WMOD00469

• Title: Environmental Impact of Large Windpower Farms
• Weather Modification - Doc Type: Report

Means of mitigating the hazard have not been investigated.
Whether the birds might be diverted from the hazard area by
flashing lights, amplified distress calls, or the like is
entirely unknown.

In the course of thei r hunti ng, most bi rds of prey pass through
the elevations above ground at which windpowered generators
operate, but they are active only by daylight. Because of the.ir
keen eyesi ght and extremely maneuverable fl i ght, it is doubtfu"1
that they would often fall victim to the moving blades. The \.dde
separation of individual machines, furthermore, would probably
not appreciably increase the proportion of the windfarm area
denied them as hunting territory. However, assumptions as to
impacts on these birds remain to be verified.

It should be noted with respect to wildlife in general that
windfarms will selectively occupy sites that are windy and
barren, sites where wildlife populations tend to be sparse
because of lack of food and shelter. This characteristic will
tend to mi nimi ze the impact of wi ndfarms on wil dl ife.

c. Noise. - Recognition of noise pollution as an environmental
factor has grown rapidly in recent years and has resulted in
intensive study and action with respect to certain outdoor noise
sources such as highways. If large windfarms were to be thought
of by the public as noise polluters, their acceptability would be
adversely affected.

People with wind-machine experience have not considered noise to
be a potential problem (Thomas, 1975; Rogers, et al., 1976) but
potential critics may not be satisfied. The following analysis
therefore aims at identifying th(~ conditions under which windfann
noise will be most prominently louder than background noise and
to relate past observations to probable worst cases. In the
worst case analysis, three elements are to be considered: the
sound power emitted by the wind machine itself; the sound power
emi tted by adj acent sources; and the sound i ntens i ty reachi ng ,a
hearer from the combination.

A wind machine generates sound power in two modes. The first
mode arises from displacement of the air as the blade momentarily
separates it. In this mode the blade behaves as an aerodynamic
monopole (Heller and Franken, 1971) and the emitted power variles
as the fourth power of the blade speed through the air. The
second mode arises from the interaction between the blade and
the flow of air to produce unsteady forces. In this mode the blade
behaves as an aerodynamic dipole and the emitted power varies as
the sixth power of the blade speed through the air. The s~pport

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