Wind Power Generation - Renewable Energy: Technology, Economics and Environment - page 343

effects on soil, water and air that these sectors are known for. Due to the strict en-
vironmental regulations, the corresponding environmental effects are relatively
low. Also, the propensity toward malfunctions during manufacturing is relatively
low - with the exception of e.g. iron and steel manufacturing.
Normal operation. Wind power stations do not directly release any toxic sub-
stances. Nevertheless, the operation of wind energy converters has certain effects
on the natural environment. The main effects are described as follows.
Audible sound. With their rotating rotors wind power stations represent acoustic
sources. Sound is primarily caused by aerodynamic noise at the rotor blades and
by sound radiation by gearbox and generator.
The latter acoustic source was considerably reduced by converter casings and
by gearless converters built over the last few years. Furthermore, the gearbox and
generator on the one hand and the nacelle on the other hand are separated in al-
most every modern converter design. Thus, nacelles no longer act as resonance
bodies and sound impact has been significantly reduced compared to converters
built in the early nineties.
Aerodynamic sound emissions of wind energy converters are due to the airflow
around the rotor blades and the rotor blade coming through the tower shadow;
they mainly occur at medium and high blade tip speeds. For the environment
solely the noise created at low and medium wind speeds is of importance, since
natural wind sound is predominant at high airflow speeds. In the past, aerody-
namic sound creation has been reduced by an optimised shape of rotor blade and
blade tip. On the whole, this and further measures have reduced sound emissions
considerably (by about 5 to 10 dB(A)) over the past decade /7-12/. This is why
wind energy converters can only be clearly distinguished from background noise
at comparatively low wind speeds.
Sound emissions created by wind power stations represent an essential parame-
ter for converter design. Favourable arrangement of wind energy converters and a
corresponding selection of converter models may allow to reduce the sound level
by approximately 10 dB(A) at constant power generation. Also the legislation for
noise reduction may prescribe certain sound levels for residential, industrial and
mixed areas which must not be exceeded. In order to obtain a building permit for
a wind energy converter or a wind park, observation of certain sound limits must
be proven by corresponding survey reports to prevent unacceptable noise.
Infrasonic sounds. Wind energy converters mainly emit aerodynamic infrasonic
sounds within a frequency range from 0.6 to 1.5 Hz. At these frequencies the hu-
man detection limit of 120 to 130 dB(A) is very high. At a distance of 120 m from
e.g. a 500 kW converter infrasonic sounds of 75 to 85 dB(A) were measured
which were reduced to 67 to 77 dB(A) at a distance of 300 m. Since certain dis-
tances have to be observed with regard to the building permit and in order to
comply with the legislation there is no impact on the population by infrasonic