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30m split wind turbine blade

Work within the course of the MEGAWIND project resulted in the design and manufacturing of a 30m blade. The geometry of the blade is optimised for maximum energy capture under high wind speed conditions, while the structural design was carried out for a split-blade concept, with the aim to facilitate logistic problems, i.e. handling during manufacturing, transportation etc. The blade is made mainly of glass/polyester, whereas for specific stiffness requirements, carbon fabric was also considered during the design phase to be used in selected blade regions. Joint design addresses the highly complex 3-D stress states around eventual discontinuities. The wind turbine blade design was conducted according to the IEC 61400-1 standard using state-of-the-art tools, extensively validated in complex terrain applications. Special emphasis was given in the conceptual and structural design of joining elements.

The concept of splitting blades is not entirely new since to our knowledge it first appears in early 80¿s with the design of a 12m blade by DLR for the DEBRA-25 wind turbine. In addition, an ongoing JOULE project, JOR3970167, entitled ¿Design, structural testing and cost effectiveness of sectional wind turbine blades¿, takes a comprehensive look at possible connection designs to test the most promising, and then to design, build and test for static and fatigue strength a prototype sectional blade.

However, it is the first time that a Class-I, IEC-61400-1 lightweight, splitting blade of the MW scale was developed and full-scale tested. Further, the structural design of the joints address the determination of the highly complex 3-D stress states and fatigue life verification was based on the use of fatigue strength criteria suitable to account for multiaxial stress states. The validation of accurate fatigue life estimation for the joining techniques through extensive laboratory sub-component testing is also innovative.

Additionally, aerodynamic blade design was performed for maximizing the Annual Energy Capture (AEC) of the rotor at a typical high wind site. AEC was evaluated using enhanced blade element theory. Design variables were the blade planform characteristics (chord and twist span-wise distribution) and the profile shapes of the blade sections. Previous work has shown that AEC can be increased in the order of 2-5% by careful redesigning for high wind sites, compared to more ¿conventional¿ solutions.

This improvement is achievable through the re-adjustment of the blade sections maximum lift combined with an appropriate displacement of the drag laminar pocket to higher lift values (profiles with increased camber) without affecting maximum loading or blade area (weight). An extended database of such profiles is available to the partners, including the geometry and the lift-drag polars of almost 200 airfoils. A genetic-algorithm-based optimiser was employed, allowing for discrete optimisation (search in the profile database to select the best suited for a specific blade section). Constraints were put on the maximum blade loading and geometry (blade area, maximum thickness of the blade sections etc.).

Technical risks are mainly related to the accuracy of the theoretical predictions for estimating fatigue life of joining structural details but extensive component testing efforts will minimize their effect. With respect to aerodynamic design, prediction uncertainties in profile load characteristics might affect the expected AEC increase.

Eventually, a 30 m long prototype of the split blade was manufactured and thoroughly tested in a full-scale testing laboratory, according to IEC 61400-23 standard. The introduced innovative design options are expected to enhance cost effectiveness of wind turbines with respect to manufacturability. Production of split blades presents significant advantages by reducing manufacturing time, space and infrastructure requirements, and by increasing production line flexibility. Existing production lines for medium size blades can easily accommodate the MW size machine blade parts. Moreover, in terms of installation, modular blades will minimize transportation costs.

Though on-site assembly increases installation time, it offers significant benefits concerning easy transportation at sites of poor accessibility and lack of infrastructure. Additionally, lower maintenance costs are expected due to modular component replacement in case of wear/damage, e.g. in case of a catastrophic lightning strike, the outer blade part might be replaced only instead of the entire blade.

The introduction of new design options in wind energy industry will impose favourable socio-economic impacts in Europe by enhancing industry competitiveness, supporting employment and promoting social cohesion by the regional industrial development.

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Reported by

University of Patras
Department of Mechanical Engineering & Aeronautics, P.O.Box 1401 Panepistimioupolis Rio
265 04 Patras
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