Environmental Engineering Reference
In-Depth Information
Council (GWEC), the energy generation capacity of the worldwide operating wind
farms increased in 2012 by about 20 % and reached a total of 283 GW [
16
].
A further growth of the wind energy industry depends mainly on the techno-
logical developments, especially considering production and construction methods
of the plants. From certification authorities, specified minimum design lifetime of
a new wind turbine is currently only 20 years, which, compared with the pro-
duction and maintenance costs economically is not sufficient enough. The main
challenge to the design of a wind turbine is caused by alternation loadings initiated
by wind, waves, and earthquake-induced dynamic excitation forces. For instance,
the load change of the rotor blades and thereby of the entire tower structure of a
wind turbine designed for 20 years corresponds to approximately 2
10
8
[
18
],
which is about 100 times more than a standard highway bridge.
The performance of a wind turbine is primarily determined by its tower height
and rotor diameter. With respect to lifetime problem and for the purpose of
building higher and more effective plants, it is necessary to improve the structural
dynamics of wind turbines. Several methods can be applied to mitigate the tower
vibrations and achieve better structural response.
12.2 Tower Vibrations
Onshore wind turbine towers carry, besides static gravity loading from nacelle,
also wind-induced loadings from rotor mainly in fore-aft direction. In side-to-side
direction wind flow causes vortex-induced tower vibrations. Furthermore, in
seismic regions turbine towers must withstand earthquake-induced extreme loads.
These dynamic loadings threaten structural safety and affect the lifetime of plants.
Wind turbine towers show different dynamic responses depending on the utilized
construction material. Hereby, besides material strength, the damping properties must
also be considered. The dynamic characteristics of a wind turbine tower rely also on
the structural properties. Especially, stiffness and mass, which determine the natural
frequencies of the tower, are important for the resonant behavior of wind turbines.
12.2.1 Wind Loading
Depending on the wind speed and turbulence intensity wind induces different
tower vibrations, which can be categorized as periodic vibrations of resonant
behavior and transient vibrations.
During normal wind conditions at low wind speeds, tower vibrations are mainly
characterized by resonant type of oscillations. At these wind speeds, the revolution
frequency of rotor blades is usually quite near the fundamental frequency of the
tower fore-aft modes. Wind turbines behave like a harmonically oscillated single
