Fundamentals of wind energy - Wind Power Generation and Wind Turbine Design - page 15

N

NNE

NNW

NE

NW

WNW

ENE

Calm
8.2%

W E

0

5%
10%
15% 20%

WSW ESE

SW SE

SSE

SSW

S

Figure 5: Wind rose diagram for wind directions.

of calm or near calm air is given as a number in the central circle. Some wind rose
diagrams may also contain the information of wind speeds.

4.2.6 Wind shear
Wind shear is a meteorological phenomenon in which wind increases with the
height above the ground. The effect of height on the wind speed is mainly due to
roughness on the earth's surface and can be estimated using the Hellmann power
equation that relates wind speeds at two different heights [33]:

a
⎛⎞

z

uz () = uz ( ) (15)

⎜ ⎝⎠

0

z

0

where z is the height above the earth's surface, z 0 is the reference height for
which wind speed u ( z 0 ) is known, and a is the wind shear coeffi cient. In practice,
a depends on a number of factors, including the roughness of the surrounding
landscape, height, time of day, season, and locations. The wind shear coeffi cient
is generally lower in daytime and higher at night. Empirical results indicate that
wind shear often follows the “1/7 power law” (i.e. a = 1/7). The values of wind
shear coeffi cient for different surface roughness are provided in [34].
Because the power output of wind turbine strongly depends on the wind speed
at the hub height, modern wind turbines are built at the height greater than 80 m,
for capturing more wind energy and lowering cost per unit power output.

5 Modern wind turbines

A modern wind turbine is an energy-converting machine to convert the kinetic
energy of wind into mechanical energy and in turn into electrical energy. In the