Researchers at the Oregon State University College of Engineering have conducted research that uncovered a way to improve the efficiency of a type of grid-scale storage crucial for a global transition toward renewable energy.
Moving towards net-zero carbon emissions means dealing with the intermittent, unpredictable nature of green power sources such as wind and solar and also overcoming supply and demand mismatches. Those challenges necessitate energy storage through means beyond pumped hydro plants, which feature a turbine between two water reservoirs of different elevations, and huge lithium-ion batteries, said OSU’s Nick AuYeung, who led the study along with Ph.D. student Fuqiong Lei.
The computer modeling study led by AuYeung found that one of those additional energy storage technologies, compressed air, could be improved via chemical reactions. This refers to reversible reactions that can absorb energy in the form of heat and subsequently conserve energy that would otherwise be lost.
The liquid and compressed air techniques harness the energy that can be accessed when needed by allowing stored air – either pressurized or cooled to a liquid form – to expand and pass through electricity-generating turbines.
“An advantage of CAES is that it allows energy to be stored at large scales, which is a hurdle for electrochemical battery technologies,” AuYeung said. “But a major challenge for traditional CAES is reaching high round-trip efficiency.”
The OSU-led team came up with a storage scheme to improve that efficiency – thermochemically recovering lost heat – and developed a mathematical model for its design and operation. Thermochemical energy storage, or TCES, has been reported to have higher energy density than other methods. This achievement was made possible by capturing heat in the form of chemical bonds, he said.
Using their model, the researchers analyzed the performance of TCES incorporated into thermal energy storage via “packed beds” – vessels filled with some kind of solid packing medium, where energy reaches the solid by means of a heat transfer fluid such as air. They suggested using alumina, ceramic or crushed rock as a filler material. Packed beds are classified as “sensible” storage because the energy in such a system is used by changing the temperature of the filler material.
“We looked at TCES with packed beds filled with rocks and barium oxides,” AuYeung said. “Our results showed a similar round-trip efficiency between beds with TCES and beds without because of the relatively low heat capacity and heat of reaction for the barium oxides. We got to 60% round-trip efficiency for both systems with a 20-hour storage time after charge. Other means of thermal storage cannot store the heat for long periods of time since they cool down.”
With the TCES material placed atop the packed beds, there was a more stable turbine air inlet temperature – higher for longer – which is a key to optimal power generation and thus desirable to utilities. In addition, AuYeung said the model shows that with future advanced materials, round-trip efficiency and storage time could improve as well.
“To better illustrate the potential of the concept, we came up with a hypothetical material with the same heat capacity as rocks but a thermochemical storage capacity three times that of barium oxides, and we looked at that hypothetical material in our model,” he said. “Results showed that a potential round-trip efficiency improvement of more than 5% can be obtained, as well as longer storage durations. Also, 45% less filler volume would be needed to achieve storage capacity similar to rock-filled beds.”
The next step in this research will be to try to find more suitable materials.