Optimum coefficient of performance and exergetic efficiency of a two-stage vapour compression refrigeration system

Abstract

This paper presents the results of an optimization study for a two-stage vapour compression refrigeration system based on the coefficient of performance (COP) and exergetic efficiency. Traditional studies have focused on the first-law performance, while those studies dealing with the second law have primarily been limited to performance analysis as opposed to performance optimization. The results of this study indicate that the use of the common approximation of the geometric mean to find the optimum interstage pressure can lead to significant errors in interstage pressure. However, an optimum COP or exergetic efficiency based on the same interstage pressure has relatively little error. This trend is valid as long as the isentropic compressor efficiencies are ‘reasonable’. Second-law optimization revealed that the optimum data curves themselves have a maxima for each set of conditions tested. This leads to the conclusion that for a given system there is an optimum set of conditions that lead to the lowest amount of exergy destruction for that system. This is shown to occur consistently for reasons that are, as yet, undetermined. Finally, polynomial equations have been fitted to the resultant optimum data for the interstage pressure, COP and exergetic efficiency. These equations allow for the reproduction of optimum points based on high-and low-pressure compressor efficiencies and condenser and evaporator pressures.