When the sun sets or the wind dies down, renewable energy sources get a lot less reliable. That is why large-scale electricity storage technologies are being pursued by a number of energy technology companies.
Although none of these technologies are commonplace, they could fill a niche in the booming solar and wind industries and make renewable energy more economically compelling.
One company with a novel approach is Massachusetts-based start-up General Compression, which is building a wind turbine that compresses air and stores it underground in caves or other geological structures. The compressed air is drawn when needed and expanded to drive electricity generators.
A document on General Compression's Web site last month that has since been removed indicated it was looking for US$30 million this fall to finance its expansion.
Earlier this month, Australian firm Cleantech Ventures made a "significant investment" in Smart Storage Pty to commercialize a hybrid battery for off-grid storage. The "ultrabattery" technology stems from research at Australia's national science agency.
Flywheels from companies like Beacon Power have been approved by regulators for maintaining a steady frequency over the grid as power demand fluctuates minute to minute. The flywheels--essentially a huge rotating cylinder--are designed to absorb energy when the grid is making excess energy and feed the energy back to meet shortfalls in supply.
For several hours of storage, utilities are testing different battery technologies. Each of these techniques has different purposes and drawbacks but are getting serious consideration, said experts.
"There's been more going on in energy storage in the last six months than in decades (prior)," said Garth Corey, an electrical storage consultant and former Sandia National Labs scientist. "There are true benefits, but we haven't had the tools to do it."
An energy bill, now making its way through the U.S. Congress, may include a provision that would boost the amount of renewable energy that electric utilities need to generate.
But even without higher renewable energy mandates, large-scale storage stands to make wind and solar--two of the fastest sources of power generation--more versatile.
Bottling wind power
Right now, electricity generators supply electricity to meet shifting demand. But unlike natural gas or coal-fired power plants, utilities cannot count on a wind farm or solar array to meet its peak demand needs, typically in the middle of the day.
One of the most promising techniques for addressing "peak power" is called compressed air energy storage (CAES), a storage method that General Compression intends to plug into.
This technique allows utilities to store hours' and even weeks' worth of electricity. The idea is to use power generators to compress air during off-peak hours, like during the middle of the night, and then tap into it later in the day, when they can command a higher price for electricity.
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 US$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.
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