Geothermal Energy: Sustainable Heating and Cooling Using the Ground

Preface

Geothermal energy systems that provide heating and cooling using the ground are increasingly applied, and represent a technology that supports sustainable use of energy. Ground-source heat pumps, thermal energy storage and district energy are components of geothermal energy systems, and have been around for over 40 years and are widely applied. But they are also undergoing research and being improved continually, and advanced systems and components, as well as advanced understanding, are expected to be developed over the foreseeable future.

In this book, geothermal energy systems that utilize ground energy in conjunction with heat pumps to provide sustainable heating and cooling are described. Information on a range of topics is provided, from thermodynamic concepts to more advanced discussions on the renewability and sustainability of closed-loop geothermal energy systems. Numerous applications of such systems are also described. Theory and analysis are emphasized throughout, with detailed descriptions of models available for vertical geothermal heat exchangers.

The book also contains many references, including some related to books and articles on various aspects of geothermal systems that are not fully covered. Some links to websites with basic freeware for ground-source heat transfer modeling and building heating loads are referenced throughout the book.

The book is research oriented, thereby ensuring that new developments and advances in geothermal energy systems are covered.

The book is intended for use by advanced undergraduate or graduate students in several engineering disciplines such as mechanical engineering, chemical engineering, energy engineering, environmental engineering, process engineering and industrial engineering. Courses on geothermal energy systems or related courses such as heat exchangers, thermal energy storage or heat pumps that are often offered at the graduate level in Mechanical Engineering or related fields may find this book useful. The information included is sufficient for energy, environment and sustainable development courses. The book can also be used in research centers, institutes and labs focusing on the areas mentioned above, by related learned societies and professional associations, and in industrial organizations and companies interested in geothermal energy and its applications. Drillers and installers as well as regulatory agencies may also be interested in the book. Furthermore, the book offers a valuable and readable reference text source for anyone interested in learning about geothermal energy systems.

The book strives to provide clear information on ground-based geothermal systems and the many advances occurring in the field in a way that makes it understandable for students, practitioners, researchers and policy makers.

Various topics are covered, from fundamentals to advanced discussions on sustainability. Many applications are described, while theory and analysis are emphasized throughout. Detailed descriptions are provided of models for geothermal heat exchangers and heat pumps. The organization of the book is intended to help the reader build knowledge in a logical fashion while working through the book, and is as outlined here. Introductory material is included in the first two chapters, with an overview of geothermal energy as a source of energy and technologies that can harvest it described in Chapter 1, and fundamentals of thermofluid engineering disciplines related to geothermal energy systems provided in Chapter 2. Information on the main components of geothermal energy systems such as heat pumps, heat exchangers, heating, ventilating, and air conditioning equipment and energy storage units are provided in Chapter 3. The next five chapters form the heart of the book, with thermal energy storage being the focus of Chapter 4, geothermal heating and cooling forming the core of Chapter 5, and design and installation considerations for geothermal energy systems being the emphasis of Chapter 6. Extensive material is provided on modeling of ground heat exchangers and heat pumps, with the modeling of ground heat exchangers including a variety of models examined in Chapter 7 and the application of the models to various relevant examples presented in Chapter 8. The thermodynamic analysis of geothermal energy systems is the focus of Chapter 9. Extensive coverage is provided on environmental and sustainability factors, as these have become increasingly germane in recent years. Environmental factors related to geothermal energy systems are covered in Chapter 10 while their renewability and sustainability are examined in Chapter 11. To close, a range of case studies for geothermal energy systems is presented in Chapter 12 that illustrate the technologies, their applications and their advantages and disadvantages.

The main features of the book are:

comprehensive coverage of ground-based geothermal energy systems;

detailed descriptions and discussions of methods to determine potential environmental impacts of geothermal energy systems and their thermal interactions;

presentations of the most up-to-date information in the area;

suitability as a good reference for geothermal heat exchangers;

a research orientation to provide coverage of the state of the art and emerging trends and recent developments;

numerous illustrative examples and case studies;

clarity and simplicity of presentation of geothermal energy systems that use the ground.

