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8/17/2019 Noise From Wind Turbines
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Training program in wind energy
and project developmentPhuket 11-15 September 2006
Module 5.2
Noise from wind turbinesNiels-Erik Clausen
Risø National Laboratory
Wind Energy Course, Phuket, 11-15 Sept 20062
Impact from noise
• Principles for calculation and approval
• Noise measurements
• Noise mitigation
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Sources of noise
• There are two potential sources of noise:
• Aerodynamical as the turbine blades passing through
the air and
• Mechanical: The gearbox and generator in the
nacelle.
• Noise from the blades is minimised by careful design
and manufacture of the blades.
• The noise from the gearbox and generator is reduced
by sound insulation and isolation materials. For a
modern wind turbine this component is normally
negligible
Wind Energy Course, Phuket, 11-15 Sept 20064
What kinds of noise do wind turbines produce?
• Wind turbines most commonly produce somebroadband noise as their revolving rotor bladesencounter turbulence in the passing air. Broadband
noise is usually described as a "swishing" or"whooshing" sound.
• Some wind turbines (older ones) can also producetonal sounds (a "hum" or "whine" at a steady pitch).This can be caused by mechanical components or,less commonly, by wind flow interacting with turbineparts. This problem has been eliminated in modernturbine designs.
Sources of noise
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Noise sources from wind turbine
Wind Energy Course, Phuket, 11-15 Sept 20066
Measurement of noise
• Noise is measured in decibels (dB). The decibel is a
measure of the sound pressure level , i.e. the
magnitude of the pressure variations in the air. An
increase of 10 dB sounds roughly like a doubling of
noise level to human ear (highly non-linear).Measurement-wise 3 dB(A) increase means a
doubling of the noise
• Measurement of noise requires that the noise signal
is some 10 dB(A) above the background noise
• The background noise from leaves, birds, and traffic
will frequently be above 30 dB(A).
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Definitions
• Sound intensity in dB = 10*log (sound power)+120 [1]
where sound power is measured in W/m2
• Sound power P = 10 0.1*(dB-120) [2]
Adding several sources
• The sound intensities are converted to sound power in
the point of reception [2]. The sound power from all
sources in the point of reception is added and converted
to sound intensity in dB(A) by [1]
• Calculation by using special software e.g. WindPro
Wind Energy Course, Phuket, 11-15 Sept 20068
Sound propagation
• The energy in the sound
waves decreases
proportional to the square
of the distance to the
source i.e. a doubling ofthe distance leaves only
¼ of the energy
• Doubling the distance
means a decrease in
sound intensity of 6 dB(A)
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Wind Energy Course, Phuket, 11-15 Sept 20069
Impact from noise
Wind Energy Course, Phuket, 11-15 Sept 200610
Impact from noise
• The sound power level from a single wind turbine isusually between 90 and 105 dB(A).
• This creates a sound pressure level of 50-60 dB(A) ata distance of 40 metres from the turbine, i.e. aboutthe same level as conversational speech.
• Ten such wind turbines, all at a distance of 500metres would create a noise level of 35-45 dB(A) withthe wind blowing from the turbine towards thereceptor. With the wind blowing in the oppositedirection the noise level would be about 10 dB lower.
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Wind Energy Course, Phuket, 11-15 Sept 200611
Impact from noise – ex. 500 kW
• Each square measures 43
by 43 metres, corresponding
to one rotor diameter. The
bright red areas are the
areas with high sound
intensity, above 55 dB(A).
The dashed areas indicate
areas with sound levels
above 45 dB(A), which will
normally not be used for
housing etc.
• The zone affected by noise
extends to 3.5 rotor
diameters to 45 dB(A) and6.5 diameters' to 40 dB(A)
Wind Energy Course, Phuket, 11-15 Sept 200612
Sound power from different turbines
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Wind Energy Course, Phuket, 11-15 Sept 200613
Background noise
Background noise
Wind Energy Course, Phuket, 11-15 Sept 200614
Impact from noise
Legal Noise Limits
• At distances above 300 m the maximum theoretical
noise level from high quality wind turbines will
generally be significantly below 45 dB(A) outdoors,
corresponding to the legislation in Denmark. (Forbuilt-up areas with several houses, a noise limit of 40
dB(A) is the legal limit in Denmark).
• Noise regulations vary from country to country. In
practice the same machine designs can be used
everywhere.
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Wind Energy Course, Phuket, 11-15 Sept 200615
Impact from noise
Low-frequency Noise
• Concerns have been raised that noise radiated fromwind turbines contains high levels of low frequencyenergy that may pose a threat to human health. Itwas suggested that symptoms included nausea,headaches and anxiety.
