The current status of control of the loads on a wind turbine has evolved from the successful ‘Danish concept’ (constant rotor speed with blade stall at high wind speeds to limit the absorbed power) to variable rotational speed combined with the adjustment of the collective pitch angle of the blades to optimize energy yield while controlling the loads. This was a big step forward: the control of the blade pitch angle has not only led to high-quality power regulation, but also to a significantly lighter blade construction due to the lower load spectrum and to a lighter gear box due to shaved torque peaks.
The next step in blade load control, individual, rather than collective, blade pitch angle adjustment, is already applied in some wind turbines. This further alleviates the rotor loads, especially the periodic loading due to yaw and wind shear. Not only the blades, but also the drive train and nacelle structure, benefit from this.
A further step, probably for the 2015 wind turbine generation with an even larger rotor size, may well be a much more detailed and faster control of the loads. Ideally, control should be possible for each blade at any azimuthal position and any spanwise station, through the use of aerodynamic control devices with embedded intelligence distributed along the span. The correspondence with the control devices on airplane wings (flaps at the leading and trailing edge and ailerons) is apparent, but the requirements for blade control devices are probably much more severe. Modern blades are very reliable, requiring only limited maintenance of the blade pitch bearing. Future blades with distributed control devices should be just as reliable, requiring no addition maintenance.
The development of this kind of technology, often named in popular terms ‘smart blades’, is the topic of this special issue. The content reflects the fact that ‘smart blades’ require an interdisciplinary development par excellence, since new knowledge on aerodynamics, control and structural layout is reported. The special issue contains contributions on:
The aerodynamics of aerofoils with control elements. Several options are available for the adjustment of lift and drag. Most of the emphasis is on flaps, but plasma actuators and microtabs are also discussed. Furthermore, one contribution addresses the alleviation of loads due to large wind gusts by an innovative pitch actuator.
Control. The control algorithms for this type of control are not yet available. Fast, real-time load identification algorithms, allowing application of predictive control techniques, is a challenging task. One contribution is dedicated to these challenges; several others discuss it in the course of the paper.
Blade material and construction. One contribution is included on passive control, by means of torsion induced deformation of the blade under loading.
Performance and stability. These should not be adversely affected by the reduction of fatigue loads; this topic is discussed in some of the contributions.
The topics addressed by the papers reflect the early stage of development where everything is still possible and the focus is on feasibility, modelling and exploration of possibilities. Issues like the reliability of the actuators, their mechanical and structural layout, the embedding of the distributed control in the overall turbine control and safety system, the consequences for the turbine design and detailed cost/benefit analyses are all to be done in the future.
We wish you much pleasure and inspiration in reading this special issue.