The aim is to control the loads on large wind turbine blades using a flexible trailing edge made of an elastic material which can be controlled by means of compressed air or hydraulics.
Wind turbine blades measuring 60 m or more can flex as much as 4-6 m in strong gusts of wind. However, the blades are also so long that there can be considerable differences in the loading from the wind gusts along the blade. In wind farms, surrounding wind turbines also exert considerable influence and generate turbulence, which has a more localised effect, Risø says.
“It is these local influences which we hope our design will help mitigate. However, in addition to our rubber trailing edge, it also calls for effective sensors and control systems which can tell the system to regulate the flaps according to the local wind conditions along the blade. Right now we are looking at different types of sensors and a trailing edge made of plastic instead of rubber,” explains Research Specialist Helge Aagaard Madsen.
The test set-up consisted of a two-metre-long wind turbine blade section with a total chord of one metre and a 15 cm rubber flap covering the entire span. The wind turbine blade incorporated a pneumatic system for controlling the flexible silicone material which the trailing edge is made of. Finally, two sensors were attached to the front of the blade which measured wind direction and speed.
“The operational principle which we have arrived at is very simple and robust, and we also believe that the manufacturing process will be so. The wind tunnel test showed, among other things, that the outward curve of the flap does not change markedly when subjected to wind loads similar to those on a real turbine blade.
“In addition, we measured the correlation between the deflection of the flap and the change in lift on the blade section. This produced figures which we can enter into our calculation models and then realistically simulate how the flap will reduce the loads on the turbine,” says Aagaard Madsen.
“A further bonus of our design is that the moulded rubber trailing edge gives us a sharp edge which produces less noise and greater output. As most blades today are manufactured in two halves and afterwards joined together, the trailing edge will always have a certain thickness. The trailing edge is then ground to make it thinner, but with our design, the blade automatically gets a completely sharp edge.”
Next step: industry cooperation
The next step in the project is to develop the technology towards a stage where the rubber wind turbine trailing edge flap is ready for testing in a full-scale wind turbine prototype model. The researchers will work on optimising the design so the deflection will be as big as possible for a given pressure in the voids within the trailing edge. Durability is another important issue, as is developing systems for supplying compressed air or hydraulic power, Risø says.
“We are in contact with a number of companies who are interested in working with us on these issues, and it is vital for the further development of the technology that we involve industry as this is where the systems will be manufactured at the end of the day,” Aagaard Madsen concludes.