Environmental Engineering Reference
In-Depth Information
Betz recognized when he wrote in 1920 that maximum energy can be extracted from
the wind if it is slowed down to one-third of its original speed. This is the only way
in which theoretically 16/27 or 59.3% of the wind's potential energy is usable.
Today this theory is called Betz' Law in honour of the man who made the
discovery.
This power factor therefore identifi es how much wind energy a wind turbine uses.
Under ideal operating conditions, optimized modern wind turbines can use just over
50% of the energy contained in wind and convert it into electric energy. They
therefore reach power factors of at least 50%, close to the maximum possible limit.
The formula notation is c
p
. The power factor is a typical parameter for wind turbines
and is also found in the data sheets of many manufacturers of wind power
generators.
If a wind turbine stops the force of the wind, it ends up changing the course of the
wind 's fl ow. Behind the wind turbine the braked wind fl ows through a large area.
The course of the wind's fl ow widens (Figure 8.5 ).
Figure 8.5
A wind turbine
course of fl ow.
In wind use a distinction is made between
■
Drag principle
■
Lift principle
For example, the old Viking ships that were equipped with square rigs worked on
the drag principle. The sail put up resistance to the wind and the wind pushed the
sail, propelling the ship forward.
With the lift principle, which is also used by modern sailboats with fore-and-aft
sails, considerably more energy can drawn from the wind than with the drag prin-
ciple. Consequently, modern wind turbines function almost exclusively on the basis
of the lift principle. Among the different system concepts, the familiar rotors with
a horizontal axle have been the most successful.
