"Current molten salt and phase-change material storage systems are only able to operate up to around 600°C, and are expensive due to their low energy densities. Our knowledge of regenerative absorption processes for CO2 separation systems was used to

By Andrew Williams

With assistance from the US DoE under the SunShot ELEMENTS program, and project partners Precision Combustion and Clariant, the Southern Research Institute (SRI) is further refining a tailored calcium-based sorbent material and heat-exchanger reactor system specifically for use within CSP facilities.

Energetic efficiency

The key objectives of the DoE-backed project are to develop and validate at 'bench-scale', a system that can store energy at 600-900°C and at a volumetric energy density greater than 1 MWhr/m3, with a roundtrip energetic efficiency greater than 98% that will maintain performance over the full 30-year life of a CSP facility.

"As the next-generation of CSP plants moves towards higher operating temperatures to achieve higher conversion efficiencies, it became apparent that a new generation of high-temperature storage needs to be developed to allow these facilities to continue to provide power in a cost-effective and dispatchable manner," says Michael Johns, vice president of engineering at Southern Research Institute.

"Current molten salt and phase-change material storage systems are only able to operate up to around 600°C, and are expensive due to their low energy densities. Our knowledge of regenerative absorption processes for CO2 separation systems was used to develop a thermochemical storage concept applicable to high-temperature CSP applications," he explains.

Calcium-based technology

According to Johns, the planned calcium-based thermal energy storage system uses 'highly reversible carbonation-decarbonation reactions' in a closed-loop fixed-bed reactor.

During peak insolation, when the solar field heat rate is greater than the capacity of the power block, a portion of the heat transfer fluid (HTF) will be diverted from the power-block to the storage system.

Thermal energy extracted from the HTF will drive equilibrium of the sorbent material towards an endothermic reaction, releasing CO2, which is stored in a closed-loop system. In the evening, or off-peak, the sorbent will be exposed to the CO2 to exothermically react, releasing energy that will heat the HTF.

"Because the storage process is completely closed-loop, there is no exchange of mass with the environment or the HTF. The only power input required is for the compression of the CO2 for storage," highlights Johns.

For him, the key technical strengths of this system are its ability to operate at high temperatures, and to vary the equilibrium temperature of the reaction based on the specific power-block.

Through control of the absolute and partial CO2 pressure, he says, the equilibrium temperature can be set anywhere within the 600-900oC temperature range, 'allowing it to be integrated with a variety of advanced high-temperature power blocks, including supercritical CO2 Brayton, ultra supercritical H2O Rankine, and dish Stirling.'

"The financial advantages come from the use of high-availability and very low-cost sorbent material-tailored calcium carbonate from basic limestone - and the high energy density of the sorbent, which allows for small containment vessels," explains Johns.

He points out that the cost of materials for these containment vessels and process piping is 'significant,' and that for low-energy density storage materials large volumes are needed.

"The SRI calcium-based system stores at energy at over 1 MWhr/m3, while current molten salt systems can only achieve 0.25-0.40 MWhr/m3," he says. "Furthermore, the Southern Research system uses an elegantly simple heat exchanger reactor system to store energy”.

“Competing systems require solids conveyors, gas phase separations, combustors, high pressure gas-liquid separations, or slurry pumps to reversibly store energy."

"Our compact and simple design limits the amount of high-temperature high-cost superalloy material required, and allows for extremely high energetic and exegetic efficiencies through the limited amount of required process steps."

Sorbent material

The main challenge, in Johns’s opinion, will be in refining a calcium sorbent material that balances the tradeoff between high energy-density, low cost, and long material lifetime.

"A baseline sorbent material has already been developed, and further refinements will ensure the suitability and performance of this material and storage system for the CSP industry," he says.

The project will consist of two phases, an initial material refinement stage at the laboratory-scale combined with systems modelling, followed by the operation of a bench-scale system that is representative of the commercial embodiment.

"This storage system is extremely modular, and as a result the scale-up can be done in relatively large steps. Once the bench-scale system is refined and validated, a MWhr-scale demonstration system will be fabricated that can be integrated with a demonstration or commercial high-temperature CSP facility,” notes Johns.

As the storage system is specifically designed for high-temperature applications, he expects the regional markets for its integration to in areas where advanced power-block CSP facilities are deployed.

To respond to this article, please write to the author, Andrew Williams.