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Unlike most commodities, electricity typically must be consumed when it is produced, so saving it up for a rainy—or exceptionally hot—day usually isn’t an option. However, several utilities now are studying new options for energy storage that could help ease peak-demand crunches and provide a boost to wind and other renewable-energy technologies.
Peak-demand periods have long frustrated electric-utility planners. During these few days—or, in some cases, a few hours—each year, consumers pull just enough electricity out of the grid to force utilities to start up expensive peaking generating plants or turn to even pricier outside suppliers to meet their customers’ needs. The alternative could be rolling brownouts to ensure overall transmission- and distribution-system safety.
Approaching this problem from the demand side, utilities are pursuing “smart-grid” technologies that would allow them to manage customers’ equipment remotely, allowing them to adjust air conditioners, dryers and refrigerators for short periods to shave the tops off those expensive peaks. Energy storage tackles the problem from the supply side, banking electricity when production costs are lower and bringing it online, as needed.
“The basic premise is the arbitrage of off-peak and on-peak energy costs,” said Kent Holst, development director of the Iowa Stored Energy Project (ISEP). Iowa’s municipal utilities are backing this effort, which will use grid-driven electric motors to compress air into an underground storage area during low-use periods. When demand rises, the air will be released and heated by an attached natural-gas unit to power two 134-megawatt (MW) generators. The design will use less than half the natural gas of a standard turbine generator and could eventually run on biofuels.
“When the economics were looked at, it came out to be a very favorable proposition,” Holst said.
Only one other such facility exists in the United States. It’s located in McIntosh, Ala., and operated by the Alabama Electric Coop.
When the ISEP comes online in approximately 2013, its output will address intermediate demand needs of Iowa’s municipal utilities—that portion of demand that falls above baseload requirements on a regular basis. Utilities seeking smaller-scale resources to meet peaking or power-smoothing needs are considering a battery approach that’s been used in Japan for 13 years.
Batteries: an ever-ready solution?
American Electric Power (AEP) is the first utility to bring these batteries online. The company will be adding at least two sodium sulfur batteries to its service territories this year. The 2MW units are the first step in a plan to have 2MW of battery storage in place by the end of the decade and 1,000MW in service by 2020.
Ali Nauri, strategic technology consultant with AEP, said the company now is seeking a small-scale wind project to test the batteries’ energy-storage capabilities. In Japan, where the batteries have been operating for more than a decade, storage is a mandatory component of any wind-power installation to even out supplies. In the United States, where more than 5,000 MW of wind capacity was added in 2007, intermittent wind supplies still pose challenges for facility developers.
“If you want to grow that sector, you have to have storage,” Nauri said.
However, large-scale wind farms require much larger storage capacity than batteries can supply affordably. A Newton, Mass.-based company called General Compression is adapting compressed-air technology to help wind-facility operators gain greater return on their infrastructure investment.
“One of the things holding back wind power is the transmission,” said Carlos Pineda, General Compression’s senior development officer. A wind farm rarely generates its rated capacity of electricity, he said, but regulators require its transmission connections be sized to that larger figure. A compressed-air generator could supplement wind-produced electricity to maximize that transmission capacity.
“You can raise the capacity of your wind resource by orders of magnitude,” Pineda said.
The General Compression approach is feasible for wind facilities that are sited over geologically appropriate settings, which could include depleted natural gas caverns, mined-out salt deposits or aquifers. The system’s design is similar to that of the ISEP plant, with natural gas used at the extraction end to heat the released air and drive a generating turbine. However, instead of drawing electricity from the grid to run air compressors, General Compression’s plan uses wind turbines.
The company’s Dispatchable Wind Power System (DWPS) is built around a proprietary compressor, for which General Compression holds an exclusive worldwide license in wind-power applications. This wind-driven compressor would replace the turbine’s generator, so air would be forced underground with no added assistance from the grid. Compressor-equipped turbines could take the place of some or all standard-model machines, based on the project’s goals.
The DWPS now is in development, and General Compression anticipates a utility-scale prototype will be operating by 2010, with projected commercial operation in 2012.
ROSS is a freelance writer located in Brewster, Mass. He can be reached at [email protected].