As more and more of our electrical energy supply is furnished by "renewables," means of storing the output of such sources becomes increasingly important to the "smartness" of the grid. How can that grid best utilize photovoltaic energy that's available only during the day, or the ups and downs of wind turbine output during weather changes? Storage is the solution.
Said NEMA President and CEO Evan Gaddis in April 2013, "Energy storage is a key enabler of the Smart Grid.... By getting us beyond the need to generate electricity at the same moment as it is used, energy storage delivers greater system efficiency, enhances reliability and resiliency, and fosters integration of renewable energy."
In the most popular storage method, energy available when demand is low is used to pump water to a higher elevation, from which gravity returns it to drive generators when demand is high.
In a similar way, atmospheric air can be put into storage at high pressure, then released to an expansion turbine for generation during periods of peak demand. The term for this is Compressd Air Energy Storage, or CAES. Just as the pumped hydro system requires a suitable water reservoir, so CAES needs a large, reasonably airtight storage area.
Underground caverns, like the "salt domes" used for the U.S. Strategic Petroleum Reserve, are well-suited for compressed air storage. These are not widely available, however. As yet only two major CAES installations are in service.
First was the 290 MW plant in Huntorf, Germany, completed in 1979. In the U.S., the only existing facility is the 110 MW McIntosh plant in Alabama operated by PowerSouth. Stored air at 1,100 pounds per square inch can be released to reach full electrical output within 14 minutes. To supplement the air flow, the two-stage expansion turbine burns natural gas fuel at about one-third the rate used by a conventional gas turbine.
Back in 2008, one of the "top thinkers in energy storage" announced a collaboration with a New Jersey utility to "develop new ways to trap windgenerated power in underground reservoirs." A $20 million investment was expected. But those "new ways" were nothing more than the CAES technology described here, and there has been no further word of that project.
In 2009, a larger CAES development was planned to use a "sandstone aquifer" 3,000 feet underground at Dallas Center, Iowa. This $400 million "Iowa Stored Energy Park" project, to be completed during 2015, was undertaken by the Iowa Stored Energy Plant Agency, a consortium of 57 municipal utilities in four Midwestern states. Its compression (storage) cycle was expected to absorb the output of as many as 150 large wind turbines running at rated RPM. The 270 MW generated output would supply more than double the maximum electrical load of downtown Des Moines.
Said Bob Schultz, in behalf of the management consulting firm retained by ISEPA to assess project viability, "The economic studies . . . show that an innovative CAES project like this can be cost-effective compared to conventional generation alternatives, and supportive of additional wind energy development in the region."
Eventually, however, geological studies of the underground aquifer site showed that it was "not suitable for the scale of project that was envisioned." Added Schultz, ". . . due to geology limitations . . . the ISEPA members have easier, less expensive, and less risky conventional alternatives to meet their customers' future electric needs." Here again, the "conventional alternatives" typically involve gas-fired thermal generation that is far less expensive than it was a few years ago. So the Iowa project was terminated on July 28, 2011. The ISEPA member firms were "to determine at a later date whether they will consider or pursue additional projects."
Meanwhile, the Alabama facility remains the U.S.'s only CAES facility, exhibiting 95% reliability. Its salt dome reservoir contains 19 million cubic feet after being "solution mined" for 629 days to remove salt. *
By Richard L. Nailen, P.E., EA Engineering Editor
(c) 2013 Barks Publications
Original headline: Turning wind into wind
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