Wind Power - Understanding Renewable Energy Systems - page 203

Figure 5.15 Rotor Blade Positions for Different Wind Speeds for a
Pitch-controlled System
Yawing
Horizontal axis wind turbines must always follow the direction of the wind, in
contrast to vertical axis wind turbines. The orientation of the rotor blades
must be chosen so that the rotor blades face the wind at the optimal angle.
This can be a problem for pitch-controlled wind turbines if the direction of the
wind changes very fast or is gusty. Hence, high power fluctuations can occur
and must be cushioned by changes in the rotor speed.
The position of the rotor can be upwind or downwind. The position of the
rotor relative to the tower for upwind turbines is before the tower in the
direction of the wind, and for downwind turbines it is behind. The
disadvantage of the downwind rotor is that the rotor blades have to
continually pass the sheltered zone of the tower. This produces high mechanical
strains and noise emissions due to turbulence from the tower and the nacelle.
Therefore, most large wind turbines are upwind turbines. Downwind turbines
have the advantage that wind pressure adjusts the rotating rotor blades
optimally to the wind. Small wind turbines can use wind vanes for passive
yawing. The wind vane moves the rotor of an upwind turbine always to a
position perpendicular to the wind.
To move a horizontal axis wind turbine in the yaw direction, the whole
nacelle with rotor, gearbox and generator must be movable on top of the tower.
Wind measuring equipment on the nacelle estimates the wind speed and
direction and a control unit decides when an electric or hydraulic yaw drive
moves the nacelle and rotor azimuth. When the nacelle reaches its optimal
position, azimuth brakes hold this position. In reality there are always small
deviations of the direction of the wind and the optimal position of the rotor.
This deviation is called the yaw angle and is usually about 5°.
Tower, Foundations, Gearbox, Nacelle and Generator
The tower is one of the most important parts of the wind power plant. It must
hold the nacelle and the rotor blades. Higher tower heights can increase the
wind generator output because the wind speed rises with height. Early in their