Flywheel

Flywheels are powered by electricity and can store electrical energy as rotating inertia. When charged, the flywheel accelerates; when discharged, the flywheel slows and the kinetic energy is converted back into electricity by a generator. Air friction is usually reduced by placing the flywheel in a vacuum, and rotating friction is minimised by using magnetically levitated bearings, resulting in high efficiencies of 90-95%. The energy output tends to be low, so flywheels are limited to short-term applications such as spinning reserve for grid frequency regulation. The fast response time, low maintenance requirements and very high cycle life (i.e. can undergo a large number of charge and discharge cycles) are also important characteristics for short-term applications to maintain power quality. In addition, flywheel storage can provide both up and down regulation during the same time period, although not simultaneously.