Topic 2: Wind Resource
Learning Outcomes
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By the end of this topic you should be able to:
Identify the types of factors that you will need to consider in selecting a suitable wind power
site
Identify global and local wind distribution patterns
Understand how to measure the wind speed at a given location
Analyse a wind rose as a way of visually representing a set of wind speed direction data
Determine the wind speed profile for a given terrain
Identify planning and environmental issues related to wind power systems.
Introduction to the Topic
If wind farm developers are to maximise the amount of wind power generated by a wind turbine,
they should situate the wind turbine in a place with high average wind speeds. In this topic you
will learn how wind speeds vary in different locations and at different heights. You will also learn
about the planning and environmental issues associated with the development of a wind farm by
analysing controversies over proposed wind farm developments in scenic areas.
Session 2.1 Identifying Wind Distribution Patterns
Wind Patterns
The power in the wind varies as the cube of the wind speed. A small difference in average wind
speed between two possible locations for wind turbines can make a large difference to the
economics of the project. It is therefore important to be aware of wind distribution patterns.
Wind patterns vary across the globe. The best winds are found at high latitudes; i.e. far from the
equator.
Winds blow across the Earth from high-pressure area to low-pressure area. However, winds don’t
travel in a straight line. The actual paths of winds—and of ocean currents, are partly a result of the
Coriolis effect. Geostrophic or global winds are related to the latitude.
The Earth contains major global wind zones: polar easterlies, westerlies, trade winds, and the
doldrums. Westerlies blow from the west at mid latitudes. Westerlies are strongest in the winter
when pressure over the pole is low. NE and SE Trade Winds From 0-30 degrees latitude.
Westerlies: From 30-60 degrees latitude (Westerlies).
Polar Easterlies: From 60-90 degrees latitude. See Figure 1.
Figure 1: Global winds.
In addition to global winds other local winds are superimposed.
Wind speeds are generally higher out at sea than on land. Wind turbines can be installed offshore,
but these installations are more expensive. Some countries are now installing their wind turbines
offshore, both to tap into the high-speed winds and to avoid difficulties associated with securing
land for wind developments. However, most wind turbines are still installed on land.
Higher wind speeds are generally found along exposed regions of the coastline, particularly those
facing the direction of prevailing winds.
The energy in the wind is also determined by local wind patterns. Note that turbulence must be
avoided.
Local wind patterns are also affected by:
• obstacles
• speed up effects from tunnels or hills.
For the Australian case you can read “Wind resource assessment in Australia: a planners guide” at
http://www.cmar.csiro.au/e-print/open/coppin_2003a.pdf
.
Figure 2 See in the figure below Australia wind map showing average wind speed at 80 m.
For all world wind maps of average wind speed, average power density and terrain roughness you
can consult the Global wind atlas at https://globalwindatlas.info/ (Figure 3).
Figure 3. Maps of Australia average wind speed at 50 m (top) and roughness length from the
global wind atlas.
Turbulence
Turbulence can be created by uneven terrain and obstacles such as buildings. Turbulent winds are
not suitable for wind generation.
Obstacles
Substantial obstacles can create turbulence and decrease the wind speed significantly.
Tunnel effect
A tunnel effect is produced when air is compressed between buildings or mountains. You can feel
this effect when you walk between tall buildings or in a narrow mountain pass. The tunnel effect
increases the wind speed.
Hill effect
Wind becomes compressed on the windy side of a hill and expands again as it soars down into the
low-pressure area on the lee side of the hill. This creates higher wind speeds on the top of the hill.
To avoid turbulence, rounded hilltops are preferable.
Park effect
When many wind turbines are assembled in an array, downstream wind speed is reduced by
upstream wind turbines. This limits the efficiency of a wind farm: the park effect. Turbines in wind
parks are usually spaced somewhere between 5 and 9 rotor diameters apart in the prevailing wind
direction, and between 3 and 5 diameters apart in the direction perpendicular to the prevailing
winds.
The effect of height
The speed of the wind will also be a function of the height from the ground. Wind turbines are
installed on towers. In measuring wind speed, it is therefore important to measure the speed at the
intended height of the wind turbine.
All these factors need to be considered in looking for locations where the wind energy will be
highest.
Figure 4 (a) Hill effect (b) Obstacles effects (c) Cost line topography (d) Park effect (e) Lay out
of wind turbines sites.
Session 2.2
Speed
Selecting a Suitable Wind Site and Measuring Wind
Characteristics of Sites
Locating a suitable wind turbine site usually involves selecting a site that offers the most suitable
combination of characteristics.
