To achieve high thermal-to-electric conversion efficiency and to make solar technologies cost-competitive with conventional electric power generation, power cycles require higher operating temperatures. Third generation (Gen3) of concentrating solar power (CSP) technology are being integrated with supercritical CO2 (sCO2) Brayton power cycle. To achieve this integration Gen3 CSP needs to operate at temperatures above 550°C requiring high-temperature advanced fluids as thermal energy storage (TES) and heat transfer fluids (HTF) operating regularly from about 540 to 720°C. Based on the thermal carrier physical state in the solar receiver, CSP stakeholders defined three potential pathways: molten salt, particle, or gaseous. Because there is no change from current state-of-the-art power tower design, molten-salt systems represent the most familiar approach; however, significant material challenges will be faced because of the increase in the maximum operating temperature from state-of-the-art nitrates at 565°C to approximately 720°C. New salts are required to operate in this higher temperature range because nitrates decompose above 600°C. Knowledge around the selection of a high-temperature molten salt chemistry is needed, as well as its impact on containment materials that can achieve acceptable strength, durability, and cost targets at these high temperatures. Reliable and accurate data of the new salt chemistry is required as well as understanding the corrosion mechanisms is essential for the establishment of this new nigh temperature molten salt technology. Corrosion mitigation approaches have been investigated to keep degradation of containment materials below 20 µm/year. From the salt handling, and thermal energy density point of views molten ternary eutectic carbonate Na2CO3/K2CO3/Li2CO3 is the best heat transfer fluid and TES for Gen3 CSP, but its cost is extremely high. Because of their low cost, and high decomposition temperatures, molten chlorides are the top candidates. However, these molten salts introduce a set of technological and engineering challenges because of their very corrosive characteristics for typical materials. Corrosion in molten chlorides is controlled in atmospheres without oxygen and water. If these impurities are present, molten chlorides become very corrosive in the liquid and vapor phases. Catastrophic mechanical failure is then possible because intergranular attack is the corrosion mode.

Molten salt chemistries and their corrosion mitigation for high temperature thermal technologies

2019 Fall National Meeting

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