Solidification study of DI water in a spherical capsule for cool thermal energy storage applications

Abstract

Cool thermal energy storage (CTES) systems utilising packed beds commonly employ spherical capsules as containers for thermal storage. In these systems, the phase change material (PCM) is solidified using a chiller, a process that consumes considerable energy. The time required for complete PCM solidification directly impacts chiller operation duration, and reducing this time can enhance chiller efficiency and lower electricity costs. Research has identified several factors influencing PCM solidification time, including the external surface heat transfer coefficient, cooling fluid temperature, capsule geometry, and PCM thermal conductivity. The present study focused on two key parameters—surface heat transfer coefficient and PCM thermal conductivity—to investigate the solidification of deionized (DI) water as a PCM. Experimental investigations were conducted, yielding significant insights. Results from experiments also supported the theoretical formula that was applied in the parametric study. Both theoretical and experimental findings revealed that approximately 50 % of the total solidification time was dedicated to freeze the central 25 % of the water within the capsule. Parametric studies indicated that the influence of the external convective heat transfer coefficient, when employing air and water-glycol mixture as heat transfer fluids, was significant up to 35 W m⁻² K⁻¹ and 500 W m⁻² K⁻¹, respectively. The findings of this study will benefit energy storage researchers and engineers, as well as those in the chemical, food, and plastics processing industries.