Experimental and numerical study of cylindrical encapsulated phase change material in packed bed thermal energy storage with expansion and shrinkage effects

Packed bed latent heat thermal energy storage (PBTES) is a promising technology for storing thermal energy with a relatively compact size and smaller temperature variation during phase change. In this study, a lab-scale lowcost PBTES with cylindrical shape encapsulation of phase change material (PCM) is designed and fabricated for evaluating the charging-alone and discharging-alone thermal performance for medium-temperature range (200-350 degrees C) latent heat storage applications. Air is used as a heat transfer fluid (HTF), and solar salt is employed as a PCM. A new numerical model incorporating shrinkage and expansion of PCM during phase change is developed by considering the packed bed as the porous medium and the melt fraction-based density variation of PCM to model the transport and solid-liquid phase change phenomena in the PBTES. The numerical model is validated with the in-house experimental results. The maximum thermal charging efficiency is found to be 67.1 % for the mass flow rate of 4.3 g/s and a maximum charging inlet air temperature of 360.9 degrees C. The maximum discharging efficiency of 86.1 % is obtained for the mass flow rate of 4.3 g/s and discharging inlet air temperature of 35 degrees C. It is found that the expansion and shrinkage effects are prominent during the phase change of PCM during the charging and discharging operations of the PBTES.