ECE and ME 4570 – Homework Assignment #10
Dr. John Wagner – Section 10: Wind Farm Feasibility Studies –
Onshore and Offshore Farm Siting with Environmental Aspects
Problem Solutions
10.1 Two wind turbines emitting 105 dB(A) at the source are located 200m and 240m away
from a location of interest, respectively. Calculate the sound pressure level in dB(A) at the
point of interest from the combined acoustic effect of these two turbines. Assume a sound
absorption coefficient of 0.005 dB(A)/m. Please select the correct answer listed below
(a, b, c, or d).
(a) 52.12dB, (b) 32.18 dB, (c) 48.92 dB, (d) 55.12 dB
Solution:
For the first turbine, the sound pressure level is calculated using
Lp =
LW − 10 log10 (2π R 2 ) − α R
where, L p is sound pressure level(dB ), R is distance from noise source
LW is power level (dB ),
α is frequency dependent sound absorption coefficient = 0.005
=
( Lp )1
105 – 10 log10 (2 × π × 2002 ) –=
0.005 × 200
50 dB ( A )
Similarly, for the second turbine one obtains
(Lp)2 = 105 – 10 log10 (2 × 𝜋𝜋 × 2402 ) – 0.005 × 240 = 48.2 dB(𝐴𝐴)
The sound pressure level from the two turbines is calculated using
=
L total 10 log10 (10
L1/10
L2/10
+ 10
=
) 52.12 dB ( A )
Correct solution is (a).
10.2 Please estimate the sound power level for a three-bladed, upwind wind turbine with the
following specifications: rated power of 500kW, rotor diameter of 40m, rotor rotational
speed of 40rpm, and wind speed of 12m/s.
Solution:
The sound power level for the turbine can be calculated using three different sound power
level prediction equations
10.3
LWA 10(log10 PWT ) + 50
=
LWA 22(log10 D) + 72
=
LWA = 50 ( log10 VTip ) + 10 ( log10 D ) − 4
From the 1st equation:
LWA= 10 (log10 (500,000) + 50= 56.99 + 50= 106.99dB
From the 2nd equation:
=
=
+ 72 107.25dB
LWA 22(log10 40)
Using the 3rd equation:
=
=
Vtip (2
π )(20)(40 / 60) 83.78 m / s
L=
50 ( log10 83.78 ) + 10 ( log10 40 ) −=
4 96.16 + 16 −=
4 108.18dB
WA
10.3 Suppose that the sound pressure level at a distance of 100m from a single wind turbine is
60dB. What are the sound pressure levels at a distance of 300m, 500m, and 1,000m?
Assume a sound absorption coefficient of 0.005.
Solution:
Sound pressure level can be calculated using the equation
Lp =
LW − 10 log10 (2π R 2 ) − α R
where L p is sound pressure level (dB ), R is distance from noise source
LW is power level (dB ),
α is frequency dependent sound absorption coefficient = 0.005
Sound power level becomes
LW =
L p + 10 log10 (2π R 2 ) + α R
60 + 10 log10 (2π 1002 ) + 0.005 ×100
=
= 108.48dB
At 300m, the sound pressure level is
Lp =
LW − 10 log10 (2π R 2 ) − α R
= 108.48 − 10 log10 (2π × 3002 ) − 0.005 × 300
= 49.5dB
At 500m, the sound pressure level is
10.4
Lp =
LW − 10 log10 (2π R 2 ) − α R
= 108.48 − 10 log10 (2π × 5002 ) − 0.005 × 500
= 44.02dB
At 1,000m, the sound pressure level is
Lp =
LW − 10 log10 (2π R 2 ) − α R
= 108.48 − 10 log10 (2π ×10002 ) − 0.005 ×1000
= 35.5dB
Distance
Sound Pressure Level
300m
49.5 dB
500m
44.0 dB
1,000m
35.5 dB
10.4 For the wind turbine description offered in problem 10.3, what are the sound pressure
levels at a distance of 300m, 500m, and 1000m for two, five, and ten turbines?
