1
Assessing noise from wind farm developments in Ireland: A consideration of critical wind
2
speeds and turbine choice
3
E.A. King, F. Pilla, J. Mahon
4
5
Abstract
6
Wind farms are becoming increasingly popular in Ireland in an effort to increase the production
7
of green energy within the state. As with any infrastructural development, wind farms must
8
consider potential environmental impacts prior to construction. One particular issue that must
9
be examined is the emission of noise from the development. In Ireland wind farm
10
developments must adhere to planning conditions that usually outline permissible noise levels
11
for both the construction and operational phases of the development. The critical wind speed
12
is often cited as the wind speed at which these limits apply. This paper examines how the
13
critical wind speed is determined and investigates its relationship with background noise levels
14
and turbine choice. The study consisted of ten one-week monitoring periods during which
15
meteorological conditions and background noise levels were simultaneously recorded. It was
16
found that the critical wind speed is non-transferable, i.e. it depends on both the turbine choice
17
and background noise environment and is specific to that particular turbine/site combination.
18
Furthermore the critical wind speed during the night-time is often different to the overall critical
19
wind speed suggesting that future noise studies should consider a range of critical wind
20
speeds, particularly for night-time noise assessments.
21
Keywords
22
Wind farm, Noise, Critical wind speed
23
24
1.
Introduction
25
Developing and investing in renewable energy is an integral part of Ireland’s sustainable
26
energy objectives (SEAI, 2010). The use of renewable energy will reduce Ireland’s carbon
27
footprint and help to achieve the target CO2 reductions set out in Directive 2009/28/EC on the
28
promotion of the use of energy from renewable sources (European Union, 2009). Over the
29
period 1990–2008, CO2 avoided through the use of renewable energy increased by 257%
30
reaching 2830 kt in 2008, while wind energy use gave rise to the largest avoidance of
31
emissions in 2008 (46%) (SEAI, 2010). Renewable energy also provides some energy security
32
as it is mainly an indigenous energy source. Furthermore, the current increasing and volatile
33
nature of energy (fossil fuels) costs demonstrate the need for reducing reliance on imported
34
fossil fuels.
1
Ireland is in an ideal position to harness energy from renewable sources due to its prevailing
2
climate and geographical position. The Sustainable Energy Authority of Ireland (SEAI)
3
reported that the installed wind capacity, as of 12 January 2010, was 1264 MW, contributing
4
to the overall renewable capacity of 1441 MW (SEAI, 2010). Statistics from the Irish Wind
5
Energy Association (IWEA) in July 2010 reported that the grid connected and operational
6
installed wind capacity on the island of Ireland was 1746.7 MW. If a 31% load or capacity
7
factor is assumed, wind energy will generate 4,743,339 MWh on average in a year (Coen and
8
Grace, 2011). Wind energy represents a growing energy source in Ireland and it is clear that
9
the amount of wind farms in Ireland will significantly increase in the coming years.
10
While this increase will yield real benefits in terms of reduced carbon emissions, the growth
11
will also lead to greater environmental concerns associated with such development. Noise and
12
visual impact are the most common negative effects related to wind farms. These are mainly
13
local in nature and generally only impact residences in the vicinity of the development
14
(Swofford and Slattery, 2010). van der Horst (2007) reports the existence of a ‘distance decay’
15
whereby levels of public concern regarding the negative impacts of a wind farm diminish with
16
distance.
17
Generally wind turbines will generate noise that may be described as a combination of tonal,
18
broadband, low frequency and impulsive sounds through various phases of operation. van
19
den Berg et al. (2008) found that noise was the most annoying aspect of wind farm
20
developments. However, the degree of this annoyance may be dependent on the level of
21
visual intrusion. Pedersen and Larsman (2008) found that in locations where wind farms are
22
perceived as having a negative impact on scenery, the probability of noise annoyance,
23
regardless of the A-weighted sound pressure level, is increased.
24
It is clear that noise resulting from a wind farm should be controlled in order to minimise the
25
related acoustic impact on surroundings. Ensuring that noise considerations form an integral
26
part of the planning process at an early stage is the most effective form of noise control as,
27
once a wind farm becomes operational, most retrofitted noise mitigation measures would
28
directly impede the production of energy. This could be achieved through a planning process.
29
Gamboa and Munda (2007), for example, report that some authorities enforce minimum
30
distances between wind turbines and residential areas that can range from 300 m to 1 km.
31
Such policies would have a direct control over the noise impact, but may unduly restrict
32
development in certain areas.
33
1.1.
Noise assessments in Ireland
34
In Ireland, wind farms developments are generally subject to number of planning conditions,
35
which impose noise limits at nearby sensitive receivers. The Irish Department of Environment,
1
Health and Local Government (DoEHGL) have issued guideline documentation regarding
2
wind farm developments (DoEHGL, 2006). These guidelines suggest that a lower fixed limit
3
of 45 dB(A) L90 or a maximum increase of 5 dB(A) above background noise at nearby noise
4
sensitive locations is considered appropriate to provide protection to wind energy development
5
neighbours. A fixed limit of 43 dB(A) is suggested for night-time periods. These limits are
6
drawn from guidance from the UK’s Working Group on Wind Turbine Noise, contained in
7
ETSU-R-97 (ETSU, 1996). The Irish Wind Energy Association’s ‘‘Best Practice Guidelines for
8
the Irish Wind Energy Industry’’ also recommends the application of the ETSU-R-97 Report
9
(IWEA, 2008). Further guidance is contained in the Irish Environmental Protection Agency
10
(EPA) guidance note on the assessment of noise from wind turbine operations at EPA licensed
11
sites (EPA, 2011).
