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A flywheel is a very heavy wheel, formerly a large spoked wheel with a heavy metal rim but now more commonly made from a carbon-fiber composite material, with a smaller cylindrical form that is only about a quarter as heavy. In both cases the principle is the same – it needs significant force to set the wheel turning, and the same to stop it from spinning. In other words, it has high angular momentum.
The result is that at high speeds it is able to store a lot of kinetic energy, which makes it a mechanical battery. That is, it stores energy in the form of kinetic energy rather than as chemical energy as does a conventional electrical battery.
Theoretically, the flywheel should be able to both store and extract energy quickly, and release it, both at high speeds and without any limit on the total number of cycles possible in its lifetime. However, their cost, weight, and energy density have been traditional concerns with flywheels. These are being addressed with advances in materials sciences and rotating system design. Environmental concerns are also driving research into flywheel energy storage systems (FESS).
Flywheels are often large and heavy because they are able to store more energy that way. On the other hand, smaller and lighter wheels are also used in many situations because they can spin much faster and thus much more kinetic energy is generated this way. Thus there are different sizes and shapes of flywheel. With the availability of modern lightweight composites and ceramics, flywheels are now usually smaller and able to spin at high speeds.
How Does a Flywheel Work?
The FESS is made up of a heavy rotating part, the flywheel, with an electric motor/generator. The inbuilt motor uses electrical power to turn at high speeds to set the flywheel turning at its operating speed. This results in the storage of kinetic energy. When energy is required, the motor functions as a generator, because the flywheel transfers rotational energy to it. This is converted back into electrical energy, thus completing the cycle.
As the flywheel spins faster, it experiences greater force and thus stores more energy.
Flywheels are thus showing immense promise in the field of energy storage systems designed to replace the typical lead-acid batteries.
For a flywheel, kinetic energy is calculated as for a spinning object, as
E = ½Iω2
I is the moment of inertia, which depends on the actual mass and the location of that mass from the spinning center – the farther out it is the higher the moment of inertia becomes.
ω is the angular velocity of the flywheel.
Thus the best flywheel in terms of moment of inertia could be one which is larger, spoked and lightweight, but with a heavy rim of metal. If the rim is twice as heavy as the original, this would store double the energy that a lighter rim would, but the mechanical limitations increase correspondingly. On the other hand, doubling the rate of spinning yields twice the angular velocity, which means the energy stored is quadrupled!
Flywheels turn on bearings which require proper lubrication to minimize frictional forces. Air resistance must also be reduced to as little as possible. For this reason, the latest development in flywheels is mounting them on low-friction bearings inside sealed metal cylinders, or even better, floating them on superconducting magnets which avoids friction almost completely and placing them inside vacuum chambers to avoid air drag as well.
The FESS is capable of generating several mW of power for brief periods. Flywheels are best suited to produce high power outputs of 100 kW to 2 mW over a short period of 12-60 seconds. The peak output, at 125 kW for 16 seconds, is sufficient to provide 2 mW for one second.
There are two basic flywheel configurations. In one type the flywheel is attached to the shaft and both rotate together. This is termed a conventional rotor. The other type consists of a flywheel spinning around a shaft which does not move, also called an inside-out rotor.
Advantages of the Conventional Rotor System
The advantages of the first system are:
- The use of smaller and lighter magnetic bearings
- Lightweight smaller motor/generator systems
- Easier removal of heat from the motor/generator, thus enabling much less heat dissipation
- Less stress on the components of the magnetic bearings and motor rotor
- Extension of the stator away from the rotating rotor, which eliminates magnetic drag losses during the charging mode (these could otherwise almost completely discharge the FESS within 12 hours), allowing the charge to be retained for a month
Advantages of the Inside-Out Rotor System
The advantages of this system:
- Compact configuration
- Only connection between the rotor and axle is through the magnetic bearing
- The stationary axle can provide columnar support
The motor/generator is typically a permanent magnet-based machine because these have higher efficiencies and are smaller for any given power rating. They also have lower rotor losses and winding inductances, making them more workable in a vacuum operating environment and suited to the rapid energy transfer typical of flywheel applications.
The energy storage itself is performed using a three-phase IGBT-based PWM inverter/rectifier setup.
Magnetic bearings are composed of permanent magnets that use repulsive force to keep the flywheel’s weight suspended, while it is stabilized with the use of electromagnets. The high-temperature superconducting magnetic bearings are preferred here as they automatically position the flywheel without requiring electrical power or a positioning control system.
An external inductor is also necessary because when used in series with the machine in charging mode, the total harmonic distortion is reduced to within normal range. The permanent magnets would otherwise offer low inductances which increase the THD and cause higher power losses and increased temperature.
The FESS has three working modes, the charging mode, the standby mode, and the discharging mode.
The benefits which make the FESS so appealing include:
- High power density
- High energy density
- Lifetime independent of charge depth or discharge cycle
- Low maintenance
- Short recharge time
- Independent of temperature due to a vacuum environment
- Environmentally friendly materials and process
However, the FESS has some issues as well:
- Requirement for sturdy and durable bearings with low frictional loss
- Mechanical limitations as energy storage increases
- Danger of fragmentation or mechanical cracking at around 700M/second
- Failure mode could be potentially dangerous
- Short discharge time
Sources and Further Reading