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 degrees 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 degrees 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 degrees C but that the achievable AT was 360 degrees C, about 90 degrees 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.

Automated summary

Thermal energy storage allows for the generation of dispatchable electricity in modern CSP plants. The use of Solar Salt as a heat transfer fluid is important for success in solar tower systems. However, Solar Salt is not used in line-focusing CSP plants due to the risk of freezing. This study presents data on nitrate salt mixtures with lower melting points and higher thermal stability, which could be potential heat transfer fluids and storage media.