12
The application of the limits outlined above is dependent on a number of variables that must
13
be taken into account when planning a wind farm. The level of noise generated by a wind
14
turbine will increase with increasing wind speeds. However, when the wind is blowing the
15
noise from the turbines may be masked by the sound of the wind itself, particularly if there is
16
an abundance of trees or vegetation in the area. In general the rate at which turbine noise
17
increases with wind speed is lower than the rate at which background noise levels increase
18
with wind speed (DoEHGL, 2006). The impact of wind turbine noise is therefore likely to be
19
greater at low wind speeds, when the difference between the noise of the wind turbine and
20
the background noise is likely to be greater. Thus planning conditions often refer to noise
21
levels at this critical wind speed, which occurs when the noise radiated by the total
22
complement of wind turbines and blades is most substantially in excess of ambient noise,
23
usually in the region where the turbine noise rises to a plateau.
24
In order to assess the existing background noise level and the manner in which this level
25
varies with wind speed, a detailed background noise survey must be undertaken. Prevailing
26
background noise levels over a range of wind speeds are recorded and at least one weeks’
27
worth of continuous noise monitoring is required to avoid results being weighted by
28
unrepresentative conditions (ETSU, 1996). By comparing the background noise with the
29
sound power of the proposed turbines over a range of wind speeds the critical wind speed
30
may then be calculated.
31
This paper examines the effect different turbines may have on critical wind speed calculations
32
and how the turbine choice may dictate the relevant noise levels at nearby sensitive receivers.
33
A range of different turbine types, combined with ten different background noise environments,
34
is assessed. The paper identifies the factors that influence the critical wind speed and
35
investigates if the critical wind speed is an appropriate consideration for wind farm
36
developments.
1
2.
Sound power levels of turbines
2
This study examines four popular turbine types (make and manufacturer) currently in use in
3
Ireland. Because of varying preferences amongst manufacturers it was not possible to obtain
4
sound power levels for identical hub heights, blade diameter and power output. Thus the
5
turbines under examination are as similar as possible; i.e. all were of a similar power output,
6
approximately 2 MW, with similar hub heights, approximately 80 m, and similar blade lengths.
7
Table 1 displays the sound power/wind speed relationship for each turbine while Fig. 1
8
displays this information graphically. The winds speeds reported are normalised to a height of
9
10 m. This height is the widely accepted reference height to report sound power levels of
10
turbines. IEC 61400-11 and ETSU-R-97 use this 10 m reference height and this information
11
is generally available from all manufacturers.
12
3.
Measurement methodology
13
A detailed representation of the background noise levels is essential for setting appropriate
14
noise limits; i.e. the limit of 5 dB(A) above background levels. This will involve an in-depth
15
background noise survey that generally requires at least one week’s monitoring. Good practice
16
would suggest at least 20–30 measurements should be taken within ± 2 m/s of the critical wind
17
speed (ETSU, 1996). This poses an interesting question to the acoustic engineer. It is
18
generally regarded that windshields will be effective up to wind speeds of 5 m/s (BS
19
4142, 1997). In higher wind speeds the wind passing over the diaphragm of the microphone
20
of the sound level meter can generate noise interference. However, in the case of background
21
noise assessments, noise measurement in wind speeds of up to 12 m/s may frequently be
22
required. Measurements carried out above 5 m/s may be influenced by the wind itself and may
23
not be a true representation of the background noise environment. This issue has previously
24
been examined by the authors (King et al., 2009) and more research regarding the impact of
25
wind-induced noise on such measurements is required.
26
For the current case a total of 10 locations were selected for examination. In each case, the
27
monitoring period was one week yielding an overall measurement period of 10 weeks
28
continuous noise measurements, with synchronised monitoring of meteorological conditions.
29
Table 1
30
Sound power levels of turbines at different wind speeds.
Sound power levels for four selected turbines
Turbine A dB(A) Turbine B dB(A)
5
97.2
103.3
6
101.6
107.3
7
103.6
108.4
8
104
108.4
Turbine C dB(A) Turbine D dB(A)
100.5
102.5
103.6
103
104.8
103.5
105.2
9
10
104
104
108.4
108.4
104
104
105.3
105.3
1
2
3
4
Fig. 1. Sound power levels of turbines.
3.1.
Site description
5
The 10 monitoring locations were spread across four counties in the southwest of Ireland,
6
where wind farm developments are quite popular. During a background noise survey
7
measurements are generally conducted at sensitive receivers near the wind farm. The site
8
layout and presence of existing noise sources will determine the location and amount of these
9
measurement positions. The aim of the background measurement survey is to determine the
10
background noise environment in the area and as such measurement locations are chosen to
11
best represent this background noise. For the current case Locations 1, 3, 5, 6, 7, 8, 9 and 10
12
were located in the rear gardens of private residences in rural locations. Where access was
13
not granted to a private residence (Locations 2 and 4), locations close to the residence in
14
private fields were used. Bushes, trees and vegetation were in abundance at each location,
15
although the microphone was positioned at least 3.5 m away from these sources. In general
16
local regional (minor) roads provided access to the sites and no major noise sources were in
17
the locality. In all cases a wind farm development was proposed in the locality.
18
3.2.
Measurement of meteorological conditions
19
A Vantage Pro2 weather station developed by Davis Instruments was used to monitor wind
20
speeds, wind direction, temperature, humidity and rainfall rate throughout each measurement
21
period. This data was logged in 10 min intervals and synchronised with the sound level meter.