We hope this book allows geothermal energy to be used more widely for the provision of heating and cooling services using the ground in a sustainable manner, using both existing and conventional equipment and systems as well as new and advanced technologies. The book aims to provide an enhanced understanding of the behaviours of heating and cooling systems in the form of ground-source heat pumps that exploit geothermal energy for sustainable heating and cooling of buildings, and enhanced tools for improving them. By exploiting the benefits of applying exergy methods to these ground-based energy systems, we believe they can be made more efficient, clean and sustainable, and help humanity address many of the challenges it faces.

October 2016

Marc A. Rosen and Seama Koohi-Fayegh

University of Ontario Institute of

Technology, Oshawa, Canada

About the Authors

Marc A. Rosen is a Professor at the University of Ontario Institute of Technology in Oshawa, Canada, where he served as founding Dean of the Faculty of Engineering and Applied Science. A former President of the Engineering Institute of Canada and the Canadian Society for Mechanical Engineering, he is a registered Professional Engineer in Ontario. He has served in many professional capacities, including Editor-in-Chief of several journals and a member of the Board of Directors of Oshawa Power and Utilities Corporation. He is an active teacher and researcher in energy, sustainability, geothermal energy and environmental impact. Much of his research has been carried out for industry, and he has written numerous books. He has worked for such organizations as Imatra Power Company in Finland, Argonne National Laboratory near Chicago, and the Institute for Hydrogen Systems near Toronto. He has received numerous awards and honours, including an Award of Excellence in Research and Technology Development from the Ontario Ministry of Environment and Energy, the Engineering Institute of Canada's Smith Medal for achievement in the development of Canada, and the Canadian Society for Mechanical Engineering's Angus Medal for outstanding contributions to the management and practice of mechanical engineering. He is a Fellow of the Engineering Institute of Canada, the Canadian Academy of Engineering, the Canadian Society for Mechanical Engineering, the American Society of Mechanical Engineers, the International Energy Foundation and the Canadian Society for Senior Engineers.

Seama Koohi-Fayegh is a Post-doctoral Fellow at the Department of Mechanical Engineering at the University of Ontario Institute of Technology in Oshawa, Canada. She received her PhD in Mechanical Engineering at the University of Ontario Institute of Technology under the supervision of Professor Marc A. Rosen. Her PhD thesis topic was proposed by the Ontario Ministry of Environment and focused on thermal sustainability of geothermal energy systems: system interactions and environmental impacts. She did her Master's degree in Mechanical Engineering (Energy Conversion) at Ferdowsi University of Mashhad, Iran, and worked on entropy generation analysis of condensation with shear stress on the condensate layer. Her thesis research won multiple awards at the school level and at the Iranian Society of Mechanical Engineering in 2009. Her research interests include heat transfer, sustainable energy systems and energy technology assessment.

Acknowledgments

The work of many of our colleagues helped greatly in the development of this book, and is gratefully acknowledged. Some of the material in this book is derived from research that we have carried out with numerous distinguished collaborators over the years. These include the following faculty members in geothermal energy and related areas:

Drs Ibrahim Dincer and Bale V. Reddy, University of Ontario Institute of Technology, Oshawa, Ontario, Canada

Drs Wei Leong and Alan Fung, Ryerson University, Toronto, Ontario, Canada

Dr Vlodek R. Tarnawski, Saint Mary's University, Halifax, Nova Scotia, Canada

Dr Robert A. Schincariol, Western University, London, Ontario, Canada

Dr Tomasz Śliwa, AGH University of Science and Technology, Krakow, Poland

Dr Frank C. Hooper, University of Toronto, Toronto, Ontario, Canada

Dr David S. Scott, University of Victoria, Victoria, British Columbia, Canada

We highly appreciate all of their efforts, as well as their thought-provoking insights.

Last but not least, the authors warmly thank their families, for their endless encouragement and support throughout the completion of this book. Their patience and understanding is most appreciated.

Nomenclature

Greek Letters

Subscripts

Superscripts

Abbreviations

Chapter 1

Introduction to Geothermal Energy

Geothermal energy systems are one option for providing energy services. They take advantage of the ground and the energy it contains. Sometimes ground energy is the basic ground at its natural temperature, which is mainly affected by ambient conditions. At other times, the ground is at an elevated temperature. Considering the current level of geothermal energy use and future energy needs, geothermal energy sources show great potential for contributing a larger fraction of the world's energy needs.

