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Contemplating compressed air storage

Shell believes that compressed air energy storage for wind plant could be viable if economies of scale can be reached. It is investing $2 billion in a wind plant of 1 GW to 1.5 GW rated capacity, or more, in the windy Texas Panhandle, backed up by another $2 billion in a compressed air energy storage system with matching capacity using underground caverns. The region has become a Mecca for wind development because of its strong winds, but the transmission grid lacks the capacity to transfer all the electricity produced to load centres (Windpower Monthly, November 2008).

Storing energy underground in periods when more electricity is being produced than can be exported to customers could be an economic answer to the problem, feels Shell. Power from wind plant would be used to compress air into caverns carved out of salt beds, with release of it later at high pressure to help turn gas-powered turbines when demand, and prices, recover in periods of low wind power production.

Shell says the site, in Briscoe County, is ideal for the experimental technology, which it hopes will be cheaper for the power system as a whole than building out transmission capacity to import conventional generation when winds drop and to export wind power when it is not needed locally. Using stored wind energy will enable Shell to reduce gas consumption in connected thermal plant by two-thirds, it says, though has no further details, saying its studies are still at an early stage.

The key to the whole idea, says Shell WindEnergy's Dick Williams, is finding a suitable salt cavern somewhere between 600 to 900 metres underground. Fresh water would be injected to dissolve the salt into brine that can be pumped out, making room for air. Shell's studies suggest that a wind farm needs capacity of at least 2 GW before compressed air storage becomes cost effective. The Briscoe County project, still several years away, could reach 3 GW.

Williams concedes that the sums are large. "It is very expensive," he says. "You're going to have to have the brine-handling facilities. You're going to have to have the compressors, generators, the substation to put it back on grid. When you add all that up, it can be a pretty sizeable number."

The particular structure of the Texas electricity market may make storage viable. As it does not have the advantage of being linked to other major power systems, the impact of wind fluctuations is more acute: electricity purchase prices in Texas fluctuate with the wind. High prices are paid when wind production is low and demand is high, while low prices prevail in windy weather. Studying the economics, says Shell, is a key part of the project.

Shell says that if anyone can make the technology work, it can. "The beauty of being a part of Shell is that we have plenty of down-hole engineers here and plenty of down-hole consultants," Williams says, noting he has personal knowledge of salt cavern storage. "We have a little bit of expertise and experience we can apply to this."

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