by Dan Fink
Photo by Tim Fecteau
This article was orginally published in the July, August and September 2005 issues of the
Energy Self Sufficiency Newsletter.
DanF writes a monthly wind power column for the ESSN, and we at
Otherpower.com highly recommended this publication for anyone
interested in renewable energy. The ESSN focuses on homebrew and
do-it-yourself projects instead of the expensive turnkey installations
seen in many other magazines. Best of all, it's free! You can download
both the current issue and back issues
Wind power is taking off in a big way worldwide, in both giant
utility-scale installations and small-scale turbines intended to power
a single home. Remote off-grid dwellers are finding wind power an
excellent supplement to solar during cloudy weather, and enjoying the
extra freedom that more power input gives, especially after dark or
during cloudy weather. On-grid folks are installing home wind turbines
to offset rising power costs, and even selling extra power back to the
from the volume of questions received about wind power, there are many
misconceptions amongst people out in the real world. Even though the
physical laws and formulas governing wind power have been well
understood for over 150 years, it has taken new fears about falling oil
production, rising gas prices, and global climate change to generate
the growing interest in wind power we have now with the general public.
In this article an attempt to explain the basics
of how power is extracted from the wind, to help readers understand how
much power they could expect from turbines of different sizes.
Meet the Writer
How small wind turbines work
wind turbine extracts energy from moving air by slowing the wind down,
and transferring this harvested energy into a spinning shaft, which
usually turns an alternator or generator to produce electricity. The
power in the wind that's available for harvest depends on both the wind
speed and the area that's swept by the turbine blades.
Mathematics ahead! But it's pretty simple, and if you are armed with a
pocket calculator and some simple formulas and concepts, you should be
able to make a wise choice in selecting a wind turbine, and be able to
reject unsuitable products and detect scams. I apologize in advance for
mixing metric and standard units!
Power available in wind (in Watts)
• AirDensity = 1.23 kg per cubic meter at sea level (1.0 here in Colorado)
• SweptArea is in square meters
• WindVelocity is in meters per second
If we work the formula for a 5-foot diameter turbine in a 10 mph wind:
• 5 feet = 1.524m
• SweptArea = pi * r2 = 1.8241m2
• WindVelocity = 10 mph = 4.4704 m/s
Power available (Watts) =
* 1.23 * 1.8241 * 4.47043 = 100.22 Watts
first key concept that this formula shows is that when the wind speed
doubles, the power available increases by a factor of 8. That means
there's very little power available in low winds. Increase the wind
speed for this 5-foot rotor to 20 mph (8.9408 m/s) and we get:
Power available (Watts) =
* 1.23 * 1.8241 * 8.94083 = 802 Watts
only way to increase the available power in low winds is by sweeping a
larger area with the blades and that's the second key concept from this
formula. Power available increases by a factor of 4 when the diameter
of the blades doubles.
If we use a 10-foot (3.048m) diameter rotor for a 7.30 m2
swept area in a 10 mph wind, we get:
Power available (Watts) =
* 1.23 * 7.30 * 4.47043 = 401 Watts
and in a 20 mph wind:
Power available (Watts) =
* 1.23 * 7.30 * 8.94083 = 3209 Watts
there's no way to harvest ALL of this available energy and turn it into
electricity. In 1919 a gentleman named Betz calculated that there's a
limit to how much power a turbine blade can extract from the wind.
Beyond the Betz Limit of 59.26% energy extraction, more and more air
tends to go around the turbine rather than through it, with air pooling
up in front. So 59.26% is the absolute maximum that can be extracted
from the available power.
are additional losses after Betz. Small wind turbine blades are never
100% efficient, even when running at their favorite speed. No
alternator or generator is 100% efficient in converting the energy in a
rotating shaft into electricity. There are friction losses from
bearings, and from any gearing that's involved in the power conversion.
