# Flywheel energy storage

A recent innovation in electrical energy storage is the use of flywheel energy storage, also called flywheel power storage.

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 material. 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

[itex]\frac{1}{2}I\omega^2[itex]

Where [itex]I[itex] is the moment of inertia of the mass about the center of rotation and [itex]\omega[itex] (omega) is the angular velocity in radian units. A flywheel is more effective when its inertia is larger, as when its mass is located farther from the center of rotation either due to a more massive rim or due to a larger diameter. However, because increasing the rotational velocity of the flywheel results in a geometric increase in stored kinetic energy (rather than a linear growth when increasing the mass), modern research focuses on making flywheels spin as fast as possible.

Flywheels are not affected by temperature changes as chemical batteries are nor do they suffer from the memory effect. Moreover, they 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. Another advantage of flywheels is that by a simple measurement of the rotation speed it is possible to know the exact amount of energy stored. However, use of flywheel accumulators is currently hampered by the danger of explosive shattering of the massive wheel due to overload.

In the 1950s flywheel-powered buses were used in Yverdon, Switzerland, and there is ongoing research to make flywheel systems that are 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 low capacity, long charge times, heavy weight, and short usable lifetimes. Flywheel systems have also been used experimentally in small electric locomotives for shunting or switching.

In 1980s Soviet engineer Nourbey Gulia had been working on flywheel energy storage. His work resulted in many original solutions for wheel suspension, sealing the vacuum chamber, rotation rate decline compensator and hydraulic transmission. However, the primary advance was the composite flywheel capable of rotation rates exceeding 40,000 rpm, running for up to a week when not loaded, and resistant to explosive destruction. Gulia's "super flywheels" were tightly wound of metal or plastic tape, which, in addition to tensile strength higher than that of moulded steel, simply unwound inside the chamber, filling it and grinding to a stop. Gulia's first wheels were made of steel tape, but the latest models used kevlar filament, wound not unlike a bobbin of thread.

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; this is commonly referred to as "flywheel explosion" since wheel fragments can reach kinetic energy comparable to that of a cannon shell. Consequently, traditional flywheel systems require strong containment vessels as a safety precaution, which increases the total mass of the device. Fortunately, composite materials tend to disintegrate quickly once broken, and so instead of large chunks of high-velocity shrapnel one simply gets a containment vessel filled with red-hot sand (still, many customers of modern flywheel power storage systems prefer to have them embedded in the ground to halt any material that might escape the containment vessel). Gulia's tape flywheels did not require a heavy container and reportedly could be rewound and reused after a tape fracture.

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.

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