Blade manufacturer LM Glasfiber in Denmark reports that the application of stall strips can delay the onset of edgewise vibrations, a problem which has plagued turbines with its 19 metre blades, causing stress cracks in some cases (Windpower Monthly, May 1997). The vibrations have occurred in winds from around 17 m/s to above 25 m/s, beyond the cut-out wind speed of most turbines. Edgewise vibrations under certain circumstances are a known phenomenon, but following damage to some blades in 1996, LM recommended a fine adjustment of vibration sensors and a reduction of the cut-out wind speed, if necessary, plus the installation of vibration detection to stop turbines if the edgewise moment of the blade exceeds the load cases on which the turbine's approval is based. Bonus Energy A/S, meantime, drew attention to the results of placing stall-strips to counteract this problem. A patent was first applied for by Bonus, but LM Glasfiber has now obtained the right to make use of the new discovery. This means that machines where frequent vibrations were a problem can now utilise blades fitted with this simple and reliable solution. If the blades are pitched slightly more in a positive direction, or if the blade is equipped with vortex generators, the performance of the turbine remains virtually unchanged.
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Senior Renewable Energy Analyst (WindGEMINI Product Lead) DNV GL Bristol (City Centre), City of Bristol