Phase diagram, thermodynamic properties and long-term isothermal stability of quaternary molten nitrate salts for thermal energy storage

Thermal energy storage (TES) is a vastly growing technique that allows for the generation of dispatchable electricity in modern concentrating solar power (CSP) plants. In solar tower systems the key to success is the use of a heat transfer fluid (HTF) and storage medium known as Solar Salt. This nitrate salt mixture of NaNO3 and KNO3 is considered thermally stable, non-toxic and environmentally friendly. In line-focusing CSP plants Solar Salt is still not being used as heat transfer fluid due to the inherent risk of freezing and related systematic failure. Accordingly, there is a need to develop nitrate salts with lower melting temperatures but yet acceptably high thermal stability. The information on molten nitrate mixtures with low melting points (⪅100 °C) is limited, especially in terms of isothermal stability tests. This work presents thermo-physical data of the complex quaternary Ca,Li,Na,K//NO3 system. Melting temperatures of more than 100 mixtures were assessed and compositions with melting points below 100 °C were identified. The heat capacity of selected mixtures was in the range of 1.5–1.6 kJ/kg K and generally increased with increasing Li-content. Thermal stability, with Solar Salt as reference salt, indicated that the stability of all mixtures did not exceed 500 °C but that the achievable ΔT was 360 °C, about 90 °C higher than that of Solar Salt. Some compositions are therefore are potential HTF and storage media but the overall Ca-content plays a crucial role in the decomposition of the quaternary mixtures during operation at high temperatures.