Research by the Oregon State University College of Engineering has found a way to improve the efficiency of a type of grid-scale storage critical to a global transition to renewable energy.
Going to net zero carbon emissions means dealing with the intermittent and unpredictable nature of green energy sources such as wind and solar power and also overcoming the disparities between supply and demand, said Nick AuYeung of the OSU, who led the study with Ph.D. student Fuqiong Lei.
These challenges, AuYeung notes, require energy storage by means other than pumped hydroelectric plants, which feature a turbine between two water reservoirs at different altitudes, and huge lithium-ion batteries.
The computer modeling study conducted by AuYeung, associate professor of chemical engineering, and Lei found that one of these additional energy storage technologies, compressed air, could be improved through chemical reactions.
Reversible reactions can absorb energy in the form of heat and subsequently retain energy that would otherwise be lost.
Results, published in Energy conversion and management, are also applicable to related technology, liquid-air energy storage, said AuYeung.
As their names suggest, liquid air and compressed air techniques harness the energy that can be accessed when needed by allowing the stored air – either under pressure or cooled in liquid form – to expand and pass through turbines that generate electricity.
However, CAES, as compressed air energy storage is typically expressed, and LAES (liquid air) score fairly low in a category known as round trip efficiency, says AuYeung. With either, only about half of the energy invested can be withdrawn – think of it as making a bank deposit of $ 1,000 but, due to various fees, only about $ 500 is available for the withdrawal.
“One advantage of CAES is that it can store energy on a large scale, which is a barrier for electrochemical battery technologies,” he said. “But a major challenge for traditional CAES is to achieve high round trip efficiency.”
In a conventional CAES process, electricity is used to compress the air, and the compressed air is stored underground in a cave or in a pressure vessel, AuYeung said. When air is compressed, its temperature rises, but this heat is generally considered waste and therefore is not recovered and unused.
“To remove air to generate electricity, it is usually heated with natural gas to increase the enthalpy of the turbine feed, the total energy of the system,” he said. “Taking into account the heat lost during storage, the result is that the overall round trip efficiency – the ratio of the turbine output work to the work consumed by compression – is only between 40% and 50%.
AuYeung and collaborators from OSU, Mississippi State University and Michigan State University proposed a storage scheme to improve this efficiency – by thermochemically recovering waste heat – and developed a mathematical model for its design and operation. An advantage of thermochemical energy storage, or TCES, over other methods is the higher energy density made possible by capturing heat in the form of chemical bonds, he said.
Using their model, the researchers analyzed the performance of TCES incorporated in thermal energy storage via “packed beds” – vessels filled with a kind of solid packing medium, where the energy reaches the bottom. solid by means of a heat transfer fluid such as air. The filling material is typically alumina, ceramic or crushed rock.
Packed beds are classified as “sensitive” storage because energy is harnessed due to the change in temperature of the fill material.
“We looked at the TCES with compact beds filled with rocks and barium oxides,” said AuYeung. “Our results showed similar round-trip efficiency between beds with TCES and beds without due to the relatively low heat capacity and heat of reaction of barium oxides. We achieved 60% round trip efficiency for both systems with 20 hours of storage time after charge. Other thermal storage media cannot store heat for long periods of time because they cool down.
Importantly, he noted, with the TCES material placed on top of the packed beds, there was a more stable turbine air inlet temperature – higher for a longer period of time – which is key to optimal energy production and therefore desirable for public services. 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 proposed a hypothetical material with the same heat capacity as rocks but a thermochemical storage capacity three times that of barium oxides, and we examined this hypothetical material in our model” , did he declare. . “The results showed that a potential improvement in round trip efficiency of over 5% can be achieved, as well as longer storage times. In addition, 45% less fill volume would be required to achieve a storage capacity similar to that of rock-filled beds. “
AuYeung said the chemistry of barium that the initial model was based on was the most obvious that researchers could think of, but it comes with a downside because it has a fairly low heat capacity.
“There are oxygen-free chemicals such as hydrates and carbonates that have the hypothetical properties – high heat capacity, high heat of reaction – that we have looked at, but so far we have not identified any for a material. ORP that works on the oxygen swing, ”he said. “A next step maybe for us, or for others with more material expertise, would be to try and discover new materials.”
Oregon State University has supported this research through OSU Advantage, which supports work related to entrepreneurship, intellectual property, and technology transfer.
The collaboration included David Korba, Like Li, and Wei Huang from Mississippi State University, who were instrumental in building the mathematical model, and Kelvin Randhir from Michigan State University, who helped to conceptual development.