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Adiabatic combustion raises the temperature of the working fluid in a power cycle and provides the source of “high-temperature heat” to the heat engine. Analysis in the previous chapter showed that adiabatic combustion reactions are irreversible, and lead to entropy generation and hence loss of availability. Isothermal reactions that operate at equilibrium with the environment avoid this loss mechanism. If carefully executed, these can lead to more efficient use of the chemical energy. One practical way to directly convert chemical energy to electricity under nearly isothermal conditions is in a fuel cell, where reactions occur in the form of an electrochemical pair, or a redox pair.
Energy is one of the most important needs of humanity. Mobility, lighting, communications, heating, and air conditioning are all energy-intensive functions that are indispensable in modern life. Industrial production, food production, and clean water require energy.
In Chapter 5 we covered gas turbine cycles and discussed conditions under which they are expected to achieve high efficiency or high specific work. With high-temperature energy sources, such as combustion of clean fuels, gas turbines offer many advantages in electricity generation, as well as high-speed propulsion. They are not, however, compatible with intermediate-temperature sources, such as lower-temperature nuclear reactors and concentrated solar thermal energy, or low-temperature sources such as concentrated solar power or geothermal energy. Two-phase Rankine cycles can be designed to operate at high efficiency while utilizing these sources. This is the subject of this chapter.
Thermomechanical energy conversion is concerned with the conversion of heat and thermal energy to mechanical work or mechanical energy. The latter may be used directly (e.g., for propulsion) or to generate electricity. Currently, the major source of thermal energy is the combustion of fossil fuels and biomass, followed by thermonuclear reactions, with geothermal energy and solar energy at much smaller scales. The conversion efficiency is directly related to the temperature of the heat source or the “quality” of the thermal energy. The temperature of the source and that of the environment determine the maximum efficiency of energy conversion cycles.
Renewable sources of thermal energy have been used to generate electricity in power plants using power cycles similar to those described in Chapters 5 and 6, although with some modifications to make them compatible for the intermediate- and lower-temperature heat sources. These renewable sources include geothermal energy and concentrated solar thermal energy. The power cycles used for these two types will be discussed in this chapter, starting with general guidelines regarding how to maintain the cycle efficiency as high as possible while using lower-temperature heat sources. For instance, using different working fluids in Rankine cycles can improve the efficiency by operating the cycle as a supercritical cycle even though the source temperature is relatively low.
Energy conversion systems, in which the energy source is a fuel or a chemical energy carrier such as hydrogen, very often involve reacting mixtures. Reactions among species in these mixtures result in the conversion of their chemical bond energy into other forms, such as thermal energy or electrical energy. In exothermic reactions, of which combustion reactions are an important subset, the stored chemical energy is converted into thermal energy, which raises the temperature of the mixture. In jet engines and rockets, the thermal energy is converted into kinetic energy in the nozzle, which produces thrust. Exothermic chemical reactions are also used in industrial operations – where thermal energy delivered at fast rates is required – and domestic and commercial heating systems. Endothermic reactions, which absorb energy, are also important. Examples of practical applications of endothermic reactions include some fuel reforming processes, the production of synthetic fuels and other chemicals, chemical storage of hydrogen, and the formation of nitric oxides in combustion. In endothermic reactions, thermal energy is supplied through heat transfer to energize the reactions. Reactions involving the conversion between chemical and thermal energies are generally called thermochemical reactions. Such reactions are the principal scope of this chapter.
In this chapter, some proposed power cycles for CO2 capture in coal power plants are presented. These, as discussed in Chapter 11, include post-combustion capture, pre-combustion capture, and oxy-combustion capture cycles. The objective of the coverage is, besides describing the processes in some of these cycles, to evaluate their efficiency and contribution of the capture process toward derating the power production. While similar in principle to those designed for use with natural gas, coal (and other solid fuels) differs in fundamental ways from natural gas because it is consumed in the solid phase and it is often contaminated with sulfur, nitrogen, and ash, among other undesirable substances. Both the nature of the fuel and the contaminants make coal more challenging to use in energy production, and more so when carbon capture is implemented. However, and given its wide availability, lower cost, and higher CO2 emission per unit of useful energy produced, it is imperative to develop this technology.
Carbon dioxide production in electric power plants depends strongly on the fuel and power cycle efficiency. Coal and natural gas generate approximately one mole of CO2 for each mole of fuel burned, but they differ in their plant conversion efficiency. Table 10.1 shows average data for CO2 production when using either fuel for electricity generation. The absolute majority of pulverized coal plants operate on steam Rankine cycles. The most efficient natural gas plants operate on combined cycles, although some operate on simple cycles.
Coal is a widely available cheap fuel that has been used extensively in heating, electricity generation, and industrial processes. Coal reserves and resources are the largest among other known fossil fuel reserves and resources. Besides carbon, hydrogen, and some oxygen, raw coal contains, among other things, sulfur, metallic compounds, mercury, and nitrogen. Technologies have been developed to utilize coal while limiting the emissions of “criteria” pollutants, including sulfur compounds, nitric oxides, mercury, and fine particulates. While increasing the cost of electricity by raising the plant capital cost and lowering its efficiency, these technologies made it possible to continue to expand the use of coal without negatively affecting air quality. More recently, coal use has accelerated significantly in developing economies. This and the fact that coal produces the largest amount of CO2 per unit of useful energy has intensified the effort to improve the overall efficiency of coal power plant and to develop technologies for CO2 capture from these plants.
