Environmental Engineering Reference
In-Depth Information
much of an issue since those are located far from dwellings, for the small house,
mounted wind turbines vibration can have signifi cant impact on people's lives
living in the house where the wind turbine is attached.
Moving on to the aerodynamic noise, there are various techniques or even tech-
nologies to decrease sound from the wind turbine blades. As somebody would
expect most of those techniques originate from designing more aerodynamic
blades [18] and adjusting the rotational speed of the turbine.
Again, revising section 'sources of wind turbine sound' we can remind our-
selves what causes aerodynamic noise and therefore suggest potential solutions.
Some of the noise causes we have discussed concern downwind designs, blade
speed and shape and interaction of the airfl ow between the tower and the wind
turbine.
Nowadays most rotors are upwind i.e. the rotor faces into the wind, reducing
the risk of causing localised fl ow instabilities that are responsible for impulsive
noise. Although there are still quite a few downwind turbines (where the rotor
faces away from the wind) in use, new improved design features have been
incorporated aiming at reducing impulsive noise such as increasing the distance
between rotor and tower.
In addition to designing upwind turbines the shape of the tower and the nacelle
are aerodynamically streamlined in order to reduce any noise that is created by the
wind passing the turbine.
To limit the generation of aerodynamic sounds from wind turbines the rotor's
rotational speed may be restricted in order to reduce the tip speeds. Large variable
speed wind turbines often rotate at slower speeds in low winds, and in increased
speeds in higher winds until the limiting rotor speed is reached. This results in
much quieter operation in low winds than a comparable constant speed wind tur-
bine. Many modern wind turbines have embedded special control programs that
reduce the infl ow angle and rpm of the rotor depending on the time of day or year,
the wind speed and the wind direction. The noise can be signifi cantly reduced at
the expense of power output.
Wind turbine blades are constantly being redesigned to make them more effi -
cient and less noisy. The broadband tip vortex noise caused by rotating wind
turbines can be tackled by giving to the blade tip an aerodynamic shape that
decreases generation of vorticity [19]. Forward sweeping into the direction of
the incoming fl ow of the blade could result in quieter operation. Figure 12 shows
graphically how design changes in the blade tip and shape can result in noise
reduction i.e. how three different blade tip geometries can produce three different
noise profi les [ 20 ].
When it comes to small wind turbines (under 30 kW) the ways to reduce noise
are similar to those for large turbines. This means that they also have often vari-
able-speed controls. The interesting fact is that small wind turbine designs may
even have higher tip speeds in high winds than large wind turbines. This can result
in greater sound generation than would be expected, compared to larger machines.
Many modern microwind turbines rotate over their vertical axis regulating power
in high winds by turning out of the wind. This additional functionality in operation
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