Performance of cascaded packed bed thermal energy storage with size-gradient phase change material capsules

Although the packed bed latent thermal energy storage (PBLTES) is promising for application in renewable energy systems and waste heat recovery, its practical application is hindered by the heat transfer deterioration in the outlet region. To address this challenge, this study investigates the performance of the PBLTES system with an arrangement of phase change material (PCM) capsules with size gradient using an experimentally validated concentric dispersion model incorporating both forced and natural convection. The results indicate that the benefit of size-gradient PBLTES can be maximized when the heat transfer fluid (HTF) flows in the same direction during charging-discharging cycles, achieving a 14.8 % reduction in cycle duration through a more uniform phase change completion at different axial positions. Besides, compared with the PCM capsule size in other region, the capsule size near the outlet region has a greater impact on the system thermal performance. However, the enhancement of heat transfer performance may come at the penalty of a higher pressure drop, which is increased by 118.3 % when the small-sized PCM capsules of 20 mm are used, while the average heat transfer rate improvement is only 11.8 %. The capsule-to-tank diameter ratio between 0.160 and 0.167 is found effective to simultaneously enhance the heat transfer rate and decrease the pressure drop compared with the non-gradient PBLTES system. These findings may provide new insights and practical guidance for optimizing sized-gradient PBLTES systems in energy storage applications.