Environmental Engineering Reference
In-Depth Information
The modern wind turbine is a development of the last few decades that utilizes the latest
technology in design and manufacture. Currently used exclusively to produce electrical power,
wind turbines are usually tied into electrical transmission and distribution system, although some
turbines are used to power remote installations. Despite the many ingenious forms of wind turbines
that have been developed, the predominant type used today is the horizontal axis machine, mounted
on a support tower, that is free to rotate so as to align the axis with the wind direction. The turbine
rotor, invariably a three-bladed propeller-like structure, drives an electric power generator through
a speed increasing gear. Because the turbine-generator must be able to rotate with respect to its
supporting tower, electrical power is transmitted through slip rings to the electrical transmission
system. Because the turbomachinery is mounted at the top of a tower 50 or more meters in height,
with the wind turbine rotating and the whole power unit swiveling into the wind, a reliable and
strong supporting structure that will withstand the highest expected storm winds is required.
A wind turbine more closely resembles an airplane propeller than it does a steam or gas turbine
rotor. The wind turbine blades are long and slender; the tip of the blade moves at a speed much
greater than the wind speed. An airplane propeller is designed to produce a large thrust while the
wind turbine must produce power, which is the product of the torque that the wind applies to the
turbine rotor times its rotational speed. Nevertheless, the wind turbine blade shape is quite similar
to that of an airplane propeller.
The source of power from the wind is the flow of air through the wind turbine. If
V
is the wind
speed, each unit mass of air possesses a kinetic energy of amount
V
2
R
2
is the area
subtended by the rotating blades of length
R
, then the mass flow rate of air through an area
A
of
the undisturbed wind stream is
/
2. If
A
=
π
is the air density. The maximum rate at which the
wind kinetic energy could be supplied by the wind flow passing through the turbine is the product
of the mass flow rate,
ρ
VA
, where
ρ
VA
, and the kinetic energy per unit mass,
V
2
V
3
A
2.
In practice, the wind turbine power available from aerodynamically perfect design is less than this
value because the action of the turbine modifies the surrounding wind flow, reducing the mass flow
rate below the value of
ρ
/
2, for a value of
ρ
/
VA
.
To illustrate this effect, Figure 7.15 shows the ideal wind flow past a turbine. Because the
turbine extracts some of the kinetic energy of the wind flow, the wind speed is reduced in the
ρ
V
t
V
V
w
R
Figure 7.15
The flow of wind through a wind turbine whose blade radius is
R
is slowed at the turbine disk
and is slowed even further in the wake region down stream of the turbine.
