Novel Molten Alkali Chloride Salts for Thermal Energy Storage up to 700 °C

Abstract

Suitable material candidates for the use as heat transfer fluid (HTF) and/or for thermal energy storage (TES) are supposed to fulfill a list of requirements concerning physical data (e.g. melting point, density, viscosity, heat conductivity and heat capacity), availability, corrosion behavior, stability as well as toxicity. Taking into account these points, several hundred mixtures have been investigated. It appeared that the commonly used mixtures of sodium and potassium nitrates are so far the most promising candidates for high-temperature heat storage applications. Nevertheless, due to their weak stability at temperatures exceeding 550 °C, nitrates could be replaced by more stable suitable materials when higher operating temperatures are required (up to 700 °C). Due to their physicochemical and thermal properties and high thermal stability, molten alkali chloride salt mixtures are potential liquids to be used for sensible thermal energy storage (TES) such as in Concentrating Solar Power (CSP) plants and in fossil fuel powered plants to increase the operation flexibility. However, the application of molten chlorides at high-temperatures involves additional challenges such as increased corrosiveness. In the present work, we focus on the molten salts purity, because it has a direct influence on the corrosion rate of metals. Because some chloride salts are hygroscopic and heating in presence of water induces hydrolysis of the salt and formation of oxides/hydroxides. These compounds increase the redox potential of the salt and so its corrosion rate.