Energy Recovery Technology for Building Applications: Green Innovation towards a Sustainable Future [1st ed.] 9783030500054, 9783030500061

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Mardiana Idayu Ahmad Saffa Riffat

Energy Recovery Technology for Building Applications Green Innovation towards a Sustainable Future

Energy Recovery Technology for Building Applications

Mardiana Idayu Ahmad • Saffa Riffat

Energy Recovery Technology for Building Applications Green Innovation towards a Sustainable Future

Mardiana Idayu Ahmad School of Industrial Technology Universiti Sains Malaysia Penang, Malaysia

Saffa Riffat University of Nottingham Department of Architecture and Built Environment Nottingham, UK

ISBN 978-3-030-50005-4    ISBN 978-3-030-50006-1 (eBook) https://doi.org/10.1007/978-3-030-50006-1 © Springer Nature Switzerland AG 2020 This work is subject to copyright. All rights are reserved by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors, and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Switzerland AG The registered company address is: Gewerbestrasse 11, 6330 Cham, Switzerland

This book is dedicated to all students of IEK516 (Sustainable Energy Sources), MSc. (Environmental Science) Universiti Sains Malaysia

Preface

Energy can neither be created nor destroyed; energy can only be transferred or changed from one form to another—Albert Einstein

This book is meant to provide guidance and basic understanding of energy recovery technology and its role in reducing energy consumption of buildings in various climatic conditions. It is written to inspire further research into energy recovery technology particularly on green innovation landscape for building applications. It can be used by senior undergraduate and graduate students, engineers, professionals, practitioners, scientists, researchers, planners, technologist and employees in the area of engineering, technology, pure science and applied sciences. It also can be used as university reference book to serve as a graduate-level textbook to meet the growing demand for new courses in renewable and sustainable materials at technical and general universities. This book is a product of many years in relation to research and case studies from the earliest beginning to the recent development in the field of energy recovery technology by focusing on buildings. It is divided into eight chapters within the energy recovery technology field with each chapter provides an in-depth technical information. Chapter 1 provides a snapshot on energy, green innovation and sustainable development goals which are discussed briefly to provide basic information to the theme of the book. There has been a dramatic increase in the world’s energy consumption, which has risen at an alarming rate. This occurs due to the global increasing pattern of building energy consumption. Implementing less utilisation of natural resources and reducing the energy consumption will be positive strategies to cope with this scenario. In addition, with the establishment of new building codes and the requirement of providing comfortable indoor environment for the occupants, adequate ventilation from fresh outdoor air must be provided to dilute the emissions of indoor air so that building space conditions will be maintained within the comfortable range. In order to create environmentally friendly buildings apart from the standard energy conservation strategies, innovative green technologies must be implemented, which vii

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suggested the application of energy recovery technology as one of the solutions. This technology aids in improving indoor environmental quality and occupants’ comfort level. It also has the capability to reduce one-third of mechanical ventilation loads. Therefore, it is the aim of Chap. 2 to provide information, guide and basic understanding of energy, building energy consumption and energy efficiency strategies in inspiring further research into this field particularly on green innovation landscape for building applications. Energy recovery includes any technique or method of reducing the input of energy to an overall system by transferring energy with one another. This energy could be in the form of sensible or latent or both. In buildings, energy recovery technology has been proven as an effective mean to reduce energy consumption and operational cost. With the aim to give an insight into the principle of this technology, an approach for understanding the technical terms in energy recovery technology field is brought up in simple ways by explaining the usage and meaning of the terms with special attention and emphasis are highlighted for building applications. The book provides the foundation of energy recovery technology pertaining to theory and mechanism. Definition, concept and working principles are presented in Chap. 3. Heat exchangers form a vital part of many processes including energy recovery application. Heat exchanger is defined as a device that is used to transfer heat or energy between two streams. The transfer involves two or more fluids which can be single or two phases depending on the exchanger type. Heat exchangers are classified into flow configurations (such as counter-flow, cross-flow, co-current flow and hybrid flows) and construction (recuperative and regenerative). Heat exchangers have been widely utilised in both cooling and heating process within various industries and fields. In an energy recovery system, heat exchanger is the heart of the system in which energy or heat and/or mass is transferred from one stream to another stream. It is the core of the system consisting of matrix containing the heat and mass transfer areas. In Chap. 4, construction method, flow configurations and heat and mass transfer mechanism as well as literatures related to heat exchangers are discussed. Recent developments of heat exchangers including application for energy recovery are also summarised and reported. In general, energy recovery systems can be commonly identified based on their classification and types. They are classified into the following: (1) application, in terms of process-to-process system, process-to-comfort system and comfort-to-­ comfort system, and (2) working mechanism, in terms of air-to-air energy recovery, earth-to-air energy recovery, earth-to-water energy recovery. In the context of types, there are five common energy recovery systems which can be found in the global market and have been extensively studied which are based on the construction of their heat exchangers. These include fixed-plate, rotary enthalpy wheel, heat pipe, run-around and thermo-siphon. Chapter 5 presents classifications and types of energy recovery systems as well as an overview of existing research in these domains. The application of energy recovery system has been proven as one of the key solutions to produce energy savings and to provide fresh outdoor air in building ventilation. The performance of the system can be evaluated in terms of efficiency

Preface

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and recovered energy (heat and mass transfer) through its heat exchanger. The efficiency can be determined using ASHRAE standard, effectiveness–NTU method and global efficiency. Meanwhile, recovered energy is calculated based on the heat and mass transfer rates of the system. Chapter 6 provides a background of the performance evaluation of energy recovery system from existing established data and previous works in the literature. Energy consumption for HVAC loads in buildings often accounts for the major portion of a country’s primary energy usage, which is still mainly relied on fossil fuels. Today, there is a great global emphasis on reducing of fossil fuel energy which has turned towards optimal use of sustainable energy technologies. There have been extensive researches carried out worldwide with the aim to improve building energy efficiency towards nearly zero carbon buildings and low carbon buildings (buildings that are specifically engineered with greenhouse gases reduction in mind). This requires a major shift in the improvement of specific systems such as ventilation, heating, cooling, day-lighting and so on. Therefore, achieving this goal will need a rethink of the conventional designs or systems currently in use. One of the strategies in realising this goal is by using integrated or hybrid system or integrated design process in typical building systems. Integrated or hybrid system is defined as the process of bringing together component sub-systems into one functional system. It usually involves integrating existing device, unit or system in such a way which focuses on improving performance, quality or added value to the existing market, customer or technology. In this context, integrating energy recovery ventilator or energy recovery system into the existing ventilation systems would be one of the effective methods towards achieving energy-efficient systems for low-energy buildings. Chapter 7 discusses the integration of energy recovery systems in various building services, highlighting on mechanical ventilation systems, natural ventilation systems, air-conditioning systems, dehumidification systems and building integrated photovoltaic systems. This work provides an overview of research, development, application and status of these systems, and finally research needs and opportunities are identified. Application of energy recovery systems continues to increase in acceptance and usage in building mechanical ventilation systems. They are designed to provide energy savings by recycling energy from the building’s exhaust air to pre-treat or pre-condition the outside air/ventilation air. By pre-treating or pre-conditioning the outside air, the load and of HVAC units can be reduced. Chapter 8 highlights the application of energy recovery systems in various building types and climatic conditions. In a nutshell, this book provides the foundation of energy recovery technology pertaining to theory and mechanism from fundamental towards engineering application in buildings. In the process of preparing and writing this book, the support provided by the individuals and institutions is noteworthy. In this context, we would like to express our appreciation to all the editorial team of Springer Nature and Springer International Publishing for their contribution in any kind of forms in bringing the book to fruition. Our thanks go to the School of Industrial Technology, Universiti Sains Malaysia and the Department of Architecture and Built Environment,

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University of Nottingham, United Kingdom, for facilitating the process of gathering material and information for publishing this book. In addition, we also appreciate all reviewers for their time reviewing the content of this book. We also thank our families for their patience and support during the preparation of this book. Our special thanks go to Ms. Zeny Amante-Roberts for scheduling meetings for two of us and also to Ms. Mok Ru Ying for helping us checking the reference list. This book would not be possible without their kind support from many aspects during the process. The work involved in this book is part of the outcomes of funded research projects, thus we would like to take this opportunity to convey our appreciation to the sponsors for the financial and technical supports. Our thanks go to Universiti Sains Malaysia Research University Grant (1001/PTEKIND/814275; 1001/ PTEKIND/811229; 1001/PTEKIND/8014124), Fundamental Research Grant Scheme (203/PTEKIND/6711574; 203/PTEKIND/6711274) and Trans-disciplinary Research Grant Scheme, Ministry of Education Malaysia (203/PTEKIND 67610003). Any opinions, findings and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the MOE and USM. We would like to thank also our Graduate Research Students/Assistants involved in the researches of energy recovery technology associated with the above-stated grants: Ms. Fatin Zafirah Mansur, Ms. Siti Masitah Abul Rahman, Ms. Tan Yih Chia, Ms. Tang Chiau Yong, Mr. Mohamad Aliff Shakir and Ms. Nor Amalin Keling. Penang, Malaysia  Mardiana Idayu Ahmad Nottingham, UK   Saffa Riffat December 2019

Contents

1 Introduction: Energy, Green Innovation and Sustainable Future������������������������������������������������������������������������������   1 1.1 Overview��������������������������������������������������������������������������������������������   1 1.2 Energy and Sustainable Development Goals��������������������������������������   2 1.3 Green Innovation: Solution to Carbon Dioxide Emissions?����������������������������������������������������������������������������   3 1.4 Summary ��������������������������������������������������������������������������������������������   4 References����������������������������������������������������������������������������������������������������   4 2 Building Energy Consumption and Energy Efficiency Strategies����������������������������������������������������������������������������������   5 2.1 Energy and Buildings��������������������������������������������������������������������������   5 2.2 Building Energy Consumption ����������������������������������������������������������   6 2.3 Energy Efficiency Strategies in Buildings������������������������������������������   7 2.4 Overview of Energy Recovery Technology as Green Innovation Towards Integrated Energy Efficient in Buildings��������������������������������������������   9 2.5 Summary ��������������������������������������������������������������������������������������������  10 References����������������������������������������������������������������������������������������������������  10 3 Definition and Working Principle of Energy Recovery Technology ��������������������������������������������������������������������������������  13 3.1 Overview��������������������������������������������������������������������������������������������  13 3.2 Definition and Concept ����������������������������������������������������������������������  14 3.2.1 Waste Heat������������������������������������������������������������������������������  15 3.2.2 Heat Transfer��������������������������������������������������������������������������  17 3.2.3 Mass Transfer��������������������������������������������������������������������������  17 3.2.4 Sensible Heat��������������������������������������������������������������������������  17 3.2.5 Latent Heat������������������������������������������������������������������������������  18 3.2.6 Enthalpy����������������������������������������������������������������������������������  19 3.2.7 Ventilation Air������������������������������������������������������������������������  19 3.3 Working Principle ������������������������������������������������������������������������������  19 xi

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3.4 Summary ��������������������������������������������������������������������������������������������  22 References����������������������������������������������������������������������������������������������������  22 4 Heat Exchanger: The Heart of Energy Recovery System����������������������  25 4.1 Overview��������������������������������������������������������������������������������������������  25 4.2 Construction Method��������������������������������������������������������������������������  26 4.2.1 Size, Heat and Mass Transfer Areas ��������������������������������������  26 4.2.2 Materials ��������������������������������������������������������������������������������  28 4.2.3 Structures��������������������������������������������������������������������������������  29 4.3 Flow Configuration ����������������������������������������������������������������������������  35 4.4 Heat and Mass Transfer Mechanism��������������������������������������������������  35 4.5 Summary ��������������������������������������������������������������������������������������������  37 References����������������������������������������������������������������������������������������������������  38 5 Classification and Types of Energy Recovery Systems��������������������������  43 5.1 General Classification ������������������������������������������������������������������������  43 5.1.1 Classification Based on Different Application������������������������  43 5.1.2 Classification Based on Working Mechanism������������������������  44 5.2 Types ��������������������������������������������������������������������������������������������������  44 5.2.1 Fixed-Plate������������������������������������������������������������������������������  45 5.2.2 Rotary Wheel��������������������������������������������������������������������������  47 5.2.3 Heat Pipe��������������������������������������������������������������������������������  51 5.2.4 Run-around ����������������������������������������������������������������������������  55 5.2.5 Thermo-siphon������������������������������������������������������������������������  61 5.3 Summary ��������������������������������������������������������������������������������������������  66 References����������������������������������������������������������������������������������������������������  67 6 Evaluating the Performance of Energy Recovery Systems��������������������  73 6.1 Overview��������������������������������������������������������������������������������������������  73 6.2 Efficiency��������������������������������������������������������������������������������������������  74 6.2.1 Sensible Efficiency������������������������������������������������������������������  75 6.2.2 Latent Efficiency��������������������������������������������������������������������  75 6.2.3 Enthalpy Efficiency����������������������������������������������������������������  76 6.2.4 Global Efficiency��������������������������������������������������������������������  76 6.2.5 Efficiency Based on Effectiveness NTU Method ������������������  77 6.3 Recovered Energy ������������������������������������������������������������������������������  83 6.4 Effects of Operating Parameters on the Performance ������������������������  83 6.4.1 Effects of Air Velocity/Airflow ����������������������������������������������  84 6.4.2 Effects of Air Conditions��������������������������������������������������������  85 6.5 Summary ��������������������������������������������������������������������������������������������  86 References����������������������������������������������������������������������������������������������������  87 7 Energy Recovery in Integrated or Hybrid Systems towards Energy-­Efficient Technologies ������������������������������������  89 7.1 Overview��������������������������������������������������������������������������������������������  89 7.2 Energy Recovery in Mechanical Ventilation System��������������������������  89 7.3 Energy Recovery Assisted Passive/Natural Ventilation����������������������  90

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7.4 Energy Recovery Coupled Air-Conditioning��������������������������������������  93 7.5 Energy Recovery Incorporated with Dehumidification System������������������������������������������������������������  93 7.6 Energy Recovery Coupled-Photovoltaic/Solar Thermal System����������������������������������������������������������������������������������  98 7.7 Summary ������������������������������������������������������������������������������������������  100 References��������������������������������������������������������������������������������������������������  102 8 Application of Energy Recovery Systems in  Various Building Types and Climatic Conditions��������������������������������  107 8.1 Application in Various Building Types��������������������������������������������  107 8.1.1 Selecting and Installing an Energy Recovery System in Residential Buildings ������������������������������������������  109 8.2 Application in Various Climatic Conditions ������������������������������������  110 8.2.1 Application in Cold Climate under Frosting and Defrosting Periods ������������������������������������������  110 8.2.2 Application in Summer and Winter Climatic Conditions��������������������������������������������������������������  113 8.2.3 Application in Tropical Climate ������������������������������������������  114 8.3 Summary ������������������������������������������������������������������������������������������  118 References��������������������������������������������������������������������������������������������������  118 Index������������������������������������������������������������������������������������������������������������������  123

About the Authors

Mardiana Idayu Ahmad  obtained her PhD in Engineering Science: Sustainable Energy Technologies at the Department of Architecture and Built Environment, Faculty of Engineering, University of Nottingham, United Kingdom, in 2011. She is currently an Associate Professor in the Environmental Technology Division, School of Industrial Technology, Universiti Sains Malaysia. Her research spans in the breadth of sustainable energy technologies and environmental management. She has always been passionate about continuing her research in a way to bridge these two fields. Her research work leads to the production of over than 100 publications nationally and internationally, including journal papers, research books, popular academic books, book chapters, conference proceedings and other publications. Saffa Riffat  is one of the world’s leading experts in sustainable technologies/eco-­ buildings. He holds the posts of Chair of Sustainable Energy and Head of Architecture, Climate and Environment Research Group at the University of Nottingham, UK. He is currently a Professor in the Department of Architecture and Built Environment, University of Nottingham, and also the President of the World Society of Sustainable Energy (WSSET). He has a wide range of experience of renewable energy/sustainable technologies, eco-cities/sustainable buildings, heat pumps/cooling systems, energy storage and heat-powered power cycles. He has published over 650 refereed papers. He has been awarded the degree of Doctor of Science (DSc) from the University of Oxford for his research contribution in the field of heat pumps and ventilation technology. He is named as the inventor on 30 International Patents.

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Chapter 1

Introduction: Energy, Green Innovation and Sustainable Future

1.1  Overview Energy is an essential commodity in human life, economic development and industrial society. Much of the world’s energy, however, is currently produced and consumed in ways that could not be sustained and significantly higher than the environmentally friendly renewable energy sources. With the increasing rate of urbanisation and escalating standard of living and quality of life, the world energy demand is predicted to increase from 50 to 80% in the next 10 years (Ahmad and Riffat, 2015). This could lead to the energy security issue which deals with timely investment to supply energy in line with economic development and sustainable environmental demand. Besides, for the last two decades, primary energy and carbon dioxide (CO2) emission have grown significantly by 49 and 43%, respectively (Tang and Tan, 2015) which linked to the worsening of the climate change indicators. Driven by higher energy demand in 2018, global energy-related carbon dioxide emissions rose 1.7% to a historic high of 33.1 Gt carbon dioxide (IEA, 2018). In building sector, for instance, as projected in The fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC), building-related greenhouse gas emissions had reached about 8.6 billion metric tons (t) CO2 equivalent (e) in 2004, and anticipated to increase to 26% by 2030, reaching 15.6 billion t CO2e under their high-growth scenario. In order to tackle this problem, sorting the issue of energy which in turn is connected to development should be considered. Technical progress, capital provision, wide range of energy sources and carriers that provide energy services to offer long-term security of supply for minimising the environmental consequences and sustainability of the whole system are the important elements to be taken into account. The question is how reliable, clean and sustainable energy sources at an affordable price to maintain national power can be secured?

© Springer Nature Switzerland AG 2020 M. I. Ahmad, S. Riffat, Energy Recovery Technology for Building Applications, https://doi.org/10.1007/978-3-030-50006-1_1

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1  Introduction: Energy, Green Innovation and Sustainable Future

1.2  Energy and Sustainable Development Goals In the year 2012, the United Nations General Assembly had designated the year as the International Year of Sustainable Energy for All. Sustainable Energy for All was formally launched as an initiative in September 2011. This was in recognition of the increasing importance of energy towards economic development and mitigation of climate change. It was then declared 2014–2024 as the Decade of Sustainable Energy for All (SE4All Decade). The Sustainable Energy for All Initiative has identified three inter-linked objectives to be realised by 2030 and pursued during the SE4All Decade, gearing towards sustainable development pertaining to energy accessibility. The three goals are: (1) to ensure universal access to modern energy services; (2) to double the rate of improvement in energy efficiency; (3) to double the share of renewable energy in the global energy mix. As a continuality from this, the United Nation (UN) Sustainable Development Summit in September 2015 adopted the 2030 agenda, which spells out 17 goals the so-called sustainable development goals (SDG) aiming to have international efforts for sustainable development until 2030—“sustainable development that meets the needs of the present without compromising the ability of future generations to meet their own needs”—1987s Brundtland Report (WCED, 1987). Under the SDG 2030 agenda, one goal which is SDG7 highlights specifically on ensuring access to affordable, reliable, sustainable and modern energy for all. Access to affordable, reliable and sustainable energy is vital in achieving many of other sustainable development goals in relation to poverty eradication through advancements in health, education, water supply and industrialisation to mitigating climate change. The SDG7 have listed the following targets to be achieved by 2030 (UNDP, 2019): 1 . Ensure universal access to affordable, reliable and modern energy services. 2. Increase substantially the share of renewable energy in the global energy mix. 3. Double the global rate of improvement in energy efficiency. 4. Enhance international cooperation to facilitate access to clean energy research and technology, including renewable energy, energy efficiency and advanced and cleaner fossil-fuel technology, and promote investment in energy infrastructure and clean energy technology. 5. Expand infrastructure and upgrade technology for supplying modern and sustainable energy services for all in developing countries, in particular least developed countries, small island developing states and land-locked developing countries, in accordance with their respective programmes of support. In addition, SD9 and SD13 also emphasise on the energy-related area gearing towards sustainability and decarbonisation in economic and industrial development. SD9 focuses on building resilient infrastructures and sustainable industrialisation as well as green innovation and; SDG13 relates to mitigation and adaptation efforts of climate change. All of these SDG goals should be approached in an integrated manner without isolating each other in order to ensure their interconnectedness for sustainability is achieved.

1.3  Green Innovation: Solution to Carbon Dioxide Emissions?

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In materialising these SDG goals, in recent years, almost all sectors within environmental and energy community from transportation, agriculture, waste management, industries, manufacturing to buildings are aiming to have “a more sustainable future”. Actions and strategies to promote clean energy and more sustainable approach for the conservation and management of natural resources are becoming one of the main focuses within these sectors. The commitment of industry along with government and other stakeholders is vital in shaping a better sustainable future that is decarbonised, affordable and secure. Redoubled efforts will be required, particularly for countries with high-energy usage and huge energy access deficits.

1.3  G  reen Innovation: Solution to Carbon Dioxide Emissions? There is an urgent drive to promote clean energy and environmentally sustainable approach to address climate change and other environmental issues towards decarbonisation. There is no doubt that climate change is one of the utmost challenges of the world today. The concentration of carbon dioxide emissions in the atmosphere has skyrocketed since the last 150 years, which gives impact on the rising of earth’s average temperature. Further, economic growth has been related to about 40% growth in carbon dioxide levels in the last two decades and only a few countries have successfully managed to reduce their emissions during this period (IEA, 2018). Carbon dioxide emissions and the improvement of environmental efficiency in relation to climate change and greenhouse gas emissions have become crucial issues across the world. In dealing with this issue, United Nations Framework Convention on Climate Change (UNFCC) established international policy goals in 1992. As a result of this, to date, 192 nations have adopted the UNFCCC goal of “stabilisation of greenhouse gas concentrations in the atmosphere at a level that would prevent dangerous anthropogenic interference with the climate system”. One of the key elements in achieving this is reducing energy consumption in all sectors while developing more sustainable approaches which have the potential to reduce carbon dioxide emissions. Studies agree that the potential for reducing carbon dioxide emissions is very promising using sustainability-oriented innovation or green innovation towards low carbon and circular economy. “Green innovation” is often used interchangeably eco-innovation, environmental innovation or sustainable innovation and is often connected with environmental technology, eco-efficiency, eco-design, low carbon technology, sustainable design, environmental design or energy-efficient technology. It is the development of products or processes, which includes a variety of ideas and concepts from environmentally friendly technological advances to socially acceptable innovative and inventive paths that contribute to sustainable development. In simple word, it refers to innovative products or processes that are able to reduce environmental impacts. Therefore,

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it is essential to develop modern, innovative and ecological technological solutions, including in the fields of energy and building technologies.

1.4  Summary Ensuring access to affordable, reliable, sustainable and modern energy for all as well as reducing global carbon dioxide emissions have become most of the countries agenda across the world. Strategies and solutions should be considered to enable countries to leapfrog to cleaner emissions using sustainability-oriented innovation or green innovation towards low carbon and circular economy. The 2030 Agenda on Sustainable Agenda has emphasised these as main key components, which should be tackled urgently on a global scale. Acknowledgements  Trans-disciplinary Research Grant Scheme of the Ministry of Education (TRGS-KPM) 203/PTEKIND/67610003.

References Ahmad, M. I., & Riffat, S. B. (2015). Building energy consumption and carbon dioxide emissions: Threat to climate change. Journal of Earth Science and Climatic Change, S3, 001. https://doi. org/10.4172/2157-7617.S3-001 IEA. (2018). Global energy & CO2 status report 2018. The latest trends in energy and emissions in 2018. Retrieved from https://webstore.iea.org/global-energy-co2-status-report-2018 Tang, C.  F., & Tan, B.  W. (2015). The impact of energy consumption, income and foreign direct investment on carbon dioxide emissions in Vietnam. Energy, 79, 447–454. https://doi. org/10.1016/j.energy.2014.11.033 UNDP. (2019). Transforming our world: the 2030 Agenda for Sustainable Development. Retrieved from https://sustainabledevelopment.un.org/content/documents/21252030%20Agenda%20 for%20Sustainable%20Development%20web.pdf WCED. (1987). Our common future. World commission on environment and development. Retrieved from https://sswm.info/sites/default/files/reference_attachments/UN%20WCED%201987%20 Brundtland%20Report.pdf

Chapter 2

Building Energy Consumption and Energy Efficiency Strategies

2.1  Energy and Buildings Buildings are one of the built environment forms, founding a necessary base or core of a country’s culture and heritage. In general, they have the lifespan of about 60–120 years with various functions, shapes and sizes – either conventionally built or modernist-typed buildings. Throughout the world, the building sector is growing at unprecedented rates approximately 30–40% of total global basic resources (Cao et al., 2016). This sector can be grouped into residential and non-residential. Within these two groups, they are categorised into several types based on their typologies: medium-rise, low-rise, seascraper, skyscraper, shed and folly. They also can be classified into two main classifications, which are based on occupancy and type of construction. Under the classification of occupancy, they can be categorised into agricultural buildings, commercial buildings, residential buildings, educational buildings, government buildings, industrial buildings, military buildings, religious buildings, transport buildings and power plants. Meanwhile, based on the type of construction, buildings are classified into five categories: five resistive buildings, non-combustible buildings, ordinary buildings, heavy timber buildings and wood-­ framed buildings. All these building categories serve several societal needs of the human habitat and are much more than a thermal envelope that transmits or conserves energy or a locus for indoor energy consumption (Ekins and Lees, 2008). Therefore, a good understanding of the buildings and energy are linked to each other is crucial for establishing adequate mitigating strategies for the building sector in the future. Throughout the literature, there are a lot of reports that can be accessed, discussing this interconnectedness which provides a glimpse into the future energy of buildings (Allouhi et al., 2015; Amasyali and El-Gohary, 2018). From these reports, energy in buildings can be categorised into two types of services which are: 1. Energy for the maintenance or servicing of a building during its estimated lifespan (relates to building services which are the systems installed in buildings for © Springer Nature Switzerland AG 2020 M. I. Ahmad, S. Riffat, Energy Recovery Technology for Building Applications, https://doi.org/10.1007/978-3-030-50006-1_2

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the purpose of comfortable, safety, functional and efficient). These include building control systems, energy distribution and energy supply either through non-­ renewable or renewable sources. 2. Energy capital that goes into production of a building (embodied energy) using various building materials; which means the total energy required or portion that would be consumed associated with the production of a building including processing, extraction, manufacturing and delivery and administrative function. These denote that energy is one of the important components and is consumed within the life cycle of a building, from the production/construction, operational, maintenance and even disposal phase. Energy consumption in buildings can be described in terms of primary energy or delivered energy. Primary energy is that which is found within the fossil fuels in the ground. Delivered energy is that which is delivered to the building. When energy is consumed, CO2 is produced and thus these energy categories in buildings can be considered as the indicator of the environmental impact of building materials, services and systems related to greenhouse gas (GHG) emission or carbon footprint.

2.2  Building Energy Consumption Over the past decade, building energy consumption continues to rise driven by urbanisation, population growth and improved living quality. Besides, the more stringent demand for better indoor environmental quality and building functions lead to increased energy consumption. The building energy consumption currently accounts for about one-third (approximately 30–36%) of the total primary energy consumption worldwide and contributes to about 40% of total direct and indirect CO2 emissions (IEA, 2018). This sector has been reported as the biggest single energy consumer amongst the other sectors in Energy Perspective, 2017 (Fig. 2.1).

Transport 28%

Buildings 30%

Other 5% Other industry 31%

Construction industry 6%

Fig. 2.1  Global final energy consumption by sector (Adapted from IEA, 2017)

2.3  Energy Efficiency Strategies in Buildings

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Within the buildings, the top 5 energy-consuming building types consume nearly half of the energy consumed by all the non-residential buildings (IEA, 2017). These include mercantile and service (15%); office (14%), education (10%); health care (8%) and lodging (6%). Without current changes in practices, the increasing trend of energy consumption in buildings is forecasted to increase in 10 years (Allouhi et al., 2015). Major factors that influence building consumption. People typically spend a great majority of their time up to 85–95% in an indoor environment (Zomorodian et al., 2016). Good indoor environment is defined by several parameters in terms of thermal comfort (thermal environmental conditions), indoor air quality, acoustic comfort and visual comfort. Hence, good indoor environment in buildings is an important component as it provides comfort, health and well-being of occupants. Thus, to fulfil the needs of good indoor environment for occupants, large energy load is imposed on buildings. From the building energy consumption percentages, parts of the major energy consumption (nearly half percentages of the total building energy consumption) in buildings are from the mechanical ventilation systems in terms of heating, ventilation and air-conditioning (HVAC). Some parts of the world highlight it as mechanical ventilation, air-conditioning (MVAC) systems. The systems are indoor climate controls that regulate humidity and temperature to provide comfortable, healthy indoor environment and adequate ventilation of buildings. It is predicted that the energy consumption of building HVAC/MVAC systems will continue to take a large proportion of the total building energy consumption. The energy consumption for an enclosed space heating or cooling to maintain thermal comfort accounts 60–70% of the total energy (Yang et al., 2014). On the other hand, about 9–10% of the building energy consumption is contributed from lighting systems (IEA, 2017). Therefore, concerning the extensive need for indoor environmental comfort and growing pressure on the CO2 emissions, seeking for routes to reduce fossil fuel consumption and increase utilisation of low-carbon energy technologies for mechanical ventilation systems, lighting systems and other building services are of particular importance. In addition, significant energy savings can be achieved in building sector by the implementation of better energy efficiency strategies through building design, operation, services and practices. On average, approximately 25–35% of the energy delivered to buildings is lost through departing ventilation air streams. In more modern buildings, the proportion of airborne energy loss can be even greater due to the higher standards of thermal insulation. Mechanical extract ventilation systems can account for a significant proportion of overall building heat loss or gain.

