5 CHAPTER FIVE
WIND ENERGY
5.1 Introduction
Wind is air in motion and the basic driving force of air movement is a difference in air pressure
between two regions as a result of the uneven heating of the earth by solar radiation. On a global
scale, these thermal effects combine with dynamic effects from the earth's rotation to produce
prevailing wind patterns. In addition to this general behaviour of the atmosphere, there is
important local variation caused by geographical and environmental factors. To harness the
wind, one needs to be familiar with its patterns and be able to select a suitable site. The ultimate
use of wind power is very dependent on the success and acceptance of machines and suitable end
use patterns.
Application of wind power
i. Electricity generation
ii. Sea transport
iii. Grain milling
iv. Water pumping
v. Heat production
Autonomous wind power systems have great potential as substitutes for oil used in heating and
water pumping or for the generation of electricity from diesel engines.
5.2 Wind shear
As the wind passes over the surface of the earth wind close to the surface is retarded due to the
roughness caused by buildings, trees and crops. The extent to which wind speed is decreased
near the surface of the earth and the resulting variation with height is called 'wind shear'. The
extent of wind shear depends on surface terrain, temperature and humidity (cold damp air has a
larger shear than warm dry air). The most commonly used equation that shows the variation of
wind speed with height is:
n
h
u 2 u1 2
h1
Where
u 2 = wind speed at new height h2
u1 = known wind speed at height h1
n = coefficient determined empirically and varies with height, time of day, season of
year, nature of terrain, wind speed, temperature, pressure and moisture.
(If site specific data are not available then the following equation for n can be used:
n a bLnu1 , where a = 0.11 during the day and 0.38 at night; b = 0.0611 during the day and
0.209 at night). The heights h1 and h2 are heights above level of zero wind: In a dense maize or
wheat field, it corresponds to the average height of the crop; in a forest, the level of zero wind
corresponds to the height of where branches of adjacent trees touch.
51
, 5.3 Siting of Wind Machines
Since wind speed generally increases with height, a wind machine should be mounted as high as
possible to develop maximum power. However, topography plays an important role in screening,
accelerating and deflecting the wind and so site-specific conditions must be considered:
Ridges of hills parallel to the seashore with progressive and moderate slops (10° to 22°) are very
favourable to wind power generation especially when they are perpendicular to the prevailing
wind direction and when they are bare. The shoulders (ends) of ridges and the upper half of the
windward face are often good sites. (The leeward side must be avoided because of possible
turbulence).
Circular hills of moderate slopes can be better sites for wind machines than ridges if the
prevailing wind direction changes very much with season. Wind is very much accelerated on the
sides of circular hills, tangent to the wind direction and such places may be considered as good
power sites if the prevailing wind direction is nearly constant throughout the year.
Cliffs with moderate slopes are also favourable especially when they are perpendicular to the
wind direction. The best sites are situated between 0.25 and 2.5 times the cliff height downwind
from the cliff but a conservative strategy is to site as close to the cliff as possible in order to be
sure that the entire rotor rotates well above the zone of turbulence.
Special care must be taken when choosing a site on the ridge, hill or cliff with slope higher than
30° because on such obstacles high turbulence may occur. If the hill is very steep, the wind
breaks away from the upper surface and considerable turbulence occurs in the wake. Turbulent
flows may cause dangerous stress in the blades of a wind turbine so that a hill of this kind does
not constitute an ideal wind power site. Whatever site is proposed, it is advisable to make
measurements at positions susceptible to be selected in order to avoid disappointment. Wind
speed data needs to be collected for the site for at least one year and the anemometer should
anemometer should be placed at the hub height of the planned machine.
5.4 Wind Speed Distribution
From anemometer records and observations, speed duration curves and speed frequency curves
may be projected. The drawing of these curves is necessary for any important design.
5.4.1 Annual speed duration curve
On the horizontal axis, the annual time during which the wind velocity exceeds a fixed value is
recorded. On the vertical axis that value itself is recorded. This is done for each year. It is then
possible to obtain an inter-annual speed duration curve which is useful for energy determination.