1
This interval period is in keeping with ETUS-R-97 guidance (ETSU, 1996). Periods where
2
rainfall was recorded were omitted for all analyses.
3
Wind speed measurements were recorded at a height of 2 m. However, wind speed will vary
4
with height above the ground level, increasing with increased height. In accordance with ETSU
5
guidance the values of wind speed were corrected to a reference height of 10 m using the
6
following equation:
7
𝑣1 ln⁡(ℎ1 ⁄𝑧0 )
=
𝑣2 ln⁡(ℎ2 ⁄𝑧0 )
8
where n is the wind speed (m/s) at a height of h1 meters above ground level, n2 is the wind
9
speed (m/s) at a height of h2 metres above ground level, z0 is the ground roughness length
10
(m). Some typical values for z0 are presented in Table 2 (ETSU, 1996).
11
Thus both background levels and the sound power levels of the turbines are referenced to a
12
wind speed at a height of 10 m above the ground. However, it is important to note that the
13
ground roughness plays an important part in determining the variation of wind speeds with
14
height. Fig. 2 displays how different values of the ground roughness, from those presented in
15
Table 2, will influence the wind speed profile. It is evident that different values of z0 will
16
significantly influence results. Given that values for z0 may be based on estimations, and
17
limited guidance is available, there exists significant potential for error. This is compounded
18
by the fact that, as wind speed at the measurement height (2 m) increases, variation is
19
increased.
20
Table 2
21
Roughness length for various types of terrain (ETSU, 1996).
Type of terrain
Water area, snow or sand surfaces
Open, flat land, mown grass, bare soil
Farmland with some vegetation
Suburbs, towns, forests, many trees and bushes
22
Roughness length z0 (m)
0.001
0.01
0.05
0.30
1
2
Fig. 2. Calculated distribution of wind speeds with height.
3
Furthermore it has been noted that there is a potential mismatch between referencing wind
4
speeds used for measurements and turbines to a height of 10 m levels unless site-specific
5
wind shear is taken into account (IOA, 2009). It has been recommended that background
6
noise levels be correlated with a derived 10 m height as opposed to measured. This would
7
result in referencing all noise levels to the wind speed at turbine hub height, although results
8
would be stated in terms of the derived 10 m height (IOA, 2009).
9
van den Berg (2004) also observed differences in the wind shear between day and night
10
periods. He demonstrated that the sound emission level might, at the same wind speed at 10
11
m height, be significantly higher (up to 18 dB) during the night-time than in the daytime. This
12
difference was attributed to different wind profiles existing throughout in the day and night
13
periods; the wind speed at night was higher than expected from the conventional extrapolation
14
from the wind speed measured at 10 m height. Van den Berg concluded that a logarithmic
15
wind profile, based only on surface roughness and not on atmospheric stability, is not a good
16
predictor for wind profiles at night. Further, wind shear coefficients are highly variable over the
17
full day and may change from less than 1/7 during the day to more than 1/2 at night over the
18
same terrain (Gualtieri and Secci, 2011).
19
3.3.
Noise measurements
20
A Svan 959 class 1 integrating sound level meter and a Svan 958 class 1 integrating sound
21
level meter were used to continuously monitor noise at each location. The sensitivities of the
22
instruments were measured before and after each measurement campaign and no drift was
23
observed. Appropriate noise levels (LAeq and LA90) were logged every 10 min simultaneously
24
with the meteorological data. All noise measurements were conducted in general accordance
25
with ISO 1996 and BS 4142 (ISO, 1996-1, 2003). In all the cases the microphone was tripod
1
mounted at a height of 1.5 m above ground and at least 3.5 m from any reflecting surface
2
other than the ground. In each case the microphone was orientated vertically. An outdoor
3
windshield was used for each test.
4
4.
Measurement results
5
4.1.
Correlating background noise levels and wind speed
6
Figs. 3–22 present the relationship between background noise levels and wind speeds
7
observed for each test location. A fifth order polynomial has been fitted to the data in keeping
8
with ETSU-R-97 guidance. The R2 value presented in each graph indicates the fraction of
9
variance observed in noise level data explained by the wind speed. If the R2 value was equal
10
to one, all changes in the noise level would be due to the wind speed; thus high levels of R2
11
are desirable. In some cases low values are reported, indicating that increased noise levels
12
are not fully explained by increased wind speed. This variance could be due to the effect of
13
extraneous noise in the locality, e.g. activity in the surroundings of the house, nearby roads,
14
etc.
15
16
Fig. 3. Background noise levels, over all time periods, correlated with wind speed for test
17
Location 1.
18
19
Fig. 4. Background noise levels, over night-time periods, correlated with wind speed for test
20
Location 1.
1
2
Fig. 5. Background noise levels, over all time periods, correlated with wind speed for test
3
Location 2.
4
5
Fig. 6. Background noise levels, over night-time periods, correlated with wind speed for test
6
Location 2.
7
8
Fig. 7. Background noise levels, over all time periods, correlated with wind speed for test
9
Location 3.
1
2
Fig. 8. Background noise levels, over night-time periods, correlated with wind speed for test
3
Location 3.
4
5
Fig. 9. Background noise levels, over all time periods, correlated with wind speed for test
6
Location 4.
7
8
Fig. 10. Background noise levels, over night-time periods, correlated with wind speed for test
9
Location 4.
1
2
Fig. 11. Background noise levels, over all time periods, correlated with wind speed for test
3
Location 5.
