R. Colin Johnson
EE Times
(07/02/2008 10:34 AM EDT)

PORTLAND, Ore. — An engineer at Sandia National Laboratories thinks compressed air stored in underground caverns could help cut in half the cost of electricity from generators.

During off-peak hours compressors pressurize underground caverns to as high as 1,200 pounds per square inch (compared to 15 psi at sea level). During peak demand hours the compressed air is used to make electricity generators more efficient.

"A [compressed air] system is like a really big battery," said Georgianne Peek. "But instead of an electro-chemical process to store electricity, we are using compressed air for savings of up to 50 percent."

Peek is project manager for Sandia National Labs at the Iowa Stored Energy Park, which includes more than 100 municipal utilities in Iowa, Minnesota and the Dakotas. Together, they account for a nominal 268 megawatt to 13,400 megawatts per hour of compressed air energy storage with a 50-hour storage capacity.

During the day, the Iowa facility harnesses energy from wind farms; at night it uses compressed air in an underground cavern. The process achieves savings as high as $5 million annually for each participating municipality.

Sandia researchers are now studying core samples from Iowa to make sure the rock cavern is airtight. Sandia specialist Steve Bauer is analyzing the geologic, hydrologic and rock physics using geomechanics to design the Iowa underground air storage cavern, thereby determining how much pressure is needed and whether too much is leaking.

"A small amount of air does leak [and] move away from the storage volume," said Peek. "However, the cap rock--the sealing rock forming the top and sides of the containment--has very low permeability, so this leakage should be small."

Recent core samples should tell Bauer's team whether the Iowa site is suitable for the compressed air system. If all goes as planned, the Iowa facility will go online in 2012. '

The Iowa facility and several others in the U.S. store compressed air during off-peak hours, then recover the stored energy by day with a compressed-air-driven heat recuperators that increase the fuel efficiency of gas turbines.

Caverns must be airtight, and must have the right dimensions to enable enough compressed air to be stored. "Pressure depends on a couple items: the depth of the geologic formation and how much energy you want to store," said Peek.

The compressed air is then mixed with fuel which is warmed by a recuperator, harvesting the exhaust heat by warming the compressed fuel in a series of parallel vents. The preheated gas more efficiently boosts the flash point, thereby making gas turbines more fuel efficient.

"Electricity compresses the air and the energy is stored as compressed air. Then the compressed air is used to run a natural gas turbine to regenerate the electricity to be put back into the grid," said Peek. "All natural gas turbine generators have a compression cycle.'However, the turbine generators used in a [compressed air energy] system are much more efficient because they use pre-compressed air."

Peek eventually wants to build an all-green compressed air facilty that would include an electric generator running exclusively on compressed air. A European Union research project is attempting to store the heat of compression and use it along with the compressed air in a steam electric generator, said Peek.'The technique is called "advance adiabatics, and would eliminate the need for fossil fuel," Peek added.

The EU effort would use an adiabatic generator, the opposite of a recuperator, to transform heat to fuel. It uses sliding-pressure turbines to harness temperature changes caused by compression to drive a steam-powered electricity generator.

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