The definition of open cycle rational efficiency is unequivocally based on the ratio of the actual shaft work output from a power plant to the maximum work that could be obtained in a reversible process between prescribed inlet and outlet states. However, the specification of this ideal process remains a matter for discussion. The “classical” definition specifies the outlet state as a “standard” atmosphere (p0,T0) with a given composition of constituent gases and an exergy of zero. The ideal work output is then the exergy of the fuel supplied, which is the sum of the Gibbs function change in reversible combustion at (p0,T0) and the work of extraction and delivery of the reactant and product species.

However, two other ideal processes may be considered in which different outlet states are specified leading to alternative definitions of rational efficiency. In one process all species enter and leave the ideal plant individually at (p0,T0) in thermal and mechanical (but not chemical) equilibrium with the environment. In the other, the exhaust is at (p0,T0) but the gas composition is the same as in the real process. A further complication occurs when water or steam is injected into a gas turbine plant. In such cases the exergy of the added water must be considered within any definition of rational efficiency. The three definitions of rational efficiency are discussed and compared and some illustrative calculations presented.

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