Solution:
The solution can be found using
N
Ltotal = 10log10 ∑10Li /10
i =1
For two turbines at a distance of 300m
N
Ltotal = 10 log10 ∑10 Li /10
i =1
= 10 log10 (1049.5/10 + 1049.5/10 )
= 52.5dB
For two turbines at a distance of 500m
N
Ltotal = 10 log10 ∑10 Li /10
i =1
= 10 log10 (1044/10 + 1044/10 )
= 47.0dB
For two turbines at a distance of 1,000m
N
Ltotal = 10 log10 ∑10 Li /10
i =1
= 10 log10 (1035.5/10 + 1035.5/10 )
= 38.5dB
10.5
Distance
Two (2) Turbines
Sound Pressure Level
Five (5) Turbines
Sound Pressure Level
Ten (10) Turbines
Sound Pressure Level
300m
400m
1,000m
52.5 dB
47.0 dB
38.5 dB
56.4 dB
51.0 dB
42.5 dB
59.5 dB
54.0 dB
45.5 dB
10.5 The 10 minute average wind speed at a specific offshore buoy is measured to be 8.5m/s at
an elevation of 10m. The buoy is located 10 km from land in the direction of the oncoming
wind. If the surface roughness length is assumed to be 0.0002 m (0.20 mm), then what is
the mean wind speed at a height of 80m? Please select the closest answer to your
calculated value as listed (a, b, c, d, or e).
(a) 5.25 m/s, (b) 10.13 m/s, (c) 15.38 m/s, (d) 29.12 m/s, (e) none of the above
Solution:
U ( z ) ln ( z / z0 )
=
U ( zr ) ln ( zr / z0 )
U (80) ln ( 80 / 0.0002 ) 12.8992
=
= = 1.192
8.5
ln (10 / 0.0002 ) 10.8198
U (80) 8.5(1.192)
=
= 10.132 (m / s )
So wind speeds at 80m will be 10.132 m/s
Correct answer is (b)
10.6 One of the environmental concerns with large wind turbines in rural areas is the impact on
the local wildlife population. As an engineer with a technical understanding of these
complex machines, you may be periodically engaged in discussions regarding the deaths of
wildlife due to the installation of wind turbines. Please write a concise essay of 500 words
(maximum) that discusses this topic and whether it merits attention?
Solution:
Wind turbines cause fatalities of birds and bats through collision, mostly with the turbine
blades. Wind turbine blades spin at around 179mph (80m/s) at the tips; it’s easy to see why they
can be potentially harmful to birds. The notion that wind turbines could be a potential threat to
bird populations was brought up when hundreds of bird carcasses where found at the California
Altmont Pass wind farm. The birds found were raptors which is an endangered species. However
this incident took place 20 years ago, when wind turbine had faster moving blades and used
10.6
lattice tower structure which attracted birds to nest. The farm was built in a major bird migration
corridor which caused the high death rates.
According to experts, wind turbine kill far less birds when compared to other human
contracted structures/ intervention of environment.
Contributing Factor
Feral and domestic cats
Power lines
Windows (residential and commercial)
Pesticide
Automobile
Lighted communication towers
Wind turbines
Number of Bird Deaths
Hundreds of millions annually
130 ~174 million
100 million~ 1 billion
70 million
60million ~ 80 million
40 million ~ 50 million
10,000 – 40,000
Global bird population: 200 to 400 billion
Wind turbines also kill far less birds when compared to other power producing technologies.
There are approximately 0.3-0.4 fatalities per kWh caused by wind turbines. It’s 5.2kwh for
fossil fuel related power generation technologies and 0.4 for nuclear energy.
Bats like birds also suffer fatalities due to wind turbines. However the number of deaths is
much lowers when compared to bat deaths by other forms of human intervention. According to
the American Wind Energy Association, it's not just pollution and habitat destruction. People
accidentally waking bats from their daytime sleep kills far more bats than wind turbines ever
could.
10.7 A wind turbine has a measured sound power level of 102 dB in an 8 m/s wind stream.
Please plot the sound pressure level as a function of distance (0 to 1500 meters) using both
hemispherical and spherical noise propagation models. Further, a series of sound
absorption coefficients including 0.0025, 0.005, and 0.010 ( dB(A)m-1 ) should be assessed.
