A typical system consists of a massive flywheel disc suspended by magnetic bearings inside a vacuum chamber to reduce friction, connected to a combination electric motor/electric generator. The wheels are generally made of high-tensile-strength fibers (such as carbon fibers) embedded in epoxy resins, or some other high-strength composite. The system stores power by driving the motor to increase the speed of the spinning flywheel. The system provides power by using the momentum of the flywheel to power the generator. The kinetic energy stored in a rotating flywheel is
Flywheels are not affected by temperature changes as chemical batteries are, and are not as limited in the amount of energy they can hold. They are also less potentially damaging to the environment, being made of largely inert or benign materials. In the 1950s flywheel powered buses where used in Yverdon, Switzerland, and there is ongoing research to make flywheel systems smaller, lighter, cheaper, and have a greater capacity. It is hoped that flywheel systems can replace conventional chemical batteries for mobile applications, such as for electric vehicles. Proposed flywheel systems would eliminate many of the disadvantages of existing battery power systems, such as capacity, charge time, and weight.
Flywheel power storage systems in current production (2001) have storage capacities comparable to batteries and faster discharge rates. They are mainly used to provide load-leveling for large battery systems, such as an Uninterruptible Power Supply.
One of the primary limits to flywheel design is the tensile strength of the material used for the disc. Generally speaking, the stronger the disc, the faster it may be spun, and the more energy the system can store. When the tensile strength of a flywheel is exceeded the flywheel will shatter, releasing all of its stored energy at once; flywheel systems require strong containment vessels as a safety precaution. Fortunately, composite materials tend to disintegrate quickly once broken, and so rather than producing large chunks of high-velocity shrapnel one instead simply gets a containment vessel filled with red-hot sand.
Further improvements in superconductors may help eliminate eddy current losses in existing magnetic bearing designs. Even without such improvements, however, modern flywheels can have a zero-load rundown time measurable in years.