Environmental Engineering Reference
In-Depth Information
9.1 Introduction
Wind energy has been developed rapidly throughout the world. According to the
World Wind Energy Report 2010 of the World Wind Energy Association, in the
year 2010, the wind capacity reached 196 GW worldwide, after 159 GW in 2009,
120 GW in 2008, and 93 GW in 2007, and the trend wherein the installed wind
capacity more than doubles every third year continues. However, as more wind
turbines are built worldwide, the number of accidents recorded increases as well.
During the last 10 years, more than 1,000 wind turbine accidents have been
reported as well as 132 accidents per year from 2007 to 2011 inclusive. As the
most critical and expensive components of the wind turbine system, wind turbine
blades often suffer damage. According to the Caithness Windfarm Information
Forum, by far the greatest number of incidents found was due to blade failure.
''Blade failure'' can arise from a number of possible sources and results in either
whole blades or pieces of blade being thrown from the turbine. A total of 234
separate incidences were found up to March 2012.
Wind turbine blades usually achieve a very long operating life of 20-30 years.
They are the only part of the turbine designed specifically for the wind energy
industry. During their operation, blades encounter complex loading with a high
number of cycles, such as aerodynamic loads, centrifugal inertia forces, changing
gravity moments, braking force, and accidental impacts as well as severe weather
such as moisture absorption, wind gusts, or lightning strikes. All of these factors
result in accumulated damage, acceleration of fatigue damage, and even sudden
blade failure, which can cause catastrophic damage to the wind turbine. At the
same time, the downtime of the blades for extended repair and maintenance
resulting from undiscovered damage can lead to large economical losses. Thus,
much research effort is now focused on real-time monitoring techniques. In recent
years, many structural health-monitoring (SHM) techniques, including vibration-
based, optical fiber sensing, and piezoelectric techniques, have been developed and
applied as important and valid tools.
Currently, a number of techniques are being investigated for damage detection
of wind turbine blades. Optical fiber Bragg grating (OFBG) sensors have been
used to measure strain at various spatial locations and to detect severe damage
modes, such as the failure of adhesive joints or the delamination and failure of
blades due to their excellent sensing and mechanical performance and their
capability of online monitoring [
1
-
4
]. OFBG sensors can be patched on the surface
of the blades or embedded into textile-reinforced composites [
5
,
6
]. They can be
integrated with other sensors as well [
7
,
8
]. However, OFBG sensors can still only
measure the local deformation, but not the damage, such as the impacted damage
or active cracks, which are far away from the location of sensors. In the meantime,
the distributed optic sensors based on Brillouin scattering show great advantages
of environmentally stable, immunity to electromagnetic interference, and distrib-
uted sensing over extremely long distances with increasingly high accuracy. The
scheme of Brillouin optical time-domain analysis (BOTDA) is much preferable