You may wish to consider the following two expressions.
𝐿𝐿𝑝𝑝 = 𝐿𝐿𝑤𝑤 − 10 log10 (2𝜋𝜋𝑟𝑟 2 ) − 𝛼𝛼𝛼𝛼 : Hemispherical noise propagation model
𝐿𝐿𝑝𝑝 = 𝐿𝐿𝑤𝑤 − 10 log10 (4𝜋𝜋𝑟𝑟 2 ) − 𝛼𝛼𝛼𝛼 : Spherical noise propagation model
In the problem description, Lp represents the noise pressure level (dB) at a distance “r”
from the source radiating at a power level Lw and α is the sound absorption coefficient.
Solution:
10.7
100
Hemispherical Noise Propagation Model
90
α=0.005
α=0.0025
80
α=0.01
Sound Level (dB)
70
60
50
40
30
20
10
0
0
200
400
600
800
1000
1200
Distance (m)
Hemispherical noise propagation model graphical results
10.8
1400
100
Spherical Noise Propagation Model
90
α=0.0025
α=0.005
80
α=0.01
Sound Level (dB)
70
60
50
40
30
20
10
0
0
200
400
600
800
1000
1200
1400
Distance (m)
Spherical noise propagation model graphical results
It can be clearly observed that according to the spherical noise propagation model, the noise level
will be slightly lower than the hemispherical model. As expected, the higher the absorption
coefficient, the lower the sound level at the same distance from the source.
10.8 When wind turbines are installed within a farm setting with multiple installations, roadway
infrastructure for erection and maintenance vehicles, and electric grid connectivity, one of
the potential environmental concerns is the local wildlife. In this written response question,
please consider at least four (4) strategies that can be implemented to protect birds from
wind turbines. Please be sure that your ideas are well structured and cohesively formulated;
you can list them using a bullet format with accompanying text statements.
Solution: When wind turbines are installed within a farm setting with multiple installations,
roadway infrastructure for erection and maintenance vehicles, and electric grid connectivity, one
of the potential environmental concerns is the local wildlife. In this written response question,
please consider at least four (4) strategies that can be implemented to protect birds from wind
turbines. Please be sure that your ideas are well structured and cohesively formulated; you can
list them using a bullet format with accompanying text statements.
10.9
Wind turbines are often located in remote regions where the average winds are consistently and
sufficiently large to support electrical power production. For isolated locations, the visual and
acoustic impact on humans will be minimal but the surrounding wildlife may be adversely
affected through collision with the blades, towers, and electrical grid equipment. To reduce the
fatalities and injuries sustained by birds, several guiding principles and actions that may be
considered by the engineering team are listed below using a bullet format.
•
•
•
•
•
•
•
•
Wind farms should not be built in migration corridors that may be determined a priori
based on historical flight path data or high concentration areas for birds
Tower structures should be considered which do not offer perch sites (e.g., lattice
structures, guyed wires) for birds
To reduce the number of rotating blades for possible bird strikes and overall number of
structures in a wind turbine farm, install larger wind machines
Minimize the electrocution of birds by burying electrical transmission lines
Relocate prey away wind turbine farms through live trapping methods to reduce the
presence of large birds of prey
Require electrical utilities to keep detailed records of bird deaths (bird mortality) and bird
population sightings (bird risk) to monitor the impact of wind turbines
Shut down the wind turbines during high migration time periods, bad weather and/or
reduced visibility to avoid bird injuries; explore pausing energy production at night time
Provide alternative habitat areas that are a considerable distance from wind turbine farms
for wildlife; create “green spaces” for birds to offset locale destruction
The eight (8) strategies mentioned above will certainly prove helpful in protecting wildlife
within the vicinity of wind turbine installations. As a final thought, every bird species has a
distinct manner of obtaining nourishment based on its diet and body structure. Foraging, or
gathering food, by birds includes scratching, gleaning, hawking, probing, and scanning to name a
few. The specific foraging method typically requires the bird to initially be in flight, at ground
level or on trees, so the presence of a wind turbine will directly impact their feeding cycle. To
use a Latin phrase, primum non nocere or “first, do no harm” should be the motto for renewable
energy sources.
