Solar Energy Technology Advances [1 ed.] 9781614704928, 9781594546143

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Copyright © 2005. Nova Science Publishers, Incorporated. All rights reserved. Tiwari, G.N.. Solar Energy Technology Advances, Nova Science Publishers, Incorporated, 2005. ProQuest Ebook Central,

Copyright © 2005. Nova Science Publishers, Incorporated. All rights reserved. Tiwari, G.N.. Solar Energy Technology Advances, Nova Science Publishers, Incorporated, 2005. ProQuest Ebook Central,

SOLAR ENERGY TECHNOLOGY ADVANCES

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Tiwari, G.N.. Solar Energy Technology Advances, Nova Science Publishers, Incorporated, 2005. ProQuest Ebook Central,

Copyright © 2005. Nova Science Publishers, Incorporated. All rights reserved. Tiwari, G.N.. Solar Energy Technology Advances, Nova Science Publishers, Incorporated, 2005. ProQuest Ebook Central,

Copyright © 2005. Nova Science Publishers, Incorporated. All rights reserved.

SOLAR ENERGY TECHNOLOGY ADVANCES

G.N. TIWARI

Nova Science Publishers, Inc. New York

Tiwari, G.N.. Solar Energy Technology Advances, Nova Science Publishers, Incorporated, 2005. ProQuest Ebook Central,

Copyright © 2006 by Nova Science Publishers, Inc.

Copyright © 2005. Nova Science Publishers, Incorporated. All rights reserved.

All rights reserved. No part of this book may be reproduced, stored in a retrieval system or transmitted in any form or by any means: electronic, electrostatic, magnetic, tape, mechanical photocopying, recording or otherwise without the written permission of the Publisher. For permission to use material from this book please contact us: Telephone 631-231-7269; Fax 631-231-8175 Web Site: http://www.novapublishers.com NOTICE TO THE READER The Publisher has taken reasonable care in the preparation of this book, but makes no expressed or implied warranty of any kind and assumes no responsibility for any errors or omissions. No liability is assumed for incidental or consequential damages in connection with or arising out of information contained in this book. The Publisher shall not be liable for any special, consequential, or exemplary damages resulting, in whole or in part, from the readers’ use of, or reliance upon, this material. This publication is designed to provide accurate and authoritative information with regard to the subject matter covered herein. It is sold with the clear understanding that the Publisher is not engaged in rendering legal or any other professional services. If legal or any other expert assistance is required, the services of a competent person should be sought. FROM A DECLARATION OF PARTICIPANTS JOINTLY ADOPTED BY A COMMITTEE OF THE AMERICAN BAR ASSOCIATION AND A COMMITTEE OF PUBLISHERS. LIBRARY OF CONGRESS CATALOGING-IN-PUBLICATION DATA

Available upon request ISBN 978-1-61470-492-8 (eBook)

Published by Nova Science Publishers, Inc. New York

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CONTENTS

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Preface

vii

Chapter 1

Introduction

Chapter 2

Solar Thermal Devices

15

Chapter 3

Solar Distillation

35

Chapter 4

Solar Greenhouse Crop Drying

57

Chapter 5

Solar Greenhouse Crop Production

83

Chapter 6

Embodied Energy Analysis of Photovoltaic (PV) System

103

Life Cycle Cost Analysis

125

Chapter 7

1

References

129

Index

135

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PREFACE This book reviews recent advanced research work in the area of flat plate collectors, solar distillation, greenhouse technology for crop drying and production and solar electric/ thermal (PV/T) systems. The basic working principle, energy balances, thermal modeling, energy and economic analysis will be discussed. An instantaneous and overall efficiency of each solar thermal and electric system are also discussed and their results compared for economic analysis. Basic knowledge of availability of solar radiation is discussed in the beginning. Life cycle cost analysis, which includes initial investment, operating cost, interest rate, salvage value and annual power output, has been considered. An energy pay back time (EPBT) for solar electric/ thermal (PV/T) system has been evaluated by evaluating embodied energy during production of solar cell, PV module and balance of system (BOS) and useful both electric and thermal energy. Thermal energy from PV module can be in the form of sensible heat either for water or for air heating system.

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Chapter 1

INTRODUCTION

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ABSTRACT Solar energy is clean and eco-environmental friendly renewable energy resource. It has a low energy density, most suitable from conservation of eco-system point of view. It has many other applications in addition to global greenhouse effect. Few of them are solar thermal namely flat plate collector, solar distillation, microclimate greenhouse technology for crop drying and production and solar electric namely photovoltaic (PV) system. The flat plate collector has many applications for domestic (bathing, cooking, space heating and swimming pool heating etc.) and commercial (cottage industries, pre-heating of boiler and hospitals etc.) sectors. It has been observed that energy pay back time (EPBT) for solar thermal and electric (PV) system is about 2-3 and 1415 years respectively. It is to be noted that Energy pay back time (EPBT) for solar thermal system is much less than solar electric (PV) system and hence solar thermal is more economical and acceptable to common users than solar electric (PV) system. However, an energy pay back time (EPBT) for solar electric (PV) system can be reduced by using it as hybrid system as photovoltaic cum thermal (PV/T) than it can also be economical and acceptable to common users. Less energy pay back time (EPBT) means low emission of CO2 in the atmosphere for production of solar electric/ thermal (PV/T) system. In this chapter, an attempt has been made to review the recent advance research work in the area of flat plate collector, solar distillation, greenhouse technology for crop drying and production and solar electric/ thermal (PV/T) system. The basic working principle, energy balances, thermal modeling, energy and economic analysis will be discussed. An instantaneous and overall efficiency of each solar thermal and electric system will be also discussed and their results will be compared for economic analysis. Basic knowledge of availability of

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2

G. N. Tiwari solar radiation will also be discussed in the beginning. Life cycle cost analysis, which includes initial investment, operating cost, interest rate, salvage value and annual power output, has been considered. An energy pay back time (EPBT) for solar electric/ thermal (PV/T) system has been evaluated by evaluating embodied energy during production of solar cell, PV module and balance of system (BOS) and useful both electric and thermal energy. Thermal energy from PV module can be in the form of sensible heat either for water or for air heating system.

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1.1 FORMATION OF ATMOSPHERE The earth is almost round in shape having a diameter of about 13000 kilometers and it came into existent some 4.6×109 years ago. The eruption of the volcanoes is generally occurred at plate boundary of the earth. During eruption of the volcanoes, various greenhouse gases namely carbon dioxide (CO2), methane (CH4), nitrous oxide (NOx), ozone (O3) and water vapor (H2O) etc. available inside ground were also discharged through the plate boundary. These discharged gases at boundary of the plate’s moves upward towards the sun due to its low density. These gases formed a layer between the sun and earth as shown in Fig. 1.1. This layer is generally referred as the earth’s atmosphere. The earth revolves around the sun once in about a year. Nearly 70% of the earth is covered by the water and remaining 30% is land. Half of the earth is lit by sunlight at a time. It reflects one third of the sunlight that falls on it. This is known as earth’s elbedo. The earth is spinning about its axis constantly. Its axis is inclined at an angle of 23.5°. As a result, the lengths of days and nights keep changing. The sun, which has an effective black body temperature (Ts) of 5777 K, is the largest member of the solar system with other members revolving around it. The sun is a sphere of intensely hot gaseous matter with a diameter of 1.39 x109 m and is, on an average, 1.5 x 1011 m from the earth. The sun is, effectively, a continuous fusion reactor. The maximum spectral intensity occurs at about 0.48 µm wavelength (λ) in the green portion of the visible spectrum. About 8.73% of the total energy is contained in ultraviolet region (λ < 0.40 µm); another 38.15% is contained in the visible region (0.40 µm < λ < 0.70 µm) and the remaining 53.12% is contained in the infrared region (λ > 0.70 µm).

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Figure 1.1. Position of sun, atmosphere and earth

Solar radiations while passing through the earth's atmosphere are subjected to the mechanisms of atmospheric absorption and scattering. The X-rays and extreme ultraviolet radiations of the sun are absorbed highly in the ionosphere by nitrogen, oxygen and other atmospheric gases; ozone and water vapors largely absorb ultraviolet (λ< 0.40 µm) and infrared radiations (λ>2.3 µm) respectively. There is almost complete absorption of short wave radiations (λ< 0.29 µm) in the atmosphere. Hence, the energy in wavelength radiation below 0.29 µm and above 2.3 µm, of the spectra of the solar radiation, incident on the earth's surface is negligible. The earth’ atmosphere has the following unique properties: •

•

It absorbs the ultraviolet and far infrared radiation and allows only radiation having wave length range between 0.29µm to 2.3 µm (short wave length radiation) and, It also does not allow radiation having wave length ≥9 µm (long wave length radiation)

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G. N. Tiwari

The phenomenon of blocking of UV radiation happens about 420 million years ago and this is referred as Global Greenhouse Effect and this allowed the plants to grow on the earth. The fossil (remains of blue-green algae and bacteria) has been found in rocks/water at least 3×109 years ago. Without the greenhouse effect, the earth would be a frozen planet with an average temperature of about -18° C (about 0°F), example 1.5 in Tiwari (2002). It is clear that for survival of living plants on the earth, there should be a favorable environment (global environment) in terrestrial region controlled by short wave length radiation transmitted by atmosphere. However, the similar effect is observed by having the transparent material over the any surface because the transparent material also behaves as the atmosphere with respect to short wavelength. The concept of trapping short wavelength radiation (thermal energy) in an enclosure has many applications. Some of them are as follows: a.

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b.

c.

d.

Flat plate collector: It is a device having insulated blackened flat surface with transparent glass window above it. This device works with micro greenhouse effect. Solar distillation: It is a device having insulated rectangular box filled with water with sloped transparent glass window. This device is referred as solar still and basically works on principle of micro greenhouse effect. Greenhouse: A micro climate can also be created by using the transparent glass/plastic house similar to global greenhouse concept. The glass/plastic house can be used for optimum growth of living plants (e.g. flowers, vegetables, etc.) for maximum crop production during season as well as in off-season (post harvest and pre-harvest period), is generally known as greenhouse technology. The greenhouse can also be used for crop drying for storage purposes and Photovoltaic (PV): It is a device used to convert short wavelength radiation into direct current (dc) electricity etc.

For optimum design of above system basic knowledge of solar radiation is required and hence brief discussion on solar radiation has been made in next section.

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1.2 SOLAR RADIATION The radiant energy flux received per second by a surface of unit area held normal to the direction of sun's rays at the mean earth-sun distance, outside the atmosphere (extra-terrestrial region), is practically constant throughout the year. This is termed as the solar constant Isc and its value is now adopted to be 1367 W/m². However, this extraterrestrial radiation suffers variation due to the fact that the earth revolves around the sun not in a circular orbit but follows an elliptic path, with sun at one of the foci. The intensity of extraterrestrial radiation measured on a plane normal to the radiation on the nth day of the year is given in terms of solar constant (Isc) as follows (Duffie and Beckman, 1991):

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ION = ISC [1.0 + 0.033 cos (360n/365)]

(1.1)

The range of wavelength radiation emitted from the sun, attenuation of its amplitude during propagation from sun to atmosphere and further attenuation of radiation in the atmosphere and the long wavelength radiation emitted from earth. has also been shown. Thus, from the view of terrestrial applications of solar energy, only radiation of wavelength between 0.29 µm and 2.3 µm is significant.

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The solar radiation, through atmosphere, reaching the earth's surface can be classified into two components: beam radiation and diffuse radiation. Beam radiation (Ib): It is the solar radiation propagating along the line joining the receiving surface and the sun. It is also referred to as direct radiation. Diffuse radiation (Id): It is the solar radiation scattered by aerosols; dust and molecules; it does not have a unique direction. The total radiation (I): It is the sum of the beam and diffuse radiation and is sometimes referred to as the global radiation. Definitions of some of the terms used in solar energy applications are as follows: Irradiance (W/m²): The rate at which radiant energy is incident on a surface, per unit area of surface. Irradiation or Radiant Exposure (J/m²): The incident energy per unit area on a surface, found by the integration of irradiance over a specified time, usually an hour or a day. Insolation is a term applied specifically to solar energy irradiation. Radiosity or Radiant Exitance (W/m²): The rate at which radiant energy leaves a surface per unit area, by combined emission, reflection and transmission. Emissive Power or Radiant Self-Exitance (W/m²): The rate at which radiant energy leaves a surface per unit area, by emission only. Albedo: The earth reflects about 30 percent of all the incoming solar radiation back to extra-terrestrial region through atmosphere. Following Singh and Tiwari (2004), the rate of beam (direct) radiation reaching in terrestrial region can be written as IN = ION . exp [-(m. ε.TR + α)]

(1.2)

where the parameters ‘m’ and ‘ε’ (Kasten, 1965 and Kasten and Young,1989) are expressed in form of m = [cos θz + 0.15 x (93.885 - θz)-1.253]-1

(1.3)

ε = 4.529 x 10-4 . m2 – 9.66865 x 10-3 . m + 0.108014

(1.4)

and

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In terms of air-mass ‘m’, integrated Rayleigh scattering optical thickness of atmosphere ‘ε’ and the Linke turbidity factor ‘TR’, the terrestrial beam radiation received on horizontal surface is expressed by classical equation as: Ib = IN cos θz = ION. exp (-m. ε.TR+ α) . cos θz

(1.5)

where θz is the solar zenith angle for a given time (Eq. (1.13), Tiwari, 2002) and α is a lumped atmospheric parameter for beam radiation, Singh and Tiwari (2004). The α determines the additional depletion in direct normal irradiance in terrestrial region due to cloudiness/haziness, transient and unpredictable changes The diffuse radiation on the horizontal surface can be rewritten in terms of constants K1 (dimensionless) and K2 (W/m2) as

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Id = K1 (ION – IN). cos θz + K2

(1.6)

where IN (W/m2) is the normal terrestrial solar radiation at the ground level given by Eq. 1.2. The constants K1 and K2 can be defined as lumped atmospheric parameters for diffuse radiation, Singh and Tiwari (2004). Further, the constant K1 can be interpreted as ‘perturbation factor’ for describing scattering out of beam traversing the lumped atmosphere and K2 can be referred as ‘background diffuse radiation’. An expression for cos θz is given by

cos θ z = cos φ cos δ cos ω + sin δ sin φ

(1.7)

 2π  ( 284 + n )  365 

δ = 23.45 sin  where

δ

and ω are given by

ω = (ST-12) 15° where

φ

and ST are latitude and local solar time respectively.

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Table 1.1.a. The evaluated atmospheric transmittance and lumped atmospheric parameters (TR , α, K1, K2) for weather type a. Month Parameter TR α K1 K2

Jan 1.84 0.16 0.43 13.9

Feb 2.22 0.14 0.37 02.01

Mar 1.35 0.23 0.29 20.62

April 2.59 0.15 0.31 14.86

May 3.18 0.13 0.38 -5.75

June 4.05 0.02 0.43 03.61

July 2.40 0.24 0.49 -69.12

Aug 3.43 0.29 0.36 28.99

Sept 3.81 0.08 0.32 17.50

Oct 3.46 0.10 0.48 -14.38

Nov 2.35 0.07 0.36 -18.68

Dec 2.47 0.02 0.43 -36.02

Nov 6.46 0.02 0.37 -02.6

Dec 3.68 0.09 0.49 -40.01

Table 1.1b. The evaluated atmospheric transmittance and lumped atmospheric parameters (TR , α, K1, K2) for weather type b. Month Parameter TR α K1 K2

Jan 2.27 0.20 0.44 -14.73

Feb 2.77 0.18 0.46 -17.70

Mar 2.87 0.18 0.42 +0.90

April 3.03 0.22 0.39 +0.15

May 5.84 0.14 0.41 +07.95

June 5.56 0.13 0.49 -24.23

July 6.08 0.23 0.49 -39.78

Aug 4.68 0.32 0.36 +14.67

Sept 4.72 0.15 0.42 -27.61

Oct 5.51 0.12 0.41 -14.88

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Table 1.1. c. The evaluated atmospheric transmittance and lumped atmospheric parameters (TR , α, K1, K2) for weather type c. Month Parameter TR α K1 K2

Jan 3.47 0.23 0.50 -30.86

Feb 4.04 0.25 0.45 -25.70

Mar 4.06 0.22 0.44 -18.73

April 4.86 0.14 0.44 -14.65

May 6.70 0.17 0.49 -43.00

June 8.05 0.09 0.48 -36.71

July 8.72 0.19 0.51 -57.43

Aug 6.91 0.26 0.40 3.37

Sept 5.64 0.24 0.45 -14.43

Oct 6.10 0.19 0.42 -36.14

Nov 6.51 0.00 0.38 -13.95

Dec 4.85 0.22 0.51 -51.69

Nov 8.86 0.06 0.33 +06.33

Dec 8.09 0.56 0.44 -48.91

Table 1.1. d. The evaluated atmospheric transmittance and lumped atmospheric parameters (TR , α, K1, K2) for weather type d. Month Parameter TR α K1 K2

Jan 7.28 0.83 0.34 -17.68

Feb 6.40 1.27 0.29 -06.56

Mar 9.08 0.36 0.44 -38.67

April 10.05 0.10 0.48 -36.76

May 9.40 0.21 0.48 -32.76

June 9.59 0.51 0.47 -64.35

July 7.94 0.73 0.43 -45.34

Aug 8.05 0.83 0.41 -42.69

Sept 7.14 0.67 0.45 -46.73

Oct 8.16 0.66 0.40 -23.29

10

G. N. Tiwari

The values of TR, α, K1 and K2 for following four weather condition of New Delhi (composite climate) a. b. c. d.

clear day (blue sky) Hazy day (fully) Hazy and Cloudy (partially) and Cloudy day (fully)

are given in tables 1.1. After knowing hourly beam (Eq.1.5) and diffuse (Eq.1.6) radiation on horizontal surface, the total radiation for any inclined (inclination= β ) with any orientation of solar thermal device (for east, south, west and north γ =90o , 0, +90o and ±180o (for detail see table1.5 of Tiwari (2002)) for a given latitude (φ) can be evaluated by using following Liu and Jordan formula, Liu and Jordan (1960).

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I t = I N cosθ i + Id R d + ρ R r (I b + Id )

(1.8)

where Rd and Rr are known as conversion factors for diffuse and reflected components respectively and ρ is the reflection coefficient of the ground (= 0.2 and 0.6 for ordinary and snow covered ground respectively). An expression for cos θ i is given by

cosθi = (cosφ cosβ + sinφ sin β cosγ ) cosδ cosω + cosδ sinω sin β sinγ + sinδ (sinφ cosβ - cosφ sin β cosγ )

(1.9)

Expressions for these conversion factors are given below: i) Rd :It is defined as the ratio of the flux of diffuse radiation falling on the tilted surface to that on the horizontal surface.

Rd =

1 + cos β 2

(1.10a)

ii) Rr : The reflected component comes mainly from the ground and other surfaces and it is given by

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 1 - cos β   Rr =  2  

(1.10b)

Singh and Tiwari (2005) have also evaluated the values of TR, α, K1 and K2 for monthly average value of beam and diffuse radiation for five Indian city given in table 1.2 by using the data given by Mani (1980).The values of TR, α, K1 and K2 obtained for different city (table 1.2) have been given in tables 1.3.

Table 1.2. The latitude, longitude, elevation and data period of various stations belonging to different Indian climate conditions Parameter

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Station Chennai Jodhpur Kolkata Mumbai New Delhi

Latitude (in degree)

Longitude (in degree)

13.00N 26.30N 22.65N 19.12N 28.58N

80.18E 73.02E 88.45E 72.85E 77.20E

Elevation (in meter above mean sea level) 16 224 06 14 216

Data Period

1957-1978 1960-1978 1957-1978 1969-1978 1957-1978

After knowing monthly average hourly beam and diffuse radiation on horizontal surface from Eqs.1.5 and 1.6 respectively for parameters given in tables 1.3 for different climatic condition (table 1.2), the total radiation for any inclined (inclination= β ) with any orientation of solar thermal device (for east, south, west and north γ =-90o, 0, +90o and ±180o for a given latitude (φ) can be evaluated from Eq. 1.8. The monthly average data can be used to optimize the design of any solar thermal devices for maximum out put. In next sections, the performance of various solar thermal and PV systems along with energy and economic analysis will be discussed.