Archaeological evidence shows that geothermal energy was first used by ancient peoples, including the Romans, Chinese, and Native Americans. They used hot mineral springs as a source of heat for bathing, cooking, and heating. The minerals in water from these springs also served as a source of healing. While such uses of hot springs have changed over time, they are still used as a source of heat for bathing in several spas around the world. With technological developments, the use of geothermal energy has expanded to deeper levels of the earth's crust, which can be used for a wider range of applications such as domestic heating and cooling, industrial processes, and electricity generation. However, only a small fraction of available geothermal energy is currently used commercially to generate electricity or provide useful heating, in part due to the current state of the technology.

Geothermal energy systems that exploit hot reservoirs in the ground (e.g., thermal springs, geysers, ground heated by hot magma) are used mainly to generate electricity and to provide heating. Such systems are common in countries such as Iceland, Turkey and others. The global operating capacity for geothermal electricity generation from such geothermal resources is about 12.8 GW as of January 2015, spread across 24 countries, and it is expected to reach between 14.5 GW and 17.6 GW by 2020 (Geothermal Energy Association 2015).

There is another type of geothermal energy system, which provides heating and cooling using the ground. That is the type of geothermal energy that is the focus of this book. Such geothermal energy systems take advantage of the energy contained in the ground in its natural state, even when it is not at elevated temperatures due to heat within the earth. This ground energy is related to the background ground temperature and includes the ground itself and groundwater.

1.1 Features of Geothermal Energy

Ground-based energy can be used in all seasons:

Ground-based energy can provide heating directly in winter, since the ground below the surface is often warmer than the air above. Such applications include space heating, greenhouse heating, aquaculture pond heating, agricultural drying, industrial heating uses, bathing and swimming, and snow melting. Sometimes the ground temperature is only adequate to provide preheating. The ground temperature can also be boosted via devices like heat pumps, allowing ground-based energy to provide heating at higher temperatures. The use of geothermal energy via ground-source heat pumps has grown considerably compared to the other applications, primarily due to the technology's ability to achieve high efficiency and to utilize groundwater and/or ground temperature anywhere in the world.

Conversely, ground-based energy can provide direct cooling in summer, since the ground below the surface is often cooler than the hot air above. Again, the ground temperature may only be adequate to provide precooling. But the ground temperature can also be lowered using heat pumps operating in a cooling mode, allowing ground-based energy to provide cooling at lower temperatures.

Although the earth's ultimate geothermal energy potential cannot be estimated based on our current level of knowledge and the unpredictability of technology development, geothermal energy systems of both types are usually classified as renewable energy forms. When such geothermal energy is utilized, the temperature of the ground is returned to its elevated temperature by heat contained within hot regions in the earth, or by the effect of the ambient conditions. Discussions of the renewability of various heat sources vary for the different technologies utilizing the energy source. For example, technologies that utilize the ground at temperatures affected by the ambient conditions can be considered renewable provided the ambient conditions are sustained. The constant heat supply from solar radiation and the sustainability of the hydrological cycle (infiltration and precipitation) guarantees a constant flow of heat to the ground and the renewability of such geothermal sources. The energy replacement often occurs on a time scale comparable with that of the extraction time scale.

Sustainable geothermal energy utilization often refers to how this energy resource is used to meet current energy needs without compromising its future utilization. Estimating the long-term response of geothermal energy sources to current utilization and production capacity levels is important if we are to understand their potential contributions to sustainable development. As a renewable energy source, geothermal energy is often viewed as a contributor to sustainable development and the broader goal of sustainability, provided that they are well designed. Being sustainable goes beyond geothermal energy being a renewable energy form, and includes many of its other characteristics:

Availability. Geothermal energy in the form of ground at elevated temperature is available in many parts of the world, especially in regions with seismic and volcanic activity. Geothermal energy in the form of ground at ambient temperature is available almost everywhere, although its temperature depends on the location and climate. Geothermal energy is available day and night, every day of the year, and can thus cover base-load energy needs and serve as a supplement to intermittent energy sources. The availability characteristics of intermittent renewable energy forms such as solar and wind are much different.

