When it comes to evaluating whether wind-generated electricity
makes sense for you and to selecting the best equipment for
your particular application and site, a host of choices and
variables are worth considering.
Will the savings be worth the up-front investment? (Generally
speaking, there is at least a break-even point compared to
conventional electricity over the estimated 20-year lifetime
of smaller-scale wind machinery. As you go bigger, the savings
over conventional power can be substantial, and don’t
forget the huge savings to the environment.)
Should I go with new or remanufactured equipment? (If a quality
rebuild from a reputable company is available to suit your
needs, you will likely save a bundle).
Should I choose a system with two rotor blades or three?
Ah, that’s a very good question.
The spin on rotor blades
Wind power guru and Organic University instructor Mick Sagrillo
minces no words when it comes to the question of rotor blades.
“Stay away from two-bladed systems,” said the founder
of Sagrillo Power and Light, a consulting firm specializing
in home-sized wind-turbine technology and educational workshops.
“They will literally tear your machine up.”
By Sagrillo’s own admission, the advice is somewhat
counterintuitive. Blades can account for up to 40 percent
of the cost of an entire wind system, so two blades over three
can offer significant savings. And two blades spin faster
under the same wind than three, meaning increased rotor speed
and more electricity produced as coils of wire inside a generator
create an electronic pulse each time they spin through a magnetic
field. But like a great vessel on high seas, a two-bladed
system is prone to the effects of yaw—in this case the
generator pivoting on its bearings as it tracks the wind—
“causing enormous strain on the generator, the blades,
the welds and the fasteners,” explained Sagrillo.
While three blades results in some level of inertia, “every
time you add a blade it causes a bit of turbulence for the
one behind it,” said Sagrillo, the stops and starts,
or blade chatter, that a two-blades system experiences as
it tries to maintain its plane of rotation make it too prone
to damage from stress.
Preventative maintenance
There’s no such thing as “maintenance-free; don’t
let anyone tell you different,” Sagrillo told the class
gathered for the prequel to the Upper Midwest Organic Farming
Conference in La Crosse, Wisconsin. “For most wind turbines
today, what maintenance involves is inspections.”
In his 17 years installing and inspecting wind machinery,
Sagrillo said, the most common breakdowns are a result of
flagrant neglect, such as failing to tighten a loose bolt.
“Nobody is on the tower for four, five, or six years
and guess what?—the turbine explodes. A bolt loosens,
a nut falls off, another bolt loosens, and pretty soon the
blade is in the tower. You’ve got two or three thousand
dollars damage, and it all could have been prevented by tightening
a bolt. The life of the system is directly related to the
involvement of the owner. You should be checking welds, fasteners,
nuts and bolts, looking for cracks…and if you find something
wrong, take care of it.”
Sagrillo also warned against going in for “maintenance
free” gimmicks such as sealed bearings. “Sealed
bearings are a sales pitch, he said. “From a maintenance
perspective, there’s nothing advantageous to them.”
Evaluating your wind resources
Winter is the optimal time for wind power, Sagrillo said,
because more molecules are present in the air. This, he said,
is why you can expect to harness 13 percent more electricity
from the wind in wintertime under the same wind speed and
at the same location than at any other time of year. It’s
also why wind power and solar power make such a great combination.
“The sun picks up when there’s no wind around;
they are remarkably complimentary, wind power and photovoltiacs.”
(As well as temperature, Sagrillo said, altitude and humidity
also affect the density of the air.)
It is not uncommon for two relatively close spots to differ
in average wind speed by a few miles per hour or more, Sagrillo
said, adding that even a small increase makes a monumental
difference in a location’s capacity for generating electricity.
This, he said, is due to the formula P=1/2dAV3 (where P=power,
D=density of the air, A=area, and V=windspeed). Because wind
speed is cubed, Sagrillo said, even in incremental increase
in wind velocity becomes hugely significant as far as capacity
to generate power. (An increase from 8 to 10 miles, he said,
can result in twice the power generated.)
