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First megawatt scale battery experiment -- Xcel tests wind storage economics

Xcel Energy, the largest utility purchaser of wind power in the United States, is the first entity in the country to experiment with megawatt-scale battery storage connected directly to a wind plant. It has installed a 1 MW sodium-sulphur battery alongside the 11 MW Minwind facility near Luverne, Minnesota. The trial, says Xcel, is aimed at mitigating wind variability, helping make wind power more dispatchable on demand.

Whether balancing variability with stored power instead of reserve power already on the system is a good idea is something Xcel is trying to find out. "While it is natural to think batteries or other storage systems might be needed to supply steady power, they are not needed to integrate wind energy into electric power systems," says Jeff Anthony of the American Wind Energy Association. "The power system essentially already has storage in the form of hydroelectric reservoirs, gas pipelines, gas storage facilities and coal piles that can provide energy when needed."

Xcel's Frank Novachek, who is running the experiment, says the project is not intended to boost integration of wind immediately, but is in preparation for when wind penetration on the Xcel system is so high to maybe justify battery storage. Minnesota requires renewables electricity to account for 30% of Xcel's retail sales by 2020. Of that, at least 25% must be generated by wind.

Novachek concedes that Xcel can comply with renewables laws without storage. "But we're so strategically placed in the country with such rich wind availability that we're hoping to be able to put even more wind on our system than the mandates require. We believe storage might help us get beyond that."

Preliminary testing of charge and discharge cycles have been underway for months and the system is functioning as planned, says Novachek. After a battery has charged, a steady megawatt of power output can flow for seven hours. The battery is 75% efficient, so 25% of stored power is lost. Novachek says the battery costs about $3 million, with additional costs for research and analysis he declines to specify.

Novachek says relatively small battery storage systems facilitate the flow of power to the grid by smoothing peaks and troughs in wind plant output. On a congested system short of grid capacity to balance wind variability with other power, a one megawatt battery can potentially facilitate many more megawatt of delivered wind power, he contends.

Xcel says its experiment is also aimed at developing wind-storage to make best use of peaking prices on the market for merchant plant selling their production directly into the wholesale market governed by the Midwest Independent System Operator (MISO), rather than through long term power purchase agreements. "As we get prices from MISO, at any given point in time we can automatically send those prices to the battery and program it to charge and discharge at certain points based on the right market signals," says Novachek. Should the battery be put into commercial operation, wholesale prices for output over the unit's lifetime would need to be consistently high enough for the unit to be profitable.

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