A recent long spell of calm weather was among several factors that contributed to rising energy prices in Europe. So, once again, the issue of how to store energy from renewable but unpredictable sources, such as wind and solar power, was put under the spotlight.
According to “Energy Storage Study — Contribution to the security of electricity supply in Europe,” an independent report by the European Commission, traditional pumped hydropower remains the most common type of energy storage reservoir in the region.
The report also describes advances in new technologies, including lithium-ion batteries, electrochemical storage such as batteries coupled with photovoltaic plants, and thermal storage of fluids with concentrating solar power plants.
Now, chemical engineers at Oregon State University (OSU), Corvallis, Ore., have turned their attention to a less-well-studied option: compressed air.
A computer modeling study led by Nick AuYeung, associate professor of chemical engineering, and PhD student Fuqiong Lei found chemical reactions could improve the efficiency of compressed air energy storage (CAES).
Their findings, published in an article in Energy Conversion and Management, also apply to a related technology, liquid air energy storage (LAES), AuYeung says.
CAES and LAES harness energy accessed by allowing stored air — either pressurized or cooled to a liquid form — to expand and pass through electricity-generating turbines. However, both lose about half of their energy when accessed, and so score poorly in round-trip efficiency (RTE).
CAES particularly suffers from low energy and exergy conversion efficiencies inherent in compression, heat loss during storage and natural gas-fired reheat prior to expansion.
The article notes isothermal and advanced adiabatic CAES previously have been proposed to enhance the efficiency of the process. Among the adiabatic schemes, for example, thermal storage can hold heat from compression process. Prior to expansion, the air can recuperate the stored thermal energy, which could reduce or even eliminate the need for natural gas-fired reheat.
Although sensible and latent heat storage have been previously investigated for this purpose, the authors say few published studies look at the use of thermochemical energy storage (TCES) for CAES.
The team propose a direct heat transfer scheme with combined TCES and sensible energy storage.
An advantage of TCES over other methods is a higher energy density made possible by capturing heat in the form of chemical bonds, notes AuYeung.
Using their model, the researchers analyzed the performance of TCES incorporated into thermal energy storage via packed beds filled with different solids. Energy reaches the solid by means of a heat transfer fluid such as air.
Such packed beds are classified as “sensible” storage because energy is harnessed by virtue of the filler material changing temperature.
“We looked at TCES with packed beds filled with rocks and barium oxides,” AuYeung explains, adding, “Our results showed a similar RTE 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% RTE 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.”
Importantly, the researchers noted TCES material placed on top of packed beds created a more-stable turbine air inlet temperature — i.e., it remained higher for longer. This is key to optimal power generation.
AuYeung says the model shows that with future advanced materials, RTE and storage time could improve as well. Possibilities here include non-oxygen chemistries such as hydrates and carbonates. While these have the required properties of high heat capacity and high heat of reaction, the researchers hit a brick ball when trying to identify one for a redox material that operates on oxygen swing.
“A next step perhaps for us, or for others with more materials expertise, would be to try to discover new materials,” concludes AuYeung.
Meanwhile, Pecho LD Energy Storage, San Francisco, Calif., an affiliate of Hydrostor, has filed an application to build a 40-MW, 3,200-MW-hour energy storage facility in San Luis Obispo County, Calif. If greenlighted, the new plant will use Hydrostor’s Advanced CAES technology to deliver 400 MW of stored energy, every hour, for eight hours; cost approximately $800 million; and be up and running by 2026.
