Chapter 10 - Exergy

Abstract

The first law of thermodynamics has provided the concept of internal energy and enthalpy, which are conserved in a physicochemical system. The second law has defined entropy, free energy, and free enthalpy as thermodynamic energy functions to indicate the possibility and capacity of advancing irreversible processes. The law of conservation of energy indicates that energy never disappears, while the second law of thermodynamics sets forth that thermal energy cannot be fully utilized in the atmospheric environment. Engineering thermodynamics has recently introduced a new energy quantity called exergy to figure out how much work or power one can utilize from a given amount of energy with respect to the natural environment. Energy is conserved in any process; whereas, exergy is dissipated in spontaneous processes. Exergy analyses are thus effective in improving the energy efficiency in practical manufacturing processes. This chapter examines the concept and property of exergy in thermodynamic systems and shows that exergy losses are in principle related to the affinity (the free enthalpy change) of irreversible processes. The chapter also discusses various examples of exergy evaluation for heat transfer, gas expansion, substance mixing, and chemical reactions.