Compatibility. Systems exploiting geothermal energy are often compatible with both centralized and distributed energy generation.

Affordability. Geothermal energy is often exploitable for heating and cooling, and for electricity generation, in an affordable manner. Of course, some geothermal systems are not economically viable, but work is ongoing on several of these to improve commercial prospects.

Acceptability. Most people are supportive of geothermal energy, in part because it is renewable and often economically viable, and also because geothermal energy systems are not intrusive and usually are invisible. This is not the case for many other renewable energy forms, such as solar and wind.

Barriers to deployment include high capital costs, resource development risks, lack of awareness about geothermal energy, and perceived or real environmental issues.

1.2 Geothermal Energy Systems

Geothermal energy systems can exploit hot reservoirs in the ground, often in the form of natural hot water or steam, to provide heating and electricity generation. The geothermal energy technologies that are used in electricity generation are flash technologies, including double and triple flash units, dry steam, and binary cycles. Electricity generation using flash technologies contribute to nearly 60% of the global market use, with dry steam and binary cycles accounting for 26 and 15% of the global market, respectively (Geothermal Energy Association 2015). Growth in use of such geothermal energy systems for heating and electricity generation is limited by their high capital costs. Geothermal development costs depend on resource temperature and pressure, reservoir depth and permeability, fluid chemistry, location, drilling requirements, size of development, number and type of plants (dry steam, flash, binary or hybrid) used, and whether the project is greenfield or expansion (10–15% less). Development costs are strongly affected by prices of commodities (e.g., oil, steel, and cement). Declines in oil and gas prices can decrease geothermal capital costs.

Geothermal energy systems that provide heating and cooling using the ambient ground are made up of various systems and components. Some of the main systems include ground-source heat pumps, thermal energy storage systems, and district energy (i.e., district heating and/or district cooling) capabilities. They also include many other components, such as compressors, heat exchangers, pumps and pipes. Ground-source heat pump systems are capable of providing heating and cooling in one unit. The capacity of a ground-source heat pump is selected based on the heating and cooling loads, the temperature of the ground, and other parameters. Since most areas do not have balanced heating and cooling loads, the capacity of the heat pump is often selected based on one load. In most regions in the USA, the heat pump capacity matches the cooling load and is oversized for the heating loads. In Europe, ground-source heat pumps are used in the residential sector to cover base heating loads and are integrated with another heating system that covers peak heating loads. The capacity of individual ground-source heat pump units ranges from about 1.5 t for small residential applications to over 40 t for commercial and institutional applications. Technology improvements in ground-source heat pumps are expected to improve the performance and lower the cost of heat pump technologies. Key components such as compressors and heat exchangers will likely provide the largest areas for improvement. The main goals for ground-source heat pumps are reducing capital costs and improving operating efficiency, while expanding the range of products for most of the heating and cooling applications and sub-markets in the building sector.

Thus, geothermal energy systems can provide heating and cooling using the ambient ground, and can exploit hot reservoirs in the ground to provide heating and electricity generation. Both types of geothermal energy are used in practise, and are finding increased application. But the use of geothermal energy systems that use the ground to provide heating and cooling services (the focus of this book) is growing at a particularly noteworthy rate. According to a recent report (Lund and Boyd 2015), the direct use of geothermal energy has experienced an annual growth of 7.7% in capacity over the 5-year period after 2010, with the highest installed thermal capacity in the USA, China, and Sweden. This growth is mainly attributable to the growing popularity of ground-source heat pumps. About 90 000 TJ/year of ground-source heat pump utilization was observed in 2010, and this grew to approximately 325 000 TJ/year by the end of 2014.

Although geothermal energy technologies have been around for over 40 years and are applied in many areas, they are continually undergoing research and development. These efforts allow for system improvements, advances in components and enhanced understanding. Such activity is likely to carry on in the future.

