Molten salts as engineering fluids – A review

Molten salts constitute an important class of fluids for high temperature applications, like catalytic medium for coal gasification, molten salt oxidation of wastes or for latent or sensible heat storage. In particular, molten alkali nitrates are being used as heat transfer/storage fluids in concentrated solar power (CSP) technologies. These kind of technologies operate in ranges of temperature for which molten salts are particularly adequate, due to its main characteristics: stability at high temperatures, low vapor pressure, liquid state in a large range of temperatures, ability to dissolve many inorganic and organic compounds, viscosity generally low (as ions are mutually independent) and high heat capacity per unit volume. For the proper design and dimensioning of heat exchangers and other ancillary equipment’s it is decisive to have accurate data for the thermophysical properties of the employed fluids. This paper reviews the available data for the relevant properties that are important to a salt system for storage and heat transfer applications. Those are the melting point, density, viscosity, heat capacity and thermal conductivity. The chosen fluids were the pure molten lithium, sodium and potassium nitrates and relevant mixtures, like the solar salt (NaNO3/KNO3: 60/40), HITEC® (a ternary mixture of NaNO3, KNO3 and NaNO2) and some new quaternary mixtures. Review reveals that there are still large discrepancies between different sets of data for the same salt systems and that it is impossible currently to recommend reference data/measuring methods that can guide the reader for a selection of the best systems. The impact of that and the potential applications are briefly discussed. The reviewed fluids have great potential for actual and future applications in renewable processes for energy storage and transformation.