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.