Studies on the performance characteristics and system optimization of cascade heat pump

Abstract

In this study, the studies of cascade heat pump performance characteristics and optimization for heating and hot water supply in extremely cold region were conducted. In general, conventional heat pump water heater system shows a limited performance at low ambient temperature whereas the heating demand increases. In order to increase the water discharge temperature, multi-stage cycle was suggested and among the multi-stage cycle, cascade cycle shows the best performance at high condensing temperature condition. In order to increase the cascade performance at low ambient temperature, R410A refrigerant which is suitable for low temperature application was adopted in bottoming cycle. The R134a refrigerant which has a higher critical temperature was adopted in topping cycle to increase the water discharge temperature. In order to increase the cascade system efficiency, several attempts were conducted. The determination of optimal refrigerant charge amount was studied by numerical simulation and experiment. The optimized coefficient of performance was obtained at the optimal charge amount condition and its corresponding degree of subcool at each cycle was suggested. The intermediate temperature which determines the pressure ratio of each cycle was also optimized by numerical anaysis based on the reverse-Carnot model. The verification of optimized numerical intermediate temperature was conducted by experiment and numerical intermediate temperature well predicted the experimental optimal intermediate temperature. The performance characteristics of cascade heat pump with water temperature lift at condenser were conducted by experiment. Despite of several advantages of heat pump than conventional boiler, slower thermal response is the weakness of heat pump water heater system. In order to increase the water discharge temperature, mass flow rate of water should be reduced. In case of decreasing water mass flow rate, the performance characteristics of cascade heat pump were obtained. The fast response of heat pump system is the key issue for hot water supply. The optimal control logic which is suitable for cascade heat pump was designed and the verification of control logic was conducted. The optimized PI controller based on the genetic algorithm showed enhanced performance than conventional PI tuning method.