Environmental Engineering Reference
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opposite sides. Near the axis, the blades overlap so that the redirected wind
can flow from one blade to the other. The rotor blades also utilize the lift
principle so that the efficiency of the Savonius rotor is little better than that of
simple drag devices. However, the efficiency is much worse than that of good
lift devices, reaching maximum power coefficients of the order of 0.25 (Hau,
2000). Savonius rotors have the advantage that they can start at very low wind
speeds. They are therefore used for ventilation purposes on buildings or utility
vehicles. Besides poor efficiency, Savonius rotors have the disadvantage of a
high material demand. Thus, they are not used in large systems.
The Darrieus rotor was developed by the Frenchman Georges Darrieus in
1929. The Darrieus rotor consists of two or three rotor blades that have the
shape of a parabola. The profile of the rotor blades corresponds to lift devices
so that the Darrieus rotor utilizes the lift principle. Due to its vertical axis the
angle of attack at the Savonius rotor changes continuously. The efficiency of
the Darrieus rotor is much above the efficiency of the Savonius rotor; however,
it reaches efficiencies of only about 75 per cent of modern rotors with
horizontal axes. A grave disadvantage of the Darrieus rotor is that is cannot
start on its own: it always needs an auxiliary starting system that can be a
drive motor or a coupled Savonius rotor.
A further development of the Darrieus rotor is the H rotor or H-Darrieus
rotor. This rotor is also called the Heidelberg rotor after the company
Heidelberg Motor. A permanent-magnet generator is directly integrated into
the rotor structure and needs no gearbox. The rotor works in the same way as
the Darrieus rotor as a lift device. The three rotor blades of the H rotor are
attached vertically. Supports to the vertical axis help the rotor maintain its
shape. The very robust H rotor was designed for the extreme weather
conditions existing in high mountains or in Antarctica.
Wind power plants with vertical axes have some advantages. Their
structure and their assembly are relatively simple. The electric generator and
the gear as well as all electronic components can be placed on the ground. This
simplifies the maintenance compared to rotors with horizontal axes. Rotors
with vertical axes need not be oriented into the wind; therefore, they are
perfectly suited for regions with very fast changes of wind direction.
However, these advantages have not resulted in a breakthrough for wind
generators with vertical axes. Today, almost all wind power plants use rotors
with horizontal axes; systems with vertical axes are only used for very special
applications. The poorer efficiency and higher material demand of systems
with vertical axes have been the deciding factors for the market dominance
achieved by horizontal axis turbines to date.
Wind turbines with horizontal rotor axis
System components
Most wind turbines generating electricity today are horizontal axis machines.
It is predominantly medium-sized enterprises that have pushed wind market
developments. Wind power plants have reached a high technical level and
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