Now it's time to look at storage that supplies a big burst of big electricity or less for longer. These systems can't send big electricity to customers all day, like pumped hydroelectric and CAES can.
Flywheels store energy by spinning. The fastest ones consist of a motor, a levitating magnet, a vacuum to nix friction and a shell for safety. When there's extra electricity available on the grid, it can run the motor, which spins the magnet. When electricity is needed, the flywheels can spin it out in minutes to hours, as the situation requires.
On the electric grid, flywheels make good quality controllers. They're good at steadying frequency, which, as we've mentioned, wobbles above and below 60 hertz in the U.S. today. It spikes when utilities make more electricity than customers use and dips when utilities make less. Flywheels change the situation because ISOs can control them directly -- eventually, they'll be automatic -- so that no one has to call Jane at power plant A and wait for her to raise or lower generation to correct the frequency problem. With fast response, the frequency can be leveled before the customer feels it. In fact, several U.S. I.S.O.s are testing flywheel pads [source: Beacon Power 1, Beacon Power 2, Beacon Power 3].
Another use for flywheels is steadying voltage on the grid. What could possibly change the voltage on those sturdy high-voltage lines? Try domino effects from power outages, downed trees and electric trains. When subway or light rail trains brake, they generate electricity, raising voltage and making current surge locally. When trains accelerate out of the station, they draw electricity, making the voltage dip and sucking current from elsewhere. Flywheels can absorb and release the current, leaving the rest of the grid undisturbed. In fact, they've been tested on New York City's subway trains [source: Kennedy].
Flywheels are also great for wind farms, where they can spin up extra electricity during gusts and spit it out during die-downs, so customers don't suffer the fluctuations.
Supercapacitors, even speedier than flywheels, store energy by separating charges. They're "super" because they store more energy than traditional capacitors, but they work the same way. When there's extra electricity, it can be used to push charges off of some metal plates and onto others, leaving some positively and others negatively charged. When electricity is needed, the plates neutralize, and charge flows, making a current. In Madrid, Beijing and other cities, cabinets full of supercapacitors buffer electric trains [source: Siemens].
Superconducting magnetic energy storage, or SMES, is another way to get rid of voltage dips and spikes on the grid. During spikes, loops of wire take up extra current, and during dips, the loops return the current to the grid. Because the wire has almost no resistance, it stores current with almost no loss.
Next up -- power storage systems many of us use on a daily basis: batteries.