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Table 1.3.a. Evaluated parameters (TR, α, K1, K2) for average weather conditions for Chennai climatic conditions Month Parameter TR

Jan

Feb

March

April

May

June

July

Aug

Sept

Oct

Nov

Dec

4.14

4.16

5.07

5.06

6.36

7.95

8.50

8.10

4.45

2.92

2.21

4.30

α K1 K2

0.26 0.38 -12.83

0.11 0.38 -7.23

0.02 0.36 -1.69

0.17 0.32 +20.10

0.19 0.35 +18.86

0.25 0.47 -58.22

0.44 0.49 -80.85

0.39 0.47 -67.90

0.62 0.38 -16.26

0.97 0.30 +16.99

1.10 0.28 +20.57

0.67 0.33 +5.03

Table 1.3.b. Evaluated parameters (TR, α, K1, K2) for average weather conditions for Jodhpur climatic conditions Month Parameter TR α K1 K2

Jan

Feb

Mar

April

May

June

July

Aug

Sept

Oct

Nov

Dec

1.38 0.39 0.31 +24.08

1.15 0.45 0.24 +51.05

4.83 -0.04 0.37 +10.87

3.71 0.28 0.45 -11.99

4.98 0.19 0.49 -27.19

5.44 0.37 0.51 -48.54

4.97 0.84 0.46 -42.26

4.90 0.87 0.43 -31.52

3.26 0.39 0.39 -11.20

2.32 0.22 0.28 +22.92

1.52 0.29 0.27 16.29

2.22 0.17 0.36 -2.56

Table 1.3.c. Evaluated parameters (TR, α, K1, K2) for average weather conditions for Kolkata climatic conditions Month Parameter TR α K1 K2

Jan

Feb

March

April

May

June

July

Aug

Sept

Oct

Nov

Dec

4.14 0.26 0.39 -21.10

4.26 0.27 0.38 -12.18

5.37 0.21 0.42 -27.94

5.79 0.30 0.45 -33.92

6.34 0.37 0.47 -39.29

6.19 1.02 0.39 -35.95

4.46 1.46 0.37 -32.61

4.86 1.33 0.36 -28.96

3.69 1.18 0.35 -15.77

2.48 0.87 0.34 -0.71

1.82 0.67 0.33 +6.51

2.95 0.41 0.35 -7.49

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Table1.3.d. Evaluated parameters (TR, α, K1, K2) for average weather conditions for Mumbai climatic conditions Month Parameter TR α K1 K2

Jan

Feb

March

April

May

June

July

Aug

Sept

Oct

Nov

Dec

1.94 0.40 0.30 +10.62

1.94 0.41 0.28 +26.41

3.05 0.31 0.32 +12.94

4.26 0.24 0.43 -23.15

5.05 0.22 0.47 -33.38

6.01 0.89 0.41 -48.70

4.69 1.83 0.39 -60.15

3.95 2.14 0.39 -55.24

4.31 1.03 0.39 -25.23

3.04 0.52 0.33 +10.25

2.32 0.35 0.26 13.39

2.84 0.21 0.30 +1.78

Table 1.3.e. Evaluated parameters (TR, α, K1, K2) for average weather conditions for New Delhi climatic conditions Month Parameter TR α K1 K2

Jan

Feb

March

April

May

June

July

Aug

Sept

Oct

Nov

Dec

2.65 0.33 0.40 -12.12

2.94 0.28 0.41 -14.51

3.39 0.25 0.43 -12.08

3.49 0.38 0.44 -08.62

4.58 0.35 0.46 -15.56

7.33 0.39 0.49 -49.59

4.49 1.03 0.40 -39.26

3.77 1.02 0.37 -24.69

4.75 0.35 0.39 -19.53

4.33 0.03 0.28 +29.68

3.60 0.04 0.16 +65.18

4.28 -0.04 0.34 +4.66

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Chapter 2

SOLAR THERMAL DEVICES

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2.1. FLAT-PLATE COLLECTOR The flat-plate collector is one of the most popular and economical solar energy collection system designed for operation in the low temperature range (ambient- 60 0C) or in the medium temperature range (ambient-100 0C). It absorbs solar energy; convert it into heat and then to transfer absorbed heat to a stream of liquid or gas. It absorbs both the beam and the diffuse radiation, and is usually planted on the top of a building or other structures. A cross-sectional view of tube in plate flat plate collector has been shown in Fig. 2.1a. It does not require tracking of the sun and requires little maintenance. It can also be used at high operating temperature only by changing the configuration of absorber by evacuated tube and hence it is referred as evacuated tubular collector. A flat-plate collector usually consists of the following components: i.

Glazing cover, which may be one or more sheets of window/toughen glass or some other radiation transmitting material? Generally, single glass window/ toughen of 3mm is preferred. ii. Tubes/evacuated-glazed tube for liquid fluid fins for air fluid or passages for conducting or directing the heat transfer fluid from the inlet to the outlet. iii. An absorber plate, which may be flat, corrugated or grooved with tubes/evacuated-glazed tube, fins or passages attached to it. An absorber plate is generally made up of copper sheet of 18 gauges.

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Figure 2.1(a) Cross-section view of tube in flat plate collector

iv. Header or manifolds, to admit and discharge the fluid. Upper and lower header is generally made up of copper pipes of diameter 1.25cm (0.0125m) and 10 mm (0.0011m) respectively. v. Bottom glass wool insulation, which minimizes heat loss from the back and sides of the collector. Its optimum thickness is about 0.05m. vi. A container or casing, which surrounds the various components and protects them from dust, moisture etc. It is generally constructed by using aluminium (Al) sheet.

2.1.1. Analysis of Flat-Plate Collectors

& ) per unit time of a collector of area Ac is The useful energy output ( Q u the difference between the absorbed solar radiation, q& ab , and the thermal loss and is given by,

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17

Q& u = Ac q& u = Ac [ q& ab - UL (Tp − Ta )]

(2.1)

where,

q& ab =

( ατ ) I (t )

An instantaneous thermal efficiency ηi of flat-plate collector is given as,

ηi =

& (Tp −Ta ) q& Q (Tp- Ta ) u =η0 −UL = ab − UL I (t ) I(t) Ac I (t ) I (t )

(2.2)

where I(t) is the intensity of incident radiation on inclined flat plate collector, α and τ are the absorptivity of the absorber and the transmissivity of glass

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cover of a flat plate collector respectively and

η0 =

ατ is the optical

efficiency of the flat plate collector . The expression for UL is given as UL =Ut+Ub , the overall heat loss coefficient. The typical value of Ub, the overall bottom heat loss coefficient for 0.10m glass wool insulation thickness of flat plate collector (K=0.04 W/mK) is 0.40 W/m2K. For uninsulated flat plate collector as shown in Fig. 2.1b, Ub=2.8+3 V (only convection due to downward facing) can be considered. The typical value of UL, the overall top heat loss coefficient for single glass cover placed at 0.10m away from absorber of flat plate collector is 6-7 W/m2 K. For unglazed flat plate collector as shown in Fig. 2.1b, UL=5.7+3.8 V (convection and radiation due to exposed surface) can be considered. The collection efficiency, defined as the ratio of the useful gain to the incident solar energy over the same period of time, is given by,

ηc =

& dt ∫Q u

Ac ∫ I (t )dt

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(2.3)

18

G. N. Tiwari

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Figure 2.1(b) Characteristics curve of a flat plate collector

2.1.2. Collector Efficiency (F/ ) and Flow Rate (FR) Factors The collector efficiency factor, F/, is defined as the ratio of actual useful

& ) to the useful heat collection rate when the heat collection rate ( Q useful collector absorbing plate (Tp) is at the local fluid temperature (Tf), i.e.

F′ =

& Q useful & Q u

T ρ =Tf

=

& Q useful Ac [q& ab - U L (Tf - Ta )]

& or, Q = F′ Ac [q& ab - U L (Tf - Ta )] useful

(2.4a) (2.4b)

where an expression for F/ for a typical tube-in-plate flat plate collector is given by (Tiwari, 2002)

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Solar Energy Technology Advances

F′ =

19

1 W UL W + π D h D + (W − D)F (2.4c)

F=

tanh [m(W - D)]/2 [m(W - D)]/2 2 m = U L /K δ

Here W, D, K, δ, F and h are distance between centre of two tubes, diameter of the tube, thermal conductivity of the tube, thickness of the tube, fin efficiency and convective heat transfer coefficient from inner tube to the fluid respectively. The typical values of m, F and F are 6, 0.98 and 0.9 (for h=300 W/m2) respectively.

& ) per unit time from Equation (2.4b) is the useful energy output ( Q useful Copyright © 2005. Nova Science Publishers, Incorporated. All rights reserved.

a flat plate collector with an effective area of Ac under natural flow of fluid through pipes attached to the absorber. For N identical collectors connected

& in parallel, the useful energy output ( Q ) per unit time will be Nuseful

& = F′ N Ac [q& ab - U L (Tf - Ta )] Q N useful

(2.4d)

Now, an instantaneous thermal efficiency ηi of flat-plate collector becomes as,

ηi =

& (Tf −Ta) q& (T -T ) Q useful = F/[ ab − UL f a ] =F/η0 −F/UL (2.4d) I(t ) I(t) I (t ) NAc I (t )

For forced mode of operation, Eq. (2.4b) becomes

Q& useful = Ac FR [q& ab - UL (Tfi - Ta )]

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(2.5a)

20

G. N. Tiwari

where Tfi is the inlet temperature of the fluid to the collector and the flow rate factor, FR , is given by

& Cf /( A c U L )][1 - exp (- A c U L F′/(m & C f )] F R = [m The numerical values of FR is always less than F/, however its value generally varies between 0.8 and 0.9 for a reasonably good flat-plate collector. An instantaneous thermal efficiency ηi of flat-plate collector under forced mode of operation is given as,

& (Tfi −Ta)  Q -   useful = FR(ατ)-ULTfi Ta=FRη0 −FRUL  I(t )   I(t)  AcI(t ) 

ηi =

(2.5b)

Equation (2.5b) is known as characteristic equation of flat plate collector and it is also known as Hottel-Whiller-Bliss (HWB) equation. The variation of ηi with

(Tfi - Ta ) I (t )

has been shown in Fig.2.1b. It is evident that the term

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FR(ατ)=FR η0 represents the gain of a flat plate collector and the term –FRUL represents the loss term from flat plate collector. If an N flat-plate collector under forced mode of operation is connected in series, then the outlet fluid temperature at Nth collector can be written as:

 q& ab  & Cf T fON =  + Ta {1− exp(- N Ac UL F′/(m  UL 

)} + Tfi exp{- NAc UL F′/(m& Cf } (2.6)

In equation (2.6), it is assumed that all flat-plate collectors under forced mode of operation are identical. i.e. i.

An overall heat transfer coefficient for all flat-plate collectors are equal ( UL1 = UL2 = ................= ULN = UL) ii. An area of all flat-plate collectors is also equal ( AC1 = AC2 = ....= ACN = AC) iii. and the collector efficiency factor of all flat-plate collectors is also same ( F/1 = F/ 2 = .............= F/ N = F/

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Solar Energy Technology Advances If the term

( N Ac UL F′/(m& C f )

21

in equation (2.6) is very small

& ) is very large or collector area (Ac) is very means either the flow rate ( m small, than equation (2.6) reduces to TfoN = Tfi. This indicated that there is no withdrawal of thermal energy from the collector. If the term

( N Ac UL F′/(m& C f )

in equation (2.6) is very large

& ) is very small or collector area (Ac) is very means either the flow rate ( m large, than equation (2.6) reduces to   q& ab + Ta  T fON =    UL This is the maximum temperature one can obtain from the collector. For N identical collectors connected in series, the useful energy output

& ) per unit time will be (Q Nuseful

Q& Nuseful = Ac {[FR (ατ )] N I (t ) - [FR UL] N (Tfi - Ta )}

(2.7a)

[

]

[ F R ( ατ )] N = FR (ατ ) 1 - (1 - K K ) N / NK K and,

[

]

[ F R U L ] N = FR U L 1 - (1 - K K ) N / NK K with K

=

) Cf m ( / FR UL Ac

K

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where,

&

The outlet fluid temperature can also be evaluated as •

T fON =

Q Nuseful •

m Cf

+ T fi

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(2.7b)

22

G. N. Tiwari

It is important to mention here that the numerical value of UL in above cases has been considered as constant. However, there is a variation in the value of UL within an accuracy of 5% at high operating temperature. Under this condition, characteristic curve as shown in Fig. 2.1b becomes nonlinear and F/ UL can be considered as

F /U L = a1 + a 2 (T f − Ta )

(2.8a)

where a1 and a2 are constants and can be determined by linear regression analysis. Now, Equation (2.4d) for an instantaneous thermal efficiency ηi of flat-plate collector becomes as,

ηi =F η0 − a1 /

(T

− Ta )

f

I (t)

− a2

(T

− Ta )

2

f

I (t )

= a0 − a1 X − a2 X 2

(2.8b)

Equation (2.8b) can be used to determine constants a0, a1 and a2 for

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known

ηi

and X=

(T

f

− Ta )

I (t )

by using multiple regression analysis. This /

helps to determine a0 = (ατ) and F U L from Eq. (2.8a) for any type of flat /

plate collector. Generally F U L is considered to be constant due to its small variation in designing of a system based on flat plate collector.

2.1.3. Energy Pay Back Time (EPBT) For natural mode F/=0.9, N=2, Ac=1m2, ατ=0.81, I(t)=500 W/m2 and Ta=25oC(annual average values), UL=6 W/m2 oC and Tf =60 oC and n=250 clear days in one year and tT=8 sunshine hours per day, the annual thermal energy available can be calculated from Eq.(2.4d) as follows: Annual thermal energy Qannul=0.9×2×1× [0.81×500-6×(60-25)]8 × 3600×250 J =2.5272×109 J=702 kWh For forced mode, a pump of 0.1kW is required to circulate the water through flat plate collector and only change in FR=0.8 and Tfi=30 oC in the

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above calculation, the annual thermal energy available can be calculated from Eq.(2.5a) as follows: Annual thermal energy Qannul=0.8×2×1× [0.81×500-6×(30-25)]8 × 3600×250 J =4.32×109 J=1200 kWh

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Annual electrical energy required to operate the pump= 0.1×8×250=200 kWh The net annual energy saved for forced mode of operation=(1200-200) kWh =1000 kWh This calculation indicates that the net annual energy saved for forced mode of operation is higher than the annual thermal energy available in natural circulation mode of operation. The embodied energy, an amount of energy required producing the materials in the present form (for more detail see section 6), of different components of 2m2 galvanized iron (G.I.) sheet and tube has been shown in Fig. 2.1c. The total embodied energy required for 2m2 flat plate collectors is 1724 kWh. The energy pay back time (EPBT) can be evaluated as the ratio of embodied energy to an annual energy saving i.e.

EPBT =

Embodied energy An annual energy saving

For natural circulation mode,

EPBT =

1724 kWh = 2.46 years 702 kWh per year

For forced circulation mode,

EPBT =

1724 kWh = 1.724 years 1000 kWh per year

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(2.9)

24

G. N. Tiwari

Absorber (Plate and tube) 51%

Insulation 18.36%

Casing 9.87%

Fitting + Paint+gasket 7.49%

Glass cover 6.86%

Stand 6.28%

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Metal 70.73%

Non-Metal 29.27%

Figure 2.1.c. Percentage of energy required by different materials used in flat plate collector.

This shows that the EPBT is less than the life of flat plate collector i.e. 10 – 15 years. However, increasing an annual energy saving can further reduce the EPBT of flat plate collector. An annual energy saving can be increased by i. Increasing insolation ii. Increasing sunshine hours and iii. Reducing overall heat loss etc.

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2.2. EVACUATED TUBULAR COLLECTOR 2.2.1. Analysis of Owens – Illinois (OI) Flat Plate Collector

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The OI collector basically consists of two coaxial tubes with evacuated space between a selective coating surface of inner copper tube and outer glass tube. The selective coating is applied to the outer surface of inner copper tube. The heat transfer fluid enters through glass delivery tube and exits from the same end of the tubes through an annular space between selectively coated copper tube and glass delivery tube as shown in Fig. 2.2a. One end of selectively coated copper tube is sealed. The tubes are connected together at one end and are placed above a reflecting surface, which reflects sunlight onto the underside of the tube. Since the overall heat loss coefficient due to only radiation in evacuated space, conduction in the glass cover, convection and radiation from the outer glass cover to ambient is very small in comparison to the flat plate collector and hence, the collector efficiency is relatively insensitive to ambient air temperature, wind velocity and operating temperature up to about 150 °C.

Figure 2.2.a. Cross-sectional view of Owens – Illinois (OI) flat plate collector

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The rate of useful heat delivered by this collector tube is given by [ Beekley and Mather 1978] •

•

Q = mC ( T f

fo − T fi

) = Fr Ac [(ατ )I eff −U L ( AL / Ac )(T fi − Ta )]

( 2.9a )

where, Ac = Absorber tube diameter times collector length =0.043m×1.067m=0.0458 m2 AL = πAc =0.1440 m2 Cf = The specific heat of the fluid Ieff = Effective solar radiation on collector Tfi = Inlet fluid temperature Tf0 = Outlet fluid temperature UL = The overall heat loss coefficient =0.77 W/ m2 0C Fr= The flow rate factor =1.0 ατ = The product of absorptivity and transmitivity of glass tube=0.81

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The outlet fluid temperature from evacuated tubular collector can also be evaluated as •

T fO =

Q •

m Cf

+ T fi

(2.9b)

An instantaneous thermal efficiency ηi of evacuated tubular collector under forced mode of operation is given as,

ηi =

& Q

Ac Ieff

(Tfi −Ta )  -  = Fr (ατ) - UL Tfi Ta  = Fr (η0 ) − FrUL Ieff  I (t) 

(2.9c)

Equation (2.9c) is similar to Eq. (2.5b). The variation of an overall heat transfer coefficient (UL) with absorber temperature and ηi with (Tfi – Ta)/ Ieff have been shown in Figs. 2.2b and c respectively. Figure 2.2(c) also shows the performance of two-cover non – selective flat plate collector for comparison. It can be inferred from Fig 2.2(b) that there is not much variation in the value of an overall heat transfer coefficient (UL) with respect

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to ambient air temperature (Tfi). However, there is sharp increase in the value of an overall heat transfer coefficient (UL) with respect to absorber plate temperature (T4) due to high operating temperature as explained earlier. In the figure 2.2c, d is the center – to - center distance of two tubes and DB is the distance of center of tube from the absorber.

Figure 2.2. (b) Variation of UL with absorber temperature and (c) characteristic curves

2.2.2. Energy Pay Back Time (EPBT) For I(t) =500 W/m2 and Ta=25oC(annual average values), n=250 clear days in one year and tT=8 sunshine hours per day, Annual thermal energy Qannul=0.0458×1×[0.81×500-0.77×3.14×(3025)]8 ×3600×250 = 0.13×109 J=36 kWh The total embodied energy for 3mm (0.003m) thickness of copper and glass tube with effective surface area of 0.144 m2 (density = 8795 kg/ m3 and energy density = 36.87 kWh/kg) and 20.5 m2 (density = 2700 kg/ m3 and

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G. N. Tiwari

energy density = 8.72 kWh/kg) for evacuated tubular collector is 152.62 kWh. Out of total embodied energy, 8.25 % is glass cover and remaining 91.75 % is copper.

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Figure 2.2(c) Characteristic curve of O/I collector

The energy pay back time (EPBT) for evacuated tubular collector can be evaluated as

EPBT =

152.62 kWh = 4.2 years 36kWh per years

The energy pay back time (EPBT) for evacuated tubular collector is higher than flat plate collector due to use of G.I. pipe and sheet in flat plate collector. The evacuated tubular collector is generally used for industrial application unlike flat plate collector. However, either increasing an annual energy saving or decreasing the total embodied energy of evacuated tubular collector can further reduce the EPBT for evacuated tubular collector. An annual energy saving can be increased by i. Increasing insolation and ii. Increasing sunshine hours etc.

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The total embodied energy of evacuated tubular collector can be reduced by i. Decreasing the thickness of copper tube and ii. Changing the design with minimum use of copper.

2.2.3. Analysis of Evacuated – Tube Collector with Heat Pipe

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The schematic view of an evacuated – tube collector with heat pipe evaporator and condenser has been shown in Fig. 2.3. The fin plate has a selective surface. The condenser of heat pipe transfers the heat to the fluid through a manifold as shown in figure. Each fin plate with heat pipe absorber is enclosed in a separate evacuated cylindrical envelope.

Figure 2.3. View of an evacuated – tube collector with heat pipe

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For steady state condition, the rate of useful thermal energy in Watt can be expressed, Norton (1992) and Tiwari (2002), as follows: •

Qu = F A q •

r

c

ab − U L

(T fi − Ta )

( 2.10 a )



where ,

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Fr =

1  U L Ac   hhm Ahm

   U L  + + 1     h ph 

hhm and hph are heat transfer coefficient between the heat pipe fluid and the manifold fluid and from fin plate to the heat pipe fluid respectively and Ahm is area of heat pipe. This indicates that Fr mainly depends on various heat transfer coefficients and an area of heat pipe and collector and its values can be more then 0.8 for UL/hph for any values of ratio of resistances more than 17. The outlet fluid temperature from evacuated tubular collector with heat pipe can also be evaluated as •

T fO =

Qu

+ T fi

•

m Cf

(2.10b)

The instantaneous thermal efficiency can be defined as •

ηi

Q (T = u = F (ατ ) − F U IAc

r

r

L

fi

− Ta I

)

( 2.10b )

Equations (2.9c) and (2.10b) are same as Eq. (2.5b) which is the Hottel – Whiller – Bliss equation of flat plate collector. A graph between ηi verses (Tfi – Ta)/ I gives a straight line with intercept Fr(ατ) and slop FrUL for all cases..

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It is important to note that this analysis is based on a single – tube module. The value of Fr increases appreciably when a module of 20 or more tubes is considered.

2.3. THERMAL PERFORMANCE OF CONCENTRATOR A cylindrical parabolic trough is a convectional optical imaging device, which is used as a solar concentrator. In this case, beam radiation received by cylindrical parabolic trough is reflected at a blackened metal tube placed at focal line as shown in Fig. 2.4. A blackened metal tube can be glazed with cylindrical glass tube having higher diameter to reduce thermal losses from the blackened tube. The heat transfer fluid flows through the absorber tube, gets heated and thud carries heat. Focal line

Receiver

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Reflector

Figure 2.4. View of a cylindrical parabolic trough concentrator

The energy balance considerations, similar to flat plate collectors, are applied to describe the performance of concentrators. The complications occur in the calculation of thermal losses due to the following reasons: i.

