Implementation of the ‘smart’ rotor concept - Wind Power Generation and Wind Turbine Design - page 470

Both twisting toward feather and to stall is possible [7, 8]. In the fi rst case the
angle of attack is lowered and thus the aerodynamic loading. In the second case the
blade is pushed into stall and is thus subjected to low lift and high drag forces. The
concept is passive and it requires very accurate fi ber placement and the implementa-
tion of expensive carbon fi bers to be successful.
Many of the systems in use today react to the rotational speed of the rotor or
generator torque. This means that that the peak loads are dampened after they
occur and that the main source of fl uctuations - the way in which the airfl ow
around the blade is converted into mechanical loading - is not controlled. Nor is
the loading of individual blades. Other concepts are passive. This is a disadvantage
because the loading of the turbine is hard to control and stability issues may arise.
Moreover, passive systems only react to the type of signals to which they are tuned
(e.g. centrifugal loading for the flexhat or blade bending with bend-twist cou-
pling). Stall control seems to be a very simple way to control the loading of tur-
bines, but it cannot mitigate load fl uctuations. Blades that go dynamically in and
out of stall are actually subjected to very high fl uctuations in loading.
Active systems, based on feedback control, could dampen all possible load fl uc-
tuations which are picked up by sensors in the blade (e.g. strain gages), regardless
of the source. The most widely implemented active load control system, variable
speed and variable (individual) pitch control on turbines, also has the potential to
actively mitigate load fl uctuations. But controlling the loads this way puts a heavy
strain on the pitch bearings and hydraulics. Furthermore, the actuation speeds are
limited because the whole blade needs to be pitched. Finally, pitching the blade
results in a change in angle of attack and thus loading over the whole span of
the blade. Having the capability to control the aerodynamics along the span would
give much greater control over the dynamics of the blade and the other components
of the turbine at a much smaller effort.
1.2 The 'smart' rotor concept
Therefore the following new concept is proposed: by controlling the aerodynamics
at different stages along the blade's span the way the blade is loaded can be con-
trolled, counteracting the disturbances and mitigating fatigue loads. This, in com-
bination with appropriate sensors that measure the loads or structural response and
a controller that computes an actuation signal, is defi ned as the 'smart' rotor con-
cept. Such an aerodynamic load control system has been intensively researched for
helicopter blades and recently also feasibility studies for wind turbine blades have
been made [9-12]. The goals of the system would be to react both to deterministic
loads, such as wind shear and tower shadow, as to indeterministic loads such as
gusts and turbulence. Most authors focus on deformable aerofoil geometry, but
there is also an effort into microtabs or Gurney fl aps by van Dam and co-workers
[13-15]. These are small tabs of a height that is in the order of 1% of the cord,
that are placed near the trailing edge, perpendicular to the fl ow. The tabs jets the
fl ow in the boundary layer away from the blade's surface, effectively changing
the circulation around it and thus the lift and drag characteristics. Depending on