Research Alliance Wind Energy presents project results 3.2.2016

Nowadays, wind turbine rotor blades can reach up to 85 metres in length and the turbines themselves can tower more than 200 metres above the ground. Consequently, rotor 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. In the case of storm winds in particular, the wind load can be so great that turbine operators are even forced to power the systems down in order to avoid damage. From a cost-efficiency perspective that is particularly bad, as stronger wind translates to higher energy yields.

The ideal solution would be rotor blades which are able to adapt their geometry to suit the local wind conditions. This is made possible by active and passive technologies which allow individual rotor blades to adjust to the prevailing wind conditions – so-called smart blades. The Smart Blades project was a joint effort between researchers from the FVWE together with the German Aerospace Center (DLR), Fraunhofer IWES and ForWind, the Center for Wind Energy Research of the Universities of Oldenburg, Hanover and Bremen aimed at investigating the effect of these technologies. The findings of this project offer turbine developers and operators new information and tools allowing them to launch more effective, more cost-efficient and more reliable system designs on the market.

Intelligent structures react to wind turbulence
When a rotor blade subject to high wind turns in such a way that it offers the wind a smaller contact surface, researchers speak about a bending-torsion coupling (BTC). As this bending is initiated by the force of the wind alone, it is described as a “passive” mechanism. The investigation focused on two approaches which 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.

In this structural approach, the glass fibres from which the rotor blade is produced are arranged in such a way that it rotates at different wind speeds, thereby the pitch is adapted locally. “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,” said 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.

Active control elements in the rotor blade
Another approach pursued by the scientists is active mechanisms which adapt the trailing edges of a rotor blade and which system operators can use to control the aerodynamic loads on a rotor blade. 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. The investigations revealed that both options effectively reduce the load on the rotor blade. However, 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 construction of demonstration blades for this concept is also planned at some point.

Flexible leading edge flaps create an optimal profile
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 enables optimal use of a rotor blade in a large wind speed range. “The advantage in this respect is in the reaction speed of the flexibility of the leading edge flap, which allows rapid influencing of the active aerodynamic forces in turbulent influx conditions,” said Michael Hölling, Smart Blades technology coordinator for rotor blades with flexible leading edge flaps at Forwind, commenting of the potential of the adaptive leading edge flap. The concept of the flexible leading edge flap was tested in a wind tunnel during the project and delivered promising results for future developments.

In addition, the researchers also assessed the cost-efficiency of the technological developments. They compared all the mechanisms with a state-of-the-art reference system with an 80-metre-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.

The Smart Blades project was one of the first large research projects conducted by the research alliance founded in 2012 to be completed successfully. “The outstanding cooperation within the group is reflected in the promising results delivered by the project. The project has shown that the different skills offered by the various partners complement each other and dovetail ideally,” stressed Ceyda Icpinar, Smart Blades project manager from the DLR Institute for Composite Structures and Adaptive Systems in Braunschweig, Germany. The successful completion of the 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.

The Research Alliance Wind Energy, founded in 2013, combines the know-how of more than 600 scientists and generates groundbreaking stimulus for the energy supply of the future. The three partners – German Aerospace Center (DLR), ForWind – the Center for Wind Energy Research of the Universities of Oldenburg, Hanover and Bremen and Fraunhofer Institute for Wind Energy and Energy System Technology (IWES) North-West – are able to successfully carry out major long-term and strategically important projects, hanks to their manpower and strong ties to major industry players, politics and research institutions.