Exergy: Energy, Environment, and Sustainable Development

Energy, entropy and exergy concepts stem from thermodynamics and are applicable to all fields of science and engineering. This chapter provides the necessary background for understanding these concepts, as well as basic principles, general definitions and practical applications and implications. Illustrative examples are provided to highlight the important aspects of energy, entropy and exergy.

The scope of this chapter is partly illustrated in Fig. 1.1, where the domains of energy, entropy and exergy are shown. This chapter focuses on the portion of the field of thermodynamics at the intersection of the energy, entropy and exergy fields. Note that entropy and exergy are also used in other fields (such as statistics and information theory), and therefore they are not subsets of energy. Also, some forms of energy (such as shaft work) are entropy-free, and thus entropy subtends only part of the energy field. Likewise, exergy subtends only part of the energy field since some systems (such as air at atmospheric conditions) possess energy but no exergy. Most thermodynamic systems (such as steam in a power plant) possess energy, entropy and exergy, and thus appear at the intersection of these three fields.

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Fig. 1.1. Interactions between the domains of energy, entropy and exergy.

Energy comes in many forms. Thermodynamics plays a key role in the analysis of processes, systems and devices in which energy transfers and energy transformations occur. The implications of thermodynamics are far-reaching and applications span the range of the human enterprise. Throughout our technological history, our ability to harness energy and use it for society's needs has improved. The industrial revolution was fueled by the discovery of how to exploit energy in a large scale and how to convert heat into work. Nature allows the conversion of work completely into heat, but heat cannot be entirely converted into work, and doing so requires a device (e.g., a cyclic engine). Engines attempt to optimize the conversion of heat to work.

Most of our daily activities involve energy transfer and energy change. The human body is a familiar example of a biological system in which the chemical energy of food or body fat is transformed into other forms of energy such as heat

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