4
5
Fig. 12. Background noise levels, over night-time periods, correlated with wind speed for test
6
Location 5.
7
8
Fig. 13. Background noise levels, over all time periods, correlated with wind speed for test
9
Location 6.
1
2
Fig. 14. Background noise levels, over night-time periods, correlated with wind speed for test
3
Location 6.
4
5
6
Fig. 15. Background noise levels, over all time periods, correlated with wind speed for test
7
Location 7.
8
9
10
Fig. 16. Background noise levels, over night-time periods, correlated with wind speed for test
Location 7.
1
2
Fig. 17. Background noise levels, over all time periods, correlated with wind speed for test
3
Location 8.
4
5
Fig. 18. Background noise levels, over night-time periods, correlated with wind speed for test
6
Location 8.
7
8
Fig. 19. Background noise levels, over all time periods, correlated with wind speed for test
9
Location 9.
1
2
Fig. 20. Background noise levels, over night-time periods, correlated with wind speed for test
3
Location 9.
4
5
Fig. 21. Background noise levels, over all time periods, correlated with wind speed for test
6
Location 10.
7
8
Fig. 22. Background noise levels, over night-time periods, correlated with wind speed for test
9
Location 10.
10
11
Table 3
12
Determining the critical wind speed for each location associated with Turbine A.
Differences—Turbine A
Wind speed
4
5
Loc 1 dB(A)
Loc 2 dB(A)
Loc 3 dB(A)
Loc 4 dB(A)
Loc 5 dB(A)
Loc 6 dB(A)
Loc 7 dB(A)
Loc 8 dB(A)
Loc 9 dB(A)
Loc 10 dB(A)
60.5
60.1
66.6
65.4
64.7
64.3
60.0
69.2
70.0
67.7
6
63.3
61.7
69.5
68.2
67.9
66.3
60.7
72.6
72.5
70.4
7
63.7
61.3
69.6
68.2
68.3
65.9
58.0
73.5
72.6
70.5
8
62.1
59.1
67.9
66.3
66.8
64.1
54.5
72.6
71.0
68.8
9
59.0
56.5
65.6
64.0
64.9
62.0
55.2n
71.2
68.8
66.2
10
54.5n
55.8n
63.2
61.8
62.9
59.8
66.6n
69.7
66.4
63.3
1
Note: The asterisk (*) denotes a value that was extrapolated from measurements as, in some
2
cases, wind speeds during the measurement period did not reach these high values. Values
3
in bold represent the estimated critical wind speed in each case.
4
5
Table 4
6
Determining the critical wind speed for each location associated with Turbine B.
Differences—Turbine B
Wind speed
4
5
Loc 1 dB(A)
Loc 2 dB(A)
Loc 3 dB(A)
Loc 4 dB(A)
Loc 5 dB(A)
Loc 6 dB(A)
Loc 7 dB(A)
Loc 8 dB(A)
Loc 9 dB(A)
Loc 10 dB(A)
66.6
66.2
72.7
71.5
70.8
70.4
66.1
75.3
76.1
73.8
6
69.0
67.4
75.2
73.9
73.6
72.0
66.4
78.3
78.2
76.1
7
68.5
66.1
74.4
73.0
73.1
70.7
62.8
78.3
77.4
75.3
8
66.5
63.5
72.3
70.7
71.2
68.5
58.9
77.0
75.4
73.2
9
63.4
60.9
70.0
68.4
69.3
66.4
59.6n
75.6
73.2
70.6
10
58.9n
60.2n
67.6
66.2
67.3
64.2
71.0n
74.1
70.8
67.7
7
Note: The asterisk (*) denotes a value that was extrapolated from measurements as, in some
8
cases, wind speeds during the measurement period did not reach these high values. Values
9
in bold represent the estimated critical wind speed in each case.
10
11
4.2.
Critical wind speed calculations
12
The critical wind speed is the speed at which noise radiated by the total complement of wind
13
turbines and blades is most substantially in excess of ambient noise. Tables 3–6 present the
14
difference between the background noise levels and turbine sound power level at each wind
15
speed for each location/turbine type combination for the overall measurement period. In each
16
case the highest value is presented in bold and the wind speed at this value is deemed to be
17
the critical wind speed.
18
It is important to note that, in some cases, the wind speeds recorded did not reach high levels,
19
particularly throughout the night period. In these cases the trend lines representing the
20
relationship between background noise levels and wind speed had to be extrapolated to
21
estimate noise levels at these high speeds. These values are identified with an asterisk (*) in
1
the tables. Take, for example, Location 7; during this measurement period wind speeds higher
2
than 8 m/s were not observed (Figs. 15 and 16) and noise levels at these higher speeds had
3
to be extrapolated beyond the measurement range. Location 10 yielded similar concern during
4
the night-time. These data must be treated with caution, and perhaps highlights a flaw with
5
current guidance. The critical wind speed cannot be determined until after noise
6
measurements have been conducted thus it is not possible to ensure at least 20–30
7
measurements are recorded within 2 m/s of the critical wind speed. Tables 7–10 repeat the
8
analysis for the night-time periods. Another potential error is highlighted in the case of the
9
night-time correlation at Location 8. This correlation yielded a low R2 value of 0.19958 (Fig.
10
18), thus the predicted relationship is not reliable. In this case noise levels were predicted to
11
decrease with increasing wind speeds above approximately 9 m/s. Thus the derived
12
correlation is only valid within the measurement range and extrapolation should be avoided.
