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. 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 system correspondingly results in an increase in the sp. A typical system consists of a flywheel supported by connected to a. The flywheel and sometimes motor–generator may be enclosed in a to reduce friction an. Compared with other ways to store electricity, FES systems have long lifetimes (lasting decades with little or no maintenance; full-cycle lifetimes quoted for flywheels range from in excess of 10, up to 10, cycles of use. In the 1950s, flywheel-powered buses, known as, were used in () and () and there is ongoing research to make flywheel systems that are smaller, lighter, cheaper and have a great.
