Thermal energy storage (TES) is crucial for managing renewable energy intermittency. Latent heat storage using phase change materials (PCMs) provides high-energy density and stable temperatures during transitions. Inorganic salts, used as PCMs and heat transfer fluids (HTF) in high-temperature applications, offer superior thermal stability, conductivity, and volumetric heat capacity, along with high fusion enthalpy.

Industrial processes contribute significantly to global CO2 emissions, with 74% of the total industrial energy demand stemming from heating. Most industrial processes operate up to 250°C. While many PCMs melt up to 120°C, few remain stable between 120°C and 250°C. The commercial HITEC salt, composed of 53% KNO3, 40% NaNO2, and 7% NaNO3, stands out with a melting point of 142°C and stability up to 631°C. Mainly used as a HTF, its PCM potential and long-term stability are underexplored. Rodriguez-Garcia et al. [1] note its possible solid-solid transitions, but full transition mechanisms are still unclear.

Our research investigates the solid-liquid transitions of HITEC and its long-term stability. We conducted extensive TGA/DSC analyses with a STA Jupiter 3 (Netzsch), involving multiple cycles at various heating rates and isothermal holds to examine the solid's stability and intrinsic thermal properties. Phase transitions were also analyzed using X-ray diffraction and synchrotron X-ray diffraction (SXRD) at the European Synchrotron Radiation Facility (ESRF-BM31) in Grenoble.

Results show HITEC remains stable after 150 cycles between 130°C and 180°C. We identified a solid-solid transition of KNO3 from orthorhombic to hexagonal forms. This transition occurs at different temperatures: 130°C initially and around 90 ± 5°C in subsequent cycles, influenced by the product's thermal history. During cooling, NaNO3 crystallizes first. To use the total energy of the solid-liquid HITEC ‘s transition, we recommend a lower temperature of 130°C. Utilizing the solid-solid energy requires a lower temperature no higher than 80°C, but further studies are required to fully understand the dynamics of this transition, as variations may occur based on thermal history.

[1] M.M. Rodriguez-Garcia; Proceedings of SWC2017/SHC2017, 2017, 1‑12.