Receiver shapes are widely variable and the radiation intensity at the receiver is not uniform. ii. The temperature being high (>1500C), edge losses and conduction effects are significant. In a steady state condition, the useful energy gain Watt, in terms of the energy transfer to the fluid at local fluid temperature, Tf is given, Tiwari(2002), by

• Ar ′ = F A U L (T f - T a a q ab − Qu Aa  •

 ) 

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(2.11a)

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G. N. Tiwari

where Aa and Ar are the aperture and the receiver area. The collector efficiency factor, F′, is given as

F′ =

or,

1/ UL 1 D0   +  D0 ln Do  UL hfi Di  2K Di  F′ = U0 / U L

where Do and Di are outer and inner diameter of absorber (copper tube) and K is the thermal conductivity of copper material. The hfi is the convective heat transfer coefficient from inner surface of the tube to the fluid The collector heat loss factor (UL) with respect to collector aperture area is defined as,

U L = q& loss /(T r - Ta )

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From equation (2.11a), an instantaneous thermal efficiency of a cylindrical parabolic trough concentrator ca be written as •

Qu = F′ ηi = Aa Ib

1  η 0 - ( RC ){U L (Tf - Ta )/I b

 } 

(2.11b)

where RC, Tr and Ta are the concentration ratio, the receiver plate and ambient temperature and η0=ατ. The collector efficiency factor, F/, is given by the ratio between thermal resistance from the receiver surface to ambient and the thermal resistance from fluid to ambient. In terms of inlet fluid temperature (Tfi) i.e. forced mode of operation, Equation (2.11b) becomes



1



{U L (Tfi - Ta )/I b } η c = FR  η 0 RC   The heat removal factor, FR, is given by

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(2.12)

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FR =

 UF / & Cf  m 1 - exp  & Cf UL   m

  

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& is the flow rate per unit area and Cf is the fluid specific heat. here m

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Chapter 3

SOLAR DISTILLATION

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3.1. IMPORTANCE OF WATER Water is the most abundant and important substance on the earth. Supply of drinking water is a major problem in underdeveloped as well as in some developing countries. Along with food and air, water is a basic need for human being for survival. Man has been dependent on rivers, lakes and underground water reservoirs for fresh water. It is the principal component of life, health and sanitation on the earth. It is absolutely essential for life and vegetation on the earth though the freshwater availability in the land areas of the earth is more than adequate to meet the current water needs. However, it is becoming scarce with time, leading to severe water crisis in many parts of the world. This is attributed mainly to uneven distribution of water resources, steep increase in population and water pollution due to industrial growth in recent past. It has been observed that an availability of water will decrease to less than half for most of the countries in the year 2025 and for some countries, like Tanzania, per capita water availability would decrease to one third of the water available in 1990. A widespread general knowledge of scientific awareness among the people helps in preventing wastage of water. Vital benefits such as sustained supply of potable water have been realized in drought prone areas adopting a scientific approach. The approach makes it possible to: i.

Optimise the water use so that it can be made available in desired time and space, ii. Avoid wastage and iii. Make it economically viable.

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G. N. Tiwari

In developing country poverty, malnutrition, unsafe drinking water and an unsanitary environment are largely responsible for epidemic and deadly diseases. Diseases related to water and sanitation has far reaching social and economic consequences. Excess brackishness causes the problem of taste and laxative effects. One of the control measures includes supply of water with total dissolved solids within permissible limits (500 ppm as WHO). This is accomplished by several desalination methods like reverse osmosis, electro dialysis, vapor compression, multistage flash distillation and solar distillation, which are used for purification of water. Among these, the solar stills can be used as desalinators for such remote settlements where salty water is the only type of moisture available, power is scarce and demand is less than 200 m3/day. On the other hand, settings of water pipelines for such areas are uneconomical and delivery by truck is unreliable and expensive. Since other desalination plants are uneconomical for lowcapacity fresh water demand, under these situations, solar stills are viewed as a means to attain self-reliance and ensure regular supply of water.

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3.2. WATER SOURCES AND WATER DEMAND More than two-thirds of the earth's surface is covered with water. Most of the available water is either present as seawater or as icebergs in the Polar Regions or rivers/lakes. Freshwater for drinking is only 2.6 percent and the rest is salty. Less than 1 percent (of 2.6 percent) freshwater is within human reach. Even this small fraction is believed to be adequate to support life and vegetation on the earth. Nature itself provides most of the required freshwater, through hydrological cycle. A very large-scale process of solar distillation naturally produces freshwater. Solar radiation falling on the surface of rivers, lakes, marshes and oceans is absorbed as heat and causes evaporation of water from these heated surfaces. The resulting vapors rise as humidity of the air above the surface and move along winds. When the air vapor mixture is cooled to the dew point temperature, condensation may occur; and the pure water may be precipitated as rain or snow. The essential features of this process are thus summarized as the production of vapors above the surface of the liquids, the transport of vapors by winds the cooling of air-vapor mixture, condensation and precipitation. This natural process is copied on a small scale in basin type solar stills. Water is essential for human life. Every activity of man involves some use of water. The most important uses of water are in three sectors, namely

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i. Domestic (6%), ii. Agriculture (92%) and iii. Industrial (2%).

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3.3. WATER POLLUTION AND ITS EFFECT According to a World Health Organization (WHO) report, around 30,000 people die from water related diseases, everyday. The report further says that in developing countries 80 percent of all illness is water related. A quarter of the children born in these countries die before the age of 5, mainly due to water related diseases. In India, it is estimated that every year 1.5 million children under the age of 5, and 4 million in total, die due to water related diseases alone. The diseases so transmitted are chiefly due to microorganisms and parasites. Cholera is one such disease. In India, during 1897 to 1907, about 370 000 people died from this disease, and thousands of Indians continue to die every year even now. According to a report of Indian Toxicology Research Centre (ITRC), about 8,000 cases of Cholera, 1 million cases of gastroenteritis and 7 million cases of dysentery were reported annually. Hence water pollution has become a major problem and singlehandedly it affects the health of people to a great extent. It is therefore, important to disinfect the polluted water. Among the non-conventional methods to disinfect the polluted water, solar distillation is the most prominent method. Comparatively, this requires simple technology and low maintenance and because of this it can be used anywhere with lesser number ofproblems. The principles of solar distillation are discussed in the next section.

3.4. PRINCIPLE OF SOLAR DISTILLATION: A STATE OF THE ART Figure 3.1 shows the various components of energy balance and thermal energy loss in a conventional double-slope symmetrical solar distillation unit (also known as roof type). It is an airtight basin, usually constructed out of fiber-reinforced plastic (FRP) with a top cover of transparent material like glass, and plastic. The inner surface of the base known as basin liner is blackened to efficiently absorb the solar radiation incident on it. There is a provision to collect distillate output at lower ends of top cover. The brackish

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G. N. Tiwari

or saline water is fed inside the basin for purification using solar energy. The working principle of the distiller unit is as follows:

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Figure 3.1. Fraction of solar flux at different components of the distillation unit.

The solar radiation after reflection [Rg.I(t),W m-2] and absorption [ α g (1-Rg). I (t), W m-2] by the glass cover is transmitted inside the enclosure of distiller unit. The transmitted radiation [(1- α g) (1-Rg).I(t), Wm-2] is further partially reflected [Rw'.I(t), W m-2,Rw'=(1-Rg).(1-ag).Rw] and absorbed ′ ′ −2 [ α w ⋅ I (t ), Wm , α w = α w 1 − α g ⋅ 1 − R g ⋅ (1 − Rw ) ] by the

(

)(

)

water mass. The attenuation of solar flux in water mass depends on its absorptivity and depth, respectively. The remaining solar radiation finally reaches the blackened surface where it is mostly absorbed and converted into thermal energy. After absorption of solar radiation at blackened surface, i.e., basin liner, most of the thermal energy is convected to water mass and the rest, which is very small (for nicely insulated bottom and sides of distillation unit), is lost to atmosphere. The water, thus, gets heated, leading to an increase of temperature difference of water and glass cover. Inside the solar still there are basically three modes of heat transfer, namely radiation, convection and evaporation from the water surface to the glass cover. The evaporated water gets condensed on the inner surface of the glass cover after releasing the latent heat. The condensed water trickles into the channels provided at the lower ends of glass cover under gravity. The collected distilled water in the

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channel is taken out of the system for appropriate use. The thermal energy received by the glass cover, which includes radiative, convective and latent heat, is lost to the ambient by convection and radiation. The distillate output of a solar still depends on design parameters, operational conditions and climatic parameters. Therefore, solar distillation is an attractive alternative because of it's simple technology; non-requirement of highly skilled labor for maintenance work and low energy consumption. As such, it can be used at any place without much problem. Based on published literature, solar distillation has been classified as follows: Passive solar still: In passive solar still, the water in the basin is generally heated by solar radiation coming through the condensing cover. The shape of condensing cover may be different for higher yield. Some of them are symmetrical (Fig. 3.1) and unsymmetrical single basin double slop, single slop and single basin, conical, spherical and multi-wick etc. solar stills. ii. Active solar still: In passive solar still, if the water in the basin is also heated by other means namely flat plate, evacuated tubular and concentrator collectors, then it is referred as active solar still.

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i.

3.5. THERMAL MODELLING OF SINGLE BASIN SYMMETRICAL DOUBLE SLOP SOLAR STILL For thermal modelling of a single basin symmetrical double slop solar still as shown in Fig. 3.1, the energy balance equation for each component of a single basin symmetrical double slop solar still should be written.

Energy balances: The basic heat flux components at various points have been shown in Fig. 3.1. The following assumptions have been made in writing the energy balance in terms of Joules per sec per m²: i. Inclination of the glass cover is very small, ii. The heat capacity of the glass cover, the absorbing material and the insulation (bottom and sides) is negligible, iii. The solar distiller unit is vapor-leakage proof and is in a quasisteady state condition and

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G. N. Tiwari iv. Absorptivity of the glass cove (condensing cover) and water mass is neglected.

The energy balance for different components of the still, Tiwari (2002), is as follows: Glass cover:

h1 ( T w − Tg )A b = h 2 (Tg - Ta )A g

(3.1)

Water mass: glass cover

h3 ( T b − T w )A b = A b (MC )w

dT w + h 1 (T w - Tg )A b dt

(3.2)

Basin liner:

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α bτ I (t ) Ab = h 3 (T b - Tw )A b + h b (T b - Ta )(A b + As )

(3.3)

where Ab and As are basin liner and water side contact area, Ag is the glass cover (condensing cover) area. The h1=hrw+hcw+hew, the total internal heat transfer coefficient from water surface to the glass cover, h2=5.8+3.8V, the convective and radiative heat transfer coefficient from the glass cover to ambient, h3, the convective heat transfer coefficient from basin liner to the water mass and hb, the overall heat transfer coefficient from basin liner to ambient thorough the bottom insulation. An expression for other heat transfers coefficients, Cooper (1973) and Malik etal. (1982), are as follows: hrw=6.5W/m20C, 1/3

(P w - Pg ) (T w + 273)   hcw = 0.884 T w - Tg + 268.9 x 103 - P w   −3 hew = 16.273 x 10 h cw

P w − Pg T w - Tg

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5144   P (T ) = exp  25.317 − , T + 273   −1

L 1  hb =  i +  , hbc=2.8 +3V  K i h bc  where hbc is the convective heat transfer coefficient from bottom of insulation to ambient for mounted solar still, Li and Ki are thickness and thermal conductivity of bottom insulation and V and P(T) are the wind velocity and partial vapor pressure at temperature T. Equations (1-3) can be solved for the water and glass cover temperatures. These equations can be rewritten after algebraic simplification in the form of

dT w + a T w = f (t ) dt

(3.4)

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where,

a =

(ατ )eff I (t ) + U L Ta UL ; f (t ) = ; (MC )w (MC )w

( ατ )eff = α b τ

h3 h3 + h b

U L = Ub + U t ; U b =

/

hb = hb [1 +

/ h 3 hb / h1 h 2 ; , = Ut / / h3 + h b h1 + h 2

Ag As / ] and h2 = h2 Ab Ab

An approximate solution of Eq. (3.4) with the above initial conditions i.e. Tw=Tw0 at t=0.0 and with the following assumptions:

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G. N. Tiwari The time interval ∆t (0 < t < ∆t) is small, The function f (t) is constant, i.e. f (t) = f (t ) for the time interval ∆t

and, ‘a’ is constant during the time, interval ∆t. can be written as

Tw =

f (t ) [ 1 − exp ( - a ∆t )] + T w 0 exp ( - a ∆t ) a

(3.5)

After knowing the water temperature, Tw, an expression for the glass cover temperature, Tg, can be obtained from Eq.(3.1) as / h1 T w + h 2 Ta / h1 + h 2

Tg =

(3.6)

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The rate of heat transfer per m2 from water surface in the basin due to evaporation is determined by

q& ew = h ew (T w - Tg )

(3.7a)

The hourly yield per m2 from a single basin symmetrical double slop solar still can be evaluated as •

m ew

•

q × 3600 = ew L

(3.7b)

where L, the latent heat of vaporisation, Fernandez and Chargoy, 1990 and Toyama, 1972, is given by (i) For operating temperatures less than 70 0C,

L = 3.1615 × 106 [1 - 7.6160 × 10−4 T ] , and (ii) For temperature higher than 70 0C

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L = 2.4935×106 [1- 9.4779×10−4 T + 1.3132×10−7 T2 - 4.7974×10−9 T3]

3.6. THERMAL EFFICIENCY OF A SINGLE BASIN SYMMETRICAL DOUBLE SLOP SOLAR STILL The thermal efficiency of solar still can be defined as the ratio of the amount of thermal energy utilized to get a certain amount of distilled water to the incident solar energy within a given time interval. Thermal efficiency of single basin symmetrical double slop solar still can be defined in the following forms: (i) Instantaneous thermal efficiency

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An instantaneous efficiency of distillation unit having water mass Mw is given by

ηi =

q& ew h ew (T w - Tg ) = I (t ) I (t )

ηi =

h ew h1g (T w - T a ) h1w + h1g

or,

(from Eq . 3.6)

After substituting the expression for water temperature, Tw, from Eq.(3.5) in the above equation, one gets

ηi =

hew h / 2 1   (T - T ) × (ατ )eff (1− e−a ∆t ) + UL w 0 a e−a ∆t  / I (t ) h1 + h2 UL   (3.8a)

Equation (3.8a) is known as characteristic equation of solar still. The variation of

ηi

with

(T w 0 - T a ) has been shown in Fig. 3.2. It is clear I (t )

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G. N. Tiwari

that the behaviour of characteristic curve of a solar still is opposite to the characteristic curve of a flat plate collector (Fig. 2.1b). This can be explained as follows: i.

The solar still gives better performance for maximum upward heat loss unlike flat plate collector and ii. Internal heat transfer coefficient is non-linear due to temperature dependent if evaporative heat transfer coefficient unlike flat plate collector.

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It is important to mention that water mass (water depth) in the basin has been considered very small.

Figure 3.2. Characteristic curve of a single slop solar still

(ii) An overall thermal efficiency The overall thermal efficiency of a single basin symmetrical double slop solar still (passive solar still) can be mathematically expressed as

η passive =

Σm &wL × 100 As ∫ I (t) dt + ( MC ) w (Tw0 − Ta )

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(3.9)

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If Tw0 is the initial basin water temperature fed into the basin and the second term in the denominator will be zero if Tw0 =Ta.

3.7. EFFECT OF CLIMATIC AND STILL DESIGN PARAMETERS ON PERFORMANCE OF PASSIVE SOLAR STILL The daily/annual yield from a single basin symmetrical double slop solar still mainly depends on climatic (solar intensity, wind velocity and ambient air temperature) and still design parameters namely, absorptivity, water depth, impurity in water mass, inclination of condensing cover and bottom insulation thickness etc. These parameters affect the temperature difference between the water surface in the basin and inner surface of glass cover (the condensing cover). As seen from Equation (3.7), the higher the value of (Tw – Tg), the greater is the daily/annual yield.

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Effect of Wind Velocity In the case of mounted solar still, wind can flow above the condensing cover and below the basin. The wind has no significant effect on the daily/annual yield due to insulating material used for basin construction. However, as the wind velocity flowing above the condensing cover increases, the convective heat loss from the glass cover to ambient increases hence the glass cover temperature decreases which increases the water-glass cover temperature difference i.e. (Tw – Tg) and hence the overall daily/annual yield. This effect is significant at lower water depth. The overall daily/annual yield varies between 1-10 %, Cooper (1969).

Effect of Solar Intensity and Ambient Air Temperature With decrease in the ambient air temperature (cooler location), the glass temperatures decrease due to fast heat release from glass cover and temperature difference between the water surface in the basin and inner surface of glass cover (the condensing cover) i.e. (Tw - Tg) increases, but there is a general fall in the overall temperature of the system, hence the

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G. N. Tiwari

output increases. The solar still becomes more effective during nighttime at higher water depth in the basin. For lower values of ambient air temperature and high insolation (solar intensity), solar still gives maximum daily/annual yield.

Effect of Bottom Insulation The optimisation of thickness of bottom insulation of basin depends on water depth in the basin. It plays very important role at higher depth due to nocturnal production of distilled water (faster evaporation in the night). The thickness also depends on the type of insulation and its compactness. For example, the basin of fibre-reinforced plastic (FRP) may be 0.003m thick.

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Effect of Condensing Cover Inclination The optimum inclination of the condensing cover varies from season to season for receiving maximum solar radiation. However, it should be optimised from annual yield per m2 point of view. It has been observed that the optimum inclination is equal to latitude of the place, Singh and Tiwari (2004) for northern part of India. These parameters should be optimised for given latitude and climatic parameters.

Effect of Impurities in the Water Impurities in the water may be of different kind e.g. arsenic, fluoride, iron content and salt concentration etc.. However, the presence of arsenic, fluoride and iron content in the water do not affect much the performance of solar still. Experiments have shown that the daily/annual output per m2 of the still falls linearly as the salt concentration of the water increases right up to the saturation point.

Effect of Water Depth (Water Mass Per M2) The daily/annual yield from a solar still initially decreases rapidly up to the depth of 0.10m and than becomes stagnate for passive solar still. This is an important parameter, which affects the daily/annual yield from a passive

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.

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solar still. The operational depth of water in the basin is about 0.05m, ElSebaii (2003). The behaviour of an overall thermal efficiency of passive and active solar still are different due to operating temperature range for a given climatic parameters. The operating temperature range in active solar still is much higher than passive solar still. It depends on the following parameters: i. Type of collectors used for active heating, ii. Number of collectors used for heating and iii. Either collector is connected in series or parallel.

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Effect of Charcoal and Black Dye According to the studies made by Akinsete and Duru (1979), the effect of charcoal is most pronounced in the mornings and on cloudy days when the values of direct radiation are low. The presence of charcoal pieces utilizes the diffuse radiation much better than the conventional unlined still. It was also seen that the charcoal lined still is relatively insensitive to basinwater depth as long as a good amount of the charcoal remains uncovered. However, it becomes very expensive from maintenance point of view due to regular cleaning of charcoal and basin of the solar still. The same problem has been observed in the case of black dye used in the basin water. The black dye has significant effect at larger depth of basin water (≥0.10 m). Since the daily/annual yield is higher at lower depth, hence the use of black dye has never been recommended in the case of solar still.

Effect of Scaling on the Basin Liner This is important parameter and its effects depend on salinity of water to be distilled. For seawater with salinity more than 50.000 ppm, the cleaning of basin water may be required more then the underground water with salinity of about 5,000 ppm. The maintenance of solar still with seawater may not be feasible due to fast reduction of absorptivity of basin liner. It has been observed that the daily/annual yield

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3.8. OTHER DESIGN OF PASSIVE SOLAR STILLS On the basis of literature survey till 2004 keeping the maintenance of solar distillation unit, an important parameter, it has been observed that the cost of daily/annual yield per m2/day in a symmetrical double slop single basin solar still (Fig.3.1) is acceptable to common users for drinking purpose, Tiwari et al. (2003). Since the daily/annual yield of solar still mainly depends on the temperature difference between the evaporative and condensing surfaces. However, some scientists had made an attempt to maximise the daily yield per m2/day in a single basin solar still in a passive mode by changing its design to get maximum temperature difference between the evaporative and condensing surfaces. Some of the developed design has been discussed in the following sections:

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Unsymmetrical Double Slop and Single Basin Solar Still with Fin In order to increase the daily yield of unsymmetrical double slop and single basin solar still, Faith and Hosny (2002) have used the fin on one of the condensing cover to increase the heat transfer from outside condensing cover to ambient for higher evaporation. This arrangement has been shown in Fig. 3.3a. The solar still is north-south orientation. It was observed that there is an increase of 55% in yield in comparison with base case. Solar radiation

Glass cover

Built-in Finned condenser

Black dyes Basin

Bottom insulation

Figure 3.3.a. Schematic view of unsymmetrical single slop and single basin solar still

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Single Slope Single Basin Solar Still with Condenser

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In a symmetrical double slop solar still, the glass cover is used for transmission of solar energy as well as for condensation of water vapor evaporated from water surface. During the condensation, the latent heat is given to the glass cover, which raises the glass cover temperature and hence reduces the overall temperature difference between the evaporative and condensing surfaces. In order to increase this difference, the condensing surface is separated from the single slop single basin solar still chamber as shown in Figures 3.3b. There is little condensation on the slopped surface, mainly condensation takes place in the attached condenser due to transfer (purging) of vapor from solar still chamber to condensing chamber. Since most of condensation is taking place in the condensing chamber, the temperature difference between glass cover and water is more, which causes faster evaporation, and distillate output is more. In this case, the still efficiency is increased by 45%. Further, the distillate output can be increased by natural circulation of vapor between evaporator and condenser.

Figure 3.3.b. Single slop solar still with condenser

Single Slope Single Basin Multiple Effect Diffusion Solar Still Tanaka et al. (2002) have improved the design of single slop solar still as shown in Fig.3.3c for higher daily yield. In this case, the outer portion of vertical wall is attached with wetted wick so that the maximum condensation takes place on inner surface due to its lower temperature in comparison with

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G. N. Tiwari

glass cover. During the process, the latent heat of condensation is released to the vertical wall. The vertical wall gets heated along with water trickling over it and further evaporation takes place and then condenses on to another metallic wall with wetted wick after releasing latent heat of condensation and process goes on and hence it is refereed as multiple effect. Tanaka et al. (2002) have studied till 3- effect diffusion type solar still. feeding saline water pn

p1 Double glass cover

Wicks Partitions Distillate

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Basin water

Distillate Brine to waste

Figure 3.3.c. Schematic view of unsymmetrical single slop and single basin solar still

Two Comportment Single Basin and Single Slop Solar Still Meukam et al. (2004) have design two comportment single basin solar still, which is shown in Fig. 3.3d, for alcohol production by distillation method. The compartment 1 has single glass cover to transmit solar radiation. It also receives the evaporated water from basin water for condensation. There is another glass cover over compartment 2, which is cooler than the glass cover of compartment 1. There is plywood over glass 2 at the gap of 2 cm so that air can flow between the space provided between plywood and glass cover 2 to cool the glass cove for faster condensation. It has been reported that there is 23% increase in alcohol production with two compartments solar still.

