Laboratory advances: Thermochemical storage for up to 1,200 °C

A parabolic trough CSP plant. So far, the temperature which can be produced by these plants is quite limited. (Photo: Jan Gesthuizen)
A parabolic trough CSP plant. So far, the temperature which can be produced by these plants is quite limited. (Photo: Jan Gesthuizen)

Engineers have developed a new system, which might be able to store heat at very high temperatures similar to the way that batteries store electrical energy. This thermochemical storage system might revolutionise energy storage for concentrated solar power plants.

Since the production of electricity from concentrated solar power (CSP) plants varies greatly depending on the intensity of solar irradiation, a dependable storage system is very important. Unfortunately, such systems are still quite expensive and can often only store molten salt at relatively low temperatures of about 200 to 600 °C. Storing much higher temperatures would greatly increase the efficiency of the whole system.

Now engineers at the University of Florida's Department of Mechanical and Aerospace Engineering, might have found a new approach to store CSP energy cheaper and more efficiently. Their approach uses thermochemical storage, which resembles a battery, as Nick AuYeung, assistant professor of chemical engineering at the OSU College of Engineering, explains. The difference being that chemical bonds of the compounds in the “battery” are used to store and release heat instead of electricity.

Work in progress

The system, which is currently being researched at the University of Florida in collaboration with Oregon State University (OSU), uses strontium carbonate (SrCO3). When this compound gets heated (i.e. heat gets stored), it decomposes into strontium oxide (SrO) and CO2. When the thermal “battery” gets discharged, the two molecules recombine to produce SrCO3 and release heat. This process can be repeated for several heating and cooling cycles.

An advance in the storage of concentrated solar thermal energy may reduce its cost and make it more practical for wider use. (Graphic by Kelvin Randhir, courtesy of the University of Florida)

At the moment, the system is still in development and in the lab the energy storage capacity of the process declined after only 45 of those cycles, due to some changes in the underlying materials. As AuYeung points out, there is still need for further research to identify ways to reprocess the materials or significantly extend the number of cycles that could be performed before any reprocessing is needed. But potentially, the system could be a real breakthrough in energy storage.

Huge potential for the future of CSP

This is because the materials used are non-flammable, readily available and environmentally safe and the new system is much smaller and could yield a tenfold increase in energy density. “With the compounds we’re studying, there’s significant potential to lower costs and increase efficiency,” said Nick AuYeung, corresponding author on the study.

Additionally, 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, and then residual heat could be used to make steam to drive yet another turbine.

“In these types of systems, energy efficiency is closely related to use of the highest temperatures possible,” explained AuYeung. “The molten salts now being used to store solar thermal energy can only work at about 600 °C, and also require large containers and corrosive materials. The compound we’re studying can be used at up to 1,200 °C, and might be twice as efficient as existing systems.”

The study has been published in the sustainable chemistry journal ChemSusChem and was supported by the SunShot Initiative of the U.S. Department of Energy.

Besides the increase of the cycle number, other refinements may also be necessary to resolve issues such as thermal shocks and check whether scaling-up works well, before a prototype could be ready for testing at a national laboratory. But while there is still some way to go, the prospect of a dependable, cheap and more efficient heat storage system is very attractive.

Tanja Peschel