Energy storage: As the flywheel spins, it stores kinetic energy. When energy is extracted from the system, the flywheel's rotational speed is reduced as a consequence of the principle of conservation of energy; adding energy to the. Flywheel Energy Storage Systems (FESS) rely on a mechanical working principle: An electric motor is used to spin a rotor of high inertia up to 20,000-50,000 rpm. It typically is used to stabilize to some degree power grids, to help them stay on the grid frequency, and to. Flywheel energy storage, an innovative mechanical energy storage method, will hold a significant position in the future energy storage field due to its unique energy conversion principles and wide application prospects.