TAH. Importance of exergy for analysis, improvement, design, and assessment. Energy and Environment 2013;2(3):335–49. Baehr HD, Schmidt EF. Definition und berechnung von brennstoffexergien (Definition and calculation of fuel exergy). Brennst-Waerme-Kraft 1963;15:375–81. Gaggioli RA, Petit PJ. Use the second law first. Chemtech 1977;7:496–506. Rodriguez LSJ. Calculation of available-energy quantities. In: Gaggioli RA, editor. Thermodynamics: Second Law Analysis. Washington, DC: American Chemical Society; 1980. p. 39–60. Szargut J. Grenzen fuer die anwendungsmoeglichkeiten des exergiebegriffs (limits of the applicability of the exergy concept). Brennstoff-Waerme-Kraft 1967;19:309–13. Sussman MV. Steady-flow availability and the standard chemical availability. Energy – The International Journal 1980;5:793–804. Ahrendts J. Reference states. Energy – The International Journal 1980;5:667–78. Bosnjakovic F. Bezugszustand der exergie eines reagiernden systems (reference states of the exergy in a reacting system. Forschung Im Ingenieurwesen 1963;20:151–2.

Further Reading Al Ali M, Dincer I. Performance assessment of integrated energy systems for HVAC applications. International Journal of Green Energy 2016;13:1342–51. Bejan A, Dincer I, Lorente S, Reis AH, Miguel AF. Porous media in modern technologies: energy, electronics, biomedical and environmental engineering. New York: Springer Verlag; 2004. p. 396. Dincer I. Refrigeration systems and applications. third ed. London: John Wiley & Sons, Ltd.; 2017. p. 727. Dincer I, Hamut HS, Javani N. Thermal management of electric vehicles. London: John Wiley & Sons, Ltd.; 2017. p. 457. Dincer I, Hogerwaard J, Zamfirescu C. Clean rail transportation options. New York: Springer Verlag; 2015. p. 223. Dincer I, Joshi A. Solar-based hydrogen production systems. New York: Springer Verlag; 2013. p. 141. Dincer I, Ratlamwala T. Integrated absorption refrigeration systems: comparative energy and exergy analyses. New York: Springer Verlag; 2016. p. 270. Dincer I, Rosen MA. Thermal energy storage systems and applications. London: John Wiley & Sons, Ltd.; 2002. p. 580. Dincer I, Rosen MA. Exergy. first ed. Oxford: Elsevier Science, Ltd.; 2007. p. 454. Dincer I, Rosen MA. Thermal energy storage systems and applications. second ed. London: John Wiley & Sons, Ltd.; 2011. p. 600. Dincer I, Rosen MA. Exergy analysis of heating, refrigerating and air conditioning. Oxford: Elsevier Science, Ltd.; 2015. p. 388. Dincer I, Rosen MA, Ahmadi P. Optimization of energy systems. London: John Wiley & Sons, Ltd.; 2017. p. 453. Dincer I, Zamfirescu C. Drying phenomena: analyses and applications. London: John Wiley & Sons, Ltd.; 2016. p. 482. Dincer I, Zamfirescu C. Sustainable hydrogen production. Oxford: Elsevier Science, Ltd; 2016. p. 479. Lucca G. The exergy analysis: role and didactic importance of a standard use of basic concepts, terms and symbols. In: A future for energy: Proc. Florence world energy research symposium; 1990. p. 295–308.

Relevant Websites https://www.eolss.net/sample-chapters/C15/E1-32-53-00.pdf Exergy Analysis for Sustainable Buildings. https://link.springer.com/chapter/10.1007%2F978-1-84882-647-2_2 Exergy Analysis of Green Energy Systems. https://www.elsevier.com/books/exergy/dincer/978-0-08-097089-9 Exergy, 2nd Edition. http://www.inderscience.com/jhome.php?jcode=IJEX Inderscience Publishers. http://www.me.unm.edu/Bmammoli/ME562_stuff/slides/motivation.pdf ME 562: Sustainable Energy – An Exergy Analysis. http://www.springer.com/gp/book/9783319046808 Progress in Exergy, Energy, and the Environment. http://cerl.uoit.ca/ University of Ontario Institute of Technology.

1.7 Energy and Exergy Efficiencies Ibrahim Dincer, University of Ontario Institute of Technology, Oshawa, ON, Canada r 2018 Elsevier Inc. All rights reserved.

1.7.1 1.7.2 1.7.3 1.7.4 1.7.5 1.7.5.1 1.7.5.2 1.7.5.3 1.7.6 1.7.6.1 1.7.6.2 1.7.6.2.1 1.7.6.2.2 1.7.7 1.7.8 1.7.8.1 1.7.8.1.1 1.7.8.1.2 1.7.9 1.7.9.1 1.7.9.2 1.7.9.3 1.7.9.3.1 1.7.9.3.2 1.7.9.4 1.7.9.5 1.7.9.5.1 1.7.9.5.2 1.7.10 1.7.10.1 1.7.11 1.7.11.1 1.7.12 1.7.12.1 1.7.12.2 1.7.12.3 1.7.12.4 1.7.12.5 1.7.12.6 1.7.12.7 1.7.12.8 1.7.12.9 1.7.13 1.7.13.1 1.7.13.2 1.7.13.3 1.7.13.4 1.7.13.5 1.7.13.6 1.7.13.7 1.7.13.8 1.7.14 1.7.15

Introduction Energy and Exergy Domains and Their Linkages to the Environment and Sustainability Efficiency and Energy Management Dimensions Thermodynamic Laws Energy Change and Energy Transfer Mass Transfer Heat Transfer Work The First Law of Thermodynamics Closed System Open System Steady flow system Unsteady uniform flow system The Second Law of Thermodynamics Entropy Entropy Balance Closed system Open system Exergy Reversibility and Irreversibility Reversible Work and Exergy Destruction Exergy Change Closed system Open system Exergy Transfer Mechanisms Exergy Balance Closed system Open system Energy and Exergy Efficiencies Energy Efficiency Efficiencies of Cyclic Devices Carnot Heat Engine and Carnot Refrigeration/Heat Pump Efficiencies of Steady Flow Devices Turbine Compressor Pump Nozzle Diffusers Throttling Valve Heat Exchanger Mixing Chamber Electric Resistance Heating Conversion Efficiencies of Common Devices Electric Resistance Heater Electric Water Heater Natural Gas Water Heater Combustion Efficiency Heating Values Boiler Efficiency Generator Efficiency and Overall Efficiency Lighting Efficiency Efficiencies of Mechanical and Electrical Devices Power Plant Efficiencies

Comprehensive Energy Systems, Volume 1

doi:10.1016/B978-0-12-809597-3.00123-1

267 267 269 271 271 272 272 272 272 272 273 273 274 274 275 276 276 276 277 277 277 277 278 278 278 278 279 279 280 280 281 283 285 285 287 290 292 293 295 296 300 300 303 303 303 304 304 304 304 305 305 305 306

265

266

Energy and Exergy Efficiencies

1.7.16 Efficiencies of Vapor Power Cycles 1.7.17 Efficiencies of Gas Power Plants 1.7.18 Efficiencies of Cogeneration Plants 1.7.18.1 Steam-Turbine Based Cogeneration Plant 1.7.18.2 Gas-Turbine Based Cogeneration Plant 1.7.19 Efficiencies of Geothermal Power Plants 1.7.20 Refrigerators and Heat Pumps 1.7.20.1 The Carnot Refrigeration Cycle 1.7.21 Second-Law Analysis of Vapor-Compression Refrigeration Cycle 1.7.22 Energy and Exergy Efficiencies of Vapor-Compression Heat Pump Cycle 1.7.23 Absorption Refrigeration Cycle 1.7.24 Efficiency Assessment of Psychrometric Processes 1.7.24.1 Balance Equations for Common Air-Conditioning Processes 1.7.24.1.1 Heating or cooling 1.7.24.1.2 Heating with humidification 1.7.24.1.3 Cooling with dehumidification 1.7.24.1.4 Evaporative cooling 1.7.24.1.5 Adiabatic mixing of air streams 1.7.25 Future Directions 1.7.26 Concluding Remarks Acknowledgments References Further Reading Relevant Websites

Abbreviations

PER SEER

Primary energy ratio Seasonal energy efficiency ratio

Nomenclature cp specific heat at constant pressure (kJ/kg K) cv specific heat at constant volume (kJ/kg K) E energy (kJ) ex specific exergy (kJ/kg) Ex amount of exergy (kJ) _ Ex rate of exergy (kW) Exdestroyed exergy destruction (kJ) g gravitational acceleration (m/s2) h enthalpy (kJ/kg) enthalpy of vaporization (kJ/kg) hfg HHV higher heating value (kJ/kg) HV heating value (kJ/kg) I current (amp) k specific heat ratio KE kinetic energy (kJ) LHV lower heating value (kJ/kg) m mass (kg) _ m mass flow rate (kg/s) n polytropic constant P pressure (kPa) PE potential energy (kJ)

q Q _ Q

W _ W z

specific heat transfer (kJ/kg) amount of heat transfer (kJ) rate of heat transfer (kW) gas constant (kJ/kg K) compression ratio cutoff ratio pressure ratio specific entropy (kJ/kg K) total entropy (kJ/K) entropy generation (kJ/K) time (s) temperature (K or 1C) specific internal energy (kJ/kg) total internal energy (kJ) specific volume (m3/kg) velocity (m/s) voltage (V) volume (m3) volume flow rate (m3/s) amount of work (kJ) rate of work or power (kW) elevation (m)

Greek letters energy efficiency Zen

Zex Zth

exergy efficiency thermal efficiency

COP EER

Coefficient of performance Energy efficiency ratio

R r rc rp s S Sgen t T u U v V V V V_

307 310 312 313 315 315 321 322 322 327 329 330 330 331 333 335 336 337 338 338 338 338 338 339

Energy and Exergy Efficiencies

Subscripts amb Cogen Comb Comp Cond CV e elect Evap Exp. valve H HP HTr in isen

1.7.1

ambient cogeneration combustion compressor condenser control volume electricity electricity evaporator expansion valve high temperature heat pump hydraulic turbine inlet isentropic

L mech out ov P R regen rev s surr th Tr w WTr 0

267

low temperature mechanical outlet overall pump refrigerator regenerator reversible source, isentropic surroundings thermal turbine water water turbine dead (environmental) state

Introduction

In any energy process, system and application, it is critically important to thermodynamically understand the system and design it, analyze its, and assess its performance through true efficiencies. In deploying thermodynamics, the description of any energy conversion system is usually followed by an appropriate efficiency definition of the system. This may be done through energy and exergy efficiencies, or energy-based coefficient of performance (COP) and exergy-based COP. A right assessment of system performance requires a careful study of efficiencies and their improvements. There are recent books [1,2] covering such efficiency definitions and their utilization for various systems and applications. For any practical system, efficiency, in general, can be defined as the ratio of desired (useful) output divided by the required input. Although this definition provides a simple general understanding of efficiency, a variety of specific efficiency relations for various energy systems and operations need to be developed. There are many conceptually different (and sometimes conflicting) efficiency definitions and equations in the open literature which may not necessarily be fully correct. There are even confusing efficiency definitions and equations which may cause wrong performance assessments. In this comprehensive contribution, the first aim is to introduce efficiency definitions correctly and discuss them for various thermodynamic closed and open systems and specific applications, and the second aim is to present numerous basic to advanced examples and case studies by correctly using efficiencies (in terms of energy and exergy efficiencies or energy and exergy-based coefficients of performance).

