ENVIRONMENTAL IMPACTS OF WIND POWER
By Emil Bedi, CANCEEand Hakan Falk, "Energy Saving Now".
In many
part of the world, there is such a dearth of electricity generation that the
public welcomes wind turbines with open arms. Where there are alternative
choices, however, environmental impact is of major significance for development.
Note that impacts may be judged as either beneficial or harmful. The impacts of
wind turbines and the factors influencing these are:
ACOUSTICS
Noise
is mostly generated from blade tips (high frequencies), from blades passing
towers and perturbing the wind (low frequencies) and from machinery, especially
gearboxes. Since noise is essentially a sign of inefficiency and because of
complaints, manufacturers have reduced noise-generation intensities greatly over
the last five years. The critical noise intensity is usually considered to be 40
dBA, or less, as judged necessary for sleeping. This level of acceptance is
usually attained at distances of about 250 m or less. However, attitudes to
noise are strongly psychological; the owner of a machine probably welcomes the
noise as a sign of prosperity; whilst neighbours may be irritated by intrusion
into “their space”.
LAND AREA AND USE
Turbines should be separated by at least five to ten tower heights; this
allows the wind strength to reform and the air turbulence created by one rotor
not to harm another turbine downwind. Consequently, only about 1 % of land area
is taken out of use by the towers and the access tracks. The taller and larger
the turbines, the greater the separation. Megawatt machines should be spaced
between half and one kilometre apart. Neither buildings nor commercial forestry
can be established between, so the land is thereafter safeguarded against such
development and can be used for agriculture, leisure or natural ecology.
VISUAL IMPACT
Wind turbines
are always visible from places in clear line of sight. The larger the machines,
the greater the distance between them. The need for a long fetch of undisturbed
wind, and the economic bias to large machines, means that machines will
potentially be visible from distances of tens of kilometres. However, at such
distances, the majority of the public will have their view obscured by hills,
trees, buildings etc. The most likely people to notice the machines on land are
walkers and pilots. For the former, beauty is in the eye of the beholder, and
for the latter there is danger for exceptionally low flying. For offshore
machines, visual impact is largely, as yet, unassessed.
BIRD STRIKE
Birds often collide with high voltage overhead
lines, masts, poles, and windows of buildings. They are also killed by cars in
the traffic. Birds are seldom bothered by wind turbines. Radar studies from
Tjaereborg in the western part of Denmark, where a 2 megawatt wind turbine with
60 metre rotor diameter is installed, show that birds - by day or night - tend
to change their flight route some 100-200 metres before the turbine and pass
above the turbine at a safe distance. In Denmark there are several examples of
birds (falcons) nesting in cages mounted on wind turbine towers. The only known
site with major bird collision problems is located in the Altamont Pass in
California. A "wind wall" of turbines on lattice towers is literally closing off
the pass. There, a few bird kills from collisions have been reported. A study
from the Danish Ministry of the Environment says that power lines, including
power lines leading to wind farms, are a much greater danger to birds than the
wind turbines themselves. Some birds get accustomed to wind turbines very
quickly, others take a somewhat longer time. The possibilities of erecting wind
farms next to bird sanctuaries therefore depend on the species in question.
Migratory routes of birds will usually be taken into account when siting wind
farms. Offshore wind turbines have no significant effect on water birds. That is
the overall conclusion of a three year offshore bird life study made at the
Danish offshore wind farm Tunų Knob.
There have been many independent studies of birds killed by rotating
blades. This undoubtedly happens, but perhaps to a similar or lower frequency
than strikes by a car, against the windows of a building or : against grid
transmission cables. Every death is regretted. The counter argument, again
attested by experts, is that land around wind turbines may provide excellent
breeding conditions. The exception to this argument is the possibility of
strikes by large migratory birds flying in the dark and by raptors intent on
their prey.
ELECTROMAGNETIC INTERFERENCE
TV, FM and radar waves are perturbed in
line of sight by electrically conducting materials. Therefore, the metallic
parts of rotating blades can produce dynamic interference in signals. It is
easy, but not necessarily cheap; to install TV and FM repeater stations to
provide another direction of signal for receivers. Radar interference is, as
yet, a largely undocumented effect, of most concern to the military. However,
wind turbines are a fact of life that has to be accepted by the military on an
international scale. There are many sites of wind turbines close to airfields,
and no significant difficulties occur.
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