Wind energy on demand
A utility-sponsored project and an ambitious company aim to store wind energy underground for sale when demand—and price—is high.
Wind doesn't blow around the clock. Because of its ephemeral nature, making wind a viable alternative energy source requires storage capacity and the ability to release that energy when demand—and market price—is high.
New technology from one company and a project in Iowa may show how to bring these two requirements together.
Compressing air for energy isn't a new idea, but its use for wind energy is. When energy demand is low, energy from wind turbines can force air into an underground aquifer. When demand is high, a utility can expand the pressurized air and transform it back into electricity by pumping it to the surface and running it through a turbine. Two compressed-air energy-storage (CAES) installations exist—one in Huntorf, Germany, and the other in McIntosh, Ala.—but neither relies on wind energy for compression or expansion.
The Iowa Association of Municipal Utilities (IAMU), which represents more than 550 electricity, water, gas, and telecommunications utilities in Iowa, is pioneering a unique wind-storage method to generate electricity and plans to have it up and running by 2011. Officials envision underground storage, in this case in a natural aquifer, as a way to take advantage of variations in the price of electricity.
"There's a lot of wind energy available in Iowa, but the inability to know
when the wind will blow is a drawback," says Kent Holst, development director for the Iowa project. "When you combine wind energy with compressed-air energy storage, you make wind energy dispatchable"—in other words, available on demand.
The project, created by IAMU, is dubbed the Iowa Stored Energy Park. The $200 million undertaking aims to harness about 75 megawatts (MW) of power from wind turbines across Iowa, supplemented with electricity bought at off-peak times delivered by existing power lines. The energy would power motors to compress air and force it 2000 feet below ground into a natural aquifer near Dallas Center, in central Iowa.
The Iowa plant should produce 268 MW when running at full capacity, which is enough to power about 75,000 homes. Most of the energy will be used by the municipal utilities in Iowa and in surrounding states that invest in the park. The rest will be sold on the grid.
Although Iowa is a prime location for wind and natural aquifers, finding a site still posed a challenge for the project team. Many geological sites are already used to store natural gas underground, Holst says. Energy park officials investigated two other sites before settling on the present location. The Dallas Center aquifer was chosen because it has all the necessary geological features, including size, depth, and structure—and isn't currently used for storage.
Now, one company hopes to commercialize the wind-energy storage method. General Compression of Attleboro, Mass., says it can bring CAES into widespread use. The company, created in March 2006, has $5 million of seed funding to establish underground storage of compressed air that can later be harvested as electricity when the price is right.
General Compression plans to offer its dispatchable wind turbine system for sale to turbine operators within the next 3 years. The system features a compressed-air wind turbine, a pipeline network that collects and stores compressed air, and a mini power plant of expanders and generators, says Michael Marcus, the company's president.
The blowing wind lifts the turbine blades, spinning the compressor. In turn, the compressor pressurizes air and pumps it down the tower into an underground network of pressurized pipes. The pipes collect and store 6–12 hours of wind-generated energy. For longer storage, the technology can also pump compressed air into a nearby salt dome, aquifer, limestone cavern, or depleted gas field, Marcus says.
To retrieve that power, the compressor is configured as an expander, which turns the change in pressure as air is released into rotary motion. The expanders are connected to electrical generators.
Should these ventures prove successful, they'll show other private operators and utility companies considering wind that the power source can also be a sound investment. "The technology doubles the rate of return of conventional wind farms, making it about twice as profitable," says David Marcus, CEO of General Compression.
More than 10,000 MW of electricity is generated from the wind in the U.S., which is enough to power 2.5 million average American homes, according to the Department of Energy's National Renewable Energy Laboratory. Wind energy could eventually provide 20% of the nation's energy needs. By providing the capability to sell that energy for the highest price possible, CAES makes wind all the more desirable.
