A general thermodynamic model for eutectics of phase change molten salts in concentrating solar power applications

The production of electric energy from solar radiation is nowadays one of the most investigated “carbon free” technologies. Concentrating solar power (CSP) plants employ thermal energy storage (TES) units in order to dispatch electrical output to match the peak demand period. Therefore, the selection of a proper heat storage material (HSM) is the key to make this technology suitable for renewable energy production. The aim of this work is to provide a general semi-predictive thermodynamic model able to detect the eutectic temperature and molar composition of phase change molten salt mixtures employed as HSM. The method is based on the experimental liquidus curves, where the non-ideal behavior of the system is expressed by the most commonly used local-composition activity coefficients models, namely Wilson and NRTL. The only conditions required are a total miscibility in the liquid phase and a quasi-total immiscibility in the solid phase. This simulation tool was validated predicting the eutectic points of several significant ternary mixtures based on nitrates, nitrites, chlorides and carbonates that are the most promising candidates as PCM in temperature range applications from 200°C to 600°C. At this purpose, the literature phase diagrams of the binary subsystems were simulated in order to obtain the fitting parameters necessary for the ternary system behaviour prediction. Then, the ternary systems were modelled and an experimental campaign was conducted to compare the experimental eutectic points with the simulated ones. The model always resulted flexible and accurate with a maximum discordance of 5% for both temperature and molar composition.