Thermodynamics of the reciprocal NaCl-KCl-NaNO3-KNO3 system

Thermodynamic properties of the reciprocal NaCl-KCl-NaNO3-KNO3 system are of interest for selecting compositions, which can be used as phase change materials in thermal energy storage. In the presented work two mixtures 7.5KCl-92.5NaNO3 and 12.5NaCl-87.5KNO3 on the diagonal sections of the reciprocal system with the liquidus temperatures of 549 K and 561 K, respectively were studied by DSC and DROP calorimetry. The heat capacities and the enthalpy increments of the solid and the liquid phases were obtained for both mixtures (∆H549 K=9.5 kJ/mol, ∆H514-549 K=16.9 kJ/mol, ∆H519-561 K=17.0 kJ/mol). The combination of these results allows the calculation of the phase transition enthalpies. DROP-calorimeter was used in a mode of slow heating rate (0.5 K/min) thermal analysis, where the DROP-calorimetric detector was applied for the direct determination of phase transition enthalpies. The thermodynamic properties were calculated using our own dataset of the reciprocal system. The analysis of our experimental and calculated results of enthalpy increments and heat capacities has confirmed that our dataset can be used for prediction of thermodynamic properties of the full reciprocal system. Based on this conclusion three additional compositions 50NaNO3-5.5KCl-44.5KNO3, 28NaNO3-12.4KCl-59.6KNO3 and 13.1NaNO3-11.2KCl-75.7KNO3 of the system with liquidus temperatures at 481 K, 510 K, and 549 K were suggested as potential phase change materials (∆H481 K=9.8 kJ/mol, ∆H496-510 K=12.7 kJ/mol and ∆H510-549 K=15.8 kJ/mol).