Implementation of the ‘smart’ rotor concept - Wind Power Generation and Wind Turbine Design - page 500

6 Conclusions and discussion
6.1 Conclusions on adaptive aerospace structures
The adaptiveness of aerospace structures is advocated with many purposes in mind;
from vibration control to reconfi guration. But the underlying idea is the same: to
obtain lighter, better performing structures. Both the smart rotor blade research
for helicopters, as the smart wing research for aeroplanes pose interesting bench-
marks. In rotor blades aerodynamic load control is mostly pursued for vibration
control. The concepts are usually based on hinged fl aps where adaptive materials
are implemented for their high power to weight ratio. Moreover, with piezoelec-
trics very high actuation frequencies are attainable. For vibration control or fl ight
control for helicopters mostly piezoelectrics are proposed. For quasi-static blade
tracking SMAs are referred to.
In smart wing research most research into aerodynamic load control is aimed at
replacing the current control systems or for reconfi guration from one fl ight mode
to another. Most concepts for control surfaces are still aimed at mechanisms rather
than integrated structures. In addition compliant mechanisms have been proposed
for morphing surfaces. Actuation of these surfaces does not necessarily have to be
done by means of adaptive materials, but they are mentioned because of their high
power-to-weight ratio. For the smart wind turbine blades, the morphing fl ap or
aileron concepts are mostly interesting.
6.2 Conclusions on adaptive materials
Two adaptive materials are of most interest: piezoelectrics and SMAs. The chal-
lenge with piezoelectrics is to suffi ciently amplify their strain and to take precau-
tions for their brittleness, in the case of PZT. Precompression and applying it in
Thunder-type benders or in mechanisms are good solutions.
The challenges with SMAs are actuation speed and controllability. The band-
width of an SMA actuator could be increased by active cooling and for its control-
lability models exist, but the material behavior is highly non-linear and dependent
on the load history. An issue with SMA material is the fatigue properties of the
material. The advantage of SMA material is that the theoretically attainable band-
width and force-displacement characteristics are very well suited for actuation in
MW-sized HAWT blades.
With all electrically controlled actuators, whether they are electro-mechanical
actuators (EMA) or adaptive materials, an issue is lightning strike.
6.3 Conclusions for wind turbine blades
From a stiffness point of view a rigorous alteration of the blade design proba-
bly not needed, depending on the blade design. Reinforcing and stiffening ele-
ments such as ribs or an additional spar could be placed around the actuator slots.
Thermoplastic materials are favorable for such a blade concept because they allow