An experimental study of freezing and melting of water inside spherical capsules used in thermal energy storage systems

This paper reports the results of an experimental study on the heat transfer during freezing (charging) and melting (discharging) of water inside a spherical capsule of the type often found in the beds of thermal (ice) storage systems used for the building air conditioning systems. Spherical capsules of different diameters and materials are tested. The aqueous solution of 35-wt% ethylene glycol is used as the heat transfer fluid (HTF). The major studied parameters are the size and material of the spherical capsule, the volume flow rate and temperature of the heat transfer fluid (HTF). The effects of these parameters on the time for complete charging/discharging, the solidified/melted mass fraction, the charging/discharging rate, the energy stored/regain, and the energy recovery ratio (ERR) are studied. The experimental results show that the energy recovery ratio is becoming better when using metallic capsules, increasing the capsule size and reducing the HTF volume flow rates.