crew working on a 32 m rotor blade on a 1.5 MW wind
© 2000 Christian Kjaer
Increases with the Swept Rotor Area
a farmer tells you how much land he is farming, he will usually
state an area in terms of hectares or acres. With a wind turbine
it is much the same story, though doing wind farming we farm
a vertical area instead of a horizontal one.
The area of the disc covered by the rotor,
(and wind speeds, of course), determines how much energy we can
harvest in a year.
The picture gives you an idea of the normal
rotor sizes of wind turbines: A typical turbine with a 600 kW
electrical generator will typically have a rotor diameter of
some 44 metres (144 ft.). If you double the rotor diameter, you
get an area which is four times larger (two squared).
This means that you also get four times as much power
output from the rotor.
Rotor diameters may vary somewhat from the
figures given above, because many manufacturers
optimise their machines to local wind conditions: A larger
generator, of course, requires more power (i.e. strong winds)
to turn at all. So if you install a wind turbine in a low wind
area you will actually maximise annual output by using a fairly
small generator for a given rotor size (or a larger
rotor size for a given generator) For a 600 kW machine rotor
diameters may vary from 39 to 48 m (128 to 157 ft.) The reason
why you may get more output from a relatively smaller generator
in a low wind area is that the turbine will be running more hours
during the year.
for Choosing Large Turbines
- There are economies of scale in wind turbines, i.e.
larger machines are usually able to deliver electricity at a
lower cost than smaller machines. The reason is that the cost
of foundations, road building, electrical grid connection, plus
a number of components in the turbine (the electronic control
system etc.), are somewhat independent of the size of the machine.
- Larger machines are particularly well suited for offshore
wind power. The cost of foundations does not rise in proportion
to the size of the machine, and maintenance costs are largely
independent of the size of the machine.
- In areas where it is difficult to find sites for more than
a single turbine, a large turbine with a tall tower
uses the existing wind resource more efficiently.
You may take a look at some megawatt-sized
wind turbines in the picture gallery.
for Choosing Smaller Turbines
- The local electrical grid may be too weak to handle
the electricity output from a large machine. This may be the
case in remote parts of the electrical grid with low population
density and little electricity consumption in the area.
- There is less fluctuation in the electricity output from
a wind park consisting of a number of smaller machines, since
wind fluctuations occur randomly, and therefore tend to cancel
out. Again, smaller machines may be an advantage in a weak electrical
- The cost of using large cranes, and building a road
strong enough to carry the turbine components may make smaller
machines more economic in some areas.
- Several smaller machines spread the risk in case of
temporary machine failure, e.g. due to lightning strikes.
- aesthetical landscape considerations may sometimes
dictate the use of smaller machines. Large machines, however,
will usually have a much lower rotational speed, which means
that one large machine really does not attract as much attention
as many small, fast moving rotors. (See the section on wind
turbines in the landscape).