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By MICk SAgrILLo
Back to the Basics:
Quantity + Quality = more electricity
More tower, more power!
mick sagrillo, sagrillo
power & light, is a small
wind consultant and educator, and serves as a wind
energy specialist for focus
on energy, wisconsin’s
renewable energy program.
Contact him at msagrillo@
My past two columns have analyzed the two “ene- mies” of a wind turbine: ground drag and tur- bulence. As those columns pointed out, ground
drag affects the quantity of the wind, while turbulence
affects the wind’s quality. In this column we’ll take a look
at just how important these variables are.
First, though, let’s briefly review the concepts of wind
quantity and quality.
©2009-2010 miCk sAgrillo
10 11 12 13
Wind turbines with
tall towers can take
advantage of higher wind
speeds, which results
in exponential increases
in power generation.
drag, Turbulence Are detrimental
Wind, like water, is a fluid and follows the rules of
fluid dynamics. A river flows fastest in the middle, where
it’s the greatest distance away from the bank and river-bed. Increased separation
reduces friction with the
rocks and earth of the riv-erbed, resulting in greater
velocity of the fluid.
This is precisely what
we want for our wind turbine: greater fluid velocity. Ground drag, the friction between the moving
wind and the fixed Earth,
The second component of the wind, quality, refers to
the turbulence caused by ground clutter. Trees and buildings cause the wind to tumble and swirl, which reduces
the energy available for conversion to electricity. In addition, turbulence causes increased wear and tear on the wind
turbine. Wind farm developers strive to site their projects
well away from farm buildings, fence rows and woodlots to
minimize the amount of turbulence from such obstacles.
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wind speed is Crucial
The phenomenal influence of these two crucial variables
becomes evident in one simple equation:
p is the power available at the turbine rotor, d is the density of the air, A is the swept area of the rotor, and V is the
wind speed. At a given location, we have no control over air
density, so for any given wind generator with a given swept
area, the only real variable is V, wind speed. Therefore, we
can rewrite the equation as p~V3.
This is stunning. It means that doubling the wind speed
does not result in a doubling of power available to the turbine (a 100 percent increase), but an 800 percent increase.
This is illustrated in the graph to the left, in which the orange
bars each represent a 10 percent increase in wind speed and
the blue bars represent power available in the wind.
The upshot of this is that very small increases in wind
will result in considerable increases in the power available
in the wind. For example, wind velocity increasing from
a paltry 8 miles per hour to only 10 miles per hour, a 25
percent increase in wind speed, results in a whopping 100
percent increase in power generation capacity. Again, this is
why wind farm turbines are mounted atop such tall towers:
to maximize the fuel available to the turbines.
lesson: Taller = Better
A wind turbine close to the treetops or to the roofline
suffers from both ground drag and turbulence. In light of
this, a shorter tower than the site requires is no bargain
because the turbine is not producing much electricity and
its life expectancy is decreased.
Nevertheless, people often rationalize a short tower
height based on the cost of the turbine. I have heard it said
that a small wind turbine in the size range of 1 or 2 kilowatts
does not justify the expense of a tall tower, which might cost
four or five times as much as the turbine. Worse yet, others
look to completely forgo a tower and mount the wind turbine
on the roof of a house. This reasoning completely ignores the
physics of fluid dynamics, which dictate the tall tower in the
first place, based on ground drag and turbulence.
To illustrate: If a given tower height has a 10 mph
wind speed, and shortening the tower results in a 20 percent decreased wind speed to 8 mph, we’ve saved money
on the tower, but we’ve also reduced the quantity of fuel
available to the wind turbine by 50 percent and increased
A taller tower invariably results in the wind turbine generating more electricity.
To quote Steve Wilke at Bergey Windpower, “More
tower, more power.”
Next time we’ll look at the rules of thumb for sizing