Performance simulation of CO2 transcritical cooling system with mechanical subcooling cycle for automobile air conditioning

In newly produced vehicles, the low global warming potential (GWP) refrigerant must be charged instead of conventional high GWP media according to the environmental regulations. For these needs, the CO2 cooling system is re-attracted as green technology in the automobile field. However, the conventional CO2 system has the drawback that it sharply declines the cooling capacity and coefficient of performance (COP) when the hot ambient conditions. To enhance the COP of a CO2 air conditioner, the mechanical subcooling cycle is introduced in this study. But the subcooling cycle requires installing additional heat exchangers. In the automobile application which has limited install space, the design of heat exchangers should be considered for adopting the subcooling cycle. Thus, in this study, numerical models were built to predict the COP by changing the heat exchangers. In addition, an optimal method is used to determine the maximum COP. The results show that the COP improves when applying the subcooling cycle, and the maximum COP is determined based on the thermodynamic states at the outlets of the gas cooler and the subcooler. Meanwhile, the refrigerant types do not substantially affect the COP. While the COP is improved with the use of larger heat exchangers and fans, the rate of improvement gradually decreases. In the optimum efficiency calculation, the power consumption of fans should be considered, and the proper heat exchanger and fan should be designed accordingly.