Theoretical study of a high-temperature heat pump system composed of a CO2 transcritical heat pump cycle and a R152a subcritical heat pump cycle

Abstract

A high-temperature heat pump system composed of a CO2 transcritical cycle and a R152a subcritical cycle is proposed for heat and cooling cogeneration. Emphasis is placed on analyzing and comparing thermodynamic performance of the combined system to the single heat pump and singe CO2 refrigeration systems working alone. Entransy dissipation rate and T-Q diagram are also applied to illustrate temperature match behavior of heating process of supply water. Besides, performance comparisons were also conducted between the systems with and without intermediate heat exchanger and between the systems with different working fluids. The studies indicate that there exist appropriate ranges of CO2 discharging pressure for CO2 transcritical cycle to meet the requirements of pinch point temperatures for heat transfer processes. Increasing CO2 discharging pressure, both the system heating and cooling capacities are increased. However, total system power consumption first slightly decreases and then increases exponentially. As for the overall system COP and exergy efficiency, both are increased with increasing CO2 discharging pressure. Under the same operating conditions, the system COP and exergy efficiency of this combined system are increased by about 54.7% and 175%, respectively, as compared to the single R152a heat pump and single CO2 transcritical cycle working alone.