Undertaking a wind power project involves a considerable investment. Deciding whether a project
is worthwhile therefore involves estimating the likely return on investment. The return one is likely
to make on a particular investment will be determined by the income that the project will generate
and the operating and maintenance costs, as well as the size of the initial investment.
The income generated by a wind turbine farm will be determined by the wind power available at
the site. Typical operating and maintenance costs are about 2% of capital cost. This will be higher
for small wind installations or for remote and offshore locations.
The costs of the investment will be determined by many factors, including:
• difficulty of the terrain
• location
• size and height of wind turbines
• grid connection costs.
Wind speed monitoring
Since the power produced by a wind turbine depends on the wind speed, a developer planning to
install a wind turbine at a given location should know the typical wind conditions at that location.
The most important parameter is the average wind speed. It is also very useful to know the wind
speed distribution. This can be determined from wind speeds measured at regular intervals (for
instance, hourly).
In many countries the national weather service monitors wind speeds at several locations. In
Australia, the national weather service is the Bureau of Meteorology (http://www.bom.gov.au).
Several other agencies may also monitor wind speeds for
various environmental monitoring programs such as soil
erosion. National weather service measure wind speeds for
meteorological purposes. It is therefore unlikely that they
will be measuring wind speeds at sites suitable for wind
power installations.
For the purpose of proving the suitability of a site for a
proposed wind power project, it is necessary that wind
speeds be monitored for at least a year. It is preferable to
monitor wind speeds at the intended hub height.
Wind speeds are measured using an anemometer. Figure
2.1 shows an anemometer.
Figure 5 Anemometer
Beaufort Scale
Wind speeds can be estimated using the Beaufort Scale. The Beaufort Wind Scale was devised in
1805 by Sir Francis Beaufort of the British Navy. The scale can be used to estimate the wind speed
on land or at sea if no other wind velocity indicator is available. The Beaufort Scale is shown in
Table 2.1. The Beaufort number is also referred to as Force, for example the Beaufort number 6
equals Force 6.
Beaufort
number
Description
Wind speed at height of 10 metres
above open flat ground
knots (kn)
m/s
km/h
0
calm
<1
0–0.2
<1
1
light air
1–3
0.3–1.5
1–5
2
light breeze
4–6
1.6–3.3
6–11
3
gentle breeze
7–10
3.4–5.4
12–19
4
moderate
breeze
11–16
5.5–7.9
20–28
5
fresh breeze
17–21
8.0–10.7
29–38
6
strong breeze
22–27
10.8–13.8
39–49
7
near gale
28–33
13.9–17.1
50–61
8
gale
34–40
17.2–20.7
62–74
9
strong gale
41–47
20.8–24.4
75–88
10
storm
48–55
24.5–28.4
89–102
11
violent storm
56–63
28.5–32.6
103–117
12
hurricane
64 +
32.7 +
118 +
Observable
effects on land
Calm, smoke rises
vertically
Smoke drift; wind
vanes unaffected
Wind felt on face;
leaves rustle; wind
vanes move
Leaves and small
move constantly;
flags flap
Papers
blow;
small
branches
move
Small trees sway
Large branches in
motion; umbrellas
used
with
difficulty
Whole trees sway
Twigs break off;
walking difficult
Slight structural
damage
Trees uprooted;
much structural
damage
Widespread
damage
Severe
and
extensive damage;
disaster
Power produced
by wind turbine at
50 metres hub
height
None
None
None
About 0 to 20% of
rated capacity
About 20 to 60% of
rated capacity
About 60 to 100%
of rated capacity
Full capacity
Full capacity
Full capacity
Wind turbine shut
down
Wind turbine shut
down
Wind turbine may
be damaged
Serious
damage
unless lowered
Table 2.1 Beaufort Wind Scale
Session 2.3 Presenting Wind Direction Data using a Wind Rose
Wind Vane
It is often useful to know the wind direction. Such information will be useful when designing the
layout of a wind farm. The wind direction is monitored using a wind vane.
Figure 5 Wind Vane
The Wind Rose
A wind rose summarises the wind speed, direction and frequency for a given location.
Figure 6 shows an example of a wind rose. The circle in the centre of the rose indicates the
percentage of time that it is calm (i.e. wind speed of less than 1 knot). Each of the sixteen branches
of the wind rose represents the wind coming from the corresponding direction. For example, the
horizontal branch on the left represents winds coming from a westerly direction.
