SMaLL WIND systems
Zoning for Small Wind Turbines
Why does a “small” wind turbine need to be so tall?
By Mick Sagrillo
One of the biggest hurdles that anyone
interested in installing a small wind
turbine faces is securing a building permit. Zoning committees are notoriously
uninformed about why wind turbine towers need to be as high as they are, but are
very tuned in to neighbor concerns about
the visibility of such structures. All too
often, a wind turbine tower is restricted
to a height limit either because a height
limit is “on the books” or to appease the
neighbors at the expense of the prospective small wind owner. In the next few
issues of SOLAR TODAY, we will explore
why small wind systems are mounted on
tall towers, as well as offer suggestions for
zoning authorities in dealing with such
installation requests.
Small wind is defined as up to and
including 100 kilowatts in capacity. This
upper limit is a fairly large “small” wind
turbine, capable of powering a farm, business, small school district or environmental center. But “small wind” also includes
residential wind equipment and cabin-sized
off-grid battery charging equipment, considerably under the 100-k W cap.
The kilowatt rating of a small wind
turbine doesn’t really tell you how much
energy the machine will produce over any
period of time. It only tells you the maximum sustained capacity of the generator.
Far more important than the generator size
is the swept area of the rotor. The swept
area is the circle through which the rotor
blades spin as they collect the energy in the
wind. Output is a function of the size of
the collector, not the size of the generator.
Just as with solar water-heating or photovoltaic systems, the larger the collector, the
more energy you can extract from your fuel
source. In our case the fuel is the wind. The
larger the swept area of the rotor, the more
electricity we generate. To quote author
Paul Gipe, “Nothing says more about the
output of a wind turbine than the size of
its rotor.”
The smallest of small wind turbines have
blades that may be only 4 feet (1.2 meters)
long (we exclude micro-turbines for portable use). However, at the upper end of
Most zoning officials
don’t know that the
common 35-foot height
limit is a relic of old
fire-safety codes.
istOcKPHOtO.cOm
the power produced depends on the mass of air moving through the rotor disc. Large rotors
make more power.
the small-wind range, 100 k W, blades can
measure 35 feet ( 10. 5 meters) long, for a
70-foot (21-meter) diameter.
The reason that tall towers are required
for wind turbines is to allow them to reach
their fuel source, the wind. Even though
you may feel some wind at ground level,
the reality is that the resource at ground
level is greatly diminished and compromised. Wind is a fluid, and as it passes
over a solid earth, friction occurs between
the two. Meteorologists call this “ground
drag.” The result of ground drag is to slow
the moving air masses near the ground
level, with increasing wind velocity as you
move away from the surface of the earth.
Since we want higher wind speeds to “fuel”
our wind turbine, it only makes sense to go
up high where the wind is strong, not near
the ground where it is greatly reduced.
Simply put, the higher you go above the
surface of the earth, the stronger, smoother
and more reliable the wind. The higher the
quantity of fuel, the more electricity you
can generate.
Most zoning officials don’t know that
the common 35-foot height limit is a relic
of old fire-safety codes. In the 19th century,
large corporations hired millions of recent
immigrants to work in factories. They often
crowded workers into squalid and dangerous buildings, with results like the 146
deaths in New York’s Triangle Shirtwaist
fire of 1911. Similar disasters prompted
passage of new building, plumbing and
fire codes. In an era before firefighters had
portable high-pressure pumps, they could
only pump water about 35 feet vertically.
Codes were written to ensure that a fire
department could douse a roof. Today’s
fire-fighting technology easily overcomes
