Through its three-year “Smart Blades” research project, the Research Alliance for Wind Energy (FVWE) says it has developed new concepts for making wind turbine rotor blades lighter and more stable.
The alliance comprises the German Aerospace Center; ForWind – the Center for Wind Energy Research of the Universities of Oldenburg, Hanover and Bremen; and the Fraunhofer Institute for Wind Energy and Energy System Technology North-West. The German Federal Ministry for Economic Affairs and Energy provided EUR 12 million for the Smart Blades project.
Nowadays, rotor blades can reach up to 85 meters in length, and the turbines themselves can tower more than 200 meters above the ground, explains FVWE.
Consequently, blades are subject to severely fluctuating wind loads due to the difference in wind distribution close to the ground and at the top end of the turbine. The result is high loads for the rotor blade material and an enormous challenge when it comes to the control system, says the alliance. In the case of storm winds, in particular, the wind load can be so great that turbine operators are forced to power the systems down in order to avoid damage.
According to FVWE, the ideal solution would be rotor blades that are able to adapt their geometry to suit the local wind conditions. This is made possible by active and passive technologies that allow individual blades to adjust to the prevailing wind conditions: so-called “smart blades.”
When a rotor blade subject to high wind turns in such a way that it offers the wind a smaller contact surface, FVWE explains, a bending-torsion coupling (BTC) can happen. When this bending is initiated by the force of the wind alone, it is described as a “passive” mechanism.
This investigation focused on two approaches that produce this effect. On the one hand, a crescent-shaped geometry was examined, and, on the other, a particular structure was employed for the material composition of the rotor blade, the alliance explains.
The glass fibers from which the rotor blade is produced are arranged in such a way that it rotates at different wind speeds; then, the pitch is adapted locally, says FVWE.
“The advantages of the mechanisms are that the blades can be built with a less robust design and are, therefore, lighter. Both processes have the potential to improve the energy utilization of wind power systems,” says Alper Sevinc, Smart Blades’ technology coordinator for the bending-torsion-coupled rotor blades at Fraunhofer IWES.
The researchers now hope to be able to investigate the mechanisms tested in the simulation on demonstration rotor blades they have already designed in a future project.
Another approach is active mechanisms that adapt the trailing edges of a rotor blade and that system operators can use to control the aerodynamic loads. In this respect, the researchers examined both flexible and rigid trailing edge flaps. The concept was inspired by the aviation industry and is comparable to the flaps on the wings of aircraft, says FVWE.
The investigations revealed that both options effectively reduce the load on the rotor blade. However, according to the alliance, the maintenance efforts required for rigid trailing edge flaps are so high due to the soiling of the moving parts that the advantages of flexible trailing edge flaps outweigh them substantially.
The researchers also considered whether a flexible leading edge flap on a rotor blade can improve the efficiency of wind turbines subject to heavily fluctuating, turbulent wind conditions. This mechanism, says FVWE, enables optimal use of a rotor blade in a large wind-speed range.
In addition, the researchers also assessed the cost-efficiency of the technological developments. They compared all the mechanisms to a reference system with an 80-meter-long rotor blade in simulations and came to the conclusion that many of the examined mechanisms could lead to improved rotor blades in the future. In a next step, the researchers hope to be able to test their results on full-scale rotor blades.
According to the alliance, the successful completion of this project has not only cleared the way for further joint activities in the field of intelligent rotor blades, but has also laid solid foundations for future projects in the wind energy sector as a whole.