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Solar Energy Technology Advances Glass 1

51

Glass 1

Condensate

Condensate collector Condensate collector

Compartment 1 Compartment 2

Solution to be distilled Blacken Metallic plate

Plywood

Ploystrene

Figure 3.3.d. Schematic view of two comportment single basin and single slop solar still

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3.9. THERMAL MODELLING OF ACTIVE SINGLE SLOP AND BASIN SOLAR STILL In the case of active solar still, the temperature difference between the evaporating and condensing surfaces is increased by feeding the additional thermal energy from other source namely the flat plate, evacuated tubular and concentrator collectors into the basin of solar still as shown in Fig. 3.4, Voropoulos et al. (2004). Many scientists have investigated such active solar still. Some of them are Rai and Tiwari (1982), Tiwari (1992) and El-sebaii (2004) etc. The flat plate collector is integrated to the basin of solar still. The water in the basin is circulated through flat plate collector either in a natural circulation mode (parallel connection) or a forced circulation (series connection) mode depending upon the requirement. In the Fig. 3.2, a heat exchanger has been used between collector and bottom of the solar still to circulate the hot water between collector and solar still without mixing of water of collector and solar still. This arrangement is most suitable under very harsh cold climatic condition (ambient air temperature goes below zero degree centigrade). The system has dual application namely distillation and water heating as pointed by Voropoulos et al. (2004). The connecting pipes are insulated to avoid thermal losses from the hot water in the pipe to ambient during hot water circulation through it. In an active solar still, the

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water in the basin is heated directly as well as indirectly through a flat plate collector. The collector should be operated only during sunshine hour. The rise in the temperature of water in the basin mainly depends on number of collectors connected in series.

Figure 3.4. A schematic view of active solar still

Energy Balance Energy balance for glass cover and basin liner of active solar still without heat exchanger will be as Eqs. (3.1) and (3.2) respectively. However, energy balance of water mass will be as:

Water Mass: Glass Cover •

Qu + h3 (T b − Tw )Ab = A b (MC )w

dTw + (Tw - Tg )Ab dt

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(3.10)

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•

where Q u is the rate of thermal energy available from either flat plate, evacuated tubular with and without heat pipe and concentrator collectors (see section 2, Eqs. 2.7a, 2.9a, 2.10a etc.). Equations (3.1), (3.3) and (3.10) can be solved for an expression of the water, glass cover temperatures, the rate of heat evaporated and hourly/daily yield for an active solar still. In the case of active solar still, an overall thermal efficiency for an active solar still ( η active ) can be written as

η active =

Σm & ew L ×100 / Ab ∫ I (t )dt + ηc Ac ∫ I (t )dt + (MC)w (Tw0 − Ta )

[

]

(3.11) where

η c and I / (t )

are collector thermal efficiency and the solar intensity

available on the collector. 20

ηo(P) ηo(A)

16 Overall efficiency (%)

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18

14

12

10

8

6 0.05

0.1 Depth (m)

Figure 3.5. The variation of η passive and η active with water depth.

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If Tw0 is the initial basin water temperature fed into the basin and the third term in the denominator will be zero if Tw0 =Ta. The variation of η passive and η active with water depth has been shown in Fig. 3.5. It is clear that an overall thermal efficiency for active solar still is lower than an overall thermal efficiency of passive solar still due to operating temperature range.

3.10. ENERGY PAY BACK PERIOD (EPBT) For Passive Solar Still In order to evaluate energy pay back period (EPBT) for single slop and single basin solar still, one need the embodied energy required to fabricate solar still with an effective area of 1m2 and annual yield from solar still. Embodied energy of a passive solar still can be calculated as follows:

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Embodied energy of 1.2 m2 glass cover =Area × thickness × density × energy density = 1.2m2 × 0.003m × 2350 kg/m3 × 8.72 kWh/kg =73.77kWh Embodied energy of fibre glass material = 10kg

× 25.64kWh/kg = 256.4kWh

Embodied energy of galvanised iron angle stand = 8 kg × 13.88 kWh/kg = 111.04 kWh Total embodied energy for solar still= 441.21 kWh If annual yield/ m2 (out put) is assumed to be 2kg and number of sunshine day in the year is considered as 250, then annual energy available from solar still will be Annual energy = 2 kg

× 250 × 2.52 × 106 J/kg = 12.60×108 J = 350kWh

So, energy pay back time = EPBT =

Embodied energy An annual energy saving

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or,

EPBT =

441.21 kWh = 1.26 years 350kWh per year

This indicates that EPBT for passive solar still is much less than expected life of passive solar still, which is about 25-30 years.

For Active Solar Still

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An embodied energy for active solar still = Embodied energy of passive solar still + Embodied energy for 2 m2 collector Following section 2.1.3, an embodied energy for 2 m2 collector is 1724 kWh, then An embodied energy for active solar still = 441.21 + 1724 = 2165.21 kWh If an annual yield per 1 m2 of active solar still is 5 kg, then Annual energy = 5 kg × 250 × 2.50 × 106 J/kg = 31.25×108 J = 868 kWh So,

EPBT =

2165.21 kWh = 2.49 years 868kWh per year

It is important to note that energy pay back time (EPBT) for active solar still is significantly higher than energy pay back time (EPBT) for passive solar still. It is due to fact that an overall thermal efficiency of active solar still is reduced due to high operating temperature range. Hence active solar still is recommended for commercial application and passive solar still should be used for domestic application.

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Chapter 4

SOLAR GREENHOUSE CROP DRYING

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4.1. IMPORTANCE OF DRYING After air and water, it is well known that food is a basic need of a human being. Food holds a key position in the development of a country. In order to avoid food losses (generally about 25%depeding on crop) between harvesting and consumption, there is need of drying of food for storage for long time use. High moisture content is one of the reasons for its spoilage during the course of storage during harvesting period. High moisture crops are prone to fungus infection, attacks by insects, pests and the increased respiration of agricultural produce. To solve this problem many drying technique have been developed. Drying has the following advantages: i. Facilitate early harvest, ii. Permits planning the harvest season, iii. Helps in long term storage, iv. Helps farmers to fetch better returns, v. Helps farmers to sell a better quality product, vi. Reduce the requirement of storage space, vii. Helps in handling, transport and distribution of crops and and viii. Permits maintaining viability of seeds. Drying helps in reducing the moisture content of a product to a level below which deterioration does not take place and the product can be stored for a definite period. The final moisture content (%,w.b.) and name of the some crops are given below:

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Final moisture content (%,w.b.) (i) ≅ 5

(ii) 10-15 (iii) ≤ 25

Name of the crop Vegetables (Green peas, Cauliflower, Carrots, Green beans, Onions, Garlic, Cabbage, Sweet potato, Gauvas, Brinjal etc. ) Paddy, Maize, Wheat, Rice, Pulses, Oil seed etc. Fruits (Grapes, Apples, Apricot, Grapes, Okra etc.)

4.2. WORKING PRINCIPLE OF DRYING SYSTEM Basically, there are three modes of drying namely,

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i. Open sun drying ii. Direct drying and iii. Indirect modes of drying In the presence of solar energy. The working principle of these modes mainly depends upon the method of solar energy collection and its conversion to useful thermal energy.

Open Sun Drying (OSD) The working principle of open sun drying (OSD) has been shown in Fig. 4.1. The short wavelength solar energy [I(t)] falls on the uneven crop surface having an area of At. A part of this energy is reflected back into the atmosphere and the remaining part is absorbed [αc I(t)] by the crop surface. The absorbed radiation is converted into thermal energy and the temperature of crop [Tc] starts increasing. These results in i. Long wavelength radiation loss, ii. Convection loss and iii. Mass transfer through moist air removal from the crop surface to ambient air.

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The convective heat loss depends on wind speed and evaporation of moisture takes place in the form of evaporative losses and so the crop is dried. Further a part of absorbed thermal energy is conducted into the interior of the product. This helps a rise in temperature and formation of water vapour inside the crop and than diffuses towards the surface of the crop and finally losses the thermal energy in the form of evaporation. In the initial stages, the moisture removal is rapid since the excess moisture on the surface of the product presents a wet surface to the drying air. Subsequently, drying depends upon the rate at which the moisture within the product moves to the surface by a diffusion process depending upon the type of the product, Sodha et.al., (1985).

Figure 4.1. Working principle of open sun drying

In open sun dying, there is a considerable loss of the crop due to many reasons. Some of them are due to rodents, birds, insects, micro-organisms and unexpected rain or storm et. Further, over drying and insufficient drying as well as discolouring by UV radiation is characteristic for open sun drying. In general, open sun drying does not fulfil the international quality standards and therefore it can’t be sold in the international market. In addition to an open sun drying, a more scientific method of solar energy utilisation for crop drying has been emerged, which is known as controlled drying or solar drying. These are as follows:

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Direct Solar Drying (DSD)

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Figure 4.2 shows the principle of direct solar crop drying. This is also referred as cabinet dryer. The cabinet dryer works on the same principle as in the case of either flat plate collector or solar still. In this case too, microclimate is created inside cabinet dryer. Solar radiation incident on the glass cover is transmitted inside cabinet dryer after reflection from the glass cover. Further, a part of transmitted radiation is reflected back in the form of short wavelength from the surface of the crop, which is again transmitted to atmosphere through the glass cover. The remaining part is absorbed by the surface of the crop. Due to the absorption of solar radiation, the crop temperature increases and the crop start emitting

Figure 4.2(a) Working principle of direct solar drying

i.

Long wavelength radiation, which is not allowed to escape to atmosphere due to presence of glass cover unlike open sun drying. Thus the temperature above the crop inside chamber becomes higher, ii. The glass cover serves one more purpose of reducing direct convective losses to the ambient which further becomes beneficial for rise in crop and chamber temperature respectively and iii. The convective and evaporative losses occur inside the chamber from heated crop simultaneously. The moisture (the vapour formed

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due to evaporation) is taken away by the air entering into the chamber from below and escaping through another opening provided at the top as shown in Fig. 4.2.

Figure 4.2(b) Schematic view of solar cabinet dryer

A cabinet dryer has the following limitations: i.

Due to its small capacity, its use is limited to small-scale applications, ii. Discolouration of crop due to direct exposure to solar radiation, iii. Moisture condensation inside glass cover reducing its transmittivity and iv. Sometimes the insufficient rise in crop temperature affecting moisture removal. Further, some of the limitations of cabinet dryer namely capacity and moisture condensation at inner surface of glass cover and cost has been removed in greenhouse drying, which is also a direct solar dryer. The greenhouse dryer has been briefly discussed below:

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Greenhouse Crop Drying

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The working principle of greenhouse crop dryer is same as the working principle of cabinet dryer. A roof type even span greenhouse with 1.20 × 0.78 m2 effective floor covering area was made of PVC pipe and UV (ultraviolet resistance ) film covering materials. The central height and height of the walls were 0.62 and 0.42 m, respectively. An air vent with an effective opening of 0.0722 m2 was provided at the roof for natural convection. For forced convection a fan of 120 mm sweep diameter with air velocity 5 m/s was provided on the sidewall of the greenhouse during the experiments. The orientation of the greenhouse was taken as east–west during the experiments due to motion of sun from east to west due south. The schematic view of greenhouse dryer is shown in Fig. 4.3. A wire mesh tray having dimensions of 0.4 × 0.24 m2 for thin layer drying can be used. The drying product can be placed on tray. The greenhouse is erected above the ground to allow the passing air though base of tray for thermal heating and extracting moist air from the product. The moist air is driven either natural mode or forced mode.

Figure 4.3(a) Schematic view of greenhouse dryer under natural convection mode

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Sun

Ta

I (t )

γa

0.60 m

Tr

Ui Ui

Tj

Fc

0.40 m hc

Tr

Tr

γr

Fn

Inlet Air Inlet Air T

r

hgr

Jaggery

1.2 m

Balance

W S 0.78 m hg∞

N E

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Figure 4.3(b) Schematic view of greenhouse dryer under force convection mode

The above design of greenhouse dryer have been used to evaluate basic internal heat and mass transfer coefficient of various crop namely onion, green peas, clouliflower, potato chips, jaggery etc. to be dried inside greenhouse, Anwar and Tiwari, ( 2001). Further, design of any size of greenhouse dryer can be done for a given capacity of the crop to be dried. The bigger greenhouse can also be used for cultivation of various crops during off-season. Others limitation of cabinet dryer namely discoloration of crop due to direct exposure to solar radiation and insufficient rise in crop temperature affecting moisture removal can be solved in indirect solar drying by either using (i) reverse absorber cabinet dryer (RACD) or conventional solar dryer.

Indirect Solar Drying (ISD) In the case of indirect solar drying, the crop is not directly exposed to solar radiation to minimise discolouration and cracking on the surface of the crop.

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(i) Reverse absorber cabinet dryer (RACD)

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Anwar and Tiwari(2001) have proposed and analysed reverse absorber cabinet dryer (RACD). The schematic view of RACD without and with glass has been shown in Fig. 4.4. In this case convective heat loss from an absorber is suppressed due to down facing of absorber. The drying chamber is used for keeping the crop in wire mesh tray. A downward facing absorber (reverse absorber) is fixed below the drying chamber at a sufficient distance ( ≅0 05m) from the bottom of the drying chamber. A cylindrical reflector is placed under the absorber fitted with the glass cover on its aperture to minimise convective heat losses from the absorber. The absorber can be selectively coated. The inclination of the glass cover is taken as 45° from the horizontal to receive maximum radiation. The area of absorber and glass cover are taken equal to the area of bottom of the drying chamber. Solar radiation after passing through the glass cover is reflected by cylindrical reflector towards the absorber. After absorption, a part of this is lost to ambient through the glass cover and the remaining is transferred to the flowing air above it by convection. The flowing air is thus heated and passes through the crop placed in the drying chamber. The crop is heated and moisture is removed through a vent provided at the top of drying chamber.

Figure 4.4(a) Schematic view of single tray reverse absorber cabinet dryer without glass

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Figure 4.4(b) Schematic view of single tray reverse absorber cabinet dryer with glass

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(ii) Conventional dryer Figure 4.5 describes another principle of indirect solar drying which is generally known as conventional dryer. In this case, a separate unit termed as solar air heater is used for solar energy collection for heating of entering air into this unit. The air heater is connected to a separate drying chamber where the crop is kept. The heated air is allowed to flow through wet crop. Here, the heat fro moisture evaporation is provided by convective heat transfer between the hot air and the wet crop. The drying is basically achieved by the difference in moisture concentration between the drying air and the air in the vicinity of crop surface. A better control over drying is achieved in indirect type of solar drying systems and the product obtained is of good quality.

Passive and Active Solar Dryer In a passive solar dryer, air is heated and circulated naturally by buoyancy force or as a result of wind pressure or in combination of both. Normal and reverse absorber cabinet dryer and greenhouse dryer operates in passive mode. The active solar dryers employ solar energy and motorized fans/pumps for air circulation. All active solar dryers are, thus, by their application, forced convection dryer. In integral type active dryers, the solar collector forms an integral part of the roof/wall of the drying/storage chamber. A distributed type active solar dryer is one in which the solar collector and

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drying chamber are separate units (Fig. 7.5). Mixed – mode type dryers are rather uncommon designs and it combines some features of the integral and distributed type.

Figure 4.5 Working principle conventional indirect solar dryer

It is important to mention that there are two type of drying namely i. Thin layer drying and ii. Deep bed drying. On the basis of experience and literature survey, it is noticed that thin layer drying is most convenient drying by using solar energy due to its intermittent nature and low grade. Thus, it is decided to discuss only thin layer drying in the various sections.

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4.4. THERMAL ANALYSIS OF CABINET/GREENHOUSE DRYER For writing the energy balance for different component of cabinet dryer (Fig. 4.2), the following assumptions have been made: i. ii. iii. iv. v. vi. vii.

The heat capacity of glass, crop tray, drying chamber wall and air have been neglected, Volume shrinkage is negligible during drying process (as in the case of jaggery), Particle to particle conduction is negligible, The heat flow inside crop is negligible, There is no condensation of water vapour in drying chamber, and There is no stratification along the depth of the crop due to small depth of crop, The thin layer drying has been considered.

The energy balance equations are as follows.

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Crop Surface If αc and τ are absorptivity of the crop and transmittivity of cover materials and Mc , Ac and Cc are mass, surface area and specific heat of crop, then energy balance of crop is

 α c τ 

i=n

∑ i =1

 dT Ai I i ( t ) = M c C c c + h( Tc − Tch ) Ac dt 

(4.1)

Drying Chamber Most of the rate of heat transfer from crop to environment air is carried away through vents at vertical wall of cabinet dryer/ roof vent of greenhouse dryer and rest is lost through side area, As, by conduction, then energy balance of chamber air is

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G. N. Tiwari h( Tc − Tch ) Ac = V1 ( Tch − Ta ) + hs As ( Tch − Ta )

(4.2)

where, Ii(t) is the rate of solar energy falling on Ai section and V1=NV/3, N is the number of air change from chamber to atmospheric air through vents/opening provided at vertical walls/roof vents and its values depends on mode of ventilation i.e either natural or forced. For natural it varies between “1-10” and for forced mode it depends on rpm of fan and its diameter. V is the volume of the chamber and the h = hrw+hcw+hew, the total internal heat transfer coefficient from crop surface to the chamber air and it can be evaluated for known crop ( Tc ) and chamber ( Tch ) temperatures by using the formula given in section 3.5 with replacement of Pg by γPch. Equations (4.1) and (4.2) can be solved for crop ( Tc ) and chamber (Tch) temperatures as done in section 3.5 for the case of solar distillation for a given design and climatic parameters.

Steady State Analysis o Drying System

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If

Tc is the crop temperature inside solar dryer, than for steady-state

condition,

dTc = 0. dt Equations (4.1) and (4.2) can be combined for the net thermal energy available to the crop to raise its temperature as •  Q u = α c τ 



i =n

∑ A I (t ) − U i =1

i i

L

Ac (Tc − Ta )

where,

U L = U t + U bs ,

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(4.3)

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Ut =

hAc V1 + hAc + hs As

Ub =

hs As V1 + hAc + hs As

69

and

Equation (4.3) can be used for both cabinet and greenhouse dryer and it is same as Eq. (2.1) of flat plate collector. The only difference is in its bahaviour. In the case of flat plate collector, the value of UL should be minimum unlike cabinet and greenhouse dryer. In the case of cabinet and greenhouse dryer, the value of UL should be maximum for faster evaporation (mass transfer) for quick drying of the crop. The characteristic equation for cabinet and greenhouse dryer can be written as •

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ηi =

Qu I (t )

= α cτ − U L

(Tc − Ta )

I (t )

(4.4)

where, i =n

I (t ) =

∑ A I (t ) i =1

i i

Ac

is the weighted solar energy.

One can observed that the gain factor ( α cτ ) and loss factor (-UL) both should be maximum for faster mass transfer unlike flat plate collector.

Energy Pay Back Time (EPBT) (i) Cabinet dryer The different materials and embodied energy used for cabinet dryer with an effective area of 0.5m2 are as follows:

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G. N. Tiwari (a) Mass of glass cover = 4 kg, Embodied energy = mass × energy density = 4×8.72=34.88 kWh (b) Mass of wood material = 10 kg, Embodied energy = mass × energy density = 10×4.95=49.5 kWh (c) Mass of galvanized iron = 1 kg, Embodied energy = mass × energy density = 1×13.88=13.88 kWh

The total embodied energy used for 0.5 m2 cabinet dryer = (34.88 +49.5+13.88) kWh = 98.26 kWh An annual useful energy for cabinet dryer = η × α cτ × I ×Ac ×N×250×10-3 kWh For dryer thermal efficiency (η) = 0.50,

α cτ

= 0.40×0.8=0.32, Ac=0.5

m2, an annual average insolation ( I ) = 500 W/m2, N = 5 sunshine hour and n =250 clear day/year. An annual useful energy for cabinet dryer = 0.5 ×0.32×500×0.5 × 5×250×10-3 kWh = 50 kWh

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So, energy pay back time = EPBT =

Embodied energy An annual energy saving

or,

EPBT =

98.26 kWh = 1.96 years 50kWh per year

(ii) Greenhouse crop dryer The different materials and embodied energy used for greenhouse crop dryer with an effective area of 1.2×0.8 = 0.96 m2 are as follows: (a) Mass of PVC pipe of diameter 1// = 0.25 kg, Embodied energy = mass × energy density = 0.25×18.9=7.225 kWh (b) Mass of wood material = 0.5 kg, Embodied energy = mass × energy density = 0.5×25.64=12.82 kWh (c) Mass of galvanized iron (nails) = 0.100 kg, Embodied energy = mass × energy density = 0.100×13.88=1.388 kWh The total embodied energy used for 0.5 m2 cabinet dryer = (7.23 +12.82+1.388) kWh = 21.50 kWh

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An annual useful energy for greenhouse crop dryer = η × α cτ × I ×Ac ×N×250×10-3 kWh For dryer thermal efficiency (η) = 0.10 due to large surface area in comparison to cabinet dryer and loss due to solar fraction,

α cτ

=

0.40×0.8=0.32, Ac=0.96 m2, an annual average insolation ( I ) = 500 W/m2, N = 5 sunshine hour and n =250 clear day/year. An annual useful energy for cabinet dryer = 0.1 ×0.32×500×0.96 × 5×250×10-3 kWh = 19.2 kWh So, energy pay back time

EPBT =

21.50 kWh = 1.12 years 19.2kWh per year

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It can be observed that energy pay back time (EPBT) for greenhouse crop dryer is much less than EPBT for cabinet dryer. However, the life of cabinet dryer (10 years) is higher than greenhouse crop dryer (4-5 years).

