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Open AccessArticle
A Systematic Approach to Exergy Efficiency of Steady-Flow Systems
by
Yunus A. Çengel
Yunus A. Çengel
Yunus Çengel is a Professor Emeritus at the University
of Nevada, Reno, USA. He received his Ph.D. [...]
Yunus Çengel is a Professor Emeritus at the University
of Nevada, Reno, USA. He received his Ph.D. in Mechanical Engineering from
North Carolina State University, and served as a faculty member at the
University of Nevada, Reno since 1984. He was the Director of the Industrial
Assessment Center and served as consultant in the areas of energy efficiency,
renewable energy, energy policies. Professor Çengel is the author or coauthor
of the several widely adopted engineering textbook such as Thermodynamics:
An Engineering Approach, whose tenth edition was published in 2024.
Mehmet Kanoğlu
Mehmet Kanoğlu
Mehmet Kanoğlu is Professor of Mechanical Engineering
at Alanya Alaaddin Keykubat University. He [...]
Mehmet Kanoğlu is Professor of Mechanical Engineering
at Alanya Alaaddin Keykubat University. He received his M.S. and Ph.D. in
mechanical engineering from University of Nevada, Reno. His research areas
include energy efficiency, refrigeration systems, gas liquefaction, hydrogen
production, renewable energy systems, geothermal energy, and cogeneration. He
has taught at University of Nevada, Reno, Ontario Tech University, and American
University of Sharjah. He is the coauthor of the books Thermodynamics: An Engineering
Approach (10th ed., 2024), Fundamentals and Applications of Renewable Energy
(2nd ed., 2023), Energy Efficiency and Management for Engineers (2020), all
published by McGraw-Hill. He is listed among the top 2% most cited scientists
worldwide since 2017.
1
Department of Mechanical Engineering, University of Nevada, Reno, NV 89557, USA
2
Department of Mechanical Engineering, Alanya Alaaddin Keykubat University, Antalya 07425, Türkiye
*
Author to whom correspondence should be addressed.
Entropy 2025, 27(11), 1108; https://doi.org/10.3390/e27111108 (registering DOI)
Submission received: 14 September 2025
/
Revised: 6 October 2025
/
Accepted: 20 October 2025
/
Published: 26 October 2025
Abstract
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 ηex = Xoutput/Xinput = 1 − (Xdestroyed+Xloss)/Xinput or the ratio of exergy recovered to exergy expended ηex = 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.
Share and Cite
MDPI and ACS Style
Çengel, Y.A.; Kanoğlu, M.
A Systematic Approach to Exergy Efficiency of Steady-Flow Systems. Entropy 2025, 27, 1108.
https://doi.org/10.3390/e27111108
AMA Style
Çengel YA, Kanoğlu M.
A Systematic Approach to Exergy Efficiency of Steady-Flow Systems. Entropy. 2025; 27(11):1108.
https://doi.org/10.3390/e27111108
Chicago/Turabian Style
Çengel, Yunus A., and Mehmet Kanoğlu.
2025. "A Systematic Approach to Exergy Efficiency of Steady-Flow Systems" Entropy 27, no. 11: 1108.
https://doi.org/10.3390/e27111108
APA Style
Çengel, Y. A., & Kanoğlu, M.
(2025). A Systematic Approach to Exergy Efficiency of Steady-Flow Systems. Entropy, 27(11), 1108.
https://doi.org/10.3390/e27111108
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