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Wind Farm Noise Impact Assessment

NOISE PREDICTIONS – SOURCE DATA AND PROPAGATION

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Source

Propagation

Receiver

The situation to be assessed ….

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VH

V10

SWLv = SPLv + 10 x log ( 4 x p x r2 ) – 6dB

Source Sound Power (IEC 61400-11)

rH

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ISO 61400-11: Frequency based data

Third octave SWL

Narrow band tonal assessment

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Sound power values?

• Tested sound power (IEC 61400-11) with reported test uncertainty σ

• Warranted values: includes “safety margin” of typically up to 2dB(A)

…… but sometimes not (commercial considerations)• Turbine specification: additional uncertainty in warranty?• Declared values: procedure of IEC 61400-14 for adding

uncertainties (1.654 σ)

N.B: test results are LAeq data, so LA90 prediction is obtained by subtraction of 2dB

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Source

Propagation

Receiver

The situation to be assessed ….

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Propagation models

• ‘Exact’ numerical methods(e.g. parabolic equation, fast field program)

• Approximate semi-analytical methods(e.g. ray tracing)

• Empirical ‘engineering’ methods(e.g. ISO 9613-2, Concawe)

Input data

Calculation implementatio

n

Output results

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Factors affecting sound propagation

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Factors affecting sound propagation - 1

• Spherical spreading of noise

• Atmospheric attenuation

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Factors affecting sound propagation - 1

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-20

-15

-10

-5

0

5

10

32 63 125 250 500 1000 2000 4000 8000

Rel

ativ

e so

un

d p

ress

ure

lev

el, d

B

> 2MW turbine at 1000m

< 1MW turbine at 500m

Factors affecting sound propagation - 1

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Factors affecting sound propagation - 2

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Factors affecting sound propagation - 2

In ISO 9613-2: modelled by ground factor coefficient G (0 to 1)

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Factors affecting sound propagation - 2

• G = 1 : soft, porous groundground covered by grass or treescultivated farming land

Not used as it will often under-predict

• G= 0: hard groundpaving, water ice, concrete, etc.Industrial ground

Robust but can over-predict, particularly with warranted source data

• G= 0.5: mixed ground

Used with warranted source data or tested data + uncertainty

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Factors affecting sound propagation - 3

Refraction effects…

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Factors affecting sound propagation - 3

Hot

Cold

Temperature lapse

Upwind

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Factors affecting sound propagation - 3

Hot

Cold

Temperature inversion

Downwind

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Factors affecting sound propagation - 4

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Factors affecting sound propagation - 4

Sound energy enters ‘shadow region’ via turbulent scattering

335 340 3450

10

20

30

40

50

60

70

80

90

100

Sound speed (m/s)

Hei

ght

(m)

0 200 400 600 800 1000 12000

10

20

30

40

50

60

70

80

90

100

Distance (m)

Limiting ray

Shadow

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Factors affecting sound propagation – 3 + 4

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Comparison of PE and ISO 9613 results

20m

200m

1000m

Harmonoise PE model (based onannual measured meteorological conditions [Salomons])

ISO 9613(based on typical downwind propagation conditions)

Percentage of time below stated noise level, %

Calc

ula

ted n

ois

e level,

dB

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Factors affecting sound propagation - 6

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Factors affecting sound propagation - 6

In downwind conditions: limited to no more than 2 to 3 dB

Terrain = noise barrier?

