Two performance indices of TES apparatus: Comparison of MPCM slurry vs. stratified water storage tank

Microencapsulated phase change material (MPCM) slurry, which has a higher thermal storage capacity than water, was introduced as a storage medium in a coil-in-tank, but the key question is whether a competitive charging/discharging rate could be achieved. In this paper, two key indices, the volumetric thermal storage capacity at a given temperature difference and the charging/discharging rate variation over time, are newly defined. From the experimental results, it is estimated that the volumetric thermal storage capacity of the MPCM slurry is nearly twice that of water in the temperature range of 8-18 °C. Finally, the overall heat transfer coefficient of the slurry storage device as a shell-tube heat exchanger was measured, and the external average convective heat transfer coefficient was also calculated. The two coefficients exhibited visible peaks during the phase change process with high-speed stirring and were much higher than that of water. However, the overall charging/discharging rates of the MPCM storage tank were observed to be much lower than the idealized stratified water storage tank (SWST), indicating that the design of an MPCM slurry thermal storage device needs to be further optimized.