Influence of the nitrite impurity on Solar Salt: Thermophysical properties and structural analysis

Solar Salt is the most widely used heat transfer and storage medium in concentrated solar power plants. However, the performance of Solar Salt and its economic viability in industrial applications is influenced by NO₂⁻, an impurity whose effect on the thermal properties of Solar Salt remains underexplored. This study involved a systematic investigation of the effects of this impurity (0.5–6.0 wt%) on the thermophysical properties (including the melting point, decomposition temperature, specific heat capacity, viscosity, thermal conductivity, and density) and the thermal stability of Solar Salt, and the influence of NO₂⁻ on the microstructure of the molten salt is elucidated. NO₂⁻ significantly affects the melting point, specific heat capacity, and thermal conductivity, but has negligible effects on the decomposition temperature, viscosity, density, and thermal stability. Notably, the thermal conductivity, which exhibits the highest sensitivity to NO₂⁻, decreases by 23.1 %, even at a low NO₂⁻ concentration of 0.5 wt%. For impurity concentrations greater than 2.0 wt%, the melting point and specific heat capacity decline significantly, for instance by 21 °C and 26.1 %, respectively, at 6.0 wt% NO₂⁻. Advanced characterization (X-ray diffraction, Raman and Fourier transform infrared spectroscopy, and scanning electron microscopy) reveals that NO₂⁻ induces significant microstructural reorganization within the molten salt system. Based on these findings, we recommend maintaining a NO₂⁻ concentration of <2.0 wt% to ensure optimal stability in terms of the thermophysical properties.