Fraunhofer Institute for Wind Energy and Energy System Technology (Fraunhofer IWES) expects to receive a EUR 10 million financial investment from German federal authorities to further develop a new method of rotor blade testing, as well as improve its test infrastructure in Bremerhaven.
At present, rotor blade prototypes are tested by moving them separately both in and perpendicular to the direction of the wind in order to obtain information about how their material behaves over their 20-year service life more rapidly – a major simplification of how loads actually occur simultaneously in the field, according to the research centre.
The method currently under development in Bremerhaven as part of the “Future rotor blade concept” research project is based on separating a blade into segments for testing – e.g. root segment and rotor blade tip. The method reportedly has two advantages: tests become possible at higher frequencies and with a more accurate load profile.
The tests are rendered even more accurate when individual sections with a critically high load and greater material thickness or strong curvatures, for example, are investigated separately. This approach not only produces more informative results, but also reduces the testing times by a calculated 30%, which means a noticeable cost saving, according to the researchers.
Since, in reality, only one rotor blade will undergo the complete blade testing procedure right through to certification, the calculative safety factors selected are not necessarily those leading to optimum costs, but rather those which can cope with the operational loads with certainty. A higher number of tests that can be realized at reasonable cost allow the safety margins to be reduced, which, in turn, means a more economical design for the rotor blades, the research centre said in a release.
At the conclusion of the first phase of the research project, which will take five years in total, the infrastructure will be operational and the test methods developed.
“It is a very ambitious schedule, but we can build on ten years of experience with mechanical test methods and a sound understanding of material properties and the behaviour of fibre composites; and the pressure in the industry to innovate is a definite incentive,” Florian Sayer, head of department, said.
While component and blade segment tests are already turning up more and more frequently in the industry’s list of requirements, the testing of critical sections is still a long way off. This is done by dividing the rotor blade according to the requirements of the investigation in order to be able to take a closer look at the critical areas. The subsequent execution of the load tests requires a complex infrastructure and profound knowledge of how complex load cases affect the structure.
A so-called hexapod test stand with a Reynolds platform to apply torsion forces and bending moments in parallel is being constructed in Bremerhaven – right next to the established complete-blade test stands and material testing laboratories. Manufacturers of rotor blades benefit from significantly shorter tests and particularly realistic load simulations, and their modified infrastructure set-up leads to lower energy costs as well, according to the research centre.