And there are magnetic drag and electrical resistance losses in the
alternator or generator.
power researcher Mike Klemen did some volunteer math to benefit the
wind power community, and came up with a web page about the Perfect
A "perfect turbine" would work right at the Betz limit, the blades and
the alternator would match perfectly at all wind speeds, and the
alternator would have no internal magnetic or electrical losses. Klemen
also averaged the efficiency of power conversion (called the
"Coefficient of Power", or Cp) of several commercial small wind
turbines, and got the figure of Cp=35% for what he calls on his page a
"Good Turbine", though none of the commercial turbines he tested were
able to reach that efficiency.
now you are already armed with the knowledge to detect a wind power
scam or a misleading advertisement and there are some out there. Caveat
Emptor (Buyer Beware), and TANSTAAFL (There Ain't No Such Thing As A
Heinlein). If an advertisement claims more poweroutput than is even
available in the wind, you are dealing with a con artist. If the power
output claims beat Betz, you are either dealing with a mis-informed
individual or company, or a con artist. And if output claims exceed
Klemen's very optimistic Cp=35% guideline, you may have found an
exceptionally good turbine, or more likely someone who is not measuring
Cp carefully enough with their data acquisition equipment. Though giant
utility-scale turbine designers are attempting to close the gap between
reality and Betz, a small turbine that gives a Cp of 35% or more would
be considered an exceptionally good design right now.
crucial concept to understand about wind power is at what speeds the
wind usually comes to us for harvest. Folks that are new to wind power
tend to think that if they put up a small, 4-foot wind turbine that's
rated 400 watts, they'll be getting 400 watts power input whenever it
gets breezy outside. In reality, at most locations the wind comes to us
at low speeds 5-15 miles per hour. And we've already learned that low
winds don't have much power in them, and that they can only make useful
power when swept with a large rotor. Advertised wind turbine "ratings"
are currently all based off of peak output in high winds which are
relatively rare. Much more important is what the turbine will do in low
winds, but this information can only be extracted from power curve
charts supplied by the manufacturer.
easy to look up the average wind speed at your geographic location with
the NREL (National Renewable Energy Laboratory) printed and online Wind
will show you your "wind zone" and the average wind speed in that zone.
It's a good way to see if wind power is even appropriate for your site,
but doesn't give you all the information you need to select a turbine
for your location and power needs.
statistics used to calculate the distribution of wind speeds are
complicated, but the results are easy to understand. In most locations
worldwide, the distribution of wind speeds keeps fairly close to a
Weibull or (simplified) Rayleigh distribution of wind speeds, shown
below. There are non-Rayleigh locations where the curve takes on other
shapes, but these are relatively rare. The distribution shown here is
the chart, the horizontal axis shows wind speed, and the vertical axis
the probability (which can be condensed down to the predicted number of
hours per year) that the wind is blowing at that speed. The area under
the graph is always equal to one, because the probability that the wind
is either blowing or not blowing is always 100%.
anemometer is an extremely valuable tool for evaluating your location
for wind potential. Anemometers can be rented, built at home, or
purchased. Some models even plot your Weibull distribution for you. See
the links at the end of this article for more anemometer information
concepts are the key to selecting the correctly sized wind turbine for
any application select your turbine based on the SWEPT AREA, not the
manufacturer's "rated output". Rated output is an almost meaningless
figure, it's simply the peak power the turbine can make in high winds.
But at what
speed does it make that rated power? Most manufacturer rated outputs
are taken at around 28-30 mph. And for how many hours per year do you
get wind at that high speed? In most locations, very few. Most winds
come in at a much lower speed, and therefore have much less power
available in them. The only way to compensate is with a bigger swept
One of otherpower.com 10-foot turbine designs in operation. This one was
built and flown by our neighbor Tim Fecteau. The web page about the
design is at:
In a 10 mph wind (very common), there are 100 Watts of power available
with a 5 foot diameter wind turbine. Betz lowers this to 59.26 Watts,
and with Klemen's "good" turbine losses we are down to at most 35 watts
of output. That's only enough power to fire up a couple of efficient CF
light bulbs. By comparison, a 10 foot turbine has 401 Watts available,
238 W with a "perfect" turbine, and 140W output in an excellent turbine
design. Much better, but not anything that's going to make your
electric meter run backwards! A "good" 20-foot turbine could possibly
give 740W at 10 mph.
