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Wind farms typically generate most of their energy at night, when most electricity demand is lowest. So a lot of that "green" energy is wasted.
So the big question is: How do you bottle that power for air conditioners and other appliances that are busiest during the day?
There are many companies moving to fill the energy gap. Using federal loan guarantees and $4 billion in "smart grid" stimulus cash, they are working on utility-scale storage units that they hope will help balance intermittent renewable sources like wind and solar and let electric grid operators match power supplies with demand.
Among the leaders is a Massachusetts company that plans to use hundreds of "flywheels" to store 20 megawatts of electricity, enough to power 200 homes for a day. Beacon Power Corp. is working with a $43 million federal loan guarantee for its $69 million storage project in Stephentown, N.Y., which is scheduled to break ground by year's end.
The plant would store cheap "off peak" electricity in 2,500-pound flywheels that turn faster than the speed of sound. When the electricity prices rise -- or when winds die -- energy can be withdrawn from the wheels and sold to the grid at a premium rate.
"It will signal a dramatic shift to a cleaner, more sustainable method of providing frequency regulation on the grid," Beacon CEO Bill Capp told the GridWeek conference in Washington last week.
Utilities today use combinations of hydropower and natural gas to "firm up" intermittent power sources, but hydropower has spatial limitations and natural gas can be expensive and polluting. Yet the need for reliable backup power will grow as states require utilities to use more renewable energy and the cost of carbon-based power rises in the face of expected climate regulations.
"There's a natural affinity between storage and renewables," said Rich Kalisch, senior director of technology initiatives at Midwest Independent Transmission System Operator, which manages nearly 100,000 miles of transmission lines in 13 states.
The nonprofit group currently manages 6,600 megawatts of wind power -- about 4 percent of its total generation -- but has about 54,000 megawatts of wind projects in development. Operators warn that those projects could threaten reliability.
"We are very concerned," Kalisch said.
Still, recognizing a need for energy storage is one thing, but proving that storage technologies can work is another.
One of the largest U.S. demonstrations of battery-based storage uses sodium-sulfur, or NaS, batteries manufactured by Japan's NGK Insulators. American Electric Power Co. Inc. has installed 7 megawatts of the bus-sized batteries to ease congestion on its transmission lines, and it has an additional 4 megawatts under development.
Meanwhile, Xcel Energy Inc. is testing a 1-megawatt NaS battery to manage its wind power in Minnesota.