Molten chloride salt technology for next-generation CSP plants: Compatibility of Fe-based alloys with purified molten MgCl2-KCl-NaCl salt at 700 °C

Abstract

Molten chlorides, such as MgCl2-KCl-NaCl, are promising advanced high-temperature (up to 800 °C) thermal energy storage (TES) materials in next-generation concentrating solar power (CSP) plants. However, their high corrosivity to commercial Fe-Cr-Ni alloys impedes the commercial applications of chloride-TES. In this work, we investigated the corrosion of two selected commercial Fe-based alloys (SS 310 and In 800H) in molten MgCl2-KCl-NaCl salt, aiming to study the feasibility of affordable Fe-based alloys instead of expensive Ni-based alloys in the chloride-TES system. The alloy samples were immersed in the liquid-Mg-purified molten salt at 700 °C for 2000 h under a protective inert gas atmosphere. After the corrosion test, SEM-EDX microstructural analysis and mass loss analysis showed that corrosion rates of the immersed alloy samples were lower than 15 µm/year, and the corrosion rates had a decreasing tendency with increasing immersion time during the 2000-hour test. To our best knowledge, this is the first experimental demonstration that corrosion rates of the Fe-based alloys in molten MgCl2-KCl-NaCl salt at 700 °C can be controlled below the target (15 µm/year) proposed by the US Department of Energy (DOE). Using affordable Fe-based alloys as main structural materials, the cost of chloride-TES (27 USD/kWh) could be comparable to that of commercial nitrate-TES (20–33 USD/kWh). Taking advantage of chloride-TES with higher operating temperature, the next-generation CSP plant could use an advanced power cycle (e.g., sCO2 Brayton) to have a much higher energy conversion efficiency, leading to a significantly lower Levelized Cost of Electricity (LCOE) than the current commercial CSP plant.