4.5.THERMAL ANALYSIS FOR REVERSE ABSORBER CABINET DRYER (RACD) Energy Balance for Thin Layer Drying Referring to Fig.4.4, the energy balance equation for different components is as follows:

For an Absorber Plate If τ, ρ/ and αp are transmittivity of the glass cover having temperature Tc, reflectivity of the reflector and absorptivity of an absorber having temperature Tp , then energy balance of an absorber is

τρ / α p I / = h pf ( T p − T f ) + hrpc ( T p − Tc ) + U t ( T p − Ta )

(4.5)

where, hpf, hrpc and Ut are convective heat transfer coefficient from absorber to working fluid, radiative heat transfer coefficient from absorber to crop and

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an overall heat transfer coefficient from absorber to ambient through glass cover respectively.

For Working Fluid (Air) The rate of heat transfer from plate to the fluid will be equal to the rate of heat delivered to the crop having temperature Tc above it i.e.

h pf ( T p − T f ) = h fc ( T f − Tc )

(4.6)

Here, hfc is the convective heat transfer coefficient from fluid to the crop.

For Crop Surface The rate of energy received from the fluid and absorber (plate) will be utilized for raising the temperature of crop and releasing the moisture from inside crop in from of vapor to the drying chamber. The energy balance is

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[h fc ( T f − Tc ) + hrpc ( T p − Tc )]Ac = M cCc dTdtc + h( Tc − Tch ) Ac

(4.7)

where h =hrc + hcc + hec, sum of radiative, convective and evaporative heat transfer coefficient from crop to the chamber and it can be evaluated for known crop and chamber temperature by using the formula given in section 3.5 with replacement of Pg by γPch. h( Tc − Tch ) Ac = V1 ( Tch − Ta ) + hs As ( Tch − Ta ) where , V1 =

NV . 3

For Drying Chamber Energy balance of chamber is same as Eq. (4.2) (4.8) Equations (4.5) - (4.8) can be combined into one order differential equation which can be solved for a given design and climatic parameters as in section 3.5 for Eq. (3.4). The hourly variation of the crop temperature and moisture removal from the crop for reverse absorber cabinet dryer (RACD) with and without glass cover have been shown in Figs. 4.6. Figure 4.6(a) shows that there is significant increase in crop temperature (≅ 10°C) due to additional solar

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energy available from glass cover of the dryer. Similar effect has been observed for moisture removal in Fig.4.6 (b). It can be observed that there is three times increase in moisture removal during peak sunshine hour due to direct availability of solar energy through glass cover. However, there is no moisture removal during low sunshine hour and off-sunshine hour as per expectation. The comparison of overall thermal efficiency of RACD dryer with and without glass cover has been also shown in Fig. 4.7. This shows that RACD with glass cover gives better results.

Figure 4.6(a) Hourly variation of crop temperature with and without glass cover

Figure 4.6(b) Hourly variation moisture removal of crop with and without glass cover

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Figure 4.7. An overall thermal efficiency of reverse absorber cabinet dryer (RACD) with and without glass cover

Energy Pay Back Time (EPBT) The different materials and embodied energy used for reverse absorber cabinet dryer with an effective area of 1 m2 are as follows: (a) Mass of glass cover = 8 kg, Embodied energy = mass × energy density = 8×8.72=69.76 kWh (b) Mass of wooden structure = 25 kg, Embodied energy = mass × energy density = 25×4.95=123.75 kWh (c) Mass of galvanized iron with stand = 15 kg, Embodied energy = mass × energy density = 15×13.88=208.20 kWh The total embodied energy used for 0.5 m2 cabinet dryer = (69.76 +123.75+208.20) = 451.71 kWh

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An annual useful energy for cabinet dryer = η × α aτ × I ×Ac ×N×250×10-3 kWh For crop drying system thermal efficiency (η) = Thermal efficiency of reverse absorber air heater × drying chamber efficiency =0.5×0.50=0.25,

α aτ = 0.90×0.8=0.72, Ac=1.0 m2, an annual average insolation ( I ) = 500 W/m2, N = 5 sunshine hour and n =250 clear day/year. An annual useful energy for cabinet dryer = 0.25 ×0.72×500×1.0 × 5×250×10-3 kWh = 112.5 kWh So, energy pay back time

EPBT =

471.71 kWh = 4.19 years 112.5kWh per year

It should be noted that EPBT for reverse absorber cabinet dryer is increased due to increase in material for construction. It can be reduced by

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i. Using all structure particularly G.I. stand by wooden material and ii. Making structure by using fiber-reinforced plastic (FRP) material which increases the life of the system as well.

4.6. ENERGY BALANCE FOR ACTIVE (CONVENTIONAL) SOLAR DRYING (ASD) SYSTEM The energy balance for ASD system will be in the following form (Fig.7.4):

Solar Air Heater Solar air heater is simply two parallel metallic sheets, which is glazed from the exposed blackened top surface and insulated from bottom. The air is allowed to flow between two sheets under forced mode to be get heated. The hot air is then allowed to the drying chamber for extraction of moisture from the crop. In order to write energy balance equation for each component, particularly absorber plate, a view of air flow over elemental length ‘dx’ has been shown in Fig. 4.8.

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L

W

x

x+dx

Tf

Tf+dTf

Figure 4.8. A view of air flow over elemental length ‘dx’

Absorber Plate

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If τ, and αp are transmittivity of the glass cover and an absorptivity of absorber having temperature Tp and W is the width of absorber, then energy balance of an absorber is

τ α p I Wdx = U t ( T p − Ta )Wdx + h pf ( T p − T f )Wdx

(4.9)

where,

 1 1  Ut =  +   h pg hga 

−1

and hpf are an overall top loss coefficient and convective heat transfer coefficient from plate to the fluid having temperature Tf. The hpg and hga are convective heat transfer coefficient from plate to the glass temperature and radiative heat transfer coefficient from glass cover to ambient air temperature.

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Working Fluid Some of the rate of heat given by plate to the fluid will be carried out by flowing air and rest is lost to ambient through bottom insulation to ambient having temperature Ta. The energy balance of flowing air will be •

h pf ( T p − T f )Wdx = m C f

dT f dx

dx + U b ( T f − Ta )Wdx

(4.10a)

where, Ub is an overall heat transfer coefficient from fluid to ambient through bottom insulation. Equation (4.10) can be solved for Tf in terms of design and climatic parameters after eliminating Tp by using Eq. (4.9) and initial condition i.e. Tf = Tfi at x=0. The outlet air temperature (Tfo) will be obtained as Tfo= Tf at x =L. Then, the rate of thermal energy available at outlet of air collector and carried out by flowing air is given by •

•

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Q u = m C f (T fo − T fi )

(4.10b)

•

The Q u , which will be in terms of Tc will be utilized by the crop in drying chamber for moisture removal.

Drying Chamber The rate of heat carried out by flowing air will be fed into the drying chamber and its energy balance will be •

m C f ( T fo − T fi ) = M c C c

dTc + h( Tc − Tch ) Ach dt

(4.11)

and, •

h( Tc − Tch ) Ach = m C f ( Tch − Ta ) + hs As ( Tch − Ta )

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(4.12)

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Now, Eq. (4.11) will be solved for Tf in terms of design and climatic parameters after eliminating Tch by using Eq. (4.12) and initial condition i.e. Tc = Tci at t=0 (as done in section 3.5 with same initial conditions). After knowing the crop temperature (Tc), the chamber (Tch), the plate temperature (Tp) and the air outlet (Tfo) can be obtained from Equs. (4.12), (4.9) and (4.10b) respectively for a given design and climatic parameters. The overall thermal efficiency of conventional and reverse absorber cabinet dryer (RACD) has been shown in Fig.4.9, Goyal and Tiwari (1997). From this figure, it is clear that there is about 10% improvement in the performance of a reverse absorber cabinet dryer (RACD) over conventional dryer. The RACD has following advantages:

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i. The overall heat loss in RACD is significantly reduced and ii. The absorptivity of absorber is unaffected due to placement of crop in RACD.

Figure 4.9 Variation of overall thermal efficiency with mass of the crop for conventional and RACD dryer

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Figure 4.9. An overall thermal efficiency of active and reverse absorber cabinet dryer.

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Coefficient of Performance The coefficient of performance (C.O.P.) of a grain dryer is expressed as follows: C .O .P . =

Tc − Ta T f − Ta

(4.13)

The variation of Heat utilization factor (H.U.F.) and coefficient of performance with time of the day has been shown in Fig. 4.10(a). It is clear that the H.U.F. increases with time of the day. It may be due to fact that the temperature decrease (Tf – Tc ) is faster in the beginning due to more moisture content in the crop and the temperature increase (Tf – Ta ) is slow at later stage due to decrease on insolation level as expected. Similarly, the coefficient of performance (C.O.P.) decreases with time of the day similar nature of the crop (Tc – Ta ) and the fluid (Tf – Ta ) temperature difference with respect to ambient air temperature.

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Figure 4.10(A) Hourly Variation Of Heat Utilization Factor (H.U.F.) And Coefficient Of Performance.

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Thermal Efficiency Instantaneous thermal drying efficiency ( ηi) of the system can be defined as follows: •

ηi =

M ev × λ 3600 × I / ( t )

× 100

(4.14)

Overall daily thermal efficiency of the system ( ηo) is defined by the ratio of heat energy utilized in vapourizing the moisture to that of heat collected bt the reverse absorber collector can be determined using the following expression,

λ ηo =

t = 24

•

∑M t =1

ev

t = 24

( 3600 )

∑I

(t )

(4.15) /

(t )

t =1

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The overall thermal efficiency depends on the amount of moisture removal from the crop for a given insolation. This also determines the drying rate. The variation of drying rate with moisture content has been shown in Fig. 4.10(b). This indicates that drying rate is more for more moisture content as expected.

Figure 4.10.(b). Variation of drying rate with moisture content

Energy Pay Back Time (EPBT) The different materials and embodied energy used for active solar drying system with an effective area of 0.5 m2 are as follows: (a) Mass of glass cover (1m2 ) = 8 kg, Embodied energy = mass × energy density = 8×8.72=69.76 kWh (b) Mass of wooden structure in air collector (1m2) and drying chamber = 35 kg, Embodied energy = mass × energy density = 35×4.95=173.25 kWh (c) Mass of galvanized iron stand for collector and drying chamber = 35 kg, Embodied energy = mass × energy density = 35×13.88=485.80 kWh (d) Black paint and other paint = 1 kg

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G. N. Tiwari Embodied energy = mass × energy density = 1×40=40 kWh

The total embodied energy used for 0.5 m2 cabinet dryer = (69.76 +173.25+485.80+40) = 768.81 kWh An annual useful energy for cabinet dryer = η × α aτ × I ×Ac ×N×250×10-3 kWh For crop drying system thermal efficiency (η) = Thermal efficiency of air collector × drying chamber efficiency =0.7×0.50=0.35,

α aτ = 0.90×0.8=0.72, Ac=1.0 m2, an annual average insolation ( I ) = 500 W/m2, N = 5 sunshine hour and n =250 clear day/year. An annual useful energy for cabinet dryer = 0.35 ×0.72×500×1.0 × 5×250×10-3 kWh = 157.5 kWh So, energy pay back time

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EPBT =

768.81 kWh = 4.88 years 157.5kWh per year

The above calculations have not included the effect of electrical energy used to operate the air blower. An energy pay back time (EPBT) for active solar cabinet dryer can be increased due to decrease in material for construction. It can be reduced by i. Using all stand structure by wooden material and ii. Making structure by using fiber-reinforced plastic (FRP) material. It can be observed that an energy pay back time (EPBT) increases as the design and structure of drying system becomes more complicated in comparison to cabinet dryer. Hence it is important to note that if there is need to change the design of dryer for efficient one than the system should have the following criteria: a. b. c. d.

Materials with less energy density should be used for construction of dryer, Materials should have higher life, Maintenance should be minimum and There should be maximum used of dryer per year.

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Chapter 5

SOLAR GREENHOUSE CROP PRODUCTION

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5.1. INTRODUCTION Greenhouse for crop production is a highly sophisticated structure, which aims at providing ideal conditions for satisfactory plant growth and production throughout the year. For satisfactory plant growth, the growth factors namely light intensity, temperature, humidity and air composition are maintained optimum inside the greenhouse for higher productivity. The maintained chamber inside greenhouse is also referred as controlled environment greenhouse. In this section, a summary and review of work on greenhouse covering its various applications, constituents of greenhouse air, classification, thermal heating and cooling will be discussed in brief, Tiwari (2003).

5.2. APPLICATIONS OF GREENHOUSE (CONTROLLED ENVIRONMENT GREENHOUSE) 5.2.1. Crop Cultivation Physical phenomenon of utilization of solar energy for crop production is that solar energy enters into the process of photosynthesis. Most of the living organisms ultimately depend upon light energy. Visible light constitutes a source of energy for plants. Light energy, carbon dioxide and

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water all enter into the process of photosynthesis through which carbohydrate is formed. The process results in growth of the plant, which can be visualized as an increase in dry matter. When all the factors such as carbon dioxide, temperature and water are optimized for photosynthesis, an optimum light intensity can be determined. Variation in any one of the parameters adversely affects the growth of the plant. When plant is grown in open field, none of the above parameters are under control (Fig.5.1a). Plants are subjected to variations in temperature and light intensity. Uncontrolled wind velocity and untimely rain impede the plant growth. The role of greenhouse effect due to transparent atmosphere for survival of all living organism on the earth can be made similar to a transparent house around the crop as mentioned earlier where a micro greenhouse effect can be created. The different heating and cooling arrangements can be integrated with greenhouse to achieve desired climate to cultivate the specific crop in any season of the year (Fig.5.1b).

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5.2.2. Soil Solarization Soil solarization is an effective method to control the damage caused by soil borne pests and plant pathogens. Solarization mainly depends on solar energy to heat the soil to a temperature, which are lethal to these organisms. It is a natural, hydrothermal process of disinfecting soil of plant pests that is accomplished through passive solar heating, Stapleton(2000). Solarization can be done by covering moist soil with a transparent plastic film for 2 to 8 week period during availability of solar radiation. Most soil borne pests and plant pathogens are mesophilic and are killed at temperatures between 40 to 60 0C. At these higher temperature, disfunction of membranes and increased respiration are responsible for the death of all soil borne pathogens. The temperature inside the greenhouse rises to nearly 55-60 0C during summer period resulting in the killing of all soil borne pests and pathogens for safe cultivation. Another application for which solarization has come into common use in the greenhouse is to disinfect the seedbeds, containerized media and cold frames, Stapleton(1995).

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Solar Energy Technology Advances

Figure 5.1a and 5.1b

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5.2.3. Poultry Like plants, controlled environment is also an important requirement for healthy growth of chicks in broiler and brooder houses. The growing chicks require changing environmental conditions, as they grow older. Supplemental heat, usually from brooders is used until heat produced by the birds is adequate to maintain desired conditions. At the early stages of growth, moisture dissipation per bird is low. Consequently low ventilation rates are recommended to prevent excessive dryness. Current recommended practices for broiler houses are as follows, Arbel et al.(1999): i. Room temperature: 15.6 to 18.3 0C, ii. Relative humidity: 50 to 80% and iii. Ventilation rate: 0.52 to 1.04 liter/s per kg of live weight in winter and 2.08 liter/s per kg of live weight in summer.

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The above environmental condition can be maintained only in a protected enclosure like greenhouse where there is provision of suitable heating and cooling arrangements.

5.2.4. Aquaculture Greenhouse pond systems provide a good alternative for maintaining water temperature in aqua cultural facilities. A shallow solar pond covered with plastic film maintains required temperature due to greenhouse effect causing the satisfactory rearing of fish and fish seeds. The increase of temperature in greenhouse pond system helps in maintaining the growth rate and survival of fingerlings in winter month. Also excessively increased temperature during summer period inside the greenhouse pond results in killing of all water borne pathogens to create safe medium for fish culture in next season. On an average there occurs the rise of water temperature by 5.2 0 C as compared with outside air temperature in a 1-m pond covered with polyethylene film, Zhu et al.(1998).

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5.3. CONSTITUENTS OF CONTROLLED ENVIRONMENT The main constituents of controlled environment in the greenhouse are temperature, light, humidity, carbon dioxide and root-medium, which are briefly described as follows:

5.3.1. Temperature Optimum temperature refers to the best temperature at which plant can grow under particular type of climatic conditions. Optimum temperatures for different crops are different, Hanan et al.(1978). Each crop has an optimum temperature at which enzymes, which are heat sensitive and responsible for bio-chemical reactions, are most active. Net growth of crop occurs when photosynthesis exceeds respiration. Hence in order to achieve high levels of photosynthesis rate, plant temperatures are kept low at night to decrease respiration rate and warmer by day to increase photosynthesis, Nelson (1985) and Kurata (1989).

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5.3.2. Light Intensity The intensity of incoming solar radiation is an important parameter for influencing the photosynthetic activity of plants. The light intensity varies from place to place but it generally varies from zero at the beginning of the day to about 100,000 to 150 000 lux (lumen/m2) around noontime. Light intensity on cloudy days is quite low which leads to poor photosynthetic process. Light intensity below 3200 lux and above 129 000 lux are not ideal for plant, Nelson (1985). But the optimum light intensity for a plant is 32000 lux. Hence solar radiation transmittance needs utmost attention while designing and constructing the greenhouse. It is also influenced by the orientation of the greenhouse and the sun elevation. The transmittance of various greenhouse types with east-west and north-south orientation in December and June is shown in Fig. 5.2, Nisen(1969). From the figure it is clear that light transmittance is higher for east-west orientation in winter and lower in summer than north-south orientation. Greenhouses with curved roofs have better transmittance than greenhouses with a pitched roof of 250 slope.

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During peak summer, some protection from the high intensity of light may be required because the high intensity raises the temperature of leaf and causes sunburning. Therefore, some type of shading screen, either over the greenhouse or inside greenhouse is provided. Spraying the greenhouse cover with a suitable shading compound such as lime water, white latex, paint with water is recommended during summer, Nelson(1985).

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Figure 5.2.

5.3.3. Humidity Like light and temperature, humidity is also an important parameter in the greenhouse climate. Absolute humidity is the amount of water vapor actually present in the air. Relative humidity inside the greenhouse should be between 60-70% for better growth of plants, ASHRAE (1997). If the plants have a well-developed root system, then relative humidity above 40% is preferred to avoid water stress conditions. Even very low relative humidity (less than 20%) can cause wilting due to higher rate of evaporation from plant. High levels of humidity can lead to yield loss for tomato crop, Bakker (1990) and Holder and Cockskull(1990). Higher humidity (above 80%) also leads to occurrence of fungal diseases within the greenhouse. Jolliet (1994) had optimized the humidity and transpiration in the greenhouse and he reported that 70-75% relative humidity was congenial for the plant for its desirable growth in the greenhouse.

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5.3.4. Carbon Dioxide Carbon dioxide is an important parameter for plant growth like water, light, soil nutrients and temperature. In photosynthesis process, a plant leaf seeks to combine molecules of CO2 and water in the presence of sunlight to form carbohydrates and oxygen. Several researchers have showed that closed greenhouse system offers good opportunity to improve production through the elevation of CO2 levels. Carbon dioxide, which comprises about 0.03 percent (300 ppm) of ambient air, is essential for plant growth. This level of carbon dioxide in atmospheric air is sufficient to meet the photosynthetic requirement of open field crops. In the closed field conditions, i.e., in greenhouse, the level of carbon dioxide rises up to nearly 1000 ppm, because respired carbon dioxide remains trapped overnight. As the sunlight becomes available, photosynthesis process begins and carbon dioxide from greenhouse air gets depleted. Owing to this, the carbon dioxide level in greenhouse even goes below 300 ppm before noon. If greenhouse air does not receive additional carbon dioxide from any other source, the plant would become carbon dioxide deficient resulting in poor growth. Carbon dioxide enrichment is therefore essential when greenhouse is sealed against infiltration particularly during winter period. Critten (1991) reported that crop yield was increased by 20-30% when carbon dioxide level was maintained from 1000-1500 ppm inside the greenhouse. The most common method of carbon dioxide supplementation is through burning of carbon fuels. Care should be taken to assure complete combustion by providing outdoor air infiltration to supply adequate oxygen levels for combustion.

5.3.5. Root Medium In addition to the above, root medium also plays an important role for cultivation of crops in pots as well in field. This is also known as growing medium for greenhouse crop cultivation. It must serve as reservoir for plant nutrients. Also it must hold water in a way that is available to plant and at the same time it provides the path for the exchange of gases between roots and the atmosphere above the root medium. Finally the root medium must provide an anchorage or support for the plant. The desirable properties of a root medium are as follows: i. Stability of organic matter ii. Maintenance of carbon : nitrogen ratio

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G. N. Tiwari iii. iv. v. vi.

Keeping desirable bulk density Capacity for moisture retention and aeration Balance of pH level Higher level of cation exchange capacity.

The growing media of soil with manure and sand with manure are used for raising crop both in pot and in field in the greenhouse. Approximately 30% of organic matter is mixed with the soil and sand for preparing good root medium.

5.3.6. Greenhouse Climate Requirement The climatic requirements for plant growth can therefore be summarized as follows, Sirjacobs(1989) and Verlodt(1990):

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i.

ii.

iii. iv. v.

vi.

