Macroencapsulated carbonate eutectic salt phase change material with high durability for high temperature heat storage

Abstract

Carbonate eutectic salt is a kind of promising phase change material (PCM) for high temperature thermal storage application, which unfortunately constrained by its significant corrosiveness and low thermal conductivity. The encapsulation of salt PCM with stable inorganic shell to make capsules of millimeter to centimeter size can solve these problems, which is unfortunately technologically difficult to be prepared. In this work, a double shell strategy was proposed to coat the salt macrospheres with inner expanded graphite (EG) layer and outer ceramic kaolin layer. All raw materials were powders, which were shaped to spheres and capsules with the help of organic binder, pressed by isostatic pressing and finally sintered at high temperature for capsule formation. In a comparative case, the single kaolin shell coated capsules leaked when they were heated over the melting point of salt since that the relatively porous kaolin shell could be infiltrated with liquid salt because of their good wettability. However, the double shells coated capsules could prevent the leakage of salt since that the inner EG layer was repellent to liquid salt. The double shells coated macrocapsules showed good stability and high durability for long-term use, as confirmed by a cycling test over 300 cycles. The eutectic salt (Na2CO3/Li2CO3 = 57/43 in weight) core PCM presented a high latent heat of 313 J/g and melting point of around 498 °C. The macrocapsules with a size of 10@1.5@3 showed a high heat storage capability of 380 J/g in a temperature range of 400–600 °C. The macroencapulated carbonate eutectic salt with good heat storage capability and good thermal cycling durability can be extensively used for high-temperature heat storage applications in solar thermal power plant and industrial waste heat recovery.