Melting and freezing of water in a spherical capsule is of practical importance in an ice storage system which is considered very promising to reduce peak electricity demand in the summer season. Heat transfer with melting and freezing of water 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 phase change material (PCM) and capsule material, and another is natural convection heat transfer in melt pool under the solid PCM. Owing to this complicated nature, there has been no reported 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 ease of installation. In this article, we present mathematical formulation and numerical results on the transient melting (charging) heat transfer in a spherical ice storage capsule. 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. In 1994, the world's largest district heating and cooling (DHC) system was opened in Yokohama, near Tokyo, in which about six million spherical ice balls were installed in two vertical storage tanks with a height of 28 meters. The results of the present analysis can be effectively applicable to that plant for a thermal design.
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