Plants grown under protected cultivation are mainly adapted to average temperatures ranging from 17 to 27 0C. Taking into account the warming-up effect of solar radiation in the greenhouse, the above temperature range can be possible without any heating arrangement in it when outside ambient temperature prevails in the range from 12 to 22 0C. If the mean daily outside temperature is below 12 0C, greenhouse is to be heated, particularly at night. When mean daily temperature is above 22 0C especially during summer, artificial cooling is necessary or cultivation in greenhouse is to be stopped. Natural ventilation is sufficient when ambient mean temperatures range from 12 to 22 0C. The absolute maximum temperature for plants should not be higher than 35-40 0C The minimum threshold for soil temperature is 15 0C. Verlodt(1990) suggests a threshold of the average night temperature as 15-18.5 0C for heat requiring plants such as tomato, pepper, cucumber, melon and beans. The safe ranges of relative humidity are from 70-80%.

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5.4. REVIEW: STATUS OF GREENHOUSE Presently there are more than 50 countries in the world where cultivation of crops are undertaken on a commercial scale under cover. However, in recent years, due to demand of off-season vegetables and export of cut flowers, the research activities in greenhouse technology all over the world have also been strengthened for many fold increase of crop yield. China has adopted plastic greenhouses in about 48000 ha. United States of America has a total area of about 4000 ha under greenhouse mostly used for floriculture, Yadav and Choudhari(1997). In central and northern Europe, most greenhouses are covered with glass, whereas most of the greenhouses in southern Europe are covered with plastic film, Elsner et al.(2000). High concentrations of greenhouses in Europe are observed in specific regions with favorable climatic conditions. The area under greenhouse in Spain is around 25,000 ha and in Italy about 20,000 ha. It is mostly used for growing vegetable crops like watermelon, capsicum, straw berries, beans, cucumbers and tomatoes. In Spain, simple tunnel type greenhouses are generally used without any elaborated environmental control equipment mostly using UV stabilized polyethylene film as cladding material. In Canada, greenhouse is used for production of flowers and off-season vegetables. The main vegetables grown in Canadian greenhouses are tomatoes, cucumbers and capsicum. Netherlands is the traditional exporter of greenhouse grown flowers and vegetables all over the world. The Dutch greenhouse industry is probably the most advanced in the world, covering about 89,600 ha. Dutch greenhouses are made mostly of glass frame. Israel is the largest exporter of cut flowers and has wide range of crops under greenhouses. Cover cultivation of cut flowers and vegetables are also grown in Turkey. In Saudi Arabia, cucumbers and tomatoes are the most important crops grown in greenhouses contributing more than 94% of the total production of crops cultivated under greenhouses. The most common cooling method adopted in these areas is evaporative cooling. In Egypt, greenhouses are in about 1000 ha area consisting mainly of plastic covered tunnel type structures. Arrangements for natural ventilation are made for regulation of temperature and humidity. In Asia, China and Japan are the largest users of greenhouses. The development of greenhouse technology in China is faster than any other country in the world. The majority of the greenhouses in China are made of local materials for the frame and flexible plastic films for glazing.

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In Japan, an area of about 40,000 ha is under greenhouse cultivation and is used for cultivation of fruit orchards, vegetables and flowers. In South Korea, more than 21,000 ha area is under greenhouses for production of flowers and fruits, Tiwari and Goyal(1998). The recent data concerning the total greenhouse area in different countries have also been shown in Table 5.1, Jenson and Malter(1995).

Table. 5.1. Greenhouse area in different countries (Jensen and Malter, 1995)

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Country Algeria Australia Belgium Bulgaria Canada Chile China Columbia Egypt England

Area in (ha) 5000 600 2400 1350 400 1600 48,000 2,600 1000 3500

Country France Greece Hungary India Israel Italy Japan Jordan Morocco Netharland

Area in (ha) 5800 4240 5500 500 2200 18500 42000 450 3000 9600

Country Poland Portugal Romania Spain South Korea Turkey United Arab USA

Area in (ha) 1500 2500 3500 25000 21000 9800 55 4250

5.5. CLASSIFICATION OF GREENHOUSES Greenhouse can be classified on the basis of i. Working principle, ii. Shape and iii. Utility and cost.

5.5.1. Based on Working Principles Based on working principles, greenhouses are classified into passive and active greenhouse. These are explained as follows:

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Passive Greenhouse Passive greenhouse mainly depend on architectural design that can be used to maximize solar gain in the winter (and minimize them in the summer) to reduce heating (and cooling) loads. The heating and cooling in passive greenhouse can be integrated as follows:

(a) Heating

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The passive heating may be due to water storage, rock bed storage, north wall, mulching, phase changing materials and movable insulation etc.

(i)Water storage The heat storage system can be placed inside the greenhouse, in plastic bags, filled with water. Water containers placed inside the greenhouse can also be used as solar collector and heat storage. The system absorbs and traps the incident solar radiation during the day. During the night, the stored heat is returned to the interior by natural convection or radiation. Grafiadellis(1987) and Gupta and Tiwari(2002) used the water as storage medium and found that the temperature of greenhouse was 5oC more than the outside condition. The study was conducted on the hot wastewater based heating of greenhouse for cultivation of ornamental plants. Mohamud(1995) carried out a study towards the potential of a solar pond and used the same as a primary heating system. AMS -Amri AI(1997) studied the effect of solar water heater. It was fixed on the interior of gable even span greenhouse. He reported that the productivity of tomato was enhanced by 46.67% in the greenhouse due to heating of the greenhouse by solar water heater.

Figure. 5.3. Passive solar greenhouse with water storage in (a) plastic bags and (b) water containers

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(ii) Rock-bed storage Rock-bed is a popular and economical heat storage material. It is prepared with gravels of 20-100 mm diameter. The storage area is placed under the greenhouse at a depth varying between 40-50 cm. The gravel can be enclosed in an insulated concrete storage enclosure. During the day, excess heat is transferred from inside the greenhouse to the underground store. A ventilator can be used to transport greenhouse air (using a fan at a rate of 5 m3/min m2) to the heat storage area. During daytime, excess heat inside the greenhouse is transferred to the under ground storage i.e., rock bed and that excess heat is utilized during nighttime for reducing temperature fluctuation in the greenhouse enclosure. At night, the process is reversed. The cool air is moved through the store, where heat is tansferred from the gravel to the colder air and then return to the greenhouse. (iii) Northwall If the north side of the greenhouse is composed of thick thermal mass, made of brick or cement blocks filled with concrete, then the transmitted solar radiation through the glazed north wall can be checked and be retained in the greenhouse for use in photosynthetic use and for increasing temperature. The concept of opaque north wall is commonly employed for east-west oriented greenhouse in northern hemisphere. Because in east-west oriented greenhouse, maximum solar radiation falls in south wall during winter period and leaves greenhouse through glazed north wall. This happens due to the movement of sun from east to west direction with low altitude angle. It is therefore necessary to make this wall insulated externally and painted black internally for thermal storage. During day time the incident solar radiation impinges on the wall and significantly raises its thermal storage. That stored energy is released during night for thermal heating in the greenhouse. Also the presence of wall substantially reduces the heat losses from the greenhouse. Santamouris et al.(1994a) tested the performance of north wall and reported that temperature inside the greenhouse were 10oC higher than ambient temperature. Singh and Tiwari(1998) reported that there is a significant effect of the thermal storage from the north wall on the plant and room air temperatures. Thermal load leveling (reducing fluctuations between maximum and minimum temperatures) decreases with increasing its isothermal mass in the case of heating and vice versa for cooling.

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(iv) Mulching Cebula (1995) studied the effect of mulching with transparent or black plastic film on soil temperature and its effect on the greenhouse production of sweet pepper. The transparent film ensured higher soil temperature during the day, while the loss of heat at night was prevented to a greater degree by the black mulch for maintaining optimum temperature.

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(v) Phase change materials Phase change materials or latent heat of materials are alternative heat storage medium. These materials have considerably higher thermal energy storage densities as compared to sensible heat storage materials and are capable of absorbing and releasing large quantity of energy (latent heat) at constant temperature, while undergoing phase changes. These are effectively used for storage and load leveling purposes in the greenhouse. The latent heat storage materials are usually placed underground, on a well-insulated area or at the north wall. In daytime, it absorbs the heat and in nighttime, it releases the heat to the greenhouse for thermal heating. Santamouris et al.(1994b) and Ismail and Goncalves (1999) used phase change material for heating a greenhouse. (vi) Movable insulation Movable insulations are usually night curtains or thermal screens, which are drawn inside or outside the greenhouse cover during nighttime in winter months to reduce heat losses to ambient resulting in the conservation of energy in the greenhouse. These movable insulations are uncovered during daytime in order to enter solar radiation into the greenhouse for thermal heating. Chandra and Albright (1980) have analytically determined the effects of night curtain on the heating requirement of greenhouse and predicted that nearly 70 percent of heating load could be saved by use of night curtain. Garzoli and Blackwell (1981) studied the effect of movable insulation that could check the exchange of long infrared radiation, emitted by the roofing material with sky during cold night. Barral et al. (1999) tested the performance of integrated thermal improvements of thermal curtains as well as thermal blankets and reported that these movable insulations were proved to be very efficient to provide the required temperature levels for the healthy growth of tomatoes and peppers during winter period. Plaza et al.(1999) also reported that the energy in the greenhouse could be saved up to about 20% by the use of thermal insulation.

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(b) Cooling During summer, high intensity of solar radiation increases the air temperature inside the greenhouse to an undesirable level due to greenhouse effect. Due to this reason, the inside environment becomes unfavorable for crop-production particularly in summer season. Therefore, thermal cooling of greenhouses is required especially in peak temperature hours during the summer. The passive and low energy cooling systems applied for building, Givoni (1991) can also be used in greenhouse system, as these are simple and inexpensive methods for cooling. However natural ventilation and reduction of solar intensity through shading can prevent the increase of temperature in the greenhouse to some extent. These are described as follows:

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(i) Natural Ventilation If the greenhouse is equipped with ventilation openings, both near the ground level and at the roof, then this type of ventilation replaces the internal hot air by external cooler one during hot sunny days with weak wind. The external cool air enters the greenhouse through the lower side openings while the hot internal air exits through the roof openings due to density differences between air masses of different temperature causing the lowering of temperature in the greenhouse. (ii) Shading The entry of direct solar radiation through the covers into the room is primary source of maximum heat gain inside the greenhouse. The entry of unwanted radiation can be controlled by the use of shading. Stretching shade cloth over the roof of greenhouse is the effective way of cutting excessive incoming radiation to the greenhouse as maximum solar radiation falls in the roof of any structure during summer period. The application of shading compounds to the cover can also be used to reduce the infrared portion of the solar spectrum responsible for enhancing the thermal energy in the greenhouse, ASHRAE (1978). The shading compounds most commonly used are in forms of lime. Caustic compounds for removing shading are used to clean the cover when light transmittance becomes essential for the plant in the greenhouse. Bailey (1981) studied the shading effect and reported that an aluminum plated mesh reduced the inside temperature by 6oC in comparison to a greenhouse without shading at an ambient temperature of 33oC. Feuermann et al.(1998) developed a computer simulation model and studied the relationship between the parameters of the liquid radiation filter greenhouse

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and its thermal performance under different climatic conditions. Shamim and McDonald (1995) carried out an experimental investigation to determine the feasibility of using liquid foam as an insulating medium between the walls of a greenhouse in a hot-arid climate, where it must act both as an insulator and as a translucent medium to attenuate thermal radiation. The results showed that the foam is effective in attenuating the thermal radiations. A 25mm layer was found to transmit only 50% of incident solar radiation to greenhouse resulting in lowering the temperature in the greenhouse enclosure as compared to the greenhouse without foam.

(c) Active greenhouse

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In active mode greenhouse, an external thermal energy available either from conventional fuel or solar energy through collector panel is fed inside the greenhouse. These greenhouses use fan and pumps with the help of electrical energy to move the working fluid in the system. It has been observed that energy pay back time (EPBT) for solar thermal system increases with use of electrical appliances to achieve controlled environment. Hence, a discussion on active solar still will be left to the reader to consult book on Greenhouse.

5.5.2. Classification on the Basis of Shape The greenhouses are available in different shapes and sizes suitable for different climatic zones prevailing in world. Each zone requires different shapes of greenhouse for providing favorable climatic condition for the growth of plants. As per the review carried out by Tiwari and Goyal (1998), it is revealed that there is need to finalize the shape of greenhouse for a given climatic condition. On the basis of shape, greenhouses are classified as single span groundto-ground type. If the single span greenhouses are joined together, then the facility is known as multi-span greenhouse. A greenhouse is said to be of lean- to- type (solarium) if its north side forms the wall of residence in northern hemisphere. On the basis of shapes greenhouses may be named as follows and these are shown in Fig.5.4: (i) Spherical dome (ii) Hyperbolic paraboloid (iii) Quonset (iv) Modified quonset

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(v) Gothic arch (vi) Mansard roof (vii) Gabic even span (viii) Gabic uneven span

Figure 5.4. Various shapes of greenhouse

The shape Quonset type greenhouse is most economical and required minimum maintenance and energy pay back time is least.

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5.5.3. Classification on Basis of Cost of Construction or Extent of Environment Control Greenhouses are available in different forms for different climate conditions. India is divided into six climatic zones. The economic feasibility of any type of greenhouse depends upon its cost (as per climatic requirement) and type of crop to be grown. In this section, classification of greenhouse is done on the basis of cost per m2 of constructed area. The greenhouses are broadly classified into following three categories, based on the extent of environmental control, Tiwari and Goyal (1998) and Sirohi and Behera(2000).

(A) Low Cost or Low Tech Greenhouse

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The low-tech greenhouse is a simple chamber made of polythene sheet of 150-200 micron. It is constructed with locally available materials such as bamboo and timber etc. Unlike conventional or high tech greenhouses, no specific control devices for regulating environmental parameters inside the greenhouse are provided. This type of greenhouse is mainly suitable for cold climatic zone. The temperature within greenhouse increases by 6-10 oC than the outside. It could be adopted for vegetable cultivation in winter season.

(B) Medium-tech Greenhouse This type of greenhouse is constructed by using galvanized iron (G.I) pipes for permanent framed structure. The greenhouse has single layer covering with UV-stabilized polythene sheet of 200-micron thickness and greenhouse cover is attached to the framed structure with the help of screws. Whole structure is firmly grouted into the ground to withstand the disturbances against wind. Exhaust fans along with thermostat are provided to control the temperature. Evaporative cooling pads and misting arrangement are also made to maintain a favorable temperature and humidity inside the greenhouse. The greenhouse frame and glazing material have a life span of about 20 years and 2 years respectively. These types of greenhouses are suitable for dry as well as warm climatic zones and can also be adopted for vegetable production.

(C) Hi-tech Greenhouse As more environment factors in the greenhouse are controlled one time then it becomes difficult to co-ordinate individual control (thermostat humidistat) to avoid overlap in system control. A simple example pertaining

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such type of situation may be like this: greenhouse thermostat indicating for heating whereas exhaust fans are still running. To avoid such kind of overlap few manufactures have started to develop controller which can perform specific segment of the control flow. A control system, basically, consists of a sensor, a comparator and an operator, a signal receiver, a comparator and or operator (to respond to the change to bring about an increase or decrease of supply). The entire control system attempt to satisfy the condition represented by the sensor, therefore, the sensor location is critical. Sensor placed in a aspirated box will result in better control then it exposed directly to the greenhouse environment. It sense and measure the variable, compare the measurement to a standard value. If needed, activate components to bring the measured variable into agreements with the standard value. Hi-tech greenhouse is a very energy intensive and hence it should be used for high value crop just as floricultures.

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5.5.4. On the Basis Greenhouse Cover Material Before the Second World War, mostly, greenhouse used to be constructed using glass as the glazing material and glasshouse and greenhouse becomes synonymous. With the recent development of different kind of material i.e. plastic film, rigid sheets, poly vinyl chloride (PVC), etc., the construction of greenhouses has been considered using glass or plastic sheet. Polyethylene, PVC, EVA, acrylic, polycarbonate fibre glass, polyester, PVF are some of the plastic material and has been used for greenhouse construction. Classification is subject to the material of construction of frame, i.e. wood bamboo, steel aluminium, reinforced concrete.

5.5.5. On the Basis of Utility Use of greenhouse is another criterion for classification. It could be retail or a wholesale greenhouse. Greenhouses for academic research interest, public Park or gardens are termed as institutions greenhouses. An over wintering greenhouse is generally an unheated facility as long as the air temperature in the greenhouse remain close to freezing. Some heating may be necessary the temperature goes much below zero.

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5.6. CONCLUSIONS The design of environmental control systems for plants is complicated owing to the interaction of many environmental variables affecting growth and production. The clear necessity of matching food production to the needs of the world’s population requires the accurate knowledge of the factors limiting primary production. Therefore the environmental factors that limit agricultural production and physiological responses of plants to environmental stresses need to be identified and understood. The fluctuating and interacting natural environment makes it nearly impossible to analyze the effects of various climatic factors on plant behaviour. The most appropriate solution for the above problem is to move research into a plant growth structure (greenhouse) with partial or complete environmental control. Attempts are mostly focused towards control of light, temperature, humidity and air composition in growth cabinet for healthy growth of plant. Hence priority needs to be emphasized regarding the intensive research in greenhouses particularly for optimum environmental control towards increased food production. On the basis of reviews made in this paper, the following conclusions may be highlighted:

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i. ii. iii. iv.

v. vi.

vii.

Optimum environmental factors for plant growth are different for different crops Design of greenhouse mainly depends on type of plant, season of crop cultivation, degree of automation and climate Principle of greenhouse effect can be better applied in fabricating solar dryer for efficient drying of crop There is a significant effect of opaque north wall on plant and greenhouse room air temperature for east-west oriented greenhouse in northern hemisphere Light transmittance is higher for east-west oriented greenhouse in winter and lower in summer than north-south orientation Ground radiation in the greenhouse can furthermore be enhanced by the presence of reflecting north wall for efficient photosynthesis to occur Solar fraction for opaque north wall needs to be incorporated for design and thermal analysis of greenhouse.

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Chapter 6

EMBODIED ENERGY ANALYSIS OF PHOTOVOLTAIC (PV) SYSTEM

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6.1. INTRODUCTION At the global level, generation of electrical energy is mostly dependent on fossil fuels. However, fossil fuels are diminishing due to extensive and continuous use for power production. This is prompted due to increase in population along with industrial development. Moreover, the cause of air pollution and environmental degradation is CO2 emissions, which takes place during burning of fossil fuels. Hence, there is a strong need to conserve fossil fuels and to explore the possibility of other alternatives. In this perspective, awareness about the utilization of renewable energy sources (such as solar energy) has gained momentum universally. In India, most part is blessed with a high level of solar radiation. The photovoltaic (PV) system converts solar radiation in to direct current (dc) electricity (in addition to thermal energy already discussed in previous sections), which can be converted in to alternative current (ac) electricity by using inverter. The electrical efficiency of PV module is reported to be around 10%, which is further reduced due to involvement of storage battery, converter, distribution through wires and efficiency of electrical appliances etc. Energy conversion through photovoltaic (PV) system is one of the most important, reliable and environmental friendly technology, which has the potential to contribute significantly to a sustainable energy system. It also

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plays an important role to mitigate CO2 emissions. In view of above, photovoltaic (PV) technology has to meet two main criteria namely: i. Cost effectiveness and ii. The maximum net annual energy yield. Here, the net maximum annual energy yield means that the sum of annual electrical energy output of PV system and annual thermal energy, if any. Alsema et. al. (2000) have evaluated the total energy requirement for manufacturing of PV system, energy pay-back time (EPBT) and also CO2 emissions. However, they have not considered the following parameters:

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i. Support structure, ii. Interval of battery replacement and iii. Efficiency of balance-of-system. They have considered the system efficiency to 14% uniformly throughout the lifetime of PV system. Krauter et al., (2004) have evaluated only the energy requirement for manufacturing the PV system and CO2 emissions without considering the above mentioned parameters. Frankl et al., (1998) have considered support structure for open field mounted and on roof top to evaluate the energy requirement for manufacturing PV system. They have considered the same life-span for battery and PV system. In this section, an attempt has been made to calculate energy pay-back time (EPBT), energy yield factor (EYE) and energy yield (EY), by considering the following parameters: i.

Evaluation of energy requirement for manufacturing a single crystalline silicon PV system for open field and roof top conditions, ii. Evaluation of energy requirement for balance-of-system (BOS), iii. Use of thermal energy from PV system and iv. Mitigation of CO2 emissions at macro and micro-level of the PV system.

6.2. ENERGY ANALYSIS In order to have complete view of energy analysis of PV system, it is mandatory to know the total electrical energy invested in preparation of each

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material thorough various processes and energy obtained in its life time. Broadly, energy can be classified as Embodied energy: It is the amount of energy in terms of kWh required to produce the material in its products form. It is also known as energy input (Ein). It can be classified into two groups: (a) Macro-level and (b) Micro-level. a. Macro-level: If lifetime of battery (consumables) and PV system are same. b. Micro-level: If lifetime of battery (consumables) and PV system are not same. ii. Energy output: It is the amount of energy obtained per year in kWh from the system (Eout).

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i.

In present energy analysis, a comprehensive study has been made for energy input and output involved in products. The overall energy performance of such products is determined by accounting all energy flows in the life cycle from resource extraction through manufacturing and product use. In the case of PV system, the gross energy requirement (Ein) is determined by adding together the energy input during resource winning, production, installation, operation and maintenance of the PV system and the other system components. This gross energy requirement is then compared with the energy output (Eout).

