Molten chloride salt technology for next generation concentrating solar power plants: corrosive impurity monitoring and corrosion mitigation

Abstract

In the next generation of concentrating solar power plants (Gen3 CSP), the envisaged higher thermal-to-electrical conversion efficiency requires high operating temperatures. However, the existing nitrate-based thermal storage system (nitrate-TES) is unable to meet the demand for operating temperatures above 700 °C, limiting the thermal-to-electrical conversion efficiency to around 40%. Molten chloride salts, such as the MgCl2-KCl-NaCl-system, are considered promising thermal storage media. However, despite the potential advantages of low cost, large operating range, and high abundance, corrosion has been a century-long challenge for chloride salts, hindering their further development as high-temperature thermal storage media and heat transfer fluids (HTFs). For the MgCl2-KCl-NaCl salt system, the MgOHCl species, has been identified as a main corrosive impurity, which is produced by a mix of the salts hygroscopicity nature and hydrolysis reactions. The implementation of quantitative analysis and subsequent removal of corrosive impurities is crucial for effective management of metal corrosion in molten chloride salts. In this dissertation, chemical and electrochemical methods are developed that can monitor corrosive MgOHCl-impurities at concentrations down to tens of ppm (parts per million) in molten chloride salts. At such low analytical concentrations, the acceptable impurity level of MgOHCl in molten chloride salt can be discussed for cost-effective Fe-based alloy quantitatively. In addition, purification methods to reduce the MgOHCl impurity concentration were developed to investigate their impact on the corrosion of commercially available and economically viable structural materials. A simple and effective magnesium-based corrosion mitigation strategy was developed to be effective in reducing the MgOHCl impurity concentration to allow the use of Fe-based alloys as structural materials. This corrosion mitigation strategy has been verified in long-term tests up to 2000 hours at 700 °C and has confirmed a potential service life of > 30 years for Fe-based alloys with corrosion rates of < 15 µm/year, which was not thought possible prior to this work. This conclusion is confirmed from several perspectives, including mass loss, microstructural analysis and concentration of corrosion products in salts. Considering the allowable stress of the material, this work indicates that Fe-based alloys, represented by austenitic stainless steels, can be the main structural materials for the hot part at 700 °C of the chloride-TES system. Moreover, the magnesium-based chloride salt mitigation strategy is used in an isothermal exposure experiment at 500 °C, confirming that ferritic-martensitic steel can also achieve a corrosion rate below 15 µm/year at this temperature. Thus, ferritic-martensitic steel can be a candidate for the structural material of the cold part in the chloride-TES system. The corrosion test results indicate excellent compatibility between the low-cost Fe-based alloy and purified MgCl2-KCl-NaCl, even at temperatures as high as 700 °C. This compatibility makes the chloride-TES system economically competitive with existing nitrate-TES systems, and opens up the possibility for integration into next-generation power cycles, such as the supercritical CO2 Brayton power cycle. This integration will potentially increase the thermal-to-electrical conversion efficiency of Gen-3 CSP from the current 40% to ~55%, thereby further reducing its levelized cost of electricity (LCOE).