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Development of a mw scale wind turbine for high wind complex terrain sites (MEGAWIND)

Exploitable results

Within the MEGAWIND project on-site manufactured towers, featuring composite material shells and concrete or other core materials are introduced for application in the wind energy sector. The design and development of an alternative wind turbine tower allowing for on-site manufacturing of the tower involved research on materials and large-scale structural tests on representative subassemblies of the tower. Composite material parts of the tower structure offer the potential of being transported to the site in low weight, small volume pre-forms and assembled on site, thus reducing the transportation costs. Whereas standard concrete and steel towers offer competitive prices in standard erection sites this is not generally the case in isolated locations or in water depths of over 10 metres. New structural solutions are possible by using advanced composite materials technology that may offer cheaper whole-life solutions by increasing the net area of commercially viable wind sites and prolong the life, or reduce the maintenance cost, of the wind generator superstructure. It is expected that a tower made from composite materials will have a number of advantages over a standard steel tower: lower maintenance costs, improved dynamic damping characteristics, extended fatigue life, reduced logistic costs for installation and, potentially, the capacity to improve on some aspects of environmental (landscape) pollution. The aim was to provide an alternative to the existing steel or concrete designs particularly where the site and accessibility conditions are difficult. The construction and manufacturing methodology aims to reduce the logistic burden (heavy cranes and ancillary equipment), usually incurred when erecting towers made up of large monolithic elements, by using light weight composite formwork segments that can be assembled on site. The innovative use of composite materials is also of interest for offshore applications. In this case the advantages are those qualities that are afforded by the low chemical inertness of fibre-reinforced composites in saline environments, as well as the reduced logistic and transportation costs. Although the tower design revolved around a land-based tower, the advantages show that such constructions may offer new structural solutions for offshore platforms in waters over 10 metres in depth. Different manufacturing solutions have been pursued during the course of the MEGAWIND project, in order to discern the optimum one. From the initial design concepts the design group defined a preliminary design, part of which was tested at component level for structural verification. The design approach to such a composite structure is based on the critical interaction between the design and the manufacturing process. Moreover, the tower design and its construction procedure address the key issues relating to the erection of large wind turbines in isolated locations: logistics of construction, long term maintenance and motivate the potential for tower self-erection. The group comprising structural expert organisations, a construction company and a materials testing research laboratory, elaborated the conceptual design of the main structural elements of the tower and provided tower design, based on the use of fibre-reinforced composites. The dynamic and structural performance of the tower was initially verified by numerical simulation. In addition to this, large-scale structural tests on representative subassemblies of the tower design were carried out before the approval of the final construction. Eventually, the group gave extra emphasis on the optimisation of the composite lamination sub-element joining as well as the manufacturing process. The final wind turbine tower design was conducted according to the IEC 61400-1 standard using state-of-the-art tools, extensively validated in complex terrain applications. A 40m height composite material tower was manufactured and the performance of the prototype was evaluated through systematic laboratory testing. Test results will be used for the design assessment of tower. The introduction of new materials and design options in the wind turbine tower industry is expected to impose favourable socio-economic impacts in Europe by enhancing industry competitiveness, supporting employment and promoting social cohesion by the regional industrial development.
Little ingenuity has been shown in the design of geared generator drives in wind turbines. The common approach is to increase the size of the gearbox to match the high dynamic torque, which occurs. This however, results in large, expensive, and not very reliable gearboxes not appropriate for turbines subjected to very turbulent wind flows as for the case of operation on high wind speed complex terrain sites. Statistics on major wind turbine failures reveal gearbox malfunction as the second main cause of wind turbines break down. In addition, available gearbox design standards do not account for the highly variable amplitude dynamic loading typical in wind turbine applications. The novelty of the work performed within the MEGAWIND project is that an integrated design methodology was used for the mechanical drive systems, to minimise dynamic overload torque. The gearbox was specifically designed for high wind speed and high turbulent conditions aiming at developing a highly reliable, low cost, low weight geared drive system for large-scale wind turbines. Special attention was given to the gearbox to achieve very low noise emission, minimizing gear noise at all power levels. Combining innovative and careful design of the whole system with very advanced gear design and stress analysis techniques of gears resulted in an improved gearbox concept for this demanding wind turbine application. The work that was carried out focused on the development of the conceptual design of novel mechanical drive system investigating alternative gearbox configurations. Evaluation of the performance of these drives followed, by analysing their dynamic performance with an appropriate dynamic model. The final selection of the most appropriate involved also the optimisation of the parameters of the mechanical drive to give minimum dynamic overload. Detailed design of the mechanical drive e.g. shafts, bearings, gears and mounting concluded the development of the gearbox. At the same time effort was put on developing a comprehensive instrumentation system to monitor dynamic shaft torques, axial rotor bearing loads, gear-case vibrations etc. The advanced instrumentation system was for the first time installed in an operating in complex terrain site 1.3MW wind turbine to closely monitor loads of the drive train developed inside the gearbox. Extensive field-testing was performed by experienced testing organisations according to the guidelines of IEC 61400-21 standard, including all necessary measurements for the evaluation of the developed concept, emphasising on the measurements taken by the intensively instrumented gearbox. Refinement of the mechanical drive system will be conducted based on these measurements. From the work conducted within the MEGAWIND project it is expected that it will have a positive effect on the development of gearboxes for the next generation wind turbines. The developed design methodology to achieve an optimal gearbox for a dedicated use in a specific wind turbine is expected to improve the cost effectiveness and the reliability considerably, not only on on-shore conditions (as in the case of complex terrain applications), but also on offshore.
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.