Heat transfer enhancement of latent heat thermal energy storage in solar heating system: A state-of-the-art review

Latent heat thermal energy storage (LHETS) has been widely used in solar thermal utilization and waste heat recovery on account of advantages of high-energy storage density and stable temperature as heat charging and discharging. Medium and low temperature phase change materials (PCMs), which always with their low thermal conductivity, are used in solar thermal utilization system, resulting in unsatisfactory heat storage performance. This paper summarizes the PCMs used in solar thermal utilization and their thermal physical parameters in different operating temperature ranges (low, medium, and high). Considering the specific cold and heat sources of the solar thermal system, the thermal performance evaluation index of the LHTES is put forward. Also several methods concerning to LHTES performance improvement are reviewed, which involve three aspects: material optimization, structure optimization and system optimization. Finally, through further discussion, some deeper problems about these optimizations are proposed and some recommendations for future research were put forward.