Exploring counterclockwise thermodynamic cycles.

A common belief is that any closed, counterclockwise cycle on a pV diagram represents a refrigerator or heat pump. It has been established that this is not the case, but previous papers on this topic have made unnecessary assumptions about the temperatures of the energy reservoirs and about how the system exchanges energy with the reservoirs. Relaxing these assumptions leads to a wide array of unexpected behaviors. In some cases, the same pV cycle can be a heat pump, a cold pump, or a Joule pump simply by changing how the system is connected to the reservoirs. This paper explores the strange world of counterclockwise cycles. Editor's note: In this paper, author Randall Knight explores the rich behavior of counterclockwise thermodynamic cycles. A counterclockwise cycle can function as a heat pump or refrigerator, but this is not the only possibility. The choice of thermal reservoirs and the choice of which reservoir heat is absorbed from and exhausted to during the cycle affect the coefficient of performance and even the basic function of the device. The author illustrates these possibilities through an analysis of counterclockwise Carnot, Kelvin, Brayton, and Stirling cycles. The analysis includes interesting possibilities, such as a cycle that operates with a single reservoir, as well as practical applications like the role of heat exchangers. The surprising results of this analysis will likely be of interest to anyone who has studied thermodynamics, and the material can easily be incorporated into any course that includes thermodynamics, from introductory physics to graduate study. [ABSTRACT FROM AUTHOR]