2.3  Energy Efficiency Strategies in Buildings In recent years, the march towards net-zero energy buildings, energy efficiency and sustainability become the priority agenda of building sector across the world. Pertaining this, most countries have shown their commitments by setting up new

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building standards, codes, policies, regulations and guidelines. For instance, in the EU, the Energy Performance of Building Directive (EPBD) was adopted in December 2002 to improve the energy performance of buildings which stresses on the development of a common framework for the potential energy savings in the buildings sector throughout Europe. In this policy, all new buildings must be nearly zero-energy buildings (ZEBs), which means these buildings have very high-energy performance with very low amount of energy required covered by renewable sources. Meanwhile, in the United Kingdom, Building Regulations, a standard for limiting heat gains and losses through elements and other parts of the building services, was developed in October 2010. There are also several advanced building design standards such as ECOhomes (BRE, UK), Passivhaus (Germany), AECB (UK) and LEED (USA). In Japan, Basic Energy Plan (BEP) was adopted in June 2010, which represents the significant statement of Japanese energy policy and energy crisis. In New Zealand, under the Building Act 204, building codes are highlighted which set out the rules for the construction, alteration, demolition and maintenance of new and existing buildings in New Zealand. While in Australia, building codes are mandatory under the Greenhouse and Energy Minimum Standards (GEMS) Act 2012. In Singapore, codes of practice for energy efficiency in buildings are regulated under the Energy Market Authority (EMA). MS 1525, Code of Practice on Energy Efficiency for Non-Residential Buildings, United Nation Development Programme (UNDP) Malaysia Building Sector Energy Efficiency Project and National Energy 1979 have been formulated in Malaysia with the principal energy objectives to ensure efficient, secure and environmentally sustainable supplies and practices of energy, including electricity. On the other hand, great attention has also been paid to the energy efficiency of buildings in China. The Ministry of Construction of the People’s Republic of China published “Public Building Energy Efficiency Design Regulation” in order to improve the energy efficiency design standard with the aim to improve the energy efficiency of public buildings by 50%. In addition to this, China’s building energy codes for residential and non-residential types have been established since 2015 which set the minimum standards for the energy efficiency of building components including envelope, heating, ventilation, HVAC/MVAC systems and the power system. These codes are mandatory for residential and commercial buildings in urban areas, while compliance with rural residential building codes is promoted through incentives. Energy Performance Norm (EPN) has been introduced in the Netherlands, a standard which is used to express the energy efficiency of a building in the number of energy performance coefficient. All these policies, standards and rating tools would encourage the building design and services to adopt energy-efficient building materials, technologies and at the same time ensure that adequate means of ventilation are provided towards the demand of energy savings. Besides, various technical strategies are adopted for energy conservation in buildings through low carbon technologies and they play a vital role in the move towards a green energy. “Low carbon technologies” is a term given to technologies that have a minimal output of GHG emissions into the environment biosphere, but specifically refers to the GHG CO2 (Tan et al., 2016). It means such technologies

2.4  Overview of Energy Recovery Technology as Green Innovation…

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have the potential to reduce the carbon intensity of processes at every stage of the energy supply chain to end-user efficiency (Tan et al., 2016). In this concept, it takes into account the energy efficiency of a system in terms of ratio between the energy input and the useful energy output considering the form of primary energy source such as fossil fuels. The examples of such technologies include renewable energy technologies using renewable energy sources, evaporative cooling, passive ventilation systems, passive cooling systems, solar photovoltaic, dehumidification system and energy recovery systems. Using low-carbon technologies, building energy consumption could be reduced up to 20–30%. Furthermore, implementation of these technologies in buildings will be a substantive contribution to the mitigation of global warming in the coming decades and improve energy conservation. However, low-carbon design can only be achieved successfully through careful design of built form and services using renewable or low-energy sources such as wind, water, solar energy and passive solutions.

2.4  O  verview of Energy Recovery Technology as Green Innovation Towards Integrated Energy Efficient in Buildings The escalating fuel costs and the increasing of stringent standards towards comfortable indoor environment require the energy consumption of buildings to be dealt efficiently. In dealing with these issues, the usage of energy recovery is essential. The application of energy recovery technology in buildings provides the most effective way to recycle waste energy and create comfortable indoor environments. It comes into widespread use in various climatic conditions across the world (Ahmad et  al., 2016; Al-Waked and Nasif, 2019). It is scientifically known as one of the energy-efficient technologies, which has the capability to reduce the power demands of building mechanical ventilation systems in terms of heating, cooling, air conditioning and ventilation loads. Figure 2.2 illustrates the link between building energy consumption, energy recovery systems and energy-efficient technologies in building services. The energy recovery technology offers energy savings to the mechanical ventilation systems and simultaneously provides better air quality to the indoor spaces. It has been reported and projected that the CO2 emission generated from the combustion of fossil fuels will continue to rise more than 40 billion metric tonnes by 2030 (IEA, 2017). By saving the energy, the energy recovery technology is eventually minimising the CO2 emission (Moffitt et al., 2012). Throughout the open literature, a plethora of research activities has been devoted to the understanding of energy recovery technology in buildings of various climatic conditions. This technology could reduce building energy consumption in various weather seasons.

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Fig. 2.2  The link between building energy consumption, energy recovery systems and energy-­ efficient technologies in building services

2.5  Summary Increasing rate in population, rising demand for building services and the need for comfortable indoor environment, together with the upward trend of living standard and quality of life, assure the energy demand will continue to rise in the future. Improving the energy efficiency of buildings is thus important to overcome this scenario. Stronger policies, technologically and commercially viable solutions are urgently needed at regional, national and international levels in the building sector. One of the strategies is by adopting the energy recovery system, which is scientifically proven as a promising technology for energy savings in building sector. Acknowledgments  Trans-disciplinary Research Grant Scheme of the Ministry of Education (TRGS-KPM) 203/PTEKIND/67610003.

References Ahmad, M. I., Mansur, F. Z., & Riffat, S. (2016). Applications of air-to-air energy recovery in various climatic conditions: Towards reducing energy consumption in buildings. In M. I. Ahmad, M. Ismail, & S. Riffat (Eds.), Renewable energy and sustainable technologies for building and environmental applications. Cham: Springer.

References

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Allouhi, A., El Fouih, Y., Kousksou, T., Jamil, A., Zeraouli, Y., & Mourad, Y. (2015). Energy consumption and efficiency in buildings: Current status and future trends. Journal of Cleaner Production, 109, 118. https://doi.org/10.1016/j.jclepro.2015.05.139 Al-Waked, R., & Nasif, M. (2019). Air to air energy recovery from HVAC systems under different membrane materials. Universal Journal of Mechanical Engineering, 7, 37–49. https://doi. org/10.13189/ujme.2019.070202 Amasyali, K., & El-Gohary, N. M. (2018). A review of data-driven building energy consumption prediction studies. Renewable and Sustainable Energy Reviews, 81, 1192–1205. https://doi. org/10.1016/j.rser.2017.04.095 Cao, X., Dai, X., & Liu, J. (2016). Building energy-consumption status worldwide and the state-­ of-­the-art technologies for zero-energy buildings during the past decade. Energy and Buildings, 128, 198–213. https://doi.org/10.1016/j.enbuild.2016.06.089 Ekins, P., & Lees, E. (2008). The impact of EU policies on energy use in and the evolution of the UK built environment. Energy Policy, 36(12), 4580–4583. https://doi.org/10.1016/j. enpol.2008.09.006 IEA. (2017). Energy technology perspectives 2017. Retrieved from https://webstore.iea.org/ global-energy-co2-status-report-2018 IEA. (2018). Global energy & CO2 status report 2018. Retrieved from https://webstore.iea.org/ global-energy-co2-status-report-2018 Moffitt, R., Murphy, J., & Stanke, D. (2012). Air-to-air energy recovery. Trane Engineers Newsletter. https://www.trane.com/content/dam/Trane/Commercial/global/products-systems/ education-training/continuing-education-gbci-aia-pdh/Air-to-Air-Energy-Recovery/appcmc046-en_material_vod.pdf. Accessed on 20th Nov 2019 Tan, Y. C., Ismail, M., & Ahmad, M. I. (2016). Turbine ventilator as low carbon technology. In M. I. Ahmad, M. Ismail, & S. Riffat (Eds.), Renewable energy and sustainable technologies for building and environmental applications: Options for a greener future (pp. 167–174). Cham: Springer International Publishing. Yang, L., Yan, H., & Lam, J.  C. (2014). Thermal comfort and building energy consumption implications—A review. Applied Energy, 115, 164–173. https://doi.org/10.1016/j. apenergy.2013.10.062 Zomorodian, Z. S., Tahsildoost, M., & Hafezi, M. (2016). Thermal comfort in educational buildings: A review article. Renewable and Sustainable Energy Reviews, 59, 895–906. https://doi. org/10.1016/j.rser.2016.01.033

Chapter 3

Definition and Working Principle of Energy Recovery Technology

3.1  Overview By general definition, energy recovery refers to obtaining, saving or recovering energy which would otherwise be lost from waste into usable heat, electricity or fuel through a variety of processes such as combustion, gasification anaerobic digestion, pyrolisation and landfill gas recovery. It is also known as “waste-to-energy”. Simply put, it is about obtaining energy from waste. In this context, the waste can be in the forms of materials or energy or heat. It includes technique or process of reducing the input of energy to an overall system by exchanging energy from one sub-system with another. Application of energy recovery principle has been applied in various systems and processes which have an exhaust stream or waste stream which is transferred from the system to its surroundings. Some of the energy in that flow of material (often gaseous or liquid) may be transferred to the make-up or input material flow. This input mass flow often comes from the system’s surroundings, which, being at ambient conditions, are at a lower temperature than the waste stream. This temperature differential allows heat transfer and thus energy transfer, or in this case, recovery. For instance, the energy recovery principle is used in desalination processes (Kadaj and Bosleman, 2018), anaerobic digestion processes (Momayez et al., 2019), water drinking systems (van der Hoek et al., 2018) and tyre recycling processes (Shulman, 2011) and building ventilation systems (Mardiana-Idayu and Riffat, 2012). Some examples of energy recovery technology include: • • • • • • • •

Heat recovery Regenerative braking Energy recovery ventilator Energy recycling Water heat recycling Heat recovery ventilation Heat recovery steam generator Cyclone waste heat engine

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Thermal diode Thermal oxidizer Thermoelectric modules Waste heat recovery units

This chapter discusses the definition, concept and mechanism of energy recovery technology for building applications.

3.2  Definition and Concept Referring to building applications, energy recovery is defined as a process of recovering or exchanging energy in terms of heat and/or mass, from a stream at a high-­ temperature to a low-temperature stream through a heat exchanger (the heart or core of the unit). In one of the earliest studies, Shurcliff (1988) defines energy recovery as a device that removes in terms of extracts, recovers or salvages heat or mass or energy from one airstream and transfers it to another airstream. Simply, energy recovery is a process in which energy in terms of heat or moisture or both are exchanged from high temperature to low temperature (sensible heat) and/or from high moisture to low moisture (latent heat) to provide sufficient ventilation air for occupied conditions. The total of both sensible and latent heat is called as enthalpy. This process occurs in two streams which are supply and exhaust streams in which waste heat (sensible and/or latent) is recovered from one stream with high temperature or high moisture to another stream with low temperature or low moisture. In this context, to understand the principle of energy recovery, it is necessary to recognise the fundamental of thermodynamics, heat and mass transfer and the relationship between sensible heat, latent heat and enthalpy. Thermodynamics is the study of the relationship between heat (or energy) and work. In thermodynamics, heat is defined as the transfer of energy to or from a thermodynamic system. This energy can be transferred either by mechanisms of conduction, convection, radiation or combination of these. In the laws of thermodynamics, heat transmits from higher temperature to lower temperature. Therefore, by extending the laws of thermodynamics into the laws of mass transfer, moisture also moves from higher vapour pressure to lower vapour pressure (Welty et al., 2014). These are the basic phenomena underlying the operation of energy recovery system. The definition and brief explanation of several important keywords of this topic are presented in the following parts.

3.2  Definition and Concept

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3.2.1  Waste Heat Waste heat is defined as the unused heat given to the surrounding environment in the form of thermal energy that is produced by a heat engine or a machine or other process in a thermodynamic process in which it converts heat to useful work. These processes give off waste heat as a result of the laws of thermodynamics. Waste heat is found in the conservation of energy equation (Eq.  3.1) for a heat engine (Dunlap, 2014).



QH = QL + W

(3.1)

where QH is the input heat to the system from a given fuel; W is the useful mechanical work attained from the system and QL is the waste heat. By knowing the input heat, and the work output by the system, the waste heat is simple to find. The thermal efficiency (η) which describes the ratio of the useful work to input energy can be found in terms of this waste heat, by Eq. 3.2:

η = 1−

QL QH

(3.2)

In the second law of thermodynamics, limitations of heat transfer are described in which it puts forward the idea that heat cannot be entirely converted into mechanical energy but waste heat must be produced. This can be explained by the Waste Heat Statement which formally known as the Kelvin-Planck Statement (Rathakrishnan, 2005). It states that “It is impossible to extract an amount of heat, represented as QH, from a hot reservoir and use it all to do work”. It implies that it is not possible to build a heat engine with 100% thermal efficiency but it must exchange heat with low-temperature heat sink as well as a high-temperature source to complete the thermodynamic cycle. In this case, the waste heat can be used by another process or a portion of heat that would otherwise be wasted can be reutilised in the same process if make-up heat is added to the system. Waste heat can be recovered either directly or indirectly. Direct recovery of waste heat is through recirculation (without heat exchanger) which is known as cheaper option but depending on location and restricted by contamination considerations. Meanwhile, indirect recovery of waste heat is using heat exchanger involving two fluid streams, which are separated by a heat transfer surface. The amount of available waste heat can be calculated from the following basic energy flow rate Eq. 3.3:

Q = SV ρCP ∆T

(3.3)

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where Q  = energy flow rate (kW); S = cross-sectional area (m2); V = flow velocity (m/s); ρ  =  fluid density (kg/m3); Cp  =  fluid specific heat capacity (kJ/kg  K) and ΔT = temperature difference between heat source and heat sink. According to literature, a significant amount (about one third) of energy produced by human activities is squandered as waste heat which contains valuable energy content (Inayat and Raza, 2019). This waste heat can be lost to the surrounding environment (atmosphere, large bodies of water) at all stages of the process such as through inefficient generation, transmission or final use of that energy. Figure 3.1 shows waste heat generated by a thermodynamic process that would otherwise be wasted can be reutilised to heat buildings. The application of waste heat in buildings has been observed for more than a century. It was started when Thomas Edison developed a commercial power plant in the United States and then he sold its steam to heat nearby buildings (Hughes, 1987). Some sources of waste heat in buildings are ventilation system extracts; boiler flue gaseous; air compressors; refrigeration plant; high-temperature exhaust gas streams from furnaces, kilns, ovens and dryers; hot liquid effluents; power generation plant; process plant cooling systems. Recently, in the effort to reduce carbon dioxide emissions and climate change, processes that utilise waste heat for other needs are becoming more relevant. With this regard, various sectors are exploring innovative technologies to capture and reuse this waste heat as renewable energy source.

Source Heat in QH

Waste heat QL

Work out W

Useful waste heat

Surrounding environment Fig. 3.1  Waste heat generated by a thermodynamic process that would otherwise be wasted can be reutilised to heat buildings

3.2  Definition and Concept

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3.2.2  Heat Transfer Heat transfer describes the flow of thermal energy (heat) due to temperature differences and the subsequent temperature distribution and changes between physical systems. Heat can be transferred by three methods: conduction in solids, convection of fluids (liquids or gases) and radiation through anything that will allow radiation to pass. Without external help, heat will always flow from hot objects to cold ones which is a direct consequence of the second law of thermodynamics. Heat transfer is transmission of thermal energy due to a gradient in temperature.

3.2.3  Mass Transfer Mass transfer is the net movement of mass from one location, usually meaning stream, phase, fraction or component, to another. Mass transfer may take place in many processes, such as absorption, evaporation, drying, precipitation, membrane filtration and distillation. Mass transfer may take place in many processes, such as absorption, evaporation, drying, precipitation, membrane filtration, either in a single phase or over phase boundaries in multiphase systems. It involves at least one fluid phase (gas or liquid), although it may also be described in solid-phase materials.

3.2.4  Sensible Heat Sensible heat is the amount of heat exchanged or absorbed or lost by a body or a substance or a thermodynamic system in which the exchange of heat changes the temperature of the body or the substance of the system without changing its phase or other variables such as volume or pressure. Simply, it is literally the heat that can be felt and can be measured using a thermometer. It is evidenced by its temperature. For instance, when heat is added to steel, the temperature of the steel increases and the increase of temperature can be measured. The general equation of sensible heat is expressed in Eq. 3.4.



 P ∆T Qsen = mC

(3.4)

where Qsen = sensible heat, Btu/h (kcal/h); m  = mass flow rate, lb/h (kg/h); Cp = specific heat of the fluid, Btu/lb-°F(kcal/kg-°C), ΔT = temperature difference/change for a stream °F (°C).

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Temperature (°C)

Phase change

Phase change

condensation

gas (water vapour)

evaporation freezing

liquid (water)

melting

solid (ice)

Energy (kJ/mol)

Fig. 3.2  Latent heat is the heat that changes the phase or state of a body or a substance or a system

3.2.5  Latent Heat Latent heat is thermal energy released or absorbed, by a body or a substance or a thermodynamic system, during a constant-temperature process. Latent heat can be understood as heat energy in concealed form which is supplied or extracted to change the phase or state of a body, a substance or a thermodynamic system without changing its temperature system. Simply, it is the heat required to convert a solid into a liquid or vapour, or a liquid into a vapour, without change of temperature. For instance, this heat contained in the airborne moisture (the energy absorbed when water is evaporated and stored in the resultant steam). The higher the room humidity, the greater the latent heat present. Latent heat cannot be measured by a thermometer. In order to have a better understanding of the term, let us consider the following example as illustrated in Fig. 3.2. Heat is added to extremely cold ice. The temperature is observed to go up which describes its sensible heat. However, when it begins to melt or evaporate, the heat is now described as latent heat (is represented by the flat parts of the line, during melting or evaporation). In this situation, adding heat to water can either raise the temperature of the water or change its phase. The heat that changes the temperature is called as sensible heat and the heat that changes the phase is called as latent heat.

3.3  Working Principle

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3.2.6  Enthalpy Enthalpy.is defined as a thermodynamic quantity equivalent to the total heat content of a system. The term was coined in the early twentieth century, in analogy with the nineteenth century by Thomas Young (Welty et al., 2014). Enthalpy is the total of both the sensible and latent heat, that is, the total heat contained in air. In other word, it is the heat content of a system. The total enthalpy of a system cannot be measured directly; the enthalpy change which is the heat that passes into or out of the system during a reaction of a system is measured instead. Enthalpy change is defined by the following Eq. 3.5.

∆H = H F − H I

(3.5)

where ΔH is the enthalpy change, HF is the final enthalpy of the system and; HI is the initial enthalpy of the system.

3.2.7  Ventilation Air Ventilation air is a process of introducing outside air into occupied spaces for dilution of indoor pollutants. It reduces occupant discomfort and complaints. A well-­ designed and ventilated area will result in high levels of human productivity and health.

3.3  Working Principle In building sector, energy recovery technology refers to a system or a device or mechanical equipment that features a heat exchanger combined with a ventilation system to provide controlled ventilation into a building which is known as energy recovery ventilator (ERV) or energy recovery unit (ERU) or energy recovery system (ERS). In some parts of the world, for commercialisation purpose, it is called as heat recovery ventilator (HRV) or heat recovery unit (HRU). The difference between ERV/ERU/ERS and HRV/HRU is commonly HRV refers to a device that is only able to transfer sensible heat without the ability to transfer latent heat. This can be generally distinguished by their heat exchanger units. However, if we look into the theoretical understanding of heat in terms of sensible and latent heat as types of energy released or absorbed in a system, both ERV and HRV can be used interchangeably and both thermodynamically illustrate the working principle of energy recovery system. Energy recovery systems have been proven as effective and economical to be operated within mechanical ventilation systems of a building. They are utilised in a

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3  Definition and Working Principle of Energy Recovery Technology

wide array of building sector either in residential and non-residential including industries, offices and commercials. As discussed earlier, energy recovery systems work based on the principle to recover a fraction of waste energy or energy loss. These systems recycle waste energy in terms of heat from the buildings’ exhaust air (indoor air) to pre-treat or pre-condition the entering outdoor (fresh air) ventilation air in commercial and residential heating, ventilation, air-conditioning (HVAC) systems. They allow the fresh air to be circulated within the indoor and outdoor air while maintaining the building’s temperature (Mardiana-Idayu and Riffat, 2011). The pre-treating or pre-conditioning of outdoor air will decrease the power demands of building HVAC loads, thus reduces energy consumption and improves the building efficiency by approximately 60–95% (Attia and Carlucci, 2015) depending on their system configurations. In addition, these systems also allow the indoor environment of buildings to maintain relative humidity of around 40–50%. The systems are capable to remove excess humidity or add humidity to the ventilating air that is being brought into the buildings. In typical situation, when outside air is cooler than inside air, heat exchanger of the system exchanges heat (sensible and/or latent) from the warmer exhaust air to the cooler supply air taken from outside. With increased temperature, fresh supply air is ventilated to the building. During hot days, warmer supply air exchanges heat (sensible and/or latent) to the exhaust air and can thus help to cool down and dehumidify the building interiors. This technology offers energy savings without neglecting the needs of indoor environment comfortable level. The only energy penalty is the power needed for the blower to overcome the pressure drop in the system. There are two main categories of energy recovery systems that are available for transferring energy from the exhaust air to the supply air and vice versa as reported in the literature (Mardiana-Idayu and Riffat, 2012). These categories are sensible and enthalpy recovery systems (Dieckmann et al., 2003). The heat transfer surfaces in sensible energy recovery system can only transfer sensible heat between the makeup and exhaust air, while in the enthalpy recovery system, it can transfer both sensible and latent heat. However, enthalpy recovery system can be more expensive and requires greater maintenance. These systems are developed either as a stand-­ alone unit or an integrated unit. The integrated unit is designed by combining a stand-alone unit with existing ventilation systems or building technologies towards the development of low-carbon systems (Mardiana and Riffat, 2013). It can be combined with various systems such as mechanical ventilation systems, passive ventilation systems, air-conditioning systems, dehumidification systems, solar photovoltaic systems and other sustainable energy technologies which are further discussed in Chap. 7. In general, typical components of a stand-alone energy recovery unit consist of several equipment namely the ducting system for supply airstream (from fresh outdoor air) and exhaust airstream (from indoor air); blower fans, one is to exhaust stale air and another one is to supply fresh air via the heat exchanger core; and a heat exchanger. The heat exchanger is the core of the system where the energy or heat and/or mass are transferred from one stream to another stream. Further explanation on the heat exchanger, the core of energy recovery system, is presented in Chap. 4.

21

3.3  Working Principle

Fresh intake air

Expelled air

Energy recovery unit

Supply air

Return air

Supply air

Return air

Fig. 3.3  Typical a stand-alone energy recovery system in building

They are often installed in a roof space or within the building structure such as walls, to recover heat from one stream before it is discharged to the other stream. Typical installation of energy recovery system in building is illustrated in Fig. 3.3. In an advance design of this system, sometimes the incoming air is filtered to reduce the incidence of pollen and dust while the outgoing air is filtered to protect the heat exchanger and internal components. This system lowers the enthalpy of the building supply during hot days and raises it during cold days by transferring energy between supply and exhaust streams. In other words, it is based on heating or cooling the incoming air via the recovered waste heat and hence, decreasing the heating or cooling loads. In the heat exchanger, the supply airstream is automatically pre-heated or pre-­ cooled (depending on the season or climatic condition) by the exhausted air and distributed to the interior part of the building. The outgoing and incoming air pass through next to each other but do not mix in the heat exchanger. Figure 3.4 illustrates the working principle of energy recovery system between two airstreams.

22

3  Definition and Working Principle of Energy Recovery Technology Expelled air Heat transfer surface

2 °C Fresh intake air 0 °C

18 °C Supply airstream

20 °C

Exhaust airstream 18 °C

Temperature

2 °C Temperature difference 0 °C Length

Fig. 3.4  Mechanism of energy recovery systems between two airstreams

3.4  Summary Energy recovery technology for building applications works based on the principle to recover a fraction of waste energy or energy loss. This technology recycles waste energy in the form of heat from the buildings’ exhaust air (indoor air) to pre-treat or pre-condition the entering outdoor (fresh air) ventilation air while maintaining the building temperature. This process is called “energy recovery” or “heat recovery”. There are two main categories of energy recovery technology which are sensible and enthalpy recovery systems. It can be developed as a stand-alone unit or as an integrated unit by incorporating with existing ventilation systems. This technology can provide better indoor air quality and reduce energy consumption in buildings. Acknowledgements  Universiti PTEKIND/8014124).

Sains

Malaysia

Research

University

Grant

(1001/ ­

References Attia, S., & Carlucci, S. (2015). Impact of different thermal comfort models on zero energy residential buildings in hot climate. Energy and Buildings, 102, 117–128. https://doi.org/10.1016/j. enbuild.2015.05.017

References

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Dieckmann, J., Roth, K. W., & Brodrick, J. (2003). Air-to-air energy recovery heat exchangers. ASHRAE Journal, 45, 57–58. Dunlap, R. A. (2014). Sustainable energy (SI ed. 1st ed.) Boston, MA: Cengage Learning. Hughes, Thomas P. (1987). The evolution of large technological systems. In The social construction of technological systems. New directions in the sociology and history of technology, edited by W. E. Bijker, T. P. Hughes and T. Pinch. Cambridge, Massachusetts & London, England: MIT Press, 51–82. Inayat, A., & Raza, M. (2019). District cooling system via renewable energy sources: A review. Renewable and Sustainable Energy Reviews, 107, 360–373. https://doi.org/10.1016/j. rser.2019.03.023 Kadaj, E., & Bosleman, R. (2018). Chapter 11—energy recovery devices in membrane desalination processes. In V.  G. Gude (Ed.), Renewable energy powered desalination handbook (pp. 415–444). Oxford: Butterworth-Heinemann. Mardiana, A., & Riffat, S.  B. (2013). Review on physical and performance parameters of heat recovery systems for building applications. Renewable and Sustainable Energy Reviews, 28, 174–190. https://doi.org/10.1016/j.rser.2013.07.016 Mardiana-Idayu, A., & Riffat, S. B. (2011). An experimental study on the performance of enthalpy recovery system for building applications. Energy and Buildings, 43(9), 2533–2538. https:// doi.org/10.1016/j.enbuild.2011.06.009 Mardiana-Idayu, A., & Riffat, S. B. (2012). Review on heat recovery technologies for building applications. Renewable and Sustainable Energy Reviews, 16(2), 1241–1255. https://doi. org/10.1016/j.rser.2011.09.026 Momayez, F., Karimi, K., & Taherzadeh, M.  J. (2019). Energy recovery from industrial crop wastes by dry anaerobic digestion: A review. Industrial Crops and Products, 129, 673–687. https://doi.org/10.1016/j.indcrop.2018.12.051 Shulman, VL. (2011). Chapter 21 - Tyre Recycling in Waste A Handbook for Management. Academic Press (pp. 297–320). https://doi.org/10.1016/B978-0-12-381475-3.10021-X Shurcliff, W. A. (1988). Air-to-air heat-exchangers for houses. Annual Review of Energy, 13(1), 1–22. https://doi.org/10.1146/annurev.eg.13.110188.000245 van der Hoek, J. P., Mol, S., Giorgi, S., Ahmad, J. I., Liu, G., & Medema, G. (2018). Energy recovery from the water cycle: Thermal energy from drinking water. Energy, 162, 977–987. https:// doi.org/10.1016/j.energy.2018.08.097 Welty, J., Gregory, L.  R., & David, G.  F. (2014). Fundamentals of momentum, heat, and mass transfer (Rev. 6th ed. pp. 768). Hoboken, NJ: Wiley.

Chapter 4

Heat Exchanger: The Heart of Energy Recovery System

4.1  Overview Heat exchangers form a vital part of many processes including energy recovery application. Heat exchanger is defined as a device that is used to transfer heat or energy between two streams. The transfer involves two or more fluids which can be single or two phases depending on the exchanger type. Heat exchangers are classified into flow configurations (such as counter-flow, cross-flow, co-current flow and hybrid flows) and construction (recuperative and regenerative). Heat exchangers have been widely utilised in both cooling and heating process within various industries and fields. Heat exchangers have a wide variety of applications in different fields of industry. There are many types of heat exchanger which can be classified according to heat transfer, number of fluids, geometry and construction type. They also can be divided into two main categories which are indirect and direct contact types (Cuce and Riffat, 2015). In indirect contact heat exchangers, hot and cold fluids are separated by a wall (heat transfer surface). In this type, no direct contact occurs between the two fluids with different initial temperatures. This heat exchanger is usually called as surface heat exchanger. In indirect contact heat exchangers, the extent of the process is limited by the surface area, and the heat transfer rate is possible through the surface. Meanwhile, in a direct contact heat exchangers, heat exchange takes place directly between hot and cold fluids. Both the hot and cold fluids flow into the same space without a separating wall (Boehm and Kreith, 1988). The heat transfer rate of direct contact heat exchangers is very high, compared to that of indirect contact heat exchangers. These types can be used in many applications which contain mass transfer in addition to heat transfer. However, the complexity of multiple components challenges their economic advantage as compared to indirect contact heat exchangers. In an energy recovery system, heat exchanger is the heart of the system in which energy or heat and/or mass is transferred from one stream to another stream. It is the © Springer Nature Switzerland AG 2020 M. I. Ahmad, S. Riffat, Energy Recovery Technology for Building Applications, https://doi.org/10.1007/978-3-030-50006-1_4

25

26

4  Heat Exchanger: The Heart of Energy Recovery System

core of the system consisting of matrix containing the heat and mass transfer areas. For energy recovery application, usually indirect contact heat exchangers are used (Cuce and Riffat, 2015). All heat exchangers work under similar basic principles namely the Zeroth, First and Second Laws of Thermodynamics irrespective of the type and design (Böckh and Wetzel, 2012). These principles describe and dictate the transference or exchange of heat and mass from one fluid to another. Therefore, understanding these fundamental principles is the most critical part in the design and development stage of energy recovery systems. Heat exchangers for energy recovery applications can be classified in several ways based on their design characteristics. These include heat and mass transfer mechanism, construction method and flow configuration. The suitability of each heat exchanger type in transferring heat between fluids is dependent on the specifications and requirements of the application. Those factors mainly determine the optimal design of the desired heat exchanger and affect the corresponding rating and sizing calculations. Some of important factors in the designing and choosing a heat exchanger are the type of fluids, the fluid stream and their properties; the desired thermal outputs; size limitations and costs.

