Theoretical analysis and experiment on combined close-contact and natural convection melting in thermal energy storage spherical capsule

Melting and freezing of a phase change material (PCM) in a spherical capsule is of practical importance in heat storage systems which are considered very promising to reduce peak electricity demand in the summer season. Heat transfer with melting and freezing of PCM in a capsule is quite complicated because of two heat transfer modes occurring within a capsule, i.e. one is close contact melting mode between PCM and capsule material, and another is natural convection heat transfer in melt pool under or above the solid PCM. Owing to this complicated nature, there has been reported no detailed analysis up to date. In comparison with the cylindrical capsule type unit, the spherical capsule unit has a great advantage from the viewpoint of the thermal performance and care of installation. In this article, the authors present a mathematical formulation and the numerical results on the transient melting (charging) heat transfer in a spherical heat storage capsule under inner wall temperature distribution. In recent years, efforts have been devoted to clarify the mechanism of close-contact heat transfer for a single enclosure with various shapes. However, there is no theoretically exact numerical simulation considering both close-contact and natural convection melting processes within a spherical capsule.