6.3. EMBODIED ENERGY FOR 1.2 KWP PV SYSTEM (a) Embodied energy 1m2 PV module A 1200 Wp (1.2 kWp.) PV system of SIEMENS make has been considerd for energy analysis. Each module with an effective area 0.6240 m2 produces 75 Wp power. There are 16 modules connected in series and parallel combination and total effective area of PV system is 10 m2. The specifications and design data of PV system are given in table 6.1. Material (silicon) required for preparing a PV module of 1m2 is 0.7245 kg. The energy and material requirements for module production are mass and area dependent. These are as follows:

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Table 6.1. Specifications and design data of 1.2 kWp PV system

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Size of a cell Area of a cell Thickness of a cell Density of silicon Mass of a single cell No. of cells in a module No. of cells in an array Total mass of cells

12.5cm × 12.5cm 156.25cm2 0.035cm 2.3gm/cm3 12.578 gm. (square shape) 36 576 7245 gm (7.245 kg)

(i)Energy required in purification and processing of material (silicon dioxide) to produce Czochralski silicon (solar grade) ingot per kg.(670 kWh), a. Production of 2.11kg MG-Si = 20× 2.11=42.2 kWh b. Production of 2.027 kg EG-Si = 100×2.027=202.7 kWh c. Production of 1.46 kg EG-Si for Cz-Si= 210×1.46=426.32 kWh d. Total embodied energy for silicon material required for 1 m2 PV module= 671.2~670 kWh (ii) Energy required for production of silicon wafers form Czochralski silicon ingot for cell fabrication per m2 (120kWh), (iii) Energy required for module assembly per m2 (190kWh), (iv) Energy required for support structure per m2 (500kWh for open field and 200 kWh for roof top) and (v) BOS components: Battery, inverter, electronic components, cables, miscellaneous, etc.

Production of Czochralski Silicon Ingot (Cz-Si) Metallurgical grade silicon (MG-Si) is created by carbothermic reduction of silicon dioxide (Si O2), ‘Quartz sand’. It is a process in which coal, coke and wood chips are heated together with silica dioxide. The energy requirement to produce 1 kg of (MG-Si) is 20 kWh (Dones and Frischknecht, 1998 and Kato, 1998). Electronic grade silicon (EG-Si) is produced from MG-Si. The energy required to produce 1 kg of EG-Si is 100 kWh and there is a 90% yield. The next step is to melt the EG-Si in a Czochralski crystal puller at 1400 oC and slowly crystallize the silicon to form a single crystal ingot of silicon. There is a total yield of 72% from Czochralski process. The energy required to produce 1 kg of Cz-Si is 292 kWh per kg. The ingot typically sliced with a thickness of 0.5 to 1 mm. So,

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energy required in purification and processing of material (silicon dioxide) to produce Czochralski silicon (solar grade) ingot per kg is equal to (20+100+292=) 412kWh. Since, material (silicon) required for preparing a PV module of 1m2 is 0.7245 kg, hence embodied energy for PV module of 1m2 = 412×0.7245=298.5 kWh.

Production of Silicon Cell

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The wafer production is to slice the ingot obtained from Czochralski process into wafers. This is carried out with a multiwire saw and abrasive slurry. There are 40-50% losses of the ingot as sawdust during the wafer production. The silicon wafer thickness is assumed to be 0.350mm (350 micron) and the losses is to be 0.300mm (300 micron). After trimming, a 156.25 cm2 silicon wafer (square shape), a pseudo square PV cell with an effective area of 142cm2 is prepared. Thus the mass of each PV cell is 11.43 gm (142×0.035×2.3) for 2.3 gm/cm3 (density of silicon). There is 91% yield. Solar cell fabrication entails a sequence of high temperature diffusion, oxidation, deposition and anneal steps. The energy required to prepare 1m2 of silicon cell is 120 kWh. These solar cells are used to make a PV module with a packing factor of 0.82 (i.e.82%) silicon and 18% open space between cells.

PV Module After testing solar cells under standard test conditions (STC: Irradiance : 1000 W/m2, cell temperature : 25oC) and sorting to match current and voltage, the 36 solar cells of single crystalline silicon are laid out into a matrix of 9×4 cells, inter connected in series with copper ribbon and encapsulated to form a module. A module consists of the following components: i. Front cover low iron tempered glass, ii. Encapsulant, transparent, insulating, thermoplastic polymer foil, the most widely used one is EVA (Ethylene vinyl acetate), iii. Solar cells, iv. Copper ribbon, v. Back cover foil of tedlar and vi. Aluminum channel. The solar cells are encapsulated between a glass front plate and back cover foil of tedlar using EVA through heat and pressure and hermetically sealed using state-of- art technology. The module is then framed with

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anodized aluminum channels on perimeter of module. These are manufactured to withstand all climatic conditions. A junction box with ON/OFF switch is fixed under the module and connected through cable to the battery. The energy requirement to prepare a module of 1m2 is 190 kWh.

(b) Embodied energy of support structure The suitable number of PV modules are mounted on metallic frame and connected in series and parallel combination to form an array. The structure is firmly grouted in open field or fixed on roof top and oriented so that the panel / array (modules) faces south and inclined with respect to the horizontal to get maximum insolation (solar intensity). It has a provision for changing the angle of inclination with respect to the horizontal in order to take care of the seasonal variation. Embodied energy of support structure of firmly grouted in open field or fixed on roof top are 500 kWh / m2 and 200 kWh / m2 respectively.

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(c) Balance-of-system (BOS) The requirements of balance-of-system (BOS) (normally considered as per kWp rating system, converted into kWh/m2 PV module area for embodied) will depend largely on the desired application. In grid connected PV system, one considers a dc-to-ac converter, cables and some panel / array support materials. A battery for energy storage will be required, since solar cells can not store the energy themselves. The embodied energy at macro-level for a typical single crystalline silicon PV module is given in table 6.2. Table 6.2 also shows the energy requirements for a typical module based on single crystalline silicon. One can observed that silicon purification and processing process is the major energy consumer. Also note the relatively high energy use for support structure with balance-of-system in open field. The embodied energy at micro-level of PV system with battery replacement at an interval of 5 and 7 years in lifetime of 35 years is given in tables 6.3(a) and 6.3(b) respectively.

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Table 6.2. Break-up of embodied energy at macro-level for a typical single crystalline silicon PV module. Processes / items Silicon purification and processing Cell fabrication Module assembly Support structure Balance-of-system (BOS) (i) Battery (ii) Inverter Total Over all O&M, electronic components, cables and miscellaneous, etc., taken into account 10% extra Total

Embodied energy (kWh / m2) Open field Roof top 670 670 120 120 190 190 500 200 46 32 1555 160

46 32 1255 130

1710

1380

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Table 6.3(a): Embodied energy at micro-level (kWh/m2) of PV system with battery replacement at an interval of 5 years Years With BOS (open field) With BOS (roof top)

5 1710

10 1756

15 1802

20 1848

25 1894

30 1940

35 1986

1380

1426

1472

1518

1564

1610

1656

Table 6.3(b): Embodied energy (kWh/m2) at micro-level of PV system with battery replacement at an interval of 7 years. Years With BOS (open field) With BOS (roof top)

7 1710

14 1756

21 1802

28 1848

35 1894

1380

1426

1472

1518

1564

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6.4. ENERGY OUTPUT OF PV SYSTEM Energy output of PV system depends on the solar radiation and various temperatures etc. Therefore, it is very site specific and variable. Proper sizing and designing of PV system is must for a reliable performance for a longer period. An annual average insolation (solar radiation) on inclined PV module for different climatic zones, at different efficiencies, different sun shine hours per day, different radiation in India taken into account, the energy output (Eout) of PV system can be calculated as : Annual average insolation (kWh) = Insolation on inclined plane (W/m2) × peak sun shine hours per day × number of clearsunny days in a year (6.1) = 800 (W/m2) × 6 (hrs) × 300 clear sunny days = 1440 KWh /m2 / year (in India)

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Similarly calculation for 1000 and 1200 W/m2 have been done. Eout of the PV system can be evaluated as: E out = Insolation (kWh /m2/ year) × system efficiency

(6.2)

The single crystalline silicon solar cells are assumed to have an efficiency of 14% (Tiwari, 2004) under standard test conditions. The solar cell efficiency is reduced to 11%, due to the following factors: i. Increased cell temperature, ii. Reduced solar intensity and iii. Dust deposition. Packing factor: It is the ratio of area occupied by PV cells in a module to the actual area of the same module.

Packingfactor(PF) =

Areaof PVcellsina module Actualareaof a module

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(6.3)

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If the cell packing factor 82% in the present case is to be considered then there is further decrease in efficiency i.e. Efficiency of PV system = efficiency of cell × packing factor

(6.4)

Further, there is an electrical loss due to inverter, transformer and connecting electrical resistance which is generally considered as 15%. In addition to these, 6% electrical losses are considered for electrical efficiency of the balance-of-system (BOS) becomes:

ηBOS = (100 – allelectrical losses)

(6.5)

or,

ηBOS = (100 – 15 – 6) = 79%

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This reduces the annual average energy output. Now, an electrical energy output from 1m2 PV panel will be as follows by using Eqs. 6.4 and 6.5. Eout = Annual average insolution × efficiency of solar cell × packing factor × efficiency of BOS (6.6) = 1440 kWh /m2/ year × 0.11×0.82×0.79 = 102.61 kWh/m2/ year (with uniform efficiency of 11%) If

efficiency

of

cell

decreases

with

η ( at , t = 0) = 11% and η (at , t = 35) = 5%

respect

to

time,

then, average efficiency

of cell ( η av.) will be 8%. Therefore, if efficiency of cell is considered as 8% in place of 11% of PV system then electrical energy output from 1m2 PV panel will be as follows (Eq. 6.6 ): Eout = 1440 kWh /m2/ year × 0.08×0.82×0.79 (with average efficiency of 8%) = 74.626 kWh/m2/year Energy output (Eout) of PV system based on a conversion efficiency of 11% and 8% for different climatic zones, different solar radiation and different sun shine hours per day is given in table 6.4. The amount of power produced by a PV module depends upon the amount of sun light it is

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exposed to. More light means more power. To intercept more sun light, a PV module must be positioned in such a way that the sun’s rays arrive at the module directly; i.e. perpendicular to its surface.

Table 6.4. Energy output of 1m2 PV system Efficiency

Eout (kWh /m2 / year

11% 8%

Solar radiation (W/m2) 800 1000 1200 Clear sunny days in a year 300 and sun shine hours per day 6 8 6 8 6 8 103 137 128 171 154 192 75 99 93 124 149 140

η

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6.5. ENERGY PAYBACK TIME (EPBT) In order to evaluate energy pay-back time of a PV system, it is necessary to have knowledge of embodied energy from resource extraction through manufacturing, product use until end of the life as well as energy output from the system per year in (kWh). Similar factors have been taken into account while calculating energy pay-back time and energy yield factor, as already discussed by Alsema et al., (2000) i.e. i.

Which process in the life cycle are evaluated regarding their energy input and output? ii. How will secondary energy carries (notably electricity) be treated in the analysis? iii. Which common property will be used in the presentation of energy requirements for PV modules? The embodied energy is compared with the energy output of the PV system. The energy pay-back time (EPBT) can be calculated by dividing the embodied energy (Ein) by the annual energy output (Eout). The energy payback time shows how long it takes before energy investments are compensated by energy yield. Since the only part of the total energy yield is reflected in the energy pay-back time, it is useful to represent also the net

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energy balance by calculating an energy yield factor by dividing the lifetime energy output (Eout, lifetime) by the gross energy requirement (embodied energy). The energy yield factor shows how much energy is obtained per unit ‘invested’ energy. However, because energy yield factor do not provide much more insight then energy pay-back time will mainly employ the latter one. In fact the energy yield can be obtained very easily by dividing the lifetime by EPBT.

Embodiedenergy(kWh/m2) EPBT(atmacro −level) = Energy output(kWh/m2 /year

(6.7)

Embodiedenergy(kWh/m2 ) EPBT(atmicro − level) = Energyoutput(kWh/m2 / year

(6.8)

Energy yieldfactor

=

Life timeenergyoutput Grossenergyrequirement

(6.9)

Energy yield

=

Life timeof thesystem EPBT

(6.10)

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and

The net annual energy yield for PV system should be greater than zero. With a positive energy yield, means that the energy output during lifetime of the PV system must be greater than embodied energy during the system’s life cycle. Using Eqs. 6.7 and 6.8, the energy pay-back time (EPBT) at macro and micro-level for different components of 1 m2 PV panel can be obtained. The EPBT at macro and micro-level for different components of 1 m2 PV panel is shown in tables 6.5 and 6.6 respectively. It is also summarized based on a conversion efficiency of 11% and 8% of PV system for different climatic zones, different solar radiation, different sun shine hours per day at macrolevel in table 6.7 and micro-level with battery replacement at an interval of 5 and 7 years in tables 6.8(a) and 6.8(b) respectively.

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Table 6.5. Energy pay-back time (EPBT) at macro-level for 1 m2 PV module Components

Ein (kWh/m2)

MG –Si 42 EG – Si 201 Cz – Si 423 Cell 120 fabrication 190 Module 734 assembly 404 BOS (open field) BOS (roof top) Total Without BOS With BOS (open field) c. With BOS (roof top)

Eout (kWh/m2/year

EPBT (Years)

η =11%

η =8%

η =11%

η =8%

103

75

0.41 1.95 4.11 1.16 1.84 7.13 3.92

0.56 2.68 5.64 1.60 2.53 9.79 5.39

9.47 16.6 13.40

13.01 22.8 18.4

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Table 6.6. Energy pay-back time (EPBT) at micro-level for 1 m2 PV module Components

MG – Si EG – Si Cz – Si Cell fabrication Module assembly BOS (open field) BOS (roof top) Total a. Without BOS b. With BOS (open field) c. With BOS (roof top)

Ein (kWh/m2)

Eout (kWh/m2/year

EPBT (Years)

η =11%

η =8%

η =11%

η =8%

42 201 423 120 190 1010 680

103

75

0.41 1.95 4.11 1.16 1.84 9.80 6.60

0.56 2.68 5.64 1.60 2.53 13.46 9.07

9.47 19.28 16.08

13.01 26.48 22.08

It is inferred from the tables 6.4 and 6.5 that EPBT is a very strong function of efficiency of solar cell, insolation of location and EPBT of the BOS. The lifetime of the PV system is generally considered between 30-40 years. The lifetime of PV system is considered to be 35 years.

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Table 6.7. EPBT at macro-level in different climatic zones [life of battery (consumables) and PV system is same] Eout (kWh/m2/year)

η Efficiency

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11% 8%

Ein(kWh/m2) With With BOS BOS (open- (roofopen) field)

Solar Radiation (W/m2) 800 1000 1200 Clear sunny days in a year 300 and sun shine hours per day 6 8 6 8 6 103 137 128 171 174 75 99 93 124 112

8 205 149

1710

1380

EPBT (year) With BOS (open field)

With BOS (roof top)

Solar Radiation (W/m2) 800 1000 1200 Clear sunny days in a year 300 and sun shine hours per day 6 8 6 8 6 8 17 12 13 10 11 8 23 17 18 14 15 11

800

6 13 18

1000

8 10 14

6 11 15

1200

8 8 11

6 9 12

8 7 9

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Table 6.8 (a). EPBT at micro-level in different climatic zones [battery (consumables) replacement at in interval of 5 years] Eout (kWh/m2/year

Ein (kWh/m2) With With BOS BOS (roof(openopen) field)

Solar Radiation (W/m2) Efficiency

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η

11% 8%

800 1000 1200 Clear sunny days in a year 300 and sun shine hours per day 6 8 6 8 6 103 137 128 171 174 75 99 93 124 112

8 205 149

986 1

1656

EPBT (year) With BOS (open field)

With BOS (roof top)

Solar Radiation (W/m2) 800 1000 1200 Clear sunny days in a year 300 and sun shine hours per day 6 8 6 8 6 8 19 15 15 12 13 10 26 20 21 16 18 13

800

6 16 22

1000

8 12 17

6 13 18

1200

8 10 13

6 11 15

8 8 11

Table 6.8 (b): EPBT at micro-level in different climatic zones [battery (consumables) replacement at in interval of 7 years] Efficiency

η

Eout (kWh/m2/year

Solar Radiation (W/m2)

11% 8%

800 1000 1200 Clear sunny days in a year 300 and sun shine hours per day 6 8 6 8 6 103 137 128 171 174 75 99 93 124 112

8 205 149

Ein (kWh/m2)

EPBT (year)

With BOS (openfield)

With BOS (open field) Solar Radiation (W/m2)

With BOS (roof top)

800 1000 1200 Clear sunny days in a year 300 and sun shine hours per day 6 8 6 8 6 8 18 14 15 11 12 9 25 19 20 15 17 13

800

1897

With BOS (roofopen)

1564

6 15 21

1000

8 11 16

6 9 13

1200

8 9 13

6 10 14

8 8 10

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6.6. CO2 EMISSIONS The average carbon dioxide (CO2) equivalent intensity for electricity generation from coal is approximately 0.98 kg of CO2 / kWh, (Watt et al, 1998). If the PV system has lifetime of 35 years, the CO2 emissions per year by each components can be calculated as :

CO2 emissions per year =

Embodied enrgy × 0.98 Lifetime

(6.11)

The CO2 emissions per year for 1 m2 PV panel in present conditions at macro and micro-level is given in tables 6.9 and 6.10 respectively.

Table 6.9. CO2 emissions per year from 1 m2 PV panel at macro-level

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Components MG – Si EG – Si Cz – Si Cell fabrication module assembly BOS (open field) BOS (roof top) Total Without BOS With BOS (open field) With BOS (roof top)

Embodied energy (kWh) 42 201 423 120 190 734 404

CO2 emissions (kg) 1.18 5.63 11.84 3.36 5.32 20.55 9.1

976 1710 1380

27.23 47.88 38.64

CO2 save/m2 in lifetime and CO2 save/m2/year is also summarized for different climatic zones, different solar radiation, at different efficiencies, different sun shine hour per day and at different period of battery replacement are shown in tables 6.11(a) and 6.11(b). Tables 6.12(a) and 6.12(b) are also summarized CO2 save /kWh in different conditions. The embodied energy and energy save at macro-level in different conditions are shown in Figs 6.1(a) and 1(b). Embodied energy and energy save at micro-level in different conditions are also shown in Figs 2.

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Table 6.10. CO2 emissions per year from 1 m2 PV panel at micro-level Components MG – Si EG – Si Cz – Si Cell fabrication module assembly BOS (open field) BOS (roof top) Total Without BOS With BOS (open field) With BOS (roof top)

Embodied energy (kWh) 42 201 423 120 190 1010 680

CO2 emissions (kg) 1.18 5.63 11.84 3.36 5.32 28.28 19.04

976 1986 1656

27.33 55.61 46.37

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6.7. RESULTS AND DISCUSSION The results of EPBT at macro and micro-level are represented in tables 4 and 5 respectively. It can be seen from the Table 4 that the EPBT at efficiency of 11% without BOS is 9.47 years, with BOS (open field) is 16.6 years and with BOS (roof top) is 13.40 years. Similarly from the Table 5 the EPBT with BOS (open field) is 19.28 years and with BOS (roof top) is 16.08 years. The actual effect of PV system in terms of CO2 emissions at macro and micro-level are represented in table 6.6 and 6.7 respectively. The CO2 emissions at macro-level with BOS is 27.23 kg / year, with BOS (open field) is 47.88 kg / year and with BOS (roof top) is 38.64 kg / year. Similarly the CO2 emissions at micro-level with BOS (open field) is 55.61 kg / year and with BOS (roof top) is 46.37 kg / year. The EPBT at macro and micro-level, CO2 emissions and save from PV system in different conditions are shown in tables [6.9-6.12(b)]. The results of EPBT, CO2 emissions and save depend on the efficiency, solar radiation, sun shine hours, location and lifetime of the PV system.

