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Back to the Basics: Ground Drag
Why roof-mounted and short-tower turbines won’t perform well.
By MICk SAGrILLO
40 January/February 2010 SOLAR TODAY solartoday.org
More than ever before, the buying public is inter- ested in generating its own electricity for either cost-saving or environmental reasons. Understandably, many people are investigating “breakthrough
technologies” that promise very low upfront costs or
reduced installation footprints. One common aspect of
these “innovative” systems seems especially attractive to
potential customers: They mount on roofs or on very short
towers. This would presumably make small wind turbines
more accessible to consumers living in urban areas where
photovoltaic panels would normally be the most practical
option for generating one’s own clean energy. In addition,
we humans are a ground-dwelling species and the prospect
of scaling a very tall tower in order to maintain a wind turbine leaves many people understandably weak-kneed. The
tower can also comprise 50 percent or more of a system’s
total cost. So a short tower has emotional appeal.
You can indeed install a wind turbine on your roof (if
your roof is strong enough and your insurance company
knows what you are doing) or on a short tower, but you
had better have a good understanding of the meager wind
resource at those locations before plunking down your
hard-earned savings. In this column, we’ll take a look at
why rooftops and short towers invariably offer poor wind-resource sites.
Imagine that you are sitting in a park down by a river.
You pick up a few small twigs to toss into the river. You
notice that a twig splashing in near the bank moves quite
slowly downstream. If you toss the twig a bit farther toward
the middle of the river, it moves a bit faster with the water.
And if you throw the twig all the way to the center of the
river, it moves quite quickly downstream.
It is a fundamental law of physics that any time two
materials move across one another, movement is slowed
by the friction between them. The greater the friction
between the two materials, the slower the movement of
the two materials relative to each other. When one of the
materials is a fluid, friction is high close to the boundary but
diminishes farther away. Let’s look at how this law applies
to the river and its bank.
The water in the river is a fluid, and the bank of the river
(and the riverbed) is a solid. The rate of flow depends on the
moving water’s location relative to the nonmoving riverbed.
Friction between the earth and the flowing water causes
the water near the bank to move quite slowly. Then, as you
move into deeper water away from the shallow bank, the
riverbed’s influence on the moving fluid decreases until you
reach the deep center channel, where the effect of friction
between the solid and the liquid is lowest. In the center of
the river, the farthest we can get from the bank, we have
smooth “laminar” flow, or we would if not for turbulence
generated by rocks on the bottom. A canoeist who wants
to move quickly downstream sticks to the center; coming
back upstream, he’ll paddle against the gentle flow near
Like water, the ever-moving atmosphere flows over a
rough friction-producing surface. Winds blowing across
the Earth demonstrate similar flow movement close to the
ground, just like water in contact with a riverbed. The big
difference for us as observers is that we can see the flow of
water, whereas we sense the flow of air only by its effect
on clouds, flags, leaves and other visible
windblown objects. As you move away
from the surface of the Earth — like moving away from a riverbed — friction is
reduced and wind speed increases. With
distance from the ground, laminar flow
of wind increases and friction with the
ground decreases. That friction is called
“ground drag.” It can be graphed as shown
Note that ground drag starts “breaking”
in the graph at about 20 meters above the
ground, or 66 feet. This is the point where
wind speeds begin increasing more quickly
as the effect of ground drag diminishes and
the laminar flow of air over air increases.
This graph demonstrates the laws of fluid dynamics, which dictate the flow behavior of wind. The graph
describes the kinetic energy in the wind that we capture
with wind turbines and convert into electricity. The greater
the wind speed, the greater the kinetic energy in the air mass
that is available to power a wind turbine.
Air flow and ground drag operate irrespective of the
technology used to capture the kinetic energy. Whether
your turbine is conventional or innovative, whether it is
has a horizontal axis or vertical axis or even some hybrid of
the two, if it’s located below the steep part of the curve, it
simply doesn’t have access to much energy.
This may seem pretty obvious, but some people still
don’t get it. These are the folks who make claims for “
technology breakthrough” devices that can mount on the roof
or the ground. Their customers also don’t get it.
Next time, we’ll take a look at turbulence created around
buildings and trees, the second problem with rooftop and
short-tower installations. ST
Mick sagrillo, a small-wind
consultant, owns sagrillo
Power & Light and is wind
energy specialist for focus
on energy, Wisconsin’s
him at msagrillo@
DAnisH WinD enerGy AssociAtion