Thermal performance of phase change material to air heat exchanger (PAHX): state of the art review

The building sector is the largest energy consumer, responsible for 40% of the global energy consumption and one-third of CO 2 emissions. Heating, ventilation, and air-conditioning (HVAC) systems constitute approximately half of this energy demand. Thermal energy storage (TES) systems offer a promising solution for reducing HVAC energy consumption by enabling free cooling, heating, and ventilation. A free-cooling system (FCS) stores cooling energy in TES at night, which is then utilized throughout the day. However, the effectiveness of FCSs is limited to regions where daily temperature fluctuations fall within thermal comfort ranges, with greater effectiveness in regions experiencing larger temperature variations. Latent heat TES utilizing phase-change materials (PCMs) is particularly advantageous because of its high energy-storage capacity with minimal changes in temperature and volume. This review examines various studies on PCM-to-air heat exchangers (PAHXs) within FCSs, highlighting key challenges such as the thermophysical properties of PCMs and the dependence of free cooling on climate. The factors influencing the PCM melting and solidification times, including the heat-transfer fluid (HTF) flow rate and the temperature difference between the HTF intake and PCM melting temperature, are also discussed. Additionally, improvements in PCM properties, encapsulation techniques, and heat exchanger designs are reviewed. This comprehensive analysis confirms the predominance of slab PAHX designs and paraffin-based PCMs. Methods such as fins, nanoparticles, and multi-PCM approaches are identified as effective strategies for enhancing the heat transfer in PCM systems.