The main concept currently in use in wind energy involves horizontal-axis wind turbines with blades of fiber composite materials. This turbine concept is expected to remain as the major provider of wind power in the foreseeable future. However, turbine sizes are increasing, and installation offshore means that wind turbines will be exposed to more demanding environmental conditions. Many challenges are posed by the use of fiber composites in increasingly large blades and increasingly hostile environments. Among these are achieving adequate stiffness to prevent excessive blade deflection, preventing buckling failure, ensuring adequate fatigue life under variable wind loading combined with gravitational loading, and minimizing the occurrence and consequences of production defects. A major challenge is to develop cost-effective ways to ensure that production defects do not cause unacceptable reductions in equipment strength and lifetime, given that inspection of large wind power structures is often problematic.

A full-scale test was performed on a Vestas V27 wind turbine equipped with one active 70 cm long trailing edge flap on one of its 13 m long blades. Active load reduction could be observed in spite of the limited spanwise coverage of the single active trailing edge flap. A frequency-weighted model predictive control was tested successfully on this demonstrator turbine. An average flapwise blade root load reduction of 14% was achieved during a 38 minute test, and a reduction of 20% of the amplitude of the 1P loads was measured. A system identification test was also performed, and an identified linear model, from trailing edge flap angle to flapwise blade root moment, was derived and compared with the linear analytical model used in the model predictive control design model. Flex5 simulations run with the same model predictive control showed a good correlation between the simulations and the measurements in terms of flapwise blade root moment spectral densities, in spite of significant differences between the identified linear model and the model predictive control design model. Full-scale test of trailing edge flaps on a Vestas V27 wind turbine D. Castaignet et al.tabs, etc.) and actuators (trailing edge flaps, microtabs, boundary layer suction or blowing jets, plasma actuators, etc.) along the blades. A detailed overview of different smart rotor concepts is given by Barlas and van Kuik. 11 Trailing edge flaps on turbine blades have been investigated for several years, as part of this smart rotor concept. Computational fluid dynamics (CFD) simulations, 12 2D aeroelastic simulations 13,14 and 3D aeroelastic simulations [15][16][17] confirmed the high potential of trailing edge flaps to reduce flapwise blade root fatigue loads. Wind tunnel tests on a blade section 18-20 as well as on a scaled turbine 21 corroborated the ability of the trailing edge flaps to reduce loads. At last, in 2010, a full-scale test was carried out on the Vestas V27 turbine located at the Risø campus of Technical University of Denmark (DTU). Only open-loop controls were tested at that time, and no active fatigue load reduction was performed. 22 This paper shows the results from the latest tests made in 2011 on the same Vestas V27 demonstrator turbine. Those tests include active load reduction achieved with a frequency weighted model predictive control (MPC), 23 and derivation of a time-invariant linear model, from trailing edge flap angle to flapwise blade root moment, with a system identification method. 24,25 The first section of this paper describes the demonstrator wind turbine. The tests performed during this test campaign are developed in the second section. The simulation models are detailed in Section 4, and the results from the field tests are presented and compared with Flex5 simulations in Section 5.

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