Solar Salt - Thermal Property Analysis - Extended Version: Report on thermo-physical properties of binary NaNO3-KNO3 mixtures in a range of 55-65 wt% NaNO3

Kurzfassung

Thermal Energy Storage (TES) plays a crucial role for the implementation of dispatchable, renewable energy systems world-wide. Molten salt storage has proven advantageous for storage units in modern CSP plants with large power and capacity levels. Solar Salt, a mixture of 60 wt% NaNO3 and 40 wt% KNO3, is the state-of-the-art storage material and heat transfer fluid and is utilized in temperature regimes between 290 °C and 565 °C in a cold- and a hot tank configuration, respectively. Typically, these salts are provided by salt suppliers using big bags (e.g., ~1000 kg) of the single salts. This study considers that SQM offers big bags with pre-mixed Solar Salt, which simplifies the organizational efforts during the TES start-up. The mixing accuracy is however important to ensure that the final mixture exhibits uniform physicochemical properties. This report investigates the impact of these defined compositional changes for binary mixtures containing 55-65 wt% NaNO3. The thermo-physical properties analysed are the heat capacity, viscosity, thermal conductivity and density, as well as the melting properties.