1.7.2

Energy and Exergy Domains and Their Linkages to the Environment and Sustainability

As stated in an earlier contribution on exergy, thermodynamics brings two important concepts, such as energy and exergy. Energy comes from the first law of thermodynamics and is treated under energy balance equation (EBE), while exergy comes from the second law of thermodynamics and is treated under exergy balance equation (ExBE). These two concepts technically govern all practical processes, and their connections to environmental impact and sustainable development are, therefore, of paramount significance. Exergy analysis is useful for improving the efficiency of energy-resource use, for it quantifies the locations, types, and magnitudes of wastes and losses. In general, more meaningful efficiencies are evaluated with exergy analysis rather than energy analysis, since exergy efficiencies are always a true measure of how nearly the efficiency of a process approaches the ideal. Therefore, exergy analysis identifies accurately the margin available to design more efficient energy systems by reducing inefficiencies. Many engineers and researchers agree that thermodynamic performance is best evaluated using exergy analysis because it provides more insights and is more useful in efficiency-improvement efforts than energy analysis alone. Measures to increase energy efficiency can reduce environmental impact by reducing energy losses. From an exergy viewpoint, such activities lead to increased exergy efficiency and reduced exergy losses (both waste exergy emissions and internal exergy consumption). A deeper understanding of the relations between exergy and the environment may reveal the underlying fundamental patterns and forces affecting changes in the environment, and help researchers deal in a better way to tackle with the environmental damages. The second law of thermodynamics is, in this regard, instrumental in providing insights into environmental impact. The most appropriate link between the second law and environmental impact has been suggested to be exergy [1], and, in part because it is a measure of the departure of the state of a system from that of the environment. The magnitude of the exergy of a system depends essentially on the states of both the system and the environment. This departure is zero only when the system is in equilibrium with its environment after reaching the reference state or dead state conditions.

268

Energy and Exergy Efficiencies

As a matter of fact exergy analysis is methodological and conceptual tool extracted from the second law of thermodynamics, and can pinpoint the losses of quality, or work potential, in a system. Exergy analysis is consequently linked to sustainability because in increasing the sustainability of energy use, we must be concerned not only with loss of energy, but also loss of energy quality (or exergy). A key advantage of exergy analysis over energy analysis is that the exergy content of a process stream is a better valuation of the stream than the energy content, since the exergy indicates the fraction of energy that is likely useful and thus utilizable. This observation applies equally at the component level, process level, and life cycle level. Application of exergy analysis to a component, process, or sector can lead to insights in how to improve the sustainability of the activities comprising the system by reducing exergy losses. Sustainable development requires not just that sustainable energy resources be used, but that the resources be used efficiently. The authors and others feel that exergy methods can be used to evaluate and improve efficiency and thus to improve sustainability. Since energy can never be “lost” as it is conserved according to the first law of thermodynamics, while exergy can be lost due to internal irreversibilities, exergy losses which represent potential not used, particularly from the use of nonrenewable energy forms, should be minimized when striving for sustainable development. Furthermore, Fig. 1 clearly summarizes the key advantages of exergy as potential for better environment and sustainable development. It is obvious that an understanding of the thermodynamic aspects of sustainable development can help in taking sustainable actions regarding energy. Thermodynamic principles can be used to design, analysis, assess and improve systems and their performances, and to better understand environmental impact and sustainability issues. For the broadest understanding, all thermodynamic principles must be used, not just those pertaining to energy. Thus, many researchers feel that an understanding and appreciation of exergy, as defined earlier, is essential in the discussions of environmental impact and sustainable development and their dimensions. Fig. 1 further illustrates what exergy does through primary actions regarding the systems and applications, covering design, analysis, assessment, and improvement which ultimately help achieve better efficiency, better resources use, better economics, better environment, and better energy security which finally all contributes to obtaining better sustainable development. Fig. 2 illustratively presents the relation between exergy and sustainability and environmental impact. Here, sustainability is seen to increase and environmental impact to decrease as the process exergy efficiency increases. The two limiting efficiency cases

Exergy

Design

Analysis

Assessment

Improvement

Better efficiency Better resources use Better economics Better environment Better energy security

Better sustainability

Fig. 1 Illustration of how exergy contributes to better environment and sustainable development.

Environmental impact

Environmental impact Sustainability

Sustainability

0

Exergy efficiency (%)

100

Fig. 2 Qualitative representation of what kind of relation exists between the environmental impact and sustainability of a process, and its exergy efficiency.

Energy and Exergy Efficiencies

269

are significant. First, as exergy efficiency approaches the full efficiency (i.e., 100%), environmental impact approaches zero (i.e., 0%), since exergy is only converted from one form to another without loss, either through internal consumptions or waste emissions. Also sustainability approaches infinity because the process approaches reversibility. Second, as exergy efficiency approaches 0%, sustainability approaches zero because exergy-containing resources are used but nothing is accomplished. Also, environmental impact approaches infinity because, to provide a fixed service, an ever increasing quantity of resources must be used, and a correspondingly increasing amount of exergy-containing wastes is emitted. The relationships between environmental impact and sustainability versus exergy efficiency may be expressed quantitatively by some examples. But first we develop the formulation needed for such an analysis. A power plant exergy efficiency may be expressed as Zex ¼

_ out W _ in Ex

ð1Þ

_ out is the net (useful) work output rate produced and Ex _ in is the exergy input rate, which is equal to the mass flow rate of where W fuel consumed times the specific fuel exergy content. The exergy input rate may also be defined in the form of thermal exergy input rate due to heat transfer rate. For refrigeration and heat pump systems, the basic efficiency approach is no longer used due to the values more than 100%, which is considered impossible for efficiency. This essentially requires another performance criterion, rather than efficiency, as COP. The exergetic COP for a refrigeration system is defined as the actual COP divided by the reversible COP under the same temperature limits: COPact ð2Þ COPrev where the reversible COP, based on the Carnot refrigerator, is defined in terms of the temperatures of the low-temperature reservoir TL and high-temperature reservoir TH as COPex ¼

COPrev ¼

TL TH

TL

ð3Þ

Connelly and Koshland [3] suggested that the efficiency of fossil fuel consumption is simply characterized by a depletion number defined as Dp ¼

_ d Ex _ in Ex

ð4Þ

which represents the relationship between the exergy destruction Exd and the exergy input Exin by fuel consumption. The relationship between the depletion factor and the exergy efficiency is Zex ¼ 1

Dp

ð5Þ

It is now important as stated by Connelly and Koshland [3] to express the sustainability of the fuel resource by a sustainability index (SI) as the inverse of the depletion number as an indication of how the magnitude of depletion (through the depletion number) affects the sustainability (through SI): SI ¼

1 Dp

ð6Þ

Example 1: As a first example, we consider a power plant using natural gas (approximated as methane) as the fuel. We express the environmental impact in terms of the amount of carbon dioxide emissions. A balanced chemical combustion equation of methane shows that for each kilogram of methane burned, 2.75 kg of carbon dioxide (CO2) is released. The specific chemical exergy of methane is 51,840 kJ/kg [3]. The amount of carbon dioxide emitted and the SI as a function of the exergy efficiency for 1 kWh of power production are plotted in Fig. 3. The trends explained in Fig. 2 generally apply to the results shown in Fig. 3. Example 2: As a second example, we consider an air-conditioner used to maintain a space at 251C (298K) when the outdoors is at 351C (308K). It is assumed that the electricity consumed by this air-conditioner is produced in a coal-fired power plant. For 1 kW of electricity produced in a coal-fired power plant, 6.38 g of SO2 and 3.69 g of NOx are emitted. In this example, we express the environmental impact in terms of the total SO2 and NOx emissions. These emissions and the SI as a function of the exergy efficiency for 1 kWh of cooling load from the space are illustrated in Fig. 4.

1.7.3

Efficiency and Energy Management Dimensions

In the analysis of any energy process, system or application, it is important to understand the difference between energy and exergy efficiencies. By considering both of these efficiencies, the quality and quantity of energy used to achieve a targeted objective is considered, and the degree to which efficient and effective use of energy resources is achieved can be understood. Improving efficiencies of energy systems is recognized as an important challenge for meeting energy policy objectives. It is necessary to note that reductions in energy consumption may generally assist in partly attaining energy security goals. In addition to efficient energy

Energy and Exergy Efficiencies

20

3.5

CO2 emission (kg)

3.0

16

2.5 2.0

12

1.5

8

1.0 4

0.5 0

20

40 60 80 Exergy efficiency (%)

Sustainability index

270

100

Fig. 3 A quantitative illustration of the relation between the carbon dioxide emission and sustainability index (SI) of power generation, and its exergy efficiency. The fuel considered here is methane, and the results are obtained for 1 kWh of power output.

20

7 16

6 5

12

4 8

3 2

4

Sustainability index

Emissions (SO2 + NOx) (g)

8

1 0

20

40 60 Exergy efficiency (%)

80

100

Fig. 4 A quantitative illustration of the relation between the SO2 and NOx emissions and the sustainability index (SI) and its exergy efficiency. The system is an air-conditioner with electricity as the work input. The results are essentially obtained for 1 kWh of cooling load.

utilization, an introduction of renewable energy technologies may significantly help resolve environmental issues, especially by considering the fact that the past climate change conference in Paris in 2015 aimed to address the global climate change by employing renewables. Note that increased energy efficiency benefits the environment by avoiding energy use and the corresponding resource consumption and pollution generation. From an economic as well as an environmental perspective, improved energy efficiency has great potential for achieving better sustainability. Furthermore, accelerated gains in efficiency in energy production and use, particularly in the power generation and utility sectors, can help reduce environmental impact and promote energy security. While there is a large technical potential for increased efficiency, there exist significant social and economic barriers to its achievement. It is necessary to give priority to the energy management policies and strategies that will yield efficiency gains. However, reliance on such policies and strategies alone is unlikely to overcome these barriers. For this reason, innovative and bold approaches are required by government, in cooperation with decision makers in the power generation industry, to realize the opportunities for efficiency improvements, and to accelerate the deployment of new and more efficient technologies. An engineer designing a system is often expected to aim for the highest reasonable technical efficiency at the lowest cost under the prevailing technical, economic, and legal conditions, and with regard to ethical, ecological, and social consequences. Exergy methods can assist in such activities, and offer unique insights into possible improvements. Exergy analysis is a useful tool for addressing the environmental impact of energy resource utilization, and for furthering the goal of more efficient energy-resource use, for it enables the locations, types, and true magnitudes of losses to be determined. Also, exergy analysis reveals whether or not and by how much it is possible to design more efficient energy systems by reducing system related inefficiencies. Exergy is also strongly related to sustainability and environmental impact. Sustainability increases and environmental impact decreases as the exergy efficiency of a process increases, as illustrated in Fig. 2. As exergy efficiency approaches 100%, the environmental impact associated with process operation approaches zero, since exergy is only converted from one form to another without loss (either through internal consumption or waste emissions). Also, the process approaches sustainability since it approaches reversibility. As exergy efficiency approaches 0%, the process deviates as much as possible from sustainability because

Energy and Exergy Efficiencies

271

exergy-containing resources (fuel, ores, steam, etc.) are used but nothing is accomplished. Also, environmental impact increases markedly because, to provide a fixed service, an ever increasing quantity of resources must be used and a correspondingly increasing amount of exergy-containing wastes are emitted to the surroundings [1]. Finally, energy and exergy efficiencies are treated as critical performance assessment tools for energy systems and applications and hence their potential improvements. However, the use of ambiguous efficiencies, which are not clearly defined, may not serve this purpose adequately. A clear, correct and effective definition and use of energy and exergy efficiencies is crucial in efficiency improvement efforts, which are often considered a key objective in energy management policies and strategies.