Each branch is made up of segments and each segment represents a wind speed range (e.g. 15-20
knots). Each wind speed range is represented by a different thickness and colour of the segment.
The length of each segment indicates the frequency of that wind speed for the given direction. The
circles on the wind rose represent 5% frequency intervals.
Table 2.2 contains wind directions and frequency data at a given site. It is based on measurements
taken over several years during the month of September. Wind speeds are given in knots (kn). One
knot is about 0.5 m/s. Wind speeds below 1 knot are considered ‘calm’. The ‘Total' column shows
the total frequency of wind speed from the given direction. The ‘Mean’ column shows the mean
wind speed from the given direction. Note that the sum of the ‘Total’ column plus the % calm add
to 100%. Figure 2.3 is the corresponding wind rose.
Direction 1 to <5
5 to <10
N
0
0.58
NNE
0
0.58
NE
0.58
4.07
ENE
1.16
4.07
E
1.16
6.4
ESE
0.58
3.49
SE
0.58
3.49
SSE
0
1.74
S
0
3.49
SSW
0
1.16
SW
0
0
WSW
0
1.74
W
0
1.16
WNW
0.58
1.16
NW
0
1.16
NNW
0
0.58
Total
4.65
34.88
Calm (<1.00 kn) = 0.58%
Site minimum (kn) = 0.00
Site maximum (kn) = 33.00
Site mean (kn) = 10.70
10 to <15
0.58
3.49
6.4
1.16
2.91
3.49
1.74
0.58
8.14
1.16
0
1.74
7.56
2.33
1.74
0.58
43.6
15 to <20
0
0
1.74
0
0.58
0.58
0
0
2.91
1.16
0.58
0
4.07
0.58
1.16
0.58
13.95
20 to <25
0
0
0
0
0
0
0
0
0
0
0
0
1.74
0
0
0
1.74
>=25
0
0
0
0
0
0
0
0
0
0
0.58
0
0
0
0
0
0.58
Table 2.2 Wind Direction and Frequency Data
(This data obtained from the Bureau of Meteorology website)
Ttotal
1.16
4.07
12.79
6.4
11.05
8.14
5.81
2.33
14.53
3.49
1.16
3.49
14.53
4.65
4.07
1.74
99.42
Figure 6 Wind rose
Direction 1 to <5
5 to <10
N
0.49
0.65
NNE
0
0.98
NE
1.31
6.53
ENE
3.1
8.65
E
5.22
10.6
ESE
2.28
3.59
SE
1.63
3.92
SSE
2.12
4.4
S
1.63
5.55
SSW
0.33
1.63
SW
0.49
0.98
WSW
0.16
2.61
W
0
1.63
WNW
0.49
1.63
NW
0.33
0.98
NNW
0
0.49
Total
19.58
54.81
Calm = 3.10%
Site minimum (kn) = 0.0
Site maximum (kn) = 29.90
Site mean (kn) = 7.99
10 to <15
0.16
0.49
2.61
1.63
1.47
0
0.16
0.16
1.14
1,14
0.33
0.49
2.28
0.65
0.33
0.16
13.21
15 to <20
0
0
0.49
0.33
0
0
0.16
0
0
0.33
0
1.31
2.12
0.49
0.33
0
5.55
20 to <25
0
0
0
0
0
0
0
0.16
0.16
0
0
0.82
1.79
0.33
0
0
3.26
>=25
0
0
0
0
0
0
0
0
0
0
0
0
0.49
0
0
0
0.49
Table 2.3 Wind Direction and Frequency Data
(This data obtained from the Bureau of Meteorology website)
Total
1.31
1.47
10.93
13.7
17.29
5.87
5.87
6.85
8.48
3.43
1.79
5.38
8.32
3.59
1.96
0.65
96.9
Mean
5.97
8.89
8.7
7.33
6.06
5.21
6.39
6.15
7.26
9.8
7.81
12.69
15.51
10.46
9.32
7.53
Session 2.4 Estimating the Wind Speed for a Particular Height
The Relationship between Wind Speed and Height
Wind speed increases with height due to friction between the air and the surface of the earth. At
the surface of the earth the wind speed will be zero. At a very great height there will no longer be
any friction effect, so the wind speed no longer increases with height.
The zone over which the wind speed increases with height is called the atmospheric boundary layer.
The thickness of the atmospheric boundary layer varies from several hundred metres to more than
a kilometre. Wind turbines operate within this boundary layer.
