Pumped hydroelectric stations use falling water to make electricity. An example of this can be seen at Raccoon Mountain in Tennessee. At the foot of the mountain, the Tennessee Valley Authority (TVA) made a lake by siphoning some of the Tennessee River.
When customers aren't using much electricity, TVA diverts electricity from other power stations to a power house inside the mountain. The electricity spins the house's turbines backwards, pushing lake water up a tunnel in the mountain to the top. After 28 hours, the upper basin is full. To make electricity, TVA opens a drain in the upper basin. Water falls straight through the center of the mountain and spins the turbines forward, generating electricity. It falls for 22 hours, steadily outputting 1,600 megawatts of electricity, matching the output of a large coal-fired plant. TVA adds this electricity to the contribution from its other plants on days of high demand [source: TVA].
Pumped hydroelectric stations are operating worldwide, outputting between 200 megawatts and 2,000 megawatts of power on peak demand days [source: Cole]. They emit no air pollution, and once charged, are online in 15 minutes, faster and greener than a peaker plant. The only problem is "we're running out of good sites for it," says Gyuk.
Compressed air energy storage (CAES) is storage for natural-gas power plants. Normally, these plants burn natural gas to heat air, which pushes a turbine in a generator. When natural gas plants are near an underground hole, like a cavern or old mine, they can use CAES. On slow days, the plant can make electricity to run a compressor that compresses outside air and shoves it into the hole underground. On days when customers need maximum electricity, the power plant can let the compressed air rush out against the turbine, pushing it, along with the normal heated air. This compressed air can help for hours, steadily adding 25 megawatts to 2,700 megawatts of electricity to the plant's output on peak demand days [source: Cole].
Keep reading to learn where else we can store energy on the grid.