Numerical study of a high-temperature thermal energy storage system with metal and inorganic salts as phase change materials

This study proposes a novel thermal energy storage (TES) concept using two phase change materials (PCMs) (inorganic salt and metal alloy) as the storage media. The metal alloy PCM is encapsulated in a tube which is inserted in the inorganic salt PCM. Thus, the metal alloy PCM serves as the heat storage material as well as the heat transfer enhanced fin for the inorganic salt PCM. After validation, a numerical model is developed to simulate the charging and discharging processes of the presented TES unit. Furthermore, the influence of the storage material selection, the phase change temperature difference between those two PCMs, and the location of heat transfer surface on the thermal behavior of the charging and discharging process are discussed. The results show that, compared with the unit filled with only salt PCM, the proposed unit can significantly reduce the charging and discharging time by 33.2% and at least 50.3%, respectively. When selecting metal PCM, it is not recommended to use the metal PCM with higher melting temperature than the selected salt PCM. With 75K melting temperature difference, the melting and solidification time of the unit was prolonged by 47.1% and 6.1%. In addition, if the unit has bottom heat transfer surface, the melting and solidification time of the unit are 356 and 1989min, corresponding to 4.5 charging power and 0.8kW discharging power.