Influence of the size of spherical capsule on solidification characteristics of DI (deionized water) water for a cool thermal energy storage system – An experimental study

• Capsule size had a significant influence on subcooling at low temperature potential.
• Freeze front moved faster in lager capsule than in smaller till the solidification of 75% mass.
• Solidification duration is almost the same for 86 and 100 mm capsules till 75% mass.
• Several fold increase in heat flux is achieved with 100 mm capsule at −12 °C.
• Combination of capsule size and temperature potential is essential for good design.

The present study aims to investigate the influence of the size of the spherical capsule on the solidification characteristics of water as the PCM (phase change material) filled with 90% of its full volume. The experiments were conducted with three different sizes of stainless steel spherical capsules filled with PCM up to 90% of its full volume, maintained at various bath temperatures. It was observed that the capsule size had a significant influence on subcooling at lower temperature driving potential and was totally eliminated at higher temperature potential, for all the capsule sizes. The freeze front moved at a faster rate in the larger capsule than in the smaller capsule till the solidification of 75% mass of PCM. This effect was more pronounced at a higher temperature driving potential. Increasing the temperature potential was not beneficial for the 74 mm capsule due to insignificant increase in heat flux. However, the increased temperature potential significantly enhanced the heat flux by several folds for the higher diameter capsules during the solidification of 75% of mass. Considering the larger size capsule with sufficient temperature potential would hence be the optimal way of designing an increased energy efficient CTES (Cool thermal energy storage) system with rapid charging and discharging.