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Assessment of Research Needs for Wind Turbine Rotor Materials Technology
In the recommendations below, specific research tasks are listed that need to be carried out. Within each category of research—materials, manufacturing, structural response, etc.—these research tasks are listed in approximate order of priority. However, we wishes to emphasize that the overall goal of an R&D program should be to develop a system to produce longer-lived, less expensive, and more efficient wind turbine rotors. Increased knowledge of the fatigue properties and fatigue failure mechanisms of blades should take precedence, but this cannot be separated from the search for better manufacturing processes or from design innovations that will either minimize the likelihood of failure or ease the aerodynamic constraints of blade shape that impede process innovation. The committee wish to emphasize that the four factors of fatigue, manufacturing, advanced materials, and design are closely interrelated in the quest to produce a more cost-effective blade.
Recommendations are classified with respect to four goals. More detailed recommendations will be found at the ends of Chapters 2, 3, 4, 5, and 6.
Goal 1: To improve the material properties and design capability so that the structure will withstand higher stresses, or the same level of stress for a much longer period of time.
Long-term fatigue data should be developed for the most common glass-reinforced plastics (GRP) laminates and critical elements under appropriate environmental conditions. The data should be carried to 108 to 109 cycles if possible, at stress ratios of R = 0.1 (tension and compression) and R = -1 (fully reversed). An extensive search of all fatigue data on GRP composites should be conducted and published in a source convenient to blade designers. This should evolve into a databank of wind turbine blade materials.
The extensive compendium of mechanical data, including fatigue data, on wood/epoxy laminates should be published and made available to domestic blade designers. Very high cycle (108 to 109) fatigue tests should be conducted on this material to compare its fatigue response with that of GRP materials.
The potential benefits for significant weight reduction in blades (about 50 to 70 percent) while maintaining required stiffness through the use of hybrid composites (in which carbon or aramid fibers are placed in critical blade locations) should be explored through design studies and limited blade testing. Critical use of cost models must be a requirement in this work because of the strong industry reluctance to utilize materials that are more expensive than E-glass/vinyl ester or wood/epoxy.
Goal 2: To lower the operating stress levels by altering the structural/configuration design.
Simple cross-sectional analyses and computer codes need to be developed for determination of sectional elastic constants in composite blades with elastic couplings. These design tools should consider blade parameters such as curvature, twist, taper, and, above all, completely general material/geometry.
An aeroelastic design code should be developed for wind turbine blades. This would permit the investigation of aeroelastic tailoring as a passive control mechanism.
An investigation of new active control techniques for wind turbine blades should be initiated. This should be aimed at a new generation design in which gust loads are essentially reduced, thereby minimizing over-design of blades.