FES is mainly used for power management rather than longer-term energy storage. FES systems store electric energy via kinetic energy by spinning a rotor in a frictionless enclosure.
The rotor is sped up or down to shift energy to or from the grid, which steadies the power supply.
There are two categories of FES: low-speed and high-speed. These systems rotate at rates up to 10,000 and 100,000 RPM (rotations per minute), respectively, and are best used for high power/low energy applications.