13
14
Table 5
15
Determining the critical wind speed for each location associated with Turbine C.
Differences—Turbine C
Wind speed
4
5
Loc 1 dB(A)
Loc 2 dB(A)
Loc 3 dB(A)
Loc 4 dB(A)
Loc 5 dB(A)
Loc 6 dB(A)
Loc 7 dB(A)
Loc 8 dB(A)
Loc 9 dB(A)
Loc 10 dB(A)
63.8
63.4
69.9
68.7
68.0
67.6
63.3
72.5
73.3
71.0
6
64.2
62.6
70.4
69.1
68.8
67.2
61.6
73.5
73.4
71.3
7
63.1
60.7
69.0
67.6
67.7
65.3
57.4
72.9
72.0
69.9
8
61.6
58.6
67.4
65.8
66.3
63.6
54.0
72.1
70.5
68.3
9
59.0
56.5
65.6
64.0
64.9
62.0
55.2n
71.2
68.8
66.2
10
54.5n
55.8n
63.2
61.8
62.9
59.8
66.6n
69.7
66.4
63.3
16
Note: The asterisk (*) denotes a value that was extrapolated from measurements as, in some
17
cases, wind speeds during the measurement period did not reach these high values. Values
18
in bold represent the estimated critical wind speed in each case.
19
20
Table 6
21
Determining the critical wind speed for each location associated with Turbine D.
Differences—Turbine D
Wind speed
4
5
6
Loc 1 dB(A)
Loc 2 dB(A)
Loc 3 dB(A)
Loc 4 dB(A)
Loc 5 dB(A)
Loc 6 dB(A)
Loc 7 dB(A)
Loc 8 dB(A)
Loc 9 dB(A)
Loc 10 dB(A)
65.3
63.7
71.5
70.2
69.9
68.3
62.7
74.6
74.5
72.4
7
64.9
62.5
70.8
69.4
69.5
67.1
59.2
74.7
73.8
71.7
8
63.3
60.3
69.1
67.5
68.0
65.3
55.7
73.8
72.2
70.0
9
60.3
57.8
66.9
65.3
66.2
63.3
56.5n
72.5
70.1
67.5
10
55.8n
57.1n
64.5
63.1
64.2
61.1
67.9n
71.0
67.7
64.6
1
Note: The asterisk (*) denotes a value that was extrapolated from measurements as, in some
2
cases, wind speeds during the measurement period did not reach these high values. Values
3
in bold represent the estimated critical wind speed in each case.
4
5
Table 7
6
Determining the night-time critical wind speed for each location associated with Turbine A.
Differences—Turbine A
Wind speed
4
5
Loc 1 dB(A)
Loc 2 dB(A)
Loc 3 dB(A)
Loc 4 dB(A)
Loc 5 dB(A)
Loc 6 dB(A)
Loc 7 dB(A)
Loc 8 dB(A)
Loc 9 dB(A)
Loc 10 dB(A)
60.4
61.0
67.2
65.8
64.2
66.2
61.8
69.1
70.4
65.9
6
62.1
61.8
70.8
68.7
67.8
67.3
61.8
72.7
73.0
69.7
7
63.0
61.4
71.4
69.2
68.5
66.5
54.1
73.8
73.4
71.6
8
63.4
60.1
69.9
67.9
67.3
64.6
33.5n
73.5
72.1
73.1n
9
61.3n
58.0n
67.7
66.3
65.2
62.3
8.0n
73.2
70.3
77.6n
10
49.9n
54.6n
65.4
64.8
62.7
59.7
83.7n
74.1
68.5
89.3n
7
Note: Again values denoted with an asterisk (*) must be treated with caution. The negative
8
values indicate an extrapolated background level that was higher than the sound power of the
9
turbine and highlights the error associated with a failure to capture background noise level
10
over a wide range of wind speeds. Values in bold represent the estimated critical wind speed
11
in each case.
12
13
Table 8
14
Determining the night-time critical wind speed for each location associated with Turbine B.
Differences—Turbine B
Wind speed
4
5
Loc 1 dB(A)
Loc 2 dB(A)
Loc 3 dB(A)
Loc 4 dB(A)
Loc 5 dB(A)
Loc 6 dB(A)
Loc 7 dB(A)
Loc 8 dB(A)
Loc 9 dB(A)
Loc 10 dB(A)
66.5
67.1
73.3
71.9
70.3
72.3
67.9
75.2
76.5
72.0
6
67.8
67.5
76.5
74.4
73.5
73.0
67.5
78.4
78.7
75.4
7
67.8
66.2
76.2
74.0
73.3
71.3
58.9
78.6
78.2
76.4
8
67.8
64.5
74.3
72.3
71.7
69.0
37.9n
77.9
76.5
77.5n
9
65.7n
62.4n
72.1
70.7
69.6
66.7
3.6n
77.6
74.7
82.0n
10
54.3n
59.0n
69.8
69.2
67.1
64.1
79.3n
78.5
72.9
93.7n
15
Note: The asterisk (*) denotes a value that was extrapolated from measurements as, in some
16
cases, wind speeds during the measurement period did not reach these high values. Values
17
in bold represent the estimated critical wind speed in each case.
18
19
20
1
Table 9
2
Determining the night-time critical wind speed for each location associated with Turbine C.
