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    A Systematic Approach to Exergy Efficiency of Steady-Flow Systems
    (Mdpi, 2025) Cengel, Yunus A.; Kanoglu, Mehmet
    Exergy efficiency is a measure of thermodynamic perfection. A device that operates reversibly has an exergy efficiency of 100 percent and is said to be thermodynamically perfect. A reversible process involves zero entropy generation and thus zero exergy destruction since Xdestroyed = T0Sgen. Exergy efficiency is generally defined as the ratio of exergy output to exergy input eta ex = Xoutput/Xinput = 1 - (Xdestroyed + Xloss)/Xinput or the ratio of exergy recovered to exergy expended eta ex = Xrecovered/Xexpended = 1 - Xdestroyed/Xexpended. In this paper, exergy efficiency relations are obtained first for a general steady-flow system using both approaches. Then, explicit general relations are obtained for common steady-flow devices, such as turbines, compressors, pumps, nozzles, diffusers, valves and heat exchangers, as well as heat engines, refrigerators, and heat pumps. For power and refrigeration cycles, five different forms of exergy efficiency relations are developed, and their equivalence is demonstrated. With the unified approach presented here and the insights provided, the controversy and confusion associated with different exergy efficiency definitions are largely alleviated.
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    Exergy Efficiency of Closed and Unsteady-Flow Systems
    (Mdpi, 2025) Cengel, Yunus A.; Kanoglu, Mehmet
    Exergy efficiency is viewed as the degree of approaching reversible operation, with a value of 100 percent for a reversible process characterized by zero entropy generation or equivalently zero exergy destruction since Xdestroyed = T0Sgen. As such, exergy efficiency becomes a measure of thermodynamic perfection. There are different conceptual definitions of exergy efficiency, the most common ones being (1) the ratio of exergy output to exergy input eta ex = Xoutput/Xinput = 1 - (Xdestroyed + Xloss)/Xinput, (2) the ratio of the product exergy to fuel exergy eta ex = Xproduct/Xfuel = 1 - (Xdestroyed + Xloss)/Xfuel, and (3) the ratio of exergy recovered to exergy expended eta ex = Xrecovered/Xexpended = 1 - Xdestroyed/Xexpended. Most exergy efficiency definitions are formulated with steady-flow systems in mind, and they are generally applied to systems in steady operation such as power plants and refrigeration systems whose exergy content remains constant. If these definitions are to be used for closed and unsteady-flow systems, the terms need to be interpreted broadly to account for the exergy change of the systems as exergy input or output, as appropriate. In this paper, general exergy efficiency relations are developed for closed and unsteady-flow systems and their use is demonstrated with applications. Also, the practicality of the use of the term exergy loss Xloss is questioned, and limitations on the definition eta ex = Wact,out/Wrev,out are discussed.