Aerodynamic load control in horizontal axis wind turbines with combined aeroelastic tailoring and trailing‐edge flaps

Abstract

This paper presents an aeroservoelastic modeling approach to investigate dynamic load alleviation in large wind turbines with composite blades and trailingedge aerodynamic surfaces. The tower and rotating blades are modeled using geometrically-nonlinear composite beams, and linearized about reference rotating conditions with potentially arbitrarily-large structural displacements. The aerodynamics of the rotor are represented using a linearized unsteady vortexlattice method and the resulting aeroelastic system is written in a state-space description that is both convenient for model reductions and control design. A linear model of a single blade is then used to design an H ∞ regulator, capable of providing load reductions of up to 13% in closed-loop on the full wind turbine nonlinear aeroelastic model. When combined with passive load alleviation through aeroelastic tailoring, dynamic loads can be further reduced to 35%. While the separate use of active flap controls and passive mechanisms for load alleviation have been well-studied, an integrated approach involving the two mechanisms has yet to be fully explored and is the focus of this paper. Finally, the possibility of exploiting torsional stiffness for active load alleviation on turbine blades is also considered. IntroductionHorizontal-Axis Wind Turbines (HAWT) have been steadily increasing in size since they were first considered for large-scale energy production, both in terms of tower height and rotor diameter [Barlas and van Kuik(2010)]. At the time of writing, the largest wind turbines in operation have rotors measuring above 120 m in diameter, but rotors of up to 160 m are already being developed. Larger blades are necessarily more flexible and, as a result, aeroelastic effects previously not seen in smaller rotors are beginning to surface [Hansen et al.(2006)Hansen, Sørensen, Voutsinas, Sørensen, This has brought about new needs in terms of modeling requirements and poses new technological challenges, in particular, with respect to an increased need for methods of load alleviation to prolong fatigue life [Bottasso et al.(2013)Bottasso, Campagnolo, Croce, and Tib Passive load alleviation through aeroelastic tailoring is attractive in its simplicity, design and is now well understood [Shirk et al.(1986)Shirk, Hertz, and Weisshaar]. It relies on a blade structure designed with bend-twist coupling (twist-towardsfeather) to reduce the angle of attack as the blade bends upwards. This mod-1

Effects of leading-edge tubercles on wing flutter speeds

“…In this study, a planar wake is sought and it has been shown that the computation of aerodynamic loads (used here for the fluid-structure coupling) displayed little differences with free-wake models [32]. Figure 3 and further details can be found in [36]. The resulting coupled LTI EoM characterizing the full aeroelastic system is…”

Model-based aeroelastic analysis and blade load alleviation of offshore wind turbines

“…The external forces and moments are represented by Q ext and are used in the coupling with aerodynamics. The primary variable η contains all the nodal displacements and rotations, ν contains the rigid-body motions and λ are the Lagrange multipliers (Ng et al, 2015). The transformation T tr (t) map base motions and the additional kinematics due to rotation of tower moment arm onto the rotor hub.…”

“…After a convergence and numerical verification study in Ng et al (2015), the finite element discretisation of the structural model contains a total of 11 tower elements including platform and 12 elements per blade, as illustrated in Figure 2. The characteristics of the turbine blade are defined using a linear interpolation of the documented properties in J. M. Jonkman et al (2009).…”

“…Also, inspection and maintenance on wind turbines happen less frequently than on air vehicles due to difficulty in ground access. Individual pitch controls can also be considered for load alleviation and as demonstrated in Ng, Palacios, Graham, Kerrigan, and Hesse (2015) for the land-based version, the load reduction performance is higher than trailing-edge flaps. However, the power required for actuation is expected to be higher with increased wear and tear on pitch bearings.…”

“…The complete three-dimensional aeroelastic model is developed using similar steps as its two-dimensional counterpart but with three-dimensional sub-modules for the structures and aerodynamics. The methodology has been explored by Ng et al (2015) for land-based turbines, coupling the vortex-lattice unsteady aerodynamics model with a multi-body composite structural description of the turbine. Here, the model will be augmented with floating hydrodynamic capabilities, accompanied with the wave/wind representations in Section 2.4 and the controller implementation in Section 2.5.…”