https://www.youtube.com/watch?v=8X2U7bDNcPM
A flywheel stores energy as rotational kinetic energy: a rotor spun to high speed in a low-friction enclosure can later drive a generator to return electricity.
Modern “electro-mechanical batteries” pair composite rotors, magnetic bearings, and vacuum housings for grid services such as short-duration storage and frequency support alongside renewables.