Compressed air boosts wind power23 May 2013
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If enough wind energy could be stored deep underground in porous rocks for later use, it may be able to power about 85,000 homes each month. Two unique methods for this have been determined by researchers from Pacific Northwest National Laboratory (PNNL) and Bonneville Power Administration (BPA) in the US. They have also identified two locations in which they will be suitable.
Compressed air energy storage plants could help save the region's abundant wind power for times when demand is high and power supplies are more strained. These plants can also switch between energy storage and power generation within minutes, providing flexibility to balance the region's highly variable wind energy generation throughout the day.
All compressed air energy storage plants work under the same basic premise. When power is abundant, it's drawn from the electric grid and used to power a large air compressor, which pushes pressurized air into an underground geologic storage structure. Later, when power demand is high, the stored air is released back up to the surface, where it is heated and rushes through turbines to generate electricity. Compressed air energy storage plants can re-generate as much as 80 per cent of the electricity they take in.
The world's two existing compressed air energy storage plants are in Alabama, USA and Germany. They use man-made salt caverns to store excess electricity. The PNNL-BPA study examined a different approach: using natural, porous rock reservoirs that are deep underground to store renewable energy.
Interest in the technology has increased greatly in the past decade as utilities and others seek better ways to integrate renewable energy onto the power grid. About 13 per cent, or nearly 8,600MW, of the Northwest's power supply comes from of wind. This prompted BPA and PNNL to investigate whether the technology could be used in the Northwest.
To find potential sites, the research team reviewed the Columbia Plateau Province, a thick layer of volcanic basalt rock that covers much of the region. The team looked for underground basalt reservoirs that were at least 457m deep, 9m thick and close to high-voltage transmission lines, among other criteria.
They then examined public data from wells drilled for gas exploration or research at the Hanford Site in southeastern Washington. Well data was plugged into PNNL's STOMP computer model, which simulates the movement of fluids below ground, to determine how much air the various sites under consideration could reliably hold and return to the surface.
Figure 1: The Columbia Hills site could house a 207MW conventional compressed air energy storage facility.
Two locations, different methods
Analysis identified two particularly promising locations in eastern Washington in the US. One location was dubbed the Columbia Hills Site and the second is called the Yakima Minerals Site.
But the research team determined the two sites are suitable for two very different kinds of compressed air energy storage facilities. The Columbia Hills Site could access a nearby natural gas pipeline, making it a good fit for a conventional compressed air energy facility. Such a conventional facility would burn a small amount of natural gas to heat compressed air that's released from underground storage. The heated air would then generate more than twice the power than a typical natural gas power plant.
The Yakima Minerals Site, however, doesn't have easy access to natural gas. So the research team devised a different kind of compressed air energy storage facility: one that uses geothermal energy. This hybrid facility would extract geothermal heat from deep underground to power a chiller that would cool the facility's air compressors, making them more efficient. Geothermal energy would also re-heat the air as it returns to the surface.
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