Differences—Turbine C
Wind speed
4
5
Loc 1 dB(A)
Loc 2 dB(A)
Loc 3 dB(A)
Loc 4 dB(A)
Loc 5 dB(A)
Loc 6 dB(A)
Loc 7 dB(A)
Loc 8 dB(A)
Loc 9 dB(A)
Loc 10 dB(A)
63.7
64.3
70.5
69.1
67.5
69.5
65.1
72.4
73.7
69.2
6
63.0
62.7
71.7
69.6
68.7
68.2
62.7
73.6
73.9
70.6
7
62.4
60.8
70.8
68.6
67.9
65.9
53.5
73.2
72.8
71.0
8
62.9
59.6
69.4
67.4
66.8
64.1
33.0n
73.0
71.6
72.6n
9
61.3n
58.0n
67.7
66.3
65.2
62.3
8.0n
73.2
70.3
77.6n
10
49.9n
54.6n
65.4
64.8
62.7
59.7
83.7n
74.1
68.5
89.3n
3
Note: The asterisk (*) denotes a value that was extrapolated from measurements as, in some
4
cases, wind speeds during the measurement period did not reach these high values. Values
5
in bold represent the estimated critical wind speed in each case.
6
7
Table 10
8
Determining the night-time critical wind speed for each location associated with Turbine D.
Differences—Turbine D
Wind Speed
4
5
6
Loc 1 dB(A)
Loc 2 dB(A)
Loc 3 dB(A)
Loc 4 dB(A)
Loc 5 dB(A)
Loc 6 dB(A)
Loc 7 dB(A)
Loc 8 dB(A)
Loc 9 dB(A)
Loc 10 dB(A)
64.1
63.8
72.8
70.7
69.8
69.3
63.8
74.7
75.0
71.7
7
64.2
62.6
72.6
70.4
69.7
67.7
55.3
75.0
74.6
72.8
8
64.6
61.3
71.1
69.1
68.5
65.8
34.7n
74.7
73.3
74.3n
9
62.6n
59.3n
69.0
67.6
66.5
63.6
6.7n
74.5
71.6
78.9n
10
51.2n
55.9n
66.7
66.1
64.0
61.0
82.4n
75.4
69.8
90.6n
9
Note: The asterisk (*) denotes a value that was extrapolated from measurements as, in some
10
cases, wind speeds during the measurement period did not reach these high values. Values
11
in bold represent the estimated critical wind speed in each case.
12
4.3.
Results analysis
13
In all cases Turbine A resulted in either the highest or joint highest value for the critical wind
14
speed. Take, for example, Location 1. If Turbine A was selected for this development the
15
critical wind speed, based on the overall measurement period, would be 7 m/s whereas all
16
other turbines would result in a critical wind speed of 6 m/s. In all but one location, Location 4,
17
the critical wind speed varied depending on the turbine choice. However, in general, the overall
18
spread of critical wind speeds was relatively small, between 5 and 7 m/s.
19
The results also suggest there is greater variance in critical wind speed during the night-time
20
periods. This might be due to less data points contained the analysis, approximately one third
21
of the overall sample size, but may also be a result of varying wind shear profiles throughout
1
the night-time periods. Furthermore it is also evident that the number of extrapolated points
2
increased during night-time analyses which may suggest inaccuracies in the analyses.
3
However results suggest the critical wind speed tends to increase throughout the night-time.
4
Again take Location 1 for example. Three turbines saw the critical wind speed rise. Thus the
5
impact of a development may be evaluated at an incorrect lower critical wind speed during
6
night periods. Consequently the predicted noise from the turbine will be lower and potential
7
impacts may be underestimated during night-time periods. This may have severe implications
8
for policy makers. One must question if the critical wind speed should be representative of all
9
wind speeds or should a higher critical wind speed be used, albeit with a lower probability of
10
11
occurring. This issue is discussed in more detail in Section 6.
5.
Noise level at nearby sensitive receiver
12
From the foregoing analysis it is evident that the choice of turbine, in combination with
13
background noise environment, will impact on the calculated the critical wind speed. In an
14
effort to investigate the associated impact this might have on noise levels at a nearby sensitive
15
receiver, the predicted noise level at a nominal test receiver, located at a distance of 200 m
16
from a proposed single turbine development, is examined.
17
5.1.
Calculation of noise attenuation
18
ISO 9613-2 presents an engineering method for calculating the attenuation of sound during
19
outdoor propagation at a distance from a point source or a collection of incoherent point
20
sources. The conditions of the ISO 9613-2 methodology represent downwind propagation in
21
all directions and thus represent a worstcase scenario. For the current case wind turbines are
22
modelled as omnidirectional point sources.
23
Sound propagating outdoors generally decreases in level the further it travels from the source
24
due to a variety of reasons: geometrical divergence of the sound, meteorological effects,
25
presence of geographical barriers, etc. Calculations are performed in octave bands and the
26
total attenuation term for each octave band may be calculated as a sum of all different
27
attenuation mechanisms.
28
5.2.
Test sensitive receiver location
29
The noise level for each location/turbine combination at a test sensitive receiver is calculated.
30
The test receiver is assumed to be 200 m from the turbine, at a height of 1.5 m and propagation
31
occurs over completely soft ground.
32
Table 11
33
Calculated noise levels at test sensitive receiver for each location and relevant critical wind
34
speed.
