Flywheel energy storage converts electrical energy into kinetic energy by accelerating a rotor, and recovers it via regenerative braking. Modern systems use magnetic bearings, vacuum chambers, and composite rotors to minimize friction and achieve high rotational speeds. Applications include power quality (frequency regulation, voltage support), uninterruptible power supply (UPS), and short-duration grid balancing. Limited commercialization exists; Beacon Power and others have deployed grid-scale flywheels.
Energy storage faces trade-offs between power, energy, and cost. Flywheels excel at high power and fast response with long cycle life; energy density is lower than batteries. Challenges include cost per kilowatt-hour, bearing and vacuum maintenance, and competition from lithium-ion for frequency regulation. Flywheel systems remain niche for power-quality and short-duration applications.