Chloride Salt Systems for High Temperature Thermal Energy Storage: Properties and Applications

Abstract

There is substantial potential to increase the operating temperatures of concentrating solar power (CSP) plants, thereby increasing the Carnot efficiency. Coupled with viable thermal energy storage (TES) strategies, this would bring us closer to achieving the goals of the U.S. Department of Energy Sunshot Initiative. Current TES media employ molten inorganic salts (namely, nitrate salts) for thermal storage, but they are limited in application to lower temperatures: generally, below 600°C. While sufficient for parabolic trough power plants, these materials are inadequate for use with the higher operating temperatures achievable in solar power tower-type CSP plants. For these higher temperatures, chloride salts are more ideal candidate storage media, either for sensible heat storage in the molten salt (e.g, a dual-tank storage arrangement) or for sensible and latent heat thermal energy storage (LHTES) as phase change materials (PCMs). Their melting points and those of their eutectic mixtures cover a broad range of potential operating temperatures, up to and including 800.7°C, the melting point of pure NaCl. This paper examines these salt systems and presents relevant properties and potential applications in high temperature (>400°C) utility scale solar thermal power generation. A preliminary screening of pure chloride salts based on available literature yields a list of promising candidate salts. Eutectic mixtures of these salts are also considered; the eutectic systems were modeled using the thermodynamic database software, FactSage. Thermophysical properties (melting point, latent heat) are summarized for each salt system. Radiative properties are also addressed, since at these temperatures, thermal radiation can become a significant mode of heat transfer. Candidate containment materials and strategies are discussed, along with the attendant potential for corrosion. Finally, cost data for these systems are presented, allowing for meaningful comparison among these systems and other materials in the context of utility scale thermal energy storage units.