Experiments on charging and discharging of spherical thermal (ice) storage elements

This paper is concerned with an experimental investigation into the dynamic behaviour of single spherical thermal (ice) storage elements. Three glass spheres having radii of 4.07, 3.5 and 3.135 cm were chosen for this study. A flowing water-glycol solution over a range of temperature varying between 4.5 and 12degreesC (during melting) and between -9.5 and -4.4degreesC (during freezing) was used as a heat transfer fluid (HTF) during the tests. The apparatus, method and results are described. Photographic means were used to characterize the water-ice interface position and its shape during discharging (melting) process. However, during charging (freezing) process a new method was devised for the same objectives. Several interesting results have been obtained from this study. Results obtained showed that the charging and discharging rates were constant with respect to the dimensionless time to at least 90% of the storage capacity of the single spherical ice storage element. These important and new results have allowed the formulation, described in the paper, of simple empirical equations describing the charging and discharging rates for a single spherical thermal (ice) storage element at any instant time period within the range of HTF temperature and spherical element size used. It is believed that these equations will be useful to colleagues interested in modelling the dynamic behaviour of thermal (ice) storage using spherical elements as phase change. Effects of the HTF temperature and capsule size on the rate of energy charged and discharged from a single spherical enclosure are presented and discussed. Copyright (C) 2002 John Wiley Sons, Ltd.