The flywheel is the heart of the system for storing kinetic energy in the form of a rotating mass, the rotor. The flywheel uses electric energy input that is stored as “energy in motion. The rotor spins in a nearly frictionless enclosure. When short-term backup power is required because utility power fluctuates or is lost, inertia allows the rotor to continue spinning and the resulting kinetic energy is converted to electricity. Most modern high-speed flywheel energy storage systems consist of a massive rotating cylinder (a rim attached to a shaft, the rotor that is supported on a stator by magnetically levitated bearings. To maintain efficiency, the flywheel system is operated in a vacuum to reduce drag.
The flywheel is connected to a motor-generator that interacts with the utility grid through advanced power electronics. In addition to the bearings, a typical flywheel system also consists of an electric motor generator and a rotor. The design of the rotor is important in determining the effectiveness and efficiency of the system. The shaft of the flywheel is connected to an electric motor generator, which is levitated in an electromagnetic field allowing the system to cycle on demand.
Typically, high-speed flywheels have either one of two types of rims: solid steel or carbon composite. The choice of rim material will determine the system cost, weight, size, and performance. Composite rims are both lighter and stronger than steel, which means that they can achieve much higher rotational speeds. The amount of energy that can be stored in a flywheel is a function of the square of the RPM making higher rotational speeds desirable.
Some of the key advantages of flywheel energy storage are low maintenance, long life (some flywheels are capable of well over 100,000 full depth of discharge cycles and the newest configurations are capable of even more than that, greater than 175,000 full depth of discharge cycles), and negligible environmental impact.
When called upon, the integrated flywheel discharges energy by using the stored energy in the rotating mass to produce output power. When the device functions as a motor, energy is supplied to the flywheel; when the device acts as the generator, energy is stored. It is often called a "flywheel battery." The amount of energy available and its duration is governed by the mass and speed of the flywheel.
Flywheels can bridge the gap between short-term ride-through power and long-term energy storage with excellent cyclic and load following characteristics. Currently, high-power flywheels are used in many aerospace and UPS applications. Today 2 KWh to 6 KWh systems are being used in telecommunications applications. For utility-scale storage a ‘flywheel farm’ approach can be used to store megawatts of electricity for applications needing minutes of discharge duration.
Flywheel technology has many beneficial properties that improve the electric grid. A flywheel can capture energy from intermittent energy sources over time, and deliver a continuous supply of uninterrupted power to the grid. Flywheels also can respond to grid signals instantly, thus delivering frequency regulation and electricity quality improvements.
A traditional fossil fuel generator responds to a grid signal in a few minutes or hours, and must be on and idling to ramp up or down as needed. A flywheel energy storage system is always on and can respond in seconds instead of minutes, with improved accuracy and no need to inefficiently ramp to achieve the right output.