1.7.4

Thermodynamic Laws

Although there are four thermodynamic laws, namely, zeroth, first, second, and third laws of thermodynamics, two of these, such as zeroth and third laws of thermodynamics, are related to specific situations and become state connected. The other two laws, namely, first and second law of thermodynamics, essentially govern the processes, systems, and applications and are considered throughout this chapter. The key sources [1,2,4] state that a conventional thermodynamic analysis involves an application of the first law of thermodynamics, also known as energy analysis. Exergy analysis is a thermodynamic analysis technique based on the second law of thermodynamics, which provides an alternative and illuminating means of assessing and comparing processes and systems rationally and meaningfully. In particular, exergy analysis yields efficiencies which provide a true measure of how nearly actual performance approaches the ideal and identifies, more clearly than energy analysis, the causes and locations of thermodynamic losses and the impact of the built environment on the natural environment. Consequently, exergy analysis can assist in improving and optimizing designs. Energy and exergy efficiencies are considered by many to be useful for the assessment of energy conversion and other systems and for efficiency improvements. By considering both of these efficiencies, the quality and quantity of the energy used to achieve a desired objective is considered and the degree to which efficient and effective use of energy resources is achieved can be understood. Improving efficiencies of energy systems is an important challenge for meeting energy policy objectives. Reductions in energy use can assist in attaining energy security objectives. Also, efficient energy utilization and the introduction of renewable energy technologies can significantly help solve environmental issues. Increased energy efficiency benefits the environment by avoiding energy use and the corresponding resource consumption and pollution generation. From an economic as well as an environmental perspective, improved energy efficiency has great potential. An engineer designing a system is often expected to aim for the highest possible system efficiency at the lowest cost under the prevailing technical, economic, and legal conditions, and with regard to ethical, ecological, and social consequences. Exergy methods can assist in such activities, and offer unique insights into possible improvements with special emphasis on the environment and sustainability. Exergy analysis is the proposed to be a useful tool for addressing the environmental impact of energy resource utilization, and for furthering the goal of more efficient energy-resource use, for it enables the locations, types, and true magnitudes of losses to be determined. Also, exergy analysis reveals whether or not and by how much it is possible to design more efficient energy systems by reducing inefficiencies [1].

1.7.5

Energy Change and Energy Transfer

Energy is thermodynamically defined as the capacity for doing work, and hence energy of a system may consist of internal energy, kinetic energy (KE), and potential energy (PE). Internal energy may cover thermal (sensible and latent), chemical, and nuclear energies. Unless there is a chemical or nuclear reaction the internal change of a system is due to thermal energy change. In the absence of electric, magnetic, surface tension, and other possible effects, the total change in system energy is written as DE ¼ E2

E1 ¼ DU þ DKE þ DPE

ð7Þ

where internal, kinetic and potential energy changes are defined as DU ¼ mðu2 DKE ¼

1 mðV2 2 2

u1 Þ V1 2 Þ

ð8Þ ð9Þ

1 z1 Þ ð10Þ mðz2 2 For most cases, the changes in KE and PE are considered negligible during a process where the energy change is due to internal energy change only as follows: DPE ¼

DE ¼ DU ¼ mðu2

u1 Þ

in J or kJ

ð11Þ

Energy per unit time is defined as the rate of energy as follows: E E_ ¼ in W or kW Dt

ð12Þ

272

Energy and Exergy Efficiencies

The specific energy is defined as the energy per unit mass as follows: e¼

E in J=kg or kJ=kg m

ð13Þ

Energy transfer to or from a system may be in three common forms through mass, heat, and work, which are briefly described below.

1.7.5.1

Mass Transfer

The mass entering a system carries energy with it and the energy of the system increases. The mass leaving a system decreases the _ (kg/s), the rate of energy entering is equal to energy content of the system. When a fluid flows into a system at a mass flow rate of m mass times energy (including three items: flow enthalpy, flow kinetic energy, and flow potential energy) of a unit mass of a flowing _ þ V 2 =2 þ gzÞ (kW) where h¼ u þ Pv and Pv is the flow energy (also called flow work) described below. fluid: mðh

1.7.5.2

Heat Transfer

It is important to note that heat is the thermal form of energy and that heat transfer takes place when a temperature difference exists within a medium or between different media. Heat always requires a difference in temperature for its transfer. Higher temperature differences provide higher heat transfer rates. Heat transfer has the same unit as energy. The symbol for heat transfer is _ (kW). If there is Q (kJ). Heat transfer per unit mass is denoted by q (kJ/kg). Heat transfer per unit time is the rate of heat transfer Q no heat transfer involved in a process, it is so called an adiabatic process.

1.7.5.3

Work

Work is a form of the energy that is transferred by a difference in pressure or under the effect of a force of any kind and is subdivided into shaft work and flow work. Work is denoted by W. Shaft work is mechanical energy used to drive a mechanism, such as a pump, compressor, or turbine. Flow work is the energy transferred into a system by fluid flowing into, or out of, the _ (kW). Work has the same unit as energy. The direction of system. The rate of work transfer (i.e., per unit time) is called power W heat and work interactions can be expressed by sign conventions or using subscripts such as “in” and “out.”

1.7.6

The First Law of Thermodynamics

So far, we have considered various forms of energy, such as heat Q, work W, and total energy E individually, and no attempt is made to relate them to each other during a process. The first law of thermodynamics, also known as the conservation of energy principle, provides a sound basis for studying the relationships among the various forms of energy and energy interactions. Based on experimental observations, the first law of thermodynamics states that energy can be neither created nor destroyed during a process; it can only change forms. Therefore, every bit of energy should be accounted for during a process. It is known that a rock at some elevation possesses some PE, and some part of this PE is converted to KE as the rock falls. Experimental data show that the decrease in PE (mgz) exactly equals the increase in KE when the air resistance is negligible, thus confirming the conservation of energy principle for mechanical energy.

1.7.6.1

Closed System

The first law of thermodynamics can be expressed for a general closed system as the net change in the total energy of a closed system during a process is equal to the difference between the total energy entering and the total energy leaving the system: Ein

Eout ¼ DEsystem

ð14Þ

dE E_ out ¼ dt

ð15Þ

In rate form: E_ in

For a closed system undergoing a process between initial and final states (states 1 and 2) with heat and work interactions with the surroundings as shown in Fig. 5: Ein Eout ¼ DEsystem ðQin þ Win Þ ðQout þ Wout Þ ¼ DU þ DKE þ DPE

ð16Þ

If there are no changes in KE and PE: ðQin þ Win Þ

ðQout þ Qout Þ ¼ DU ¼ mðu2

u1 Þ

ð17Þ

The balance equations are thermodynamically necessary for any system to be written for mass, energy, entropy and exergy which are presented below.

Energy and Exergy Efficiencies

273

Boundary of the system Qin Wout

Mass, m state 1 to state 2 Q out

Win

Fig. 5 A general closed system with heat and work interactions.

Boundary of the system

min

Wout

msystem = Constance

Qin Qout

Win

mout Fig. 6 A general steady-flow control volume (CV) with mass, heat and work interactions.

The mass balance equation (MBE) of the closed system, as shown in Fig. 5 is as follows: MBE: m1 ¼ m2 ¼ constant

ð18Þ

where m is the mass of the system and the subscripts 1 and 2 refer to the first and the second states of the closed system. The EBE describing the energy interactions of the closed system as shown in Fig. 5 is written as follows: EBE: m1 u1 þ Qin þ Win ¼ m1 u1 þ Qout þ Wout

ð19Þ

where u is the specific internal energy of the closed system, Q is the heat transfer, and W is the work. The subscripts in and out refer to the energy getting into and out of the boundary of the closed system.

1.7.6.2

Open System

The system is considered open if mass can enter or leave the controlled volume of the system. Another description of the open system is a system with a control volume (CV), which means that the volume of the system remains constant throughout the process. The two types of an open system discussed in this chapter are based on whether the mass inside the CV is constant or changes with time.

1.7.6.2.1

Steady flow system

An open system is described as a steady flow (i.e., steady state steady flow) system if the mass inside the controlled volume of the open system does not change with time. Let us consider a CV involving a steady-flow process. Mass is entering and leaving the system and there is heat and work interactions with the surroundings (Fig. 6). During a steady flow process, the total mass and energy content of the CV remains constant, and thus the total energy change of the system is zero. Then the first law of thermodynamics is expressed as dE ¼0 E_ out ¼ dT E_ in ¼ E_ out     V2 V2 _ in þ W _ out þ W _ in þ m _ out þ m _ hin þ in þ gzin ¼ Q _ hout þ out þ gzout Q 2 2

E_ in

ð20Þ

If there are no changes in KE and PE: _ in þ W _ out þ W _ in þ mh _ out þ mh _ in ¼ Q _ out Q

ð21Þ

The mass, energy, entropy, and exergy balance equations are to be written for any open system. In this regard, the MBE of the open system operating at a steady flow process shown in Fig. 6 is as follows: MBE: m _ in

_ out ¼ dmsystem =dt ¼ 0 m

ð22Þ

274

Energy and Exergy Efficiencies

Boundary of the system

m in

Qin

msystem and energy varies with time

Wout Win

Qout

mout Fig. 7 A general unsteady-flow process with mass, heat and work interactions.

where m_ is the mass flow rate entering or leaving the CV based on the subscripts in and out respectively and msystem is the mass of the system. The right side of the MBE shows the time rate change of the mass inside the CV and it equals to zero since the mass inside the controlled volume remains constant (steady flow process). The EBE describing the energy interactions of the open system operating in a steady flow process shown in Fig. 6 is written as follows: _ in þ W _ in EBE: m _ in hin þ Q

_ out hout m

_ out Q

_ out ¼ dEsystem =dt ¼ 0 W

ð23Þ

_ is where h is the specific enthalpy of the flowing mass entering or leaving the controlled volume, Q_ is the heat transfer rate, and W the work rate. The right side of the EBE for the open system operating in a steady flow process is the rate change of the energy in the system which equal to zero. The energy of the system changes with location in the system but does not change with time.

