Experimental investigation of a low-temperature packed-bed thermal storage system with graphite-based composite phase change material

The thermocline latent heat packed bed system has the potential to be used for >200 degrees C high-temperature, 100-200 degrees C medium-temperature, and <100 degrees C low-temperature thermal energy storage applications. However, the inherent shortcoming of phase change material (PCM) is its low thermal conductivity, which inevitably restricts the heat transfer between the fillers and the heat transfer fluid. This study aims to investigate the thermal performance of the packed bed thermal energy storage (PBTES) system at mass flow rates of Re = 23-68 by filling the tank with high thermal conductivity composite PCM. An encapsulated expanded graphite/stearic acid composite PCM with a cylindrical aluminum shell was designed to prevent leakage and used as filler. The thermal conductivity of the composite is 305.7 % higher than that of pure PCM, demonstrating a higher thermal storage rate for a single capsule. Results show that this composite used as filler leads to a faster heat transfer rate between heat transfer fluid and solid fillers, decreasing the charging time by approximately 23.8 %, and keeping a more stable and distinct phase change platform at 55-60 degrees C. Moreover, an appropriate increase in the mass flow rate can facilitate heat transfer and improve the thermal performance of PBTES. Nevertheless, the thermal storage capacity utilization is reduced accordingly. Compared to the system with pure PCM fillers, the system with composite fillers exhibits a higher charging efficiency of 86.27 %, a higher stored thermal energy of 0.448 kWh, and a higher utilization rate at the threshold temperature. Overall, the thermal conductivity of filler is one of the primary parameters to determine the thermal performance of the thermocline latent heat packed bed system.