Wind Turbine Design: Basic Load Considerations
Water pumping windmill, South Australia, Photograph Soren Krohn
© 1997 DWIA
Water pumping windmill Whether you are building wind turbines or helicopters, you have to take the strength, the dynamic behaviour, and the fatigue properties of your materials and the entire assembly into consideration.
Extreme Loads (Forces)
Comodoro
Comodoro Rivadavia, Argentina (NEG Micon 750 kW turbines) Photograph Soren Krohn
© 1998 DWIA
Rivadavia Wind turbines are built to catch the wind's kinetic (motion) energy. You may therefore wonder why modern wind turbines are not built with a lot of rotor blades, like the old "American" windmills you have seen in the Western movies.
Turbines with many blades or very wide blades, i.e. turbines with a very solid rotor, however, will be subject to very large forces, when the wind blows at a hurricane speed. (Remember, that the energy content of the wind varies with the third power (the cube) of the wind speed).
Wind turbine manufacturers have to certify that their turbines are built, so that they can withstand extreme winds which occur, say, during 10 minutes once every 50 years.
To limit the influence of the extreme winds turbine manufacturers therefore generally prefer to build turbines with a few, long, narrow blades.
In order to make up for the narrowness of the blades facing the wind, turbine manufacturers prefer to let the turbines rotate relatively quickly.
Fatigue Loads (Forces)
Wind turbines are subject to fluctuating winds , and hence fluctuating forces. This is particularly the case if they are located in a very turbulent wind climate.
Components which are subject to repeated bending, such as rotor blades, may eventually develop cracks which ultimately may make the component break. A historical example is the huge German Growian machine (100 m rotor diameter) which had to be taken out of service after less than three weeks of operation. Metal fatigue is a well known problem in many industries. Metal is therefore generally not favoured as a material for rotor blades.
When designing a wind turbine it is extremely important to calculate in advance how the different components will vibrate, both individually, and jointly. It is also important to calculate the forces involved in each bending or stretching of a component.
This is the subject of structural dynamics, where physicists have developed mathematical computer models that analyse the behaviour of an entire wind turbine.
These models are used by wind turbine manufacturers to design their machines safely.
Structural Dynamics: An Example *)
A 50 metre tall wind turbine tower will have a tendency to swing back and forth, say, every three seconds. The frequency with which the tower oscillates back and forth is also known as the eigenfrequency of the tower. The eigenfrequency depends on both the height of the tower, the thickness of its walls, the type of steel, and the weight of the nacelle and rotor.
Now, each time a rotor blade passes the wind shade of the tower, the rotor will push slightly less against the tower.
If the rotor turns with a rotational speed such that a rotor blade passes the tower each time the tower is in one of its extreme positions, then the rotor blade may either dampen or amplify (reinforce) the oscillations of the tower.
The rotor blades themselves are also flexible, and may have a tendency to vibrate, say, once per second. As you can see, it is very important to know the eigenfreqencies of each component in order to design a safe turbine that does not oscillate out of control.
*) A very dramatic example of structural dynamic forces at work under influence of the wind (undampened torsion oscillations) is the famous crash of the Tacoma Bridge close to Seattle in the United States. You may find a short movie clip (700 K) on the disaster on the Internet.
© Copyright 1997-2003 Danish Wind Industry Association
Updated 19 September 2003
http://www.windpower.org/en/tour/design/index.htm
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