Engineering molten MgCl2–KCl–NaCl salt for high-temperature thermal energy storage: Review on salt properties and corrosion control strategies

Abstract

Conventional thermal energy storage (TES) media and heat transfer fluids (HTFs) currently used in commercial concentrated solar power (CSP) plants are nitrate-based molten salts with working temperature up to about 565 °C. Current interest in raising the working temperature in next-generation CSP for higher energy conversion efficiency leads to chloride-based molten salt technology (>700 °C). MgCl2–KCl–NaCl salt is selected to be such a TES/HTF material due to its excellent salt properties, abundance and low costs. In this review, survey and evaluation of its exact eutectic composition by evaluation of the phase diagram as well as most relevant salt properties for design, construction and operation of a test loop and projection of a real TES/HTF system are presented. They include minimum melting temperature, vapor pressure, heat capacity, density, thermal conductivity and dynamic viscosity. The working temperature range 420–800 °C is suggested for the TES/HTF system using this salt. Moreover, the recommended values of the heat capacity, density, thermal conductivity and dynamic viscosity in the suggested working temperature range are given. In addition, corrosion control strategies for this salt including corrosion monitoring and mitigation techniques are briefly reviewed, since controlling the salt corrosivity is one main concern of the engineering aspects using the molten chlorides at high temperatures. Overall, this review is expected to supply reliable values of the most relevant salt properties and recommend the most promising corrosion control strategies for further material studies, as well as modeling, simulation, detailed design and construction of molten MgCl2–KCl–NaCl salt TES/HTF systems.