1.7.6.2.2

Unsteady uniform flow system

In a steady-state open system, there is no variation of the energy with time inside the CV, however, in an unsteady uniform flow system the energy inside the control varies with time. Charging and discharging processes for tanks, containers, reservoirs, etc. may be analyzed or modeled as unsteady flow processes. An unsteady flow process with increasing mass time-dependently within the system is shown in Fig. 7. Assuming uniform flow conditions, the mass and energy balance relations may be expressed as mout ¼ m2

min Ein Qin þ Win þ min



V2 hin þ in þ gzin 2



ð24Þ

m1

Eout ¼ DEsystem

Qout

Wout

  V2 mout hout þ out þ gzout ¼ m2 u2 2

m1 u1

ð25Þ

The mass, energy, entropy, and exergy balance equations can be written by equalizing the inputs and outputs for the system considered. The MBE of the open system operating in an unsteady uniform flow process shown in Fig. 7 is as follows: MBE: m _ in

_ out ¼ m

dmsystem ¼ m2 dt

ð26Þ

m1

The EBE describing the energy interactions of the open system operating in an unsteady uniform flow process shown in Fig. 7 is written as follows _ in þ W _ in EBE: m _ in hin þ Q

1.7.7

_ out hout m

_ out Q

_ out ¼ dEsystem =dt ¼ m2 u2 W

m1 u1

ð27Þ

The Second Law of Thermodynamics

Energy is treated as a conserved property, and no process is known to have taken place in violation of the first law of thermodynamics. Therefore, it is reasonable to conclude that a process must satisfy the first law to occur. However, as explained in the previous sections satisfying the first law alone does not ensure that the process will actually take place. It is important to go one step ahead to consider the second law of thermodynamics, which is about nonconservation of exergy. It is clearly known that processes proceed in a certain direction and not in the reverse direction. The first law places no restriction on the direction of a process, but satisfying the first law does not ensure that the process can actually occur. This inadequacy of the first law to identify whether a process can actually take place is remedied by introducing another general principle, the second law of thermodynamics. It is indicated later in this chapter that the reverse processes discussed above violate the second law of thermodynamics. This violation is easily detected with the help of a property, called entropy. A process cannot occur unless it fully satisfies both first and second laws of thermodynamics. It is important to note that the use of the second law of thermodynamics is not limited to identifying the direction of processes. The second law also asserts that energy has quality as well as quantity. The first law is concerned with the quantity of energy and the transformations of energy from one form to another with no regard to its quality. Preserving the quality of energy is a major concern to engineers, and the second law provides the necessary means to determine the quality as well as the degree of

Energy and Exergy Efficiencies

275

degradation of energy during a process. As discussed later in this chapter, more of high-temperature energy can be converted to work, and thus it has a higher quality than the same amount of energy at a lower temperature. The second law of thermodynamics is also used in determining the theoretical limits for the performance of commonly used engineering systems, such as heat engines and refrigerators, as well as predicting the degree of completion of chemical reactions. The second law is also closely associated with the concept of perfection. In fact, the second law defines perfection for thermodynamic processes. It can be used to quantify the level of perfection of a process, and point the direction to eliminate imperfections effectively. Energy has quality as well as quantity. More of the high-temperature thermal energy can be converted to work. Therefore, it results with “the higher the temperature, the higher the quality of the energy.” Large quantities of solar energy, for example, can be stored in large bodies of water called solar ponds at about 350K. This stored energy can then be supplied to a heat engine to produce work (electricity). However, the efficiency of solar pond power plants is very low (under 5%) because of the low quality of the energy stored in the source, and the construction and maintenance costs are relatively high. Therefore, they are not competitive even though the energy supply of such plants is free. The temperature (and thus the quality) of the solar energy stored could be raised by utilizing concentrating collectors, but the equipment cost in that case becomes very high. Work becomes a more valuable form of energy than heat since 100% of work can be converted to heat, but only a fraction of heat can be converted to work. When heat is transferred from a high-temperature body to a lower temperature one, it is degraded since less of it now can be converted to work. For example, if 100 kJ of heat is transferred from a body at 1000K to a body at 300K, at the end we will have 100 kJ of thermal energy stored at 300K, which has no practical value. But if this conversion is made through a heat engine, up to 1 300/1000¼ 0.70 ¼ 70% of it could be converted to work, which is a more valuable form of energy. Thus 70 kJ of work potential is wasted as a result of this heat transfer, and energy is degraded [4]. Example 3: Consider the heating of a room by the passage of electric current through a resistor. Note that the first law dictates that the amount of electric energy supplied to the resistance wires be equal to the amount of energy transferred to the room air as heat. Now let us attempt to reverse this process. It will come as no surprise that transferring some heat to the wires does not cause an equivalent amount of electric energy to be generated in the wires. There are numerous forms of statements related to the second law of thermodynamics. Two most common statements of these ones are listed as follows:

•

•

The Kelvin–Plank statement: it is impossible to construct a device, operating in a cycle, for example: a heat engine, which accomplishes only the extraction of heat from a high-temperature source and its complete conversion to work without rejecting any heat to the surroundings. This simply shows the impossibility of having a heat engine operating with an energy (thermal) efficiency of 100%. The Clausius statement: it is impossible to construct a device, operating in a cycle, for example: a refrigerator or heat pump, which transfers heat from the low-temperature side (evaporator) to the high-temperature side (condenser) and producing no other effects. This simply shows the impossibility of running a refrigerator or heat pump without work input.

1.7.8

Entropy

The second law of thermodynamics often leads to expressions that involve inequalities. An irreversible (i.e., actual) heat engine, for example, is less efficient than a reversible one operating between the same two thermal energy reservoirs. Likewise, an irreversible refrigerator or heat pump has a lower COP than a reversible one operating between the same temperature limits. In any process that is not reversible there is some entropy generated or created during an irreversible process, and this generation is due entirely to the presence of irreversibilities. The entropy generated during a process is called entropy generation and is denoted by Sgen. Noting that the difference between the entropy change of a closed system and the entropy transfer is equal to entropy generation. Note that the entropy generation Sgen is always a positive quantity or zero. Its value depends on the process, and thus it is not a property of the system. Also, in the absence of any entropy transfer, the entropy change of a system is equal to the entropy generation. The principle of the change of the entropy in a system can be expressed as the entropy of an isolated system during a process always increases or, in the limiting case of a reversible process, remains constant. In other words, it never decreases. This is known as the increase of entropy principle. Note that in the absence of any heat transfer, entropy change is due to irreversibilities only, and their effect is always to increase entropy. Entropy is an extensive property, thus the total entropy of a system is equal to the sum of the entropies of the parts of the system. An isolated system may consist of any number of subsystems. A system and its surroundings, for example, constitute an isolated system since both can be enclosed by a sufficiently large arbitrary boundary across which there is no heat, work, or mass transfer. Therefore, a system and its surroundings can be viewed as the two subsystems of an isolated system, and the entropy change of this isolated system during a process is the sum of the entropy changes of the system and its surroundings, which is equal to the entropy generation since an isolated system involves no entropy transfer. Since no actual process is truly reversible, we can conclude that some entropy is generated during a process, and therefore the entropy of the universe, which can be considered to be an isolated system, is continuously increasing. The more irreversible a process, the larger the entropy generated during that process. No entropy is generated during reversible processes. The increase of entropy principle does not imply that the entropy of a

276

Energy and Exergy Efficiencies

system cannot decrease. The entropy change of a system can be negative during a process, but entropy generation cannot. The performance of engineering systems is degraded by the presence of irreversibilities, and entropy generation is a measure of the magnitudes of the irreversibilities present during that process. The greater the extent of irreversibilities, the greater the entropy generation. Therefore, entropy generation can be used as a quantitative measure of irreversibilities associated with a process. It is also used to establish criteria for the performance of engineering devices.

1.7.8.1

Entropy Balance

Entropy is a thermodynamic property which is a measure of molecular disorder or randomness of a system, and the second law of thermodynamics states that entropy can be created but it cannot be destroyed. Therefore, the entropy change of a system during a process is greater than the entropy transfer by an amount equal to the entropy generated during the process within the system, and the increase of entropy principle for any system is expressed as Sout þ Sgen ¼ DSsysten

Sin

ð28Þ

where S is the entropy, and the subscripts in, out, and system refer to entering, leaving, and that of the system. This relation is often referred to as the entropy balance and is applicable to any system undergoing any process. The entropy balance relation can then be stated as: the entropy change of a system during a process is equal to the net entropy transfer through the system boundary and the entropy generated within the system. There are two quantities, such as heat and mass, which lead to entropy be transferred. The entropy transfer by heat is defined as Q T

ð29Þ

Smass ¼ ms

ð30Þ

Sheat ¼ The entropy transfer by mass is given by

When two systems are in contact, the entropy transfer from the warmer system is equal to the entropy transfer into the cooler one at the point of contact. That is, no entropy can be created or destroyed at the boundary since the boundary has no thickness and occupies no volume. Note that work is entropy-free, and no entropy is transferred by work.

1.7.8.1.1

Closed system

Regarding the entropy balance equation (EnBE) for a closed system shown in Fig. 5, the EnBE on this closed system can be written in a general form as     Q Q þ Sgen ¼ mðs2 s1 Þ ð31Þ T in T out Since the closed system has energy interactions only through two forms, heat and work and since work interactions do not lead to the generation of entropy then the EnBE of a closed system contains only entropy entering the system due to heat transfer, entropy generated due to the irreversibilities and change of the entropy of the system, respectively.

1.7.8.1.2

Open system

The entropy rate balance for a steady flow open system can be expressed in the rate form as S_ in

S_ out þ S_ gen ¼ dSsystem =dt

ð32Þ

Now, consider the CV as shown in Fig. 6 with a steady-flow process. The entropy balance on this CV can be expressed in a general form as _ Q _ in þ ms T in

_ Q T out

_ out þ S_ gen ¼ 0 ms

ð33Þ

In Eqs. (31) and (33), T represents the temperature of the boundary at which heat transfer takes place. If the system is selected such that it includes the immediate surroundings, the boundary temperature becomes the temperature of the surroundings. Therefore, one can use the surrounding ambient temperature in these equations. For the unsteady-flow process shown in Fig. 7, the EnBE can be expressed as     Q Q þ min sin mout sout þ Sgen ¼ m2 s2 m1 s1 ð34Þ T in T out In recent decades, much effort has been spent in minimizing the entropy generation (irreversibility) in thermodynamic systems and applications.

Energy and Exergy Efficiencies

1.7.9

277

Exergy

The attempts to quantify the quality or “work potential” of energy in the light of the second law of thermodynamics have resulted in the definition of the property named exergy. This makes exergy as an indication of work potential and quality of energy, rather than quantity. In this regard, exergy analysis appears to be a potential thermodynamic tool based on the second law of thermodynamics, which provides critical alternative and illuminating means of assessing and comparing processes and systems rationally and meaningfully. In particular, exergy analysis yields efficiencies which provide a true measure of how nearly actual performance approaches the ideal and identifies, more clearly than energy analysis, the causes and locations of thermodynamic losses and the impact of the built environment on the natural environment. Consequently, the answer of what exergy can do is analysis, improvement, design and assessment (AIDA) as recently introduced in Ref. [5]. Energy and exergy efficiencies are both considered by many to be useful for the assessment of energy systems and applications and for potentially efficiency improvements. By considering both of these efficiencies, both quality and quantity of the energy used to achieve a given objective are considered together in harmony, and the degree to which efficient and effective use of energy resources is achieved can be understood. Improving efficiencies of energy systems is an important challenge for meeting energy policy objectives. Furthermore, the useful work potential of a given amount of energy at a specified state is called exergy. It is also called the availability or available energy. The work potential of the energy contained in a system at a specified state, relative to a reference (dead) state, is simply the maximum useful work that can be obtained from the system. A system is said to be in the dead state when it is in thermodynamic equilibrium with its environment. At the dead state, a system is at the temperature and pressure of its environment (in thermal and mechanical equilibrium); it has no KE or PE relative to the environment (zero velocity and zero elevation above a reference level); and it does not react with the environment (chemically inert). Also, there are no unbalanced magnetic, electrical, and surface tension effects between the system and its surroundings, if these are relevant to the situation at hand. The properties of a system at the dead state are denoted by subscript zero, for example, P0, T0, h0, u0, and s0. Unless specified otherwise, the dead-state temperature and pressure are taken to be T0 ¼251C and P0 ¼ 1 atm (101.325 kPa). It should be noted that a system has zero exergy at the dead state. It is important to note that the exergy of a system at a specified state depends on the conditions of the environment (the dead state) as well as the properties of the system. Therefore, exergy is a property of the systemenvironment combination and not of the system alone. Altering the environment is other way of increasing exergy, but it is definitely not an easy alternative. The work potential or exergy of the KE of a system is equal to the KE itself since it can be converted to work entirely. Similarly, exergy of PE is equal to the PE itself. On the other hand, the internal energy and enthalpy of a system are not entirely available for work, and only part of thermal energy of a system can be converted to work. In other words, exergy of thermal energy is less than the magnitude of thermal energy.