1.3 Outline of the Book

In this book, geothermal energy systems are described that utilize ground energy in conjunction with heat pumps to provide heating and cooling, in a sustainable fashion. Various topics are covered, from thermodynamic fundamentals to advanced discussions on renewability and sustainability. Many applications of such systems are also described, while theory and analysis are emphasized throughout. Detailed descriptions are provided of models for vertical geothermal heat exchangers, and a strong focus is placed on closed-loop geothermal energy systems.

In this chapter, an introduction to geothermal energy as a source of energy and technologies that can harvest it is provided. Some key features of geothermal energy systems, such as its renewability and sustainability, as well as some of its advantages are briefly described. The main components of such systems are reviewed. The aim of this chapter is to provide the reader with basic information to help develop an understanding of the overall scope and range of material that is included in this book.

In Chapter 2, fundamentals of thermodynamics, heat transfer and fluid mechanics that are related to geothermal energy systems are provided to familiarize readers with these topics and prepare them for subsequent chapters. A good knowledge of thermodynamics is important to understanding geothermal energy, especially heat pumps. Facets of thermodynamics most relevant to geothermal energy systems and their applications are introduced and particular attention is paid to the quantity exergy and the methodology derived from it, exergy analysis. Aspects of heat transfer relevant to geothermal energy systems are introduced to provide the reader with a good grounding in heat transfer, which is central to geothermal energy utilization and its application. The three main modes of heat transfer are considered: conduction, convection, and radiation. A good grounding of fluid mechanics helps in understanding geothermal energy systems, as fluid flow problems often arise, so elements of fluid mechanics relevant to geothermal energy systems are also introduced. Finally, basic concepts about the ground are presented, since such material is fundamental to understanding ground-based geothermal systems, including information on ground temperature range and gradients, ground properties, and the existence of ground-based ecosystems and their sensitivity to human activity in the ground.

Chapter 3 provides background information on components of geothermal energy systems such as heat pumps, heat exchangers, heating, ventilating and air conditioning (HVAC) equipment and energy storage units. An understanding of these technologies is needed to analyze them and the larger energy systems that they comprise. The devices examined include heat pumps, which are cyclic devices that transfer heat from a low-temperature medium to a high-temperature medium; heat exchangers, which are devices for heat exchange processes between two media; HVAC equipment, which provides heating, cooling, humidification and dehumidification for spaces, and energy storage systems that permit harvested or otherwise available energy to be stored until such a time when it is needed or desired.

Chapter 4 focuses on thermal energy storage (TES) concepts, theory and applications. Details on thermal storage types, operation and applications are provided, for both heat and cold storage. The main thermal storage types, sensible, latent and thermochemical, are covered. A focus is placed on underground thermal energy storages, which normally are sensible storages, as they can store both hot and cold energy in the ground and thus are often integral to geothermal energy systems. Common types of underground thermal energy storage are described: soil and earth bed, borehole, aquifer, rock cavern, container/tank, and solar pond. Finally, the integration of thermal energy storage with heat pumps is examined, as such systems can be particularly beneficial for heating and cooling applications.

Geothermal heating and cooling is the focus of Chapter 5. Ground-based energy can provide heating in winter and cooling in summer, in partial or full manners. Ground-source heat pumps normally form the basis of such systems and therefore are extensively discussed. Emphasis is placed on geothermal heat exchangers (also called ground, underground and ground-coupled heat exchangers), since they facilitate the exchange of heat between a fluid and the ground. For completeness, high-temperature geothermal systems are also described, including systems that essentially use the ground as a heat source for electricity generation and heating.

General information on design considerations for geothermal energy systems and procedures for the installation of ground-source heat pump systems are provided in Chapter 6. The material on how systems are designed describes the relation to building loads and weather. Procedures in designing systems with unbalanced loads are also discussed. Building energy calculations, which are important first steps in designing any space heating and cooling system, are covered, as are building and heat pump performance considerations, heating and cooling calculations, and ground heat injection and extraction. Finally, the economics of geothermal systems, which vary according to type and application, is described.

The modeling of ground heat exchangers is examined in Chapter 7. Various models have been reported for heat transfer in borehole heat exchangers and their coupling with HVAC and building energy systems. Both analytical and numerical approaches are considered, and parameters such as moisture migration and groundwater flow, relevant boundary conditions, and solution errors are described. Groundwater and how