Improving the working fluid based on a NaNO3-KNO3-NaCl-KCl molten salt mixture for concentrating solar power energy storage

Abstract

The thermo-physical properties of a working fluid (WF) strongly affect the energy efficiency and economic performance of a concentrating solar power plant (CSP) with thermal energy storage (TES). Consequently, the use of molten chloride salts instead of the current solar salt (NaNO3-KNO3 64–36% mol) has been extensively proposed. Nevertheless, the strong corrosiveness of magnesium chloride, needed to reduce the chloride-based WF melting point to CSP operational level, has inhibited this shift’s industrial-economic applicability. In this work, sodium and potassium nitrate and chloride transition paths are proposed by gradually incorporating NaCl and KCl into the current solar salt. This path is expected to allow CSP working temperature to increase gradually, obtaining benefits at a controlled industrial risk. Five levels of chlorides –from 0 to 50% mol Cl- – were studied on a solar salt, evaluating their main thermo-physical properties. The results show that the addition of chlorides in all evaluated ranges increased the solar salt’s thermal stability, ranging from 592 ?C to 642 ?C (50% mol Cl-). The melting temperature remained within the ranges of interest for CSP applications, varying between 211 ?C (14% mol Cl-) and 243 ? C (50% mol Cl-). The viscosity and heat capacity showed a significant dependence on the chloride content and temperature. Finally, the wider operative temperature range and the enhanced energy density through chloride addition are positive outcomes that would improve CSP efficiency and reduce the volume of and capital expenditures for the TES system. Conversely, the viscosity and phase miscibility can be of concern at operative low-temperature levels.