Heat Transfer Characteristics of Gas Cooler in a CO ₂ Automobile Heat Pump System

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%. Using this model, the air and refrigerant temperature distribution as well as the heat transfer coefficient along the refrigerant flow path under the condition of -20 ^°C air inlet temperature and 9 MPa refrigerant inlet pressure were predicted and analyzed. When the refrigerant thermodynamic state is in the pseudo-critical region, the refrigerant specific heat capacity and heat transfer coefficient will suddenly increase to a high level. Results show that after passing through four slabs of gas cooler, the air temperature can be increased from -20 ^°C to an average outlet temperature of 39.2 ^°C. Furthermore, the impact of the number of gas cooler slabs on the heat rejection performance of gas cooler was studied. Results show that with the increase of slab number, the heat rejection performance of gas cooler first increased rapidly, then increased slowly, and finally approached to the heat transfer limit. The lowest heat transfer temperature difference occurs at the last segment of the gas cooler.