DOE’s Office of Fossil Energy is working through its new Advanced Energy Storage Program to improve and foster the widespread use of energy storage integrated with fossil energy applications leading to facility flexibility, power grid resiliency, cost savings, and reduced greenhouse gas emissions.

One class of energy storage technology with potential for long durations and integrating with fossil assets is mechanical energy storage. Mechanical energy storage takes excess or low-cost energy and converts it into potential energy for subsequent discharge to the grid. As an example, Compressed Air Energy Storage (CAES) technology may offer an easy means of storage and power generation. It uses off-peak cheap electricity to compress air and store it in a pressurized storage reservoir. When electricity is needed at peak demand, the air is withdrawn, combusted, and expanded to drive an electric generator. With this system, air can be pre-heated by recovering heat from the compressor train or by burning fuel, such as natural gas, to improve efficiency.

In a CAES system, the compression energy is separately supplied and thus the full output of the turbine can be used to generate electricity during expansion. This means CAES plants consume only about one-third the fuel of a conventional simple cycle combustion turbine, resulting in cost savings due to reduced fuel use. As a result, the system produces only about one-third the pollutants such as carbon dioxide (CO₂) and nitrogen oxides per kilowatt hour generated.

An example of a CAES system in action can be seen in a plant built by the Alabama Electric Cooperative in McIntosh, Alabama. Commissioned in 1991, the 110-megawatt generating capacity facility has provided storage services ever since. A single salt cavern is used for on-site storage at this facility, storing enough compressed air for 26 hours of generation at full power.

“Mechanical energy storage technologies are a good example of the Lab’s work on sustainably powering the future using our nation’s resources,” NETL Technology Manager Briggs White said. “By harnessing the air we breathe, combined with our fossil fuels, the potential exists to store energy so it can be used to improve the flexibility of large critical facilities, increase system resiliency, and drive down costs for industry and consumers while lowering CO₂ emissions. The task ahead of us is making these technologies more economical for investors.”

The Alabama CAES plant was the result of a feasibility study in the 1980s.  Addressing projected load growth was one of the drivers for then owner Alabama Electric.  The company’s conclusion was it would improve the capacity factor of the~600 MW coal-fired Charles R. Lowman electrical generation plant by using the new CAES concept, the first example in the United States.

Improving capacity factor remains a driving force for CAES today as in the early 1990s. Just as electricity from the Lowman coal plant was used for the CAES process, the concept could make use of coal generated electricity at other sites throughout the nation by using commercially available equipment.

The Electrical Power Research Institute was a prime mover in the study and funded the project. DOE contributed the design for the storage space — a geologic cavern, which was modeled after the Strategic Petroleum Reserve.

Going forward, NETL is exploring ways to reduce the cost of adopting CAES on a large scale, with the right underground storage vessels in geologic formation providing potential options. Currently the cost of the storage vessel is the main factor limiting the development of CAES technology.

NETL’s work in mechanical energy storage comes as part of DOE’s Energy Storage Grand Challenge. The goal of this initiative is to create and sustain global leadership in energy storage utilization and exports, with a secure domestic manufacturing supply chain that doesn’t depend on foreign sources of critical materials. Leveraging resources from across all of DOE, the Grand Challenge builds on the $158 million Advanced Energy Storage Initiative announced in President Trump's Fiscal Year 2020 budget request.

The U.S. Department of Energy’s National Energy Technology Laboratory develops and commercializes advanced technologies that provide reliable and affordable solutions to America's energy challenges. NETL’s work supports DOE’s mission to advance the national, economic, and energy security of the United States.