Copper-Alumina Capsules for High-Temperature Thermal Energy Storage

High corrosivity, leakage and oxidation of metallic phase change materials (PCMs) have limited their applications in high-temperature thermal energy storage (TES) systems, regardless of their favorable benefits for high-temperature TES applications of over 1000 °C. To overcome these major challenges, this work presents a facial paraffin sacrificial layer approach for directly encapsulating copper (Cu) sphere PCM with alumina (Al2O3) shell, considering a buffer inner cavity. The cavity is formed by the decomposition of paraffin layer through a pre-sintered process. It plays a key role to accommodate the volume expansion of Cu core, thereby preventing the breakage of the shell and the leakage of the liquid PCM. A series of macrocapsules with different sizes (9.5 mm-21 mm) containing Cu core, cavity and Al2O3 shell are successfully produced using the paraffin sacrificial layer method and deploying two-step heat treatment. The experimental results show that the Al2O3 shell possesses a good structure, which can prevent the leakage of Cu core. The Al2O3 shell also has a strong compatibility with Cu core without any chemical reaction between two materials. At a temperature range of 1000-1100 °C, the calculated mass and volume energy storage densities of the PCM macrocapsule with 21 mm outer diameter are found to be 222 kJ/kg and 745 J/cm3 , which are 1.83 and 1.76 times, respectively, higher than those for Al2O3 ceramic. After thermal cycle tests, the encapsulated Cu PCM shows a superior shape stability, chemical stability and thermal durability, which can be applied in long-term thermal storage system for hightemperature TES systems.