Engineers at Oregon State University (OSU) have identified an approach for storing concentrated solar thermal energy that could reduce its cost and make it more practical for wider use.

In contrast to conventional solar photovoltaic cells that produce electricity directly from sunlight, solar thermal generation is developed as a large power plant in which acres of mirrors reflect sunlight onto a solar receiver. That energy is used to heat a fluid that in turn drives a turbine to produce electricity.

The advance reported by OSU researchers is based on an innovation in thermochemical storage in which chemical transformation is used in repeated cycles to hold heat, use it to drive turbines and then be re-heated to continue the cycle. In practice, this might be done over a 24-hour period with variable levels of solar-powered electricity available at any time of day, as dictated by demand.

According to Nick AuYeung, an assistant professor of chemical engineering in the OSU College of Engineering, thermochemical storage resembles a battery in which chemical bonds are used to store and release energy. However, in this case, the transfer is based on heat, not electricity.

The system hinges on the reversible decomposition of strontium carbonate into strontium oxide and carbon dioxide, which consumes thermal energy. During discharge, the recombination releases the stored heat. These materials are nonflammable, readily available and environmentally safe.

An advance in storing concentrated solar thermal energy could reduce its cost. Image credit: University of Florida/Kelvin Randhir.An advance in storing concentrated solar thermal energy could reduce its cost. Image credit: University of Florida/Kelvin Randhir.The proposed system would work at such high temperatures that it could first be used to directly heat air, which would drive a turbine to produce electricity. Residual heat then could be used to make steam to drive yet another turbine.

In comparison to existing approaches, the researchers say, the new system could allow a 10-fold increase in energy density, as it is physically much smaller and would be cheaper to build.

“In these types of systems, energy efficiency is closely related to use of the highest temperatures possible,” AuYeung says. “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. The compound we’re studying can be used at up to 1,200 degrees and might be twice as efficient as existing systems."

In laboratory tests, one concern arose when the energy storage capacity of the process declined after 45 heating and cooling cycles due to changes in the underlying materials. Further research will be needed to identify ways to reprocess the materials or extend the number of cycles that can be performed before any reprocessing is needed, AuYeung says.

Other refinements may also be necessary to test the system at larger scales and resolve issues such as thermal shock before a prototype is ready for testing at a national laboratory, he says.

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