Enhanced melting in geometries suitable for thermal energy storage

Phase-change materials (PCMs) are able to store large amounts of heat but have low thermal conductivity. Heat transfer in PCMs can be enhanced by fins that enlarge the heat transfer area. However, when the PCM melts during the storage unit charging, a layer of liquid is growing at the fins creating an increasing thermal resistance that impedes the process. The present work aims to demonstrate that charging of a latent-heat thermal storage unit may be considerably affected by achieving a so-called close-contact melting (CCM), which occurs when the solid phase is approaching a heated solid surface, and a thin liquid layer is formed beneath the solid phase. Although CCM was extensively studied in the past, its possible role in finned storage units has not been explored, and the present work aims to fill this gap. Specifically, the close-contact melting of a horizontal semi-cylinder placed on a heated horizontal surface is analyzed by means of a theoretical model. The results are expressed in terms of the time-dependent melt fraction and Nusselt number, showing their explicit dependence on the Stefan and Fourier numbers. For comparison, melting of an identical semi-cylinder heated from above is modeled using a quasi-steady approach. It is found that close-contact melting shortens the melting time drastically. The general approach developed in the present study may be applied to more complex geometries, e.g. those in which the heated surface is not horizontal or where there are a number of heated surfaces or fins.