we double the wind speed to 20 mph, the exponential increase in power
available becomes apparent 280 possible Watts from a "good" 5-footer,
1,100W from a 10-footer, and 5,900W from a 20-footer. Now we are
talking some real power for a sailboat or cabin (the 5-foot machine),
an off-grid home (the 10-foot machine), or an on-grid house trying to
offset the power bill (the 20-foot machine). Of course it varies by
location, but on a good wind power day that most people would call
breezy, the wind will usually be between 10 and 20 mph.
you are armed with some realistic expectations of how much power
different sized turbines can make at different wind speeds. You should
be able to whip out your pocket calculator at any time and envision how
much power is possible. And here's where some less scrupulous retailers
and manufacturers try to promote wind power misconceptions to boost
sales but now you are immune to hype! Some examples are listed below,
and each future part of this series will have some hype-proof and
misconception-busting information related to its topic. These examples
are all fictional but you'll find great similarities to real ads if you
prowl Ebay and Google for wind turbines.
Watt Wind Turbine only $500! More power than $1,600 worth of solar
panels! No Reserve! That 400 Watt wind turbine rating will be peak
output, probably in 28-30 mph winds. Check your Rayleigh distribution
chart, and estimate how many hours per year that you'll see 30 mph
winds. Not many! While in the meantime your 400 Watts of solar panels
will be making the full 400 Watts for many hours per week. Comparing
solar to wind is like comparing apples to oranges. A good renewable
energy system will use BOTH solar and wind.
remarkable innovation can make 3,000 Watts of power in 13 mph winds,
all from an unobtrusive 6 by 6 foot box! Get out the calculator, and
you'll see that they are claiming over 7 times more power output than
is even available in the wind. It's a scam for sure, because it defies
the laws of physics.
want a big, unsightly wind turbine in your yard? Buy 3 of these small,
400 Watt turbines, and get as much power as a big, ungainly and
expensive 1,200 Watt unit for half the price! Think back about power
available in the wind and its factor of 8, and swept area and its
factor of 4. At the rare rated peak output, which is probably around 30
mph, the 3 small 400W turbines might indeed be making near the same
power as one big 1,200W model. But what about performance in your far
more common low winds? The large turbine would be spinning and making
some power, while the small ones most likely sit still, making nothing.
900 Watt Wind Turbine! Under 3 feet in diameter!! No Reserve!!!
quick check with pocket calculator shows some problems. Running the
power available in the wind equation at 30mph, even Klemen's
exceptionally "good" turbine guideline gives only 600 Watts output with
a 3-foot rotor, so something is fishy here. A quick check of the
turbine's specs shows that that that 900W rating is for "65 mph
winds" which are extremely rare. Commercial turbines are all given
their ratings at around 30 mph that seems to be the industry standard.
last example of hype again shows exactly why you should select a wind
turbine based on swept area and not rated power output. There has been
much controversy and discussion on the internet recently regarding
rated output, since folks that are flying wind turbines can observe
with their own power meters how useless peak power ratings at 30 mph
really are, and how infrequently that output level occurs. The problem
is that wind turbines manufacturers NEED some way to quickly and easily
present their performance data to the public.
Sample power curve diagram:
Wind turbine instantaneous power curve chart from a very reputable
This one is for a Bergey XL.1, an 8.2-foot diameter, 1,000 Watt machine.
example, I could rate a nice 10 foot turbine at only 400 Watts, and
that would very accurately reflect what it can make in common 15 mph
winds. But no one would buy it, since they could get a 400 Watt turbine
with only a 4-foot diameter from another manufacturer for a fraction of
the cost and are not aware that the turbines are rated at vastly
different windspeeds. The 10-footer would make far more power in all
winds and far more power per year, but by comparing the peak output
data the uninformed buyer would assume they would perform the same.
Really, the only way to accurately rate wind turbines is by comparing
their measured power curves, and most consumers are not willing to sit
down and crunch the math.
area is the most important concept in choosing a windturbine, but it's
very difficult to get that concept across to an under-informed public.
YOU now understand some of the issues involved in figuring how much
power is available in the wind with different sizes of wind turbines.