4.2  Construction Method 4.2.1  Size, Heat and Mass Transfer Areas Typical energy recovery design aims to enhance heat and mass transfer rates and the ratio of the actual heat and mass transfer rates to the maximum possible heat and mass transfer rate while minimising cost, size, weight, pressure drop and overall thermal resistance. The heat and mass transfer rates are proportional to the heat and mass transfer surface areas, and surface area is proportional to pressure drop and initial cost. Initial cost, operational cost, maintenance components and net saving of energy recovery system significantly depend on the size of the system specifically its heat exchanger component (Nasif et al., 2010). Heat exchanger consists of heat and mass transfer areas. Heat and mass transfer areas are the heat and mass transfer surfaces that have direct contact with heat and mass transfer fluids. Within these areas, heat or energy is transferred. Therefore, physical conditions of a heat exchanger including dimensions/size (length, width, height) and surface area are pivotal to the overall efficiency and cost of an energy recovery system. Increasing heat and mass transfer area of a heat exchanger would increase the efficiency of energy recovery system. However, this would also increase bulk and cost of the system. To ensure best performance can be achieved, optimisation of heat and mass transfer area of energy recovery system is essential. Many studies have been found emphasising the optimisation of heat and mass transfer area of various energy recovery systems. For instance, an optimisation analysis was carried out by

4.2  Construction Method

27

Söylemez (2000) to estimate optimum heat transfer area for energy recovery application involving three different fixed-plate heat exchangers with different configurations (counter-flow, parallel-flow and single fluid). The results indicated that the heat transfer area and size of the heat exchanger affected the energy recovery performance in terms of efficiency. From the study, it was evident that the energy recovery efficiency increased as the heat transfer area increased. In the context of heat pipe recovery unit, size is termed by the number of rows. It was proven that as the number of row increased, the efficiency of the unit also increased (Manz and Huber, 2000). The explanation for this is with increasing number of rows, the overall heat transfer area is increased (Yua, 2001). In rotary wheel energy recovery system, the concern is addressed on the optimum rotation speed to maximise the heat transfer rate per unit area. The rotation speed is affected by channel wall thickness of the rotary wheel in which the higher the thickness, the lower the optimum speed (Niu and Zhang, 2002). The performance of rotary wheel energy recovery system is also affected by the channel wall thickness (Zeng et  al., 2017). Zhang and Niu (2002) presented a numerical simulation study of rotary wheel energy recovery system with the objective to maximise the heat transfer rate of frontal surface area of the system. Results showed that airflow with higher heat capacity rate occupied smaller frontal area. Dallaire et al. (2010) reported a study of a conceptual optimisation of rotary wheel with a numerical model. In the study, internal structure was modelled as a porous medium. Optimisation of design variables in terms of length and porosity was carried out. Results indicated that the performance of the system was influenced by design variables and the optimal values of length and porosity could be obtained. Appendages that are also known as fins can provide extended surface of a heat exchanger which directly connected to the primary surface of the heat exchanger and thereby increase the heat transfer area (Zhang, 2009a, b). The addition of appendages or fins reduces thermal resistance on fluid side and as a result net heat transfer from the surface increases. With this regard, Söylemez (2008) presented a thermoeconomic optimisation method to estimate optimum length of a finned heat pipe for energy recovery purpose by incorporating it with an integrated overall heat balance method. The combined methods were used based on fin effectiveness to compute maximum energy savings from an energy recovery system. On the other hand, it was reported in Luo et al. (2014) that efficiency of a fin-tube heat pipe type for energy recovery application of dehumidification system increased with the increase of the length in the airflow direction. In the study, it was discovered through calculation method that the best length of the airflow direction was around 0.3 m for optimisation. However, further validation is needed towards the application of the system in real practice. In addition, fins staggered arrangement also enhances the heat transfer of fluids in the heat pipe energy recovery system.

28

4  Heat Exchanger: The Heart of Energy Recovery System

4.2.2  Materials Heat exchanger materials used for energy recovery application have developed gradually along with heat transfer technologies over the past decades. High thermal conductivity and high mass diffusivity of materials are the prerequisite conditions for high performance of heat and mass exchangers used for energy recovery application (Zhang, 2009a, b; Zeng et al., 2017). Material durability is important to evaluate economic performance of energy recovery system. Design theories and fabrication methods are two important factors in the selection of material for energy recovery application. In addition, material durability is also essential to evaluate economic performance of energy recovery system. Requirements for heat exchangers for energy recovery application, such as improved efficiency cost-effectiveness and sustainability, demand reliable materials and consequently there has been a continuous search for improved materials. Heat exchangers were first constructed using metal type materials including copper, aluminium or steel (Tadrist et al., 2004). Design of classical metal heat exchangers is limited by production technology and have a number of well-known shortcomings such as high weight and cost; low resistance to fouling and corrosion. For energy recovery application in building ventilation systems, they are only capable to transfer sensible heat. With the aim to overcome fouling and corrosion, polymer-­based materials take place in the construction of heat exchangers, which was first introduced by DuPont (Githens et al., 1965). These types of heat exchangers have many advantages and have been extensively studied by researchers in a wide range of applications (Reay, 1989; Chen et al., 2016). They have low weight and cost and good chemical resistance and excellent fouling characteristics due to surface smoothness. These characteristics give a better thermal performance during operation as compared to metallic heat exchangers. In addition, as compared to regular shape of metal heat exchangers, these types of exchangers come with almost arbitrary shapes. The production and fabrication of these heat exchangers involve less energy which is approximately 2  times lower than the classical metal types, making them more environmentally attractive (El-Dessouky and Ettouney, 1999). A thorough review on the recent developments in polymer heat exchangers for various applications is reported in Chen et al. (2016). The polymer-based heat exchangers have a great potential to be used for energy recovery applications particularly with humidification and dehumidification systems (T’Joen et al., 2009). Another material that is commonly considered in the construction of heat exchangers is fibre. With hydrophilic characteristics and sufficient pore sizes, these fibre materials are capable to transfer both sensible and latent heat simultaneously (Dallaire et  al., 2010). They have lower thermal conductivity than metal heat exchangers. In addition, they have strong absorption ability with less resistance (Idicula et al., 2006). However, these types of heat exchangers are not strong enough to be constructed in a fixed-plate form. Their lifespans are also short and easily damage or deform by continuous exposure to high moisture content or being soaked with water (Fend et al., 2004). To overcome these shortcomings and gearing towards

4.2  Construction Method

29

sustainability, enhanced fibre materials incorporating polymeric matrix composites have brought new chance of production in the development of heat exchangers for energy recovery application. The combination of fibre and polymeric-based materials has overcome the drawbacks of polymer-based materials in terms of low thermal conductivities and low overall heat coefficients (Chen et al., 2016). With the aim of improved overall heat coefficients, several researches have been carried out pertaining to polymeric hollow fibre heat exchangers for various applications such as Bourouni et al. (1997); Astrouski et al. (2015) and Yan et al. (2014). With the focus to simultaneously transfer both sensible and latent heat the heat exchangers at high performance, attention has been shifted towards membrane-­ based materials. These materials are a new class of materials in the development of heat exchangers, which can be produced from organic materials such as polymers and liquids as well as inorganic materials (Kistler and Cussler, 2002). For energy recovery application, they establish a competitive technology because of high capability as total or enthalpy recovery systems which have the capability to transfer both sensible and latent heat at high-performance rate as compared to other materials (Min and Su, 2010). Due to these advantages, they have attracted much research attention in recent years (Jafarizave et al., 2019). Throughout the literature, a plethora of research and development efforts have been carried out pertaining to all of these materials for energy recovery application. Table 4.1 summarises the outcomes and conclusions from various studies of heat exchanger materials for this purpose.

4.2.3  Structures Heat and mass transfer enhancement is one of the major considerations that attract much attention these days. It is proven that enhanced surfaces are able to give higher heat transfer coefficient as compared to unenhanced surfaces (Abou Elmaaty et al., 2017). Heat and mass transfer surfaces can be enhanced by adding extended surfaces or by employing interrupted surfaces involving the structures of the surfaces. These include wick, fin and corrugated structures. 4.2.3.1  Wick Wick is one of the structures which can enhance heat and mass transfer area of a heat exchanger. It is a structure that uses capillaries to move working fluid from condenser back to the evaporator section in a heat pipe exchanger. There are various types of wick structures including axially grooves on the inner heat pipe vessel wall, screen/wire and sintered powder metal. The present of wick structures in the heat pipe exchanger will increase a surface area and thereby increases the effectiveness of the exchanger; however, variable wick does not substantially improve heat transfer (Yua, 2001; Joung et al., 2008). Figure 4.1 illustrates the wick structure in a heat

30

4  Heat Exchanger: The Heart of Energy Recovery System

Table 4.1  Summary of selected studies pertaining to heat exchanger materials for energy recovery application in buildings Metal type Manz and Experimental and numerical investigations on energy recovery (duct/heat Huber (2000) exchanger) system by adding aluminium fins on the heat transfer surfaces were carried out in this study. The results showed that by adding the aluminium fins, heat recovery up to 70% at a duct/heat exchanger length of 6 m could be achieved for building ventilation Abd El-Baky Heat pipe recovery units consisted of 25 copper tubes with evaporator and condenser sections finned with 50 square aluminium sheets of 0.5 mm thickness and were used in this work. Results denoted that aluminium fins were effective Mohamed transporters of heat from the exhaust airstream to the supply airstream of energy (2007) recovery system. From the literature, it can be concluded that the operating limitations of metal type materials in some applications have created the need to develop appropriate structures to increase heat and mass transfer as well as efficiency of exchanger for energy recovery application Polymer based Rousse et al. Theoretical and experimental analysis for using plastic heat exchanger as a (2000) dehumidifier with the aim to recovery heat/energy from ventilation system in greenhouse for agriculture industry was carried out. The heat exchanger was designed as corrugated and flexible thermoplastic drainage tubing with four thermoplastic tubes wrapped around a central tube. Results indicated that the average efficiency of 84 and 78% for air volumetric change rates of 0.5 and 0.9 change/h were obtained Kragh et al. Performance investigation of a polycarbonate fixed-plate heat exchanger for (2007) energy recovery application was carried out in this study. The performance was evaluated terms of sensible heat for cold climates. Results showed that the heat exchanger was capable of continuously defrosting itself capable of continuously defrosting itself at outside air temperatures well below the freezing point while still maintaining a high efficiency Gendebien A study was carried out on a polystyrene heat exchanger for energy recovery et al. (2013) application through modelling and experimental approaches. Thermal performance of the heat recovery device in dry conditions was presented. A comparison between prediction by the model and measurements was conducted. Results showed that the same order of magnitudes in terms of thermal performance was achieved between simulation and experimental studies Zhang (2010) An analytical solution for heat mass transfer of heat or energy recovery was studied by using a hollow fibre with a diameter of 1–3 mm and contact area between the two airstreams of 1000 m2/m3. Results of the study showed that using this fibre, the heat and moisture exchange effectiveness has the potential to attract commercial interest Zhao et al. A numerical analysis of a novel polymeric hollow fibre heat exchanger for (2013) low-temperature applications was carried out using FLUENT. The heat transfer coefficient of PP fibres was predicted to be achieved at 1109 W/m2K with inside and outside fibre diameters of 0.6 mm and 1 mm, respectively (continued)

4.2  Construction Method

31

Table 4.1 (continued) Chen et al. (2016)

Three different modules of polymer hollow fibre heat exchanger were fabricated and tested in the laboratory testing conditions in this study. The effects of various parameters on the overall heat transfer coefficient including flow rates, numbers of fibres, the effectiveness of heat exchanger, the number of heat transfer unit (NTU) and the height of transfer unit (HTU) were presented. The experimental obtained overall heat transfer coefficient and overall conductance per unit volume for the heat exchangers were compared with these of metal heat exchangers. Results indicated that the heat exchangers could offer a conductance per unit volume of 4 × 106 W/m3K, which was two to eight times higher than the conventional metal heat exchangers. It had good potential to substitute substitutes for metal type heat exchanger for energy recovery application in buildings Membrane based Zhang and A study to compare the performance of different core materials and flow Jiang (1999) arrangements of heat or energy recovery ventilator was carried out. Sensible, paper and membrane core with cross and counter-flow were compared and results indicated that the sensible core had the lowest enthalpy effectiveness, while membrane-based core with counter flow arrangement had the best performance Zhang Composite supported liquid membrane (CSLM) which used a liquid membrane (2009a, b) to selectively transfer moisture of energy recovery system was proposed in this study. In solving the weaknesses of traditional hygroscopic paper as plate and fin materials, CSLM was used as the plate material. Comparative study was also made between paper-fin and paper-plate, and paper-fin and membrane-plate. Results indicated that the latent effectiveness of the membrane-plate core is 60% higher than the traditional paper-fin and paper-plate core, due to high moisture diffusivity in the CLSM Min and Su A mathematical model was built to predict the thermal–hydraulic performance of (2010) a membrane-based energy recovery ventilator. Calculations were conducted to investigate the effects of the membrane spacing (channel height) and membrane thickness on the ventilator performance under equal fan power conditions. Results showed that for a fixed fan power, as the channel height increased, the total heat transfer rate initially increased, after reaching a maximum at a certain channel height, turns to decrease. A larger fan power leads to a larger total heat transfer rate, with the maximum total heat transfer rate occurring at a smaller channel height. As the channel height or the fan power increased, the enthalpy effectiveness decreased. When the membrane thickness increased, both the total heat transfer rate and enthalpy effectiveness decreased Hemingson A study on steady-state performance of a run-around membrane energy et al. (2011) exchanger (RAMEE) for a wide range of outdoor air conditions was presented. Results indicated that the effectiveness values were significantly dependent on outdoor conditions which result in some effectiveness values exceeding 100% or being less than 0% for several of the outdoor air conditions investigated Zhang (2012) This study presented a thorough review which covered fundamental and engineering approach on the progress of heat and moisture recovery with membranes (continued)

32

4  Heat Exchanger: The Heart of Energy Recovery System

Table 4.1 (continued) Liu et al. (2013)

Wang et al. (2015)

Kho et al. (2017)

Al-Waked et al. (2018)

In this study, poly (vinyl chloride)/sodium montmorillonite (Na+-MMT) hybrid membranes with varied Na+-MMT content were introduced using casting processes for potential use in total heat recovery ventilation systems. Even though this hybrid membrane had high potential applications in energy recovery system, this material still has some defects such as lower water vapour permeability, moisture and enthalpy exchange effectiveness compared to polymeric ionic or cellulose membranes This study demonstrated the energy-saving potential of integrating membrane-­ based energy recovery ventilators at zone level into the conventional variable air volume systems for commercial buildings. Building energy simulations were used to evaluate various operation strategies and potential energy savings for four selected climatic conditions including Minneapolis, Atlanta, Baltimore and Miami. Three cases implemented with energy recovery ventilators were simulated using EnergyPlus and their results were compared with baseline cases in compliance with ASHRAE standard 90.1. Results indicated that the annual HVAC energy savings potential could reach up to 18–49% with this technique for various climatic conditions Total energy savings potential of novel energy recovery ventilator developed with a semi-permeable membrane which could transfer both heat and moisture was investigated. A conjugate heat and mass transfer model subject to tropical climate condition was analysed by both analytical and numerical methods. The three-dimension energy recovery ventilator model was comprehensively studied by CFD simulation for analysis of critical parameters, such as velocity, temperature, humidity of supply and exhaust airflows. Results showed that both sensible and latent effectiveness could be gained very high. Even that the latent effectiveness was lower than sensible effectiveness, the energy-saving impact by dehumidification was considerable in the tropical climate Thermal performance enhancement of membrane-based energy recovery ventilators (ERV) under turbulent flow conditions is investigated utilising the computational fluid dynamics (CFD) approach. The standard k–ε model was adopted with the enhanced wall treatment option to simulate conjugate heat and mass transfer across the membrane. A user-defined function was developed and incorporated into FLUENT to simulate the heat and mass transfer processes across a variable resistance 60 gsm membrane. A mesh sensitivity analysis was conducted and the developed CFD model was validated against an in-house experimental data. The performance of the investigated ERV was tested under different number of flow channels, flow configurations, weather conditions and air flowrates. Results have shown that face velocity is more significant than flow separator in affecting the thermal performance of the investigated ERVs with a ratio of almost 5 to 1. Furthermore, the layout of the quasi-counter flow might present a preferable overall option over the L-shape hybrid flow option. The final decision would be dependent on the HVAC system in-use and the higher priority between pressure drop, thermal energy recovered, manufacturability and/or installation (continued)

4.2  Construction Method

33

Table 4.1 (continued) Jafarizave et al. (2019)

Qiu et al. (2019)

Numerical study of the new membrane shapes including curvature and circular, using COMSOL multiphysics was performed. These novel duct structures were used to improve the performance of the membrane-based heat exchanger for energy recovery application. The model considered the mechanism of heat and mass transfers for turbulent flow. The friction factor, Nusselt numbers and Sherwood numbers in different Reynolds numbers for each cycle were calculated. The model was validated with available experimental data. Results showed that the total heat exchanger with curvature duct was more effective than other heat exchangers An energy exchange efficiency prediction approach based on a multivariate polynomial regression model was proposed to predict the energy exchange efficiency of the membrane-based heat exchanger. A simplified numerical efficiency calculation model of a cross-flow air-enthalpy exchanger was trained with a small sample of experimentally measured efficiency data. Results showed that the proposed approach could predict the energy exchange efficiency with absolute deviation limits within ±8.0%. By comparing to detailed numerical calculation methods, the proposed approach required less membrane characteristic information and calculations therefore, .tit was concluded that the proposed approach could simulate the heat exchanger performance over different conditions and help sizing the heat exchanger with less requirements

ug

Modulated Wick

Pg

ul

Prototype Heat Pipe

(Pipe Inner Radius) Ls

pl (Uniform Wick)

R DR

d ws Pipe Wall

Ts,o

Le wg qe

Ts,i Evaporation

Tlg(pg)

Fig. 4.1  Schematic of wick structure of a heat pipe exchanger (Hwang et al. (2007)

34

4  Heat Exchanger: The Heart of Energy Recovery System

pipe exchanger in a study by Hwang et al. (2007). Heat and/or mass transfer in the heat exchanger can be improved by additional wick structures; however, variable wicks or their thickness do not significantly improve heat transfer. 4.2.3.2  Fin Fin structure in a heat exchanger has the similar function as wick structure, which is to increase the surface area and consequently to increase the total heat transfer rate (Picon-Nuñez et al., 1999). Fin structures often used in fixed plate heat exchangers (Abu-Khader, 2012; Niroomand et  al., 2019). There are varieties of fin types in terms of geometry shapes such as plate-fin and tube-fin. Sinusoidal plate-fins are amongst the popular structures for total or enthalpy exchangers as shown in Fig. 4.2 (Zhang, 2012). The presence of fin structures in a heat exchanger not just enhances heat and/or mass transfer but could also increase mechanical strength. The position of fin structures also gives a meaningful effect on Nusselt number as stated in Mehrizi et al. (2013). The characteristics of fin structures in terms of pitch, height and length are the important design parameters of a heat exchanger for energy recovery application. 4.2.3.3  Corrugated Corrugation has long been seen as an effective means to enhance performance or mechanical strength of materials or surfaces in engineering field. It has been exploited in various applications and academic research including heat exchanger construction. In the heat exchanger construction, corrugated structures are designed as internal channels to intensify heat and mass transfer. It poses similar geometry shape as chevron plates used in conventional metallic heat exchanger as shown in Fig. 4.3 (Zhang and Chen, 2011). In the construction of corrugated structures especially for fixed-plate heat exchanger, the following geometrical parameters are important to be considered: corrugation depth, corrugation angle and corrugation

Plates

Fins

Fig. 4.2  Sinusoidal plate-fins are amongst the popular structures for total or enthalpy exchangers (Zhang, 2012)

35

4.4  Heat and Mass Transfer Mechanism

z

β

x

Fig. 4.3  Schematic of the section in the cross-corrugated exchanger (Zhang and Chen, 2011)

depth (Abou Elmaaty et al., 2017). These parameters have significant impact on the heat and mass transfer of the heat exchangers. These structures have efficient heat exchange capabilities even with very minimum wall thickness (Zhang and Chen, 2011). Numerous studies have been carried out to investigate the performance of heat exchangers with corrugated structures for energy recovery application such as Okada et  al. (1972); Focke et  al. (1985); Stasiek et  al. (1996); Islamoglu and Parmaksizoglu (2004); Abou Elmaaty et al. (2017) and Al-Zahrani et al. (2019).

4.3  Flow Configuration In the previous sections, a review of material types in the construction of heat exchangers for energy recovery applications is presented. This section emphasises on flow configuration of heat exchangers. Flow configuration is also known as flow arrangement. It refers to the direction of movement of the fluids within the heat exchanger in relation to each other. Flow configurations have a direct impact on the efficiency or effectiveness of heat exchanger (Kandlikar and Shah, 1989). There have been many studies concerning flow configurations in heat exchangers for energy recovery applications (Mardiana and Riffat, 2013). There are four principal flow configurations employed by heat exchangers: co-current flow, counter-current flow, cross-flow and hybrid flow (Table 4.2).

4.4  Heat and Mass Transfer Mechanism There are two types of heat transfer mechanisms employed by heat exchangers: 1. Single-phase

36

4  Heat Exchanger: The Heart of Energy Recovery System

Table 4.2  Flow configurations employed by heat exchangers: co-current flow, counter-current flow, cross-flow and hybrid flow Flow configuration Co-current flow

Description Co-current flow is also referred to as parallel flow. Within this flow configuration, working fluids are moving in parallel with the same direction as each other. This configuration typically has low efficiency as compared to cross-flow and counter-flow. This flow configuration is not commonly used for energy recovery application Typical efficiency: Less than 40% In cross-flow, working fluids flow perpendicularly to one another. The efficiency of heat exchangers which employ this flow configuration falls between that of counter-flow and co-current flow heat exchangers Typical efficiency: 40–80%

Cross-­flow

Counter-­flow

Hybrid flow Hybrid flow cr flo oss zo w ne

sos cr w flo ne zo Counter-flow zone

Counter-flow is arranged by taking into account that the working fluids are moving in the opposite direction to each other. It is the most common configuration that is employed in heat exchanger. It exhibits highest efficiency, high heat and/or mass transfer between fluids and hightemperature change as compared to co-current flow and cross-flow. Typical efficiency: more than 80–95% Hybrid flow exhibits some combination of the characteristics of co-current flow, cross-flow and/or counter-flow. Hybrid flow is typically used to accommodate the limitation of an application (such as space, costs or temperature requirements). It is also used to give a better performance of a heat exchanger. Typical efficiency: 80–95%

Related studies Kim (2019) Yaïci et al. (2013) Zhou and Pei (2013) Camilleri et al. (2015)

Zafirah and Mardiana (2016) Anisimov et al. (2015) Min and Su (2010) Mangrulkar et al. (2019)

Yaïci et al. (2013) Jin et al. (2018) Pacak et al. (2019) Al-Zubaydi and Hong (2018)

Zhang (2010) Nasif et al. (2010) Al-Waked et al. (2013) Liu et al. (2016) Mardiana-­ Idayu and Riffat (2011)

4.5 Summary

37

In single-phase heat exchangers, the fluids do not undergo any phase change throughout the heat transfer process, meaning that both the warmer and cooler fluids remain in the same state of matter at which they entered the heat exchanger. For example, in water-to-water heat transfer applications, the warmer water loses heat which is then transferred to the cooler water and neither changes to a gas or solid. 2. Two-phase heat and mass transfer In two-phase heat exchangers, fluids do experience a phase change during the heat transfer process. The phase change can occur in either or both of the fluids involved resulting in a change from a liquid to a gas or a gas to a liquid. Typically, devices which employ a two-phase heat transfer mechanism require more complex design considerations than ones which employ a single-phase heat transfer mechanism. Some of the types of two-phase heat exchangers available include boilers, condensers and evaporators. Based on the design characteristics indicated above, Cuce and Riffat (2015) listed several different variants of heat exchangers available in the market as illustrated in Fig. 4.2. These include shell and tube heat exchangers; double pipe heat exchangers; plate heat exchangers; condensers, evaporators and boilers. However, not all of these type of heat exchangers are suitable or commonly used for energy recovery applications in building sector. The most common types of heat exchangers used in energy recovery applications in building are fixed-plates, heat pipes, rotary wheel and run-around. These are further discussed in Chap. 5.

4.5  Summary Heat exchangers are heat transfer devices that exchange thermal energy between two or more fluids which can be single or two phases. Heat exchangers play a significant role in the operation of many systems including energy recovery units for building application. It is known as the heart of the system. The characteristics of heat exchanger in terms of construction method, flow configurations and heat and mass transfer mechanism are very important to enhance the overall system performance. Construction method of heat exchangers in terms of size, heat and mass transfer areas, materials and structures are predominant factors in the heat and mass transfer enhancement. Additionally, flow configurations have direct influence on the efficiency or effectiveness of heat exchangers. Throughout the literature, numerous works have been carried out in relation to performance investigations on the characteristics of heat exchanger either numerical or experimental or combination of these. Although a great deal of works can be found in the open literature, a gap still exists between research results and practical application. Therefore, future works should be established in this area concerning the studies of heat exchangers for energy recovery application within building environment.

38 Acknowledgements  Universiti PTEKIND/8014124).

4  Heat Exchanger: The Heart of Energy Recovery System Sains

Malaysia

Research

University

Grant

(1001/ ­

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Chapter 5

Classification and Types of Energy Recovery Systems

5.1  General Classification 5.1.1  Classification Based on Different Application Energy recovery systems can be used for both new and retrofit applications in at least three different areas: process-to-process energy transfer, process-to-comfort energy transfer and comfort-to-comfort energy exchange (Sauer and Howell, 1981). Process-to-process system: In process-to-process system, heat is captured from process exhaust airstream and transferred to process supply airstream. This system generally recovers sensible heat and does not transfer latent heat. Process-to-comfort system: In this system, heat is captured from a process exhaust airstream in building to makeup air during cold seasons. The process-to-­ comfort system generally only recovers sensible heat. This is effective during winter months, but requires modulation during spring and autumn to prevent overheating the building. Usually, no energy recovery is made during summer. Comfort-to-comfort system: Energy recovery system in comfort-to-comfort lowers the enthalpy of building supply air during warm conditions and raises it during cold conditions. This system transfers both sensible and latent energy. The system transfers sensible heat from the warmer airstream to the cooler airstream. It also transfers moisture from the airstream with the higher humidity ratio to the airstream with the lower humidity ratio. The directions of humidity and heat transfer are not necessarily the same.

© Springer Nature Switzerland AG 2020 M. I. Ahmad, S. Riffat, Energy Recovery Technology for Building Applications, https://doi.org/10.1007/978-3-030-50006-1_5

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5  Classification and Types of Energy Recovery Systems

5.1.2  Classification Based on Working Mechanism Energy recovery system for building applications can be classified into several categories based on the working mechanism of its heat exchanger. This section discusses three major classifications which are air-to-air energy recovery, earth-to-air energy recovery and earth-to-water energy recovery. Air-to-air energy recovery: Air-to-air energy recovery is the most common category of energy recovery system that utilises air as its working fluid. In this system, heat or moisture or both are exchanged from high temperature to low temperature (or from high moisture to low moisture) for the purpose of pre-conditioning outdoor air prior to supplying the conditioned air to the occupied space. It employs air-to-air heat exchanger. This system can be used as a direct system or as part of ventilation system to include air heating, air cooling, air circulating, air cleaning and humidifying or dehumidifying. In most cases, the utilisation of this system can be performed with a minimal building changes or system alterations. Earth-to-air energy recovery: Earth-to-air energy recovery is a non-conventional technique which utilises the soil temperature under the ground level to draw ventilation supply air through buried ducting system by means of earth coupling. In this system, the fluctuations of ambient air temperature are significantly reduced by the soil temperature in the ground level which is practically constant at depth of 5–8 m (Chel and Tiwari, 2010). The incoming air will be heated in the cold season and cooled in the hot season which provides space conditioning of buildings throughout the year. These systems usually operated to suit mechanically ventilated buildings but can also be driven by natural stack ventilation. They have been used as energy-­ efficient method of pre-heating and pre-cooling of ventilation supply air to a building over the past several decades (Al-Ajmi et al., 2006). An intense review of this system for building applications is presented by Bordoloi et al. (2018). Earth-to-water energy recovery: Similar to earth-to air energy recovery, earth-to-­ water energy recovery also utilises soil temperature under the ground level through buried ducting system through earth coupling technique. However, in this system, water (heavily salted water/brine liquid) is used as working fluid to reject or absorb heat to or from the ground. A coil is housed before the air inlet of energy recovery system. A recent study was carried out by Kappler et al. (2019) by developing a prototype of the system with the aim to examine heat exchange between buried water tank and the soil and it was found that their proposed technique is feasible to provide natural air-conditioning of buildings.

5.2  Types Types of energy recovery systems are based upon the type of energy to be recovered, the proximity of the airstreams, the amount of cross-contamination that can be tolerated and the cost of equipment and systems. The most common types of energy

5.2 Types

45

recovery systems used in building ventilation system can be segmented into fixed-­ plate, rotary enthalpy wheel, heat pipe, run-around and thermosiphon. These types are categorised based on the construction types of heat exchangers.

