Optimal operation of phase change material integrated condenser under various subcooling conditions

When phase change materials (PCMs) are integrated with vapor compression cycle (VCC) condensers for thermal energy storage, system efficiency and reliability depend on both stable operation during the PCM phase transition and dynamic performance over the full operating cycle, as the PCM absorbs both sensible and latent heat. Although condenser subcooling strongly influences VCC performance and PCM melting behavior, systematic experimental investigations of this coupling remain limited. This study introduces the concept of a nominal subcooling degree (NSD), defined under steady-state refrigerant charge conditions, to evaluate subcooling effects in a PCM-integrated condenser. Experimental results indicate that the optimal NSD for steady-state operation is 5 K, resulting in a maximum coefficient of performance (COP) of 4.2. However, under long-duration operation, the optimal NSD shifts to 0.8 K. At NSD = 0.8 K, COP decreases by 31.3% between 0.5 h and 5.5 h, compared with a 44.2% reduction at NSD = 5 K. Higher NSDs are found to intensify non-uniform PCM melting and to accelerate performance degradation. These results reveal the dynamic coupling between refrigerant subcooling and the thermal behavior of the PCM, demonstrating that optimal subcooling depends on the operating duration. The findings provide practical guidance for subcooling management and system design, including the use of auxiliary subcoolers, to improve the efficiency and durability of PCM-integrated condenser systems.