Attainability of the Carnot efficiency with real gases in the regenerator of the refrigeration cycle

Abstract

Improving efficiency is an enduring effort for all work-heat conversion cycles. Ideal regenerators working with ideal gases bring about a lossless work and heat transfer over a temperature gradient, but real gases give rise to an “intrinsic” heat loss in regenerators because of the time-averaged enthalpy flow associated with the pressure dependence. Real gas effects play a vital role on the coefficient of performance (COP) of regenerators of refrigeration cycles working at the temperatures close to or below the critical point. The “intrinsic” heat loss of real gases degrades the theoretical COP of regenerators to as low as 1% of the Carnot efficiency. In this paper, an approach of heat input or removal aiming to improve the COP is proposed. The theoretical analysis of this approach reveals the underlying mechanism. It is shown that the theoretical COP of an ideal regenerator working with a real gas applying this approach is identical to the Carnot efficiency. A simplified approach of heat input is further analyzed. The Carnot efficiency can be attained under certain circumstances, and it is possible to obtain over 90% of the Carnot efficiency with a discrete method. The theory of improving the COP with the approach of heat input in discrete regenerator locations is supported by the experiment results found in the relevant literature. This new approach provides a potential way to significantly improve the efficiency of the regenerator of the refrigeration cycle working at the temperatures close to or below the critical point. This approach may further provide a reference for studies of the heat pump cycle and the engine cycle working with real gases.