Civil Engineering Reference
In-Depth Information
4
Wind Speed
and Energy Distributions
The wind turbine captures the wind's kinetic energy in a rotor consisting of
two or more blades mechanically coupled to an electrical generator. The
turbine is mounted on a tall tower to enhance the energy capture. Numerous
wind turbines are installed at one site to build a wind farm of the desired
power production capacity. Obviously, sites with steady high wind produce
more energy over the year.
Two distinctly different configurations are available for the turbine design,
the horizontal axis configuration (
Figure 4-1
)
and the vertical axis configura-
tion (
Figure 4-2
)
. The vertical axis machine has the shape of an egg beater,
and is often called the Darrieus rotor after its inventor. It has been used in
the past because of specific structural advantage. However, most modern
wind turbines use horizontal-axis design. Except for the rotor, all other com-
ponents are the same in both designs, with some difference in their placement.
4.1
Speed and Power Relations
The kinetic energy in air of mass “m” moving with speed V is given by the
following in SI units:
1
2
2
Kinetic Energy
=⋅ ⋅
m
V
joules.
(4-1)
The power in moving air is the flow rate of kinetic energy per second.
Therefore:
1
2
=⋅
(
)
⋅
2
Power
mass flow rate per
second
V
(4-2)
If we let P = mechanical power in the moving air
ρ
= air density, kg/m
3
A = area swept by the rotor blades, m
2
V = velocity of the air, m/s