1.7.9.1

Reversibility and Irreversibility

Both reversibility and irreversibility are two guiding concepts which are highly important in the analysis of thermodynamic processes and systems. The reversibility refers to a process during which both the system and its surroundings can be returned to their initial states. The irreversibility is associated with the entropy generation and the destruction of exergy, and during an irreversible process both the system and its surroundings cannot be returned to their initial states because of the irreversibilities occurring (e.g., friction, heat rejection, electrical and mechanical effects, etc.).

1.7.9.2

Reversible Work and Exergy Destruction

The reversible work Wrev is defined as the maximum amount of useful work output or the minimum work input for a system undergoing a process between the specified initial and final states in a totally reversible manner. Any difference between the reversible work Wrev and the actual work Wu is due to the irreversibilities present during the process, and this difference is called irreversibility or exergy destroyed. It is expressed as Ex d ¼ Wrev;out

Wout

or

Exd ¼ Win

Wrev;in

ð35Þ

Note that irreversibility is a positive quantity for all actual (irreversible) processes since Wrev ZW for work producing devices and Wrev rW for work-consuming devices. Irreversibility can be viewed as the wasted work potential or the lost energy to do useful work. It represents the energy that was supposed to be converted to work potential, but was not. It is important to note that such lost work potentials manifest themselves in environmental degradation and avoidable emissions. The smaller the irreversibility associated with a process, the greater the work that is produced (or the smaller the work that is consumed). The performance of a system can only be improved by minimizing the irreversibilities associated with it.

1.7.9.3

Exergy Change

In order to study the change of exergy during processes, two forms of the thermodynamic systems are considered here in the forms of closed and open systems as detailed below.

278

Energy and Exergy Efficiencies

1.7.9.3.1

Closed system

For a closed system where the mass contained inside the boundary of the system remains constant and may, in general, possess KE and PE, as well as in the absence of electric, magnetic and surface tension effects, the total energy of a closed system becomes equal to the sum of its internal energy, KE and PE. Note that KE and PE themselves are treated same as for kinetic exergy and potential exergy. In this regard, the exergy of a closed system of mass m is given by Ex ¼ ðU

U0 Þ þ P0 ðV

V0 Þ

T0 ðS

S0 Þ þ mV 2 =2 þ mgz

ð36Þ

where the properties with the subscript zero represents those at the dead state. On a unit mass basis, the closed system (or nonflow) exergy is expressed as ex ¼ ðu

u0 Þ þ P0 ðv

v0 Þ

T0 ðs

s0 Þ þ V 2 =2 þ gz

ð37Þ

From the previous two equations the exergy change of a closed system during a process is simply the difference between the final and initial exergies of the system. DEx 1-2 ¼ ðU2

U1 Þ þ P0 ðV2

V1 Þ

S1 Þ þ mðV22

T0 ðS2

V12 Þ=2 þ mgðz2

z1 Þ

ð38Þ

For stationary closed systems, the changes in both KE and PE become zero, which results in KE and PE terms dropped out or canceled. Note that the exergy change of a closed system or a fluid stream represents the maximum amount of useful work that can be done (or the minimum amount of useful work that needs to be supplied if it is negative) as the system changes from state 1 to state 2 in a specified environment, and represents the reversible work Wrev. It is independent of the type of process executed, the kind of system used, and the nature of energy interactions with the surroundings. Also note that the exergy of a closed system cannot be negative, but the exergy of a flow stream can at pressures below the environment pressure P0.

1.7.9.3.2

Open system

The total exergy of a flowing fluid, also called flow (or stream) exergy, and is given by ex ¼ ðh

h0 Þ

s0 Þ þ V 2 =2 þ gz

T0 ðs

ð39Þ

Note that the same specific exergy symbol is used for flowing and non-flowing mass, and that is because the definition of the enthalpy is the internal energy plus the flow energy, and for a non-flowing mass its flow energy is zero resulting in the reduction of enthalpy to internal energy only, except for the cases where there is boundary movement, such as piston-cylinder mechanisms. Then, the change in the exergy content of a fluid stream as it undergoes a process from state 1 to state 2 becomes Dex1-2 ¼ ex2

ex 1 ¼ ðh2

h1 Þ

T0 ðs2

s1 Þ þ ðV22

V12 Þ=2 þ gðz2

z1 Þ

ð40Þ

For the fluid streams with negligible KE and PE, both KE and PE terms are omitted, and the resulting change in the exergy content of a flowing mass is written as follows: Dex1-2 ¼ ex 2

1.7.9.4

ex1 ¼ ðh2

h1 Þ

T0 ðs2

s1 Þ

ð41Þ

Exergy Transfer Mechanisms

Heat transfer (Q) from a source at T to a system or surrounding environment is always accompanied by exergy transfer ExQ in the following form: ExQ ¼ ð1

ðT0 =TÞÞQ

ð42Þ

Exergy is defined as the useful work potential, and the exergy transfer by work is equivalent to the actual work as follows: ExW ¼ W

ð43Þ

and for boundary work, ExWb ¼ W

Wsurr ¼ P0 ðV2

V1 Þ

where P0 is the atmospheric pressure and V1 and V2 are the initial and final volumes of the system. The exergy transfer by mass is   Exmass ¼ m  ex ¼ m ðh h0 Þ T0 ðs s0 Þ þ V 2 =2 þ gz

1.7.9.5

ð44Þ

ð45Þ

Exergy Balance

The nature of exergy is opposite to that of entropy in a way that exergy can be destroyed, but it cannot be created. Therefore, the exergy change of a system during a process is less than the exergy transfer by an amount equal to the exergy destroyed during the process within the system boundaries. Then the decrease of exergy principle can be expressed as Exin

Exout

Ex d ¼ DEx system

ð46Þ

Energy and Exergy Efficiencies

279

In rate form, _ in Ex

_ out Ex

_ d ¼ ðdEx=dtÞ Ex CV

ð47Þ

_ d is the exergy destruction rate. where Exd is the exergy destroyed and Ex This relation is referred to as the exergy balance and can be stated as the exergy change of a system during a process is equal to the difference between the net exergy transfer through the system boundary and the exergy destroyed within the system boundaries as a result of irreversibilities. Exergy can be transferred to or from a system by heat, work, and mass. Irreversibilities such as friction, mixing, chemical reactions, heat transfer through a finite temperature difference, unrestrained expansion, nonquasi-equilibrium compression or expansion always generate entropy, and anything that generates entropy always destroys exergy. The exergy destroyed is proportional to the entropy generated, and it is then expressed as Exd ¼ T0 Sgen

ð48Þ

Here, the exergy destruction during a process can easily be determined by this equation. The other method is to calculate it by writing the ExBE for a specific process or a specific component and extracting the exergy destruction term from this balance equation.

1.7.9.5.1

Closed system

A closed system, in general, may possess KE and PE as the total energy involved. The exergy change of a closed system during a process is simply the exergy difference between the final state (e.g., state 2) and the initial state (e.g., state 1) of the system. For a stationary closed system involving heat input Qin and boundary work output Wout as shown in Fig. 8, the mass, energy, entropy, and exergy balances (with the negligible changes in KE and PE) can be expressed as MBE: m1 ¼ m2 ¼ constant

ð49Þ

EBE: m1 u1 þ Qin ¼ m2 u2 þ Wout

ð50Þ

EnBE: ðQin =Ts Þ þ Sgen þ m1 s1 ¼ m2 s2

ð51Þ

ExBE: ðQin ð1

ðTo =Ts ÞÞÞ þ m1 ex1 ¼ Wout þ m2 ex2 þ Exd or ðTo =Ts ÞÞÞ þ m1 ex1 þ PV1 ¼ PV2 þ m2 ex2 þ Exd

ðQin ð1

ð52Þ

where u is internal energy, s is entropy, Ts is source temperature, T0 is the dead state (environment) temperature, Sgen is entropy generation, P is pressure, and V is volume. The exergy of a closed system is either positive or zero, and never becomes negative.

1.7.9.5.2

Open system

For a CV involving a steady-flow process with heat rate input and power output as shown in Fig. 9, the mass, energy, entropy, and exergy balances (with the negligible changes in KE and PE) can be expressed as _ out MBE: m _ in ¼ m

ð53Þ

_ in ¼ m _ out _ out hout þ W EBE: m _ in hin þ Q

ð54Þ

_ in =Ts Þ þ m _ in sin þ S_ gen ¼ m _ out sout EnBE: ðQ

ð55Þ

_ in ð1 ExBE: Q

_ out þ Ex _ d _ in exin ¼ m _ out ex out þ W ðT0 =Ts ÞÞ þ m

Moving boundary

Wout

Fixed mass from initial state 1 to final state 2

Fig. 8 A closed system involving heat input Qin and boundary work output Wout.

Qin

ð56Þ

280

Energy and Exergy Efficiencies

Boundary of the system

m in Qin Wout mout

Fig. 9 A control volume (CV) involving heat input and power output.

where specific exergy of a flowing fluid (i.e., flow exergy) with respect to the reference state is given by ex ¼ ðh

h0 Þ

T0 ðs

s0 Þ

ð57Þ

In these equations, KE and PE changes are assumed to be negligible. Most CVs encountered in practice, such as turbines, compressors, heat exchangers, pipes, and ducts operate steadily, and thus they experience no time-dependent changes in their mass, energy, entropy, and exergy contents as well as their volumes. The rate of exergy entering a steady-flow system in all forms (heat, work, mass transfer) must be equal to the amount of exergy leaving plus the exergy destroyed.

1.7.10

Energy and Exergy Efficiencies

It is critically important to assess the performances of energy systems and applications in a conceptually correct manner. We can do this thermodynamically by employing both energy and exergy efficiencies for most of them, except that we use energetic- and exergetic-based coefficients of performance equations for refrigeration and heat pump systems. We will further treat these systems in the following sections. In general, the efficiency for any system (not necessarily to be a thermodynamic system) is necessary to measure of its effectiveness or performance. One may find many approaches in the open literature for this particular purpose. Some may be correct, some may be incorrect. For us, it is important to act by the concepts, rather than by senses or preferences. The very general form of efficiency is defined as the ratio of desired (or useful) output divided by the required input as follows: Efficiency ¼ desired ðusefulÞ output=required input

1.7.10.1

ð58Þ

Energy Efficiency

The definition of energy efficiency is based on the first law of thermodynamics. It is denoted by Z. It may take different forms and different names depending on the type of the system. It may be written as Zen ¼

Energy output Enout ¼1 ¼ Enin Energy input

Enloss Enin

ð59Þ

where Enin ¼ Enout þEnloss

ð60Þ

or in rate form: Zen ¼

_ out En ¼1 _ in En

_ loss En _ in En

ð61Þ

An alternative way of expressing energy efficiency becomes through Zen ¼

Energy recovered Enrecovered ¼1 ¼ Enexpended Energy expended

Enloss Enexpended

ð62Þ

where Enin ¼ Enout þEnloss

ð63Þ

_ recovered En ¼1 _ expended En

ð64Þ

or in rate form, Zen ¼

_ loss En _ expended En

Both Eqs. (59) and (62) may be used to find the energy efficiency of a system but one may be more appropriate than the other depending on the system and application. They may turn out to be equivalent in some cases, while different in other cases.

