Winter winds howl off the Dakota prairie through Minnesota, turning the 1,100 megawatts worth of wind turbines in Xcel Energy's system in that state. By 2020, the utility expects to more than triple that amount in a bid to avoid more polluting energy sources. But the wind doesn't always blow and, even worse, it often blows strongest when people aren't using much electricity, like late at night.

So Xcel Energy, Inc., has become one of the first utilities in the U.S. to install a giant battery system in an attempt to store some of that wind power for later. "Energy storage might help us get to the point where we can integrate wind better," says Frank Novachek, director of corporate planning for the Minneapolis-based utility with customers in Colorado, Kansas, Michigan, Minnesota, New Mexico, the Dakotas, Oklahoma, Texas and Wisconsin. "The overall cost of electricity might be lower by using energy storage."

The energy storage in question—a series of sodium–sulfur batteries from Japan's NGK Insulators, Ltd.—can store roughly seven megawatt-hours of power, meaning the 20 batteries are capable of delivering roughly one megawatt of electricity almost instantaneously, enough to power 500 average American homes for seven hours. "Over 100 megawatts of this technology [is] deployed throughout the world," Novachek says. The batteries "store wind at night and they contract with their utility to put out a straight line output from that wind farm every day."

That removes one of the big hurdles to even broader adoption of wind power: so-called intermittency. In other words, the wind doesn't always blow when you want it to, a problem Texas faced earlier this year when a drop in wind generation forced cuts in electricity delivery. But with battery backup, the 11-megawatt wind farm outside Luverne, Minn., can deliver a set amount of electricity at all times, making it more reliable or, in industry terms, base-load generation. Plus, the battery effectively doubles the wind farm's output at any given moment—both the megawatt being produced by the wind farm itself (that would otherwise have gone to charging the battery) and the megawatt delivered by the battery.

But it is expensive, costing roughly $3 million per megawatt plus millions for start-up and testing. "Right now, they're a little too expensive," Novachek says. But "it's getting in the ballpark where it looks like the economics might be there. Testing will help us understand the value."

So far the battery has been through five charging and recharging cycles and testing will continue through next year, Novachek says. Other utilities, including the Long Island Power Authority in New York State and American Electric Power in Ohio, have used similar or the same batteries to better manage their grids, but this would represent the first battery to store wind power in the U.S.

The battery is not the only storage experiment Xcel Energy is running: It has been testing using electricity from wind and solar installations to generate hydrogen and then burn the hydrogen in a generator to turn it back into electricity when as needed. And the utility has paired with the city of Boulder, Colo., to test plug-in hybrid electric cars as a means of providing electricity during the day when people are at work and not driving.

"The Midwest is a great [wind] resource and we are strategically placed to use that and reduce our carbon footprint," Novachek notes, by replacing some of the 16 coal-fired plants and 28 natural gas power plants the company now operates. "New technologies that are out there might really help us get more green than people had hoped—and energy storage is one of those."

* Editor's Note (posted 12/30/08): As noted by a few of our commenters, it would have been useful to know the specific total cost of the battery system. We asked Xcel Energy for that figure, but they would not say, and NGK Insulators could not be reached for comment.