5.2.1  Fixed-Plate Fixed-plate energy recovery is the most common type of energy recovery system. Fixed-plate energy recovery consists of alternate layers of thin plates, which are stacked together and formed by spacers and separators. Plates and separators are sealed by welding, gluing, cementing or combination of these. It is constructed as individual solid panel with several internal airstreams or channels (Fig. 5.1). Fixed-­ plate heat exchangers are typically made of aluminum because of its resistance to corrosion, ease of fabrication, heat transfer characteristics and cost-effectiveness. There are also steel and polymer-containing materials available commercially. In most cases, fixed-plate energy recovery systems deal with sensible energy, but there are some water-permeable materials that allow the transfer of moisture, such as polymeric membranes and treated paper (Min and Su, 2010) (Fig.  5.2). Both sensible and latent heat is transferred from warm airstream to colder airstream. As the fixed-plate heat exchanger is a static device, there are no air leakages between incoming and outgoing streams. It is also common that the fixed-plate heat exchanger contains condensate drains in order to remove condensate water and possible washing water. Cross-flow fixed-plate heat exchangers are also able to work under cold conditions. Still, pre-heating of supply air has to be considered in order to avoid problems with frost and ice. This kind of procedure increases the overall energy consumption of the building. The plates maybe smooth or may have some form of corrugation. It operates by transferring thermal energy from one airstream Fig. 5.1 Fixed-plate energy recovery

Exhaust air out

Supply air to indoor space

Fresh air in

Return air from indoor space

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5  Classification and Types of Energy Recovery Systems

a

OUTDOOR AIR

INDOOR AIR

EXHAUST AIR

b

SUPPLY AIR

Membrane Exhaust out o d Supply out

Supply in Computational domain yr Exhaust in

xr

z y

x

Fig. 5.2  Fixed-plate energy recovery systems deal with sensible energy, but there are some water-­ permeable materials that allow the transfer of moisture, such as polymeric membranes and treated paper (Min and Su, 2010)

to other airstream via plate heat exchanger surfaces. Typical efficiency of sensible heat transfer is 50–80% and airflow arrangements are counter-flow, cross-flow and parallel flow (ASHRAE, 2005). Fixed-plate types provide an excellent means of achieving highly efficient energy recovery because their high heat transfer coefficients, coupled with counter-current flow, enable them to produce close end-­ temperature differences (Lamb, 1982). The performance of plate type heat exchangers can be influenced by several factors such as: (1) plate types and configurations (different height, pitch and orientation of corrugation and chevron angles) (Abu-Khader, 2012); (2) flow pattern

5.2 Types

47

(Gherasim et al., 2011) and (3) heat and mass exchange material (Nasif et al., 2010). In terms of the plate types and configurations, different height, pitch and orientation of corrugation and chevron angles of plates have been found to have a significant effect on heat transfer coefficient (Khan et  al., 2016). A correlation to estimate Nusselt number as a function of Reynolds number is presented in Khan et al. (2016). Nusselt number is found to increase with increasing Reynolds number and chevron angle. Additionally, by applying a type of porous membrane as heat and mass transfer material and designing a part of counter-flow section, thermal effectiveness of a fixed plate heat exchanger was found to be about 75% of sensible and 65% for latent (Nasif et al., 2010). Fixed-plate energy recovery has various typical geometry shapes such as rectangular, cubical and circular. Diamond shape has been introduced in a study by Mardiana-Idayu and Riffat (2011), which is the enhanced version of the conventional shape of fixed-plate (Fig. 5.3). Studies on fixed-plate energy recovery: There are numerous previous works that were done on fixed plate type air-to-air energy recovery systems. Selected studies pertaining to fixed-plate energy recovery systems are summarised in Table 5.1.

5.2.2  Rotary Wheel Rotary wheel is also known as a thermal wheel or heat wheel or an enthalpy wheel, which consists of a rotating air-permeable storage mass (heat exchange matrix material/media) fitted in a casing. The system is driven by a small electric motor with a belt drive system. The rotating wheel is the heat exchanger of the system which is positioned within two airstreams (supply and exhaust). The wheel is slowly rotated with the rotation speed of 3–15  rpm. The motor of the system is usually equipped with inverter speed controller to control outgoing air temperature and to stop the motor if heat exchange is not required. Rotary wheels are widely used to recover energy, both sensible and latent heat especially in hot–humid climate and

Fig. 5.3  Fixed-plate heat exchanger is the enhanced version of typical geometry shape used for energy recovery application in buildings

48

5  Classification and Types of Energy Recovery Systems

Table 5.1  Studies on fixed-plate energy recovery systems References Nasif et al. (2010)

Description of studies In this study, thermal performance of a fixed-plate enthalpy/membrane energy recovery system was investigated using experimental approach. The heat exchanger of the system utilised a 60 gsm Kraft paper as the heat and moisture transfer surface for HVAC energy recovery. Performance of the system in terms of sensible, latent and total efficiencies was evaluated. Annual energy consumption analysis was also performed using in-house modified HPRate software for the air-conditioning system coupling energy recovery system in hot and humid climate In this work, enthalpy recovery system was Mardiana-­ developed and performance in terms of Idayu and Riffat (2011) efficiency and recovered energy of the system was investigated. Efficiency data generated from experiments were compared with effectiveness–NTU method

Han et al. (2007)

In this study, the effects of outdoor weather conditions on the performance of a fixed-plate energy recovery ventilator were investigated. Experimental investigations were conducted to evaluate the efficiency of the system by varying outdoor temperature and humidity in summer and winter conditions Persily Fixed-plate experiment was conducted to (1982) test the effectiveness of a cross-flow heat recovery which was constructed of plates and fins made from treated paper capable of moisture transfer. The heat recovery efficiency was determined by comparing the actual heat loss to that expected due to the mechanically induced ventilation Zhang and Experimental investigation of the Jiang (1999) performance of rectangular cross-flow fixed-plate made of membrane to study the heat transferred to recover energy in air-conditioning system was conducted in this study

Findings Results showed that the efficiency of the system increased with decreasing air velocity. From the energy consumption analysis, it was found that less energy was consumed by the air-conditioning system coupled with energy recovery system Annual energy savings up to 8% was achieved

Results showed that the efficiency up to 66% was achieved for sensible energy and nearly 59% for latent energy comparison of efficiency with effectiveness–NTU method showed both were in good agreement. Recovered energy was achieved up to 167 W at 3.0 m/s air velocity with 4.3 °C temperature difference Results showed that sensible efficiency gave larger values than in summer conditions due to the heat generation by an internal fan

It was found that the exchanger (core of heat recovery) recovered 55–60% of the heat contained in the out-going airflow depending on the fan speed

They have developed a numerical model and validated the data against the experimental results and found that the highest heat transfer occurred near the inlet area (continued)

5.2 Types

49

Table 5.1 (continued) References Lu et al. (2010)

Fernandez-­ Seara et al. (2011)

Shen et al. (2016)

Description of studies This study developed and investigated the performance of a plastic film plate energy recovery ventilator that works under cross-flow mode by both simulation and experiment Experimental analysis of an energy recovery unit equipped with fixed-plate heat exchanger made of a sensible polymer for balanced ventilation systems in residential buildings was carried out in this study In this paper, performance analysis was conducted to evaluate a hybrid model of heat recovery in liquid desiccant dehumidification system under various air mass flow rate using experimental and numerical approaches. The waste heat of the exhausted regenerating air was recovered by a fixed-plate heat exchanger

Anisimov et al. (2015)

This study focused on numerical simulation and analysis of coupled heat and mass transfer under ice formation conditions of a cross-flow fixed-plate heat exchanger for energy recovery application. The simulation was carried out using mathematical model and validated against experimental data

Al-Zubaydi and Hong (2018)

This study presented the experimental investigation of air-to-air heat exchangers employed for heat recovery ventilation in cooling mode. The two main objectives of this research are to design, fabricate and testing two polymers heat exchangers of different plate geometries and to evaluate and compare the thermal performance two quasi-counter flow plate heat exchangers. The key aims were to evaluate the effect of the surface geometry of the plates heat exchanger on the performance parameters specified in ANSI/ASHRAE Standard 84 and ANSI/AHRI Standard 1060 and narrow the gap of the limited experimental comparison of polymers sensible heat exchanger in cooling mode. The experiments were conducted on two polymer heat exchangers, one with a flat plate and the other with a dimpled surface plate

Findings The results indicated that the thin film vibrated when airflow passed through the channels which enhanced heat exchange performance Results showed that relative humidity decreased from 95% to around 34%, the heat transfer rate was 672 W and 80% of thermal efficiency Results showed that the largest relative error between experimental and numerical analysis was 10.03%. The regenerating performance was improved with heat recovery. The fixed-plate heat exchanger was able to recover about 16–19% of the waste energy. The system with heat recovery contributed to 14–18% energy savings Results showed a satisfactory agreement between numerical and experimental data. It was found that the efficiency of the heat exchanger was sensitive to various inlet conditions such as airflow relative humidity and difference of thermal efficiency The experimental results showed that the cooling capacity of the dimpled surface heat exchanger as ventilation heat recovery system in cooling mode was 50–60% better than that of the flat surface plate heat exchanger. In addition, the sensible efficiency of the dimpled surface heat exchanger was higher than that of the flat surface plates heat exchanger at lower air velocities and higher air initial temperatures. The highest COP was 6.6 achieved with dimpled surface heat exchanger under primary air operating temperature of 32.6 °C

50

5  Classification and Types of Energy Recovery Systems

often utilised in mechanical air-conditioning and dehumidification application (Jani et  al., 2016). With the purpose to recover energy pertaining to relative humidity reduction in air-conditioning and dehumidification systems, it is therefore also known as desiccant wheel (O’Connor et  al., 2016, 2017). The system has been proven as one of the most efficient solutions to handle moisture level carried by the ventilation air (Chen and Yu, 2017). The system operates intermittently between a hot and a cold air that flow in opposite directions within exhaust and supply airstreams (Fig. 5.4). Sensible heat is transferred as the heat exchange material/media picks up and stores heat from the hot airstream and releases it to the cold airstream as the wheel rotates. Latent heat is exchanged as the heat exchange material/media condenses moisture from the airstream with higher humidity ratio and simultaneously releases the heat. Moisture is then released through evaporation into the airstream with a lower humidity ratio. In this application, the heat exchange material/media is utilised to transfer moisture from one airstream to another through the process of adsorption and desorption (Nóbrega and Brum, 2009). Thus, waste heat in the form of sensible and/or latent, from one airstream in transferred to the heat exchange matrix material and from the matrix material to another airstream via heat and/or mass transfer mechanism. The energy recovery rate of this system is a function of the rotational speed of the wheel. In selecting the heat exchange material/media of the system, air contaminants, dew point temperatures, exhaust air temperature and makeup air properties should be considered. Depending on application, the heat exchange material/media can be metal (aluminium, copper, stainless steel), synthetic fibre, polymers, membrane-­ based materials, molecular sieves, zeolites, silica gels or materials treated with a desiccant such as lithium chloride or alumina. The systems are known for their high performance with efficiency of above 80% (Mardiana-Idayu and Riffat, 2012). The high performance of the systems is depending on their flow arrangement that is counter-flow (Chen and Yu, 2017); compact surface area (Zeng et al., 2017); heat exchange material (Jani et al., 2016); rate of air mixing (Tu et al., 2013) and speed

Fig. 5.4  The system operates intermittently between hot and cold air that is flowing in opposite directions within exhaust and supply airstreams

5.2 Types

51

rotation (Ruivo et al., 2015). These factors play important roles in the heat and mass exchange of the system (Zeng et al., 2017) and are related to the rotation speed, wheel thickness and air velocity. In order to achieve better performance of the system (Tu et al., 2013; O’Connor et al., 2015) summarised that high heat capacity of heat exchange material of higher than 500 J/(kg °C) and low conductivity of lower than 10 W/(m °C) are preferred. Meanwhile, the allowed air mixing rate is within 5% with the consideration of optimal rotation speed. The main limitation of rotary wheel energy recovery is the potential of leakage and carryover leading to cross-contamination or airborne contaminants such as carbon dioxide and small particulate matter between supply and exhaust airstreams due to wheel rotation. This carryover reduces the dilution ventilation effectiveness. To reduce the effect of cross-contamination a purge section can be incorporated, which circulates a portion of the fresh air back to the system in order to remove contaminants before it rotates into the supply airstream (Ruan et al., 2012). In addition, filters can be installed to separate contaminants. Unfortunately, the purge section in a rotary wheel energy recovery system leads to some heat losses. Another disadvantage is the potential of recirculation of air in an unintended direction. Due to these limitations, these systems are unsuitable for hospitals and clean rooms where contaminated air should be totally led out and isolated airstreams are required. In conjunction to this, Hemzal (2006) carried out a performance study of rotary wheel energy recovery system and found that the penetration of airflow (leakage) through the untightness was connected with contamination and heat transfer which affected the efficiency of the system. Studies on rotary wheel: Researches on this field keep active since 1980s which involve many theoretical and experimental aspects for building applications. Table 5.2 summarises some selected studies in this area. Throughout the literature, it can be seen that with the capability to transfer heat and moisture, nowadays rotary wheel has become one of the successful alternatives to the conventional mechanical dehumidification system for building applications.

5.2.3  Heat Pipe Heat pipe is sealed self-contained and evacuated tubes filled with a heat transfer working fluid. It transfers heat from one point to another with a corresponding small temperature difference (Kreith and Bohn, 1997) with a counter-flow arrangement. The unit is divided into two sections which are evaporator and condenser (Fig.  5.5). In these sections, heat energy exchange between exhaust and supply airstreams takes place. In operation, latent heat of vaporisation is transmitted from the evaporation section to the condenser section in a closed-loop cycle that is continuous with the presence of temperature difference. Capillary action of wick in the tubes or gravitational force drives the condensed working fluid to return back to the evaporation section (Yau and Ahmadzadehtalatapeh, 2010). Heat pipe energy recovery can achieve thermal

52

5  Classification and Types of Energy Recovery Systems

Table 5.2  Summary of selected studies on rotary energy recovery systems in the open literature References Sauer and Howell (1981)

San and Hsiau (1993)

Simonson and Besant (1999a)

Simonson and Besant (1999b)

Zhang and Niu (2002)

Ghodsipour and Sadrameli (2003)

Description of studies Evaluation of energy requirement of rotary wheel energy recovery system was carried out using AXCESS energy analysis that involved a two-story office building for a full year using local weather data This study analysed the effect of NTU and Bi numbers on the performance of a rotary desiccant wheel recovery system which included axial heat and mass resistance The fundamental dimensionless groups of air-to-air energy wheels that were able to transfer simultaneous sensible and latent heat were developed in this study. The dimensionless groups were derived from the governing non-linear and coupled heat and moisture transfer equations In this study effectiveness correlations were developed to predict sensible, latent and total effectiveness of air-to-air energy wheels In this study, performances of energy wheels used for dehumidification and enthalpy recovery were analysed and compared. The analyses were conducted by considering; a two-dimensional, dual-diffusion transient heat and mass transfer model that took into account the heat conduction, the surface and gaseous diffusion in both the axial and the thickness directions. Effects of the rotary speed, the number of transfer units and the specific area on the performance of the systems were investigated Theoretical study on the effect of dimensionless parameters on the effectiveness of rotary wheel system was carried out and validated with experimental results

Findings Results showed that the energy recovered by the system was around 2.5 times greater than the energy recovered by a sensible energy recovery system It was found in the study that the parameter, (λ42Ntu/Bi), was the important factor governing the axial heat conduction and mass diffusion effect of the system Results showed that the dimensionless groups were found to be functions of the operating temperature and humidity of air-to-air energy wheels

Results showed that the correlations were in good agreement with simulation data within ±2.5%

Results indicated that the optimum rotary speed for dehumidification process was much lower than the enthalpy recovery process. This was due to temperature variations of the system during the dehumidification processes that were larger than during the enthalpy recovery process

The simulation results and the experimental data were in good agreement

(continued)

5.2 Types

53

Table 5.2 (continued) References Description of studies Nóbrega and Brum Modelling and simulation studies (2009) were performed to investigate heat and mass transfer in a rotary/ enthalpy wheel and sensible energy recovery systems

Abe et al. (2006)

Dallaire et al. (2010)

Ruan et al. (2012)

O’Connor et al. (2016)

Chen and Yu (2017)

Findings Results showed that the sensible energy recovery systems were less efficient than enthalpy wheels which were depending on outdoor air conditions. The enthalpy wheels were found to be more efficient compared to the sensible energy recovery systems Results showed that the In this study, an analytical model effectiveness increased as the time was developed to predict the constant and wheel speed increased, effectiveness of rotating counter-­ flow air-to-air energy wheels taking and the uncertainty in the effectiveness reduced as the wheel into account time constant of the speed and time constant increased. It wheel and uncertainty in the time was found that the effectiveness of constant the systems within the uncertainties of ±3% to ±5% In this study, portrayed a conceptual Results showed that the performance optimisation of rotary wheel with a of the system was significantly affected by design variables and the numerical model was presented in optimal values of length and which its internal structure was porosity could be obtained modelled as a porous medium. Optimisation of design variables in terms of length and porosity was carried out Results showed that the wheel depth In this study, an enthalpy wheel and rotation speed had positive system with a purge section to influence on the performance of the prevent carryover of contaminants system. However, increasing the was investigated by developing a wheel depth and the rotation speed one-dimensional transient model. The performance of the system with degraded the system performance purge air was studied under summer and winter design conditions Results showed that reduction of In this study, a novel design of a relative humidity up to 55% was rotary desiccant wheel was achieved and pressure drop was also conceptualised and tested using maintained below 2 Pa simulation and experimental approaches. Traditional honeycomb matrix structure of the system was re-designed to give better performance This study presented an analysis on Results indicated that the system the performances of rotary desiccant under high temperature, high wheel based hybrid air-conditioning humidity and high temperature, low humidity conditions could satisfy system with natural cold source in two typical outdoor meteorological the indoor environment demand conditions (continued)

54

5  Classification and Types of Energy Recovery Systems

Table 5.2 (continued) References Shahsavar and Khanmohammadi (2019)

Description of studies Energy and exergy performance and optimisation of three building integrated systems namely BIPV/T-TW system, the conventional BIPV/T collector, and the convectional TW system were carried out in this study. The systems were capable of pre-­ heating/pre-cooling the ambient fresh air in winter/summer and also producing electricity. Comparisons were made on the basis of energy and exergy analysis

Evaporator

Findings It is observed that the BIPV/T-TW system had the best energy performance amongst the considered systems in all months of the year, while its exergy performance was lower than the BIPV/T system. It was found that annual useful energy and exergy gained by the optimised system was 196.31 MWh and 30.15 MWh, which was 563.8% and 1394.1% higher than the un-optimised system, respectively

Condenser

Heat transfer

Working fluid Heat in

Heat out

Fig. 5.5  Typical heat pipe device

e­ fficiency between 45 and 55% (Chaudhry et al., 2017). This efficiency is influenced by inlet temperature in the evaporator; the air velocity flow in the evaporation and condenser; the geometry fin including arrangement of the tubes and their diameters and working fluids (Srimuang and Amatachaya, 2012). Heat pipes are widely recognised as one of the most excellent passive heat transfer technologies that have effective thermal. Amongst the advantages of heat pipe energy recovery systems are they have high thermal conductivity that enables the transportation of heat whilst maintaining almost uniform temperature along its heated and cooled sections. These devices are also flexible in terms of sizes which make them suitable for various applications. Due to having no moving parts, these devices only require a very minimum maintenance which is periodic cleaning. Besides, large quantity of heat can be transported through a small cross-sectional area over a considerable distance without additional power input to the system.

5.2 Types

55

Other advantages of heat pipe devices are ease of assembly and installation, versatility, scalability and adaptability of design. Further advantages include low overall thermal resistance, small pressure drops in the external fluid flows (hot and cold side), chemical compatibility (especially at higher temperatures) and availability of many suitable materials for implementation in heat pipe devices. With these characteristics, heat pipe energy recovery devices are suitable to be used in naturally ventilated buildings and passive stack systems where a low-pressure loss is essential (Shao et al., 1998). Since heat pipes are passive systems requiring no power to operate, heat pipe devices have great potential for application in both commercial and industrial markets, particularly in the field of HVAC.  These devices have been proven to have great potential to be incorporated in air-conditioning system to recover a portion of energy. A thorough review on the applications of these devices in air-conditioning systems in the tropics is presented in Yau and Ahmadzadehtalatapeh (2010). From the review, the application of horizontal heat pipe devices in terms of energy savings, dehumidification enhancement and condensate drainage in the air-conditioning systems is recommended. Heat pipe devices used to recover energy within air-conditioning system was tested in Ahmadzadehtalatapeh and Yau (2011). A schematic diagram of the test chamber is shown in Fig. 5.6. Studies on heat pipe energy recovery: There are a number of valuable researches pertaining to heat pipe devices can be found in the open literature for energy recovery applications. A comprehensive review on heat pipe exchangers for energy recovery application is presented in Shabgard et al. (2015). Table 5.3 summarises some selected studies in this area. From the existing studies, more widespread usage of heat pipes is anticipated as cost is reduced through high-volume manufacturing.

5.2.4  Run-around Run-around energy recovery is a system consisting of two separate heat exchangers and a coupling liquid (heat transfer fluid) positioned within supply and exhaust airstreams to recover heat or energy as illustrated in Fig. 5.7. It also refers to run-­ around loop or run-around coil. The heat exchangers are linked by a closed-loop hydronic system that includes counter-flow ducting/piping and pumping systems to circulate the heat transfer fluids (Emerson, 1983). In the system, heat is transferred from exhaust to supply air using the heat transfer fluids such as ethylene glycol and water solution (Forsyth and Besant, 1988). An expansion tank is usually installed to accommodate the expansion and contraction of heat transfer fluids. The main highlight of this system is that supply and exhaust duct can be physically separated, not necessarily to be side by side, thus it can be installed even in different part of buildings. This provides maximum possible flexibility and gives this system an advantage over other available energy recovery systems especially

56

5  Classification and Types of Energy Recovery Systems

a

Measuring Station

Exhaust Fan

Local Panel

HPHX (Condenser) 4

Air Outlet

Return Air

3

Return Grille 1

2 HPHX (Evaporator)

Supply Diffuser Cooling Coil

Blower

Sensor Line For Heater Control Box

Electric Heater Inside Sensor

Heater Control Box

Outdoor Air

Filter

Data logger

b

PC

Evaporator Section

*:1,2,3 and 4 are measuring stations

Adiabatic Section

Condenser Section

31.75 mm

35 cm

27.5 mm

42 cm

18 cm Front View

4 rows

6 rows

Heat Pipe Tubes

8 rows

Fig. 5.6  Schematic diagram of the test chamber of heat pipe energy recovery devices in air-­ conditioning system Ahmadzadehtalatapeh and Yau (2011)

when cross-contamination is a concern. Additionally, with the airstreams not connected to each other (no air leakages between the supply and exhaust airstreams), makes this system suitable for application with hygienic reasons such as hospitals, chemical laboratories, food and pharmaceutical industries. The distance between the two airstreams is only limited by the economics of the ducting/piping and pumping systems. However, designing of this system is quite challenging as consideration of the heat transfer rates between fluids should be prioritised. This associates with the needs to have high heat transfer rates while maintaining less energy to fulfil HVAC

5.2 Types

57

Table 5.3  Summaries of some selected studies in this area References Description of studies Shao and Riffat (1997) In this study, performance of heat pipe recovery system in natural ventilation systems assisted by passive stacks was investigated using simulation and experimental approaches

Shao et al. (1998)

Lin et al. (2005)

Yau (2007)

Findings Experimental results showed that the efficiency of the system was found to be 50% with a stack flow speed of 0.5 m/s and the pressure loss across the system was about 1 Pa. Results of computer simulation showed that heat pipes located at the bottom of the stack produced greater insertion flow loss than those located at the top and heat pipes located next to the stack walls gave rise to less insertion flow loss than those in the centre It was found that the efficiency of In this study, experimental the systems decreased with investigation under laboratory conditions and computer simulation increasing air velocity. Sensible efficiency of close to 50% was were carried out on low pressure-­ recorded using a single-bank loss heat pipe recovery devices (single-bank plain-fin and double-­ plain-fin unit and the efficiency of a double-bank unit was 40% banks) which were installed in passive ventilation system where a higher than that of a single bank unit. The pressure loss coefficient low-pressure loss was crucial reduced as velocity increased (the reduction was found to be around 10% over the entire range of velocity in the passive ventilation system Results showed that the CFD In this study, heat pipe energy recovery unit was designed studied modelling was able to predict thermal performance of the heat for dehumidification using pipe unit and could be used to computational fluid dynamics modelling and simulation approach optimise the design of the heat pipe fin stack It was found that the sensible In this study, the performance of heat ratio of the system was heat pipe recovery system for influenced by inlet air state in tropical building air-conditioning terms of dry-bulb temperature, systems was experimentally relative humidity and air velocity. investigated The system was capable to enhance dehumidification performance of air-conditioning system (continued)

58

5  Classification and Types of Energy Recovery Systems

Table 5.3 (continued) References Ahmadzadehtalatapeh and Yau (2011)

Calautit and Hughes (2016)

Wang et al. (2016)

Diao et al. (2017)

Description of studies In this effect of heat pipe heat exchangers on the existing air conditioning system of a hospital ward located in Malaysia, a tropical region to recovery energy was studied. The present research employs the transient system simulation software (TRNSYS) was employed to investigate the hour-by-hour performance of the system in terms of supply duct air and indoor air conditions in the ward space. Energy consumption, power savings, supply duct air and indoor air with heat pipe heat exchangers incorporated in the air conditioning system were simulated and the results were compared with the existing system This study investigated the performance of wind towers incorporated with heat-pipe recovery system using numerical analysis and wind tunnel experiments for validation

Findings Results showed that the system with heat pipe heat exchangers was able to provide acceptable air conditions based on ASHRAE standards. A considerable amount of energy and power could also be saved

Results showed that the system capable of fulfilling the required ventilation rates above an inlet air velocity of 1 m/s. The integration of heat pipes had a positive effect on thermal performance of the wind tower as it could raise the supply air up to 4.5 K Results showed that the average A heat pipe was proposed in air-conditioning system which used heat recovery efficiency of the system in winter is 21.08%, heat pipe for heat recovery was proposed. Efficiency of the system while 39.2% in summer. The energy consumption analysis was evaluated for winter and indicated that the system had summer conditions. Energy consumption of the system was then potential for energy-saving advantage analysed using meteorological parameter of Hefei city, China Results showed that efficient heat In this study, a small flat heat pipe heat recovery device which applied transfer of the system posed high heat recovery efficiency, high flat micro-heat pipe array with reliability, low resistance, and welded, serrated and staggered fin suitable volume. The maximum on its surface was designed for high heat recovery efficiency and potential application in residential COP were recorded at r78% and buildings. Performance of the 91.9%, respectively system in terms of heat recovery efficiency, coefficient of performance (COP), device volume and influencing factors that affected the performance was analysed (continued)

5.2 Types

59

Table 5.3 (continued) References Xue et al. (2019)

Burlacu et al. (2018)

Zhou et al. (2018)

Jafarinejad et al. (2019)

Description of studies In this study, a novel heat pipe energy recovery system was developed to recover energy from exhaust air. The heat pipe unit was inserted through wave plated. The system was experimentally investigated. The performance of the system associated with temperature difference, heat transfer rate and energy efficiency ratio (by considering a few cities in China) was investigated

Findings Results showed that the heat transfer rate and energy efficiency ratio of the increased with the increase of temperature difference between indoor and outdoor in both winter and summer conditions. Temperature effectiveness was recorded maximum at 62% in winter conditions and 70% in summer conditions. The annual energy efficiency ratio of the system ranged from 3.67 to 12.72 by taking into account the system operated in selected cities in China Results showed that building Heat transfer and energy energy performance increased performance of heat pipe energy with the decrease of carbon recovery system for building application in Europe were analysed dioxide emissions. The results of the study were in close using simulation approach. The system was simulated to be used for correlation with the European Directives covering the domestic hot water and thermal improving energy efficiency and agent preparation; preheating/ the reduction of the energy heating the air from ventilation consumption of buildings systems in buildings and as a primary agent of the existing heating system Results indicated that heat In this study, a triple-loop pump-­ transfer capacity and coefficient driven heat pipe system was developed for energy recovery from of performance increased with indoor and outdoor temperature exhaust air in buildings. The difference, and the variation of thermal performances of the single-loop and triple-loop systems temperature effectiveness depended on working conditions. were studied and compared using experimental approach under winter The energy recovery performance of the triple-loop system was and summer conditions better than the single-loop system Results indicated that the A novel integrated multistage heat recovery system through a heat pipe integrated system had a better performance as compared to the device to precool the fresh air and afterwards through a condenser-side stand-alone system with less than a year of operational payback mixing box heat recovery unit in period order to deplete all the recovery potentials of the ventilated air and precool the condenser cooling air was modelled and analysed. The performance of the integrated system was compared with stand-alone system

60

5  Classification and Types of Energy Recovery Systems

Outdoor air

Supply air heat exchanger

To indoor

Exhaust air

Exhaust air heat exchanger

To outdoor

Pump

Valve

Fig. 5.7  Components of basic run-around energy recovery system

loads in buildings which eventually give a significant impact on the operating and capital costs. Another factor is the additional electricity cost which is required to pump heat transfer fluids. Selection of heat transfer fluids with anti-freeze mixture such as glycol-based is important if the system is to be used in certain climates to protect from freezing. Heat transfer fluid with anti-freeze mixture fluid usually decreases the specific capacity of the fluid and increases the viscosity and pump energy consumption which could reduce the overall efficiency of the system. This system typically can only transfer sensible heat between the airstreams. A new run-around energy recovery system that able to simultaneously transfer heat and moisture was introduced by Fan et al. (2006) coupled with a lithium bromide solution. Another system with similar mechanism to transfer to heat and moisture was proposed by Seyed-Ahmadi et  al. (2009a, b), which used semi-permeable membranes in each heat exchanger with an aqueous salt solution coupling liquid that was pumped between the heat exchangers as illustrated in Fig. 5.8. Steady-state performance of this system with the aim to correlate sensible and latent efficiency was then investigated by Akbari et al. (2012a, b). The typical efficiency of run-around energy recovery system is between 45 and 65% (Mardiana-Idayu and Riffat, 2012). Like the other energy recovery types, the efficiency of the run-around energy recovery system is also greatly dependent on outdoor air conditions. As stated by Hemingson et al. (2011), (1) the efficiency of the system increases with increasing air temperature when the enthalpy of outdoor air is greater than the enthalpy of indoor air and (2) the efficiency of the system increases with decreasing air temperature when the enthalpy of outdoor air is lower than the enthalpy of indoor air. In addition, flow rate of pumped fluid, airflow, size of heat exchanger and inlet operating conditions also give significant impacts on the

5.2 Types

61 Pipe Lines Supply Storage Tank Exhaust Liquid Circulating Pump

Supply Exchanger

Exhaust Exchanger

Outside Air

Exhaust Air

Air From Space

To Cooling Equipment

Supply Liquid Circulating Pump Exhaust Storage Tank

Fig. 5.8  Semi-permeable membranes in each heat exchanger with an aqueous salt solution coupling liquid that was pumped between the heat exchangers (Seyed-Ahmadi et al., 2009a, b)

efficiency of the system. If these parameters are carefully selected, the overall efficiency of the system can be up to 70% (Fan et al., 2006). Studies on run-around energy recovery: With the unique characteristics and advantages to be used in buildings that are remotely located and as coupled sub-­ system using a coupling fluid, a number of research has been performed on run-­ around energy recovery systems. One of the first studies on the run-around system was carried out by London and Kays (1951). From the study, it was shown that the system had its optimum performance when the heat capacity rates of the air and coupling liquid were equivalent at a constant NTU.  Table  5.4 summarises some selected studies in this area.

