Xcel Energy has taken a big step toward the pot of gold at the end of the battery storage of renewable energy rainbow with the successful testing of a battery big enough to store wind power-generated electricity for 500 homes.

"This is critical technology," Forbes Black, a battery technology and energy storage systems engineer, said of Xcel's work. "We're going to have to figure out ways to store energy for when renewables are not generating and this sounds like a really good step in that direction."

Xcel is testing a sodium sulfur (NaS) battery at an 11-megawatt (MW) wind farm near Luverne, Minnesota. The 80-ton battery made by NGK Insulators Ltd. of Japan is a constellation of twenty 50-kilowatt modules the size of two semi trailers. It stores 7.2 megawatt-hours of electricity in total and can instantly absorb or generate one megawatt of power.

The sodium sulfur battery, Black observed, is likely both simple and inexpensive. The key, he said, "is storing the most kilowatt-hours per dollar that you possibly can. You need big cheap batteries."

"My doubts," he added, "are scale-related. I am not sure any battery technology right now can be used for grid-scale energy storage. There is a huge expense related to that."

Frank Novachek, Xcel Energy's Director of Corporate Planning and the manager of the wind-to-battery storage project, explained how the preliminary results of the field test that began in October 2008 unequivocally prove the technical capability of the batteries to store wind. They also confirm Black's doubts.

"We don't know what the right battery price is," Novachek said about the use of battery storage for shifting wind energy from off-peak availability (when electricity is least expensive) to peak demand availability (when electricity is most expensive). "But we do know it's too high for looking at the time-shifting aspects alone."

But time shifting is only one of battery storage's many uses. Deferring the need for a transmission/distribution system upgrade is a much more cost-effective application, according to Novachek. Renewable capacity is going to waste and being left uninstalled in resource-rich places around the U.S. because new transmission is so expensive and red-tape-intensive. But, in West Virginia, AEP, Novachek said, has relieved a distribution bottleneck sufficiently to have deferred the need for new wires through the use of a one-megawatt sodium sulfur battery.

Battery storage is also cost-effective when it is used to ease wind's variability. "You use the battery to slow down any changes in the output of the wind farm," Novachek explained. A small amount of instantly available capacity allows more time for secondary generation sources to ramp. "If the wind picks up really quickly, the battery would start charging," he said. "If the wind drops off, you use the battery to discharge to the grid."

There are other ways grid operators can use battery storage to integrate wind and solar more efficiently into the transmission system. One is a crucial but very technical use of battery storage that has to do with balancing voltage. Another is the precise mixing of battery-stored electricity into minute-by-minute grid supply fluctuations.

Xcel and its partners at the University of Minnesota have only just begun to understand the ideal ratio of battery storage capability to total project capacity needed for effective ramp rate control. "It's somewhere between one and five megawatts for this wind farm," Novachek said of the ratio derived from the eleven-megawatt Minnesota wind farm field test. "We were only able to test one- and five-megawatt capacities. I think it's probably closer to one. I don't think you need half the size of the wind farm."

The right ratio also depends on what the project and grid operators want the storage to accomplish.

"In Japan," Novachek explained, "NGK -- the people who provided our sodium sulfur battery -- have a facility where they have a 30-megawatt battery tied to a 50-megawatt wind farm."

The Japanese transmission system does not integrate wind and other variable renewables like the U.S. system will. "The island's grid system will only accept power if it's guaranteed at a constant output," Novachek said. "So they charge the batteries at night and then they use the batteries for a constant output during the day. As the wind goes down and up, the batteries go up and down to make sure the sum of their output is constant."

In the U.S., as long as there is a mix of fossil, nuclear and renewable energies being integrated, grid operators will be more concerned with controlling the rate at which the electricity supply changes from one source to another. "If you can control the rate of change to where the overall system can respond," Novachek said, "that will facilitate wind integration."

The most sophisticated uses of battery storage will come, Novachek contends, when wind and the other renewables are "somewhere greater than 30 percent" of the power on the grid. That allows for the several years Novachek foresees as still necessary to study the technology, understand its parameters and bring down the costs about which Black was rightly concerned.

"The functionality is there to do the things we need to have done for both solar and wind variability," Novachek said. "The big issues out there now are the technical issues associated with getting the cost down." He isn't certain when that can happen. "Every situation is going to have its own price point."