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We propose a new optimisation framework developed for the investigation of innovative wind turbine blade designs. The design of wind turbines has progressively evolved over recent decades as part of an ongoing effort to provide economically competitive solutions for wind energy production. In particular, rotors have increased in size so as to capture more wind energy while limiting installation costs. At the same time blade designers have had to continually improve the structural efficiency of blades in order to accommodate higher extreme and fatigue loads resulting from growing rotor diameters. Modern wind turbine designs are the result of these incremental improvements, limiting financial risks but also confining the design space and effectively reducing opportunities for more radical innovation. In this paper, we enable the wider exploration of the wind turbine blade design space by means of a new optimisation framework. For that purpose we develop and combine state-of-the-art tools for the aero-servo-elastic analysis and optimisation of wind turbines aiming to explore the uncharted design space resulting from decades of incremental changes. Our framework relies on the use of B-spline surfaces and lamination parameters to provide a compact and continuous means of describing blade structures, also enabling the use of gradient-based optimisers. This structural parameterisation is further combined with beam and shell finite element models to provide further confidence in preliminary structural designs. The proposed framework is presented and verified herein. Validation results show good agreement with the modern large scale DTU 10 MW blade design. Additionally, the coupled bend-twist behaviour of the beam model is found to agree well with higher fidelity finite element model predictions.

Abstract. An in-depth study has been completed to study the effects of slender, flexible blades in combination with high rotor speed operation on load mitigation, targeted at cost reductions of the structural components of large wind turbines, consequently lowering the levelized cost of energy. An overview of existing theory of sensitivity of turbine fatigue loading to the blade chord and rotor speed was created, and this was supplemented by a proposed theory for aboverated operation including the pitch controller. A baseline jacket-supported offshore turbine (7MW) was defined, of which the blade was then redesigned to be more slender and flexible, at the same time increasing rotor speed. The blade redesign and optimisation process was guided by cost of energy assessments using a reduced loadset. Thereafter, a full loadset conform IEC61400-3 was calculated for both turbines. The expected support structure load reductions were affirmed, and it was shown that reductions of up to 18.5 % are possible for critical load components. Cost modelling indicated that turbine and support structure CapEx could be reduced by 6%. Despite an energy production reduction of 0.44% related to the thicker airfoils used, the blade redesign led to a reduction in Cost of Energy. IntroductionA key challenge to improve the economics of offshore wind energy -and thereby sustained public and political support for large programs -is lowering the cost of the structural components of large wind turbines and their support structures. The hypothesis is that increasing design rotor speeds in combination with significant reductions to blade chord lengths and blade stiffness can mitigate loading on these components, hence reducing their cost and wind farm cost of energy. The authors have completed an in-depth assessment to these effects, the results of which are presented here.The study was conducted throughout 2013 in the context of the DNV GL Garrad Hassan FORCE project (FOR Reduced Cost of Energy), a research project aimed at fully integrated design of large offshore wind turbines. This FORCE project consisted of two phases: phase I, where the use of advanced control strategies was demonstrated, and phase II, which focused on the blade redesign and is discussed in this paper. This research aims to compliment other studies in the field of integrated turbine design [3] [4].The research discussed in this paper uses two approaches. The first approach, described in section 2, is a theoretical approach; an investigation to the scaling relations that describe the dependence of turbine fatigue loading to changes in design rotor speed and blade chord lengths. The second approach, described in section 3, is a full design study comparing a high speed, slender rotor turbine to a baseline turbine, using aero-hydro-elastic time history based load calculations and assessment of cost effects using cost models.