5.2.5  Thermo-siphon Thermo-siphon energy recovery device consists of heat exchangers which pose similarities as heat pipes that works based on passive heat exchange. The system employs a fluid changes phase as part of the heat exchange process (as it is heated or cooled) without any mechanical force (no moving parts) (Sauciuc et al., 1995; Ersöz and Yıldız, 2016). Typical thermo-siphon device consists of an evaporator, a condenser, interlinking piping system and a heat transfer fluid (Fig. 5.9). The change of phase in the device happens due to temperature difference and the force of g­ ravity,

62

5  Classification and Types of Energy Recovery Systems

Table 5.4  A summary of some selected studies of run-around energy recovery systems References Emerson (1983)

Description of studies A run-around energy recovery system was designed. Performance of the system was investigated in relation to temperatures of heat transfer fluids (primary and secondary fluids) and their rates of circulation

Wang (1985)

A straight-forward explicit thermal design procedure of run-around energy recovery system was proposed in this study. The design procedure took into consideration exhaust and supply airflow rates, temperature conditions and pressure drops through heat exchanger A numerical simulation to investigate the performance of run-around energy recovery with two heat exchangers using aqueous glycol as a coupling liquid was developed. The study highlighted the sensitivities of the system associated with design variables such as flow rates, dimensions of the system and operating conditions

Forsyth and Besant (1988)

Dhital et al. (1995)

Findings Results showed that the performance of the system was sensitive to the rate of circulation of secondary fluid. The optimum rate changed with changes in the flow rates of the primary fluid or in the fouling resistances. The study also suggested a method for determining the rate of circulation at minimum cost of heat exchangers Smallest possible heat exchangers of the system could be obtained by the proposed procedure with appropriate pressure drops and minimum number of rows. It was suggested that a final adjustment may lead to the least expensive and most efficient design

The results showed that to achieve maximum overall efficiency, fully developed turbulent flow in the exchangers was important. The system efficiency under low liquid flow Reynolds number conditions could drop by up to 50% when a very high concentrated ethylene glycol was used in the system. Performance of the system in terms of efficiency was dependent on the flow rate of heat transfer fluids within the heat exchangers In this study, the effect of utilisation of Results indicated that the building run-around energy recovery system on energy performance with run-around energy recovery system was better than performance with regard to energy consumption and energy life cycle cost without the system the utilisation of the system reduced annual electrical energy of an office building was carried out. consumption, natural gas consumption Results were compared with the and total energy costs by up to 3.5%, building performance without the 40.7% and 4.8%, respectively. Boiler system. The system was used for pre-heating and pre-cooling the supply and chiller sizes can be reduced by up to 32.6% and 8%, respectively. It was ventilation outdoor air. The building performance operated using the system also found the ventilation rate into the was simulated and analysed using DOE building with the system increased by several hundred percentages compared 2.1 D programmes based on Chicago, to the minimum rate required by Denver, Edmonton and Fort Worth climatic conditions. Maximum outdoor ASHRAE Standard 62–1989, without increasing energy costs air ventilation rate of the system without any potential increase of energy consumption or energy cost for the building was also presented (continued)

5.2 Types

63

Table 5.4 (continued) References Fan et al. (2006)

Vali et al. (2009)

Seyed-­ Ahmadi et al. (2009a, b)

Khizir et al. (2010)

Description of studies A two-dimensional steady-state mathematic model for a run-around energy recovery system made of two cross-flow exchangers was developed to study the heat and moisture transfer of the system. Lithium bromide solution was used as the coupling liquid in the system A two-dimensional steady-state numerical model was developed in this to investigate the heat transfer of a run-around energy recovery system with two combinations of heat exchangers: counter and cross (counter/cross) flow between parallel plates or membranes with a coupling liquid that was anti-freeze aqueous ethylene glycol solution. A finite difference method was used to solve the steady-state equations of continuity, momentum and heat transfer. A new effectiveness correlation for counter/cross-flow configuration was also developed Transient behaviour of run-around energy recovery system that was able to transfer both sensible and latent heat simultaneously, which was carried out in these studies. Part I focused on model formulation and verification. Part II emphasised on sensitivity study for a range of initial conditions. These included the number of heat transfer units, thermal capacity ratio, heat loss/ gain ratio, storage volume ratio and the normalised initial salt solution concentration

In this study, a run-around energy recovery system with micro-porous membrane heat exchangers was designed and tested for HVAC applications. The system performance was evaluated by testing it with various air and solution flow rates under standard summer and winter operating conditions

Findings The overall efficiency of the system was found to be dependent on the flow rate of pumped fluid and airflow; the size and design of each exchanger and the inlet operating conditions. Results showed that with appropriate characteristics an overall efficiency of 70% could be achieved for the system Results showed that that the efficiency/ effectiveness of this the system was within efficiency/effectiveness of similar run-around energy recovery systems with either two cross-flow heat exchangers or two counter-flow heat exchangers. It was also found that for a given total surface area of the heat exchangers, the highest overall sensible effectiveness was achieved with the exchangers which had a small exchanger aspect ratio and relatively small solution flow inlet and outlet lengths Results of Part I showed that for the simultaneous sensible and latent heat transfer of the system a comparison between numerical model results and experimental measurements obtained from laboratory testing for both sensible and latent efficiency indicated satisfactory agreement at different operating conditions. Whilst results of Part II showed that the storage volume ratio and the initial salt solution concentration had significant impacts on the transient response of the system and heat transfer between the system and the surrounding environment which could change the system quasi-steady conditions significantly Results showed that under summer conditions, total system efficiency increased with increasing solution flow rate, but decreased as the air flow rate increased. Under winter conditions, total system efficiency indicated a little change with changes in the air and solution flow rates. Total efficiency of the system was between 50 and 55% (continued)

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Table 5.4 (continued) References Description of studies Hemingson In this study, steady-state performance et al. (2011) of a run-around energy recovery with membrane-based heat exchanger was carried out for a wide range of outdoor air conditions. The system was numerically simulated and the sensible and latent efficiency/effectiveness values corresponding to the maximum total energy efficiency/effectiveness for different outdoor air conditions were presented Wallin et al. In this study, a comparative study for (2012) three different cases of run-around energy recovery systems (a traditional system; a three-stage on/off controlled pumped retrofitted system and; a variable-capacity heat pump retrofitted system) was carried out using TRNSYS software

Findings It was found that the efficiency/ effectiveness values were very dependent on outdoor air conditions which resulted in values of efficiency/ effectiveness exceeding 100% or less than 0% for several outdoor air conditions analysed in this study. The heat and moisture transfers affected latent and sensible performances of the system, respectively

Results showed that by retrofitting a well-designed three-stage heat pump and variable-capacity heat pump retrofitted to the system, the annual energy recovery rate increased from nearly 20% as compared to the traditional system. The modelling also showed that a well-designed three-stage heat pump and variable-capacity heat pump could cover about 77–81% of the total ventilation heating demand Results showed that: (1) the mean Akbari et al. Application of neural networks to absolute difference between the results predict (1) the transient performance (2012a, and (2) the steady-state performance of of numerical model and neural network 2012b) models for different locations were a run-around energy recovery system 0.5 °C and 0.2 gv/kga for sensible and with membrane heat exchangers were performed in these studies latent models, respectively which indicated satisfactory agreement for energy exchange computations; (2) the root mean squared error between the finite difference and neural network models were 0.05 °C and 2 × 10−5 kgv/ kga, showing satisfactory agreement for energy exchange computations Results showed that numerical and Ge et al. In this study, performance of run-­ (2013) around energy recovery system carried experimental analyses were in good agreement. The run-around energy out using numerical and experimental recovery system with equal-sized approaches under various operating conditions. The efficiency/effectiveness supply and exhaust heat exchangers had highest efficiency/effectiveness in most of the system under balanced and conditions for both balanced and unbalanced airflow conditions was unbalanced airflow. Optimisation evaluated using analytical model. Optimisation studies for heat exchanger control of the solution flow rate could size and solution flow rate of the system enhance the annual energy recovery rate to nearly 7% were carried out (continued)

65

5.2 Types Table 5.4 (continued) References Patel et al. (2014)

Description of studies Experimental investigations were performed at different operating conditions using two run-around energy recovery prototypes. In the study, sulfur hexafluoride (SF6) was used as tracer gas to test air leakage, and toluene (C7H8) and formaldehyde (HCHO) were used to test VOC transfer fraction

Findings It was found that the exhaust air transfer ratio values for both prototypes were insensitive to changes in airflow rate, solution flow rate, latent efficiency and environmental conditions. The exhaust air transfer ratio of SF6 was 0.0 ± 3.6% for both prototypes, which means the air leakage is negligible. The transfer of C7H8 was 2.3–3.4 ± 3.5%, while the transfer of HCHO was 4.5–6.4 ± 3.6% in the prototypes Rasouli In this study, a steady-state run-around It was found that the system could reduce the annual heating energy by et al. (2014) energy recovery system with 60% in cold climates and annual membrane-based heat exchangers was simulated for a hospital building in cold cooling energy by 15–20% in hot climates. It had a payback of 1–3 years and hot climates using TRNSYS and in both cold climates and in hot MATLAB programmes. Heating and cooling energy loads as well as carbon climates it depended on the pressure drop across the heat exchangers. The dioxides emissions were analysed system could reduce up to 10% and 25% in hot and cold climates, respectively Lu et al. Field measurements based methodology It was suggested in the study that the proposed new methodology could (2016) supported by the power–law relationship of air-side heat transfer was possibly be applied to other types of air-to-air energy recovery systems proposed in this study with the aim to assess the performance of run-around energy recovery systems Vapour header Condenser

Condensate return line Static liquid level

Fig. 5.9  Thermosiphon energy recovery

Evaporator

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which causes the heat-transfer fluid to circulate between the evaporator and the condenser units (Shabgard et al., 2015). Even though this system is similar to heat pipe type, it is different in two ways: (1) it has no wick and only relies on the differences of fluid density caused by temperature differences and (2) the tubes are dependent upon nucleate boiling to change phase from liquid to vapour. Studies on thermo-siphon energy recovery: To date, there have been very limited studies on the thermo-siphon for energy recovery application in buildings. One of the earliest studies was carried out by Sauciuc et al. (1995). In the study, two-phase closed thermo-siphon devices for energy recovery application working under various operating conditions were tested and their thermal performance was measured for mean evaporator wall temperatures between 100 and 250 °C. Results indicated that an increase in thermo-siphon diameter affects the boiling mechanism from saturated film boiling to nucleate boiling. Further experiments to determine the optimum pipe diameter for the energy recovery application was suggested in the study. About a decade after that, Abu-Mulaweh (2006) carried out design and performance investigations of a thermo-siphon for waste heat recovery to recover heat rejected from an air-conditioning system were carried out in this study. Results indicated that the design of the thermo-siphon energy recovery system was capable to recover heat from air-conditioning system. Throughout the open literature, it can be seen that there is an immense gap between studies in this area. Research in this area is very scanty might be due to practicality of the devices which make them less favourable as compared to the other available energy recovery devices particularly for building ventilation system. Amongst the major drawbacks of the devices, they need to be mounted in such a way so that vapour can rise up and liquid can flow down to the evaporator without any bending in the tubing for the liquid to pool. Additionally, the device must be completely airtight to ensure the heat transfer process takes effect. Nevertheless, these devices have a great potential in the solar water heating applications and storage radiative cooling.

5.3  Summary Energy recovery systems for building applications can be categorised into their classification and types. In general, there are two classifications of energy recovery systems which are based on: 1. Application, in terms of process-to-process system, process-to-comfort system and comfort-to-comfort system. 2. Working mechanism in terms of air-to-air energy recovery, earth-to-air energy recovery, earth-to-water energy recovery. In the context of types, there are five common energy recovery systems which can be found in the global market and have been extensively studied which are based on the construction of their heat exchangers. These include fixed-plate, rotary enthalpy wheel, heat pipe, run-around and thermosiphon. Throughout the l­ iteratures,

References

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Table 5.5  A comparison analysis of energy recovery types based on efficiency ranges and advantages System type Fixed plate Rotary wheel Heat pipe Run around

System efficiency 50–80%

Advantages Compact, highly efficient due to high heat transfer coefficient, no cross-contamination, can be coupled with counter-current flow which enabling to produce close end-temperature differences Above 80% High efficiency, capability of recovering sensible and latent heat 45–55%

45–65%

No moving parts, no external power requirements, high reliability, no cross-contamination, compact, suitable for naturally ventilated building, fully reversible, easy cleaning Does not require the supply and exhaust air ducts to be located side by side, supply and exhaust duct can be physically separated, no cross-contamination

many works have been carried out since 1980s involving various types. A comparative analysis of energy recovery types by their efficiency ranges and advantages is presented in Table 5.5. Acknowledgements  Fundamental Research Grant Scheme, Ministry of Education Malaysia (203/PTEKIND/6711574; 203/PTEKIND/6711274).

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International Journal of Heat and Mass Transfer, 52(25), 6000–6011. https://doi.org/10.1016/j. ijheatmasstransfer.2009.07.012 Shabgard, H., Allen, M.  J., Sharifi, N., Benn, S.  P., Faghri, A., & Bergman, T.  L. (2015). Heat pipe heat exchangers and heat sinks: Opportunities, challenges, applications, analysis, and state of the art. International Journal of Heat and Mass Transfer, 89, 138–158. https://doi. org/10.1016/j.ijheatmasstransfer.2015.05.020 Shahsavar, A., & Khanmohammadi, S. (2019). Feasibility of a hybrid BIPV/T and thermal wheel system for exhaust air heat recovery: Energy and exergy assessment and multi-­objective optimization. Applied Thermal Engineering, 146, 104–122. https://doi.org/10.1016/j. applthermaleng.2018.09.101 Shao, L., & Riffat, S. B. (1997). Flow loss caused by heat pipes in natural ventilation stacks. Applied Thermal Engineering, 17(4), 393–399. https://doi.org/10.1016/S1359-4311(96)00029-4 Shao, L., Riffat, S. B., & Gan, G. (1998). Heat recovery with low pressure loss for natural ventilation. Energy and Buildings, 28(2), 179–184. https://doi.org/10.1016/S0378-7788(98)00016-4 Shen, S., Cai, W., Wang, X., Wu, Q., & Yon, H. (2016). Hybrid model for heat recovery heat pipe system in liquid desiccant dehumidification system. Applied Energy, 182, 383–393. https://doi. org/10.1016/j.apenergy.2016.08.128 Simonson, C.  J., & Besant, R.  W. (1999a). Energy wheel effectiveness. Part II. correlations. International Journal of Heat and Mass Transfer, 42(12), 2171–2185. Simonson, C. J., & Besant, R. W. (1999b). Energy wheel effectiveness: Part I—Development of dimensionless groups. International Journal of Heat and Mass Transfer, 42(12), 2161–2170. https://doi.org/10.1016/S0017-9310(98)00325-1 Srimuang, W., & Amatachaya, P. (2012). A review of the applications of heat pipe heat exchangers for heat recovery. Renewable and Sustainable Energy Reviews, 16(6), 4303–4315. https://doi. org/10.1016/j.rser.2012.03.030 Tu, R., Liu, X-H., Jiang, Y. (2013). Performance comparison between enthalpy recovery wheels and dehumidification wheels. International Journal of Refrigeration, 36(8), 2308–2322. Vali, A., Simonson, C. J., Besant, R. W., & Mahmood, G. (2009). Numerical model and effectiveness correlations for a run-around heat recovery system with combined counter and cross flow exchangers. International Journal of Heat and Mass Transfer, 52(25), 5827–5840. https://doi. org/10.1016/j.ijheatmasstransfer.2009.07.020 Wallin, J., Madani, H., & Claesson, J. (2012). Run-around coil ventilation heat recovery system: A comparative study between different system configurations. Applied Energy, 90(1), 258–265. https://doi.org/10.1016/j.apenergy.2011.05.012 Wang, J. C. Y. (1985). Practical thermal design of run-around air-to-air heat recovery system. Journal of Heat Recovery Systems, 5(6), 493–501. https://doi.org/10.1016/0198-7593(85)90216-4 Wang, H., Zhou, S., Wei, Z., & Wang, R. (2016). A study of secondary heat recovery efficiency of a heat pipe heat exchanger air conditioning system. Energy and Buildings, 133, 206–216. https:// doi.org/10.1016/j.enbuild.2016.09.061 Xue, L., Ma, G., Zhou, F., & Wang, L. (2019). Operation characteristics of air–air heat pipe inserted plate heat exchanger for heat recovery. Energy and Buildings, 185, 66–75. https://doi. org/10.1016/j.enbuild.2018.12.036 Yau, Y.  H. (2007). Application of a heat pipe heat exchanger to dehumidification enhancement in a HVAC system for tropical climates—A baseline performance characteristics study. International Journal of Thermal Sciences, 46(2), 164–171. https://doi.org/10.1016/j. ijthermalsci.2006.02.006 Yau, Y. H., & Ahmadzadehtalatapeh, M. (2010). A review on the application of horizontal heat pipe heat exchangers in air conditioning systems in the tropics. Applied Thermal Engineering, 30(2), 77–84. https://doi.org/10.1016/j.applthermaleng.2009.07.011 Zeng, C., Liu, S., & Shukla, A. (2017). A review on the air-to-air heat and mass exchanger technologies for building applications. Renewable and Sustainable Energy Reviews, 75, 753–774. https://doi.org/10.1016/j.rser.2016.11.052

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Zhang, L. Z., & Jiang, Y. (1999). Heat and mass transfer in a membrane-based energy recovery ventilator. Journal of Membrane Science, 163, 29–38. Zhang, L. Z., & Niu, J. L. (2002). Performance comparisons of desiccant wheels for air dehumidification and enthalpy recovery. Applied Thermal Engineering, 22(12), 1347–1367. https://doi. org/10.1016/S1359-4311(02)00050-9 Zhou, F., Duan, W., & Ma, G. (2018). Thermal performance of a multi-loop pump-driven heat pipe as an energy recovery ventilator for buildings. Applied Thermal Engineering, 138, 648–656. https://doi.org/10.1016/j.applthermaleng.2018.04.104

Chapter 6

Evaluating the Performance of Energy Recovery Systems

6.1  Overview The growing attention to energy savings has contributed to more widespread use of energy recovery systems for building applications. The systems have been shown to be effective in reducing HVAC load in various building types either through experimental analysis or computational modelling approach. Energy recovery performance can be evaluated in terms of efficiency and recovered energy (heat and mass transfer) through its heat exchanger. There are several existing standards and methods used in testing the performance of the system. These include: • ISO 16494:2014 Heat Recovery Ventilators and Energy Recovery Ventilators— Method of Test for Performance. This International Standard prescribes a method of testing the ventilation and energy-related performance of heat recovery ventilators (HRVs) and energy recovery ventilators (ERVs) that do not contain any supplemental heating (except for defrost), cooling, humidification or dehumidification components. • Air-Conditioning, Heating and Refrigeration Institute (AHRI) Standard 1061— Standard for Performance Rating of Air-to-Air Exchangers for Energy Recovery Ventilation Equipment. This standard is established for air-to-air exchangers intended for use in air-to-air energy recovery ventilation equipment (AAERVE): It is intended for the guidance of the industry, including manufacturers, designers, installers, contractors and users. However, this standard does not apply to the rating and testing of heat exchangers joined by circulated heat transfer medium (run-around loop). A run-around loop employs liquid-containing coils connected in a closed loop and placed in each of two or more airstreams. This chapter provides a background of the performance evaluation of energy recovery system.

© Springer Nature Switzerland AG 2020 M. I. Ahmad, S. Riffat, Energy Recovery Technology for Building Applications, https://doi.org/10.1007/978-3-030-50006-1_6

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6  Evaluating the Performance of Energy Recovery Systems

6.2  Efficiency Energy recovery efficiency is described in the forms of sensible (heat), latent (moisture) and total (heat and moisture/mass) with respect to balanced airflow; steady-­ state conditions and no heat transfer between the heat exchanger and its surroundings and no gains from cross-leakage. This can be studied by a simple mass and energy balance. It is also defined by the ratio of actual energy recovered to theoretical energy that could be recovered. According to ASHRAE (2000), efficiency of energy recovery is determined by three conditions: 1. Sensible energy (heat) transfer, which is calculated from the dry bulb temperature difference between intake and exhaust airstreams 2. Latent energy (mass/water vapour) transfer, which is computed by the humidity ratio difference between intake and exhaust airstreams and 3. Total energy (heat and moisture/mass) transfer, which is computed by enthalpy difference between intake and exhaust airstreams The energy recovery efficiency is thus defined as (Eq. 6.1):

ε=

Actual transfer ( of heat, mass or energy ) Maximum possible transfer between airstreams

(6.1)

And therefore the three forms of energy recovery efficiency are further derived as (Eq. 6.2):

ε=

ws ( x2 − x1 )

wmin ( x3 − x1 )

(6.2)

where: ws is the intake airflow wmin is the smaller airflow between intake airflow and exhaust airflow x stands for either sensible heat (temperature), latent heat (humidity ratio) or total enthalpy (ASHRAE 2005) and; the numbers 1, 2 and 3 are the point at the inlet of supply airstream, outlet of supply airstream and return indoor conditioned airstream, respectively. For a balanced airflow energy recovery system, ws and wmin are of the same values; hence they can cancel one another. The calculation process relating physical and thermodynamic properties such as relative and specific humidity, temperature (wet and dry bulb), pressure (air and vapour), air density and enthalpy are quite tedious. Therefore, psychrometric chart is introduced to assist the calculation process involving these variables. The following sub-sections describe the definition and calculation of energy recovery efficiency according to ASHRAE (2005) in terms of sensible efficiency;

6.2 Efficiency

75

latent efficiency and enthalpy efficiency; global efficiency (Roulet et al., 2001) and effectiveness–NTU method based on Nusselt and Sherwood correlations.

6.2.1  Sensible Efficiency The sensible efficiency (εsen) is also known as temperature efficiency, which is expressed by temperature changes of supply and exhaust air. It can be calculated by the following Eq.6.3:

ε sen =

Tin − Tsu Tin − Tre

(6.3)

where: εsen: Sensible efficiency (%) Tin: Temperature of supply air before entering the heat exchanger of energy recovery system (temperature of intake air) (°C) Tsu: Temperature of supply air before entering the heat exchanger of energy recovery system (temperature of supply air) (°C) Tre: Temperature of return air from the indoor space (temperature of return air) (°C)

6.2.2  Latent Efficiency The latent efficiency (εlat) is also known as moisture transfer efficiency, which is calculated based on the amount of humidity ratio which can be expressed in the following Eq. 6.4:

ε lat =

ωin − ωsu ωin − ωre

(6.4)

where: εlat: Latent efficiency (%) ωin: Humidity ratio of supply air before entering the heat exchanger of energy recovery system (humidity ratio of intake air) (g/kg) ωsu: Humidity ratio of supply air before entering the heat exchanger of energy recovery system (humidity ratio of supply air) (g/kg) ωre: Humidity ratio of return from the indoor space (humidity ratio of return air) (g/ kg)

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6  Evaluating the Performance of Energy Recovery Systems

6.2.3  Enthalpy Efficiency The enthalpy efficiency (εtot) or total transfer efficiency refers to the sum of heat and moisture transfer, which is presented by Eq. 6.5.

ε tot =

H in − Hsu H in − H re

(6.5)

where: εtot: Total enthalpy efficiency (%) Hin: Enthalpy of supply air before entering the heat exchanger of energy recovery system (enthalpy of intake air) (kJ/kg) Hsu: Enthalpy of supply air before entering the heat exchanger of energy recovery system (enthalpy of supply air) (kJ /kg) Hre: Enthalpy of return from the indoor space (enthalpy of return air) (kJ/kg)

6.2.4  Global Efficiency Global efficiency (ηG) was first introduced by a study in Roulet et al. (2001) by suggesting that the calculation of energy recovery efficiency should take into account air fresh airflow, infiltration and exfiltration rates of a building. The proposed global efficiency took into consideration the whole system including ventilated building and its ventilation equipment. Figure 6.1 schematically illustrates the network representing the whole system. In the system, outdoor air (o) entered the inlet grille (i) and was blown through energy or heat recovery system (ER) where it was either heated or cooled. Often, in the calculation of energy recovery efficiency, nominal efficiency of the energy recovery system itself (εER), is used. However, according to the study, by taking into account the whole system, the global efficiency was not equal to the nominal efficiency of energy recovery system. By considering measured data in the geographical and climatic conditions of Switzerland and Germany, in the study, it was found that with the factors of infiltration and exfiltration rates, the global efficiency was between 60 and 70% for the energy recovery systems having 80% nominal energy recovery efficiency. It was proven from the study that global efficiency equals the energy recovery efficiency only if there was no exfiltration and there was neither external nor extract-to-supply recirculation. Otherwise, global efficiency was smaller than energy recovery efficiency. Therefore, global efficiency of energy recovery system was significantly dependent on air infiltration and exfiltration rates. Jokisalo et al. (2009) also reported that infiltration and exfiltration had significant impact on global efficiency of energy recovery system for building applications. Results from these studies are very useful in the improvement of energy recovery

6.2 Efficiency

77

The other part of ER system

Exhaust duct

Infiltration air

Extract air

Atmosphere

inf AHU

Re

o

Outdoor air

6 re

e Rie

1

i

Inlet grille

5

ER 2

x Rxx

rs 3 Subsequent heating or cooling

4

Ventilated space

7

a

s exf Supply duct Exfiltration air

Fig. 6.1  Simplified network representing the whole system ventilated building and its ventilation equipment (Roulet et al., 2001)

system to ensure design airflow rates could be achieved. However, more studies should be carried with different geographical and climatic conditions of other regions or countries.

6.2.5  Efficiency Based on Effectiveness NTU Method Another approach to investigate the performance pertaining to efficiency of energy recovery system is by using effectiveness–NTU method. This approach takes into account heat exchange analysis in the heat exchanger of energy recovery system by using dimensionless parameters: heat capacity rates, effectiveness and number of transfer unit (NTU). This approach is often used in the situation when there is insufficient information pertaining to the calculation method using log–mean temperature difference (LMTD). The LMTD method is used when mass flow rates, intake air and exhaust air conditions of airstream in the heat exchanger are specified. However, this method requires tedious iterations and is not practical. In an attempt to eliminate the iterations from the solution of such problem, Kays and London (1984) introduced the effectiveness-NTU method, which greatly simplifies the heat exchanger analysis. Since then, it has been the most convenient method to predict performance of a heat exchanger (Zhang, 2009) and to calculate the sensible and latent effectiveness for heat exchangers analysis (Navarro and Cabezas-Gómez, 2007). The effectiveness of heat exchanger is highly depending on NTU number. When the NTU is small, the effectiveness is low and when the NTU is large, the effectiveness is high. In other words, the effectiveness increases with increasing

78

6  Evaluating the Performance of Energy Recovery Systems

NTU number (Niu and Zhang, 2002). The main limitation of this method associates with the non-uniform heat and mass transfer coefficient over the surface of heat exchangers. However, the equations and routines for this method are widely discussed in many references (Ge et al., 2013). The effectiveness of a heat exchanger can be defined by calculating its maximum possible heat transfer that could be achieved. It is dimensionless quantity between 0 and 1. In the heat exchanger, one fluid will experience the maximum temperature difference which is the temperature difference (ΔT) between inlets of hot (Th, i) and cold airstreams (Tc, i) (Eq. 6.6). ∆T = Th ,i − Tc ,i



(6.6)



The effectiveness (ε) is the ratio between the actual heat transfer rate (Qact, sen) and the maximum possible heat transfer rate (Qmax) (Eq. 6.7)

ε sen =

Qact Qmax

(6.7)



Let C = m cp ,

Qact ,sen = Ch ( Th ,i − Th ,o ) − Cc ( Tc ,i − Tc ,o )



(6.8)

When the fluid of cold stream undergoes the maximum temperature difference, the maximum possible heat transfer can be calculated as (Eqs. 6.9 and 6.10):

Qmax = Cmin ( Th ,i − Tc ,i )



Qact ,sen = ε Cmin ( Th ,i − Tc ,i )

(6.9)



(6.10)

For any heat exchanger, the effectiveness (ε), can be shown as (Eq. 6.11):



 C  ε = f  NTU, min  C max  

(6.11)

For a given geometry, the effectiveness (ε) can be calculated using correlations in terms of the heat capacity rate ratio ( Cr) (Eq. 6.12). Cr =

Cmin Cmax

(6.12)

and the number of transfer unit (NTU) is indicative of the size of a heat exchanger (Eq. 6.13)

6.2 Efficiency

79

NTU =

UA Cmin

(6.13)

where: U is the overall heat coefficient and A is the total surface area According to Kays and London (1984), for a cross-flow heat exchanger, when both airstreams are unmixed, the sensible effectiveness associated to heat transfer (sensible heat) in the heat exchanger can be calculated as (Eq. 6.14):

ε sen = 1 − exp {NTU 0.22 [exp ( −NTU 0.78 ) − 1]}





(6.14)

For a counter-flow heat exchanger, the effectiveness is as follows (Eq. 6.15):

ε sen =

1 − exp (−NTU (1 − Cr ,sen )

1 − Cr ,sen exp(−NTU (1 − Cr ,sen )

(6.15)

when Cr, sen = 1, the sensible effectiveness is presented as (Eq. 6.16):



ε sen =

NTU 1 + NTU

(6.16)

The heat exchanger also has an ability to transfer moisture in the air which is defined as latent heat transfer. The moisture transfer capability (latent recovered energy,QL) for latent heat transfer can be calculated by the following Eq.  6.17 (Incropera et al., 2013): Qlat =

( ω i − ωe )  1 1   +γ+  kC   kH

(6.17)

where: k is the convective moisture transfer coefficient and ω is humidity ratio Theoretical investigation for convective moisture transfer coefficient can be obtained from Sherwood correlation where it is derived by using Chilton–Colburn analogy (Eq. 6.18):

Sh = Nu.Le

−1

3



(6.18)

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6  Evaluating the Performance of Energy Recovery Systems

where: Sh is the Sherwood number and Le is Lewis number Sherwood number, Sh represents the effectiveness of convective moisture transfer in the surface of heat exchanger. Sherwood number is calculated by using Eq. 6.19: Sh =

kD Dva

(6.19)

where: Dva is diffusivity of vapour in air. Lewis number is expressed as the relative magnitude of heat and mass diffusion in the thermal and concentration boundary layers and is defined from Incropera et al. (2013) as (Eq. 6.20): Le =

α Dva

(6.20)

The convective moisture transfer coefficient can be calculated using the following Eq. 6.21.



k=

h· Dva ·Le λ

1

3

(6.21)



Similar to the definition of number of transfer unit for heat transfer, the number of transfer unit for moisture transfer can be calculated using the following Eq. 6.22 (Kadylak et al., 2009): NTU lat =

U lat A Cmin

(6.22)

The total moisture transfer coefficient (UL) can be calculated using Eq. 6.23:



1 1 1 = +γ + U lat kh kc

(6.23)

where: 1 1 + is the convective moisture transfer resistance on the hot and cold k h kc airstreams

6.2 Efficiency

81

γ is the moisture diffusive resistance in membrane and can be calculated as (Eq. 6.24) (Niu and Zhang, 2002):

γ=

δ ψ Dva

(6.24)

where: ψ is the coefficient of moisture diffusive resistance in membrane and is expressed as (Eq. 6.25):

ψ=

(

106 −9 + 10 10 e(

Φ φ) 2

2

5294 T )

ω

(6.25)

Zhang and Niu (2002) derived the relation between relative humidity,  Φ and humidity ratio, ω by using Clapeyron equation, it is expressed as (Eq. 6.26):



 e5294(T +273.15)  Φ=  ω  106  

(6.26)

For instance, in the context of cross-flow heat exchanger with unmixed airstreams, the latent effectiveness associated with moisture or mass transfer (latent heat) can be calculated using (Eq. 6.27):



  NTU lat 0.22   ε lat = 1 − exp   0.78   exp − NTU C C − 1 ( r lat )    r 

(6.27)

Based on Eqs. 6.23–6.26, moisture transfer coefficient influences moisture diffusivity resistance in the heat exchanger. Coefficient of ψ reflects the operating conditions of temperature and relative humidity on the moisture resistance. The convective moisture transfer coefficient (k) is related to the heat exchanger characteristics. Studies related to effectiveness–NTU method: There are various works that have been carried out pertaining to the effectiveness–NTU method of heat exchangers. Table 6.1 shows several studies related to this.

