Flywheel energy storage (FES) 101

Flywheel energy storage (FES) works by accelerating a rotor (flywheel) to a very high speed and maintaining the energy in the system as rotational energy. The energy is converted back by slowing down the flywheel.

Most FES systems use electricity to accelerate and decelerate the flywheel, but devices that directly use mechanical energy are being developed.[1]

Advanced FES systems have rotors made of high strength carbon-composite filaments that spin at speeds from 20,000 to over 50,000 rpm [2] in a vacuum enclosure and use magnetic bearings. Such flywheels can come up to speed in a matter of minutes -- much quicker than some other forms of energy storage.[2]

Source: Wikipedia