Investigation into the Application of Straight-Fibre Variable Stiffness Laminates in Wind Turbine Blade Design

Abstract

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.