Load Mitigation with Bending/Twist‐coupled Blades on Rotors using Modern Control Strategies

Abstract

The prospect of installing blades that twist as they bend and/or extend on horizontal axis wind turbines provides opportunities for enhanced energy capture and/or load mitigation. Although this coupling could be achieved in either an active or a passive manner, the passive approach is much more attractive owing to its simplicity and economy. As an example, a blade design might employ coupling between bending and twisting, so that as the blade bends owing to the action of the aerodynamic loads, it also twists, modifying the aerodynamic performance in some way. For reducing loads the blades are designed to twist towards feather as they bend. For variable‐speed pitch‐controlled rotors, dynamic computer simulations with turbulent inflow show that twist coupling substantially decreases fatigue damage over all wind speeds, without reducing average power. Maximum loads also decrease modestly. For constant‐speed stall‐controlled and variable‐speed stall‐controlled rotors, significant decreases in fatigue damage are observed at the lower wind speeds and smaller decreases at the higher wind speeds. Maximum loads also decrease slightly. As a general observation, whenever a rotor is operating in the linear aerodynamic range (lower wind speeds for stall control and all wind speeds for pitch control), substantial reductions in fatigue damage are realized. Copyright © 2002 John Wiley & Sons, Ltd.

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).…”

Beam section stiffness properties using a single layer of 3D solid elements

“…A concept developed in the late 1980's for reducing loads seen by wind turbines is to promote blade twisting to decrease the angle of attack when subject to a wind gust with the use of biased 27 layups in the blade spar cap [16,17]. This final example of the present paper concerns the analysis of the two-cell cross-section of a wind turbine blade shown in Fig.…”

On synthetic jet actuation for aerodynamic load control

“…Passive techniques for fatigue load alleviation on wind turbine blades can be based on the coupling of elastic deformation modes by careful selection of composite fiber orientation. For example, in bend-twist coupling, increased bending of the blade due to increased wind speed induces an extra twist of the blade (along the pitch axis), such that the increase in the angle of attack and lift force is limited [30,31]. In tension-twist coupling, the same is achieved when blades are elongated due to centrifugal forces.…”