That's 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 isthat 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 $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, say 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 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 $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 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."
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.