Thermal characterisation of an innovative quaternary molten nitrate mixture for energy storage in CSP plants

Abstract

Enhancements to energy storage systems developed for solar thermoelectric technologies can yield considerable increases in efficiency for this type of renewable energy. Important improvements include the design of innovative storage fluids, such as molten salts possessing low melting points and high thermal stabilities.This research examines the design of an innovative quaternary molten nitrate mixture, with the goal of improving the solar salt used currently as an energy storage fluid in CSP plants. This quaternary salt, which contains different weight percentages of NaNO3, KNO3, LiNO3 and Ca(NO3)2, exhibits better physical and chemical properties than the binary solar salt (60% NaNO3+40% KNO3) currently used. The melting points, heat capacities and thermal stability of the quaternary mixtures were studied by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). In addition to DSC and TGA tests, viscosity and electrical conductivity measurements were carried out for the quaternary mixtures at different temperatures.The new salt was designed by taking into consideration the risk of solid species formation at high temperatures when calcium nitrate is present (which requires that the wt% does not exceed 20%) and the costs of LiNO3. These boundaries set the maximum wt% of LiNO3 to values below 15%.Finally it was determined that the proposed quaternary mixture, when used as a heat transfer fluid (HTF) in parabolic trough solar power plants, is able to expand plants׳ operating range to temperatures between 132 and 580°C.