Implementation of the ‘smart’ rotor concept - Wind Power Generation and Wind Turbine Design - page 471

whether they are placed on the top or bottom side of the aerofoil the lift can be
reduced or increased and thus the loading of the blade can be controlled. van Dam
has made several studies into the topic, including CFD analysis and wind tunnel
experiments, determining the optimal confi guration and testing the performance
of the device. Another 'add on' device under investigation is the synthetic jet [16].
With this, air is sucked out of, and jetted back into the boundary layer at a very
high rate. This can be implemented to control fl ow separation, but when placed
near the trailing edge, the device has an effect similar to the microtab at low angles
of attack [17].
Here the possibilities will be addressed to implement the spanwise distributed
devices into the structure of rotor blades. For wind turbine blades a solution that
leads to an integrated adaptive structure is sought because of maintenance and
weight considerations. Ideally a construction would be obtained that can deform to
the designer's wishes by dissolved actuators. This deformation can be, e.g. a fl ap-
ping motion of the trailing edge, blade twist or activating a control surface like the
microtab. The focus will be on adaptive materials for actuation purposes because
they are not subjected to wear and require no lubrication. Therefore their imple-
mentation will lead to a structure that can stay in service with little inspection and
maintenance. Moreover, adaptive materials offer a very high power/weight ratio
[18, 19]. In this section fi rst aerodynamic control concepts found in other struc-
tures that are aimed at controlling aerodynamic forces will be addressed. These are
aircraft wings and helicopter rotors. Secondly the most widely applied and most
suitable types of adaptive materials will be discussed, namely piezoelectrics and
SMAs. And fi nally some considerations regarding control and structural integration
are addressed.
2 Adaptive wings and rotor blades
One research fi eld in which adaptive structures are heavily researched, is aero-
space. SMAs are already used to deploy space antenna's or as fasteners that do
not require bolting [19, 20]. In addition, both for fi xed wing and rotor craft many
concepts have been considered and build to control the aeroelastic behavior of the
machines. In this section the 'smart' wing and helicopter rotor concepts that proof
to be a good benchmark and example for wind turbine rotors will be discussed.
2.1 Adaptive aerofoils and smart wings
A very comprehensive explanation of adaptive aerofoils is presented by Campanile
[21]. Campanile presents in a historic overview that an adaptive wing was fi rstly
aimed at in man's fi rst attempts to fl y and that even the Wright's brother's air-
craft had morphing wings for fl ight control. Campanile asserts that because of the
ever more severe demands for load carrying capability, maneuverability and speed
(thrust), the tasks were separated in the wing box, fl aps or ailerons and engines.
But according the Campanile reintegration should be sought and it will lead to
lighter, aerodynamically more effi cient structures. In the end that is also the goal