March 1, 2018
Compared with other rotor blade prototypes measuring more than 80 m in length from industrial customers, this test object is small, nevertheless it is certainly keeping IWES´ scientists on their toes: for the very first time, a rotor blade with bend-twist coupling was tested whose design was developed by Fraunhofer IWES in a previous project. When subject to aerodynamic loads, the prebend of the blade results in mechanical deformation during operation, the purpose of which is to mitigate the loads on the overall structure. Following further tests, three identical models will be constructed by the German Aerospace Center (DLR) and shipped to the USA. The blades will then be installed there on a research turbine for a measurement campaign lasting several months. These activities will all take place within the scope of the “SmartBlades2” research project.
The static rotor blade test is always highly anticipated by blade manufacturers: The prototype’s structural design must be able to withstand the load of a simulated extreme gust. No certifiers are involved in the test this time, as this is a demonstrator rotor blade from a research project, which the test engineers at the test bench are very familiar with – after all, they designed it on the computer. In addition to testing of the two flap- and edgewise directions, testing of a rotor blade with bend-twist coupling (BTC) also comprises a torsion test. “Although the setup for the rotor blade torsion test is similar to the scenario for static testing, it is in fact more complex as the additional deformation has to be measured precisely,” said IWES test engineer Tobias Rissmann summarizing the particular challenge of the test. During the subsequent dynamic tests, the stresses incurred over the entire service life of the rotor blade spanning 20 years are simulated in a drastically reduced time frame.
Making effective use of BTC with very large blades
The concept of bend-twist coupling has been the focus of research in the wind sector for some time now. The coupling of the thrust load with the torsion of the rotor blade structure offers the advantage of being able to react immediately to gusts and thus reducing the pressure on the turbine structure without the need for additional control actuators. The challenge with BTC lies in accurately predicting the dynamic behavior of the rotor blade and its structural implementation. Considering very large blades in particular, the possibilities offered by classic blade control systems are limited as they are too slow and cannot react to local gusts. As part of the “SmartBlades2” project, the members of the Wind Energy Research Alliance: DLR, ForWind, and Fraunhofer IWES together with six industrial partners – GE Global Research, Henkel, Nordex, Senvion, SSB Wind Systems, Suzlon Energy, and WRD Wobben Research and Development – are examining, among other things, how BTC can be used reliably and effectively on very large blades. The aim of the project is to boost the economic efficiency of wind energy even further.
To this end, two approaches can be taken: The use of BTC blades on wind turbines in the development stage makes the turbine lighter compared to a turbine with conventional blades, due to the reduced load on the structure. This, in turn, means a drop in material and logistics costs while maintaining the same energy yield. In existing turbines, the use of BTC blades allows the rotor diameter to be increased without having to strengthen the other turbine components. This results in an increase in revenue thanks to a greater wind yield.
In the previous project, “SmartBlades1”, simulation tools for the blade design were both developed and extended. During the current project, these have now been assembled and validated by means of experimentation. “The simulation tools available today for rotor blade design are only able to take torsional rigidity into account to a limited extent. The tools used in the “Smart Blades1” project enable BTC to be integrated specifically into the design process,” explained the IWES technology coordinator for BTC blades, Dr. Elia Daniele. A rotor blade is being produced and tested on this basis in the “SmartBlades2” project. The next step now involves measuring the effect of bend-twist coupling on the entire wind turbine in a field test.
Measuring campaign in the USA over several months
The aim of the accompanying measuring campaign is to determine whether the load reduction thanks to BTC seen in the simulation can be recreated under real-life conditions and whether its effect is as pronounced as expected. Under the supervision of the IWES Group for accredited and certified field measurements, a test campaign will be performed, in part using the newly developed “Aeroprobe system”, on the test turbine from the US project partner National Renewable Energy Laboratory (NREL). Two pressure sensors on the surface of the blade will measure the flow dynamics around the rotor blades. Moreover, the acceleration on the blade tip and deformation in the blade during operation will be detected.
The SmartBlades2 rotor blade functions as a means of demonstrating this technology with the purpose of assessing its usability on large commercial blades. The German Federal Ministry for Economics Affairs and Energy (BMWi) has provided funding for the project to the tune of €15.4 million.
Project partners: DLR, Fraunhofer IWES, ForWind, GE Global Research, Henkel AG & Co. KGaA, Nordex SE, Senvion Deutschland GmbH, SSB Wind Systems GmbH & Co. KG, Suzlon Energy Ltd., and WRD Wobben Research and Development GmbH
Funding: German Federal Ministry for Economics Affairs and Energy (BMWi) (grant no. 0324031 A/B/C/D/E/F/G/H)
Project Duration: 06/2016 - 11/2019