Influence of Blade Structural Deformation on Wind Turbine Rotor Performance

Abstract eng:
The efficiency of wind turbine rotors depends heavily on aerodynamic performance of blades. To create torque from wind, airfoils are used to define the skin geometry of composite blades in modern wind turbines. An important key design aspect of blades is determining the right twist angle along the blade where airfoils operate with maximum lift to drag ratios to optimize the blade aerodynamic efficiency. This is directly related to effective angle of attack of airfoils in desired operating conditions and variation in effective angle of attack influence the aerodynamic performance of airfoil, blade and naturally rotor. In this regard, it is also important to understand the structural behavior of composite wind turbine blades to design them to operate in highest efficiency under desired operating conditions. Composite structures are built by a combination of fiber and matrix materials. The orientation of fibers and lamination sequence have great effect on mechanical properties of the structure. A coupling between different modes of deformation e.g. bend-twist or extension-twist can exist due to fiber orientation and lamination sequence. Wind turbine blades operate under aerodynamic, gravitational and inertial loads and under these loads the blade structure deforms and twist angle changes according to coupling mechanisms. This is a mutual interaction phenomenon which should be treated iteratively considering the change in twist angle will also affect the aerodynamic loading on the blade. In first phase of this study, Blade Element Momentum (BEM) theory is used for design, calculation of aerodynamic loading and performance of a wind turbine blade. BEM theory is a proven, fast and effective method of aerodynamic analysis of blades. In second phase, to represent the structural behavior of the composite blade, a monocoque cross section approach based on thin walled composite structure theory is used. This is followed by development of a FE beam model to reduce the 3D structural properties to 1D beam elements for the sake of computational efficiency. In the last phase, interaction between aerodynamic and structural models is constructed and solved iteratively to understand the effects of loading and structural coupling on the performance of turbine rotor.

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