Review of the solubility, monitoring, and purification of impurities in molten salts for energy storage in concentrated solar power plants

Thermal Energy Storage (TES) for Concentrated Solar Power (CSP) applications is a vital part of bringing green technologies to cost parity with traditional fuel-based power. Eutectic salt mixtures are highly suitable for use in TES. However, they contain impurities that can detrimentally impact their performance and corrosion characteristics when stored in a metallic container. This review will present a summary of findings that delve into the characterization, quantification, and, most importantly, the mitigation and control of these contaminants. Some of the common impurities discovered in commercially available salts include O-2, H+ and OH-, all of which highly accelerate corrosion. Other contaminants include chloride, fluoride, perchlorate, sulfate, carbonate and nitrite/nitrate compounds. Purification, monitoring, and control of impurities are presented; including heat processing, gas sparging, additives, acid consumption analysis, voltammetry methods, and physical filtration. Finally, recommendations are made for the most appropriate strategies for mitigating impurities for CSP applications.