Influence of Impurity SO4 2− on the Thermal Performance of Molten Nitrates Used for Thermal Energy Storage

Solar salt system, mainly composed by 60 wt.% NaNO3 and 40 wt.% KNO3, is the most optimized molten nitrates used for thermal energy storage. This paper aims to investigate the influence of the amount of impurity SO42- on thermal performance of solar salt, including thermodynamic parameters like melting point, liquidus temperature, thermal decomposition temperature, thermal stability, and thermophysical properties such as density, viscosity at different temperature. The melting point and thermal decomposition temperature of the mixtures were measured by Differential Scanning Calorimeter (DSC) and Thermo-gravimetric (TG), respectively. The liquidus temperature was obtained by a cooling curve, and the density was determined with a homemade device based on the Archimedes principle. Additionally, viscosity measurements were obtained using a Dynamic Mechanical Analyzer. It was found that: SO42- makes no negative influence on melting point, liquidus temperature and density. The results of viscosity show that controlling the SO42- level within 0.1 wt.% in nitrates is necessary. For the thermal stability at 565 degrees C, SO42- increases the mass loss of nitrates a little more than pure nitrates, as well as easily forms insoluble compounds with cations, especially Na+. Meanwhile, the mass loss mechanisms was confirmed in especial by the vaporization of the nitrates, followed by the release of O-2. In conclusion, it is critical to maintain SO42- content below 0.01 wt.% for addressing flow problems and reducing instability at high temperature.