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6  Evaluating the Performance of Energy Recovery Systems

Table 6.1 Summary of selected studies pertaining to effectiveness–NTU method of heat exchangers References Zhang and Niu (2002)

Niu and Zhang (2002)

Nellis and Pfotenhauer (2005) Wetter (1999)

Description of studies Performance investigation in relation to effectiveness was carried out of a membrane-based heat exchanger. It was found that the sensible effectiveness is a function of NTU, the number of transfer units for heat, while the latent effectiveness is a function of NTUlat, the number of transfer units for moisture. A fixed value of NTU for sensible heat could be used to determine the NTUlat for moisture In this study, fundamental dimensionless groups for coupled heat and moisture transfer of a cross-flow of enthalpy exchanger were theoretically investigated. Results from the theoretical investigation were validated with experimental data. The finite difference of numerical solutions for the model was used to study heat and the moisture transfer in the exchanger. Results indicated that the sensible effectiveness was mainly determined by the NTU of the exchanger. Meanwhile, the latent effectiveness was affected by material of the exchanger and its operating parameters An analytical study on the effectiveness–NTU method for a counter-flow heat exchanger which was subjected to an external heat transfer was carried out A simple simulation model of air-to-air plate heat exchanger for energy recovery application was developed in this study. This model was used to calculate annual energy calculations by applying effectiveness–NTU relations to parameterise the convective heat transfer of the heat exchanger A study on the effectiveness-NTU computation with a mathematical model for cross-flow heat exchangers was performed

Navarro and Cabezas-­ Gómez (2007) Mathew and A study on application of the effectiveness–NTU relationship to parallel flow Hegab (2010) micro-channel heat exchangers subjected to external heat transfer was conducted. Thermal performance of parallel flow micro-channel heat exchangers was carried out and an equation for determining the heat transfer between the fluids was formulated. It was observed that irrespective of the heat capacity ratio, for a specific NTU, as external heating decreased, the effectiveness of the hot and cold fluids increased. In addition, at a given NTU, reduction in heat capacity ratio improved the effectiveness of the fluids. From this study, it can be concluded that the model developed in this study could be used to predict the axial temperature as well as the effectiveness of the fluids in parallel flow micro-channel heat exchangers, operating in the laminar flow regime, subjected to external heat flux. However, the model was limited to incompressible and single-phase working fluids of micro-channel flow applications Ahmad et al. Heat transfer and effectiveness analysis based on a physical model of a (2015) cross-flow heat exchanger were performed in this study. The transferred heat and effectiveness (ε) in terms of temperature and NTU method were calculated for both hot and cold streams. This study suggested as air velocity increased, the transferred heat through heat transfer plates increased. As the air temperature of hot exhaust stream increased, both the transferred heat and effectiveness decreased

6.4  Effects of Operating Parameters on the Performance

83

6.3  Recovered Energy Recovered energy of energy recovery system is calculated based on the heat moisture and total transfer rates that occur within the system. It is also defined as the amount of energy consumed to pre-cool or pre-heat the incoming air (Mardiana-­ Idayu and Riffat, 2011). There are three forms of recovered energy of energy recovery system: sensible recovered energy (Qsen), latent recovered energy (Qlat) and total (enthalpy) recovered energy (Qtot). Sensible recovered energy (Qsen) is calculated using the following Eq. 6.28. Qsen = maCp ( Tin − Tsu ) = maCp ( Tex − Tre )





(6.28)

Latent recovered energy (Qlat) is defined as the change of humidity ratio from the airflow and is expressed as (Eq. 6.29) Qlat = ma hfg (ωin − ωsu ) = ma hfg (ωex − ωre )





(6.29)

Total recovered energy (Qtot) can be calculated using the following Eq. 6.30:

Qtot = ma ( H in − Hsu ) = ma ( H ex − H re )



(6.30)

where: ma is the mass flow rate of air Cp is the specific capacity of air hfg is enthalpy for evaporation T is temperature ω is the humidity ratio and H is specific enthalpy of the air

6.4  Effects of Operating Parameters on the Performance Performance of energy recovery system is affected by physical and operating parameters. The physical parameters include size and heat transfer area, fans and ducting, materials and structure which have been discussed in Chap. 4. The main operating parameters that play significant role on the energy recovery performance are air velocity/airflow rate and air conditions in terms of temperature and relative humidity (Mardiana and Riffat, 2013).

84

6  Evaluating the Performance of Energy Recovery Systems

Table 6.2  Selected studies pertaining to the effect of air velocity/airflow in the performance of heat exchanger References Zhong et al. (2014)

Fernández-­ Seara et al. (2011) Liu (2008)

Shao et al. (1998) Nasif et al. (2010)

Al-Waked et al. (2013)

Hviid and Svendsen (2011) Yaïci et al. (2013)

Description of studies A study based on experimental investigation was performed to investigate the effect of various airflow rates on sensible and latent efficiency. Results denoted that when the airflow rates increased, efficiency decreased. In addition, heat and mass transfer coefficients accelerated with the increase of airflow rates. A study on various airflow rate was performed on plate heat exchanger for potential energy recovery application. Results indicated that the effectiveness decreased when the airflow rate increased. The system has lowered down from 94 to 78% with the airflow rate started from 50 m3/h and finished at 175 m3/h. An experimental investigation was carried out with air velocity ranged from 0.1 to 15 m/s. It was recorded at 0.1 m/s that the sensible and latent effectiveness was achieved at 89.46% and 96.67%, respectively. However, the decreasing trend of the effectiveness gradually reduced when the air velocity exceeded 4 m/s. A study on heat pipe recovery system with low-pressure loss for natural ventilation air velocity was tested between 0.5 and 1 m/s. The results indicated that the efficiency decreased with increasing air velocity. An experimental investigation on thermal performance of enthalpy heat exchanger for energy recovery application was conducted. Measurements were carried out for air velocity ranging from 0.3 to 2.89 m/s. Results showed that as the air velocity increased, the effectiveness declined. It was proven in this study that the effectiveness of the heat exchanger for energy recovery application decreased as the airflow rate increased. From this study, it was found that maximum variations of 1.8 and 3.7% in sensible and latent effectiveness, respectively, were observed. Heat transfer performance against airflow rates was studied in this report. It was found that sensible efficiency and the effectiveness–NTU method decreased with increasing airflow rates increased. The sensible efficiency was observed to be reasonably consistent with the effectiveness–NTU method results A numerical analysis of a membrane-based energy recovery ventilators using computational fluid dynamics (CFD) was carried out to study the effect of air velocity on the effectiveness of two types heat exchangers of the system (co-current and counter flow); the study simulated the typical summer/winter Canadian conditions. Results showed a decrease in the effectiveness with the increase in supply/exhaust air velocity. It was also found that effectiveness in the summer condition was higher than the winter condition.

6.4.1  Effects of Air Velocity/Airflow Air velocity/airflow has a significant impact on energy recovery performance. It has been reported that when air velocity or airflow rate decreases, efficiency of energy recovery system decreases (Zhong et  al., 2014). In contrary, recovered energy increases with increasing airflow rates (Zafirah and Mardiana, 2014). It was observed in a study by Manz and Huber (2000) that unintentional airflows had an impact on a ventilation unit with energy recovery system. These intentional airflows had reduced the performance of the system in combination with unintentional heat flows through its casing.

6.4  Effects of Operating Parameters on the Performance

85

Various studies have been conducted in relation to the effect of air velocity/airflow on the performance associated with its heat exchanger. Table 6.2 summaries a few selected studies pertaining to the effect of air velocity on the performance of heat exchangers for energy recovery application. From the literature, numerous studies on numerical, simulation and experimental works can be found pertaining to the effect of air velocity/airflow on the ­performance of heat exchanger for energy recovery application in buildings. Most of these studies have been focused on the effect of air velocity/airflow in balanced inlet and outlet stream. It can be seen that the lower air velocity/airflow can eventually result in better efficiency due to better heat and moisture transfer at higher retention time in heat transfer surface.

6.4.2  Effects of Air Conditions Outdoor climatic conditions, i.e. outdoor temperature and outdoor humidity, are two main factors affecting the performance of energy recovery system. It has been denoted that a high difference between indoor and outdoor temperature possess a better efficiency of energy recovery system (Liu et al., 2010; Zhou et al., 2007). A study on energy recovery performance with various weather and temperature set points was explored in Zhou et al. (2007) under climatic conditions of Shanghai and Beijing. It was found that as the outdoor climatic conditions approached indoor climatic conditions, the system was indicated to be uneconomical or less beneficial. Besides, the system denoted to have a better performance during hot season since the temperature differences between indoor and outdoor during this season were larger as compared to cold season. It was reported in Zhong et al. (2014) that the sensible efficiency showed less sensitivity towards various temperature values than latent efficiency. Throughout the literature, many studies have been performed related to the investigations of the effects of air conditions on the heat and mass transfer of heat exchanger for energy recovery application. For instance, performance of energy recovery system with various seasonal weathers and temperatures was investigated in Zhou et al. (2007). It was found that the system was able to recover both sensible and latent heat in both during summer and winter conditions. In addition, for the system, heating recovery during summer conditions gave more significant impact as compared to cooling recovery during winter conditions. On the other hand, an experimental study on the performance of energy recovery system in Mardiana-­ Idayu and Riffat (2011) found that the temperature differences had a significant impact on the recovered energy. The study indicated that the increase of temperature difference ranged from 2.9 to 4.3 °C resulted in the increase in sensible, latent or total enthalpy in terms of efficiency. Lu et al. (2010) investigated the performance of energy recovery by applying cross-flow mechanism and induced film vibration on the flat plate-type energy recovery and it was found that the efficiency had slightly changed by the temperature difference. A theoretical study conducted by

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6  Evaluating the Performance of Energy Recovery Systems

Min and Su (2010) investigated an energy recovery system in hot–humid climatic conditions with three different outdoor air temperatures which were 32, 35 and 38 °C and outdoor air humidity of 50%. It was recorded that the changes in outdoor temperature caused almost the same change in latent and sensible heat. The study also indicated that the sensible efficiency showed little changes in hot–humid climate with the outdoor temperature and insignificant reduction as the outdoor humidity increased. In terms of relative humidity, an increase of relative humidity resulted in an increase of latent efficiency, while a decrease in temperature also increases the latent efficiency (Boardman and Glass, 2013). Rasouli et al. (2013) stated that in various climatic conditions, optimum energy recovery where latent-to-sensible ratio was highest could be found when the outdoor enthalpy or temperature was greater than that of the indoor air. Nam and Han (2016) concluded that a decrease in o­ utdoor temperature and an increase in the indoor humidity could increase the sensible efficiency of the system. As discussed in the previous subchapters, temperature and humidity ratio difference between inlet and outlet stream is often the key to determine energy recovery efficiencies, hence the effect of operating parameters especially the outdoor air conditions will always be the key factors pertaining to the performance of energy recovery systems.

6.5  Summary Energy recovery performance can be evaluated in terms of efficiency and recovered energy (heat and mass transfer) through its heat exchanger. The efficiency can be determined by applying three established methods: (1) ASHRAE standard method which can be defined in terms of sensible/temperature efficiency, latent efficiency and enthalpy (total) efficiency; (2) effectiveness–NTU method which can be defined by the calculation of NTU and heat capacity rate ratio when flow arrangement is known and/or (3) global efficiency which takes into account the ex-filtration and in-filtration rate of the ventilation system. Recovered energy of energy recovery system is calculated based on the heat, moisture and total transfer rates that occur within the system in the form of: (1) sensible recovered energy, latent recovered energy and total (enthalpy) recovered energy. The energy recovery performance system is affected by several operating parameters pertaining to air velocity/airflow rate and air conditions in terms of temperature and relative humidity. Acknowledgements  Universiti PTEKIND/8014124).

Sains

Malaysia

Research

University

Grant

(1001/

References

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References Ahmad, M. I., Yatim, Y. M., & Masitah, A. (2015). Heat transfer and effectiveness analysis of a cross-flow heat exchanger for potential energy recovery applications in hot-humid climate. Energy Research Journal, 6(1), 7–14. https://doi.org/10.3844/erjsp.2015.7.14 Al-Waked, R., Nasif, M.  S., Morrison, G., & Behnia, M. (2013). CFD simulation of air to air enthalpy heat exchanger. Energy Conversion and Management, 74, 377–385. https://doi. org/10.1016/j.enconman.2013.05.038 ASHRAE (Ed.). (2005). ASHRAE handbook of fundamentals. Atlanta, GA: American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc.. Boardman, C., & Glass, S.  V. (2013). Moisture transfer through the membrane of a cross-flow energy recovery ventilator: Measurement and simple data-driven modeling. Journal of Building Physics, 38(5), 389. https://doi.org/10.1177/1744259113506072 Fernández-Seara, J., Diz, R., Uhía, F. J., Dopazo, A., & Ferro, J. M. (2011). Experimental analysis of an air-to-air heat recovery unit for balanced ventilation systems in residential buildings. Energy Conversion and Management, 52(1), 635–640. https://doi.org/10.1016/j. enconman.2010.07.040 Ge, G., Ghadiri Moghaddam, D., Namvar, R., Simonson, C. J., & Besant, R. W. (2013). Analytical model based performance evaluation, sizing and coupling flow optimization of liquid desiccant run-around membrane energy exchanger systems. Energy and Buildings, 62, 248–257. https:// doi.org/10.1016/j.enbuild.2013.03.017 Hegab, H. (2010). Application of effectiveness-NTU relationship to parallel flow microchannel heat exchangers subjected to external heat transfer. International Journal of Thermal Sciences, 49, 76–85. https://doi.org/10.1016/j.ijthermalsci.2009.06.014 Hviid, C. A., & Svendsen, S. (2011). Analytical and experimental analysis of a low-pressure heat exchanger suitable for passive ventilation. Energy and Buildings, 43(2), 275–284. https://doi. org/10.1016/j.enbuild.2010.08.003 Incropera, F. P., Lavine, A. S., Bergman, T. L., & DeWitt, D. P. (2013). Principles of heat and mass transfer. Hoboken: Wiley. Jokisalo, J., Kurnitski, J., Korpi, M., Kalamees, T., & Vinha, J. (2009). Building leakage, infiltration, and energy performance analyses for Finnish detached houses. Building and Environment, 44, 377–387. https://doi.org/10.1016/j.buildenv.2008.03.014 Kadylak, D., Cave, P., & Mérida, W. (2009). Effectiveness correlations for heat and mass transfer in membrane humidifiers. International Journal of Heat and Mass Transfer, 52(5), 1504–1509. https://doi.org/10.1016/j.ijheatmasstransfer.2008.09.002 Kays, W. M., & London, A. L. (1984). Compact heat exchangers (3rd ed.). New York: McGraw-Hill. Liu, S. (2008). A novel heat recovery/desiccant cooling system. PhD Thesis, University of Nottingham. Liu, J., Li, W., Liu, J., & Wang, B. (2010). Efficiency of energy recovery ventilator with various weathers and its energy saving performance in a residential apartment. Energy and Buildings, 42(1), 43–49. https://doi.org/10.1016/j.enbuild.2009.07.009 Lu, Y., Wang, Y., Zhu, L., & Wang, Q. (2010). Enhanced performance of heat recovery ventilator by airflow-induced film vibration (HRV performance enhanced by FIV). International Journal of Thermal Sciences, 49(10), 2037–2041. https://doi.org/10.1016/j.ijthermalsci.2010.06.001 Manz, H., & Huber, H. (2000). Experimental and numerical study of a duct/heat exchanger unit for building ventilation. Energy and Buildings, 32(2), 189–196. https://doi.org/10.1016/ S0378-7788(00)00043-8 Mardiana, A., & Riffat, S.  B. (2013). Review on physical and performance parameters of heat recovery systems for building applications. Renewable and Sustainable Energy Reviews, 28, 174–190. https://doi.org/10.1016/j.rser.2013.07.016 Mardiana-Idayu, A., & Riffat, S. B. (2011). An experimental study on the performance of enthalpy recovery system for building applications. Energy and Buildings, 43(9), 2533–2538. https:// doi.org/10.1016/j.enbuild.2011.06.009

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Min, J., & Su, M. (2010). Performance analysis of a membrane-based energy recovery ventilator: Effects of membrane spacing and thickness on the ventilator performance. Applied Thermal Engineering, 30(8), 991–997. https://doi.org/10.1016/j.applthermaleng.2010.01.010 Nam, S.-H., & Han, H. (2016). Computational modeling and experimental validation of heat recovery ventilator under partially wet conditions. Applied Thermal Engineering, 95, 229–235. https://doi.org/10.1016/j.applthermaleng.2015.11.023 Nasif, M., Al-Waked, R., Morrison, G., & Behnia, M. (2010). Membrane heat exchanger in HVAC energy recovery systems, systems energy analysis. Energy and Buildings, 42(10), 1833–1840. https://doi.org/10.1016/j.enbuild.2010.05.020 Navarro, H., & Cabezas-Gómez, L. (2007). Effectiveness-ntu computation with a mathematical model for cross-flow heat exchangers. Brazilian Journal of Chemical Engineering, 24, 509. https://doi.org/10.1590/S0104-66322007000400005 Nellis, G., & Pfotenhauer, J. (2005). Effectiveness-NTU relationship for a counterflow heat exchanger subjected to an external heat transfer. Journal of Heat Transfer, 127, 1071. https:// doi.org/10.1115/1.2010496 Niu, J. L., & Zhang, L. Z. (2002). Effects of wall thickness on the heat and moisture transfers in desiccant wheels for air dehumidification and enthalpy recovery. International Communications in Heat and Mass Transfer, 29(2), 255–268. https://doi.org/10.1016/S0735-1933(02)00316-0 Rasouli, M., Ge, G., Simonson, C. J., & Besant, R. W. (2013). Uncertainties in energy and economic performance of HVAC systems and energy recovery ventilators due to uncertainties in building and HVAC parameters. Applied Thermal Engineering, 50(1), 732–742. https://doi. org/10.1016/j.applthermaleng.2012.08.021 Roulet, C.-A., Heidt, F. D., Foradini, F., & Pibiri, M.-C. (2001). Real heat recovery with air handling units. Energy and Buildings, 33, 495–502. https://doi.org/10.1016/S0378-7788(00)00104-3 Shao, L., Riffat, S. B., & Gan, G. (1998). Heat recovery with low pressure loss for natural ventilation. Energy and Buildings, 28(2), 179–184. https://doi.org/10.1016/S0378-7788(98)00016-4 Wetter, M. (1999). Simulation model air-to-air plate heat exchanger. Berkeley: Simulation Research Group, Building Technologies Department, Lawrence Berkeley National Laboratory. https://doi.org/10.2172/7352 Yaïci, W., Ghorab, M., & Entchev, E. (2013). Numerical analysis of heat and energy recovery ventilators performance based on CFD for detailed design. Applied Thermal Engineering, 51(1), 770–780. https://doi.org/10.1016/j.applthermaleng.2012.10.003 Zafirah, M. F., & Mardiana, A. (2014). Design, efficiency and recovered energy of an air- to-air energy recovery system for building applications in hot-humid climate. International Journal of Science and Research, 3(9), 1803–1807. Zhang, L.-Z. (2009). Heat and mass transfer in plate-fin enthalpy exchangers with different plate and fin materials. International Journal of Heat and Mass Transfer, 52(11), 2704–2713. https:// doi.org/10.1016/j.ijheatmasstransfer.2008.12.014 Zhang, L. Z., & Niu, J. L. (2002). Performance comparisons of desiccant wheels for air dehumidification and enthalpy recovery. Applied Thermal Engineering, 22(12), 1347–1367. https://doi. org/10.1016/S1359-4311(02)00050-9 Zhong, T., Li, Z., & Zhang, L. (2014). Investigation of membrane-based total heat exchangers with different structures and materials. Journal of Membrane and Separation Technology, 3, 1–10. Zhou, Y. P., Wu, J. Y., & Wang, R. Z. (2007). Performance of energy recovery ventilator with various weathers and temperature set-points. Energy and Buildings, 39, 1202–1210. https://doi. org/10.1016/j.enbuild.2006.12.010

Chapter 7

Energy Recovery in Integrated or Hybrid Systems towards Energy-Efficient Technologies

7.1  Overview According to the International Energy Agency, the building sector accounts for 36% of global final energy consumption and approximately 40% of total direct and indirect carbon dioxide emissions (IEA, 2019). This significantly motivates the development of near net-zero energy buildings through the establishment of renewable energy, low carbon technologies and passive design. In addition, HVAC energy is predicted to increase as the building fabric energy performance improves to meet the demand for ventilation standards in terms of providing good indoor air quality. There is a need therefore to incorporate energy-efficient technologies and developing ventilation systems for low energy buildings that are capable to meet the demand of energy savings and providing the necessary levels of indoor air quality to the occupants. In this context, integrating energy recovery ventilator or energy recovery system into the existing ventilation systems would be one of the effective methods towards achieving energy-efficient systems for low energy buildings. In all energy recovery systems, the underlying principle is to minimise the energy needed to pre-­ heat or pre-cool incoming air by re-using the heat or energy from the outgoing air (Shurcliff, 1988). This chapter discusses the integration of energy recovery systems in various building services; highlighting mechanical ventilation systems, natural ventilation systems, air-conditioning systems, dehumidification systems and building integrated photovoltaic systems.

7.2  Energy Recovery in Mechanical Ventilation System Mechanical ventilation systems are active systems that provide better indoor air quality in buildings by extracting stale air and/or by supplying fresh air. Fresh air is provided into a building usually by fans or heat pumps. These systems can be © Springer Nature Switzerland AG 2020 M. I. Ahmad, S. Riffat, Energy Recovery Technology for Building Applications, https://doi.org/10.1007/978-3-030-50006-1_7

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r­ etrofitted in buildings to promote fresh air into the indoor spaces and remove any latent heat. With the aim to improve indoor air quality and provide energy savings in buildings, many studies have been carried out associated with integrated systems of energy recovery with mechanical ventilation units. This integrated unit is called as mechanical ventilation heat/energy recovery system (MVHR). The system is driven by electric power and utilises forced airflow to deliver and extract air into multiple spaces with heat exchange occurring in a central air handling unit (Adamu and Price, 2015). In this system, the energy recovery system is used to recover heat that is usually wasted. It works by supplying and extracting airflows in and out of buildings to ensure a good quality of indoor air is achieved. The efficiencies of these systems typically lie between 60 and 95% (Mardiana-Idayu and Riffat, 2012). The systems have been widespread in recent years due to their relation with low carbon buildings (Adamu and Price, 2015). Table 7.1 shows several selected studies pertaining to integrated mechanical ventilation with heat/energy recovery systems.

7.3  Energy Recovery Assisted Passive/Natural Ventilation Passive ventilation is also known as natural ventilation that makes use of natural forces such as wind and thermal buoyancy. It involves the process of supplying air to and removing air from an indoor space or a building without using mechanical or active systems. Usually fresh air is provided to a building through openings such as windows, vents and doors as a result of pressure differences arising from natural forces (Etheridge, 2015). These ventilation systems work to regulate the internal air temperature. In buildings, there are two types of natural ventilation: (1) wind driven ventilation and (2) buoyancy-driven ventilation. In most countries, mechanical ventilation with heat/energy recovery system is often desirable. However, the disadvantages of this system include the usage of energy to power the system, high maintenance (Guerra-Santin et al., 2013) and the complexity of ducting throughout the system (Adamu and Price, 2015). In this context, passive natural with heat/ energy recovery system is one of the potentials which could be used as a low-energy alternative (O’Connor et al., 2016). This system has obvious advantages in terms of low energy consumption and low installation costs as it does not require fan power or major energy consumption. In this system, heat in the outlet air from natural ventilation is recovered and is used for pre-heating the inlet air. Most of the heat/energy recovery assisted passive or natural ventilation systems are designed by exploiting the buoyant nature of warm indoor air without depending on mechanically-driven airflow. The design concepts have been reported in several studies (Gan and Riffat, 1997; O’Connor et al., 2016; Dorizas et al., 2018) but very limited evidence can be found pertaining to wide-scale practical or commercial application of these systems (Adamu and Price, 2015; https://ventive.co.uk). One of the commercial applications of these systems is Passive Ventilation with Heat Recovery (PVHR™) which is a patented system/method of delivering ­consistent airflow while drastically reducing heat loss using passive-stack based on

(continued)

Results showed that heat/energy recovery ventilation system could save up to 74% energy savings of the ventilation loss with investment payback within 12 years

Akbari and Oman (2013)

In this paper, the impact of a heat/energy recovery in mechanical ventilation system ventilator on the energy consumption and indoor radon in residential buildings was investigated in a detached house in Stockholm, Sweden. The performance of the heat recovery ventilation system was examined with respect to radon mitigation and energy savings

Experimental results showed that the heat/energy recovery A novel mechanical ventilation heat/energy recovery (MVHR) with the capability of continuously defrosting itself without using supplementary heating suitable for cold or arctic climates was constructed efficiency was still high and capable to defrost below the freezing point in this study

Kragh et al. (2007)

In this study, it was found that the integrated mechanical double heat recovery pump system was the most efficient system for energy savings for the studied building

Nguyen Overall performance of mechanical ventilation heat pump system with heat/energy recovery during et al. (2005) forced ventilation in a building was investigated in this study. The methods for recovering sensible heat during the ventilation process were evaluated experimentally in four types of ventilation: types A, B, C and D. Results were compared with the case of without heat recovery ventilation. For type A, no heat recovery was used while in type B, a separate sensible heat recovery was used. In type C, single heat recovery was used for recovery heat in an integrated heating-­ventilation and in type D, double heat recovery was used to recover heat and heat pump as the second heat recovering mechanism

An advanced mechanical ventilation heat/energy recovery (MVHR) incorporating heat pump system was Results indicated that the air changes per hour complied with the developed in this study ASHRAE Standard to maintain indoor air quality. The system also had low capital and low maintenance costs

Riffat and Gillott (2002)

Results showed that a traditional exhaust ventilation system could use more energy than a ventilation system with a heat/energy recovery system. The efficiency of heat/energy recovery had a significant effect on the building energy consumption

Results showed that by using these units, rooms can be efficiently ventilated at a good level of thermal comfort. Temperature efficiencies of up to 78% were found at low levels of electric energy input

Performance of single-room ventilation units with recuperative or regenerative mechanical ventilation heat/energy recovery by means of experiments and numerical simulations was conducted in this study. Ventilation efficiency, thermal comfort, heat recovery and electric energy input were analysed.

Manz and Huber (2000)

Jokisalo A simulation study using dynamic thermal modelling of various mechanical supply and exhaust et al. (2003) ventilation systems incorporating heat/energy recovery in typical Finnish residential apartment buildings in Finland was performed. The systems were based on cost-efficient realistic components available in the market. From this study, energy efficiency in a residential building could be improved remarkably by using the system

Results demonstrated that the use of mechanical ventilation systems with heat/energy recovery unit offered a practical, cost-effective and energy-­efficient means of improving indoor air quality and produced energy savings

Description of studies

Nazaroff In this study, a mechanical ventilation system with heat/energy recovery (MVHR) was carried out with et al. (1981) the aim to control indoor radon concentrations in an energy research house in Maryland

References

Table 7.1  Selected studies pertaining to integrated mechanical ventilation with heat/energy recovery systems

7.3  Energy Recovery Assisted Passive/Natural Ventilation 91

Results indicated that the heat recovery energy efficiency of the system for supply air was about 77%. It was a good result as compared to the given efficiency in the air handling unit’s passive house certificate (which is about 75%). The regression analysis showed that the thermal efficiency was dependent on ambient conditions Results showed that the system had higher energy performances in a wide range of temperatures than other system analysed.

