Effects of sodium nitrate concentration on thermophysical properties of solar salts and on the thermal energy storage cost

•Optimization of the energy cost of CSP with different mixtures of solar salts.•Different percentages of NaNO3 and KNO3 in the solar salt mixture were studied.•Increasing the % NaNO3 increases Cp but increases melting temperature.•The impact of this change is analysed in a 85 MWe plant with 13 h of storage.•A LCOE reduction of 0.6% for the new mixture with higher NaNO3 found. Thermal energy storage (TES) systems are key components of concentrating solar power plants in order to offer energy dispatchability to adapt the electricity power production to the curve demand. Nitrate molten salts are the storage media used today in concentrated solar power plants. They are also used as heat transfer fluid (HTF) in the molten salt tower (MST) technology. Traditional MST plants work in the temperature range of 240–565 °C using the so-called solar salt, a mixture of 60–40 wt% of NaNO3 and KNO3. This study wants to optimize the thermal energy storage cost of the solar concentration technology by analysing different mixtures of solar salts, using different percentages of NaNO3 and KNO3 in the mixture. The new mixtures seek a reduction in the cost of the storage material while optimizing its physical and chemical properties. The study shows how an increase in the proportion of sodium nitrate for a new binary solar salt to 78–22 wt%, produces an increase in the heat capacity of the mixture by reducing the necessary inventory of salts in the system. However, the new salt presents an increase in the melting point, going from 240 °C to 279 °C, which makes the operation of the system difficult. The impact on the cost of this optimization in the performance of a commercial plant was analysed. The plant chosen to evaluate the impact is a tower technology plant with 85 MWe power and 13 h of storage. The study shows a LCOE reduction of up to 0.6% for the new mixture with higher sodium nitrate.