Advanced studies on thermal stability and alteration of molten nitrate salts used as heat storage media

Solar based thermal energy storage using molten salts is a highly developed process allowing for dispatchable generation of green electricity or other forms of energy. Varieties of storage material combinations facilitate thermal energy storage in a wide range of 100°C up to 1000°C making it feasible not only for solar applications but also waste heat management in industrial processes. Traditionally, solar salt ((Na,K)NO3) or HitecXL ((Ca,Na,K)NO3) are employed as heat storage media in such systems. HitecXL exhibits a significantly lower melting temperature of 131°C compared to 220-250°C in solar salt making it a promising heat transfer medium too e.g. for parabolic trough power plants due to the lower risk of freezing in system parts. The thermal properties molten salts are widely known and accepted however, thermal stability limits and decomposition mechanism under manifold conditions are scarce. In this work we demonstrate that isothermal storage at different scales and subsequent analysis of the salt composition and its thermal properties is the most reliable strategy to assess stability limits of different nitrate salts. Results of isothermal storage are compared to often-quoted TG measurements which only offer a comparative rather than quantitative view on thermal stability limits. Acquired data reveals rapid changes of nitrate-nitrite equilibria, depending on the atmospheric conditions accompanied by the formation of oxide species and eventually further reactions with atmospheric CO2 leading to carbonate formation in the molten salt. Furthermore, the first ever reported series of isochoric storage experiments using Solar Salt and salt post-analysis is reported. Different experimental conditions are employed and compared to classical isothermal storage experiments. Results indicate that the decomposition of Solar Salt engenders moderate pressure build-up over the first tens of hours but eventually stabilizes. The combination of isochoric and isobaric experiments allows for an utterly unique assessment of molten salt stability under relevant conditions.