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An automobile heat pump system with conventional refrigerant (HFC-134a or HFO-1234yf) suffers significantly diminishment of heating capacity and system efficiency as the ambient temperature decreases. Natural refrigerant CO2 (GWP = 1) is considered as a promising alternative to HFC-134a in automobile air conditioning (MAC) applications with environmentally friendly advantage. In addition, CO2 automobile heat pump system is a promising heat pump technology for EVs with great heating advantages in a cold climate. This study aims to investigate the supercritical heat transfer characteristics of a compact micro-channel gas cooler applied in an automobile CO2 heat pump system. A simulation model of automobile gas cooler was developed by using segment-by-segment method, and validated by experimental results from Series Gas cooler (SGC) and One Gas cooler (OGC) CO2 heat pump systems. The error of heating capacity between calculated results and experimental results was less than 7%.

This study investigates the cycle performance and potential advantages of the replacement of fin-and-tube evaporator with parallel flow micro-channel evaporator, in R134a roof-top bus air conditioner (AC) system. The heat exchangers for bus AC system are featured by a stringent space height limitation. The configuration of inclined four piece or six piece micro-channel evaporators was proposed to satisfy this space requirement, instead of original two piece fin-and-tube evaporators. Additionally, the individual superheat control method with thermostatic expansive valve (TXV) in each evaporator was adopted to improve refrigerant distribution. Three kinds of micro-channel evaporators were designed and equipped in an 8-m roof-top bus AC system. Except the replacement of evaporators, TXV and connecting pipes, other cycle components were kept same.