Turbine A
Turbine B
Turbine C
Turbine D
Overall dB(A)
Night dB(A)
Overall dB(A)
Night dB(A)
Overall dB(A)
Night dB(A)
Overall dB(A)
Night dB(A)
Location 1
42.77
43.17
46.47
47.57
41.67
39.67
42.77
44.37
Location 2
Location 3
Location 4
Location 5
Location 6
Location 7
Location 8
Location 9
Location 10
40.77
42.77
40.77
42.77
40.77
40.77
42.77
42.77
42.77
40.77
42.77
42.77
42.77
40.77
40.77
42.77
42.77
42.77
46.67
46.67
46.67
46.67
46.67
46.67
46.67
46.67
46.67
46.47
46.47
46.47
46.47
46.47
42.47
47.57
46.47
47.57
39.67
41.67
41.67
41.67
39.67
39.67
41.67
41.67
41.67
39.67
41.67
41.67
41.67
39.67
39.67
41.67
41.67
42.17
42.77
42.77
42.77
42.77
42.77
42.77
43.97
42.77
42.77
42.77
42.77
42.77
42.77
42.77
42.77
42.77
42.77
43.97
1
2
5.3.
Prediction results
3
Table 11 presents the calculated noise level at the test receiver, for each turbine/location
4
combination. In each case calculations are performed at the critical wind speed identified for
5
each specific turbine/location combination. In all cases, with the exception of Location 4,
6
Turbine C resulted in the lowest noise level at the sensitive receiver. Location 4 was the only
7
location where both Turbine A and Turbine C resulted in the same critical wind speed, at all
8
other locations Turbine C yielded a lower critical wind speed than Turbine A. Table 11 also
9
highlights the issue of varying critical wind speeds at night-time. Consider the use of Turbine
10
D at Location 1. The overall critical wind speed was found to be 6 m/s while the night-time
11
critical wind speed was 8 m/s (from Tables 6 and 10). The corresponding noise level at the
12
receiver is then 42.77 and 44.37 dB(A). Thus the impact may be underestimated by almost 2
13
dB (A) solely through the use of an incorrect critical wind speed.
14
6.
Discussion
15
National guidance on the assessment of noise from wind farms must be welcomed as it assists
16
with maintaining an appropriate level of consistency across wind farm noise assessments.
17
The current baseline noise assessment methodology provides a good description of the
18
background noise environment, which will enable accurate compliance monitoring surveys.
19
However this study has highlighted a number of issues that need more consideration by
20
relevant authorities, particularly associated with the calculation of the critical wind speed. It
21
was found that different critical wind speeds may be calculated through the night-time period
22
compared to the overall assessment period. Furthermore, the critical wind speed was found
23
to be a nontransferable value i.e. it depends on both the turbine choice and background noise
24
environment and is specific to that particular turbine/site combination.
25
It is clear that more research is needed in a number of areas associated with wind farm noise
26
assessments. This paper highlights a number of such issues including:
1

It is necessary to measure noise in high wind speeds but current windshields may not
2
be considered to be effective at such high speeds. The contradiction here is clear and
3
further research is needed.
4

5
6
There is a need for a standard, validated method to account for wind shear in
referencing noise levels to a height of 10 m.

The critical wind speed used for assessments may vary depending on the time of day,
7
particularly during the nighttime. It may be argued that the probability of an increased
8
critical wind speed occurring is higher and as such it might not be appropriate to base
9
predictions on a wind speed that has a low probability of occurring. However, in
10
keeping with the general policy of the precautionary principle, the authors believe such
11
a worst case scenario should be considered.
12
Furthermore, a recent review conducted in the UK, analysing how noise impacts are
13
considered in the determination of wind farm planning application, highlighted a number of
14
different interpretations of ETSU-R-97 (Hayes McKenzie, 2011). Examples include a number
15
of different approaches to measuring background noise levels and suggestions that
16
background noise measurements are not required until planning consent is given. The report
17
concludes that any subsequent guidance on best practice should be more prescriptive on the
18
approach to background noise measurements, and interpretation of data, as background
19
measurements not only form the basis of any assessment but are likely to determine the noise
20
limits used in any eventual planning conditions on noise issues. The same report also
21
acknowledges the IOA methodology to assess wind shear but notes that, as the IOA
22
methodology has no official status, it would be appropriate for any best practice guidance to
23
confirm an appropriate way of dealing with wind shear issues, as this is fundamental to the
24
assessment procedure (Hayes McKenzie, 2011). Although the report reviewed wind farm
25
developments in the UK it is likely that similar conclusions might be drawn for Irish
26
developments. The Hayes McKenzie review did not consider the interpretation of the critical
27
wind speed.
28
Overall it is clear that more definitive guidelines are required for the development of wind farms
29
in Ireland. Guidance to date lacks a detailed consideration of environmental noise. For
30
example, current guidance does not offer any recommendation on the number of
31
measurement points required at the boundary of the development and seasonal effects are
32
not considered whatsoever. Any revised guidance should address the issues highlighted in
33
this paper, but should also consider other associated concerns. Aerodynamic modulation is
34
another particular issue in which more guidance is required. In a review of noise from wind
35
farms in 2006 it was concluded that the measured level of aerodynamic modulation is greater
36
that expected or assumed within ETSU-R-97 (Hayes McKenzie, 2006). Furthermore, it is
1
questionable if the A-weighted indicator is an appropriate noise index for assessing noise
2
exposure (St Pierre, and Magire, 2004) and may be particular unsuitable for wind farm
3
developments. Finally, the consideration of environment noise should be highlighted at an
4
early stage in the planning process. A strategy similar to that proposed by King and O’Malley
5
(2011) for the treatment of road traffic noise could be adopted. They recommend the
6
consideration of noise at an early stage in the planning process and early attention should be
7
paid to possible noise amelioration that may be inherent in a proposed road scheme.
8
7.
