Rapid charging for latent heat thermal energy storage: A state-of-the-art review of close-contact melting

Latent heat thermal energy storage (LHTES) using phase change materials (PCM) has been considered a promising technique for improving the energy efficiency of thermal systems. However, a LHTES unit often suffers from low power density, e.g., low energy charging rates, because of the low thermal conductivity of common PCM like paraffin. Close-contact melting (CCM) has long been studied as a special melting process featuring very high heat transfer rates as a result of the low thermal resistance across the thin melt film between the PCM and the heating surface. It was shown in the literature that utilization of CCM could reduce the charging time of a well-designed LHTES container by even 4 times. Towards increasing the power density of LHTES units, this review paper focuses on the progress of CCM studies, including fundamental mechanisms, applications, and enhancement methods. Available theoretical models, experimental observations, as well as numerical approaches were reviewed systematically. The research gaps between investigations and applications were discussed, and the main challenges on both mechanistic understanding and utilization of CCM were also outlined. Last, some recommendations for future researches on CCM as a pathway for realizing rapid energy charging of PCM-based LHTES units were proposed.