Transient Simulation, Mobile Air Conditioning, Secondary Loop System, R-134a, R-152a, R-1234yf
Since the conventional refrigerant R-134a is being phased down due to its high GWP, finding a suitable replacement refrigerant with low GWP and a system design is of great importance. However, most of the alternatives are either flammable or more expensive. Therefore, to ensure the safety of passenger and reduce the refrigerant charge, a secondary loop system with coolant loop on both condenser- and evaporator-side was proposed. In the secondary loop system, the evaporator and condenser exchange heat with air through cabin cooler and radiator, respectively. The secondary loop system has more advantages than the direct expansion system such as easy applicability of heat pump operation and battery thermal management. In this study, transient models were developed for both direct expansion system and secondary loop system in Dymola. The simulation results show that the coefficient of performance of the secondary loop system is lower than that of direct expansion system due to high pressure ratio and high compressor revolution speed when two types of systems provide similar cooling capacity. Moreover, the performances of the system using R-134a, R-152a, and R-1234yf were evaluated and compared to that of conventional direct expansion system using R-134a under the US06 driving cycle condition. Though large fluctuation is observed on the condenser capacity in the direct expansion system, the evaporator capacity is very stable. In overall, R-152a has better performance than R-1234yf and is a good candidate as an alternative refrigerant but the secondary system needs more efficiency enhancement options to compete with current R-134a direct expansion system.