The wind speed gradient function relates wind speed to height. If we know the wind speed at a
reference height, we can estimate the wind speed at another height using this function:

 z 

v = v ref  
 z ref 
where v is the wind speed at height z
vref is the wind speed at height zref
 is the wind shear exponent.
The wind speed gradient function is approximate only. Wind farm developers recommend that
monitoring of wind speeds be done at the hub height of the planned wind turbine.
The value of the wind shear exponent varies with the terrain. Table 2.4 gives typical values for the
wind shear exponent for different types of terrain.
Typical wind shear exponent, 
Terrain
open sea
0.1
low grass
0.13
low scrub
0.19
forest, urban areas 0.32
Table 2.4 Wind Shear Exponent for Various
Terrains
Figure 6 Vertical profile for wind speed 25 m/s at 500 m.
Figure 6 shows how the wind speed varies with terrain. A wind speed of 25 m/s at 500 m is assumed
as reference. For flat terrain, where the wind shear component is low, the wind speed at a height of,
say, 50 m is much higher than for rough terrains.
Another method of estimating the wind speed gradient is the log law
ln (z ) − ln (z 0 )
v = v ref 
ln (z ref ) − ln (z 0 )
where z0 is called the roughness length. It is a measure of the roughness of the terrain. The roughness
length can vary from 0.0002 m for smooth water to over a metre for cities with tall buildings. A
typical value for open agricultural land is 0.03 m.
Typical values of roughness length for a variety of terrains can be found in the Table below.
Terrain Description
z0 (m), Winter /
Summer
Open Water
α,
Wind
Shear
Exponent
0.10
Pasture/Hay
0.19
0.01 / 0.15
Deciduous Forest
0.43
0.5 / 1.3
Mixed Forest
0.43
0.9 / 1.3
Evergreen Forest
Lake, ocean and smooth
hard ground
Foot high grass on level
ground
Tall crops, hedges, and
shrubs
Wooded country with
many trees
Small town with some
trees and shrubs
City
area
with
tall
buildings
0.43
1.3 / 1.3
0.1
0.15
0.2
0.25
0.3
0.4
0.001 / 0.001
Session 2.5 Wind Power: Planning and Environmental Issues
Wind power development projects may encounter opposition from residents. Such opposition has
stopped many proposed developments. This has become a problem for developers of wind power
systems. Developers may avoid sites with excellent wind resources, if there is likely to be
community opposition, moving instead to less windy sites where the local community is more
supportive.
The main issues that objectors to wind power developments focus on are:
▪visual impact
▪Environmental
▪noise
▪potential to kill birds.
The visual impact of wind turbines is often the most serious issue. Wind turbines are very large
structures: the total height of a large wind turbine (to the tip of the blade) can be up to around 100
metres—equivalent to the height of a 25-story building. The locations with the highest winds are
often the most scenic, and protestors often argue that the installation of wind turbines will diminish
an area’s natural beauty. Wind farms can comprise many tens, or even hundreds, of wind turbines;
thus, they often occupy quite a large area.
Environmental
Land use: wind farm may cover a large area of land, many land uses such as agriculture are
compatible with it Impact on wildlife: birds and bats
Weather change: Wind farms may affect weather in their immediate vicinity. Spinning wind turbine
rotors generate a lot of turbulence in their wakes. This turbulence increases vertical mixing of heat
and water vapor that affects the meteorological conditions downwind.
Pollution: permanent magnets used in some wind turbines makes use of neodymium. Primarily
exported by China, pollution concerns associated with the extraction of this rare-earth element.
Modern wind turbines do not use permanent magnet anymore.
Noise
Typical noise level is 50 dB(A) at 100 m. Human hearing: 20 – 20,000 Hz
Sound levels always decline with distance, inverse square of the distance. Wind farms are therefore
several km from houses.
Most of the wind turbine's noise is created by airflow through the blades.
Most of the noise comes from the outer parts of the blades while the blades moved downward.
Potential to kill birds
Issue of bird deaths due to collisions with wind turbines are also quite serious.
Summaries of ways to minimize the impact of wind farms on birds.
Specific recommendations are:
• Use existing data on migratory and other movements of wildlife to develop predictive
models of risk.
•
Use new and emerging technologies, including radar, acoustics, and thermal imaging, to fill
gaps in knowledge of wildlife movements.
•
Identify specific species or sets of species most at risk in areas of high potential wind
resources.
•
Avoid bird migration corridors and areas of high bird concentrations (micro habitats or fly
zones)
•
Use fewer, larger turbines
•
Minimize number of perching sites on turbine towers
•
Bury electrical lines