Environmental Engineering Reference
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drawings by Iranian students compare the diferent concepts: on
the left, the Persian windmill with a DD and a vertical axis, and on
the right, the drive system of the European windmill with four
wings at a horizontal axis and the transmission gear down to the
millstones.
In form of post mills, tower windmills and Dutch windmills,
horizontal systems have been used till the 19th century. They were
more efficient than their Persian and Chinese predecessors, but
much more complex in their construction. Usually the rotor had
four wings and was mounted horizontally; a wooden gear wheel
connected the upright shaft to further gear wheels that turned the
millstones. They are archetypes for modern wind turbines. The
much simpler Persian ancestral windmills, and hence the DD, were
overseen for a long time.
19.2
The Drive Train Evolution
Since wind turbines used to produce electricity were introduced,
a big variety of construction principles also appeared. It is true
especially for the rotor, which rotates with two, three or multiple
blades along a horizontal or vertical axis and converts kinetic
energy from the air flow into mechanical energy. Therefore the
power of resistance is used in the rotor. Increased conversion
efficiencies close to 50% can be achieved with rotor blades that use
the aero-dynamical buoyancy, as birds or airplanes do.
In a similar manner, diferent concepts for the conversion from
mechanical to electrical energy have been established. All necessary
components, including the generator, together nowadays are
called drive train. The conventional, dispersed drive train is well
established, and until 2010 can be found in 80% of all worldwide
produced wind energy converters that are used for electricity
production.
Thereby the enormous rotor moment in the size of, for example,
1 500 000 Nm at a 3 MW turbine is reduced by the factor 100 in
a three-step gearbox, while the rotational speed is increased by
that factor. That has the special advantage that a fast-running
generator can be used, which is available in diferent varieties
by most manufacturers of electrical machinery. The generator is
small, light and has a very high electrical efficiency (Fig. 19.4).
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