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Back to the Basics: collector size
Power is proportional to rotor swept area.
By MICK SAGrIllo
mick sagrillo (msagrillo
@ wizunwired.net) of
sagrillo power & light is a
small wind consultant
Several fundamental concepts about small wind tur- bines are often misunderstood. The first misconcep- tion is about the power available in the wind and what
diminishes that power: ground drag and turbulence. The
second misunderstanding concerns the importance of tower
height in maximizing the fuel (that is, the wind) available to
the wind turbine. These are wind resource and siting issues,
and we’ve covered these concepts in the last four columns.
However, equally misunderstood is the importance of
the size of a wind turbine’s rotor — that is, the blades and
hub that extract the energy in the wind and convert it to
electricity for our use.
rotors capture energy
I often get inquiries from people who have come across
ads for an inexpensive wind device with a very small rotor
that promises to generate incredibly large amounts of electricity relative to its cost. They are intrigued by claims of a
breakthrough technology offering the promise of “
never-before-seen efficiencies.” Consumers are unfamiliar with
the nuances of small wind technology, and that unfamiliarity is compounded by a misunderstanding of wind resource
and siting. They’re understandably confused.
The rotor of a wind turbine is made up of the blades that
spin and capture energy in the wind that passes through them.
Some rotors are traditional horizontal-axis devices that typically sport two or three blades. Others are vertical-axis systems of various blade configurations. Still others are hybrids of
these two orientations. Regardless, it is the rotor that extracts
the kinetic energy in the wind and converts it to rotational
momentum used to drive an electric generating device.
Source: Wind Power for the Homeowner, by
longer blades increase a
wind turbine’s swept area,
which can lead to a dramatic rise in energy output.
small rotor = small Output
It is well understood with other renewable technologies
that the size of your collector determines the amount of
renewable energy that you can collect and convert to some
useful purpose. Let’s use solar water-heating collectors as an
example. One 4-foot by 8-foot solar water collector has an
area of 32 square feet ( 3 square meters). It can collect only
the amount of sunlight that falls on it, no more. The collector is limited in the amount of hot water it can process,
based on the amount of sunlight it collects.
If we double the area exposed to the sunlight by adding
a second solar collector, we double the amount of sunlight
that can be collected, resulting in a doubling of the amount
of hot water that can be pumped. This is pretty straightforward: The amount of solar energy that can be extracted
from the sunlight is proportional to the size of the solar
collector used. Simple stuff!
The same holds true for a wind turbine. A small rotor
can only extract small amounts of kinetic energy out of the
wind and generate small amounts of electricity. The amount
of energy that can be extracted at a given wind speed is
proportional to the size of the rotor, period. No magic can
happen beyond the simple mathematics of the swept area
of a wind turbine’s rotor.
Copyright © 2010 by the American Solar Energy Society Inc. All rights reserved.
increase swept area for more energy
Swept area is defined as the circle delineated by the
rotating blades of the rotor. The only way to extract more
energy at a given wind speed is to increase the area that the
rotor sweeps. Increasing rotor area is quite easily accomplished: Simply increase the length of the blades.
The results of increasing blade length are quite dramatic
due to the fact that the area of a circle is proportional to the
square of the radius of the circle. In the case of a wind turbine,
the radius is the length of one blade. As shown in the diagram
at left, doubling the length of the blades results in a four-fold
increase in the volume of wind the rotor can capture and convert to rotational momentum used to drive the generator.
The output of a wind turbine depends primarily on the
amount of fuel available (the wind resource) and on the
size of the collector utilized to harvest that fuel (swept area
of the rotor). Unfortunately, one confounding factor often
thrown into the mix is the wind turbine’s maximum generating capacity or peak electrical output. While the size of
the generator is important, it is often very misunderstood
from the perspective of determining how much electricity
can be generated by the wind system. For any given wind
speed, generator size is of no consequence (provided it’s
large enough to control rotor output) because it is the rotor
diameter that determines the amount of energy that can be
extracted. In other words, a huge generator bolted to a small
rotor can only generate small amounts of electricity.
The following table, adapted from author Paul Gipe,
makes a good rule of thumb for estimating the generator
capacity of a typical horizontal axis wind system:
Nominal Rotor Nominal Power
Diameter in Feet Rating
4 100 watts
8 800 watts
12 2 kilowatts (k W)
24 10 k W
32 20 k W
50 40 k W
70 100 k W
Don’t be deluded into thinking you can generate huge
amounts of electricity with a small rotor — it simply is not
going to happen. To quote Paul Gipe, “Nothing says more
about the output of a wind turbine than its rotor.” ST