Rational assessment and selection of air source heat pump system operating with CO₂ and R407C for electric bus

Heat pump technology has gained surging interests due to its abilities in reducing the battery burden of the electric bus. Here, three heat pump systems for an 8 m bus are compared, including a R407C system, a basic transcritical CO₂ system (CO₂-BASE), and a transcritical CO₂ system with an expansion-compression integrated machine (CO₂-Advanced). The results demonstrate that the optimal ratio of the theoretical volume of the expander to the compressor in expander-compressor machine can improve the coefficient of performance (COP) of the CO₂-BASE system in all cooling conditions, with a range of 0.165–0.190. The COP of the CO₂-Advanced system has 13.59% improvement than that of the R407C system at most in cooling mode. In addition, the performances of the three systems run in five typical climates in China are compared. The comparison result shows that the CO₂-BASE system is more suitable for Severe Cold Zone and Mild Zone, and the R407C system performs better in Hot summer and Warm winter Zone. However, the CO₂-Advanced system, which has the best energy-saving effect in all climate zones in China, has a great potential to replace the existing R407C system for electric buses.