Results indicated that the tested system improved the thermal comfort conditions (7.6–9.5 °C) as compared to the houses without the system. Energy savings of up to 19% over the space heating season was also reported In comparison with a conventional and heat recovery assisted mechanical system, the integrated multistage heat recovery systems led to 6.53% of energy savings and 6.26% of energy savings improvement, respectively, while prospers from less than an operational year of payback period

Results showed that the heat/energy recovery efficiency of MVHR was the most decisive factor in rating the performance of the combined system with an air preheater. On the other hand, the reduction of the initial defrosting need was significant. The obtained results showed that the defrosting need in a building located in central Sweden in two cases of an MVHR system equipped with a rotary heat exchanger/plate heat exchanger was reduced to one-third. The defrosting need was reduced by 50% in northern Sweden for both cases

In this study, a mechanical ventilation system with heat recovery integrated by heat pump was analysed. The study was carried out to assess the energy performances of the system during the control of temperature in the winter season. Tests were performed at different temperature values of simulated outdoor air (−5, 0, 5 and 10 °C) and a fixed (reference) indoor simulated temperature (20 °C). Each trial was performed with a ventilation flow rate of 535 m3/h. The coefficient of performance of the overall system (COPs) was 9.50 at 0 °C, 8.86 at 5 °C and 6.62 at 10 °C respectively. A comparison with other prototypes found in literature and with a common commercial system was also made

Thermal simulations (TRNSYS + TRNFlow) were performed on a Norwegian house typology and a French house typology equipped with mechanical ventilation with heat recovery (MVHR) system. In this study, seven different climates were considered ranging from mixed to sub-arctic conditions in order to evaluate the impact of the system towards energy consumption for space heating and thermal comfort

Fucci et al. (2016)

Cablé et al. (2019)

Jafarinejad This study proposed and analysed a novel integrated multistage heat/energy recovery system that able to et al. (2019) recover ventilated air heat through a heat pipe heat recovery unit to pre-cool the fresh air and afterwards through a condenser-side mixing box heat recovery unit in order to deplete all the recovery potentials of the ventilated air and pre-cool the condenser cooling air. A comprehensive mathematical model was presented and an algorithm was developed to quasi-dynamically simulate and assess the novel integrated multistage heat/energy recovery system. The integrated system was compared with the conventional system and each recovery system stand-alone

Nourozi This study focused on reducing the defrosting need by preheating the incoming cold outdoor air to et al. (2019) MVHR during the coldest days. The effects of preheating the incoming air to MVHR on ventilation heat load and annual ventilation heating demand were also investigated

Description of studies

Kamendere In this study, evaluating the efficiency of mechanical ventilation systems with heat recovery (MVHR) in et al. (2015) renovated multi-family residential buildings was carried out. A comparison with natural ventilation systems was also performed

References

Table 7.1 (continued)

92 7  Energy Recovery in Integrated or Hybrid Systems towards Energy-Efficient…

7.5  Energy Recovery Incorporated with Dehumidification System

93

natural ventilation systems. This system transfers the thermal energy from exhaust air to fresh incoming air. It works by using wind power and natural buoyancy for passive airflow. It was first developed in 2013 at Imperial College London to retrofit houses with ventilation systems that remove stale air, damp and pollutants. The concept has since been expanded for usage in various building types such as schools, offices and new-build housing projects https://ventive.co.uk). Table 7.2 shows several selected studies pertaining to integrated energy recovery with passive/natural ventilation systems.

7.4  Energy Recovery Coupled Air-Conditioning An air-conditioning system works to provide suitable indoor environmental conditions in terms of temperature, humidity and fresh air. With rapid economic development, the air-conditioning of buildings recently signifies a considerable portion of the total energy consumption in the world. An increase of more than 5 billion air-­ conditioning systems in between the years of 2016 and 2050 in buildings is predicted by The International Energy Agency, which is double than the existing installed units (IEA, 2019). Air-conditioning system is used to remove heat and moisture from a building or an indoor space to improve indoor air quality and comfort of occupants. With the increasing concern on energy demand and conservation, energy-efficient air-conditioning systems become more and more important. Pursuing the goal to reduce energy consumption in buildings, energy recovery coupled air-conditioning systems have been proven as a promising alternative to conventional air conditioning units (need citation). A lot of different studies have been undertaken to investigate and evaluate the performance of these systems (Mardiana-­ Idayu and Riffat, 2012). Table 7.3 shows several studies pertaining to the energy recovery assisted air-conditioning systems.

7.5  E  nergy Recovery Incorporated with Dehumidification System Excess moisture in an indoor space can encourage the growth of microbes, such as mould, fungi and bacteria, especially in regions that experience high humidity levels. The presence of these biological agents in the indoor spaces is due to dampness when sufficient moisture is available and inadequate ventilation which leads to Sick Building Syndrome (SBS) (Adams et  al., 2016). Since most people spend about 90% of their time indoors (Klepeis et al., 2001), thus these conditions would directly influence the life quality; health; productivity and performance of the occupants. In order to overcome this, consideration should be given in providing adequate ventilation to ensure that good indoor environmental quality is achieved in indoor spaces

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without compromising the occupants’ comfort level. One of the efficient solutions to meet this demand is by the utilisation of heating, ventilation and air­conditioning (HVAC) systems or normally known as air-conditioning and mechanical ventilation (ACMV) in tropical climate regions. ASHRAE standards recommend that the HVAC/ACMV systems must be able to keep a humidity ratio below 0.012 and the relative humidity between 30 and 60% in occupied spaces (ASHRAE, 2016). However, the use of these systems requires a large amount of energy depending on the efficiency of the system components, climatic conditions and whether the systems are appropriately sized for their host indoor spaces. Energy consumption used by HVAC or ACMV systems in the building sector accounts for approximately 50–60% of the total final energy demand across the world (Mardiana and Riffat, 2015). This percentage is predicted to increase due to the rapid growth of energy consumption in line with the rising living standards and urbanisation support for shifting to a modern lifestyle. In addition, concerns about the environment and climate change have enforced substantial changes in how energy should be transmitted and utilised in buildings or indoor spaces. Hence, the need for green energy-efficient technologies with less utilisation of natural resources would be an ideal choice to reduce energy consumption and mitigate environmental impact. In order to create an idea of environmentally friendly building aside from the standard energy conservation approach, innovative technologies must be implemented in the building. To respond to these demands, together with the increasing requirements for indoor comfort, more sustainable energy-efficient heating, cooling, ventilation and dehumidification technologies are needed. Therefore, improving the effectiveness and efficiency of humidity control is crucial in maintaining healthy and comfortable indoor air climates while reducing energy consumptions. With this regard, dehumidification systems can provide energy-efficient solutions and have a successful track record for building and industrial applications for more than 60 years (Pesaran, 1994; Niemann and Schmitz, 2019). With the advancement of technology, hybrid technology consists of energy recovery coupled dehumidification systems that have been developed to address the above challenges (Zhao et al., 2016). This technology is one kind of innovative and green technology, which employs low energy sources to meet the demand for energy conservation and comfortable working or living spaces for building occupants. It has a minimum ­dependency on fossil fuel and is environmentally friendly. It is designed to remove moisture from the air to control humidity in buildings while reducing energy consumption. The benefits of this technology are better humidity control, more efficient latent load removal, and reduction of peak electric demands (Rambhad et al., 2016). Table 7.4 gives a summary of selected studies in this area.

7.5  Energy Recovery Incorporated with Dehumidification System

95

Table 7.2 Selected studies pertaining to integrated energy recovery with passive/natural ventilation systems References Gan and Riffat (1997)

Description of studies In this study, a project on heat pipe recovery with solar-assisted natural ventilation in a glazed solar chimney as an integral part of the system with heat/ energy recovery efficiency was carried out. The heat pipe recovery unit used was a heat exchanger consisting of externally finned sealed pipe containing a volatile fluid such as methanol An analytical and experimental analysis of heat recovery concept that was developed for passive ventilation systems suitable for temperate climates

Findings It was found that by installing the heat pipe recovery in the chimney of the tested building, the airflow rates increased and the thermal buoyancy effect decreased. It was suggested that in order to achieve the required airflow rates in naturally ventilated buildings with heat recovery, wind force should be used The total pressure loss and Hviid and temperature exchange efficiency of Svendsen heat recovery was measured and (2011) found to be 0.74 Pa and 75.6%, respectively for a design airflow rate of 560 L/s It was concluded in the study that O’Connor In this study, six different heat/energy heat pipes and rotary thermal wheels et al. (2016) recovery devices were analysed and posed a good potential for integration compared for suitability for integration due to high thermal efficiency and into passive ventilation systems low-pressure loss across the heat/ energy recovery device in comparison to the other types Dorizas Ventilation performance of a passive Preliminary results showed that the et al. (2018) ventilation system with heat recovery operation of the ventilation system (PVHR) based on in-situ monitoring in a was more sensitive to changes in primary school in London was carried out. wind speed and direction than to The study involved long-term (15-month) buoyancy. The assessment of the monitoring of temperature, relative ventilation performance of such humidity and carbon dioxide (CO2) natural ventilation systems depending concentrations in both the classrooms and solely on wind and buoyancy was complicated as they were dynamic the outdoor environment. In addition, short term (1 and 2 weeks) observational systems that constantly balanced with the surrounding conditions monitoring was performed in two classrooms at the ventilation system level and classroom level, during both the heating and non-heating seasons. Temperatures and air velocities were measured within the PVHR system’ Calautit In this work, a novel design by integrating The system was able to provide et al. (2019) a passive heat/energy recovery device into adequate ventilation and had the potential to reduce demand on space a wind-catcher was proposed and heating systems. Recovery of 3 K investigated using numerical and from the exhaust airstream to the inlet experimental analysis. The proposed system incorporated a rotary thermal heat airstream was found which could generate energy savings up to 20% in recovery in the wind-catcher channel heating costs

Yau and Ahmadzadehtalatapeh (2010) Ahmadzadehtalatapeh and Yau (2011)

Gong et al. (2008)

Abd El-Baky and Mohamed (2007)

References Gu et al. (2004)

Findings The system was capable to recover energy

(continued)

Results showed that the temperature changes of fresh and return air increased with increasing the inlet temperature of fresh air. The effectiveness and heat transfer for both evaporator and condenser sections were also increased to about 48% when the inlet fresh air temperature was increased to 40 °C. The enthalpy ratio between the heat/ energy recovery and conventional air mixing was increased approximately 85% with increasing fresh air inlet temperature Results showed that the system could improve cooling A new heat/energy recovery technique for an air-­conditioning/ and heating effects, and was able to recover condensing heat-pump (AC/HP) system was proposed in this study. The new heat for heating sanitary water. From experimental technique employed a compound air-cooling and water-cooling results, it was found that the system was able to perform condensing module to replace the traditional sole air-cooling stably and flexibly under various conditions with a large condensing module of the AC/HP system. The system adopting this coefficient of performance (COP) about 6.0 technique was called as a multi-function AC/HP system It was concluded that the application of heat pipe This study focused on reviews on the application of heat pipe exchangers as heat/energy recovery could save energy exchangers for heat/energy recovery in air-conditioning of tropical and enhance dehumidification in the tropical climate climate In this study, the effect of heat pipe exchanger for heat/energy recovery Results showed that the system with the added eight-row on the existing air conditioning system of a hospital ward located in a heat pipe heat exchangers was able to provide convenient tropical region was investigated. Transient system simulation software and healthy air into the ward space according to the ASHRAE recommendations. It was also found that a (TRNSYS) was employed in the study. The impact on energy considerable amount of energy and power could be saved consumption, power savings, supply duct air and indoor air of the system was simulated and the results were compared with the existing system

Description of studies An energy recovery system using phase change materials to store the rejected (sensible and condensation) heat from the air-conditioning system was developed and studied In this study, heat pipe exchangers were used for heat/energy recovery applications in the air-conditioning system. The exchangers worked to cool the incoming fresh air of the system. Thermal performance and effectiveness of the combined system were studied

Table 7.3  Selected studies pertaining to the energy recovery assisted air-conditioning systems

96 7  Energy Recovery in Integrated or Hybrid Systems towards Energy-Efficient…

Description of studies In this study, a combination of heat pipe heat exchanger (HPHE) and air-conditioning (AC) system was developed to realise secondary heat/ energy recovery. Meteorological parameter of Hefei city was used as a reference. Energy consumed by the system was analysed

In this study, a new-type pulsating heat pipe heat exchanger (PHP-HE) was developed to recover energy from exhausted air of the airconditioning system

Experimental investigation of a ground-coupled air conditioning system with assisted enthalpy recovery was performed in this study under the winter mode of a temperate climate region based on measured data. Performance of the combined system was evaluated in terms of energy demand evaluating the performance of the investigated system in terms of energy demand required for air treatment, especially air humidification

References Wang et al. (2016)

Yang et al. (2019)

Niemann and Schmitz (2019)

Table 7.3 (continued) Findings It was found that the average heat/energy recovery efficiency of the HPHE AC system in winter was 21.08%, while 39.2% in summer. The secondary heat energy recovery HPHE AC system had an energy-­saving advantage Results indicated that the PHP-HE operated well both in the horizontal and bottom heated modes, and started up under a small temperature difference (less than 4.0 °C) between the fresh and exhaust air. The overall effectiveness of this PHP-HE was in the range of about 30 and 50%, depending on operation conditions such as the installation angle, fresh air temperature and wind speed. It was concluded that the heat pipe exchanger was beneficial for energy recovery application in the air-­conditioning system Results showed that the system was capable of providing highly comfortable indoor air conditions during winter. The combined system could be operated efficiently as a renewable heat source

7.5  Energy Recovery Incorporated with Dehumidification System 97

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7  Energy Recovery in Integrated or Hybrid Systems towards Energy-Efficient…

7.6  E  nergy Recovery Coupled-Photovoltaic/Solar Thermal System Solar energy is one of the renewable energy sources which can be applied to produce thermal energy through solar thermal collector and produce electrical energy through photovoltaic (PV) collector. It consists of several components including solar panel, solar inverter, mounting, cabling and other accessories to set up a working system. Nowadays, in building sector solar energy is used for both applications (generating electrical energy and heating) at the same time. This concept is called Table 7.4  gives a summary of selected studies in this area References Zhang et al. (2005)

Description of studies In this study, a mechanical dehumidification with a membrane-based total heat exchanger for energy recovery was developed. Thermodynamic modelling and annual energy consumption analysis of the combined system were carried out.

Findings Results indicated that the annual energy requirement of the system was 4.15 × 106 kJ per person, with 33% energy savings as compared to the conventional system without energy recovery Results showed that the system Zhang In this work, four independent air (2006) dehumidification systems with energy recovery with total energy recovery consumed very minimum units were proposed. The systems were primary energy. Energy savings compared with a mechanical dehumidification system without an energy recovery unit. Energy for the four systems were in a similar order analysis was performed on the systems Results indicated that the Liang et al. An independent air dehumidification system (2010) with membrane-based total energy recovery was system had a COP of 6.8 under proposed in this study. A prototype of the system nominal operating conditions with total energy recovery. The was developed under laboratory conditions. performance was robust to Performance of the system was evaluated outside weather conditions Zhao et al. A hybrid solid desiccant dehumidification cycle Results showed that the thermal (2016) with an internal energy recovery based sorptive COP of the system was improved to almost twice the material coated heat exchanger (SCHE) was thermal COP of the proposed in this study. This system realised conventional system without waste heat recovery from exhausted air in the HRD. The hybrid system could regeneration process, in which an energy be a potential alternative with recovery device (HRD) was adopted. The effective operation in high performance of the proposed system was compared with the conventional system without humidity and temperature areas energy recovery unit Results indicated that both An enthalpy recovery device using a liquid Zhang desiccant solution with a cross-flow pattern was recovery efficiency and exergy et al. efficiency improved with the investigated in this study and its simulation (2016) model was built. Performance of the system was increase of stage number. Data from this study could be used to evaluated in terms of recovery efficiency and design an optimised enthalpy exergy efficiency recovery device using liquid desiccant

7.6  Energy Recovery Coupled-Photovoltaic/Solar Thermal System

99

Gr hr

he Cond.

PVe

he2 hr2 he3

Fig. 7.1  Represents the heat transfer and energy balances that occur on the heat recovery unit as a system Bazilian et al. (2002)

building integrated photovoltaic/thermal (BiPV/T) systems, which have emerged in the 1990s (Corbin and Zhai, 2010). These systems have emerged since 2000 due to their prospect to fulfil the requirement design of net-zero energy buildings through the usage of solar energy. These systems are not just able to generate electricity, but also produce heat, which is typically wasted. BiPV/T system can be integrated with heat/energy or thermal recovery unit or the system itself can be enhanced for heat/ energy recovery application to pre-heat incoming air especially for the winter season. The heat/energy or thermal recovery unit can take advantage of the waste heat, thus providing cogeneration for the system. A heat/energy recovery unit of an integrated BiPV/T was modelled in a study by Bazilian et al. (2002). The model was described as two dimensional and steady-state. Heat transfer and energy balance that occurred on the heat/energy recovery unit of the system is shown in Fig. 7.1. The model was developed to solve the output and PV cell temperatures given the ambient conditions in terms of ambient air temperature, relative humidity, irradiation on the surface of the BiPV array, wind speed and input temperatures. By integrating the system with heat/energy recovery unit or enhance the system towards heat/energy recovery application, the advantages are more certain than conventional BiPV/T systems. A typical design of photovoltaic/thermal (PV/T) system with heat/energy recovery unit, consisting of five main parts: (1) living space (room); (2) desiccant dehumidification and regeneration unit; (3) air-conditioning system; (4) PV/T air heating collector and (5) air mixing unit which is shown in Fig.  7.2 as studied in Sukamongkol et  al. (2010). The system was designed to recover heat from the condenser of an air-conditioning system for regenerating desiccant to be used to reduce the energy use of an air-conditioned room. A study in Karava et al. (2012) found that the flow of air in a canal underneath the BiPV/T system was able to provide a significant recovery for solar radiation as thermal energy. On the other part, the panel was cooled by recovered heat from the

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7  Energy Recovery in Integrated or Hybrid Systems towards Energy-Efficient…

PV panel. This could increase the electricity generation of the system. Therefore, this integrated system could provide a great potential to be incorporated into the building which consumes zero energy; however, this technology is not in common use (Biyik et al., 2017). Table 7.5 presents a summary of selected existing studies in this area.

7.7  Summary Motivated by the aim towards near-net-zero buildings and to meet the demand of ventilation standards, integrated energy recovery systems have become a spotlight in various building services including mechanical ventilation, natural ventilation, air-conditioning, dehumidification and building integrated photovoltaic. Research

4 G, Vwind

m–a.con

3

m–a.PV/T m–a,PV/T Ta, fa

Ta, fa

Tt2out, ft2out 5

Condenser

m–a,con Ta,con, fa,con

Outdoor Indoor

Evaporator

m–a.r3 Tr3, fr3

1

Ta,mix fa,mix

m–a,r2 m–w,Tw Dehumidification unit

m–a,r1,Tr1, fr1

m–a,mix

Tr2, fr2 Regeneration unit

2

m–a,mix Ta,out fa,out

Fig. 7.2  Schematic diagram of a condenser heat recovery with a hybrid photovoltaic/thermal (Sukamongkol et al., 2010)

7.7 Summary

101

Table 7.5  presents a summary of selected existing studies in this area References Maffezzoni et al. (2009)

Description of studies A multiphysics model of a hybrid solar panel equipped with a solar concentrator and a cooling interface with heat/energy recovery capability was developed in this study. Temperature profile along the solar cells could be predicted as a function of cooling strategy. Using a macro model developed in this study, electrothermal simulation of the system could be conducted Bazilian et al., In this study, a numerical model was created to simulate the performance of a 2002 residential-scale building integrated photovoltaic (BiPV) heat/energy recovery system. The model was created in the engineering equation solver software package (EES). Performance of the system was investigated in the context of heat transfer. Results indicated that the model could assist to stimulate a modular heat/energy recovery unit addition to a building integrated photovoltaic system and could be customised facilitate the needs of occupants Crawford et al. A heat/energy recovery unit was combined with a BiPV system to take (2006) advantage of waste heat and provided cogeneration. In this study, two different photovoltaic (PV) cell types were combined with a heat/energy recovery unit and analysed in terms of their life-cycle energy consumption to determine the energy payback period. Results showed that energy payback periods were between 4 and 16.5 years, depending on the BiPV system Sukamongkol An investigation of the condenser heat/energy recovery unit with a et al. (2010) photovoltaic/thermal (PV/T) air heating collector was conducted in this study. Experimental work along with the procedure to study the validity of a developed simulation model in predicting the dynamic performance of the system was carried out to investigate the validity of developed simulation models in predicting the dynamic performance of the system. It was found that the model was in good agreement with the results observed in experimental investigation. The thermal energy generated by the system could produce warm dry air as high as 53 °C and 23% relative humidity. About 6% electricity of the total solar radiation could be obtained from the PV/T collector of the system. This system could save energy by approximately 18% with the use of a photovoltaic thermal combined with the heat/energy recovered from the condenser unit Corbin and An experimentally validated computational fluid dynamics (CFD) model of a Zhai (2010) building integrated photovoltaic/thermal (BIPV/T) collector was studied to analyse the effect of active heat/energy recovery on cell efficiency. Results showed that cell efficiency could be raised by 5.3%

works on the integrated mechanical ventilation heat/energy recovery systems have been focused on theoretical and experimental investigations. The integration of heat/energy recovery technology into passive/natural ventilation has a great potential to reduce energy demand in buildings but further investigation is needed to optimise the recovery devices for simple installation with high efficiency. By ­incorporating energy recovery units with dehumidification systems, a significant amount of moisture could be removed from the air while reducing energy consumption. Additionally, the integration of energy recovery system with building-integrated photovoltaic/thermal is very promising towards low carbon building; however, this technology is not in common use. Throughout the literature, it can be concluded that research on practical application and economic analysis of these

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systems is still quite lacking. More works on the combination of energy recovery system with low carbon technologies such as evaporative cooling, passive ventilation design and building integrated photovoltaic/thermal should be carried out in the future. Acknowledgements  Universiti PTEKIND/8014124).

Sains

Malaysia

Research

University

Grant

(1001/

References Abd El-Baky, M.  A., & Mohamed, M.  M. (2007). Heat pipe heat exchanger for heat recovery in air conditioning. Applied Thermal Engineering, 27(4), 795–801. https://doi.org/10.1016/j. applthermaleng.2006.10.020 Adams, R. I., Bhangar, S., Dannemiller, K. C., Eisen, J. A., Fierer, N., Gilbert, J. A., et al. (2016). Ten questions concerning the microbiomes of buildings. Building and Environment, 109, 224– 234. https://doi.org/10.1016/j.buildenv.2016.09.001 Adamu, Z., & Price, A. (2015). Natural ventilation with heat recovery: A biomimetic concept. Buildings, 5, 405–423. https://doi.org/10.3390/buildings5020405 Ahmadzadehtalatapeh, M., & Yau, Y. H. (2011). The application of heat pipe heat exchangers to improve the air quality and reduce the energy consumption of the air conditioning system in a hospital ward—A full year model simulation. Energy and Buildings, 43(9), 2344–2355. https:// doi.org/10.1016/j.enbuild.2011.05.021 Akbari, K., & Oman, R. (2013). Impacts of heat recovery ventilators on energy savings and indoor radon in a Swedish detached house. WSEAS Transactions on Environment and Development, 9, 24–34. ASHRAE. (2016). HVAC Systems and Equipment (SI). In M. S. Owen (Ed.), American Society of Heating, Refrigerating and Air-Conditioning Engineers 2016 Handbook. United States. Bazilian, M. D., Kamalanathan, H., & Prasad, D. K. (2002). Thermographic analysis of a building integrated photovoltaic system. Renewable Energy, 26(3), 449–461. https://doi.org/10.1016/ S0960-1481(01)00142-2 Biyik, E., Araz, M., Hepbasli, A., Shahrestani, M., Yao, R., Shao, L., et al. (2017). A key review of building integrated photovoltaic (BIPV) systems. Engineering Science and Technology, an International Journal, 20(3), 833–858. https://doi.org/10.1016/j.jestch.2017.01.009 Cablé, A., Georges, L., Peigné, P., Skreiberg, Ø., & Druette, L. (2019). Evaluation of a new system combining wood-burning stove, flue gas heat exchanger and mechanical ventilation with heat recovery in highly-insulated houses. Applied Thermal Engineering, 157, 113693. https://doi. org/10.1016/j.applthermaleng.2019.04.103 Calautit, J., O’Connor, D., Shahzad, S., Calautit, K., & Hughes, B. (2019). Numerical and experimental analysis of a natural ventilation windcatcher with passive heat recovery for mild-cold climates. Energy Procedia, 158, 3125–3130. https://doi.org/10.1016/j.egypro.2019.01.1011 Corbin, C.  D., & Zhai, Z.  J. (2010). Experimental and numerical investigation on thermal and electrical performance of a building integrated photovoltaic–thermal collector system. Energy and Buildings, 42(1), 76–82. https://doi.org/10.1016/j.enbuild.2009.07.013 Crawford, R. H., Treloar, G. J., Fuller, R. J., & Bazilian, M. (2006). Life-cycle energy analysis of building integrated photovoltaic systems (BiPVs) with heat recovery unit. Renewable and Sustainable Energy Reviews, 10(6), 559–575. https://doi.org/10.1016/j.rser.2004.11.005

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Chapter 8

Application of Energy Recovery Systems in Various Building Types and Climatic Conditions

8.1  Application in Various Building Types Energy recovery system has been widely used in many types of buildings across many countries of the world such as China (Kang et al., 2010; Zhou et al., 2007), Canada (Rafati Nasr et  al., 2014), Finland (Pavel, 2010) and the Mediterranean region (Jaber, 2012). This indicates that the systems can be used effectively in various locations. An energy recovery system was installed in a supermarket located in China during the winter season (Kang et al., 2010). It was found that latent heat was not suitable for ventilation energy savings in the region due to the internal high moisture emissions. A simulation of an energy recovery system in an animal housing facility based on hourly weather data for one year was investigated in Freund (2003). The findings showed that more than 80% of the heating energy and 45% of the cooling energy could be saved with the installation of the system. Studies by Milbrandt (2008) and VanGeet and Reilly (2006) indicated that the systems worked well to recover energy in laboratories that led to energy savings. Fan and Ito (2012) explored the application of the energy recovery system in typical office space equipped with air-­conditioning during summer climate in Japan and found that the system provided significant energy and environmental benefits. Kragh et al. (2007) and Nielsen et al. (2009) conducted an investigation ofthe life cycle assessment of residential ventilation units with an energy recovery system. Their findings denoted a net positive impact in terms of energy savings. A study by Jaber (2012) proved that energy recovery system could save up to 17% of annual energy demand in typical Jordanian residential buildings. Report by Justo Alonso et al. (2015) showed that the systems had potentials in apartment buildings of cold climate. There are many reasons to install an energy recovery system, but the best reasons are to recover energy and reduce utility costs (Zhang & Zhang 2014). For instance, a building needs a cooling system of about 68 metric tons which requires 40% outdoor air (12,000 ft3/min of outdoor air and 18,000 ft3/min return air). By installing a 12,000 cubic feet energy recovery system per minute, the building requires only a © Springer Nature Switzerland AG 2020 M. I. Ahmad, S. Riffat, Energy Recovery Technology for Building Applications, https://doi.org/10.1007/978-3-030-50006-1_8

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8  Application of Energy Recovery Systems in Various Building Types and Climatic…

41 metric ton of mechanical ventilation unit to work together. This results in reduced mechanical ventilation unit size. It ranges from 3  months to 3  years in terms of payback period, which varies depending on the size of the energy recovery systems, building locations and climatic conditions. In a workplace or an indoor environment with a high concentration of occupants and required plenty of fresh air, the installation of energy recovery system could help to lower the carbon dioxide levels and provide comfort to the occupants. The system also helps to control odours as the stale air is exhausted out of the building and fresh air is brought in. When fresh air is brought into a building, conditioned air is exhausted back outside to equalise the pressure. With these qualities, they can be used in most commercial building applications such as offices, schools, universities, fitness centres, religious buildings, auditoriums, hospitals, retail shopping centres, function halls and other types of buildings that have high occupant density. Some applications have severe requirements and great care must be taken to prevent cross-­ contamination between airstreams. For instance laboratory fume hood exhaust, industrial applications with fumes or smoke (e.g. welding operations), industrial applications with toxic or noxious exhaust, restaurants or kitchens and specialised hospital treatment areas. In these situations, the airstreams must be physically isolated to ensure there is no cross-contamination. Apart from energy savings, the main market driver behind the increasing usage of energy recovery technology is the impact of codes and standards. Some examples of standards are: ASHRAE Standard 62.1 ASHRAE Standard 62.1–2019 was first published in 1973 as Standard 62 which outlines minimum ventilation rates and other measures intended to provide IAQ that is acceptable to human occupants. It is titled “Ventilation for Acceptable Indoor Air Quality” and is a major pillar of the rating system, as well as many local ventilation codes. This standard dictates how much ventilation air must be brought into a building. For most buildings constructed since 1989, this equates to 15–20 ft3/min per person. As older buildings are being rehabbed and brought up to code, more outside air needs to be brought into these buildings and conditioned. ASHRAE Standard 62.2 ASHRAE Standards 62.2–2019 is the recognised standard for ventilation system design and acceptable indoor air quality (IAQ). It sets a minimum standard for ventilation. It is formed in 1996 for the residential ventilation standard of buildings three stories and less, which is titled “Ventilation and Acceptable Indoor Air Quality in Residential Buildings”. It is reviewed and revised every 3 years. The first version of Standard 62.2 was published in 2003, the second in 2007, and the most current version is ANSI/ASHRAE Standard 62.2–2019. (ANSI stands for the American National Standards Institute. ANSI certification of a standard means that a specific consensus process was used to develop and maintain the standard and is required for the adoption of a standard into codes).