10.9 Researchers from Spain (refer to Hurtado et al., “Spanish Method of Visual Impact
Evaluation in Wind Farms”, Renewable and Sustainable Energy Reviews, vol. 8, pp. 483491, 2004) have developed an analytical approach that provides a numerical value for a
factor that represents a coefficient of the affected population. Please review this academic
paper and give an evaluation on the possible use of this methodology.
Solution:
The Spanish method of visual evaluation in wind farms, by Hurtado et al. (2004) is an
attempt to objectively rate the visual impact a wind farm would have over a population. As the
authors discuss, many factors must be considered when determining the location and size of a
wind farm. Specific rules exist that mainly take into consideration conservation of the
environment and the noise generated by the multi-domain machines. However, the visual impact
caused by a wind farm has no norms for regulation. One of the main reasons for this is the
10.10
subjectivity involved in evaluating visual impact. In response to these needs, Hurtado et al have
proposed a method that is described below.
The first step taken is data preparation. Geographical information system (GIS) based
software is used to obtain a solid three-dimensional view from a digital geographical map, which
was previously treated by means of a CAD software package. This data is used to prepare the
visual impact evaluation matrix (VIEM) which is composed of several coefficients. The first
coefficient, a, is the visual coefficient of wind farm from a village. As the visibility of the wind
farm will be different from different parts of the village, a median value is measured as
𝑋𝑋
∑𝑛𝑛𝑖𝑖=1 𝑖𝑖
𝑊𝑊𝑊𝑊
𝑎𝑎 =
𝑛𝑛
where n is the number of areas inside the village with different views, Xi is the number of
windmills visible from area i, and WM is the total number of windmills in the farm.
The second coefficient, b, is the visibility coefficient of the village from the wind farm which
is a ratio of the houses visible from the wind farm to the total number of houses in the village.
The third coefficient, c, is the visibility coefficient of the wind farm taken as a cuboid. This
factor basically takes the angle of view into consideration and is calculated as 𝑐𝑐 = (𝑣𝑣)(𝑛𝑛) where
the factor, v, has a value of 1 for a frontal, 0.50 for a diagonal and 0.20 for a longitudinal angle
of view, and n is the quantity factor incorporating the number of windmills. The last two factors,
d and e, are the distance and population factors respectively. They have values which range
between 0.10 and 1.10; please refer to tables provided in the paper for specific magnitudes.
After all the coefficients in the VIEM have been assigned, the matrix is evaluated. The
authors have given three methods of evaluation.
a) Partial assessment 1; 𝑃𝑃𝑃𝑃1 = (𝑎𝑎)(𝑏𝑏)(𝑐𝑐)(𝑑𝑑)
b) Partial assessment 2; 𝑃𝑃𝑃𝑃2 = (𝑎𝑎)(𝑏𝑏)(𝑐𝑐)(𝑑𝑑)(𝑒𝑒)
c) Total assessment; 𝐶𝐶 = ∑𝑚𝑚
𝑖𝑖=1
(𝑎𝑎)(𝑏𝑏)(𝑁𝑁𝐻𝐻𝑚𝑚 )
𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁
; where C is the total coefficient of affected
people, NHm is the population of the village, and NTHE is the total number of people in
the area.
The values of PA1 or PA2 ranges between 0.0 and 1.0 where a value nearing zero implies
minimal visual impact and a value nearing one implies deep visual impact on the nearby
population indicating that the installation would not be preferable.
Overall, the method presented by the authors is an attempt to realize a clear and objective
solution to a subjective, often emotional, problem. Even though the approach seems convincing,
further research should be completed before it may be applied to real world cases. One possible
means of gaining credibility would be to evaluate existing wind farms near human settlements
and calculate their score. Once a considerable number of wind farms are assessed, experts in the
field and the local population affected could be asked for their opinions on the visual impacts
caused by these farms through surveys and questionnaires. Next, the two sets of results could be
correlated. Since quantifying visual impact will always have a subjective touch to it, it may be
necessary to correlate this method with the opinion of experts and the general population affected
by the wind farm. Regardless, the proposed method is comprehensive and definitely a step in the
right direction of rigorous analysis.
10.11