Optimal performance analysis of irreversible cycles used as heat pumps and refrigerators

A general irreversible cycle model is used to investigate the optimal performance of a class of heat-driven pumps affected by the three main irreversibilities, which are finite-rate heat transfer between the working fluid and the external heat reservoirs, internal dissipation due to the working fluid and heat leakage between heat reservoirs. The coefficient of performance of the cycle system is taken as an objective function for optimization. Some equivalent parameters are introduced so that the relevant calculation is simplified. The coefficient of performance versus dimensionless heating load curves describing the general performance characteristics of heat-driven heat pumps are plotted. The optimal temperatures of the working fluid at the maximum coefficient of performance are determined. Moreover, it is expounded that the irreversible cycle model is very useful. The optimal performance not only of a class of irreversible heat-driven refrigerators but also of an irreversible Carnot heat pump and refrigerator may be directly analysed by using the cycle model. The results obtained may provide some new theoretical bases for the optimal design and operation of two classes of real heat pump and refrigerator systems driven by 'low-grade' heat energy and 'high-grade' work