Effect of Gradient Phase-Change Temperature on Thermal Performance of Floor Heating Systems with Encased Phase-Change Materials

As phase-change materials (PCMs) are increasingly utilized in radiant floor heating system (RFHS), their benefits in reducing temperature variations and improving energy efficiency have been clearly proven. However, existing studies have primarily focused on the thermal behavior of PCMs with a single phase change temperature, while the application of gradient PCMs in encased floor systems remains underexplored. This study proposes an optimized design integrating gradient PCMs into a single-layer encased RFHS. Using CFD simulation, the effects of phase change temperature gradients in the inner and outer PCM layers on the thermal behavior of the RFHS were systematically investigated. The results indicate that the phase transition temperature of the outer PCM layer significantly influences the surface temperature response speed, average temperature, and temperature fluctuation. In contrast, the phase change temperature of the inner PCM layer primarily affects the heat dissipation rate and high-temperature retention duration. By optimizing the phase change temperature gradients of the inner and outer PCM layers, the optimal configuration was identified (outer layer: 32–37°C; inner layer: 38–43°C). Compared to conventional designs, the optimized system achieved a 9.89% increase in phase change rate, a 24.32% extension in high-temperature duration, a 1.8% rise in average temperature, and a 5.41% reduction in temperature fluctuation. This study offers theoretical guidance and technical support to improve the thermal efficiency of encased RFHS and promoting their application in green buildings.