Air-stable, low-cost molten chloride storage media, stored in container with slow-growing oxide scales, could be useful for air-compatible, >750C thermal energy storage.Concentrated solar power (CSP) could compete with fossil fuels, if conversion efficiencies could be improved by operating at higher temperatures and employing thermal energy storage technologies. High temperature energy storage in the form of molten salts could allow solar energy to become a responsive and continuous source of electricity even when the sun isn’t shining. But not all earth-abundant molten salts are chemically stable in ambient air and can corrode storage tanks and pipework.
Now researchers from Purdue University have shown that a particular type of earth-abundant molten salt, based on a seawater-derived calcium chloride-sodium chloride liquid, is chemically stable in ambient air up to 750°C and, when used with the right type of containment material, produces minimal corrosion [Caldwell et al., Materials Today (2021), https://doi.org/10.1016/j.mattod.2021.02.015 ].
“Stable high-temperature molten salts and a corrosion-resistant tank material are needed to store heat from sunlight for cost-effective and sustained electricity generation,” explains Kenneth H. Sandhage, who led the effort.
Other low-melting, earth-abundant molten salts containing MgCl2 being considered as potential thermal storage media for CSP are not stable in ambient air at 750°C and corrode metal alloy containers and pipework. Alternative CaCl2-NaCl molten salts do not oxidize at 750°C in ambient air. Moreover, when these molten salts come into contact with specific oxide-bearing metals, such as NiO-bearing Ni, minimal corrosion occurs. The key to this material’s corrosion resistance is the formation of oxide scales, which protect the metal from direct contact with corrosive molten salts.
“These oxidation-resistant molten chloride salts can be used as chemically-robust liquids for storing solar thermal energy at high temperatures in ambient air, so that electricity may continue to be produced from CSP plants at night and on cloudy days,” says Sandhage. “Because these particular molten salts are oxidation resistant, the pipes and tanks used to pump and store [them] do not need to be perfectly airtight, which simplifies the design, reliability, and cost of pipes and storage tanks.”
The CaCl2-NaCl molten salts are less expensive than the molten nitrate salts currently used in CSP plants and can be used at higher temperatures. The researchers’ thermodynamic calculations predict that alternative ternary and quaternary chloride salt compositions, with lower melting points, should also be oxidation resistant, potentially broadening their useful temperature range. Other oxide-forming metal alloys, that are more mechanically robust than Ni/NiO, will also be explored by the team.
“We are hoping that our earth-abundant, oxidation-resistant, molten chlorides will be used instead of molten nitrate salts or oxidation-prone MgCl2-bearing molten salts for low-cost, reliable thermal energy storage at high temperatures with the aim of lowering the cost of dispatchable electricity produced by such plants,” says Sandhage.