Reversible and irreversible heat engine and refrigerator cycles

Abstract

Although no reversible thermodynamic cycles exist in nature, nearly all cycles covered in textbooks are reversible. This is a review, clarification, and extension of results and concepts for quasistatic, reversible and irreversible processes and cycles, intended primarily for teachers and students. Distinctions between the latter process types are explained, with emphasis on clockwise (CW) and counterclockwise (CCW) cycles. Specific examples of each are examined, including Carnot, Kelvin and Stirling cycles. For the Stirling cycle, potentially useful task-specific efficiency measures are proposed and illustrated. Whether a cycle behaves as a traditional refrigerator or heat engine can depend on whether it is reversible or irreversible. Reversible and irreversible-quasistatic CW cycles both satisfy Carnot's inequality for thermal efficiency, η≤ηCarnot. Irreversible CCW cycles with two reservoirs satisfy the coefficient of performance inequality K≤KCarnot. However, an arbitrary reversible cycle satisfies K≥KCarnot when compared with a reversible Carnot cycle operating between its maximum and minimum temperatures, a potentially counterintuitive result.