Ca(NO₃)₂—NaNO₃—KNO₃ Molten Salt Mixtures for Direct Thermal Energy Storage Systems in Parabolic Trough Plants

Molten salts are currently the only thermal energy storage media operating with multiple hours of energy capacity in commercial concentrated solar power (CSP) plants. Thermal energy is stored by sensible heat in the liquid phase. A lower melting point in the range of 60–120 °C and a decomposition temperature above 500 °C are desired because such a fluid would enhance the overall efficiency of the plants by utilizing less energy to keep the salt in the liquid state and by producing superheated steam at higher temperatures in the Rankine cycle. One promising candidate is a multicomponent NaNO₃—KNO₃—Ca(NO₃)₂ molten salt. Different compositions have been reported in literature as the best formulation for CSP plants based on melting temperature. In this paper, the National Renewable Energy Laboratory (NREL) presents the handling, preparation, thermal properties, and characterization of different compositions for this ternary nitrate salt, and comparisons are drawn accordingly. This system has a high tendency to form supercooled liquids with high viscosity that undergo glass formation during cooling. When the proportion of Ca(NO₃)₂ decreases, the formulations become more thermally stable, the viscosity goes down, and the system increases its degree of crystalline solidification. Differential scanning calorimetry (DSC) tests showed the presence of a ternary eutectoid solid–solid invariant reaction at around 100 °C. The eutectic invariant reaction was resolved between 120 and 133 °C as reported in the literature. Based on DSC and viscosity results, the best composition would seem to be 36 wt. % Ca(NO₃)₂—16 wt. % NaNO₃—48 wt. % KNO₃, which showed a low solidification point.