Energy and Exergy Efficiencies

281

The definition of exergy efficiency is based on the second law of thermodynamics. It is also called second-law efficiency or exergetic efficiency. Some sources also call it effectiveness. Here, we use exergy efficiency and second-law efficiency interchangeably. Sometimes the effectiveness will be used in a different meaning for the performance of some devices, such as refrigerators and heat pumps. Exergy efficiency may take different forms depending on the type of the system. It is denoted by Zex. Here, exergy efficiency is generally expressed as follows: Zex ¼

Exergy output Exout ¼1 ¼ Ex in Exergy input

Ex d Exin

ð65Þ

or in rate form, _ out =Ex _ in ¼ 1 Zex ¼ Ex

_ d =Ex _ in Þ ðEx

ð66Þ

where _ in ¼ Ex _ out þ Ex _ d Ex

ð67Þ

Conceptually, the second-law efficiency appears to be a measure of perfection where thermodynamic perfection becomes the reversibility. The second-law dictates that no process can be better than a corresponding reversible process (in everyday terms, nothing can be more perfect than perfect), as that would be a violation of the second law. Therefore, reversible operation is the best possible mode of operation of a device, and thus it is natural that the second-law efficiency be 1 or 100% for operations that involve no irreversibilities or imperfections. This sets the upper limit for second-law efficiency, and the current practice essentially adheres to this. One can establish that the second-law efficiency of a device or process becomes zero if it destroys the entire exergy. The exergy efficiency is commonly defined as follows: Zex ¼ ðExergy outputÞ=ðExergy inputÞ Kanoglu et al. [2] have introduced a generalized approach in defining a common efficiency as given in the following form: Zex ¼ ðExergy recoveredÞ=ðExergy expendedÞ In this regard, the exergy efficiency is now formulated as Zex ¼

Ex recovered ¼1 Exexpended



Ex d Ex expended



ð68Þ

!

ð69Þ

or in rate form: _ recovered Ex ¼1 Zex ¼ _ expended Ex

_ d Ex _ expended Ex

where _ recovered þ Ex _ d _ expended ¼ Ex Ex

ð70Þ

or alternately, Zex ¼

_ delivered Ex ¼1 _Exconsumed



_ d Ex _ consumed Ex



ð71Þ

_ expended represents the portion of the exergy coming from the resource. It is the shaft work input in the case of compressor, where Ex and the decrease in the exergy of steam (difference between exergy values at inlet and outlet) in the case of a steam turbine. Exergy recovered is the portion of the expended exergy that is retained as exergy which is the portion that is saved from destruction within the system during the process. Both Eqs. (65) and (68) may be used to find exergy efficiency of a system. In this chapter, we will provide exergy efficiency formulation based on both approaches. However, for the reasons explained above, we recommend using exergy recovered/exergy expended approach (Eq. 68). It should be mentioned that Kotas [6] provides an exergy efficiency relation as the ratio of the desired exergy output to the exergy used (so called: rational efficiency). Here, exergy output is all exergy transfer from the system, plus any by-product that is produced by the system, while exergy used is the required exergy input for the process to be performed. This is similar to exergy efficiency definition as the ratio of product to the fuel where the fuel represents the resources expended to generate the product: Zex ¼

Product Fuel

ð72Þ

Here, both product and fuel must be expressed by exergy terms. Note that most researchers utilize Eq. (65) for calculating exergy efficiencies, while Eq. (68) is rarely used.

1.7.11

Efficiencies of Cyclic Devices

A heat engine is a device which is essentially used to convert heat to work, such as a steam power plant, a gas-turbine power plant, an internal combustion engine, a solar thermal plant, and a geothermal power plant. We now consider a heat engine as shown in

282

Energy and Exergy Efficiencies

High temperature heat source QH Heat engine Wnet, out QL Low temperature heat sink Fig. 10 A schematic diagram of basic heat engine.

TH

QH Refrigerator or heat pump Win QL TL

Fig. 11 A schematic diagram of basic refrigeration or heat pump (heat pump).

_ H . Some of this heat is Fig. 10. In this heat engine, the high-temperature resource at TH supplies heat to the heat engine at a rate of Q _ L is rejected to a low-temperature medium at TL. The energy efficiency of this _ out and the remaining heat Q converted to work W cycle is called thermal efficiency, and is expressed as the ratio of work produced to the heat supplied: Zth ¼

Wout ¼1 QH

QL QH

ð73Þ

We now consider a refrigeration or heat pump as shown in Fig. 11, such as a household refrigerator or an air-conditioning _ L is system used for cooling and a heat pump for heating, which are some known examples of these cycles. Here, heat at rate of Q _ H is rejected to the high-temperature reservoir at TH. A absorbed from the low-temperature reservoir at TL and heat at rate of Q _ in is needed for the operation of the cycle. The cycle is called a refrigerator if the purpose is to keep the lowpower input W temperature space at TL and it is called a heat pump if the purpose of the cycle is to keep the high-temperature medium at TH. Because the desired output is different for a refrigerator and heat pump, their efficiencies are defined differently. If we utilize the general definition of efficiency “desired output/required input” in this case, the performance of a refrigerator and a heat pump can be expressed by their COP: _ _L Q Q COPen;R ¼ L ¼ ð74Þ _ in _H Q _L W Q

COPen;HP ¼

_H _H Q Q ¼ _ in _H Q _L W Q

ð75Þ

Due to the values of COP more than unity, we cannot treat them in the way we have the efficiencies coming out. However, the main conceptual approach remains the same, as such that the performance of the units confirms the useful output divided by the required input.

Energy and Exergy Efficiencies 1.7.11.1

283

Carnot Heat Engine and Carnot Refrigeration/Heat Pump

A heat engine which consists of all reversible processes is called a reversible heat engine or a Carnot heat engine. The thermal efficiency of a Carnot heat engine may be expressed by the temperatures of two reservoirs with which the heat engine provides heat to convert into mechanical work (as illustrated in Fig. 10): TL TH

Zth;rev ¼ 1

ð76Þ

where TH is the source temperature and TL is the sink temperature where heat is rejected (i.e., lake, ambient air, etc.). This is the maximum thermal efficiency a heat engine operating between two reservoirs at TH and TL can have. Because all processes in Carnot cycle is reversible, the cycle can be reversed. In this case we obtain reversed Carnot cycle. A refrigerator or heat pump operating on reversed Carnot cycle (Fig. 11) would have the maximum COP values at the given operating temperatures TL and TH, and they are expressed as COPR;rev ¼

COPHP;rev ¼

TL TH

TL

TH TH TL

ð77Þ

ð78Þ

Example 4: Consider two heat engines running with a thermal efficiency of 30%. One of the engines (engine A) receives heat from a source at 600K, and the other one (engine B) from a source at 1000K. Both engines reject heat to a medium at 300K. At the first glance, both engines seem to be performing equally well. However, when we take a second look at these engines in light of the second law of thermodynamics, we see a totally different picture. The Carnot heat engine efficiency of for the operating temperature of the engine A and engine B are as follows:   T0 300K ¼ 50% Zth;rev;A ¼ 1 ¼1 Tsource A 600K   T0 300K Zth;rev;B ¼ 1 ¼1 ¼ 70% Tsource B 1000K Heat engine A has a 50% useful work potential relative to the heat provided to it, at the specified temperature heat engine B has 70%. We can see now engine A is performing closer to the ideal performance than engine B. Therefore, we can say that heat engine B is performing poorly relative to heat engine A even though both have the same thermal efficiency. This is a simple example of the power of the second law of thermodynamics (exergy) in finding the real performance of systems. Example 5: In this example, we calculate the second-law efficiencies for the heat engines presented in Example 1. It is obvious from Example 1 that the first-law (thermal or energy) efficiency alone is not a realistic measure of performance of engineering devices. To overcome this deficiency, we define an exergy efficiency (or second-law efficiency) for heat engines as the ratio of the actual thermal efficiency to the maximum possible (reversible) thermal efficiency under the same conditions: Zex ¼

Zth Zth;rev

ð79Þ

Based on this definition, the exergy efficiencies of the two heat engines discussed above become Zex;A ¼

0:30 ¼ 0:60 ¼ 60% 0:50

0:30 ¼ 0:43 ¼ 43% 0:70 That is, engine A is converting 60% of the available work potential to useful work. This ratio is only 43% for engine B. The second-law efficiency can be expressed for work producing devices, such as a turbine as the ratio of the useful work output to the maximum possible (reversible, hence ideal) work output: Zex;B ¼

_ out =W _ rev;out Zex;WPD ¼ W

ð80Þ

where the subscript WPD refers to work producing devices. This definition is more general since it can be applied to processes (in turbines, piston–cylinder devices, etc.) as well as to cycles. Note that the exergy efficiency cannot exceed 100%. We can also define an exergy efficiency for work-consuming noncyclic (such as compressors) and cyclic (such as refrigerators) devices as the ratio of the minimum (reversible) work input to the useful work input: _ rev;in =W _ in Zex;WCD ¼ W

ð81Þ

where the subscript WCD refers to work consuming devices. For cyclic devices, such as refrigerators and heat pumps, it can also be expressed in terms of the coefficients of performance as Zex;R and HP ¼ COPen;actual =COPen;rev

ð82Þ

284

Energy and Exergy Efficiencies

Moving boundary

Wout

From initial state 1 to final state 2

Qin

Win = 50 kJ kg−1 Fig. 12 A piston cylinder mechanism with the energies added and removed from the boundary of the piston cylinder device.

It is necessary to note that the reversible work should be determined by using the same initial and final states as in the actual process. Example 6: A piston cylinder device as shown in Fig. 12 has air in it at a pressure of 400 kPa, work entering the system through the motion of a paddle wheel with an amount of 50 kJ/kg of air inside the piston cylinder, thermal energy is added from a heat source with a temperature of 1001C to the system transferring the system from state 1 to state 2 and the piston moves so that the temperature of the system remains constant at 171C, through the process the volume contained in the system increased to three times its original volume. Calculate (1) the boundary work, (2) the thermal energy added to the system, (3) the entropy generation, and (4) the exergy destruction. Assumptions: Here are some assumptions made for analysis:

• • •

The air trapped inside the piston cylinder device is treated as an ideal gas. The changes in the KE and PE are neglected. The specific heat is assumed to be constant throughout the process.

Solution: Analyzing the piston cylinder device using the balance equations and they are written as follows: MBE: m1 ¼ m2 ¼ constant EBE: m1 u1 þ Qin þ Win;paddle ¼ m2 u2 þ Wout;b EnBE: m1 s1 þ Qin =Ts þ Sgen ¼ m2 s2 ExBE: m1 ex 1 þ ð1 ðT0 =Ts ÞÞQin þ Win;paddle ¼ m2 ex2 þ Wout þ Exd 1. Boundary work: since the temperature of the system does not change and the pressure remains constant due to the piston motion and production of boundary work then m1u1 ¼m2u2, which means that the EBE reduces to: qin þ win;paddle ¼ wout;b For the boundary work produced by the system, since it is operating at a constant temperature, we use the following equation to calculate the boundary work:   v2 kJ kJ wout;b ¼ RTln  290K  lnð3Þ ¼ 91:4 ¼ 0:287 v1 kg=K kg 2. The heat (thermal energy) added to the system is calculated as follows: then substituting the boundary work in the EBE: qin þ 50

qin ¼ 91:4

kJ kJ ¼ 91:4 kg kg 50 ¼ 41:4

kJ kg

Energy and Exergy Efficiencies

285

3. The entropy generation is calculated as follows: form the EnBE: m1 s1 þ Qin =Ts þ Sgen ¼ m2 s2 s1 þ qin =Ts þ sgen ¼ s2

5:274 þ

41:4 þ sgen ¼ 6:417 100 þ 273:15

sgen ¼ 1:032

kJ kg=K

4. The exergy destruction is calculated based on the relationship between the exergy destruction and the entropy generation and it is utilized as follows: ex d ¼ T0 sgen ¼ 298:15  1:032 ¼ 307:6

1.7.12

kJ kg

Efficiencies of Steady Flow Devices

In this section we provide numerous efficiencies commonly used for steady-flow devices, such as turbines, compressors, pumps, nozzles, diffusers, and heat exchangers. A power plant or a refrigeration system may consist of a number of these steady-flow devices, and improving the performance of these devices would improve the performance of the entire plant or system.

