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: For a directory listing of sites, see Directory:Flywheels

A flywheel is a mechanical battery (a mechanical means of storing energy - simply a mass rotating about an axis).



Flywheels store energy mechanically in the form of kinetic energy. Kinetic energy is energy of motion. The kinetic energy of an object is the energy it possesses because of its motion. As energy is transferred into a flywheel, as it spins, it builds up kinetic energy that can be released when necessary. The flywheel has been used since ancient times, the most common traditional example being the potter's wheel. In the Industrial Revolution, James Watt contributed to the development of the flywheel in the steam engine, and his contemporary James Pickard used a flywheel combined with a crank to transform reciprocating into rotary motion.

A flywheel is a heavy rotating disk used as a storage device for kinetic energy. They come as an alternative energy storage device. Flywheels resist changes in their rotational speed, which helps steady the rotation of the shaft when an uneven torque is exerted on it by its power source such as a piston-based, (reciprocating) engine, or when the load placed on it is intermittent (such as a piston-based pump). Flywheels can also be used by small motors to store up energy over a long period of time and then release it over a shorter period of time, temporarily magnifying its power output for that brief period. Recently, flywheels have become the subject of extensive research as power storage devices see flywheel energy storage.

How They Function

Like a wound up rubber band, a flywheel stores energy. When the energy is neeeded, the flywheel slows down its rotation and releases the stored energy. A momentum wheel is a type of flywheel useful in satellite pointing operations, in which the flywheels are used to point the satellite's instruments in the correct directions without the use of thrusters.

The kinetic energy stored in a rotating flywheel is

: E = \frac{1}{2} I \omega^2

where I\, is the moment of inertia of the mass about the center of rotation and

\omega\,(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. Note the similarity of the above formula to the kinetic energy formula E = mv^2/2\,, where linear velocity v\, is comparable to the rotational velocity, and the mass m\, is comparable to the rotational inertia.

Flywheels can take an electrical input to accelerate the rotor up to speed by using the built-in motor, and return the electrical energy by using this same motor as a generator. Flywheels are one of the oldest and most common mechanical devises in existence. They may still prove to serve us as an important component on tomorrow's vehicles and future energy needs. Flywheels are one of the most promising technologies for replacing conventional lead acid batteries as energy storage systems for a variety of applications, including automobiles, economical rural electrification systems, and stand-alone, remote power units commonly used in the telecommunications industry. Recent advances in the mechanical properties of composites has rekindled interest in using the inertia of a spinning wheel to store energy.

In addition to energy density, flywheel energy storage systems (FES) also offer several important advantages over chemical energy storage. The rate at which energy can be exchanged into or out of the battery is limited only by the motor--generator design. Therefore, it is possible to withdraw large amounts of energy in a far shorter time than with traditional chemical batteries. Indeed, research into exploiting this property of FES systems to get short, intense bursts of energy is ongoing with the most notable projects being a magnetic tank gun and a fusion ignition system. Of course it is also possible to quickly charge FES batteries making them desirable for application in electric cars where the charge time could be dropped from a matter of hours to a matter of minutes.

Image:Spoked flywheel animation.gif

Flywheels store energy very efficiently (high turn-around efficiency) and have the potential for very high specific power compared with batteries. Flywheels have very high output potential and relatively long life. Flywheels are relatively unaffected by ambient temperature extremes.


Current flywheels have low specific energy. There are safety concerns associated with flywheels due to their high speed rotor and the possibility of it breaking loose and releasing all of it's energy in an uncontrolled manner. There are losses in converting electrical energy to mechanical and back to electrical. Flywheels are a less mature technology than chemical batteries, and the current cost is too high to make them competitive in the market.

Role In Alternative Energy

Flywheels already play a role in some futuristic engine designs and are presently used by some utilities to temporarily store energy that is used to provide electricity when demand spikes. Flywheels might one day play a role in conjunction with other energy storage devices, such as capacitors and batteries, to deliver the energy necessary to power an automobile or dwelling. In the world of venture capital, the term "flywheel" is used to represent the recurrent, margin-generating heart of a business.

External articles and references

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In the News

Pentadyne's Flywheel Enhances 100 kVA to 500 kVA Medical-Grade Power Systems - New system combines the Mission Critical Power Solutions' MedAC(TM) uninterruptible power system (UPS) with Pentadyne Power Corporation's Voltage Support Solution(TM) (VSS 120) flywheel energy storage system. (RedNova July 11)

See Also

Directory:Flywheels at PESWiki

Directory:Batteries at PESWiki

- Directory

- Main Page

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