Energy storage flyers are useful in power conditioning applications, ie when there is a discrepancy between the power generated and the power required by the load. Two examples of this mismatch are a temporal mismatch and a mismatch in magnitude. The use of a steering wheel in a hybrid vehicle, for example, allows the engine to be designed to provide only the power needed to overcome steadystate losses and not have the inefficiencies that occur when the engine must also provide power for maximum acceleration .
Flywheel energy storage systems (FESS)use the input of electrical energy that is stored as kinetic energy. Kinetic energy can be described as "energy of motion", in this case the movement of a rotating mass, called a rotor. The rotor rotates in an almost frictionless enclosure. When reserve power is required in the short term because the grid power fluctuates or is lost, the inertia allows the rotor to continue to rotate and the resulting kinetic energy is converted into electricity. Most modern high-speed flywheel energy storage systems consist of a massive rotating cylinder (a rim attached to an axle) that rests on a stator by magnetically levitated bearings. To maintain efficiency, the flywheel system is operated in vacuum to reduce drag. The flywheel is connected to a motor-generator that interacts with the power grid via advanced power electronics. Some of the major advantages of steering wheel storage are low maintenance, long life (some flywheels are capable of more than 100,000 full depth discharge cycles and new configurations are capable of even more than that, over 175,000 Full depth discharge cycles), and an insignificant environmental impact. Flywheels can bridge the gap between short-circuit through power and long-term energy storage with excellent cyclic and load characteristics. Typically, users of high-speed flywheels should choose between two types of tires: solid steel or carbon composite. The choice of rim material will determine the cost, weight, size and performance of the system. Composite tires are lighter and stronger than steel, which means they can reach much higher rotational speeds. The amount of energy that can be stored in a flywheel is a function of the square of the RPM which makes the highest desirable rotational speeds. Today, high power flywheels are used in many aerospace and UPS applications. Currently 2 kW / 6 kWh systems are used in telecommunications applications. For industrial scale storage, an "inertia farm" approach can be used to store megawatts of electricity for applications requiring minutes of discharge duration.