1.7.12.1

Turbine

A fluid is expanded in a turbine to produce power. Steam and gas turbines are considered here. Turbines are normally wellinsulated so that their operation can be assumed to be adiabatic. The performance of an adiabatic turbine is usually expressed by isentropic (adiabatic) efficiency. Consider a turbine with inlet state 1 with temperature T1 and pressure P1 and an exit state 2 with temperature T2 (or steam quality) and pressure P2 as shown in Fig. 13. The power output from this compressor would be maximum if the fluid is expanded reversibly and adiabatically (i.e., isentropically) between the given initial state and given exit pressure. The isentropic efficiency is the ratio of actual power to the isentropic power, which is the power produced by the same turbine if it had an isentropic efficiency of 100%. _ actual =W _ isen Zisen;Tr ¼ W

ð83Þ

Fluid with high pressure and temperature State 1 at T1 and P1 Turbine

Wout

State 2 at T2 and P2 Fluid is expanded Fig. 13 Schematic diagram of a turbine.

286

Energy and Exergy Efficiencies

_ is the mass flow rate of fluid and hs is the enthalpy of the fluid at the turbine outlet if the process was isentropic and the where m subscript Tr refers to turbine. This enthalpy may be obtained from exit pressure and exit entropy (equal to inlet entropy). KE and PE changes are neglected. Exergy efficiency of an adiabatic turbine may be determined from “exergy recovered (produced or obtained)/exergy expended” approach. In this case, the exergy resource is steam, and exergy expended is the exergy supplied to steam to turbine, which is the decrease in the exergy of steam as it passes through the turbine. Note that the exergy recovered or produced or obtained is the shaft work. Taking state 1 as the inlet and state 2 as the outlet, the second-law efficiency (and hence exergy efficiency) is defined as Zex;Tr-1 ¼

_ recovered _ out _ out _ ðh1 h2 Þ m Ex W W ¼ ¼ ¼ _ 2 _Exexpended _Ex 1 Ex _ rev _ ðh1 h2 T0 ðs1 m W

s 2 ÞÞ

ð84Þ

or Zex;Tr-1 ¼ 1

_ d Ex ¼1 _Ex expended

_ d Ex ¼1 _Ex1 Ex _ 2

_ rev W _ 1 Ex

_ out W _Ex 2

ð85Þ

The exergy efficiency definition based on the “exergy out/exergy in” approach is Zex;Tr-2 ¼

_ 2 _ out _ out þ Ex _ out Ex W W ¼ a _Ex _Ex 1 _ rev W

ð86Þ

A third definition only assumes power output as the product and inlet exergy as the input: Zex;Tr-3 ¼

_ out W ¼1 _ 1 Ex

_ d Ex ¼1 _ 1 Ex

_ rev W

_ out W _Ex1

ð87Þ

Note that the first definition (Eq. 84) is consistent with the general definition for the second-law efficiency of WPDs (the ratio of actual work to reversible work), but the second and third definitions (Eqs. 86 and 87) are not. Also, the first definition satisfies both bounding conditions for the second-law law efficiency: it is 100% when actual work equals reversible work, and 0% when actual work is zero (and thus the entire expended exergy is destroyed). It should be noted that isentropic efficiency and second-law efficiency are different definitions. In the isentropic efficiency, the actual process is compared to an ideal isentropic process between actual initial state and an assumed hypothetical exit state while in the exergy efficiency, the actual process is compared to an ideal reversible process between actual inlet state and actual exit state is used. Consequently, close but different values for isentropic and exergy efficiencies are obtained. Example 7: Consider an adiabatic steam turbine, as shown in Fig. 14, with the following inlet and exit states: P1 ¼ 10,000 kPa, T1 ¼5001C, P2 ¼ 10 kPa, x2 ¼ 0.95. Taking the dead-state temperature of steam as saturated liquid at 251C, determine isentropic efficiency and exergy efficiency based on different approaches.

State 1 at T1 = 500 °C and P1 = 10 MPa

Steam turbine

Wnut

State 2 at x2 = 0.95 and P2 = 10 kPa

Fig. 14 A schematic diagram of the steam turbine.

Energy and Exergy Efficiencies

287

0.85 State 1 at T1=500°C and P1=10 MPa

Steam turbine

0.8 Wout

ex,1, ex,2, ex,3, isen

ex,2 0.75

State 2 at x2=0.95 and P2=10 kPa

ex,1 0.7

isen

0.65 ex,3 0.6

0.55 5000

7000

9000

11,000

13,000

15,000

P1 (kPa) Fig. 15 Effect of turbine inlet pressure on the isentropic and second-law efficiencies.

Solution: The various efficiencies are to be determined from Eqs. (83), (84), (86), and (87). Before proceeding further, there is a need to write the balance equations: _2 MBE: m _1 ¼m _ out _ 2 h2 þ W EBE: m _ 1 h1 ¼ m _ _ 2 s2 EnBE: m _ 1 s1 þ Sgen ¼ m _ out þ Ex _ d _ 2 ex 2 þ W ExBE: m _ 1 ex 1 ¼ m Using the EBE to calculate the specific power produced by the steam turbine as follows: h1 ¼ h2 þ wout 3374 ¼ 2464 þ wout wout ¼ 909:8 kJ=kg Then using the ExBE, the exergy destructed through the expansion process is calculated as follows: ex 1 ¼ ex 2 þ wout þ ex d ex d ¼ 1412

151:1

909:8 ¼ 350:7 kJ=kg

Then by using the exergy efficiency definitions these are the resulting efficiencies: Zisen;Tr ¼ 0:708;

Zex;Tr-1 ¼ 0:722;

Zex;Tr-2 ¼ 0:752;

Zex;Tr-3 ¼ 0:6445

That is, the second-law efficiency is 75.2% based on Eq. (86) and it is 72.2% based on Eq. (84). In Eqs. (86) and (87), the exergy of the steam at the turbine exit is part of the exergy destroyed by the turbine. However, the turbine should not be held responsible for the exergy it did not destroy as part the processes associated with power production. With the first definition, the difference between the exergies of the inlet and exit steams is used for the exergy expended in the system. The effect of turbine inlet pressure on isentropic efficiency (Eq. 83) and three forms of exergy efficiencies (Eqs. (84)–(87)) is investigated while maintaining the exit conditions constant (Fig. 15). The efficiencies based on four definitions are considerably different. However, isentropic efficiency and the second-law efficiency by Eq. (84) are more appropriate forms. Interestingly, values of these two efficiencies are close to each other.

1.7.12.2

Compressor

A compressor is used to increase the pressure of a gas, as shown in Fig. 16. A power (essentially work) input is needed for this compression process, and the compressor is treated as a work consuming device. The performance of an adiabatic compressor is usually expressed by isentropic (adiabatic) efficiency. Consider an adiabatic compressor with inlet state 1 and an exit state 2. The power input to this compressor would be minimum if the gas is compressed reversibly and adiabatically (i.e., isentropically) between the given initial state and given exit pressure. The

288

Energy and Exergy Efficiencies

Fluid exists with higher pressure

State 2 at T2 and P2

Compressor

Win

State 1 at T1 and P1 Fluid with low pressure Fig. 16 A schematic diagram of a compressor showing the inlet streams, exit streams, and the energy interactions.

isentropic efficiency is then the ratio of the isentropic power to the actual power: Zisen;comp ¼

_ isen _ ðh2s W m ¼ _ actual _ ðh2 m W

h1 Þ h1 Þ

ð88Þ

_ is the mass flow rate of the gas and h2s is the enthalpy of the fluid at the compressor outlet if the process was isentropic where m and the subscript comp. refers to compressor. This enthalpy may be obtained from exit pressure and exit entropy (equal to inlet entropy). KE and PE changes are neglected. If the gas may be modeled as an ideal gas with constant specific heats, the isentropic power input is determined from "  k 1 # k kR ð T T Þ kRT P 2 1 1 2 _ isen ¼ m _ _ 1 ð89Þ W ¼m k 1 P1 k 1 where k is the specific heat ratio (k¼ cp/cv). Its value is 1.4 for air at room temperature. The gas is sometimes cooled through the compression process in a nonadiabatic compressor to reduce power input. This is because the power input is proportional to specific volume of the gas and cooling the gas decreases its specific volume. The isentropic efficiency cannot be used for a nonadiabatic compressors. Instead, an isothermal efficiency may be defined as Zen; isoth; comp ¼

_ isoth W _ W actual

where the subscript isoth refers to isothermal process. The power input for the reversible, isothermal case is given for an ideal gas with constant specific heats to be   P _ isoth ¼ mRT _ 1 ln 2 W P1

ð90Þ

ð91Þ

where R is gas constant, T is the inlet temperature of the gas, and P1 and P2 are the pressures at the inlet and exit of the compressor, respectively. In some cases, a reversible, polytropic process may be used as the ideal process for the actual compression. Then, a polytropic efficiency may be defined as Zen;ptc; comp ¼

_ ptc W _ actual W

where the power input for the reversible, polytropic process is given for an ideal gas with constant specific heats to be " n 1 # nRðT2 T1 Þ nRT1 P2 n _ _ _ 1 ¼m W ptc ¼ m n 1 P1 n 1 where n is the polytropic exponent and its value changes between one and specific heat ratio k.

ð92Þ

ð93Þ

Energy and Exergy Efficiencies

289

State 2 at T1 = 400K and P1 = 600 kPa

Air Compressor

Win

State 1 at T1 = 280K and P1 = 100 kPa

Fig. 17 A schematic diagram of the steam turbine.

Exergy efficiency of a compressor may be determined from “exergy recovered/exergy expended” approach. Here the resource is shaft work input, which is exergy expended by the compressor, and exergy recovered is the exergy supplied to the working fluid of the compressor, which is the increase in the exergy of fluid as it passes through the compressor. Again taking state 1 as the inlet and state 2 as the outlet, the exergy efficiency is expressed as

Zex; comp ¼

_ 1 _ recovered _ 2 Ex _ rev Ex Ex W ¼ ¼ _Ex expended _ _ in W in W

ð94Þ

or Zex; comp ¼ 1

_ d Ex _ consumed Ex

¼1

_ d Ex ¼1 _ in W

_ in W

_ rev W

_ in W




ð95Þ

The conventional definition based on the “exergy out/exergy in” approach is Zex; comp ¼

_ out _ 2 _ rev Ex Ex W a ¼ _ in _ in _ 1þW _ in Ex Ex W

ð96Þ

Note that the definition of Eq. (94) is consistent with the general definition for the second-law efficiency of work-consuming devices (the ratio of reversible work to actual work), but the definition of Eq. (96) is not. Also, the definition in Eq. (94) satisfies both bounding conditions for the second-law efficiency: it is 100% when the exergy increase of the working fluid equals actual work input, and 0% when the fluid experiences no increase in exergy as it passes through the compressor (and thus the entire expended exergy is destroyed). Example 8: Consider an air compressor, as shown in Fig. 17, that compresses air entering the compressor at an atmospheric pressure and a temperature of 280K, and the air exits the compressor at a pressure and a temperature of 600 kPa and 400K. The air mass flow rate of the air entering the compressor is around 1 kg/s, the compressor losses heat through its walls by an amount of 40% of the total work rate running the compressor. Calculate (1) the work consumed by the compressor and (2) the energy and exergy efficiencies of the compressor. Assumptions: Here are a few assumptions made for analysis and calculations:

• • •

No heat losses occur in the compression process. The changes in the KE and PE of the flowing air are neglected. The air is treated as ideal gas.

