Optimization‐based study of bend–twist coupled rotor blades for passive and integrated passive/active load alleviation

Abstract

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

Modal properties and stability of bend–twist coupled wind turbine blades

“…The reduced frequency response to mean wind speed variations of the blade root flapwise moment (see Fig. 18) concurs with reduced fatigue loads observed by Lobitz et al (1999), Lobitz and Veers (2003), Verelst and Larsen (2010), and Bottasso et al (2013). Flap-twist to feather coupling also reduces the tower bottom fore-aft moment (see Fig.…”

“…9) and for the edge-twist to feather coupled blade the backward whirling mode becomes the lowest damped mode. Flap-twist to feather coupled blades have been reported to reduce fatigue loads of the flapwise blade root bending moment of the blade (Lobitz et al, 1999;Lobitz and Veers, 2003;Verelst and Larsen, 2010;Bottasso et al, 2013). To investigate the load alleviation in frequency domain, the frequency response of the flapwise blade root bending moment to mean wind speed variation between 0 and 2 Hz for steady-state operation at mean wind speeds of 5, 10, 15, and 20 m s −1 is shown in Fig.…”

“…The motivation behind bend-twist coupling in wind turbine blade applications has mainly been load alleviation. Fatigue load reductions in the range of 10-20 % have been reported for flap-twist to feather coupled blades (Lobitz et al, 1999;Lobitz and Veers, 2003;Verelst and Larsen, 2010;Bottasso et al, 2013).…”

Bend-twist coupling potential of wind turbine blades

“…Presently there is only limited amount of information available in the literature on the BTC magnitudes that are feasible to achieve in modern wind turbine blade structures [3,4]. It is also of particular interest how introduction of BTC would affect such important blade structural properties as bending stiffness and torsional stiffness.…”

Bend-Twist Coupling Characteristics Calculations of 5MW Wind Turbine Blades based on 3D Shell Model

“…Reference [2] also confirm that bend-twist coupling blade using off-axis carbon in the skin will induce extra twist and alleviate load. Reference [3] design coupled blades where fibers are rotated only on the outboard part of blade and demonstrate this blade can achieve good load mitigation capabilities.…”