Energy and Exergy Efficiencies

Abstract

Energy and exergy efficiencies are considered to be useful for the assessment of energy conversion systems and for improved performance. By considering both of these efficiencies, the quality and quantity of the energy used to achieve a given objective is properly accounted for, and the degree to which efficient and effective use of energy resources is achieved can be assessed. Improving efficiencies of energy systems becomes an important challenge for meeting energy policy objectives. Efficiency is a true measure of the effectiveness or performance of a system. It may take different forms but it can generally be defined as the desired output over the required input. In this chapter, energy- and exergy-based efficiencies are defined for cyclic devices including heat engines, refrigerators, and heat pumps. Various efficiency definitions based on both the first and second laws of thermodynamics are provided for steady-flow devices such as turbines, compressors, resistance heaters, and heat exchangers.