This paper presents the development of a structural optimization process for the design of future large thermoplastic wind turbine blades. The optimization process proposed in this paper consists of three optimization steps. The first step is a topology optimization of a short untwisted and non tapered section of the blade, with the inner volume used as the design domain. The second step is again a topology optimization, but on the first half of a blade to study the effect of non symmetry of the structure due to blade twist and taper. Results of this optimization step are then interpreted to build a shell model of the complete blade structure to perform composite size optimization based on a minimum mass objective subjected to constraints on deflection, composite strength and structural stability. Different blade models using ribs are then optimized and compared against conventional blade structure (box spar structure without ribs and single web structure without ribs).The use of ribs in wind turbine blade structures, which is more adapted to thermoplastic composite manufacturing than for thermoset composites, leads to slightly lighter blades than conventional blade structures.

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As the design process of a wind turbine blade is highly iterative, one needs to do the same calculations several times. During that process, the kind of structural model use must be chosen carefully trying to obtain a good compromise between precision and model setup and computation time. This paper compares four blades structural models of different levels of complexity. These models are compared to each other and with experimental results with respect to their abilities to analyze blade cross-sectional properties, natural frequencies, deflection, strains, buckling strength and composite strength. This comparison shows that even if the 3D shell finite element model is the more precise and is the only one that can manage the regions of the blade where the cross-sectional shape changes quickly, models based on strength of material models give accurate results. Even the simpler model, based on blade shape simplification, gives conservative and accurate results at a very low computational cost.

A method for structural analysis of thin-walled composite beams like wind turbine blades is presented. This method is based on the Nonhomogeneous Anisotropic Beam Section Analysis (NABSA) which consists in discretizing the beam cross section using finite elements. The proposed implementation uses 3-node line cross-sectional finite elements with nodes having rotational degrees of freedom to describe the cross-sectional warping displacements. Solutions obtained using this approach were verified against the corresponding analytical or numerical solutions. Agreement was very good to excellent for the computation of cross-sectional properties and distribution of stresses, strains and warping displacements for a broad range of possible composite beam behaviors including geometric and material couplings, open sections, multicell sections, and arbitrary laminates. For thin-walled layered structures, the proposed method provides models with fewer degrees of freedom than equivalent models based on a two-dimensional discretization of cross sections using triangular or quadrilateral elements such as conventional NABSA or VABS which suggests that computation time could be reduced.

This article first presents a description of the different load types to which a wind turbine blade is subjected. Analytical equations are derived to express blade loads from operation parameters of the wind turbine (rotor and nacelle velocities and accelerations; pitch, coning, tilt, and azimuth angles; blade mass properties; turbine geometry). This allows a better understanding of the contribution of each of these parameters to the total load on a blade. A difficulty arises for transferring the loads computed by an aeroelastic model (a one-dimensional model of the blade) to a three-dimensional finite element model of the blade. A method is proposed for that purpose. It consists in applying the aerodynamic loads using RBE3 elements and applying gravitational and inertial loads as volume forces. Finally, an example of this method used for the design of a 10 kW wind turbine blade is presented.