The Aerodynamics Of The Wind Turbine – Part 4

WHAT HAPPENS WHEN THE WIND SPEED CHANGES?

The description so far was made with reference to a couple of examples where wind speed was at a constant 10 m/s.

We will now examine what happens during alterations in the wind speed.

In order to understand blade behavior at different wind speeds, it is necessary to understand a little about how lift and drag change with a different angle of attack. This is the angle between the resulting wind ÒwÓ and the profile chord.

In the drawing below the angle of attack is called ÒaÓ and the setting angle is called ÒbÓ.

The setting angle has a fixed value at any one given place on the blade, but the angle of attack will grow as the wind speed increases.

The aerodynamic properties of the profile will change when the angle of attack ÒaÓ changes. These changes of lift and drag with increasing angles of attack, are illustrated in the diagram above used to calculate the strength of these two forces, the lift coefficient ÒCLÓ and the drag coefficient ÒCDÓ. Lift will always be at a right angle to the resulting wind, while drag will always follow in the direction of the resulting wind.

We will not enter into the formulas necessary to calculate these forces, it is enough to know that there is a direct connection between the size of ÒCLÓ and the amount of lift.

Both lift and drag abruptly change when the angle of attack exceeds 15-20

degrees. One can say that the profile stalls. After this stalling point is reached, lift falls and drag increases. The angle of attack changes when the wind speed changes.

To further study these changes, we can draw diagrams, shown to the right, illustrating three different wind speeds ÒvÓ (5, 15 and 25 m/s) from our previous cross section, this time near the blade tip of a 450 kW wind turbine.

This situation is rather convenient as the setting angle ÒbÓ near the wing tip is normally 0 degrees. The head wind from the movement ÒuÓ is always the same, as the wind turbine has a constant rotational speed controlled by the grid connected generator (in these situations we do not consider the small generator used on certain small wind turbines). The free air flow ÒvÓ has three different values and this gives three different values of the

resulting wind ÒwÓ across the profile.

The size of ÒwÓ does not change very much, from 50 m/s at a wind speed of

5 m/s to 52 m/s in a 25 m/s wind. The reason for this relatively minor change is due to the dominating effect of the head wind.

However, the angle of attack ÒaÓ between the resulting wind and the chord

of the blade changes from 6 degrees at a wind speed of 5 m/s to 16 degrees at 15 m/s to 27 degrees at 25 m/s. These changes are of great importance for determining the strength of the aerodynamic forces.

Studying the diagram showing the lift coefficient ÒCLÓ and the drag coefficient ÒCDÓ we may note the following:

¥ At a wind speed of 5 m/s (A), the angle of attack is 6 degrees. The lift coefficient is 0.9 and the coefficient of drag is 0.01. Lift is therefore 90 times

greater than drag, and the resultant force ÒFÓ points almost vertically at a right angle to the mean relative wind ÒwÓ.

¥ At a wind speed of 15 m/s (B), the profile is almost about to stall. The angle of attack is 16 degrees. The lift coefficient is 1.4 and the coefficient of

drag is 0.07. Lift is now 20 times drag.

¥ At a wind speed of 25 m/s (C), the profile is now deeply stalled, the angle of attack is 27 degrees, the lift component is 1.0 and the component of lift is 0.35. Lift is now 3 times greater than drag. We can therefore note the following:

¥ During the change of wind speed from 5 to 15 m/s there is a significant increase in lift, and this increase is directed in the direction of rotation. Therefore power output of the wind turbine is greatly increased from 15 kW to 475 kW.

¥ During the change of wind speed from 15 to 25 m/s, there is a drop in lift accompanied by an increase in drag. This lift is even more directed in the direction of rotation, but it is opposed by drag and therefore output will fall slightly to 425 kW.