Energy storage

13 Jul 2009: Report
The Challenge for Green Energy:How To Store Excess Electricity

For years, the stumbling block for making renewable energy practical and dependable has been how to store electricity for days when the sun isn't shining and the wind isn't blowing. But new technologies suggest this goal may finally be within reach.by jon r. luoma“Why are we ignoring things we know? We know that the sun doesn’t always shine and that the wind doesn’t always blow.” So wrote former U.S. Energy Secretary James Schlesinger and Robert L. Hirsch last spring in the Washington Post, suggesting that because these key renewables produce power only intermittently, “solar and wind will probably only provide a modest percentage of future U.S. power.”Never mind that Schlesinger failed to disclose that he sits on the board of directors of Peabody Energy, the world’s largest private-sector coal company — a business with much to lose if a solar- and wind-powered future arrives. But at least he and his co-author got it partly right. The benefits from wind and solar are mostly intermittent — so far. But the pair somehow missed the fact that a furious search for practical, affordable electricity storage to beat that intermittence problem is well underway.


Cost will be key for determining which battery or other storage technologies prevail.

A more robust world of solar and wind power might be better served by some sort of giant battery — or, more likely, many of them, widely distributed. The basic concept has been proven. Since 2003, the world’s largest battery backup has been storing energy for an entire city: Fairbanks, Alaska. Isolated as it is, and not part of any regional electricity grid, the metropolitan area of about 100,000 residents needs an electricity backstop more than most: In its sub-zero winters, pipes can freeze solid in as little as two hours. Six years ago, the city installed a huge nickel-cadmium battery, the same technology used for years in laptop computers and other portable devices.Housed in a giant warehouse, the 1,300-metric ton battery is larger than a football field, and can crank out 40 million watts of power. Still, the Fairbanks battery provides only enough electricity for about 12,000 residents for seven minutes. That was enough to prevent 81 blackouts in the city in the battery’s first two years of operation.Yet effective storage of electricity from solar or wind arrays that generate power equivalent to one large coal plant implies batteries on a breathtaking scale — hundreds of units the size of the Fairbanks array.

One possible answer? In Japan, so-called “flow” batteries have been used for years to store backup power at industrial plants. Conventional batteries store energy in chemical form. With flow batteries, charged chemicals storage tanks, allowing still more chemical to be charged and pumped away, then pumped back into the active portion of the battery and drawn down as needed. One big advantage: Battery “size” can be expanded by simply adding more chemicals and more storage tanks. In 2003, the local utility on small King Island, off the coast of Australia, installed a large flow battery to sop up and later release excess power from a wind farm.

One big advantage: Battery “size” can be expanded by simply adding more chemicals and more storage tanks. In 2003, the local utility on small King Island, off the coast of Australia, installed a large flow battery to sop up and later release excess power from a wind farm.As with the alternative generation technologies, cost will be key for determining which battery or other storage technologies might prevail. Aside from such typical economic concerns as raw material and maintenance costs and durability, storage technologies all face some losses in “round-trip efficiency.” Inevitably, some energy is lost as it goes into storage, and more is lost as it comes out. Right now, hopes are riding high on lithium ion batteries, because they have impressive round-trip efficiencies, can pack in high densities of energy, and can charge and discharge thousands of times before becoming degraded. Because of those attributes, lithium-ion battery technology has become increasingly dominant in laptop computers and cell phones. On a far larger scale, a powerful lithium ion battery pack powers the pricey all-electric Tesla Roadster, and is slated to power the plug-in hybrid Chevy Volt next year.

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