290

Energy and Exergy Efficiencies

Solution: The first step in solving this example is to write the balance equations as follows: _2 MBE: m _1¼m _ out _ in ¼ m _ 2 h2 þ Q EBE: m _ 1 h1 þ W _ 2 s2 EnBE: m _ 1 s1 þ S_ gen ¼ m _ in ¼ m _ d þ Ex _ _ _ 2 ex 2 þ Ex ExBE: m _ 1 ex 1 þ W Qout 1. The power consumed during the compression process is needed to be calculated in this section. Using the EBE to calculate the specific power consumed by the compressor is obtained as follows: _ out þ m _ in ¼ Q _ 2 h2 W

_ 1 h1 m

Here, the properties of the stream entering and leaving the compressor are obtained from air properties tables or using engineering equation solver (EES), which contains the data base for the properties of most of the fluids through a large range of temperatures and pressures, and the properties are tabulated in Table 1. The work input needed for the compressor is calculated as _ in þ m _ in ¼ 0:4  W _ 2 h2 W

_ 1 h1 m

_ in ¼ 0:4  W _ in þ 1  401:4 W

1  280:5

_ in ¼ 201:5 kW W 2. The energy and exergy efficiencies of the compressor are then calculated as follows:

1.7.12.3

gen ¼

ðm2 h2 m1 h1 Þ 1  ð401:4 280:5Þ ¼ 0:6 ¼ 60:0% ¼ _ out 201:5 W

gex ¼

ðm2 ex2 m1 ex1 Þ 1  ð166:6 ð 0:5579ÞÞ ¼ 0:8295 ¼ 83:0% ¼ _ 201:5 W out

Pump

A pump is used to increase the pressure of a liquid. A power input is needed for this process. The liquid may be considered to be an incompressible fluid and the power input for the isentropic case may be determined from specific volume and pressure data. A schematic diagram of a pump is shown in Fig. 18. Table 1

The properties of the inlet and the exit streamsa

State point

P (kPa)

T (1C)

h (kJ/kg)

s (kJ/kg/K)

ex (kJ/kg)

Reference state 1 2

101.3 100 600

25 7.0 127

298.6 280.5 401.4

5.696 5.637 5.482

NA 0.5579 166.6

a

The reference environment temperature and pressure are taken as 251C and 1 atm.

State 2 at T2 and P2

Pump

State 1 at T1 and P1

Win Fig. 18 A schematic diagram of a pump.

Energy and Exergy Efficiencies

291

When the changes in PE and KE of a liquid are negligible, the isentropic efficiency of a pump is defined as Zisen; p ¼

_ isen _ ðP2 mv W ¼ _ actual _ ðh2 m W

P1 Þ h1 Þ

ð97Þ

where v is the specific volume of the liquid, and it is usually taken at the pump inlet and the subscript p refers to pump. One may also use Eq. (88) to determine isentropic efficiency of a pump. However, because the temperature change across a pump is small, it is difficult to get an accurate measurement of temperatures, and the corresponding enthalpy values are not dependable. For this reason, efficiency of a pump should be determined from the measurements of specific volume, pressures, and the actual power input. If the enthalpy value at the pump exit h2 is needed, it is usually determined from a knowledge of the pump isentropic efficiency or from the measurement of actual power input. The exergy efficiency of a pump can be determined from Eq. (94) as the reversible power divided by the actual power as follows: Zex; p ¼

_ 1 _ recoverd _ 2 Ex _ rev _ ðh2 m Ex Ex W ¼ ¼ ¼ _ expended _ in _ in Ex W W

h1

T0 ðs2 _ in W

s 1 ÞÞ

ð98Þ

It is necessary to note again that it may be difficult to get the reliable values for enthalpy and entropy due to small temperature change across the pump. Therefore, it is reasonable to assume that reversible power input will be approximately equal to the isentropic power input. Then, the definitions for the exergy efficiency and isentropic efficiency become equal. The operation of a hydraulic turbine is similar to that of a pump. Then the efficiency of a hydraulic turbine may be expressed by modifying Eq. (97) as Zisen; HTr ¼

_ actual _ actual W W ¼ _ isen _ ðP1 P2 Þ mv W

ð99Þ

Here, the subscript HTr refers to hydraulic turbine. This is also called hydraulic efficiency, and it may be considered as an isentropic efficiency definition for a hydraulic turbine. Example 9: Consider an adiabatic cryogenic turbine, as shown in Fig. 19, used in natural gas liquefaction plants. The liquefied natural gas (LNG) enters a cryogenic turbine at 30 bar and 1601C at a rate of 20 kg/s and leaves at 3 bar. If 115 kW power is produced by the turbine, determine the efficiency of the turbine. Take the density of LNG to be 423.8 kg/m3.

State 1 at T1 = −169°C and P1 = 30 bar

Cryogenic turbine

Wout

State 2 at P2 = 3 bar

Fig. 19 A cryogenic turbine considered in the example.

292

Energy and Exergy Efficiencies

Solution: The maximum possible power which can be obtained from this turbine for the given inlet and exit pressures can be determined from _ _ isen ¼ m ðP1 W r

P2 Þ ¼

20 kg=s ð3000 423:8 kg=m3

300ÞkPa ¼ 127:4 kW

Given the actual power, the efficiency of this cryogenic turbine is defined and calculated as Zisen ¼

1.7.12.4

_ actual 115 kW W ¼ ¼ 0:903 ¼ 90:3% _ isentropic 127:4 kW W

Nozzle

A nozzle is essentially an adiabatic device because of the negligible heat transfer and is used to accelerate a fluid. Therefore, the isentropic (i.e., reversible and adiabatic) process serves as a suitable model for nozzles. The isentropic efficiency of a nozzle is defined as the ratio of the actual KE of the fluid at the nozzle exit to the KE value at the exit of an isentropic nozzle for the same inlet state and exit pressure as follows (Fig. 20): Zisen; Nz ¼

KEexit; actual V2 ¼ 22 V2s KEex; isentropic

ð98Þ

where the subscript Nz refers to nozzle. When the inlet velocity is negligible, the isentropic efficiency of the nozzle can be expressed in terms of enthalpies: Zisen; Nz ¼

h1 h1

h2 h2s

ð99Þ

A nozzle is built to convert the enthalpy of a fluid to KE – just like a turbine being built to convert the enthalpy of a fluid to shaft work. In a nozzle, the exergy recovered is the increase in the KE of the fluid and exergy expended is the exergy decrease of the fluid stream (but without taking into consideration the exit KE of the fluid stream, which corresponds to shaft work in a turbine). Then the exergy efficiency of a nozzle may be defined using an “exergy recovered/exergy expended” approach as Zex; Nz ¼

_ recovered Ex _ expended Ex

ð100Þ

and in a more explicit manner it becomes _ recovered Ex Zex; Nz ¼ ¼ _ expended h1 Ex

V22 2

h2

V12 2

T0 ðs1

s2 Þ þ V12 =2

¼

V22 2

h1

h2

T0 ðs1

s2 Þ

ð101Þ

Note that expended exergy corresponds to the exit KE in a reversible process. When the nozzle is reversible and adiabatic, the exergy efficiency becomes 100%, as expected. For adiabatic nozzles, isentropic and exergy efficiencies are identical. For nonadiabatic nozzles, the nominator of Eq. (101) can be modified to include the exergy of the heat transferred. Example 10: Steam at the entrance of an adiabatic nozzle, as shown in Fig. 21, has a pressure of 600 kPa, at a temperature of 500K and with a velocity of 120 m/s. The adiabatic nozzle has an area ratio of 2:1, which will result in an exit velocity of 380 m/s. Calculate the (1) exit temperature, (2) exit pressure, and (3) the exergy efficiency of the process. Nozzle

V1

V2 >>V1

Fig. 20 Nozzles are shaped so that they can convert pressure energy into kinetic energy (KE).

Energy and Exergy Efficiencies

293

Adiabatic nozzle

P1 = 600 kPa T1 = 500K V1 = 120 m/s

V2 = 380m/s

Fig. 21 Schematic diagram of the adiabatic nozzle in example 10.

Solution: The first step in solving a thermodynamics device is to write the four balance equations, and for this example they are as follows: _ 2 ¼ constant MBE: m _1¼m     V2 V2 _ 2 h2 þ 2 EBE: m _ h1 þ 1 ¼m 2000 2000 _ 2 s2 EnBE: m _ 1 s1 þ S_ gen ¼ m _ d _ 2 ex 2 þ Ex ExBE: m _ 1 ex1 ¼ m 1. The enthalpy of the air at the inlet of the nozzle is equal to 503.2 kJ/kg, and by using the EBE the enthalpy of air at the exit of the nozzle is as follows:     1202 3802 503:2 þ ¼ h2 þ 2000 2000 h2 ¼ 438.0 kJ/kg which has a corresponding temperature of 437K. 2. To calculate the exit pressure and since the air is considered as an ideal gas than the ideal equation for the first and the second stages of air through the nozzle are derived from the MBE is as follows: _1 ¼m _ 2m

P2 ¼

A1 V1 A2 V2 A1 V1 A2 V2 ¼ ¼ v1 v2 RT1 =P1 RT2 =P2

A1 T2 V1 437  120  600 ¼ 331 kPa  P1 ¼ ð2Þ  A2 T1 V2 500  380

3. The exergy efficiency is calculated as follows:

Zex; Nz ¼

1.7.12.5

_ recovered Ex ¼ _ expended h1 Ex

V22 2

h2

T0 ðs1

V12 2

s2 Þ þ

V12 =2

¼

3802 2000

503:2

438:0

1202 2000

298ð5:709

2

120 5:742Þ þ 2000

¼ 79:7%

Diffusers

Diffusers are considered as steady flow devices that increase the pressure of fluids by reducing their KE or in other words reducing the fluid moving velocity. These devices they usually do not introduce any extra energy, such as heat or work and they also do not extract work or heat out of the fluids that pass through them. What they do is just change forms of energy that the fluid possesses and usually they are designed to be adiabatic. The isentropic efficiency of a diffuser is the ratio of the final KE of fluid exiting the diffuser if the process was isentropic to the actual KE of the fluid exiting the diffuser and it is as follows (Fig. 22): Zisen; Df ¼

KEexit; isen 2 ¼ V22 =V2isen KEexit; actual

ð102Þ

where the subscript Df refers to the diffuser. Diffusers are built to convert the KE of the fluid into enthalpy increase of that fluid. Which means that the exergy recovered during the velocity reduction process is the increase in the fluid exergy that has the enthalpy

294

Energy and Exergy Efficiencies

Diffuser

V2