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Flywheel energy storage

kersThe use of a flywheel as an energy storage device is not new. The conventional heavy, metal flywheel as connected to the crankshaft of an internal combustion engine, is used as an energy storage device, releasing its energy in order to maintain momentum during the idle strokes of the engine. The mechanical KERS system has been made successful by producing a flywheel of low mass, small size, running at high speeds.
The energy stored in a flywheel is given by the following formula:

E = ½ I?2
E = flywheel energy
I = moment of inertia of the flywheel (ability to resist changes in its rotational velocity)
? = the rotational velocity (omega)

The flywheel as used in a mechanical KERS system works by accelerating the flywheel to a very high speed and maintaining the energy in the system as rotational energy. When energy is extracted from the system, by pressing the KERS button (on the steering wheel) the flywheel's rotational speed is reduced as a consequence of the principle of conservation of energy. Energy added to the system, under braking, results in an increase in the speed of the flywheel.

The flywheel is a composite construction, made of high strength carbon-fibre materials. This provides the flywheel with a high strength-to-density ratio. The use of lightweight modern materials allows the flywheel to rotate at speeds in excess of 70,000 rpm and these high rotational speeds occur whilst running in a vacuum enclosure. The benefits of running in a vacuum, are that aerodynamic drag and windage losses are eliminated, allowing the rotor to spin for longer, whilst it can also reach its operational speed quicker. The system as operated by Flybrid can come up to its operational speed of 64,000 rpm in less than one second, however at these high operational speeds, the flywheel undergoes an increase in diameter.

Carbon fibre composite flywheels have a higher tensile strength than steel and are substantially lighter. One of the primary limits to flywheel design is the tensile strength of the material used for the flywheel. Generally speaking, the stronger the disc, the faster it may be spun and the more energy the flywheel system can store. When the tensile strength of a flywheel is exceeded the flywheel will shatter, releasing its stored energy all at once. Fortunately, composite materials tend to disintegrate quickly into red-hot powder once broken, instead of large chunks of high-velocity shrapnel. Nevertheless the flywheel used in KERS has a strong containment vessel as a safety precaution.

Tensile strength is indicated by the maximum value of its stress-strain curve and is generally indicated when necking of the test specimen occurs. The value of the tensile strength obtained this way, does not depend on the size of the test specimen. It is, however, dependent on the preparation of the specimen and the temperature of the test environment and material.

Tensile strength is an important parameter for engineering materials used in structures and mechanical devices. Tensile strength is specified for materials such as alloys and composite materials to name but a few.

Flywheels are not affected by temperature changes as are chemical rechargeable batteries. An advantage of a flywheel energy storage system; is that by a simple measurement of the rotational speed, it is possible to know the exact amount of energy stored.

Written by Eric Smart.

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