Experimental study on refrigerant charge amount and optimal high pressure of a CO2 heat pump system for an electric bus

In order to investigate the influence of refrigerant charge amount and discharge pressure on a CO2 heat pump system of an electric bus, an experiment setup was built. Tests were carried out in an enthalpy difference chamber to obtain the coefficient of performance and heating capacity of the system at each operating condition. The influence of operation parameters-including refrigerant charge amount, ambient temperature, outlet temperature of gas cooler, and compressor speed-was considered in this experiment. The results showed that the system performance was influenced by charge amount, and the minimum charge amount was 6.8 kg. The charge amount was almost independent of the ambient temperature, but the system performance was more sensitive to the charge amount at low temperatures. The optimal high-pressure relationship for this CO2 heat pump is p(opt) = 0.162T(gc.out) - 0.016T(amb) + 2.668, which was fitted considering both the outlet temperature of the gas cooler and the ambient temperature. Meanwhile, the low-temperature performance test (-22 & DEG;C) showed that this CO2 heat pump system can meet the operation requirements of city buses at low temperature.