Environmental Engineering Reference
In-Depth Information
The major causes of wind turbine downtime have been operation and mainte-
nance (O&M) work and faults. The occurrence of O&M work and faults has been
variable. Some sites have downtown of no more than 43 hours per month, while
others have had as much as 127 hours (NREL, 2000). O&M downtime includes
all troubleshooting, inspections, adjustments, retrofits, and repairs performed on the
turbines. Faults generally require no more than a reset and most can be performed
remotely. Increased downtime for O&M work and fault reasons means more traffic
to and from the turbine farm.
Fast-forwarding to wildlife effects resulting from wind turbine noise alone, it is
important to point out that a bird's inability to detect turbine noise at a close range
may also be problematic. The threshold for hearing in birds is higher than that for
humans at all frequencies, and the overlap in the discernible frequencies between
species indicates that birds do not filter out other species simply by being unable to
detect theme (i.e., birds can hear songs of other species). In their environment, birds
must be able to discriminate their own vocalizations and those of other species apart
from any background noise (Dooling, 1982). Calls are important in the isolation
of species, pair bond formation, precopulatory display, territorial defense, danger,
advertisement of food sources, and flock cohesion (Knight, 1974).
For the average bird, within a signal frequency range of 1 to 4 kHz, noise must
be 24 to 30 dB above the ambient noise level in order for a bird to detect it. Turbine
blade and wind noise frequencies generally fall below the optimal hearing frequency
of birds. Additionally, by the inverse square law, the sound pressure level decreases
by 6 dB with every doubling of distance. Therefore, although the sound level of the
blade may be significantly above the ambient wind noise level and detectable by
birds at the source, as the distance from the source increases and the blade noise
level decreases toward the ambient wind noise level, a bird may lose its ability to
detect the blade and risk colliding with the moving blade.
Some researchers have attempted to blame avian collisions with turbine blades
on birds' inability to divide their attention between surveying the ground for prey
and monitoring the horizon and above for obstacles; that is, the birds are so busy
searching the ground that they do not notice the turbines. This hypothesis derives
from substituting our knowledge of human vision for that of avian vision. Humans
are foveate animals; that is, we search the visual world with a small area of the
retina known as the fovea, which is our area of sharpest vision, like someone
searching a dark room with a narrow-beam searchlight. This results from our very
low ratio (approximately 1:1) of photoreceptors to ganglion cells in the macular
region of the retina. Outside the macular region, the ratio of receptors to ganglion
cells increases progressively to 50:1 to 100:1, and our visual acuity drops sharply.
Birds and many other animals, on the other hand, have universal macularity, which
means that they have a low ratio of receptors to ganglion cells (4:1 to 8:1) out to
the periphery of the retina. They maintain good acuity even in peripheral vision.
In addition, raptors possess the specialization of two foveal regions: one for frontal
vision and one for looking at the ground. Moreover, birds have various optical
methods for keeping objects at different distances simultaneously in focus on the
retina. Because of these considerations, failure to divide attention seems like an
unlikely hypothesis.
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