Ecological, exergetic efficiency and heating load optimizations for irreversible variable-temperature heat reservoir simple air heat pump cycles

Thermodynamic optimization of an irreversible air heat pump with variable-temperature heat reservoirs and hot- and cold-side counter-flow heat exchangers has been studied. The expressions of the heating load, the exergetic efficiency and the ecological function of the heat pump cycle are derived. Performance comparisons among exergetic efficiency optimization, ecological optimization and traditional heating load optimization objectives are done. The effect of the pressure ratio of the compressor, the allocation of heat exchanger inventory and the heat capacity rate matching between the working fluid and the heat reservoirs on the optimal performance of the cycle has been investigated by detailed numerical examples. When the performance optimization of the cycle is carried out by selecting the pressure ratio, three optimization objectives give simultaneously attention to the coefficient of performance (COP). The pressure ratio should be the one that is little bigger than the optimum pressure ratio corresponding to maximum COP, however, the results of three optimization objectives are consistent by optimizing the allocation of heat exchanger inventory and optimizing the heat capacity rate matching between the working fluid and the heat reservoirs. The optimum allocations of heat conductance are close to each other, and they are all less than 0.5. The results may provide guidelines for the design and optimization of practical air heat pump plants.