Over the last decades, wind turbine blades have continuously grown in size to harvest further power from the wind. Longer blades have an increased structural mass and thus suffer from increased gravitational and inertial loads. Current developments in the field of wind turbine blade design strive for lightweight, high-bending stiffness, yet cost-effective structures. An approach to drive down mass is through the combined optimisation of aerodynamic and structural properties of a blade, which is commonly referred to as aeroelastic tailoring. Current structural optimisation strategies of wind turbine blades could benefit from existing aerospace structural tailoring approaches, such as straight-fibre variable stiffness design. Straight-fibre variable stiffness, also referred to as multi-patch laminate blending, consists in partitioning a structure into constant laminate regions that are locally optimised. Those regions are then blended to bring back continuity, thus ensuring structural integrity and manufacturability. Locally optimising regions allows the stiffness to be tailored to the specific needs of each region and allows the definition of load paths within the laminate. Laminate blending enables further structural tailoring through variable stiffness composite design while preferring the use of conventional laminate patches over more costly continuous fibre angle variation. The present research aims to investigate the structural performance potential for straight-fibre variable stiffness laminates in wind turbine blade design. This research objective was tackled in two phases. First, a design methodology is proposed to couple a wind turbine aeroelastic optimisation framework with a laminate blending design algorithm. Second, the proposed design framework was applied to a blade section to evaluate the achievable structural performance when straight-fibre variable stiffness laminates are introduced in large-scale structures. The design framework for straight-fibre variable stiffness design of a blade section first optimises the lamination parameter distribution for each laminate region of the design. Then a conversion of the lamination parameters design to a stacking sequence is performed, followed by the enforcement of laminate blending constraints. To quantify the performance of the blending approach, the DTU 10MW reference turbine blade was selected as the baseline structure. Specifically, a section of this blade was assessed, taken at the maximum chord location. At this site, the largest trailing-edge panels are present, and structural requirements such as buckling are critical. The objective of the laminate blending optimisation was extended to maximise the buckling performance of the considered section while matching a tip deflection requirement. When refining the number of laminate regions present on the trailing-edge panels, improvements in buckling performance up to 68% are achieved, with a limited increase in tip deflection of 4.8%. Overall, the methodology presented in this research highlights the potential improvements achieved with straight-fibre variable stiffness laminates when applied to a blade section. Further research is recommended in refining the modelling of a blade structure, namely to define sandwich materials and shear webs. Even though a methodology to evaluate the aeroelastic response of a full blade with variable stiffness structures is presented, this methodology was not assessed. The application of this approach to a full blade design could highlight the tailoring potential of stiffness variation and load redistribution enabled through laminate blending for aeroelastic applications.

Material bend-twist coupling has been widely studied in the research community as a passive control mechanism for a wind turbine. However, there is a lack of research in incorporating it in the rotor design process, with little research being conducted for its application to small wind turbines. The present study focuses on this issue by including bend-twist coupling in the design of a 500W wind turbine by using a combination of parametric studies and a multidisciplinary constrained optimisation approach. By doing so, it aims to establish the effectiveness of bend-twist coupling as a tool for passive load alleviation in small wind turbines. The effectiveness is tested through obtaining a significant decrease in the flapwise blade root bending moment accompanied by only a marginal decrease in the AEP, when compared with the baseline uncoupled turbine. The reference blade is designed under limitations imposed by the rules of the Small Wind Turbine Design Contest. Bend-twist coupling is introduced in the blades with a fixed aerodynamic design. The rotor performance is analysed inHAWCStab2. The internal structure of the blade is created with the intention of producing flexible blades with a single composite material used throughout the blade. Carbon-epoxy and glass-epoxy FRPs are considered as the material to be chosen in the unidirectional laminae. The cross-sectional stiffness analysis is conducted using BECAS for a range of fibre layup angles in both carbon-fibre and glass-fibre blades. Carbon outperformed glass for all fibre angles with regard to the amount of coupling seen in the crosssections. Apart from flapwise bend-twist coupling, other secondary torsion couplings are also present. A load response study is carried out for varying positive fibre layup angles in both carbon-fibre and glass-fibre blades, under steady wind conditions for the operational wind speed range. An increase in the flapwise blade tip displacement and a reduction in the flapwise bending loads with increasing fibrelayup angles is observed for both glass-fibre and carbon-fibre blades. The HAWTOpt2 aero-structural design tool using OpenMDAO as its core is utilised to implement the optimisation. The spanwise fibre layup angle and laminate thickness distribution are the only design variables that were allowed to be manipulated by the optimiser. The objective function is comprised of two different individual objective functions weighed according to the situation. The optimisation cases failed due to inaccurate gradients of the objective function and constraints. Due to time constraints the manufacturing of the blade and, static load and wind tunnel testing were not carried out.