Energy, exergy, and Second Law performance criteria

Performance criteria, such as efficiencies and coefficients of performance, for energy systems, are commonly used but often without sufficient understanding and consistence. The situation becomes particularly incoherent when simultaneous energy interactions of different types, such as work, heating and cooling, take place with a system. Also, the distinction between exergy and Second Law efficiencies is not clearly recognized by many. It is attempted here to clarify the definitions and use of energy and exergy based performance criteria, and of the Second Law efficiency, with an aim at the advancement of international standardization of these important concepts. r 2006 Elsevier Ltd. All rights reserved.