52
WIND ENERGY
5.1 Introduction
Wind is air in motion and the basic driving force of air movement is a difference in air pressure
between two regions as a result of the uneven heating of the earth by solar radiation. On a global
scale, these thermal effects combine with dynamic effects from the earth's rotation to produce
prevailing wind patterns. In addition to this general behaviour of the atmosphere, there is
important local variation caused by geographical and environmental factors. To harness the
wind, one needs to be familiar with its patterns and be able to select a suitable site. The ultimate
use of wind power is very dependent on the success and acceptance of machines and suitable end
use patterns.
Application of wind power
i. Electricity generation
ii. Sea transport
iii. Grain milling
iv. Water pumping
v. Heat production
Autonomous wind power systems have great potential as substitutes for oil used in heating and
water pumping or for the generation of electricity from diesel engines.
5.2 Wind shear
As the wind passes over the surface of the earth wind close to the surface is retarded due to the
roughness caused by buildings, trees and crops. The extent to which wind speed is decreased
near the surface of the earth and the resulting variation with height is called 'wind shear'. The
extent of wind shear depends on surface terrain, temperature and humidity (cold damp air has a
larger shear than warm dry air). The most commonly used equation that shows the variation of
wind speed with height is:
n
h
u 2 u1 2
h1
Where
u 2 = wind speed at new height h2
u1 = known wind speed at height h1
n = coefficient determined empirically and varies with height, time of day, season of
year, nature of terrain, wind speed, temperature, pressure and moisture.
(If site specific data are not available then the following equation for n can be used:
n a bLnu1 , where a = 0.11 during the day and 0.38 at night; b = 0.0611 during the day and
0.209 at night). The heights h1 and h2 are heights above level of zero wind: In a dense maize or
wheat field, it corresponds to the average height of the crop; in a forest, the level of zero wind
corresponds to the height of where branches of adjacent trees touch.
51
, 5.3 Siting of Wind Machines
Since wind speed generally increases with height, a wind machine should be mounted as high as
possible to develop maximum power. However, topography plays an important role in screening,
accelerating and deflecting the wind and so site-specific conditions must be considered:
Ridges of hills parallel to the seashore with progressive and moderate slops (10° to 22°) are very
favourable to wind power generation especially when they are perpendicular to the prevailing
wind direction and when they are bare. The shoulders (ends) of ridges and the upper half of the
windward face are often good sites. (The leeward side must be avoided because of possible
turbulence).
Circular hills of moderate slopes can be better sites for wind machines than ridges if the
prevailing wind direction changes very much with season. Wind is very much accelerated on the
sides of circular hills, tangent to the wind direction and such places may be considered as good
power sites if the prevailing wind direction is nearly constant throughout the year.
Cliffs with moderate slopes are also favourable especially when they are perpendicular to the
wind direction. The best sites are situated between 0.25 and 2.5 times the cliff height downwind
from the cliff but a conservative strategy is to site as close to the cliff as possible in order to be
sure that the entire rotor rotates well above the zone of turbulence.
Special care must be taken when choosing a site on the ridge, hill or cliff with slope higher than
30° because on such obstacles high turbulence may occur. If the hill is very steep, the wind
breaks away from the upper surface and considerable turbulence occurs in the wake. Turbulent
flows may cause dangerous stress in the blades of a wind turbine so that a hill of this kind does
not constitute an ideal wind power site. Whatever site is proposed, it is advisable to make
measurements at positions susceptible to be selected in order to avoid disappointment. Wind
speed data needs to be collected for the site for at least one year and the anemometer should
anemometer should be placed at the hub height of the planned machine.
5.4 Wind Speed Distribution
From anemometer records and observations, speed duration curves and speed frequency curves
may be projected. The drawing of these curves is necessary for any important design.
5.4.1 Annual speed duration curve
On the horizontal axis, the annual time during which the wind velocity exceeds a fixed value is
recorded. On the vertical axis that value itself is recorded. This is done for each year. It is then
possible to obtain an inter-annual speed duration curve which is useful for energy determination.
52