And the one way to increase wind speed at any given site
is to go higher.
 |
| Relative
costs of wind energy systems* |
| Make
and model |
Capacity
in watts |
Turbine
cost |
Tower
height/type/cost |
Inverter/
storage |
Bergey
Windpower
XL 1 |
1,000 (1kW)
|
$1,890 |
NRG 84-foot
tilt-up, $1,310 |
$3,500 |
Bergey
Windpower
Excel |
10,000 (10kW)
|
$13,500 |
120-foot guyed
(lattice), $13,500 |
$9,400 |
| Jacobs 31-20
|
20,000 (20kW) |
$19,000 |
120-foot free-standing,
$19,00 |
$4,500 |
|
“As wind speed increases the amount of work that can
be done increases as well,” Sagrillo said. “And
as you increase away from the surface of the earth, wind speed
increases dramatically. That’s why wind farms are in
the 200-foot range, and they’re going to be approaching
300. To illustrate the cost-effectiveness of going higher,
Sagrillo calculated the cost of putting two small-scale electricity-generating
windmills on 30-foot towers (around $63,000) versus setting
up the same model windmill on a 120-foot tower ($36,000) in
order to generate roughly the same amount of power.
“If you want to increase your output, increase your
height,” Sagrillo said. “It’s always, always,
always cheaper, and the reason is that the fuel isn’t
down low; it’s up high.”
How high can you go?
A tower can be anywhere from half to four times the cost
of the rest of your system, Sagrillo said. “What determines
the tower height is the obstacles in your area.”
“There are essentially two enemies to a wind generator.
The first one is called ground drag, getting caught up in
the earth’s zone of friction. You get into a laminar
flow, just air over air, above the surface of the earth.”
Computer generated models help determine the wind resources
at a specific site, Sagrillo said, and an investment in a
professional wind audit is a sound idea before investing in
a lot of expensive equipment and positioning it wrong. (Sagrillo
himself teaches wind-assessor training for the Midwest Renewable
Energy Association.)
“Everybody’s got solar, it’s a democratic
renewable, but wind is very site-specific. The problem with
wind, you hear, is that wind is very unreliable as opposed
to solar or hydro. But it’s not about the reliability,
it’s about the tangibles. In reality, we don’t
know on a day-to-day basis what the wind is going to do, but
we do know on a seasonal basis.” This, he suggested,
makes wind every bit as reliable as solar. [Editor’s
note: A recent AP story posted on the Environmental News Network
http://www.enn.com/news/2004-04-28/s_23255.asp tells of a
new project in Norway where a combination of wind turbines,
hydrogen generators, and fuel cells produce clean electricity
year-round.]
“The other enemy of a wind generator is something called
turbulence.” [Turbulence is a swirling agitation of
the air as the wind hits a physical barrier such as a building,
tree, or hill]. “We actually use turbulence to prevent
some wind-blown problems,” Sagrillo said, giving the
example of planting trees for windbreaks to prevent erosion
and snowdrift.
To keep turbulence from interfering with rotor performance,
Sagrillo said, “the rule of thumb is that all three
blades have got to be a minimum of 30 feet higher than anything
within 500 feet, or 30 feet above the tree line, whichever
is higher.
“And the thing you have to remember about trees is,
they grow—towers don’t. So you’ll need to
know the mature height of your trees in 20 to 30 years, the
life of the wind system.”
If the mature tree line is 60 feet and the blade length is
10 feet, that means a tower height of 100 feet minimum, Sagrillo
said. “Below that you are going to have turbulence,
and that turbulence is going to eat up your wind generator.”
Sagrillo sketched a number of geographical scenarios to demonstrate
how topography, prevailing winds, and accompanying turbulence
might affect location. For a single hill rising from a plain
he selected the peak as the optimum location and the base
of the lee or downwind side (where turbulence would be greatest)
as the worst. For a bluff facing prevailing winds, he suggested
placing the tower at least 200 feet back from the ledge, just
outside of the zone of turbulence.
