Comment on “Not all counterclockwise thermodynamic cycles are refrigerators” [Am. J. Phys. 84, 413–418 (2016)

Abstract

Contrary to what Dickerson and Mottmann [Am. J. Phys. 84, 413–418 (2016)] state, the temperatures at which a refrigerator's working fluid absorbs heat need not always lie below those at which it expels heat; nor must a refrigerator's thermodynamic cycle have two adiabats. Moreover, what Dickerson and Mottmann call a “comparison Carnot cycle” cannot always be defined. These conclusions are illustrated here using a counter-clockwise Stirling cycle without regeneration. A refrigerator's cold reservoir can absorb some heat and its hot reservoir can expel some heat, so long as the net heat flow is still out of the cold reservoir and into the hot reservoir.