Impact of Liquid Reservoir on Charging Performance in R290 Heat Pump Systems for Electric Vehicles

This study investigated the effects of refrigerant charge on the cycle states and performance of an R290 heat pump system equipped with a liquid receiver for electric vehicles in both economizer and vapor-injection modes. Experimental results revealed a distinct “buffer period” during the charging process， within which the system cycle state remains essentially unchanged and the COP reaches its maximum. Outside this period， evaporating pressure， subcooling， and evaporator inlet dryness fraction increased with charging， whereas compressor suction and discharge superheats decreased. The buffer-period charge ranges differed between the two modes—260-290 g for the economizer cycle and 270-300 g for the vapor-injection cycle—primarily owing to differences in the internal refrigerant density distribution. Based on the buffer-period duration， the effective liquid storage ratios of the receiver were calculated to be 35.6% and 36.1%， respectively， providing a quantitative metric for evaluating the receiver performance. Further analysis identified a wider “performance plateau period，” during which the system cooling capacity and COP remained nearly constant， allowing quantification of the optimal charge range. By comparing the performance plateau periods of both cycles， the optimal common charge under all operating conditions was determined； for this system， it is 270 g. Extreme-condition tests showed that the system maintains good heating and cooling performance at -20 ℃ heating mode and 43 ℃ cooling mode， confirming the effectiveness of the proposed method.