The giant rotor blades are one central part of a wind turbine. Researchers developed a rotor blade that makes more efficient use of large fluctuations in wind strength using a passive bend-twist coupling (BTC) that adapts continuously to the wind forces acting on the rotor blade. When the wind loads become too high, the coupling reduces the forces affecting the structure.

A camera-reflector system detects any deformation in the blade's three main axes. (©Fraunhofer IWES, Pascal Hancz)

The rotor blades of conventional wind turbines react to changing wind strengths very slowly. The pressure acting upon the blade pointing upward, for example, can be very different from the pressure on the lower blade. Conventional rotor blades cannot compensate for a single gust of wind, as they are too rigid to twist. This means if there is a gust when the wind is already strong, the turbine operators turn the rotor blades completely away from the wind. This results in long downtimes during which no electricity is produced.

The “intelligent” BTC blade developed as part of this project is swept back while the blade tip is offset slightly to the rear in the direction of rotation. The 20-meter-long rotor blade is therefore able to rotate slightly around its own axis should strong gusts occur, in order to mitigate the wind pressure to a certain degree. This reduces the forces acting upon the blade and, ultimately, the entire turbine. By using BTC blades on a new turbine, the overall turbine weight can be reduced as the structure is subjected to lower loads. In the case of existing turbines, the retrofitting of BTC blades allows the rotor diameter to be increased without having to adapt the other turbine components.

The BTC blade in the extreme load test. The loads are applied via three hydraulic cylinders. (©Fraunhofer IWES, Pascal Hancz)

To test the design, deformation along the three main axes was monitored using a visual measurement system. Angle sensors were also used to ensure that the force was indeed introduced vertically to the blade axis. During the subsequent dynamic tests (fatigue tests), the stresses incurred over the entire service life of the rotor blade spanning 20 years are simulated within a drastically reduced time frame.

Upon completion of the test bench testing, three identical BTC rotor blades will be installed at the foot of the Rocky Mountains for a field test on a research turbine from the project partner, National Renewable Energy Laboratory (NREL). The aim of subsequent measurements is to demonstrate whether passive twisting performs as expected in real, open-air operation. Two pressure sensors on the surface of the blade measure the flow dynamics around the rotor blades. The flow on the rotor blade is also made visible by strands of wool, allowing researchers to determine the aerodynamic conditions precisely. Within the blade, further sensors work to measure the acceleration at the blade tips, while camera reflector systems detect any deformation.

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