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The smart blade in the extreme load test: the loads are applied via three hydraulic cylinders
© Pascal Hancz, IWES
Research projects revisited

The optical graduated grids on the hall’s wall detect deformation in the three main axes of the blade
© Pascal Hancz, IWES

The wind optimises the rotor blade

With increasing blade lengths, the aerodynamic vibration loads on rotor blades also increase. Conventional construction and control concepts are also increasingly reaching their limits. One possible solution is rotor blades with bending-torsion coupling. In strong winds, they not only bend but also twist axially. Such a demonstrator rotor blade is currently on the test stand. Once the measurements are completed, identical units will be installed on three research wind turbines.

Conventional blade pitch systems for rotor blades reach their limits at very long blade lengths because they react too slowly – especially to local gusts. This is why the concept of bending-torsion coupling for rotor blades has already had the wind industry’s attention for some time now. In these blades, the shear load is coupled to the axial rotation (torsion) of the blade. This has the advantage that the blade can react immediately to gusts without additional control technology, relieving the turbine structure overall. The particular challenge lies in precisely calculating the dynamic behaviour of the rotor blade in advance, and the resulting structural implementation.

The research project is called Smart Blades2 and aims to further increase the economic efficiency of wind turbines and to develop large blades using the new concept. There are two approaches for greater economic efficiency: Newly designed wind turbines can be designed lighter from the outset – compared to previous systems – thanks to the bending-torsion blades. This saves material and logistics costs. Longer blades can be installed on existing wind turbines by using blades with bending-torsion coupling without the need to reinforce the entire wind turbine. The larger rotor diameter also increases the amount of electricity generated.

The research project is carried out collaboratively by the Research Alliance for Wind Energy, with members including the Fraunhofer Institute for Wind Energy Systems (IWES), the German Aerospace Centre (DLR) and the ForWind research partnership. In addition, seven industrial partners, mainly wind turbine manufacturers, are participating in the project.

From test stand to field test

The 20 metres long demonstrator rotor blade had already been designed on the computer during the previous project. On the test stand, not only two edgewise and flapwise directions are each tested, but also torsion. IWES test engineer Tobias Rissmann describes the challenges: “The setup for the torsion test of the rotor blade is similar to the static test scenario, but requires more effort for an exact measurement of the additional deformation.” In the course of the test cycle, the demonstrator blade will be exposed to the stresses of 20 years of turbine operation in time-lapse.

After completion of the investigations on the test stand, a field trial lasting several months will begin. Three identical blades with bending-torsion coupling will be installed in the United States to research wind turbines. The National Energy Laboratory (NREL) there is an international cooperation partner. The IWES group for certified field measurements is responsible for the management of the measurements. The researchers want to clarify to what extent the load reductions simulated in the computer can also be proven in practice. The recently developed Aeroprobe system is employed for the measurements. Two pressure sensors measure the flow around the rotor blade surfaces. In addition, the acceleration at the blade tips and the deformation of the blades are recorded during operation.

Predecessor project Smart Blades1

The simulation programs and calculations originate from the predecessor project Smart Blades1. The simulation tools used until then were only able to reproduce the torsional stiffness of a blade to a limited extent. Only the tools developed in Smart Blades were able to do this. The demonstrator rotor blade has now been built based on this computer-aided design. The BINE Projektinfo brochure entitled "Better bend than break" (16/2016) presents the predecessor project.



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