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Factors affecting sound propagation - 6

“Valley effect”: +3 dB

hm ≥1.5 abs(hs-hr)/2

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Measurement studies

• 3 Separate wind farm sites, both located in rural environments and each comprising more than 20 turbines

• Two-speed turbines rated at over 2MW peak generating capacity, 60 to 70m hub height

• Type 1 sound meters continuously sampling statistical and equivalent data at varying distances, with double wind-shield arrangements

Published 2007-2009

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Measurement locations: from approximately 100m to 750 m from nearest turbine

• Very flat terrain• Minimal vegetation• Propagation path covered by peat bog• Ground prone to flooding – saturated for duration of survey

Wind Farm Envelope

NSite B

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Site B

Single site wind speed (G = 0 / 754m)

Site B - Measurement Location 4 (754 m)

30

35

40

45

50

55

60

8 9 10 11 12 13 14

Hub Height Wind Speed (m/s)

Tot

al L

A90

Noi

se L

evel

(dB

)

Measured Downwind

Predicted

Poly. (Predicted)

Poly. (MeasuredDownwind)

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Turbine specific wind speed (G = 0 / 754m)Site B - Measurement Location 4 (754 m)

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35

40

45

50

55

60

8 9 10 11 12 13 14

Hub Height Wind Speed (m/s)

Tot

al L

A90

Noi

se L

evel

(dB

)

Measured Downwind

Predicted

Upwind (±45º)

Poly. (Predicted)

Poly. (MeasuredDownwind)Poly. (Upwind (±45º))

Site B

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Turbine specific wind speed (G = 0 / 754m)

Site B - Measurement Location 4 (754 m) - G=0, variable WS

30

35

40

45

50

55

60

8 9 10 11 12 13 14

Hub Height Wind Speed (m/s)

To

tal L

A9

0 N

ois

e L

evel

(d

B)

Upwind Trendline

Predicted WTG Noise

Measured Total Noise

Site B

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Turbine specific wind speed (G = 0 / 754m)Site B - Measurement Location 4 (754 m) - G=0, variable WS

30

35

40

45

50

55

60

8 9 10 11 12 13 14

Hub Height Wind Speed (m/s)

To

tal L

A90

No

ise

Lev

el (

dB

)

Upwind Trendline

Measured WTG Noise(Corrected)

Predicted WTG Noise

Measured Total Noise

Site B

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Site B - Measurement Location 4 (754 m) - G=0, variable WS

30

35

40

45

50

55

60

8 9 10 11 12 13 14

Hub Height Wind Speed (m/s)

To

tal L

A90

No

ise

Lev

el (

dB

)

Upwind Trendline

Measured WTG Noise(Corrected)

Predicted WTG Noise

Measured Total Noise

Turbine specific wind speed (G = 0 / 754m)

+2dB(A) “warranty” margin added

~5dB

Site B

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Main measurement locations: from approximately 100m to 820

m from nearest turbine

• Lightly undulating but acoustically flat terrain• Minimal vegetation surrounding turbines, forestry close to

locations• Ground cover of mixed soft ground and flooded areas

N

Wind Farm Envelope

P1P2

P3P4

P5

Predictions are made for:• high speed mode only• G=0.5• tested sound power data +

1dB(Stated test uncertainty)

Site C

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Location P4: 700 m distance

Turbine only (predicted)

Turbine + background

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20

25

30

35

40

45

50

55

60

65

3 4 5 6 7 8 9 10 11 12Wind Speed at 10m (m/s)

Noi

se L

eve

l L90

dB

(A)

Measured (turbine + background)

Predicted turbine (G=0.5)

Poly. (Measured (turbine + background))

Site C

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Additional research

Evans and CooperComparison of predicted and measured wind farm noise levels and implications for assessments of new wind farms

Paper Number 30, Proceedings of ACOUSTICS 2011 2-4 November 2011, Gold Coast, Australia

“Steady slope” “Concave”

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Recommendations For use and application of ISO 9613-2 to WTN

• LA90 = LAeq – 2 dB

• Use spectrum data if available

• 4m receiver height

• 10 degrees / 70% humidity

• Do not use G=1• G=0.5 recommended,

with source levels which incorporate test uncertainties

(as in most warranties or “declared“ values as per IEC 61400-14)

• For propagation across a valley add +3dB

• Terrain screening: no more than -2dB

• State all assumption and input data

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Thank you