Onshore and offshore, rotor blades need to be able to withstand rapidly changing wind directions and variable wind intensities. But how can rotor blades be better equipped for such situations than previously? Researchers have designed so-called smart blades that can adapt to changing wind conditions passively or with active components – and at the same time maximise the energy yield of the wind turbine.

Rotor blades now measure up to 85 metres and the towers reach heights of over 200 metres – with a rising trend. As the rotor diameter lengthens, the energy yield also increases. However, the blades are subjected to loads from wind gusts and wind shear close to the ground and near the upper part of the wind turbines. These cause considerable stress on the materials. At the same time, wind turbine manufacturers are reaching their limits in terms of the scalability, as the aerodynamic vibrational loads also increase with increasing blade lengths. Existing control systems are barely capable of balancing out the loads on longer blades. In the case of larger wind turbines, the weight then becomes the decisive factor. Although mathematically the output would be quadrupled if the rotor blade were to double in size, this would also lead to an eightfold increase in the weight – which would also lead to higher costs. Therefore the material consumption, weight and costs need to be reduced and the wind turbines made more efficient and economical. A greater energy yield could be achieved if the aerodynamically related extreme and fatigue loads were reduced; this would then enable longer rotor blades to be constructed with the same weight. Wind turbines equipped with longer blades are particularly required in weak wind areas. This is the only way to ensure a high yield, which is ultimately significantly more important than a higher rated capacity.

Ideal would therefore be rotor blades that combine all advantages: they should be lightweight, durable but also cost-efficient and flexible. This is what scientists have been working on from the German Aerospace Centre (DLR), the Fraunhofer Institute for Wind Energy and Energy Systems IWES and the Center for Wind Energy Research (ForWind). The researchers have developed intelligent rotor blades called smart blades. These can better adapt their geometry to local wind effects than previous blade concepts. Within the „Smart Blades“ research project, the research group has investigated three different technologies. These include passive and active concepts that adapt and change the aerodynamic behaviour to relieve the structure.

If a rotor blade not only bends in a strong wind but also twists axially, experts refer to bend-twist coupling (BTC). In order to minimise the wind loads, the researchers examined two different approaches to this concept: geometric and structural BTC. The scientists analysed conventional rotor blades and compared them with socalled sickle blades. The investigations showed that flexible, 80-metre-long blades with geometric BTC reduce the loads compared with conventional blades.

It was also shown that the torsional deformations of the sickle blades in the outer blade area already significantly impact on the wind turbine‘s output and loads once the rated wind speed is reached. In the case of high wind velocities, the flow separation also occurs later.

With the structural approach, a special means for constructing the rotor blade ensures this effect. The fibre layers are laid not only longitudinally but also anisotropically, in a diagonal direction from the leading to the trailing edge of the blade. These blades are also able to not only bend through changes in the load but also twist around their axis. The blades thus change their pitch and passively counteract the load change. „The loads can be reduced much more using the geometric bend-twist coupling approach,“ says project manager Dr Jan Teßmer from DLR, in summarising the results so far for the passive approach. The research consortium wants to test the methodological effects of the smart blades with a 20-metre-long demonstration blade on a real wind turbine.