Materials challenges for concentrating solar power

A heat transfer fluid (HTF) is a major component in the system for concentrating solar power systems (CSP) to make electricity. The HTF carries thermal energy from the solar concentrator to a steam generator. Currently hydrocarbon oils or alkali-nitrate-based eutectic molten-salt mixtures are used as the HTF in CSP systems, but these materials have limited operating temperature range, which limits efficiency. Hydrocarbons are limited to 250 °C and alkali-nitrate salts are stable only below 600 °C. Using abundant inexpensive materials to make an HTF which is stable to 1,300 °C and compatible with a metal housing, like a Hastelloy nickel alloy, is desired. Design rules are given which tell how the desired goals can be met, which leads to mixing abundant ionic chloride salts, like NaCl and KCl, which boil at temperatures higher than 1,400 °C, with low-melting (~200 °C) covalent metal halides, such as AlCl3 or ZnCl2, to give low-melting (m.p.<250 °C) eutectic mixtures, which are stable at high temperatures. To have negligible corrosion of the metals which house the eutectic, the component eutectic should have more negative reduction potentials than metals in the salt housing. Accordingly, the ternary K-Na-Zn chloride molten-salt mixtures in the alloy metal housing should be stable. However, corrosion of the metal housing is seen, especially at higher temperatures. The corrosion rates of housing alloys in molten salt in the presence of or excluding air have been experimentally determined at different temperatures. Indications are that the corrosion of the metal is not due to the salt itself but dissolved impurities like water and oxygen.