Experimental investigation on the cooling performance of direct and secondary loop CO₂ air conditioning systems for electric vehicles

As a natural refrigerant, CO₂ is considered one of next generation refrigerants of electric vehicle (EV) applications for excellent environmental and heating performance. The integration and safety performance of CO₂ systems could be enhanced by secondary loop configuration, which is highly valuable for practical applications. This paper investigates the cooling performance characteristics of direct (DAC) and secondary loop (SLAC) transcritical CO₂ air conditioning systems based on liquid cooled gas coolers (LCGC). A series of steady-state experiments were carried out for both systems to study the effects of compressor speed, EXV opening, outdoor air velocity, and indoor air flow rate on the cooling performances. Furthermore, the exergy destruction of each component and the exergy efficiency of both systems were analyzed based on the experimental results, showing that the evaporator needs to be better designed to balance heat transfer performance with pressure loss performance. The results showed that the cooling capacity and COP of SLAC system were reduced by up to 9.1% and 10.8% under the same compressor speed, compared with the DAC system. This study presents experimental references for further research and practical extension of the CO₂ mobile air conditioner.