Environmental Engineering Reference
In-Depth Information
13.1 Introduction
In the last years the need for optimization procedures to design higher and higher
wind turbines, even offshore, in a cost-effective way and of control techniques to
reduce the wind-induced structural demand has definitely grown. Most of the
scientific literature on this topic is related to passive control strategies, often based
on the use of tuned mass or tuned liquid dampers.
Herein, the possible use of a semi-active (SA) control technique is investigated,
based on the use of magnetorheological (MR) dampers, also performing large-
scale shaking table tests. Previous researches on SA control of wind turbines are
not much based on numerical simulations.
Kirkegaard et al. [
1
] for the first time explored the possibility of using MR
dampers to control a wind turbine, numerically evaluating their effectiveness when
driven by a classical optimal clipped control algorithm. The proposed idea was
pioneering and, also because of this, it results to be very interesting, even if
difficult to be implemented in a real case. As a matter of a fact, the authors
consider the installation of an MR device at the base of the tower, in vertical
position, so as to be able to be solicited by relative vertical displacements induced
by the top movement of the turbine to which the damper should be mechanically
connected. Also an experimental test model has been built by the said authors,
even if adopted for passive tests only (constant voltage fed to the MR damper at
the maximum level—''passive-on'' condition). The wind turbine model was a 3 m
high steel frame with a 200 kg top mass. The MR damper was connected to the
shaking table and to the top of the frame structure by a steel bar designed so as to
avoid buckling. Comparing numerical (SA) and experimental (passive) results in
terms of top displacement, appreciable improvements in the SA strategy are
highlighted by the authors.
Karimi et al. [
2
] and Luo et al. [
3
-
5
] showed the effectiveness of SA control for
floating wind turbines by using tuned liquid column dampers (TLCD). This kind of
device is generally used as a passive damper, even if it may turn into an SA
damper with the addition of a controllable valve. With a control logic based on an
H
?
feedback methodology, the authors proposed to adopt the orifice opening
according to the structure response and loading conditions. Luo et al. also explored
the possibility of using MR fluids within the TLCD, rather than a common viscous
fluid, so leading to a ''smart'' TLCD [
6
,
7
]. The numerical simulations reported in
the above papers show that this kind of control strategy may lead to a strong
reduction of the top displacement.
Arrigan et al. [
8
] considered SA tuned mass dampers (STMD) to control wind
turbine blades in flapwise vibrations. Four STMDs were added to the wind turbine
numerical model, one to each blade tip and one at the nacelle to control the
response of each component. Simulations made by the authors showed a signifi-
cant reduction in displacement response of the system for turbulent wind loading.
A successful response reduction under a steady wind load was demonstrated.
