Enhanced close-contact melting by tuning container configurations for fast-charging latent heat storage systems

Abstract

Latent heat energy storage (LHES) systems using energy-intensive phase change material (PCM) have gained increasing attention for solar thermal utilization and industrial waste heat recovery. Close-contact melting (CCM), which maintains a small gap between the unmelted PCM and the heated surface, is known to allow high-power LHES. For enclosed PCM containers, however, the contribution of CCM to the total charging process and its dependence on the container configurations remain unclear. In this work, the effects of CCM on the melting of a paraffin wax in rectangular containers with various geometrical and thermal configurations were studied numerically. The results showed that reducing the container height-to-width ratio from 6.25 to 0.25 improves the melting contribution of CCM from 38% to 87%, with a corresponding 65% reduction of the melting time. As the aspect ratio decreases, the gravity center of the unmelted PCM moves closer to the heated bottom surface, and the liquid film thickness slightly grows. In comparison to the top and side walls, heating from the bottom wall was demonstrated to be more efficient to facilitate CCM, which drastically shortens the melting time. When elevating the bottom wall superheat from 18 to 30°C, the melting fraction can be generalized using the dimensionless group FoSte^0.9, and the maximum mean power density for LHES reaches 500 W/kg. These findings can serve as practical design guidelines for enhancing the CCM mechanism in a passive manner to realize fast-charging LHES systems.