TURBINE OPERATION/ENERGY OUTPUT
Adding wind speeds and directions
The wind which hits the rotor blades of a wind turbine will not come from the direction in which the wind is blowing in the landscape, i.e. from the front of the turbine. This is because the rotor blades themselves are moving.
To understand this, consider the picture of a bicycle which is equipped with a blue banner, or a wind vane, to indicate the direction of the wind: if there is completely calm weather and the bicycles moves forwards with, say, 7 m/s, the bicycle will be moving through the air at 7 m/s. On the bicycle we can measure a wind speed of 7 m/s relative to the bicycle. The banner will point straight backwards, because the wind will come directly from the front of the bicycle.
Now, look at the bicycle again directly from above and assume that the bicycle moves forward at a constant speed of, once again, 7 m/s. If the wind is blowing directly from the right, also at 7 m/s, the banner will clearly be blown partly to the left, at a 45 degree angle relative to the bicycle. With a bit less wind, e.g. 5 m/s, the banner will be blown less to the left and the angle will be some 35 degrees. As can be sawn from the picture, the direction of the wind, the resulting wind as measured from the bicycle, will change whenever the speed of the wind changes.
What about the wind speed measured from the bicycle? The wind is, so to speak, blowing at a rate of 7 m/s from the front and 5-7 m/s from the right. With a little use of geometry or trigonometry it can be found that the wind speed measured on the bicycle will be between 8.6 and 9.9 m/s.
To study how the wind moves relative to the rotor blades of a wind turbine, there are fixed red ribbons to the tip of the rotor blades of our model wind turbine and yellow ribbons some 1/4 out the length of the blade from the hub. Then the ribbons are let to float freely in the air (in the cartoon we abstract from the air currents created by the blades themselves and the centrifugal force).
The two images show one view from the side of the turbine and another view from the front of the turbine. Since most wind turbines have constant rotational speed, the speed with which the tip of the rotor blade moves through the air, called the tip speed, is typically some 64 m/s, while at the centre of the hub it is zero. At 1/4 out the length of the blade, the speed will then be some 16 m/s. The yellow ribbons close to the hub of the rotor will be blown more towards the back of the turbine than the red ribbons at the tips of the blades. This is obviously because at the tip of the blades the speed is some 8 times higher than the speed of the wind hitting the front of the turbine.
Why are rotor blades twisted?
Rotor blades for large wind turbines are always twisted. Seen from the rotor blade, the wind will be coming from a much steeper angle (more from the general wind direction in the landscape), as you move towards the root of the blade and the centre of the rotor.
As shown in the previous page on stall, a rotor blade will stop giving lift, if the blade is hit at an angle of attack which is too steep. Therefore, the rotor blade has to be twisted, so as to achieve an optimal angle of attack throughout the length of the blade. However, in the case of stall controlled wind turbines in particular, it is important that the blade is built so that it will stall gradually from the blade root and outwards at high wind speeds.
Additional (background) information on Rotor Aerodynamics look at the article: Review of the present status of Rotor Aerodynamics.
Changing the wind speed changes with direction relative to the rotor blade
In this picture one rotor blade has been taken off from its hub, and it is shown from the hub towards the tip, at the back side (the lee side) of the rotor blade. The wind in the landscape blows between, say 8 m/s and 16 m/s (from the bottom of the picture), while the tip of the blade rotates towards the left side of the picture.
In the picture it can be seen how the angle of attack of the wind changes much more dramatically at the root of the blade, yellow line, than at the tip of the blade, red line, as the wind changes. If the wind becomes powerful enough to make the blade stall, it will start stalling at the root of the blade.
Now, the rotor blade is cut at the point with the yellow line. In the next picture the gray arrow shows the direction of the lift at this point. The lift is perpendicular to the direction of the wind. As shown, the lift pulls the blade partly in the preferable direction, i.e. to the left. It also bends the rotor blade somewhat, however.
Rotor blade airfoils
As shown already, wind turbine rotor blades look a lot like the wings of an aircraft. In fact, rotor blade designers often use classical aircraft wing airfoils as cross sections in the outermost part of the blade. The thick airfoils in the innermost part of the blade, however, are usually designed specifically for wind turbines. Choosing airfoils for rotor blades involves a number of compromises including reliable lift and stall characteristics, and the airfoil's ability to perform well even if there is some dirt on the surface (which may be a problem in areas where there is little rain).
- Review of the present status of rotor aerodynamics