Macroencapsulation of sodium chloride by a double layer strategy for high temperature phase change thermal storage over 800 °C

Abstract

To solve the key problems of high corrosiveness, easy leakage and low thermal conductivity of sodium chloride as phase change material (PCM) for high-temperature heat storage, this study reports a double-shell coating method to prepare centimeter-sized macrocapsules. The capsules are made of powder raw materials with the help of organic adhesives such as polyvinyl pyrrolidone and carboxymethyl cellulose for shape formation, in which the inner expanded graphite (EG) layer and the outer Al2O3 layer are coated on the salt core spheres and finally are sintered formed at high temperatures. The double-shell capsules can prevent salt from leakage because the inner EG layer is repellent to liquid salt, in contrast, the single Al2O3 shell is penetrated by liquid salt due to their wettability. After 100 cyclic tests, the double-shell capsules have good stability and high durability for long-term use. In order to improve the heat storage density of the capsules, the method of isostatically pressing the cores to increase the compactness is studied. At 200 MPa, the capsules in size of 20@2@2 show a high heat storage capacity of 1024.54 J/cm3 and 551.56 J/g from 600 to 900 °C. The double-shell encapsulated salt PCM can be widely used in high-temperature heat storage over 800 °C in solar thermal power plants and industrial waste heat recovery.