Exergy Analysis for Energy Conversion Systems

Description

When we “conserve energy,” do we aim at maximum or minimum mechanical work? Why is it necessary for even the best electric power plants to dissipate and waste more than 50% of the energy in their fuels? What is the maximum amount of useful mechanical work we can get from an energy resource? How can we produce and transport liquefied natural gas with the minimum amount of energy-resource consumption? Exergy – the measure of useful mechanical work that may be extracted from energy resources – offers answers to these and several other questions on energy conversion and utilization. In the pages of this book we analyze the energy conversion processes starting with two important realizations: first, that the vast majority of energy conversion processes take place in the terrestrial environment, which is the only naturally occurring reservoir of heat, work, and mass; second, that all energy conversion processes occur because there are materials, the energy resources, that exist not in thermodynamic equilibrium with the environment. These two realizations and the laws of thermodynamics, lead us to the concept of exergy – the key to the understanding of all energy conversion and conservation processes. Using the exergy concept, this book provides a new, simple, comprehensive, rigorous, and holistic approach for the analysis of energy conversion and conservation processes. Following a simple and comprehensive exposition of exergy and its relationship to energy resources, the book offers several practical engineering cases and examples on the application of exergy to energy conversion systems that utilize our naturally occurring energy resources – fossil fuels, nuclear fuels, solar, wind, and geothermal. Among the engineering systems that are analyzed are: steam and gas electricity generation units; jet engines; nuclear reactors; heat exchangers; cogeneration; geothermal power plants, including Organic Ranking Cycles (ORCs); biomass as a fuel; photovoltaics; solar thermal systems; wind turbines; and fuel cells. What are colloquially called energy conservation systems are examined under a new perspective, the maximum work principle, which emanates from the laws of thermodynamics and is intricately connected to the exergy concept. The exergetic analysis reveals practical methods to reduce the power supplied to engineering systems and provides benchmarks for the consumption of the least amount of resources in actual processes. Examples and cases on the application of the exergy methodology include: natural gas compression and transport; refrigeration; liquefaction; pasteurization; drying of foodstuff; water desalination; lighting; heat, ventilation and air-conditioning; transportation with internal combustion engine vehicles, electric vehicles and fuel-cell vehicles; energy storage; and petroleum refining. Chapters 1–4 of this book follow the conventional exposition of exergy as a thermodynamic concept and its implications for the operation of engineering systems. These chapters offer a succinct exposition of the basic concepts and the laws of thermodynamics; the thermodynamically rigorous development of the exergy concept and its consequences for the several primary energy sources currently used by the human society; analyses of several power-producing systems including those utilizing renewable energy sources; analyses of power-consuming systems including the establishment of benchmarks for the optimum operation of systems; and analyses of the engineering systems used for transportation. A unique aspect of this book is the inclusion of three chapters on nontraditional thermodynamics subjects of current interest: 1. Biological systems (Chapter 5), a chapter that includes an exergy analysis of the human body as a thermodynamic system and explains in detail and with several examples the exergetic processes of metabolism, thermic effects, the conversion of nutrients to energy, and the production of mechanical power by the muscles. 2. The effects of energy resource utilization on the environment and the ecosystems, including the concept of eco-exergy. Chapter 6 explores the connections and implications of the exergy concept for a cleaner environment and sustainable development. 