Dancing in the wind

I have discussed in other post the phenomenal growth of the dimensions of wind turbines in the last 2 decades. Bigger rotors, taller towers and more MW has been the industry trend year after year.

There is some evidence that we are reaching the limit – blades of more than 50m length pose significant logistic challenges, while steel tower more than 100 meters tall can be subject to strong vibrations and dangerous oscillations under certain circumstances.

Such vibrations can be induced by several external sources such as an unbalanced rotor, an earthquake or the wind itself.

They are dangerous because they can damage the turbine due to fatigue loading (the weakening of materials due to cyclical loads). Some type of foundation can also partially lose stiffness – for instance monopile foundations.

Additionally, these vibrations can also trigger resonance phenomenons in the tower – you can follow this link to see of how “soft soft” and “stiff” tower are designed based on the blade passing frequency.

You can see a good full scale example of this problem in the video above and read here more about wind turbine vibrations.

There are several technical solutions currently being studied to dampen the tower reducing the vibrations.

Among the most interesting concept that I have seen I would mention tuned mass dampers – basically an auxiliary mass connected to the structure with spring and dashpots (viscous friction dampers), friction plates or similar energy dissipating elements.

These dumpers are called “tuned” because they have been designed keeping in mind the natural oscillation frequencies of the structure they have to protect. The two main parameters are the spring constant and the damping ratio: by varying them it is possible to damp harmonic vibrations.

I do not know if tuned mass dampers that can work with the first fundamental frequency of  industrial size wind turbines (below 1 Hz) are currently available – however I have found quite a lot of  studies on the topic.

A similar technological solution is the tuned liquid column damper. In this case a liquid inside an U shaped tank. By varying the geometry of the tank and the depth of the liquid different damping frequencies can be achieved.

The main benefits of this solution are the geometrical flexibility (you have to put the dumper somewhere inside the tower or the nacelle – I can assure you that the space there is very reduces) and low cost.

Another variant is the pendulum damper. In this solution, the length of the pendulum is calculated to match the fundamental frequency of the WTG.

Mass Damper (a) and Pendulum Damper (b)
Copyright O. Altay, C. Butenweg, S. Klinkel, F. Taddei
Vibration Mitigation of Wind Turbine Towers by Tuned Liquid Column Dampers
Proceedings of the 9th International Conference on Structural Dynamics, EURODYN 2014