“It depends on the prevailing winds,” he said.
“You want to know the prevailing winds and capitalize
on them in fall, winter, and spring. You are looking for the
most consistent winds.” (There are two exceptions to
the fall, winter, and spring rule, Sagrillo said: the southeastern
part of the United States and the far southwest, where wind
can be a considerable resource in summer as well.)
Tower of power
“The three most common mistakes” when installing
wind equipment, Sagrillo quipped, are “too short a tower,
too short a tower, and too short a tower.”
Residential and small-farm-size towers come in three general
types, he said, free-standing, guyed (supported with cables
or guy wires), and “somewhere in between.”
Free-standing towers typically have three legs (sometimes
four), are purchased and assembled in 20-foot increments,
and are supported by diagonal and horizontal latticework or
trusses (and lots of concrete below). “This is the most
expensive tower you can buy,” Sagrillo said. These towers
taper up from the bottom; the lighter gauge construction material
used, the more space the base of the tower will occupy. These
towers also have ladders built into them so that the wind
equipment can be accessed and serviced once it’s installed.
New heavy duty towers, Sagrillo said, sell for about $80 a
foot while used ones go for half that price or less. Used
light duty towers cost about $15 to $25 per foot, he said,
while new ones cost about twice as much.
Guyed towers are considerably less expensive but also take
up more space down below, Sagrillo said. They also weigh much
less than a free-standing tower, he said, offering the example
of a typical 10-foot section of a lattice-style guy tower
weighing in at just 70 pounds. “The guy wires go out
typically about 75 percent of the height,” Sagrillo
said. (This compared to a typical light duty free-standing
tower where the height-to-base ratio is 4 or 5 to 1 and a
heavy duty free-standing tower where the base to height ratio
is just 9 or 10 to 1. Guy wires attach to the pole at various
incremental heights and go out to three equidistant points
forming an equilateral triangle around the base of the tower.
Guy towers also require the least amount of concrete, are
easy to climb (if they are of the lattice type), and can be
had (including all hardware) for about $15 a foot (new ones
for two to three times that figure).
The biggest drawback of guy towers besides space requirements
(if that happens to be a factor), Sagrillo said, is that those
available on the market today are only designed to handle
up to about a 10kW system.
The “somewhere in between” tower to which Sagrillo
referred is a tilt-up tower—a good choice if you are
somewhat acrophobic—typically costing somewhere in between
the price of a free-standing and a guyed tower. “The
advantage of a tilt-up tower is you don’t have to climb
it,” he said. The trouble is, you can’t climb
it, and breaking one down is no small task. “Not being
able to climb it can be a problem if you just want to check
something,” Sagrillo conceded.
Tilt-up towers are supported by guy-wires going off in four
different directions (forming a square around the base rather
than a triangle) and are typically raised and brought down
with the help of a gin pole (a braced lifting arm) and a vehicle,
Sagrillo said.
To wrap up his lecture on tower types, Sagrillo laid out
the costs of a typical 120-foot guyed tower ($6,800, plus
another $1,000 for concrete and rebar), a 120-foot tilt-up
tower ($8,500 to $9,000, plus $3,000 in concrete and rebar)
and a 120-foot free-standing tower (around $19,000, plus an
additional $5,000 to $6,000 for concrete and steel). (These
are tower-only estimates.)
Although the wind is a force for generating electricity,
it’s also a force to be reckoned with.
“The wind sees the tower as a lever,” Sagrillo
said, reminding us of the familiar and relevant equation that
work equals force times distance. “The wind sees an
opportunity to knock that tower over; it’s trying to
knock it over, and it’s trying to pull that concrete
out of the ground.”
Coming up next: Lightning, stray voltage
and earth currents; the downside of wind power; wind energy
and the Farm Bill; incentives, regulations, programs and support.
Dan Sullivan is senior editor at The New Farm. Mick Sagrillo
is owner of Sagrillo Power and light, a Wisconsin-based consulting
firm specializing in home-sized wind-turbine technology and
educational workshops.
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