Upscaling of wind turbine blades
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The wind turbine industry is rapidly growing, and wind turbines are growing in size accordingly. For the wind turbines in the past and up to the present commercial size, aerodynamic forces have basically governed all blade designs. However, as the turbines continue to grow in size, the gravity forces should therefore be carefully included in the design process for large wind turbine blades. In the present study, the aerodynamic and gravity forces together with their corresponding stress levels have been simulated for3 different wind turbine sizes with diameter of 60 m, 90 m and 120 m. Two different upscaling models have been used for this simulation. A simple upscaling model uses analytical functions to obtain first estimates for the forces, bending moments and stresses. An advanced upscaling model couples a blade element momentum theory together with beam considerations. An aerodynamic analysis is performed in the advanced upscaling model within the framework that the structural analysis requires. Analytical functions are used in this analysis to describe the airfoils of the turbine blade. The aerodynamic performance is evaluated with respect to the annual energy production. For both models, a box spar of Glass Reinforced Plastics within the airfoil carries the loads. A sandwich material surrounds the airfoil. In addition, a trailing edge spline is inserted to ensure edgewise bending strength in the advanced upscaling model. Results from the simulations show that wind turbines between 60 m and 90 m in diameter are structurally designed to withstand the aerodynamic flapwise stresses. Simulations of the wind turbine with a diameter of 120 m show that the edgewise gravity stresses become more severe than the aerodynamic flapwise stresses for the inner aerodynamic sections. Hence, a design shift occurs for these inner sections. To reduce the edgewise stresses, a redesign of the blade geometry is therefore needed. Several redesign suggestions are implemented and discussed. Based on the simulation results and the redesign suggestions for wind turbine with a diameter of 120 m, scaling recommendations for the blade geometry are obtained. Scaling recommendations for both the blade planform, included chord and twist, and the blade airfoils presented. The airfoil parameters include relative thickness, position of maximum thickness, camber, positon of maximum camber and nose radius of the airfoil. In addition, the structural geometry within the airfoil, including spar geometry and spar material, are also subjected to scaling guidelines. The scaling recommendations presented in this study apply for wind turbines between 60 m and 120 m in diameter.