Storing sun and wind power

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An initial CAES facility in Alabama is using the stored compressed air in conjunction with a traditional natural gas turbine. Gas turbines use about two-thirds of their fuel to compress air, according to the Electricity Storage Association. The CAES system pre-compresses the air, making the turbines more efficient.

A similar system is being tested at the Iowa Stored Energy Park, where a group of municipally owned utilities intend to store compressed air in porous sandstone. It will be released, as needed, to help turn a traditional gas turbine.

The Iowa project is expected to cost $200 million and operate by 2011 with the capacity to store 200 megawatts of power, enough for several days.

Both the Iowa and Alabama installations can draw air to make power within 15 minutes and make a gas turbine roughly 40 percent more efficient.

General Compression's plan is to combine underground storage with on-turbine air compression. Its product designs call for an air compressor to be built into the nacelle of the turbine, the container behind the blades at the top of the tower.

Rather than feed the air to a traditional gas turbine to make electricity, General Compression envisions using another device called an expander, which, when combined with heat, will be able to generate power.

The company plans to make a 1.5-megawatt "dispatchable wind turbine" the size of turbines typically used in large wind farms. It intends to have prototype systems operating in 2010 and commercial products by 2012, according to David Marcus, who spoke at the Conference on Clean Energy in Boston last month.

One obvious restraint on CAES is available geological formations to store the compressed air. But Marcus said there are enough locations, such as depleted gas fields in Texas, to last for several years. Wind farms could be located directly above underground storage or, potentially, the compressed air could be sent through existing natural gas pipelines.

"(CAES) is probably the most viable, large-scale energy storage potential on the market right now," Corey said. "It's looking really attractive because the volume of storage is definitely available already. There are a lot of salt domes that will handle 300 psi (pounds per inch) of compressed air."

Industrial-size batteries
In the absence of underground storage to provide on-demand renewable energy, truck-size battery packs can do the trick.

Smart Storage Pty of Australia is seeking to combine the high-energy density of a supercapacitor with well-understood lead-acid batteries to make a single unit capable of storing large amounts of electricity.

The company claims that its batteries will be able to charge up to 50 percent more power and will last three times longer than other lead-acid batteries. Cleantech Ventures chose to invest in the company because the technology can be commercialized relatively easily.

"Our technology development path is directed towards manufacturing in existing lead-acid battery plants," Andrew Pickering, a principal at Cleantech Ventures, said in a statement.

VRB Power Systems has been testing its "flow battery" for several years with utilities in the U.S., Ireland, and Australia, where its batteries have been used to shore up wind power.

The company's vanadium-based batteries use two tanks of an electrolyte, which flows over a stack of fuel cells to generate electricity. The system is capable of storing hours or even days of power and can last longer than traditional lead-acid batteries, it says.

Ohio-based utility American earlier this year said it is purchasing sodium sulfur (NAS) batteries from NGK Insulators of Japan. One of these multi-megawatt batteries--part of a plan to install 25 megawatts of storage this decade at the utility--will be attached to a wind farm next year.

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