Yaw Control

From Wind wiki

Yaw control involves capturing the power of the wind by orienting the rotor to the wind direction. At times, a marked loss of power occurs because of a yaw angle, an angle deviation between the wind direction and rotor axis. There are three ways in which the rotor can be oriented into the direction of the wind:

1. Aerodynamic Yawing

Aerodynamic yawing means orienting into the wind by using wind vanes or fan-tail wheels. Using a wind vane is the simplest way to capture the wind. However, this is only useful in small wind turbines. If a wind vane is used in large turbines, it must be uneconomically large. Yawing with the use of a fan-tail wheel is also used in smaller turbines, although this has a much older history than wind vanes. Fan-tail wheels have been used in Dutch windmills. However, fantails can be disadvantageous because they are relatively expensive. Another drawback is the vibrations around the azimuth axis caused by the teeth of the worm gear during the yawing moment.

2. Active Yawing

Active yawing is done through the use of a motor as a solution to the impossibility of yawing without sufficient wind.

3. Free Yawing

Free yawing happens when the downwind rotor is used to yaw independently. If the rotor is in a downwind position, the point of attack is at the yaw axis, right behind the axis of rotation of the tower head. With the wind force, the aerodynamic forces produce a moment of restoration on the rotor within a very wide yaw angle range. However, it is still a question whether the aerodynamic restoring moment is strong enough for free yawing to happen.

For the rotor around the yaw axis to move, several moments of resistance must first be overcome. These moments are inertial and gyroscopic moments and the frictional moments that occur in the towerhead bearings. These moments and the aerodynamic forces occur due to the uneven flow in the rotor swept area caused by the increase in wind speed as the altitude goes higher or the other way around. The said moments affect the balance of moments with respect to yawing both in a supporting sense and in an impeding sense. Moreover, especially in two-bladed rotors, unavoidable and periodically alternating rotor moments around the yaw axis occur.

Since there is a complex balance of forces and moments in wind turbines, some design parameters were made in order to achieve a free yawing capability:

a. One of the design parameters with effect on yawing is the cone angle of the rotor blades. This improves the stability around the axis of yaw in the wind rotor.

b. Incorporating blade-pitch angle coupling arrangement frequently in the rotors can also influence yawing. This helps the asymmetrically blown rotor find a yaw position with a new equilibrium.

c. Having a tilted angle in the rotor axis with respect to the horizontal also helps.

With the design parameters mentioned above, the following preconditions must be present in order for wind turbines to have a free yawing capability: rotor positioned downwind, without tilted rotor axis if possible, but with a cone angle to the blades. The rotor should be two-bladed with cyclic blade pitch actuation or a three-bladed rotor.