Autonomous flap for smart wind turbines

The amount of energy produced by wind turbines in Europe is steadily increasing, but the process is anything but constant. Smart wind turbines actively control their load: they measure vibrations, use flaps to change aerodynamic forces, which damp the blades. This improves their efficiency and life-cycle cost. But what happens when they break? The whole blade needs to be replaced. Lars Oliver Bernhammer, PhD candidate in the Faculty of Aerospace Engineering, has developed an innovative autonomous flap as part of his thesis.

“Offshore turbines are very hard to access. You need a boat to get to the wind farm, and good weather to get onto the turbine, and the boat needs to be very big to replace turbine blades. People have tried to build flaps in such a way that they never fail, which is hard to do, a lot of things can go wrong. There have been all kinds of attempts to integrate flaps into the blades, but they are not easy to move or control. We felt it would be easier if you just replace part of the blade,” explained Bernhammer. Because the new flap concept is autonomous it can be easily exchanged without any impact on the rest of the blade, and this helps reduce maintenance costs. “We’re talking about something 7.5 metres x 0.5 metre maybe, which is much less than a 75 metre x 5 metre blade.”

The autonomous flap is a plug-and-play unit, which only requires a simple clip attachment to the blade. It functions as its own energy source through vibration harvesting, exploiting aeroelastic instabilities. It contains sensors and a controller, and is self-actuated; when the controller is switched on the blade can be brought to a standstill. “Since only very few load cases contribute to structural damage, the overall damage can be reduced,” said Bernhammer.

Bernhammer collaborated with the Technical University of Denmark, Technion in Israel and Sandia National Labs in the USA. The autonomous flap has been patented, although there is no immediate economic interest, and Bernhammer successfully defended his thesis on October 12, 2015. He now plans to move to Denmark to work in industry doing more applied research in the field.