Until the mid-1800s, when fossil fuels became the major source of energy for heating, electricity production, and transportation, biomass derived from plants and animal products was the primary source of energy (heating and lighting by fires and candles). The explosive growth in the use of fossil fuels powered the Industrial Revolution and provided enormous improvements in the standards of living wherever it became the dominant source of primary energy. However, biomass still supplies nearly 10% of the primary energy worldwide, and is a major source in rural communities. Although only a small percentage of available biomass resources are used for energy production, the total potential exceeds 4.5 EJ (exajoules; 10 joules). Early use of biomass was likely to have been for fires. More recently, it has been for heating by combustion for domestic use; electricity production by combustion in power plants; and for the production of biofuels such as ethanol and biodiesel by bioconversion or thermochemically. The contribution of biomass to primary energy consumption, both in quantity and form and as a fraction or absolute amount, depends strongly on the level of economic development and geographic location. In 2003, it was 26% of the total consumption in the developing world, but only 4% in the developed world. During 2010–2012, the US consumed 62 terawatt-hour/year (TWh/y) of bio-energy, followed by Germany, which consumed 37 TWh/y. During the same period, China and India consumed 27 and 3.4 TWh/y, respectively. The majority of bio-energy, close to 90% of the total, is produced using solid feedstock in the form of energy crops like sugar and corn, or lignocellulosic material, with the rest in the form of gaseous fuels produced from landfills and bio-digesters.
In Chapters 5 and 6, we discussed gas turbine cycles and Rankine cycles used in power generation, with a focus on how to improve the cycle efficiency and recover the maximum availability from the primary energy source. We also discussed the conditions under which one chooses to build a plant running on a gas cycle or a two-phase cycle, and related these to the characteristics of the primary energy source. High-temperature energy sources can be effectively utilized in a gas turbine cycle, which exhausts its stream at atmospheric pressure (unless the cycle is closed, but this is not currently practiced). Lower-temperature sources must use two-phase Rankine cycles, and must be operated in a closed cycle mode to achieve the desired efficiency. In both cases, reheat and regeneration are effective approaches to raising the thermal efficiency.
Thermodynamics is central to the analysis of energy conversion processes and systems. Although excluding rate processes, equilibrium thermodynamics’ analysis can be used to examine the efficiency and specific work of a process or a series of processes executing work and heat transfer interactions with other systems, experiencing mass transfer, undergoing chemical and electrochemical reactions, or a combination of all of these events. Non-equilibrium and rate processes can indeed impact efficiency, and are necessary to determine the power as well as other performance measures such as size and emissions. Non-equilibrium effects will be examined in later chapters. In this chapter, the basic laws of equilibrium thermodynamics are reviewed, with an emphasis on some of the origins of the different statements, the meaning of the quantities appearing in these laws, the most relevant forms of the laws to be used in analysis of energy conversion, and some conclusions regarding how these systems should be designed. The early coverage is independent of the working fluid, and focuses on the energy conversion process. Pure substance, ideal gases, and mixtures of ideal gases and their equations of state are also mentioned.
The performance of fuel cells at finite current, or finite power, is presented in this chapter, focusing on the sources of loss, how each loss mechanism is modeled, and how the design parameters and operating conditions contribute to each. In particular, we examine the role of chemical kinetics and transport processes in fuel cell efficiency. At finite current, fuel cells cannot achieve the ideal thermodynamic efficiency, corresponding to the maximum work or the Gibbs free energy of the overall reaction, due to a number of intrinsic loss mechanisms. These include: (1) non-electrochemical, or thermochemical, reactions, occurring on the surfaces or within the fuel channel; (2) potential loss associated with finite-rate electrochemical reactions; (3) decrease in reactants concentrations because of finite-rate transport processes; and (4) losses associated with the transport of ions and electrons across different elements. All of these mechanisms depend on the current drawn from the cell. Some small losses are observed even at open-circuit conditions, mostly due to electron and fuel leakage across the electrodes. Modeling these losses is tackled in some detail in this chapter.
This unique textbook equips students with the theoretical and practical tools needed to model, design, and build efficient and clean low-carbon energy systems. Students are introduced to thermodynamics principles including chemical and electrochemical thermodynamics, moving onto applications in real-world energy systems, demonstrating the connection between fundamental concepts and theoretical analysis, modelling, application, and design. Topics gradually increase in complexity, nurturing student confidence as they build towards the use of advanced concepts and models for low to zero carbon energy conversion systems. The textbook covers conventional and emerging renewable energy conversion systems, including efficient fuel cells, carbon capture cycles, biomass utilisation, geothermal and solar thermal systems, hydrogen and low-carbon fuels. Featuring numerous worked examples, over 100 multi-component homework problems, and online instructor resources including lecture slides, solutions, and sample term projects, this textbook is the perfect teaching resource for an advanced undergraduate and graduate-level course in energy conversion engineering.