Experimental comparison between finned-tube and micro-channel heat exchangers in a transcritical CO₂ air-conditioning and heat pump system used in rail vehicles

Transcritical CO₂ heat pumps represent a high-efficiency and sustainable solution for railway vehicle heating, ventilation, and air conditioning (HVAC) systems, yet their application is hindered by challenges related to overall system performance enhancement and winter frost mitigation. To address the unique constraints of restricted installation space and dynamic aerodynamic loads in rail vehicles, this study systematically evaluates the impact of replacing conventional finned-tube heat exchangers (FTHE) with micro-channel heat exchangers (MCHX) on steady-state performance and dynamic frost/defrost behavior. A full-scale rail air-conditioning experimental platform was developed to conduct a comparative analysis between a hybrid configuration and a conventional all-FTHEs configuration across multiple representative operating points. Steady-state results demonstrate that the hybrid configuration offers superior thermodynamic advantages under most conditions. At the 35 °C standard cooling condition, the hybrid configuration achieves a maximum COP improvement of 8.37% by increasing the value from 2.03 to 2.20 through optimized air-side thermal resistance. In the analysis of dynamic frosting and defrosting characteristics, the results indicate that while both MCHX and FTHE exhibit similar first frosting stabilization times of approximately 40 min, their reverse-cycle defrosting performance differs significantly. Due to capillary effects and melt-water retention inherent in the micro-channel geometry, the MCHX requires approximately 60 s for complete defrosting, whereas the FTHE completes the process in only 40 s. This study confirms that MCHX improves system steady-state efficiency, and FTHE has a notable advantage in defrosting speed.