The modern electric grid is getting some help from some admittedly old-fashioned technology.
Flywheels and compressed air don't sound as sexy as wind turbines and solar cells, but the latter probably won't go mainstream without the former.
"The growth of renewables has posed a problem," said Imre Gyuk, program manager for energy storage research at the United States Department of Energy (DOE). "It used to be that the load [demand on the electric grid] was unpredictable, and generation would try to follow it." As wind and solar installations proliferate, supply also has become unpredictable.
Electric High-Wire Act
For electric grid operators, "Now it is a balancing act," Gyuk said during the annual conference of the American Association for the Advancement of Science in Washington, D.C.
(Related from National Geographic magazine, "The Grid: Can We Fix the Infrastructure That Powers Our Lives?"
Because wind power is generated only when the wind blows, and solar energy isn’t collected on a cloudy day, technologies that can store extra power when it's not being used and mete it out when needed are becoming increasingly important.
"For this balancing act, storage is ideal," Gyuk said.
Power operators are using giant batteries to store electricity on the grid. But they can be expensive.
(Related: "Texas Pioneers Energy Storage in Giant Battery.")
So scientists are taking another look at old technologies that use basic principles of physics and the laws of motion to hold onto energy until it’s needed.
On one end of the storage spectrum are technologies that act like enormous batteries: storing large amounts of energy over hours—usually at night—and releasing it back onto the grid when demand is higher during the day.
On the other end are frequency regulation technologies that help cope with much smaller ups and downs over the course of a day. If hundreds of homes turn on their air conditioners at the same time, the demand for power goes up instantly.
Gene Hunt, of Tyngsboro, Massachusetts-based Beacon Power, notes that fossil fuel generators typically provide frequency regulation services, but they can’t do it quickly. It takes a big coal plant about five minutes to respond to a grid operator's request, but that’s an eternity in the world of electricity. The signals come in to power plants every four to six seconds. "It's like driving a cruise ship," he said. "By the time you've turned right, there might have been seven signals saying to turn left."
His company is building a more nimble frequency regulator based on flywheel technology in Stephentown, New York. A flywheel is a wheel or cylinder combined with a motor. The motor spins up the wheel with excess electricity from the grid. When power is needed, the process is reversed, and the wheel's spinning runs the motor, converting kinetic energy back to electricity. Beacon Power’s system aims to use 200 flywheels networked together to store excess energy from the grid and dispense it when needed. The flywheels can respond to a signal from the grid in four seconds.
According to a DOE paper, Gyuk said, using "fast storage" technologies such as Beacon's flywheels could reduce carbon output by 70 percent.
Just this week, Beacon announced it had reached a new milestone in the construction of the Stephentown facility: it's now running at half capacity, or 10 megawatts. The other 10 megawatts will come online in the second quarter of this year, Hunt said.
Into Not-So-Thin Air
Beacon's flywheel facility can dispense power for up to 15 minutes, but if a power plant wants to store energy for a longer period of time, it can do so by pumping water uphill. When the energy is needed later, the water flows back downhill, powering turbines that generate energy.
This so-called "pumped-storage hydroelectricity" is one of the most common forms of electricity storage now being used on the grid. But the DOE is looking into cheaper systems that rely on compressed air instead of water.
A compressed air energy storage (CAES) facility would use cheap, off-peak electricity to pump air into an underground cave or aquifer for storage when demand is low. The system then would mete out the air later when demand goes up, likely using a gas turbine to heat the air as it exits the cavern. The electricity that pumped the air in the first place would have been generated by wind, so "you're using off-peak wind, which might conceivably have been 'spilled' otherwise," said Gyuk.
Such a facility would require favorable geology, Gyuk said, but "every state in the union has a possibility for compressed air storage."
There are only two compressed air energy storage projects on the grid worldwide: one in Huntdorf, Germany, which has been running for more than three decades, and another in McIntosh, Alabama, which started up in 1991. But, with DOE funding, two more are being studied, in California and in New York. "We are doubling the world's capacity," Gyuk said. Studies have been completed for a third project, in Iowa, which is now seeking funding. And a cavern in Norton, Ohio, could theoretically store up to 2,700 megawatts of power.
Gyuk admits that most of these technologies are in their early stages, simply because storage hadn't been a concern until recently. "Ten years ago, there was no knowledge of storage among utilities. Two years ago, it exploded. Everybody's now interested. The aim of my program . . , is to make storage ubiquitous on the grid."
(Related: "Frozen Fish Help Germany Reel In Wind Power")