For a normal test of a prototype rotor blade, the simultaneously occuring loads in the field are simplified. As part of the “Future rotor blade concept” research project, scientists at Fraunhofer IWES are developing new methods that provide significantly more realistic data and allow a load-appropriate design to be produced. The Federal Ministry for Economic Affairs and Energy and the Federal State of Bremen intend to invest € 10 million to further develop methodological expertise and an innovative test infrastructure in Bremerhaven.
Reliable and economical determination of the operational robustness of XXL blades
Better safe than sorry: 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.
This is the starting point for the rotor blade experts at Fraunhofer IWES. Separating a blade into segments for testing - e.g. root segment and rotor blade tip - 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 innovative approach not only produces more informative results, but also reduces the testing times by a calculated 30%, which means a noticeable cost saving.
Test infrastructure to be operational by the middle of 2018
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. Florian Sayer, head of department, commented on the time frame as follows: “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.”
Rotor blade for detailed investigations
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.
From the overall perspective of the wind power industry, these test facilities play their part in reducing the energy production costs: When developers have a sure foundation on which they can employ greater creative freedom to develop a blade design optimised for efficiency and reliability, the economic efficiency of wind power utilization and thus its development potential increases.
Britta Rollert | Fraunhofer Institut für Windenergie und Energiesystemtechnik IWES
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