David Johnston at PaddlingInstructor.com has put up a video of a Danish wind turbine blowing up in the wind. Direct youtube link here.
Aeolian energy is huge here in Denmark and we get a significant part of our electricity from the wind. We have wind turbines plastered all over the landscape, so much that there are associations working against building more wind turbines for environmental reasons. They do make some low frequency noises, a deep humming kind of sound, and they can break in high winds if not maintained and managed correctly. For that reason still more wind turbines are being placed in wind farms off shore, where the visual impact is less (seen from land), the noise doesn’t annoy that much and a breakage is less likely to harm somebody.
The wind farm on the picture is at Middelgrunden, off Copenhagen harbour. It consists of 20 medium sized wind turbines. It is dwarfed by the much larger Horns Rev wind farm in the North Sea and the Nysted wind farm in the Baltic Sea.
Modern wind turbines are huge, and they keep getting bigger. In the photo to the left there’s a small sailing boat behind the base—compare with photo above for overall size of these medium sized wind turbines.
A modern wind turbine can have a main tower of over 100 m in height, and wings of over 70 m. It means the tip of a wing will travel well over 300 m per rotation, which is why the wind turbines often seem to spin very slowly, even in brisk winds. The house on top of the tower contains a massive gear box, so the turbine can get the most energy out of the wind even when turning slowly.
The speed of the rotation is regulated by turning the blades, and in a storm the blades will normally be turned so the wind turbine stops and the wind just blows by. It is kind of ironic that when the wind gets too strong, the wind turbines will shut down and stop producing power. This is probably what failed in the exploding wind turbine in the video.
In the video the wind turbine isn’t stopped as it should be by turning the blades parallel to the wind and it therefore starts to spin too fast. With increased rotational speed, the wing tips will start travelling close to the speed of sound, while the inner parts of the wing still travel much slower, having a small circle to travel for each rotation. As the wing tip nears the sound barrier, the stress in the wing will cause it to break at some point, and when one wing breaks, the whole construction breaks. The tower is torn down by the unbalanced rotation of the remaining parts of the wings.
There’s been at least two cases of run away wind turbines this winter. This one in Denmark, and another one in Sweden, both wind turbines produced and maintained by Vestas, the world’s largest producer of wind turbines.