Conclusion
9
The first Irish wind farm development was completed in 1992. Wind farms have been widely
10
developed since and the installed wind energy capacity has now reached over 1264 MW.
11
National guidance on the development of wind farms has been released and, where possible,
12
has drawn heavily from international best practice. However, the manner in which
13
environmental noise from wind farm developments may be considered is in need of revision.
14
This paper presents a summary of the current approach to background noise assessments for
15
wind farm developments in Ireland. In particular it focuses on the calculation of the critical wind
16
speed. A selection of turbines from different manufacturers, typical of wind farm developments
17
in Ireland, was examined to assess the impact the turbine choice may have on noise
18
assessments. It is noted that different turbines and background noise configurations will yield
19
different critical wind speeds. Overall it may be concluded that:
20

The critical wind speed is dependent on both the turbine choice and the background
21
noise environment. It is a non-transferable value and must be re-calculated if different
22
turbines are used for a development or if turbines are used in different background
23
noise environments.
24

The critical wind speed may change throughout the night-time period. Of the 40
25
turbine/location combinations examined, 25% of tests yielded a different critical wind
26
speed throughout the night-time period.
27
Thus it may be concluded that noise assessments should not be restricted to the sole use of
28
the critical wind speed but rather consider a range of critical wind speeds, particularly in the
29
case of a night-time noise assessment. Such a consideration may also allow a separate wind
30
shear be calculated throughout the nighttime period. It is also apparent that further detailed
31
guidance on the assessment of noise from wind turbines in Ireland is required.
32
Current guidance is limited and is likely to suffer from different interpretations, in a similar
33
manner to the UK’s experience.
34
1
References
2
BS 4142, 1997. Rating industrial noise affecting mixed residential and industrial areas.
3
Coen, D., Grace, P., 2011. Wind energy feasibility study—small scale wind farms. In:
4
Cummins, E., Curran, T. (Eds.), Biosystems Engineering Research Review1649-475X16,
5
UCD.
6
Department of Environment, Health and Local Government (DoEHGL), 2006. Wind Energy
7
Development Guidelines.
8
Environmental Protection Agency (EPA), 2011. Guidance not on Noise Assessment of Wind
9
Turbine Operations at EPA Licensed Sites (NG3).
10
ETSU, 1996. The assessment and rating of noise from wind farms. Report from the Working
11
Group on Noise from Wind Turbines (UK), ETSU-R-97.
12
European Union Directive, 2009/28/EC on the proportion of the use of energy from renewable
13
sources and amending subsequently repealing Directives 2001/77/ EC and 2003/30/EC. No.
14
L 140/16.
15
Gamboaa, G., Munda, G., 2007. The problem of wind farm location: a social multicriteria
16
evaluation framework. Energy Policy 35 (3), 1564–1583.
17
Gualtieri G., Secci S., 2011 Comparing methods to calculate atmospheric stabilitydependent
18
wind speed profiles: a case study on coastal location. Renewable Energy 36, 2189–2204.
19
Hayes McKenzie, 2006. The measurement of low frequency noise at three UK Wind Farms,
20
Contract Number W/45/00656/00/00, on behalf of the Department of Trade and Industry (UK)
21
(2006).
22
Hayes McKenzie, 2011. Analysis of how noise impacts are considered in the determination of
23
wind farm planning applications. Research Contract 01.08.09.01/492A for the Department of
24
Energy and Climate Change (UK). Institute of Acoustics (IOA), 2009. Acoustic Bulletin, 34(2)
25
March/April, 2009.
26
Irish Wind Energy Association (IWEA), 2008. Best Practice Guidelines for the Irish Wind
27
Energy Industry.
28
ISO 1996-1, 2003. Acoustics—Description, measurement and assessment of environmental
29
noise, Part 1: Basic quantities and assessment procedures.
30
King E.A., Mahon J., Pilla F., Rice H.J., 2009. Measuring noise in high wind speeds: evaluating
31
the performance of wind shields. In: Proceedings of Internoise 2009, Ottawa, Canada.
32
King, E.A., O’Malley, V.P., 2011. Lessons learnt from post EIS evaluations of national road
33
schemes in Ireland. Environmental Impact Assessment Review 32 (1), 123–132.
1
Pedersen, E., Larsman, P., 2008. The impact of visual factors on noise annoyance among
2
people living in the vicinity of wind turbines. Journal of Environmental Psychology 28 (4), 379–
3
389.
4
St Pierre R.L., Maguire D.J., 2004. The impact of A-weighting sound pressure level
5
measurements during the evaluation of noise exposure. In: Proceedings of Noise-Con 2004,
6
Baltimore, MD, USA.
7
Sustainable Energy Authority of Ireland (SEAI), 2010. Renewable Energy in Ireland, 2010
8
Update.
9
Swofford, J., Slattery, M., 2010. Public attitudes of wind energy in Texas: local communities
10
in close proximity to wind farms and their effect on decisionmaking. Energy Policy 38 (5),
11
2508–2519.
12
Van den Berg F., Pedersen E., Bouma J., Bakker R., 2008. Visual and acoustic impact of wind
13
turbine farms on residents. WINDFARMperception Final Report (FP62005-Science-and-
14
Society-20, Project no. 044628). van den Berg, G.P., 2004. Effects of the wind profile at night
15
on wind turbine sound. Journal of Sound and Vibration 277, 955–970.
16
Van der Horst, D., 2007. NIMBY or not? Exploring the relevance of location and the politics of
17
voiced opinions in renewable energy siting controversies. Energy Policy 35 (5), 2705–2714.