352
10 Wind Turbines
relative thickness are necessary, typically 17 % at the tip to 35 % at the hub, because
of strength. The blades are rigidly fixed to the hub, but have a high bending flexibility. With two-blade turbines, articulated rotors are typically applied. The rotor
hinges then around an axis perpendicular to the turbine shaft. This enables absorption of the force difference due to the higher wind velocity on the upper blade and
the lower wind velocity on the lower one. The rotor has a small cone angle in order
to reduce the bending moment on the hub. The centrifugal force moment thus compensates the blade force moment. Sufficient clearance from the tower requires a tilt
angle of the shaft (small slope compared to the horizontal direction).
The rotor shaft is borne in a nacelle housing the gearbox (if present) and the
generator. The rotational speed depends on the turbine size and is about 25 rpm
with a 1000 kW turbine and about 18 rpm with a 2500 kW turbine. The tip speed
varies from about 60 to 75 m/s with 40–80 m diameter rotors. The rotor is mostly
set upwind of the tower. Downwind types hardly ever occur anymore. A downwind
rotor follows spontaneously changes in wind direction, but a big disadvantage is
that the rotor blades turn through the wake of the tower. This causes efficiency loss
and serious fatigue load. Upwind rotors are preferred therefore. They require an
active yaw mechanism. The nacelle is on top of the tower, mostly on a gliding ring
on shoes with synthetic covering and with an internal gear, driven by a yaw motor
and controlled by the wind direction detected with a vane on the nacelle. Computer
control is applied, as sudden changes of wind direction shall not be reacted to immediately. A brake is applied to spare the yaw mechanism. The tower is usually a steel
tube tower. Large towers may have a concrete lower part. Small systems sometimes
use a lattice tower.
Figure 10.4 represents the section of a nacelle of a turbine with blade pitch control. It is a two-blade turbine of 500 kW that is no longer manufactured. Its parts
are: hub cover, pitch control mechanism (the pitch angle is the angle of the blade
chord with the axial direction on a reference radius), gearbox, generator, mechanical brake, gliding ring for yaw movement, yaw motor.
At wind speed above the rated value, the available wind power exceeds the
maximum generator power. The power possibly yielded by the rotor must then be
reduced. There are two common systems to achieve this: pitch control and stall
control. Because of the lower blade speed near the hub (see velocity triangle in
Fig. 10.2), the relative velocity is lower as well. Equal energy capture in all streamtubes requires a constant product of the blade speed and the tangential component
of the lift. Relative velocity is high at the tip, so that generation of the required
lift needs only a small chord and a small angle of attack. A large chord and a great
angle of attack are required at the hub. Thus the blade is close to the tangential
direction near the tip, but at the hub it is much nearer to the axial direction. Due to
the variation of the angle of attack with the radius, the torsion of the blade is lower
than the angle variation of the relative velocity. Blade torsion typically amounts
to about 10°. As the angle of attack is already very high at the hub in rated conditions, a small increase of the wind velocity causes separation at an unaltered blade
10 Wind Turbines
relative thickness are necessary, typically 17 % at the tip to 35 % at the hub, because
of strength. The blades are rigidly fixed to the hub, but have a high bending flexibility. With two-blade turbines, articulated rotors are typically applied. The rotor
hinges then around an axis perpendicular to the turbine shaft. This enables absorption of the force difference due to the higher wind velocity on the upper blade and
the lower wind velocity on the lower one. The rotor has a small cone angle in order
to reduce the bending moment on the hub. The centrifugal force moment thus compensates the blade force moment. Sufficient clearance from the tower requires a tilt
angle of the shaft (small slope compared to the horizontal direction).
The rotor shaft is borne in a nacelle housing the gearbox (if present) and the
generator. The rotational speed depends on the turbine size and is about 25 rpm
with a 1000 kW turbine and about 18 rpm with a 2500 kW turbine. The tip speed
varies from about 60 to 75 m/s with 40–80 m diameter rotors. The rotor is mostly
set upwind of the tower. Downwind types hardly ever occur anymore. A downwind
rotor follows spontaneously changes in wind direction, but a big disadvantage is
that the rotor blades turn through the wake of the tower. This causes efficiency loss
and serious fatigue load. Upwind rotors are preferred therefore. They require an
active yaw mechanism. The nacelle is on top of the tower, mostly on a gliding ring
on shoes with synthetic covering and with an internal gear, driven by a yaw motor
and controlled by the wind direction detected with a vane on the nacelle. Computer
control is applied, as sudden changes of wind direction shall not be reacted to immediately. A brake is applied to spare the yaw mechanism. The tower is usually a steel
tube tower. Large towers may have a concrete lower part. Small systems sometimes
use a lattice tower.
Figure 10.4 represents the section of a nacelle of a turbine with blade pitch control. It is a two-blade turbine of 500 kW that is no longer manufactured. Its parts
are: hub cover, pitch control mechanism (the pitch angle is the angle of the blade
chord with the axial direction on a reference radius), gearbox, generator, mechanical brake, gliding ring for yaw movement, yaw motor.
At wind speed above the rated value, the available wind power exceeds the
maximum generator power. The power possibly yielded by the rotor must then be
reduced. There are two common systems to achieve this: pitch control and stall
control. Because of the lower blade speed near the hub (see velocity triangle in
Fig. 10.2), the relative velocity is lower as well. Equal energy capture in all streamtubes requires a constant product of the blade speed and the tangential component
of the lift. Relative velocity is high at the tip, so that generation of the required
lift needs only a small chord and a small angle of attack. A large chord and a great
angle of attack are required at the hub. Thus the blade is close to the tangential
direction near the tip, but at the hub it is much nearer to the axial direction. Due to
the variation of the angle of attack with the radius, the torsion of the blade is lower
than the angle variation of the relative velocity. Blade torsion typically amounts
to about 10°. As the angle of attack is already very high at the hub in rated conditions, a small increase of the wind velocity causes separation at an unaltered blade