the property exergy serves as a valuable tool in determining the quality of energy and comparing the work potentials of different energy sources or systems. the evaluation of exergy alone, however, is not sufficient for studying engineering devices operating between two fixed states. this is because when evaluating exergy, the final state is always assumed to be the dead state, which is hardly ever the case for actual engineering systems. the isentropic efficiencies discussed in chap. 7 are also of limited use because,