3. The mathematical optimization of engineering systems and processes, based on the exergy concept. A very important part of Chapter 7 is the uncertainty quantification of the optimization variables and the propagation of uncertainty in the optimum solution. The inclusion of many cases and solved examples in every chapter further explains the application of exergy to dozens of significant engineering systems and processes. Problems at the end of every chapter offer a challenge and an opportunity for students and professionals to hone their analytical skills and appraise their ability to apply the exergy methodology to a variety of practical engineering systems and processes. A number of individuals have helped in the writing of this book. First among them are my students in the courses on Thermal Science, Thermodynamics, Sustainable Energy, and Advanced Thermodynamics, which I have taught in five universities during the last 40 years. I have learned from them more than they have learned from me. I am very thankful to my colleagues at these universities as well as to other colleagues I regularly meet during conferences for many fruitful and animated discussions on thermodynamics, energy utilization, and the environment. The arrangements of the WA “Tex” Moncrief Chair of Engineering at TCU have afforded me the opportunity to devote a significant fraction of my time to this book. The Cambridge University Press staff in New York, Steven Elliott and Julia Ford, patiently answered all my inquiries and gave me guidance. I am also very much indebted to my own family, not only for their constant support, but also for lending a hand when this was needed. My wife, Laura, and our three children, Emmanuel, Dimitri, and Eleni, are a constant source of inspiration and were always ready to help. I owe to all my sincere gratitude. About the Author page x Preface xi List of Symbols xiii List of Abbreviations xvii 1 Introduction 1 Summary 1 1.1 Energy – Whither Does It Come? Whence Does It Go? 1 1.2 Fundamental Concepts of Thermodynamics 4 1.3 First Law of Thermodynamics 8 1.4 Second Law of Thermodynamics 12 1.5 Practical Cycles for Power Production and Refrigeration 16 1.6 A Note on the Heat Reservoirs 22 Problems 23 References 24 2 Exergy 25 Summary 25 2.1 General Observations on the Capacity of Engines to Perform Work 25 2.2 The Model Environment 29 2.3 Maximum Work – Exergy of Closed Systems 30 2.4 Maximum Power – Exergy of Open Systems 35 2.5 Exergy of Chemical Resources – Fossil Fuels 40 2.6 A Note on Semipermeable Membranes 46 2.7 Exergy of Black Body Radiation 48 2.8 Exergy of the Water and the Wind 50 2.9 Exergy of Nuclear Fuel 54 2.10 Lost Work and Power – Exergy Destruction 57 2.11 Exergetic Efficiency – Second Law Efficiency 62 2.12 Characteristics of the Exergy Function 64 2.13 Models for the Reference Environment 65 2.14 An Operational Definition of Chemical Exergy 73 2.15 A Brief Historical Background 74 Problems 75 References 76 3 Energy Conversion Systems and Processes 79 Summary 79 3.1 Heat Exchangers 79 3.2 Vapor Power Plants 92 3.3 Gas Turbines 95 3.4 Cogeneration 100 3.5 Jet Engines 105 3.6 Geothermal Power Plants 108 3.7 Fuel Cells 119 3.8 Photovoltaics Systems 126 3.9 Solar Thermal Systems 136 3.10 Wind Turbines 141 Problems 144 References 146 4 Exergy Consumption and Conservation 147 Summary 147 4.1 Energy Conservation of Exergy Conservation? 147 4.2 Maximum Negative Work – The “Minimum Work” 151 4.3 Refrigeration and Liquefaction 155 4.4 Drying 161 4.5 Petroleum Refining 164 4.6 Water Desalination 169 4.7 Exergy Use in Buildings 171 4.8 Exergy Consumption in Transportation 179 4.9 Energy Storage 187 Problems 197 References 199 5 Exergy in Biological Systems 201 Summary 201 5.1 Photosynthesis 201 5.2 Land Biomass 206 5.3 Aquatic Biomass 212 5.4 Animal and Human Systems 214 5.5 Nonequilibrium Thermodynamics of Biological Systems 238 5.6 Entropy Production and Exergy Destruction in Humans 242 Problems 246 References 247 6 Ecosystems, the Environment, and Sustainability 251 Summary 251 6.1 Environmental Effects of Energy Usage 251 6.2 Ecology and Ecosystems 252 6.3 The Natural Environment 258 6.4 Exergy, the Natural Environment, and Ecosystems 261 6.5 Sustainable Development 266 Problems 270 References 270 7 Optimization and Exergoeconomics 273 Summary 273 7.1 Mathematical Optimization Models – Duality 273 7.2 Definitions of Relevant Economic Variables 277 7.3 Time Value of Money – Annualized Cost, Net Present Value 278 7.4 Thermoeconomics and Exergoeconomics 282 7.5 Uncertainty and Other Limitations 289 Problems 293 References 294 Index 296