Wind Power to Compressed Air Energy Storage

Photo: PB Energy Storage Services, Inc.
Cavern wellhead for the first CAES plant in the U.S.

General Compression is a start-up company based in Attleboro, Mass. that is looking to commercialize a concept: storing energy from wind turbines as compressed air. This blog previously mentioned CAES (Compressed Air Energy Storage) as a means of overcoming the problem of intermittency with renewable energy sources.

Martin LaMonica1 informs that CAES is not a new concept — there are two compressed-air storage facilities in operation, one in Germany and one in Alabama, neither fueled by wind turbines — but as companies look to meet growing demand for clean energy there is greater development of methods to store kinetic energy.

Last week General Compression received a $5 million round of seed funding. Whereas the propeller normally turns and inboard electric generator, “General Compression plans to break with that basic design,” reports the CNET Staff Writer. The plan is to substitute an air compressor in the nacelle, which is the housing that encompasses the gears and generator.

The lift provided by the airfoil is the force which drives the main shaft. Such force can create tremendous stress on the gear box, which converts the shaft revolutions to a speed needed by the generator. Given that there have been failures in some gearboxes, with considerable expense for replacement, much consideration is given to the design of the drive train. Driving a compressor rather than a generator seems to add greater uncertainty in MTBF (Mean Time Between Failure).

Instead of electricity from wind turbines, the plan is to pump highly compressed air into storage. The energy stored in the pressurized air becomes a spinning reserve; when power is needed the pressurized air is used to drive an electricity generator, ideally when demand and pricing is higher.

For example, if the wind is blowing hardest at 11 at night, a wind farm operator could store the energy generated from the wind and release it at 10 o’clock the next morning when demand for power starts spiking up.

In advocating for compressed air energy storage of wind power, company president Michael Marcus states that a disadvantage to an intermittent source is that supply is nonscheduled. As previously noted, the greater variability and uncertainty introduced by wind plants have been shown to increase system operating costs by up to about $5/MWH at wind penetration levels up to 20%. The greatest part of this cost is associated with the uncertainty introduced into day-ahead unit commitment due to the uncertainty in day-ahead forecasts of real-time wind energy production.

While that may be, other industry observers note that utility companies profit from rate increases when there is peak demand and thus want to discourage competition from distributed energy resources. Thus, with the cost of renewable energy sources becoming lower and traditional backup / standby sources increasing, there is more incentive to 1) resolve the problems inherent with various approaches to local storage and 2) incur the additional costs to create adequate storage. A carbon tax could spur significant development in energy storage and load control technologies associated with intermittent, renewable energy.

The General Compression concept is that the highly pressurized air is pumped by the wind turbine down into underground storage, such as caves or depleted gas wells.

The company now has a prototype device and plans to build a large-scale version of put it through testing later this year. The plan is to test the “compressor array” in a turbine in the field next year, Marcus said… The compressor was designed by Mechanology, a compressor research and development firm which spun off General Compressor in 2005 and remains a shareholder.

Besides air, other storage strategies include pumping water or other liquids or flywheels. Any of these approaches require comprehensive cost analysis. The Iowa Stored Energy Park, projected to cost $200 million and funded primarily by municipalities, will store compressed air in an underground aquifer. In this case, the gear required for the operation was already available. According to Kent Holst, the project’s development director, the Iowa large scale CAES project includes modifying equipment previously used to store natural gas underground. “The most difficult part was finding a usable geologic structure. Several are already being used for natural gas storage,” Holst said.

The General Compression concept does not need to be on such a scale, the cost associated with compressed air energy storage should be compared not only with other approaches, such as pumped hydroelectric storage, flywheels or batteries, but also with other strategies to meet peak demand.

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