Ceramic–molten salt composites (CPCMs) for high-temperature thermal energy storage: Improving sinterability and thermal stability by using solid wastes as skeletons

Abstract

Molten salts are ideal high-temperature thermal energy storage materials for solar energy generation; however, disadvantages such as leakage, corrosive behavior, and low thermal conductivity limit their widespread application. The use of ceramic skeletons to encapsulate molten salts by the mixing-sintering method is a promising approach to overcome the above shortcomings. However, the CPCMs fabricated by the mixing-sintering method usually have high porosity and are easily deformed at temperatures above the melting point of the loaded salt. In this study, two solid wastes, ferrotitanium slag and waste glass, were first used as skeleton materials to encapsulate molten Na2SO4–NaCl salt; herein, waste glass was also introduced as a modifier to promote the sinterability and thermal stability of the CPCMs. The results demonstrated that the waste glass addition not only improved the sinterability via the viscous flow mechanism but also enhanced the compressive strength, thermal conductivity, thermal cycling stability, and high-temperature structural stability of the CPCMs. Additionally, ferrotitanium slag had good compatibility with the Na2SO4–NaCl salt. The waste glass reacted with the molten salt to form a new compound, (Na0.8Ca0.1)2SO4. Although the synthesis of (Na0.8Ca0.1)2SO4 increased the melting temperature of the CPCMs by 3–4 °C, it improved the melting enthalpy of the CPCMs that was beneficial for compensating the salt loss during thermal cycling. In summary, an eco-friendly and low-cost thermal energy storage material was developed in this study, and more importantly, a promising approach to improve the sinterability and thermal stability of CPCMs was proposed.