Introduction to Wind Energy Systems by Hermann-Josef Wagner & Jyotirmay Mathur

Author:Hermann-Josef Wagner & Jyotirmay Mathur
Language: eng
Format: epub
Publisher: Springer Berlin Heidelberg, Berlin, Heidelberg

5.5 Load Considerations for Turbine Selection

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. Metal fatigue is a well known problem in many technical goods. Due to combined reasons of fatigue and mass, metal is, therefore, generally not preferred 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 mathematical computer models have been developed that analyse the behavior of an entire wind turbine. These models are used to design the machines safely.

As an example, a 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. Consequently, it is very important to know the eigenfrequencies of each component in order to design a safe turbine that does not oscillate out of control.

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