Thermochemical storage, in which chemical transformation is used in repeated cycles to hold heat, and drive turbines to create electricity, and then be re-heated to continue the cycle. This might be done over a 24-hour period as dictated by demand.
Unlike conventional solar photovoltaic cells, concentrated solar thermal (a.k.a. concentrated solar power, or CSP) uses huge arrays of mirrors to focus light, typically onto a tower, for temporarily storing the energy, which is more cost-effective than batteries. (See Australian researchers set new world record in solar-energy efficiency.)
Storage of this type helps eliminate one of the key factors limiting the wider use of solar energy: The need to deliver the electricity immediately. The development overcomes a limitation in thermochemical energy storage. “In these types of systems, energy efficiency is closely related to use of the highest temperatures possible,” said Nick AuYeung, an assistant professor of chemical engineering in the OSU College of Engineering and corresponding author of a paper in ChemSusChem, a professional journal covering sustainable chemistry.
Thermochemical storage functions like a battery, in which chemical bonds are used to store and release heat (not electrical) energy. However, the molten salts now being used to store solar thermal energy can only work at about 600 degrees centigrade, and also require large containers and corrosive materials, he explained. “The compound we’re studying can be used at up to 1,200 degrees, and might be twice as efficient as existing systems. There’s a significant potential to lower costs and increase efficiency.”
AuYeung said the new OSU system is based on the reversible decomposition of strontium carbonate into strontium oxide and carbon dioxide, which consumes thermal energy. During discharge, the recombination of strontium oxide and carbon dioxide releases the stored heat. These materials are nonflammable, readily available, and environmentally safe.
In comparison to existing thermochemical approaches, the new system could also allow a ten-fold increase in energy density (energy storage per unit volume), and it’s physically much smaller and would be cheaper to build. The proposed system could first be used to directly heat air, which would drive a turbine to produce electricity, and then residual heat could be used to make steam to drive yet another turbine.
However, in laboratory tests, the current energy storage capacity of the process declined after 45 heating and cooling cycles, due to some changes in the underlying materials. Further research will be needed to identify ways to reprocess the materials or significantly extend the number of cycles that could be performed before any reprocessing was needed, AuYeung said.
Other refinements may also be necessary to test the system at larger scales and resolve issues such as thermal shocks, he said, before a prototype could be ready for testing at a national laboratory.