Theoretical and experimental investigation on the effects of internal heat exchangers on a reversible automobile R744 air-conditioning system under various operating conditions

This paper presents a theoretical cycle analysis and an experimental study of the effects of internal heat exchangers (IHXs) on reversible automobile R744 air conditioning systems. The cycle analysis shows that the system efficiency benefits most with a high-efficiency IHX in the transcritical mode where the gas cooler exit temperature is high, while the IHX generally has a trivial or even negative effect on the system performance in the subcritical mode. The experimental results reveal that with the IHX, the optimal refrigerant charge is reduced from 1450 g to 1300 g, and the optimal high-side pressure is reduced by 0.24 to 0.67 MPa. Moreover, for both basic and IHX systems, it is experimentally confirmed that one linear control equation can relate the COPmaximizing high-side pressure with the gas-cooler/condenser exit temperature and functions well in both trans- and subcritical modes. That reveals the potential of combing the high-side pressure control in the transcritical mode and the subcooling control in the subcritical mode. Agreed with the theoretical analysis, the IHX has a negligible effect on COP in the subcritical mode in the experiments, while it improves the system efficiency by up to 13.8% in the transcritical mode. The highest benefit is gained in the pull-down condition that has high ambient and cabin temperatures and requires fast cooling.