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Table 6.11 (a). Calculation of CO2 emissions and save [battery (consumables) replacement at in interval of 5 years] Total CO2 emissions (kg) Efficiecy

η

Solar radiation (W/m2)

800 1000 11% 1200 800 1000 8% 1200

Sun shine hours/day

6 8 6 8 6 8 6 8 6 8 6 8

* coal based thermal power plant

Total Eout (kWh/m2) in lifetime (35 years)

3605 4795 4480 5985 5390 7175 2625 3465 3255 4340 3920 5215

If total CO2 emissions, if Eout is generated by TPP*

3533 4699 4390 5865 5282 7031 2572 3396 3190 4253 3842 5111

Net CO2 save (kg)

During manufacturing of PV system

CO2 save / m2 if total Eout is generated by PV stystem

CO2 save/m2/year by PV system

With BOS (open field)

With BOS (open field) 1587 2753 2444 3914 3336 5083 626 1450 1244 2310 1896 3165

With BOS (open field) 45 79 70 112 95 145 18 41 35 66 54 90

1946

With BOS (roof top)

1623

With BOS (roof top) 1914 3076 2767 4242 3659 5408 949 1773 1567 2629 2219 3488

With BOS (roof top) 54 88 79 121 104 154 27 51 45 75 63 100

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Table 6.11 (b). Calculation of CO2 emissions and save [battery (consumables) replacement at in interval of 7 years] Efficieny

η

Solar radiation (W/m2)

800 11%

1000 1200 800

8%

1000 1200

Sun shine hours/day

6 8 6 8 6 8 6 8 6 8 6 8

Total Eout (kWh/m2) in lifetime (35 years)

3605 4795 4480 5985 5390 7175 2625 3465 3255 4340 3920 5215

Total CO2 emissions (kg)

Net CO2 save (kg)

If total Eout is generated by TPP*

During manufacturing of PV system

CO2 save / m2 if total Eout is generated PV system

CO2 save/m2/year by PV system

With BOS (open field)

With BOS (roof top)

1856

1533

With BOS (open field) 1677 2843 2354 4009 3426 5175 716 1540 1334 2397 1986 3255

With BOS (open field) 48 81 72 114 98 148 20 44 38 68 57 93

3533 4699 4390 5865 5282 7031 2572 3396 3190 4253 3842 5111

With BOS (roof top) 2000 3166 2857 4337 3749 5498 1039 1863 1657 2720 2309 3578

With BOS (roof top) 57 90 82 124 107 157 30 53 47 78 66 102

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Table 6.12 (a).: Calculation of CO2 emissions and save [battery (consumables) replacement at in interval of 5 years] Efficiency

η

Solar radiatio n (W/m2)

800 11%

1000 1200 800

8%

1000 1200

Sun shine hours/day

6 8 6 8 6 8 6 8 6 8 6 8

Total Eout (kWh/m2) in lifetime (35 years)

3605 4795 4480 5985 5390 7175 2625 3465 3255 4340 3920 5215

Total CO2 emissions (kg)

Net CO2 save (kg)

If total Eout is generated by TPP*

During manufacturing of PV system

CO2 save /m2, if total Eout is generated by PV system

CO2 save/kWh

With BOS (open field)

With BOS (roof top)

1946

1623

With BOS (open field) 1587 2753 2444 2939 3336 5085 626 1450 1244 2307 1896 3165

With BOS (open field) 0.44 0.57 0.54 0.60 0.62 0.71 0.24 0.42 0.38 0.58 0.48 0.61

3533 4699 4390 5865 5282 7031 2572 3396 3190 4253 3842 5111

With BOS (roof top) 1914 3076 2767 3262 3659 5408 949 1773 1567 2630 2219 3488

With BOS (roof top) 0.53 0.64 0.62 0.65 0.68 0.75 0.36 0.51 0.48 0.60 0.57 0.67

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Table 6.12 (b). Calculation of CO2 emissions and save [battery (consumables) replacement at in interval of 7 years] Efficieny

η

Solar radiation (W/m2)

800 11%

1000 1200 800

%

1000 1200

Sun shine hours/day

6 8 6 8 6 8 6 8 6 8 6 8

Total Eout (kWh/m2) in lifetime (35 years)

3605 4795 4480 5985 5390 7175 2625 3465 3255 4340 3920 5215

Total CO2 emissions (kg)

Net CO2 save (kg)

If total Eout is generated by TPP*

During manufacturing of PV system

CO2 save /m2, if total Eout is generated by PV system

CO2 save/kWh

With BOS (open field)

With BOS (roof top)

1856

1533

With BOS (open field) 1677 2843 2534 3029 3426 5175 716 1540 1334 2397 1986 3255

With BOS (open field) 0.46 0.59 0.56 0.61 0.63 0.72 0.27 0.44 0.41 0.55 0.51 0.62

3533 4699 4390 5865 5282 7031 2572 3396 3190 4253 3842 5111

With BOS (roof top) 2000 3166 2857 3352 3749 5498 1039 1868 1657 2720 2309 3578

With BOS (roof top) 0.55 0.66 0.64 0.67 0.69 0.77 0.39 0.54 0.51 0.63 0.59 0.69

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6.8. CONCLUSION

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It is observed that with positive energy yield (EN), the energy pay-back time (EPBT) is in the range of 7-26 years as indicated in tables [6.9-6.10 (b)]. The EPBT depends upon solar radiation, efficiency of PV system and BOS. It is a good indicator of the CO2 mitigation for PV system. It is also observed form the tables 6.12(a) and 6.12(b) that the CO2 emissions per kWh from PV system is very much low in comparison with 0.98 kg CO2 emissions per kWh from coal based thermal power plant because the CO2 emissions from PV system occur almost entirely during system manufacturing, and not during system operation. The results also show that the net CO2 save from present PV technology are in the range of 18-157 kg /m2 /year or in other words 0.24-0.77 kg /kWh. So, the PV system is environment friendly in comparison to other source of energy used for power generation.

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Chapter 7

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LIFE CYCLE COST ANALYSIS In this section life cycle cost analysis of solar system will be discussed briefly by considering the effect of energy pay back time (EPBT) of solar system. If EPBT of solar system is very- very small in comparison with life of the system, then life cycle cost analysis becomes straight forward, otherwise EPBT should be subtracted from the life of the system for life cycle cost analysis. Figure 7.1 shows the cash flow diagram of solar system for life cycle cost analysis. In this case, P0, n and R are assumed to be the initial investment, life of the system and annual operation and maintenance.

Figure 7.1. Cash flow diagram without EPBT

Annualized cost of Fig. 7.1, Tiwari (2002), can be written as Annualized cost = R + P0 FPR,i,n

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(7.1)

126

G.N. Tiwari

where,

FPR ,i ,n

i (1 + i ) n = (1 + i ) n − 1

If np is the EPBT of solar system, then cash flow diagram shown in Fig. 7.1 is modified and it is shown in Fig. 7.2.

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Figure 7.2.a. Cash flow diagram with EPBT

The present worth (P0) of Fig. 7.2a after EPBT becomes present worth of Fig. 7.2b for remaining life of the sysrem i.e. (n-np) and it is expressed as Present worth at npth time ( Pnp) = (P0 + R FRP,i,np). FPS,i,np where,

FRR ,i ,n =

(1 + i ) np − 1 np and FPS ,i ,np = (1 + i ) np i (1 + i )

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(7.2)

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Figure 7.2.b. Modified cash flow diagram of Fig. 7.2b

The annualized cost of cash flow diagram as shown in Fig. 7.2b can be expressed as Annualized cost with EPBT = R + Pnp.FPR,i, (n-np)

(7.3)

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where,

FPR ,i ,n =

i (1 + i ) (1 + i )

( n−n p )

(n−n p )

−1

Example7.1: Calculate energy pay back time (EPBT) of flat plate collector for following parameters: P0 =Rs.15,000.00, R=Rs.100.00, n=15 years , np=3 years, i= 0.10 (10%) Solution: Without EPBT From Eqation (7.1), annualized cost without EPBT will be Annualized cost = 100 + 15,000 × 0.131 = Rs. 2065.00 With EPBT From Eqation (7.2), one has Present worth at npth time ( Pnp) = (15,000 + 100 × 2.4869) × 1.331 = Rs 20,2960.00

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G.N. Tiwari From Eqation (7.3), one has Annualized cost with EPBT = 100 +20, 296 × 0.1467 = Rs. 3077.42

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It can be observed that an annualized cost with EPBT is significantly increased. Hence it is concluded that life cycle cost analysis is incomplete without considering the effect of EPBT for all solar systems.

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[12] Cebula S.;. Black and transparent plastic mulches in greenhouse production of sweet pepper. I. Thermal conditions and the vegetative growth of plants. Folia Horticulture, 1995, 7(2), 51-58. [13] Chandra, P.; Albright, L.D.; Analytical determination of the effect on greenhouse heating requirements of using night curtains. Trans. ASAE, 1980, 23(4),994–1000. [14] Cooper, P.I.; Digital simulation of transient solar still processes. Solar Energy, 1969, 12, 313. [15] Cooper, P.I.; Digital simulation of experimental solar still data. Solar Energy, 1973, 14, 451. [16] Critten, DL.; Optimization of CO2 concentration of a greenhouse: a modeling analysis for the lettuce crop. J. agric. Engng. Res., 1991, 48, 261-271. [17] Dones, R.; Frischknecht, R.; Life cycle assessment of PV system: Results of swiss studies on energy chains, progresses in photovoltaic. Research Applications 1998, 6, 127. [18] Duffie, J.A.; Beckman, W.A.; Solar Engineering of Thermal Processes. New York, John and Wiley and Sons; 1991. [19] El-Sebaii, A.; A Effect of wind speed on active and passive solar stills. Energy Conversion and Management, 2003, 45 (7-8), 1187-1204. [20] Elsner, B.; von, Briassoulis, D.; Waaijenberg, D.; Mistriotis, A.; Zabeltitz, Chr. Von, Gretraud, J.; Russo, G.;. Sauy-Cortes, R.; Review of structural and functional characteristics of greenhouse in European union countries, Part I: Design Requirments. J. Agric. Engg. Research, 2000, 75(1), 1-16. [21] Fath, H.E.S.; Hosny, H.M.; Thermal performance of a single-sloped basin still with an inherent built-in additional condenser. Desalination, 2002, 142, 19-27. [22] Fernandez, J.; Chargoy, N.; Multistage, indirectly heated solar Still. Solar Energy. 1990, 44(4), 215. [23] Feuermann, D.; Kopel, R.; Zeroni M.; Levi, S.; Gale J.; Evaluation of a liquid radiation filter greenhouse in a desert environment. Transactions of the ASAE, 1998, 41(6), 1781-1788. [24] Frankl, P. ; Masini, A. ; Gamberale, M. ; Toccaceli, D. ; Simplified life cycle enalysis of PV system in buildings present situations and future trends. Progress in photovoltaic: Research Applications 1998, 6 (2), 137–146. [25] Garzoli, K.; Blackwell, J.; An analysis of the nocturnal heat loss from a single skin plastic greenhouse. J. Agric. Engg. Res., 1981, 26, 203– 214.

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[26] Givoni, B.; Performance and applicability of passive and low energy cooling systems. Energy and Buildings, 1991, 17,177-199. [27] Goyal, R.K.; Tiwari, G.N.; Parametric study of a reverse flat plate absorber cabinet dryer: a new concept. Solar Energy, 1997, 60, 41. [28] Grafiadellis, M.; PE tubes filled with water inside the greenhouse. In: Von Zabeltitz C, editor, Greenhouse heating with solar energy. FAO, Rome, 1987, 89-93. [29] Gupta, A.; Tiwari, G.N.; Computer model and its validation for prediction of storage effect of water mass in a greenhouse: a transient analysis. Energy Conver. And Mgnt, 2002. [30] Hanan, J.J.; Holley, W.D.; Goldsberry, K.L.; Greenhouse Management; Springer-Verlag, Berlin 1978. [31] Holder, R.; Cockskull, K.E.; Effects of humidity on the growth and yield of glasshouse tomatoes. J. Hort. Sci., 1990, 65, 31–39. [32] Ismail, K.A.R.; Gonclaves, M.M.; Thermal performance of a PCM storage unit. Energy conversion and Management, 1999, 40,115-138. [33] Jensen, M.H.; and Malter, A.J.; Protected Agriculture- A global review. World bank technical paper, 1995, No.253, 157. [34] Jolliet, O.; A model for predicting and optimizing humidity and transpiration in greenhouses. J. Agric. Engg. Res., 1994, 57, 23–37. [35] Kasten, F.; A new table and approximate formula for relative optical air mass. Arch. Meteorol. Geophys. Bioklimatel Ser. B., 1965, 14, 206223. [36] Kasten, F.; Young, AT.; Revised optical air mass tables and approximation formula. Appl. Opt; 1989, 28, 4735-4738. [37] Kato, K.; Murata, A.; Sakuta, K.; Energy pay-back time and life cycle of CO2 emissions of residential PV power system with silicon PV module. Progress in photovoltaic: Research Applications, 1998, 6, 105. [38] Krauter, S.; Ruther, R.; Considerations for the calculations of green house gas reduction by photovoltaic solar energy. Renewable Energy, 2004, 29, 345–355. [39] Kurata, K.; Simulation of inside air temperature, humidity and crop production in an energy conservation greenhouse. Acta Horticulture, 1989, 245, 339-345. [40] Liu, B.Y.H.; Jordan, R.C.; The interrelationship and characteristic distribution of direct, diffuse and total solar radiation. Solar Energy, 1960, 4(3), 1-19. [41] Malik, M.A.S.; Tiwari, G.N.; Kumar, A.; Sodha, M.S.; Solar Distillation, UK, Pergamon Press, 1982.

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[58] Stapleton, J.J.; DeVay, J.E.; Soil solarization. In: Reuveni R, editor. Novel approaches to Integrated Pest Management, Lewis Publisher, Boca Raton, 1995’ 309-322. [59] Tanaka, H.; Nosoko, T.; Nagata, T.; Experimental study of basin-type, multiple-effect. Diffusion-coupled solar still. Desalination, 2002, 150. 131-144. [60] Tiwari, G.N.; Recent trends in Solar Distillation, Chapter II, Solar Energy and Energy Conservation, Wiley Eastern Ltd., New Delhi. 1992, 32-149. [61] Tiwari, G.N.; Solar Energy: Fundamental, Design, Modelling and Applications. New York, CRC Press, and Pangbourne England, Alpha Science International Ltd., 2002. [62] Tiwari, G.N.; Greenhouse Technology for Controlled Environment, Pangbourne England, Alpha Science International Ltd. 2003. [63] Tiwari, G.N.; Goyal, R.K.; Greenhouse Technology. Narosa publishing house: New Delhi, 1998, 252-311. [64] Tiwari, G.N.; Singh, H.N.; Tripathi, R.; Present status of solar distillation. Solar Energy, 2003, 75, 367. [65] Tiwari, G.N.; Ghosal, M.K.; Renewable Energy Resources: Basic principle and applications. Pangbourne England, Alpha Science International Ltd.,2005. [66] Verlodt, H.; Greenhouses in Cyprus, protected cultivation in the Mediterranean climate, FAO, Rome, Italy, 1990. [67] Voropoulos, K.; Mathioulakis, E.; Belessiotis, V.; A hybrid solar desalination and water heating system. Desalination, 2004, 164, 189195. [68] Watt, M.; Johnson, A.; Ellis, M.; Quthred, N.; Life cycle air emission from PV power systems. Progress in photovoltaic : Research Applications 1998, 6, 127. [69] Yadav, I.S.; Chaudhari, M.L.; Progressive Floriculture, The House of Sarpan (Media), Banglore (India), 1997, 15-20. [70] Zhu, S.; Deltour, J.; Wang, S.; Modeling the thermal characteristic of greenhouse pod system. Aquaculture Engineering, 1998, 18, 201.

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INDEX

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A absorption, 3, 5, 38, 60, 64 aerosols, 6 Al, 16 alcohol, 50, 132 alcohol production, 50 algae, 4 aluminum, 97, 108 ambient air, 25, 27, 45, 46, 51, 58, 76, 79, 89 animals, 129

B bacteria, 4 blocks, 95 burning, 89, 103

C cables, 106, 108, 109 Canada, 91, 92 carbohydrate, 84 carbon, 2, 83, 87, 89, 117 cation, 90

cell, 2, 106, 107, 110, 111, 114 chemical reactions, 87 cladding, 91 classification, 83, 100, 101, 132 climatic factors, 102 CO2, 1, 2, 89, 103, 104, 117, 118, 119, 120, 121, 122, 123, 130, 131 coal, 106, 117, 119, 123 coating, 25 coke, 106 combustion, 89 composite, 10, 132 composition, 83, 102 compounds, 97 concrete, 95, 101 condensation, 36, 49, 50, 61, 67 conductivity, 19, 32, 41 conversion, 10, 58, 103, 111, 113, 131 copper, 15, 16, 25, 27, 29, 32, 107 covering, 62, 83, 84, 91, 100 crop production, 4, 83, 131 cultivation, 63, 84, 89, 90, 91, 92, 94, 100, 102, 129, 132, 133

D damage, 84 degradation, 103 density, 2, 27, 54, 90, 97, 107

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deposition, 107, 110 diffusion, 49, 50, 59, 107, 133 diffusion process, 59 distillation, 1, 4, 36, 37, 38, 39, 43, 48, 50, 51, 68, 132, 133 distilled water, 38, 43, 46 dry matter, 84 drying, 1, 4, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 69, 72, 75, 77, 80, 81, 82, 102, 129

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E earth’s atmosphere, 2 efficiency, 1, 17, 18, 19, 20, 22, 25, 26, 30, 32, 43, 44, 47, 49, 53, 54, 55, 70, 71, 73, 74, 75, 78, 79, 80, 81, 82, 103, 104, 110, 111, 113, 114, 118, 123 electrical resistance, 111 emission, 1, 6, 133 energy, 1, 2, 3, 5, 6, 11, 15, 16, 17, 19, 21, 23, 24, 27, 28, 29, 31, 37, 38, 39, 40, 43, 49, 52, 54, 55, 58, 59, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 80, 81, 82, 83, 84, 95, 96, 97, 98, 99, 101, 103, 104, 105, 106, 107, 108, 109, 110, 111, 112, 113, 117, 118, 123, 125, 127, 129, 130, 131, 132 energy density, 1, 27, 54, 70, 74, 81, 82 energy transfer, 31 environment, 4, 36, 67, 83, 86, 87, 97, 98, 100, 101, 102, 123, 130, 132 enzymes, 87 evaporation, 36, 38, 42, 46, 48, 49, 50, 59, 61, 65, 69, 88 exposure, 61, 63 extraction, 75, 105, 112

fluid, 15, 16, 18, 19, 20, 21, 25, 26, 29, 30, 31, 32, 33, 71, 72, 76, 77, 79, 98 France, 92 freezing, 101 freshwater, 35, 36 fuel(s), 89, 98, 103 fusion, 2 future, 130

G gravity, 38 greenhouse gas(es), 2

H health, 35, 37 heat, 2, 15, 16, 17, 18, 19, 20, 24, 25, 26, 29, 30, 31, 32, 33, 36, 38, 39, 40, 41, 42, 44, 45, 48, 49, 50, 51, 52, 53, 59, 63, 64, 65, 67, 68, 71, 72, 76, 77, 78, 80, 84, 86, 87, 90, 94, 95, 96, 97, 107, 129, 130, 132 heat capacity, 39, 67 heat release, 45 heat removal, 32 heat transfer, 15, 19, 20, 25, 26, 30, 31, 32, 38, 40, 41, 42, 44, 48, 65, 67, 68, 71, 72, 76, 77, 129, 132 hydrothermal process, 84

I India, 37, 46, 92, 100, 103, 110, 132, 133 insulation, 16, 17, 39, 40, 41, 45, 46, 77, 94, 96 irradiation, 6

F fabrication, 106, 107, 109, 114, 117, 118 films, 91 fish, 86

L lakes, 35, 36 leakage, 39

Tiwari, G.N.. Solar Energy Technology Advances, Nova Science Publishers, Incorporated, 2005. ProQuest Ebook Central,

Solar Energy Technology Advances light, 83, 87, 88, 89, 97, 102, 111 light transmittance, 87, 97

M manure, 90 mass, 7, 38, 40, 43, 44, 45, 52, 63, 67, 69, 70, 74, 78, 81, 82, 95, 105, 106, 107, 131 Mediterranean climate, 132, 133 melt, 106 membranes, 84 microclimate, 1, 60 microorganisms, 37 mixing, 51 modeling, 1, 130 moisture, 16, 36, 57, 58, 59, 60, 61, 63, 64, 65, 72, 73, 75, 77, 79, 80, 81, 86, 90 moisture content, 57, 58, 79, 81 momentum, 103

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N Netherlands, 91 nitrogen, 3, 89 nitrous oxide, 2 nutrients, 89

O optimization, 132 organic matter, 89, 90 orientation, 10, 11, 48, 62, 87, 102 oxidation, 107 oxygen, 3, 89 ozone, 2, 3

P PCM, 131 PE, 131

137

pH, 90 photosynthesis, 83, 87, 89, 102 pollution, 35, 37, 103 polycarbonate, 101 polyester, 101 polyethylene, 86, 91 polymer, 107 polythene, 100 Portugal, 92 precipitation, 36 prediction, 131 productivity, 83, 94 propagation, 5 properties, 3, 89 purification, 36, 38, 106, 107, 108, 109 PVC, 62, 70, 101

Q quality, 57, 59, 65, 129

R radiation, 2, 3, 4, 5, 6, 7, 10, 11, 15, 16, 17, 25, 26, 31, 36, 37, 38, 39, 46, 47, 50, 58, 60, 61, 63, 64, 84, 87, 90, 94, 95, 96, 97, 102, 103, 110, 111, 112, 113, 117, 118, 119, 120, 121, 122, 123, 130, 131 reduction, 47, 97, 106, 131 regression, 22 research, 1, 91, 101, 102 resistance, 32, 62 Romania, 92

S salinity, 47 sawdust, 107 scattering, 3, 7 seeds, 57, 86 shade, 97 shape, 2, 39, 98, 99, 106, 107

Tiwari, G.N.. Solar Energy Technology Advances, Nova Science Publishers, Incorporated, 2005. ProQuest Ebook Central,

138

G.N. Tiwari

silica, 106 silica dioxide, 106 silicon, 104, 105, 106, 107, 108, 109, 110, 131 soil, 84, 89, 90, 96 solar cells, 107, 108, 110 solar system, 2, 125, 126, 128 solution, 41, 102 Spain, 91, 92 steel, 101 surface area, 27, 67, 71

U USA, 92, 132 UV, 4, 59, 62, 91, 100 UV radiation, 4, 59

V validation, 131 vegetation, 35, 36 vinyl chloride, 101

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T temperature, 2, 4, 15, 18, 20, 21, 22, 25, 26, 27, 30, 31, 32, 36, 38, 41, 42, 43, 44, 45, 46, 47, 48, 49, 51, 54, 55, 58, 59, 60, 61, 63, 68, 71, 72, 73, 76, 77, 78, 79, 83, 84, 86, 87, 88, 89, 90, 91, 94, 95, 96, 97, 100, 101, 102, 107, 110, 129, 131 thermal analysis, 102 thermal energy, 2, 4, 21, 22, 23, 27, 30, 37, 38, 39, 43, 51, 53, 58, 59, 68, 77, 96, 97, 98, 103, 104 thermal resistance, 32 timber, 100 transport, 36, 57, 95

W wastewater, 94 water, 2, 3, 4, 22, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 49, 50, 51, 52, 53, 54, 57, 59, 67, 84, 86, 88, 89, 94, 131, 133 water resources, 35 water vapor, 2, 3, 49, 88 wires, 103 wood, 70, 101, 106 World Health Organization (WHO), 37

X X-rays, 3

Tiwari, G.N.. Solar Energy Technology Advances, Nova Science Publishers, Incorporated, 2005. ProQuest Ebook Central,