noise from wind turbines

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


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


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


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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Impact from noise


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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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)


Sound power from different turbines


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Background noise

Background noise


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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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.


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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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.


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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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


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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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.


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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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.


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

noise from wind turbines

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