When the wind is roaring in Minnesota, it can power more than 250,000 homes.

But when the wind dies, so does the power supply.

Intermittent wind makes the power source difficult to integrate into the national power grid – so far. “Now you have it, now you don’t,” critics say.

But, one utility is hoping to overcome the intermittency challenge by installing a powerful battery to capture excess wind power on windy days, and supply it to customers when the winds are calm. 

The single sodium-sulfur battery can power up to 500 homes for more than seven hours, said Frank Novacheck, director of corporate planning for Minneapolis-based Xcel Energy.  The battery is manufactured by the Japanese company NGK Insulators.

“The batteries can go for hours,” Novacheck said. “If the wind dies down real quickly you can call upon the battery source. It acts like a shock absorber.”

Xcel plans to become the first utility to use NaS (the chemical symbols for sodium and sulfur) technology for wind power storage when it plugs the battery into its wind farm in Luverne, Minn. The battery is ready to be shipped from Japan and the company plans to have it installed by November, Novachek said. 

The wind farm in Luverne is an ideal testing grounds for the battery because of its relatively low-energy capacity, compared to large scale wind farms like Houston-based Horizon Wind Energy’s 396-megawatt wind farm in McLean County in downstate Illinois.

“This way we can test the storage device with a wind farm that’s more to its size,” Novacheck said. 

Terms of the agreement could not be discussed, but NaS electricity costs about 10 percent more per kilowatt than electricity from new coal-powered facilities. Industry experts say the technology is still too costly for widespread use and will have to add more capacity before it is more widely installed.

“I think until an economics case can be made for storage, you won’t see it widely embraced,” Novachek said.

The project is slated to receive a $1 million grant from Minnesota’s Renewable Development Fund, pending approval by the Minnesota Public Utilities Commission.  Renewable energy standards in Minnesota say 20 percent of electricity must come from wind by the year 2020.

Today, wind energy produces about 1 percent of energy nationwide. Total renewable and non-renewable power capacity in the summer of 2006 was about 1 million megawatts, according to data from the federal Energy Information Administration.

In 2007, however, wind power accounted for 35 percent of the country’s new energy capacity, said Christine Real de Azua, spokeswoman for the wind power trade group American Wind Energy Association.  Real de Azua said power storage is not a prerequisite for wind to become a more dependable energy source, but it doesn’t hurt either.

“It’s only natural for utilities to be looking into that kind of research,” said Real de Azua. “Storage is an additional way of adding reliability.” 

Xcel is optimistic that the technology could play a more integral role in the future.  The batteries can last up to 15 years, are free of harmful emissions and can be recycled when they expire. Unlike lead acid batteries, the sodium and sulfur inside NaS batteries are non-corrosive and protected by a double-wall encasement. 

Each 1-megawatt unit takes up the space of a double-decker bus, which is comparable to the space of a power plant needed to produce a comparable amount of energy.

“It’s not going to be dramatically larger on a megawatt basis than the footprint of a power plant,” he said.

Xcel has tried other means of storing wind energy at its farms in Colorado. Water is pumped uphill to a reservoir in times of peak energy production and later released against an electromagnetic turbine much like a hydroelectric dam. 

Wind farms in Texas take advantage of the region’s geological formations to store compressed gas underground, which is released in times of low wind to produce electricity.

The NaS battery is already being used in other ways by the energy industry. Columbus, Ohio-based American Electric Power installed the first megawatt-class NaS battery to be used on the U.S. electric distribution system at a substation near Charleston, W.Va. 

The battery is charged during the night to provide energy during the day when demand is high, a strategy known as “peak shaving.” The batteries also provide relief for the company in times of power outages.

Last September, AEP ordered three more NaS batteries from NGK, part of the company’s goal to have 1,000 megawatts of storage capacity on its system in the next decade. The company hopes to install one of its new batteries on a wind farm, said Ali Nourai, manager of distributed resources at AEP.

“Our goal is to tie it up with wind, but we’re not ready yet,” Nourai said, citing the battery’s low capacity compared to that of AEP’s wind farms.  “We’re going to do it, just not this year or next year.”

Nourai said NaS batteries are reasonably space efficient and they are easily transported on the backs of flat bed trucks. The cost is the only thing preventing more utility companies from bringing them online. 

“The reasons they are not running with it like us is that storage is not cheap,” Nourai said.  “Most utilities look at the short-range payback, and this doesn’t pay back in two years.”

AEP estimates the cost of installing the three NaS batteries, including site preparation, equipment and control systems, will be about $27 million.

“This is what is strategically important to AEP,” Nourai said. “It’s the premium we are paying to ensure our future, but it’s not a short term economic benefit.”      
  
With pending legislation to set limits on carbon emissions, non-renewable energy like coal, oil and natural gas-fired electricity may soon become more expensive, however.  Utility companies will look to non-emitting, renewable sources like wind and solar power as a solution.

But this will only happen if they can provide sustained, reliable electricity.

“The wind industry is emphatically saying energy storage isn’t necessary, and that is not correct,” Nourai said.  “It [NaS] is not a curiosity anymore.”