Investigation on the performance of a high-temperature packed bed latent heat thermal energy storage system using Al-Si alloy

High-temperature energy storage system plays an important role in improving the efficiency of the concentrated solar power plants. The latent heat thermal energy storage (LHTES) is one of the most competitive thermal energy storage approaches because of the large heat storage density and approximately constant temperature during the phase change process. The Al-Si alloy has been considered as the high-temperature phase change material (PCM) used in the LHTES system because of its advantages such as large latent heat, suitable phase change temperature, high thermal conductivity and cost-effective. A three-dimensional numerical model of the packed bed LHTES system, using Al-25 wt%Si alloy as the PCM and air as the heat transfer fluid (HTF), is built to investigate the performance of the system based on the enthalpy-porosity model and surface-to-surface radiation model. The numerical model is validated through comparing with the experimental results reported in literature. The performance of the system is evaluated from the aspects of charging/discharging time, energy transfer efficiency and mean power. The results indicate that the PCM shows better performance than the rock in the energy storage system due to the involvement of latent heat and high thermal conductivity of the PCM. The influences of the radiation heat transfer and inlet temperature of the HTF on the system performance are also investigated. It is found that the radiation heat transfer shows a significant effect on the heat transfer in the high-temperature LHTES system. The temperature difference between the inlet temperature and phase change temperature dominates the charging/discharging time because the phase change time accounts for a large proportion in the total charging/discharging time. The effect of the wall thickness of the thermal energy storage unit is also studied and the results suggest that the wall thickness of the thermal energy storage unit cannot be neglected because of its relatively large thermal resistance.