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109

8.1.1  S  electing and Installing an Energy Recovery System in Residential Buildings Installing an energy recovery system or energy recovery ventilator in a residential building helps to recover some of the energy that would otherwise be lost which leads to reduced energy cost. In the hot and dry conditions, the system helps to pre-­ cool and dehumidify the air that is coming in. In the winter, the system pre-heats the air. The system itself only requires very little energy but it can improve the ventilation of indoor space or a room. The following steps give basic guidelines in selecting an energy recovery system for the residential building. Calculation of required airflow Calculation of required airflow or fresh airflow is very important as the first step. For any type of ventilation system, the amount of fresh airflow can be calculated according to ASHRAE 62.2–2010 standard. Energy recovery units are typically sized to ventilate a building/indoor space at a minimum of 0.35 air changes per hour (ACH). To calculate minimum cubic feet per minute (CFM) requirements, simply take the square footage of the building/indoor space and multiply by the height to get cubic volume. Then, divide by 60 and multiply by 0.35 as shown in Eqs. 8.1 and 8.2. CFM =

ACH × Volume of indoor space 60 min

(8.1)

CFM =

0.35 × Volume of indoor space 60 min

(8.2)





Choose efficient equipment Energy recovery systems consist of internal fans which run continuously or for many hours a day. It is important to choose a model that can provide appropriate airflow while consuming less energy. To choose an efficient model, find a unit with a sensible efficiency of at least 80%. Next, calculate its efficiency in moving the air. This is expressed in CFM per Watts (ft3/W). To calculate this, divide net airflow in CFM by the power consumed in Watts. The efficiency should be at least 1.25 CFM/Watts (ft3/W). Location of exhaust and fresh air supply points Air exhaust points should be located in the bathroom, kitchen, utility room and other high moisture areas (intended to remove stale and humid air or moisture). In order to mix fresh air throughout the building, supply points should be positioned at a considerable distance from the exhaust points. Ducting system It is best for an energy recovery system to have its own dedicated ducting system. A typical energy recovery system for a residential house has four ducting networks. Two ducts lead from the system to the outdoor in which one duct pulls in fresh outdoor air, while the other duct exhausts stale air. Two more ducts lead from the system to diffusers in the house in which one duct distributes fresh air to the living room and bedrooms, while the other duct pulls stale air from the

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b­ athrooms and kitchen. In ensuring its best performance, careful planning of locating and integrating the ducting system is required.

8.2  Application in Various Climatic Conditions Energy recovery systems have been widely used in various climatic conditions and countries to ventilate buildings and improve comfort levels. A combination of these systems with the existing ventilation particularly under tight building envelope is the most energy-efficient method of providing fresh air to occupants. Waste energy from the heated or air-conditioned air which would commonly escape via cracks and holes of the building envelope is instead passed through the heat exchanger of the energy recovery system. This energy is extracted and supplied to the incoming fresh air in the winter conditions or exhausted to the outdoors in the summer conditions, which significantly can reduce heating and cooling energy consumption. Further details on energy recovery systems in various climatic conditions such as cold, summer, winter and tropical climates are discussed in the following sections.

8.2.1  A  pplication in Cold Climate under Frosting and Defrosting Periods Cold climate poses an average temperature of below −3 °C in the coldest months and an average temperature above 10 °C in the warmest months. Energy consumption for ventilation and space heating is extensive in the cold climate. In this climate, the outdoor air that is brought into a building is typically dry and heat losses in the ventilation systems are significant. The relative humidity value in the indoor spaces also might fall below 20%, which is far lower than the acceptable condition. Therefore, maintaining healthy and comfortable indoor air conditions in a cold climate can be challenging. Most of the buildings in the cold climate rely on air leakage through the building envelope to provide sufficient ventilation. As building envelopes are tighter and with the stringent ventilation requirements of ASHRAE 62.1 and 62.2 Standards, improved mechanical ventilation by energy recovery systems is necessary to provide adequate ventilation and thermal comfort to the indoor spaces while minimising energy usage. Energy recovery systems work very effectively when the outside air is very cold. This is due to the high temperature and humidity differences between the outdoor and indoor air that create a high energy transfer rate. Table  8.1 shows several selected studies on the application of energy recovery systems in a cold climate. With the systems, about 40–60% of humidity that would normally be extracted out is transferred to the fresh incoming air and assisting to maintain the relative humidity at a comfort level. However, the effectiveness of the system is greatly affected by

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Table 8.1  selected studies on the application of energy recovery systems in a cold climate References Phillips et al. (1989) Bilodeau et al. (1999)

Kragh et al. (2007)

Beattie et al. (2015)

Rafati Nasr et al. (2015)

Anisimov et al. (2015)

Zhang and Fung (2015)

Description of studies In this study, algorithms were developed to predict the energy performance of an energy recovery system under various effectiveness operating conditions for several frost control and defrost strategies Frost formation in rotary heat and mass exchangers was analysed based on experimental and numerical approaches. Experiments revealed that glazed frost, whose density and thermal conductivity were larger than that of rough frost, prevails in rotary exchangers operating in cold climates. Results indicated that the absolute humidity is the prevailing parameter to characterise the frosting phenomenon In this study a counter-flow heat exchanger for energy recovery application in a cold climate that can continuously defrost without supplementary heating was developed with low-pressure loss. The heat exchanger efficiency was experimentally investigated. The heat exchanger was found capable of continuously defrosting itself with the outside temperature below freezing point with high efficiency This paper presents performance findings of an energy recovery system that used permeable water-vapour cores for net-zero energy homes in Arctic regions. Experimental tests were conducted using a purpose-built experimental setup where the supply of inlets and exhaust air conditions were controlled. The results and conclusions aided in the design and evaluation of a heat/energy exchanger for Arctic regions Performance of two cross-flow heat exchangers during frosting and defrosting periods were experimentally investigated in this study. The investigations were carried out under different operating conditions. Values of frosting limit, defrosting time ratio effects of the two defrosting methods on energy consumption of ventilation in three cold cities in the United States (Saskatoon, Anchorage and Chicago) were reported. Results showed that the outdoor air during the pre-heating method was better than the outdoor air during the bypassing method. The largest energy consumption under frosting conditions reduction was achieved in Saskatoon city as compared to the other two cities The paper reported numerical simulation and analysis of coupled heat and mass transfer under ice formation conditions in a cross-flow plate heat exchanger of an energy recovery system. The presented model in this study was validated against experimental data. The obtained results showed satisfactory agreement with the data obtained from the experimental measurements. Computer simulation results showed that the efficiency gains were sensitive to different inlet conditions and enable estimation of the safe operating conditions for different inlet return airflow parameters, based not only on the exhaust air temperature but also on the exchanger’s relative humidity and different thermal efficiency In this study, an energy recovery ventilator was studied under a testing house located in Toronto, Ontario. The studies covered a series of experimental investigations during normal and defrost operations. Frost resistance within a range of winter temperatures was also evaluated. Results showed that the operational of the energy recovery ventilator without the defroster recorded a frost resistance of up to −16 °C (continued)

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Table 8.1 (continued) References Liu et al. (2016)

Description of studies A simplified theoretical model was developed in this study to predict the inlet conditions under which frosting formed in the flat plate heat/energy exchangers for an energy recovery application. The model used the exchanger design parameters and operating conditions to determine the frosting limit. Experimental experiments are performed to verify the model for the frosting limits. It was found that the predicted frosting limits had good agreements with experiments, and energy exchangers had a lower risk of frosting than the heat exchangers. The model could be applied to improve the design of exchangers to reduce or avoid frosting for energy recovery application in cold climates In this study, a theoretical analysis of a cross-flow plate heat exchanger used for Jedlikowski and Anisimov energy recovery under frosting operating conditions in a cold climate was presented. Three selected techniques of frost control were studied on the basis of (2017) the original model ε-NTU model. For the cross-flow plate heat exchanger, frost-free operating conditions were determined under different opening rates of bypass damper and face and bypass dampers. It was found that the implementation of the specific model of heat exchanger output and the values of critical outdoor temperatures for various operating conditions under which the onset of frosting occurs in the cross-flow flat plate heat exchanger depended on the heat and mass transfer conditions performed at two crucial points on the surface of the return air channel Patil et al. Heat transfer characteristics and thermal performance enhancements of heat (2017) exchangers for energy recovery application of thermal systems under frosting, defrosting, and dry/wet operating conditions in cold climate were studied. The heat exchangers under different operating conditions were analysed and thoroughly reviewed using theoretical, numerical and experimental methods. Heat transfer enhancement of the heat exchangers was also suggested Pacak et al. In this paper, a theoretical analysis of power demand calculation for freeze (2019) prevention methods of counter-flow heat exchangers used in energy recovery from exhaust air in the air handling unit (AHU) under sub-zero operating conditions was studied. Two main frost prevention techniques (preheating and bypassing the outdoor airflow) were analysed and compared. Results showed that the bypass method was distinguished by lower energy recovery efficiency compared to the preheating technique.

frosting conditions which commonly occurs inside its heat exchanger (Liu et al., 2015). A thorough review on frosting in heat exchangers of energy recovery systems and frost control strategies is presented in Rafati Nasr et al. (2014). From the report in general, the formation of frost in cold regions occurs could be due to several reasons such as blockage of airflow channels; increase in pressure drop in the heat exchanger; decrease in airflow rate; increase in electricity power input of fans and; decrease in the heat transfer rate between two airstreams. Table 8.1 shows several selected studies on the application of energy recovery systems in a cold climate.

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8.2.2  Application in Summer and Winter Climatic Conditions Summer and winter are parts of the four seasons of a year that are distinguished by special climate conditions in terms of light, temperature and weather patterns across the world. The beginning date of summer and winter vary according to climatic characteristics. When it is summer in the Northern Hemisphere, it is winter in the Southern Hemisphere, and vice versa. Seasonal energy consumption patterns of buildings vary regionally in summer and winter conditions. In these two conditions, space heating and/or cooling is the main contributor to energy consumption. For instance, in a typical residential building, an average usage of about 700 kWh per month during winter and approximately 500  kWh during summer are reported. Thus, a cost-effective means is vital to reduce energy consumption without reducing indoor environmental quality. For such reasons, energy recovery systems or energy recovery ventilators have been installed in buildings to recover the energy within ventilation loads. How an energy recovery system works in winter A building must have sufficient insulation and air sealing to keep as much heat contained inside as possible and avoid cold drafts during winter. This can aid in terms of comfort and energy consumption, but it also contributes to the development of indoor pollutants and that air can be too humid or even too dry. In this respect, opening windows or doors to get some fresh air is not an option as it would ruin the purpose of having an insulated building and the energy that is dedicated to the heating system energy would go wasted. With the tighter building envelopes and building codes and a minimum amount of fresh air for proper ventilation to be fulfilled and thus mechanical ventilation is typically required (Guillén-Lambea et al., 2017). An energy-efficient solution to these problems is to use an energy recovery system which supplies the fresh outdoor air into a building and simultaneously exhausting the stale indoor air. When the energy recovery system is integrated into a building’s ventilation system, it draws outdoor fresh, cold air from outdoor then runs it through a heat exchanger where it moves through indoor stale, warm air in the heat exchanger, as the indoor and outdoor airflow through the system, the heat is transferred from the warmer airstream to the cooler airstream. In this way, the energy used to heat the air is recovered. Simply put, in the winter, the warmer indoor air helps to pre-heat the entering cold outdoor air. How an energy recovery system works in summer Energy recovery systems or ventilators are also useful in summer conditions. The uniqueness of the energy recovery system designs is that they can work in two directions in terms of warming up cold outdoor air or cooling down hot indoor air without any adjustment. During hot weather or condition, when an energy recovery system is integrated within the ventilation network, the outdoor fresh warm air is passed through a heat exchanger and heat exchange takes place where it loses its heat to the indoor air that has been cooled down by the air conditioning system. It works by exchanging the indoor air with fresh outdoor air while recovering part of the heat and moisture difference in the airstreams. Simply put, the heat and humidity are outside and the system will

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keep most of it from getting inside with the ventilation air. As we know, the system works based on temperature and relative humidity differences, so if the indoor air and the outdoor air have about a similar temperature and relative humidity ranges, the operational of the system is not recommended. In some part of the countries around the world air-conditioning system is used during summer, which makes the indoor air more cooler and dryer, therefore it is feasible to operate the energy recovery system within this particular conditions. Throughout the literature, it is evident that a huge portion of heating and cooling energy provided to indoor spaces is exhausted to the outdoor environment and wasted. Therefore, energy recovery systems in one of the best solutions in achieving energy savings during both summer and winter, while improving humidity control for indoor spaces. Since they reduce the workload on heating and cooling systems, smaller equipment/systems can be used which would reduce the capital expense. The systems also reduce the usage of external energy inputs making them relevant to be applied in passive building design. Many studies can be found in the literature pertaining to the investigations and applications of energy recovery systems in summer and winter conditions. Table  8.2 summarises several selected studies on the application of energy recovery systems in summer and winter conditions.

8.2.3  Application in Tropical Climate A tropical climate in the Köppen climate classification is a non-arid climate. Much of the equatorial belt within the tropical climate zone experiences hot and humid weather. There is abundant rainfall of approximately 103 in. a year, and during certain periods, thunderstorms can occur every day as a result of an intense heating of the earth's surface which causes the formation of clouds (cumulus and cumulonimbus). In this climate, humidity usually hovers between 70 and 90% every day. In the tropical climate, there are often only two seasons: a wet season (tropical wet climate) and a dry season (tropical wet and dry climate). Areas with hot and humid tropical climate never experience frost, and changes in the solar angle are small. The temperature remains relatively constant (hot) throughout the year with the mean temperatures of all 12  months usually experience warmer than 18  °C with high intensity of sunlight. However, due to evaporation and rain formation, temperatures in the tropical climate rarely exceed 35 °C; a daytime maximum of 32 °C is more common. Meanwhile, at night the abundant cloud cover restricts heat loss, and minimum temperatures fall no lower than about 22 °C. This high level of temperature is maintained with little fluctuation throughout the year. With continuously experiencing hot and humid weather, the regions within tropical climate have high temperatures and high humidity levels leading to an enormous discomfort. Hence, there is a necessity to provide cross ventilation between indoor and outdoor conditions to facilitate a better thermal comfort level (Yau et al., 2012). With the goal to provide a better thermal comfort level, air-conditioning mechanical ventilation (ACMV) systems have been used which contribute to about 60% total

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Table 8.2  Selected studies on the application of energy recovery systems in summer and winter conditions References Description of studies Ouazia et al. A study on an energy recovery system in the real building was carried out using (2019) summer temperature including humid summer day in Ottawa. It was found that the system was capable of reducing air conditioning energy in terms of electricity consumption (up to 20%). About 12% reduction from the total percentage in the aspect of cooling electricity consumption was recorded Fan and Ito This study explored the impacts of inlet and outlet openings arrangement through (2012) an energy recovery system on energy consumption in typical office building space equipped with air-conditioning during summer climate in Japan. Building energy simulation (BES) and computational fluid dynamics (CFD) were integrated in analysing the consumption. Results indicated for the case of the under-floor-type ventilation system, the effectiveness and impact of integrating CFD and BES approaches with non-uniform temperature distribution, thermal stratification was formed in the space of the studied building This study reported several energy recovery systems in various climatic Guillén-­ conditions including winter and warm summer conditions with medium relative Lambea et al. (2017) humidity levels. The effectiveness of the energy recovery system with the aim of optimising the air conditioning energy demand of dwellings located in several cities in the south of Europe was analysed. Analysis of undesirable operation of the system and the impact of free-cooling on the air conditioning system were also studied Bao et al. This study reported the influences and potential application of an air filter on the (2016) performances of an energy recovery system in three different modes based on experimental approaches in five climate zones of China including summer and winter conditions. The motivation of the study sparked due to the haze problems in China that affected the indoor conditions of buildings and the comfort of occupants. Results showed that the system met the required threshold value of 75 μg/m3, but the installation of the air filter reduced the airflow rate and increased the power of the fan Zhao et al. In this study, a simulation model of an energy recovery system based on generic (2007) dynamic building energy simulation and EnergyPlus was developed. Performance of the system was investigated using different indoor temperature set-points for Beijing and Shanghai weather in China during summer and winter conditions. The ratio of energy recovery to energy supply by HVAC devices was also studied. Simulation results showed that the seasonal average of the ratio was linear with indoor temperature set-points. The availability of the system in Shanghai was better than in Beijing during the winter. In summer, the utilisation of the system was uneconomical if the indoor temperature set-point was higher than 24 °C. The indoor temperature set-points had reverse effects on the availability of an energy recovery system in the mid-season Kang et al. In this study, critical temperatures of energy recovery systems for supermarkets in (2010) winter were recommended and discussed for the four cities in different climate zones of China. It was reported that the temperature of fresh air in winter can be categorised into three regions, that is, recovery region, transition region and impermissible recovery region. Results indicated that the latent energy recovery was not suitable for ventilation energy savings in supermarkets in winter. The feasibility of sensible energy recovery in supermarkets depended on the outdoor climate and fresh air flow rate (continued)

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Table 8.2 (continued) References Choi et al. (2018)

Description of studies In this study, an analysis of energy recovery efficiency under actual operating conditions, long-term field measurements were performed in a residential building in the winter of Korea. The results showed that the enthalpy recovery efficiencies fluctuated between 25 and 70% depending on the outdoor conditions. The sensible energy recovery efficiencies were between 30 and 65% and were proportional to the temperature difference between indoors and outdoors. The heat exchange efficiency was not constant but varied according to changes in indoor and outdoor conditions under actual operating conditions

energy that is mainly used to reduce temperature and moisture levels in an indoor environment (Ahmad et al., 2015; Chua et al., 2013; Herath et al., 2020). To reduce the burden of ACMV energy loads, energy recovery systems have been introduced in the tropical climate in maintaining good indoor air quality and thermal comfort in indoor spaces while reducing energy consumption (Liu, 2008; Kho et al., 2017) and operating cost of the conventional ACMV systems (Ahmad et al., 2016). Energy recovery systems can transfer both sensible and latent heat from the incoming fresh air to the outgoing air, and thus aids in reducing the load (the ventilation) of the airconditioning system (Zafirah & Mardiana 2016; Ouazia et al., 2019). On the other hand, Hilmersson and Paulsson (2006) in their study explained that the thermal load of the air-conditioning could decrease when the air was dehumidified by an energy recovery system. A comparison study was performed between an air-conditioning system coupled with an energy recovery unit and a conventional air-conditioning system without an energy recovery unit based on energy analysis on energy analysis resulted in up to 8% annual energy consumption saving in tropical climate (Nasif et al., 2010). It is apparent that the systems have a significant contribution towards energy savings by reducing the latent load in the hot and humid environment. By developing energy recovery systems, their main goal is to tackle the problem of high humidity levels. However, several challenges are faced with the installation. For instance, the performance of the system in terms of efficiency and recovered heat would be seriously compromised under the extreme conditions of hot and humid environments, which may in turn undermine the final energy savings effect. Numerous studies have been carried out to analyse the performance of the system with the aims to thoroughly investigate their features and feasibilities in the tropical climatic conditions (Nasif et al., 2010; Rafati Nasr et al., 2014; Delfani et al., 2012; Pavel, 2010). Min and Su (2010) established a mathematical model to predict the performance of a membrane-based energy recovery system in a tropical climate with three different temperatures of 32, 35 and 38 °C and relative humidity of 50%. They concluded that the total heat transfer rate increased with a large total surface area. In another study, Zafirah and Mardiana (2014) analysed the performance of an energy recovery unit under different temperatures and air velocity in a hot and

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humid climate. Efficiency up to 85% was obtained at a temperature of 40 °C and an air velocity of 1 m/s. Masitah et al., (2015) discovered that the air velocity had a large influence on efficiency and recovered heat of the energy ­recovery system. Yang et al. (2015) carried out experiment on energy recovery system coupling with an air-conditioning system to test the applicability in sports centre in Xiamen, China, stated that energy savings could achieve as high as 240.37 kW or 34.5% of total energy consumption for an air-conditioning system. Al-Waked et  al. (2015) studied three models of the membrane-based air-to-air energy recovery system for building applications in hot and humid environment using CFD simulation and found that the system could resist moisture transportation. A summary of the findings from these studies is shown in Table 8.3.

Table 8.3  Summary of findings regarding performance of energy recovery systems in the tropical climate References Zhang and Niu (2001) Liu et al. (2010) Rasouli et al. (2010) Min and Su (2011) Nasif and Al-Waked (2014) Zafirah and Mardiana (2014) Masitah et al. (2015) Yang et al. (2015) Ahmad et al. (2016) Kho et al. (2017) Herath et al. (2020)

Description of findings More than 50% of the energy consumed by conditioning fresh air could be saved annually Latent heat recovery indicated a significant impact in affecting energy savings Huge amount of cooling energy was required to dehumidify the humid outdoor air Membrane moisture resistance and thermal resistances were dependent on outdoor air temperature and humidity Energy recovery system reduced energy usage about 0.9 and 1.4 GJ in Tokyo and Miami, respectively as compared to the conventional air conditioning system Recovered energy increased when the airflow rate was increased

Effectiveness decreased as air velocity increased. Energy savings achieved as high as 240.37 kW or 34.56% of total energy usage in a mechanical ventilation system Energy recovery could ventilate out the stale indoor air and draws in the same amount of fresh outdoor air High value of sensible and latent effectiveness could be gained with the latent effectiveness was lower than sensible effectiveness Percentage energy saving of the energy recovery system increased when the temperature and the relative humidity of outdoor fresh air increased. A simple payback period of a building which was supplied with 20% fresh air varied from 1.1 to 4 years depending on fresh air mass flow rate

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8.3  Summary Energy recovery systems are used throughout the world to ventilate and improve comfort levels of different building types. They have received increasing interest from researchers and industry over the past 30 years. This system is scientifically proven as one of the key energy-efficient technologies that can deal with the growth of energy usage in buildings while maintaining indoor air quality. It retains heat and moisture from exchanged air in winter or cold weather, whereas in summer and hot-­ humid areas it pre-conditions and dehumidifies the incoming air. A lot of studies have been carried out associated with this technology which varies from fundamental to applied investigations. It can be seen over the last decades that there has been a rapid development of the system in different building types and climatic conditions. However, studies on performance-based weather data, economic and energy analyses, computational simulation and integrated systems are quite limited. In relation to frost formation in heat exchangers of energy recovery systems, more studies are needed to emphasise simultaneous sensible and latent heat transfer within the exchangers and performance under supply air temperatures less than −20 °C. Additionally, investigations on physical components of the system in terms of heat exchanger materials, fans, ducting systems for both sensible and latent recovery applications in various climatic should be further explored. More future works should also be carried out pertaining to the feasibility of the systems in medium and high rise buildings. Acknowledgements  Universiti PTEKIND/8014124).

Sains

Malaysia

Research

University

Grant

(1001/

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Index

A Air changes per hour (ACH), 109 Air-conditioning and mechanical ventilation (ACMV), 94, 114 Air-conditioning system, 55, 93, 96, 97, 117 Air handling unit (AHU), 112 Air-to-air energy recovery, 44 Air-to-air energy recovery ventilation equipment (AAERVE), 73 ASHRAE Standard, 86, 108 B Basic Energy Plan (BEP), 8 Building energy consumption, 6, 9, 10 CO2 emissions, 7 HVAC/MVAC systems, 7, 8 indoor environment, 7, 9 low-carbon technologies, 9 ventilation air streams, 7 Building energy simulation (BES), 115 Building integrated photovoltaic/thermal (BiPV/T) systems, 99 Building sector, 1, 5 Building types applications, 108 commercial building applications, 108 cooling energy, 107 energy and environmental benefits, 107 energy recovery system, 107, 108 energy savings, 108 life cycle assessment, 107 mechanical ventilation unit, 108 residential buildings, 109 ventilation energy, 107

C Carbon dioxide emissions, 3, 4 Chilton–Colburn analogy, 79 Climatic conditions cold climate, 110–112 energy-efficient method, 110 summer, 113–116 tropical climate, 114, 116, 117 winter, 113–116 Cold climate, 110–112 Comfort-to-comfort system, 43 Comparison analysis, 67 Computational fluid dynamics (CFD), 115 Construction method airflow direction, 27 appendages, 27 channel wall thickness, 27 conceptual optimisation, 27 energy recovery design, 26 energy savings, 27 heat and mass transfer, 26 operational cost, 26 wheel energy recovery system, 27 Corrugation, 34 Cross-flow fixed-plate heat exchangers, 45 Cubic feet per minute (CFM) requirements, 109 D Defrosting, 110–112 Dehumidification system, 28, 93, 94, 98 Double pipe heat exchangers, 37

© Springer Nature Switzerland AG 2020 M. I. Ahmad, S. Riffat, Energy Recovery Technology for Building Applications, https://doi.org/10.1007/978-3-030-50006-1

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Index

124 E Earth-to-air energy recovery, 44 Earth-to-water energy recovery, 44 Effectiveness NTU method Chilton–Colburn analogy, 79 Clapeyron equation, 81 convective moisture transfer coefficient, 80 counter-flow heat exchanger, 79 cross-flow heat exchanger, 79, 81 heat exchange analysis, 77 heat exchangers, 82 heat transfer rate, 78 LMTD method, 77 moisture transfer capability, 79 sensible effectiveness, 79 thermal and concentration boundary layers, 80 Efficiency energy recovery, 74 enthalpy, 76 global, 76 heat exchanger, 74 latent, 75 physical and thermodynamic properties, 74 sensible, 75 Energy, 1 buildings, 5 climate change, 2 industry, 3 literature, 5 materialising, 3 SD9 and SD13, 2 SDG, 2 SDG7, 2 sustainable energy, 2 Energy-consuming building types, 7 Energy consumption, 6 Energy-efficient technologies, 3, 9 air-conditioning system, 93, 96, 97 carbon dioxide emissions, 89 coupled-photovoltaic/solar thermal system, 98, 99, 101 dehumidification system, 93, 94, 98 energy recovery system, 89 integrating energy recovery ventilator, 89 mechanical ventilation systems, 89, 91, 92 net-zero energy buildings, 89 passive/natural ventilation systems, 90, 93, 95 Energy Market Authority (EMA), 8 Energy Performance Norm (EPN), 8 Energy Performance of Building Directive (EPBD), 8 Energy recovery, 9 Energy recovery application, 34

Energy recovery systems, 19–21, 27, 43 building applications, 73 in building ventilation system, 45 computational modelling approach, 73 experimental analysis, 73 fixed-plate, 45 performance effects of air conditions, 85, 86 effects of air velocity/airflow, 84 sensible, 83 testing, 73 Energy recovery technology application, 13 building sector, 19 definition, 14 electricity, 13 enthalpy, 19 examples, 13 heat transfer, 17 latent heat, 18 literature, 16 low-temperature heat sink, 15 mass transfer, 17 mechanism, 22 moisture, 14 sensible heat, 17 thermal efficiency, 15 thermodynamics, 14 ventilation air, 19 waste heat, 15 Energy recovery ventilators (ERVs), 19, 73, 84, 111, 113 Energy sources, 1 Enthalpy, 19, 76 F Fin structure, 34 Fixed-plate energy recovery, 45 geometry shapes, 47 heat transfer coefficients, 46 performance, 46 procedure, 45 sensible energy, 45 steel and polymer, 45 studies, 48–49 Fixed-plate energy recovery systems, 45, 46 Flow configurations, 35, 36 Frosting, 110–112 G Global efficiency, 76 Global final energy consumption, 6

Index Green innovation climate change, 3 Greenhouse gas (GHG) emission, 6 H Heat and mass transfer, 73, 78, 80, 85, 86 types, 35 Heat exchanger materials, 28, 30–33 Heat exchangers, 28 both cooling and heating process, 25 definition, 25 direct contact, 25 energy recovery applications, 26 energy recovery system, 25 indirect contact, 25 mechanism, 26 structures corrugation, 34 fin structure, 34 flow arrangement, 35 wick, 29 thermodynamics, 26 types, 25 Heat pipe, 51 advantages, 54, 55 applications, 55 comprehensive review, 55 device, 54, 55 geometry fin, 54 vaporisation, 51 Heat pipe energy, 51 Heat pipe energy recovery system, 27 Heat pipe exchanger, 33 Heat recovery, 91, 92, 100 Heat recovery ventilators (HRVs), 73 Heat transfer thermal energy, 17 Heating, ventilation and air conditioning (HVAC) systems, 20, 94 Hot and humid, 114, 116, 117 Humidification, 28 Hydrophilic characteristics, 28 I Indoor air quality (IAQ), 108 Integrated system, 90, 92, 100 International Energy Agency (IEA), 89, 93 K Kelvin-Planck Statement, 15

125 L Latent efficiency, 75 Latent heat, 18 Log–mean temperature difference (LMTD), 77 Low-carbon technologies, 9 Low-energy sources, 9 M Mass transfer, 17 Material durability, 28 Mechanical ventilation heat/energy recovery system (MVHR), 90 Mechanical ventilation systems, 89, 91, 92 Modern buildings, 7 N Number of transfer unit (NTU), 78 Nusselt number, 47 O Operating parameters air conditions, 85 air velocity/airflow, 84 Optimisation analysis, 26 P Passive Ventilation with Heat Recovery (PVHR™), 90 Passive/natural ventilation systems, 90, 93, 95 Photovoltaic (PV) collector, 98 Polymer-based materials, 28, 29 Process-to-comfort system, 43 Process-to-process system, 43 Pulsating heat pipe heat exchanger (PHP-HE), 97 R Relative humidity, 81, 83, 86 Renewable energy sources, 98 Reynolds number, 47 Rotary wheel, 47 adsorption and desorption, 50 disadvantage, 51 flow arrangement, 50 limitation, 51 motor, 47 studies, 51–54 Rotary wheel energy recovery system, 27

Index

126 Run-around energy recovery, 55 airstreams, 60 characteristics, 61 components, 60 designing, 56 efficiency, 60 Run-around energy recovery systems, 62–65 S Semi-permeable membranes, 61 Sensible efficiency, 75 Sensible heat, 17 Sick Building Syndrome (SBS), 93 Single-phase heat exchangers, 37 Sinusoidal plate-fins, 34 Solar radiation, 99 Stand-alone energy recovery system, 21 Steady-state performance, 60 Summer condition, 113–116 Supply airstream, 21 T Temperature, 74, 75, 78, 82, 83, 85, 86 Thermal conductivity, 28 Thermodynamic, 14, 16 Thermosiphon energy recovery, 65 Thermo-siphon energy recovery device, 61 application, 66 building ventilation system, 66 design and performance, 66 Transient system simulation software (TRNSYS), 96

Tropical climate, 114, 116, 117 Two-phase heat exchangers, 37 U United Nations Framework Convention on Climate Change (UNFCC), 3 United Nations General Assembly, 2 V Ventilation air, 19 W Waste heat, 15 application, 16 direct recovery, 15 energy equation, 15 energy flow rate, 16 sources, 16 thermodynamics, 15 Waste-to-energy, 13 Wick structures, 29 Winter condition, 113–116 Working principle energy recovery system, 21 enthalpy recovery, 20 ERV and HRV, 19 HVAC loads, 20 Z Zero-energy buildings (ZEBs), 8