This work is concerned with the design of wind turbine blades with bend-twist-to-feather coupling that self-react to wind fluctuations by reducing the angle of attack, thereby inducing a load mitigation effect. This behavior is obtained here by exploiting the orthotropic properties of composite materials by rotating the fibers away from the pitch axis.The first part of this study investigates the possible configurations for achieving bend-twist coupling. At first, fully coupled blades are designed by rotating the fibers for the whole blade span, and a best compromise solution is found to limit weight increase by rotations both in the spar caps and in the skin. Next, partially coupled blades are designed where fibers are rotated only on the outboard part of the blade, this way achieving good load mitigation capabilities together with weight savings. All blades are designed with a multilevel constrained optimization procedure, on the basis of combined cross-sectional, multibody aero-servo-elastic and three-dimensional finite element models.Finally, the best configuration of the passive coupled blade is combined with an active individual pitch controller. The synergistic use of passive and active load mitigation technologies is shown to allow for significant load reductions while limiting the increase in actuator duty cycle, thanks to the opposite effects on this performance metric of the passive and active control solutions. ABBREVIATIONSADC actuator duty cycle AEP annual energy production BTC bend-twist coupling CAD computer-aided design DEL damage equivalent load DLC design load case EOG extreme operative gust FEM finite element method HAWT horizontal axis wind turbine IPC individual pitch control LQR linear quadratic regulator PID proportional integral derivative SQP sequential quadratic programming

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Wind turbine multidisciplinary design optimization is currently the focus of several investigations because it has showed potential in reducing the cost of energy. This design approach requires fast methods to evaluate wind turbine loads with a sufficiently high level of fidelity. This paper presents a method to estimate wind turbine fatigue damage suited for optimization design applications. The method utilizes a high-order linear wind turbine model. The model comprehends a detailed description of the wind turbine and the controller. The fatigue is computed with a spectral method applied to power spectral densities of wind turbine sensor responses to turbulent wind. In this paper, the model is validated both in time domain and frequency domain with a nonlinear aeroservoelastic model. The approach is compared quantitatively against fatigue damage obtained from the power spectra of time series evaluated with nonlinear aeroservoelastic simulations and qualitatively against rainflow counting. Results are presented for three cases: load evaluation at normal operation in the full wind speed range, load change evaluation due to two different controller tunings at normal operation at three different wind speeds above rated and load dependency on the number of turbulence seeds used for their evaluation. For the full-range normal operation, the maximum difference between the two frequency domain-based estimates of the tower base lateral fatigue moments is 36%, whereas the differences for the other sensors are less than 15%. For the load variation evaluation, the maximum difference of the tower base longitudinal bending moment variation is 22%. Such large difference occurs only when the change in controller tuning has a low effect on the loads. Furthermore, results show that loads evaluated with the presented method are less dependent on the turbulent wind realization; therefore, less turbulence seeds are required compared with time-domain simulations to remove the dependency on the wind realization used to estimate loads. Copyright

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This article presents the aeroelastic optimization of a 10MW wind turbine 'smart blade' equipped with active trailing edge flaps. The multi-disciplinary wind turbine analysis and optimization tool HawtOpt2 is utilized, which is based on the open-source framework Open-MDAO. The tool interfaces to several state-of-the art simulation codes, allowing for a wide variety of problem formulations and combinations of models. A simultaneous aerodynamic and structural optimization of a 10 MW wind turbine rotor is carried out with respect to material layups and outer shape. Active trailing edge flaps are integrated in the design taking into account their achieved fatigue load reduction. The optimized 'smart blade' design is compared to an aeroelastically optimized design with no flaps and the baseline design.