• Low frequency noise' is the term used to describesound energy in the region below about 200Hz. Therumble of thunder and the throb of a diesel engineare both examples of sounds with most of theirenergy in this low frequency range.
Wind Energy Course, Phuket, 11-15 Sept 200616
Impact from noise
• The additional term 'infrasound' is also often used todescribe sound energy in the region below 20Hz.
Almost all noise in the environment has componentsin this region although they are of such a low levelthat they are not significant. Noise which has most of
its energy in the 'infrasound' range is only significantif it is at a very high level, far above normalenvironmental levels
• For a healthy young adult the range of hearing isoften quoted as extending from 20Hz to 20,000Hzalthough the sensitivity of the ear varies significantlywith frequency and is most sensitive to sounds withfrequencies between around 500Hz and 4000Hzwhere the majority of information in speech signals iscontained.
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Wind Energy Course, Phuket, 11-15 Sept 200617
Impact from noise
• It has been shown by measurements of wind turbinenoise undertaken in the UK, Denmark, Germany and
the USA over the past decade, and accepted byexperienced noise professionals, that the levels ofinfrasonic noise and vibration radiated from modern,upwind configuration wind turbines are at a very lowlevel; below the threshold of perception, even forthose people who are particularly sensitive to noise.
Wind Energy Course, Phuket, 11-15 Sept 200618
Noise abatement
What have manufacturers done to reduce wind turbinenoise?
• Most rotors are upwind: A wind turbine can be either"upwind" (that is, where the rotor faces into the wind)or "downwind" (where the rotor faces away from the
wind). Today, almost all of the commercial windmachines on the market are upwind designs, and thefew that are downwind have incorporated designfeatures aimed at reducing impulsive noise inducedfrom tower shadow (e.g. by increasing the distancebetween rotor and tower).
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Wind Energy Course, Phuket, 11-15 Sept 200619
Noise abatement
• Soundproofing in nacelles has been increased: Thegenerator, gears, and other moving parts located inthe turbine nacelle produce mechanical noise.Soundproofing and mounting equipment on sound-dampening buffer pads helps to deal with this issue.
• Wind turbine blades have low noise design: Turbineblades are constantly being redesigned to make themmore efficient and less noisy. Already in the mid-1990’ies LM introduced blades with a special “twist”to reduce generation of aerodynamic noise
Wind Energy Course, Phuket, 11-15 Sept 200620
Noise abatement
• Gearboxes are specially-designed for quiet operation:Wind turbines use special gearboxes, in which thegear wheels are designed to flex slightly and reducemechanical noise. In addition, special sound-dampening buffer pads separate the gearboxes fromthe nacelle frame to minimize transmission ofvibrations to the tower
• Towers and nacelles are streamlined: Streamlining(aerodynamic shape to tower and to the nacelle)reduces any noise that is created by the wind passingthe turbine.
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Wind Energy Course, Phuket, 11-15 Sept 200621
Noise abatement
• Many manufacturers have ”low-noise” programs in
the control system, where the inflow angle and rpm of
the rotor is reduced. The noise can be significantlyreduced at the expense of power output.
• This can be implemented in the control program at
certain times a day or year, at certain wind speeds,
certain wind directions etc.
Wind Energy Course, Phuket, 11-15 Sept 200622
Recommendations
What can be done to reduce the likelihood of a noiseproblem from a wind project?
• A noise analysis should be carried out based on theoperating characteristics of the specific wind turbine,
the terrain in which the project will be located, andthe distance to nearby residences.
• Also, pre-construction noise surveys can beconducted to find out what the normally-occurringbackground noise levels are at the site, and todetermine later on what, if anything, the wind projecthas added to those levels.
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Wind Energy Course, Phuket, 11-15 Sept 200623
Recommendations
• The most common method for dealing with apotential noise issue, is to require a minimumdistance between any of the wind turbines in theproject and the nearest residence, that is sufficient toreduce the sound level to a regulatory threshold.
• Some permitting agencies have set up noisecomplaint resolution processes. In such a process,typically, a telephone number through which theagency can be notified of any noise concern is madepublic, and agency staff work with the project ownerand concerned citizens to resolve the issue. Theprocess should include a technical assessment of thenoise complaint to ensure its legitimacy.
Wind Energy Course, Phuket, 11-15 Sept 200624
Calculation of noise impact
• 3 wind turbines
• Total height 100 m
• Hub height 60 and 67 m
• Source 104 dB(A)
• WindPro ver. 2
www.emd.dk