The next article in this series will cover the details of why wind
turbine blades are shaped as they are, how electricity is generated by
that spinning shaft, and how wind turbines regulate their output and
protect themselves from extreme winds. Remember that factor of 8
increase in power when wind speed doubles? This massive increase in
power can blow a wind turbine apart, and the turbine design must be
able to compensate to keep incoming power at a reasonable level.
Links you may find interesting
Excellent Danish wind power tutorial website,
including formulas and online calculation programs for wind power:
Home-built wind turbine resources
Wind turbine building information
Hugh Piggott's superb home-built wind turbine website
The otherpower.com discussion board,
where wind turbine builders worldwide meet to
information and photos
Homebuilt data-logging anemometer information
Inexpensive logging anemometers and Windows wind data logging software
Fancy logging anemometers with Rayleigh functions
Surviving high winds
wind turbines must have a way to deal with this massive increase in
available power as the wind speed goes up. In Part 1 of this series
(see ESSN July 2005), we discussed the distribution of wind speeds, and
how most wind comes to us at lower speeds. So, manufacturers try for
the best performance between 7 and 30 mph, and design the turbine to
simply "survive" winds higher than that while still producing near peak
power. If the turbine was allowed to keep making power over 30 mph, it
would - but only to the maximum power production rating of it's
generator or alternator, which can't harvest much more power beyond
that rating - so the huge amount of extra power in the wind will cause
overheating, overspeeding, and possibly burn out the generator or cause
the turbine to shed a blade.
very effective way to regulate incoming power is variable pitch blades.
The blades can rotate in the hub and change the angle at which they hit
the wind. All large utility-scale turbines use this method, regulated
by sensors and active controls. Only a few small turbines use variable
pitch blades, notably the
Jacobs has been building the system since the 1920s, and you
can still buy one new! The system is not high-tech, but is
extremely effective - the blade pitch changes mechanically
a flyball governor and centrifugal force. In low winds, the blade pitch
is very steep, and at peak output the blade pitch is very flat - this
matches the blade's angle of attack to the apparent wind (more on
apparent wind later). If winds increase more, the blades pitch
past flat, causing aerodynamic stall to prevent overspeeding.
is the most common high wind regulation technique in small wind
turbines. The turbine frame is designed with a built-in offset, and the
tail or the generator head is hinged both upwards and inwards. When
windspeed starts to approach the generator's maximum power output
capacity, the tail or head folds up, yawing the machine at an angle to
the wind.This reduces the effective swept area and thus the available
power to the maximum power output level of the generator, so it
continues to make peak power while furled. When wind speed drops, the
tail or head drops back into a normal configuration via gravity and
tracks the wind straight on once again.
very small wind turbines use flexible plastic blades that bend, twist
and flutter when power input gets too high for the generator to handle.
This technique is effective, but also noisy. Some of the extra power in
the wind is being turned directly into noise, and the sound of blades
fluttering at high speed is very distinctive. It's only used on very
small turbines, and is effective only using modern plastic blades that
are highly resistant to fatigue.
Mechanical and air brakes
regulation techniques are no longer used in commercial turbines because
they are very noisy and prone to mechanical failure from fatigue, rust,
and ice. Nevertheless, I have to admit it's exciting watching and
hearing a 1930s vintage Wincharger deploy its air brakes during a gale!
wind turbines should have some mechanical or electrical way to shut
them down (stop the blades from spinning) during severe weather events.
These can including shorting the alternator phases, a crank that turns
the tail into fully-furled position, or a mechanical brake. There's no
sense in abusing your expensive turbine and tower by letting the
machine run during a hurricane, severe thunderstorm, or tornado, since
the machine will make no more power in 100 mph winds than it will in 30
mph winds if it is furling properly.
you are working with a tiny "science fair project" windmill that's
capturing wind from an electric fan, some sort of regulation is needed
will fall off! Beware of any wind turbine whose builder
claims that it doesn't need to furl because it is built so sturdily
(tested to 100+mph!). But how many times and for how long can it
withstand such abuse? Also beware if the builder advises you to lower
the turbine to the ground if high winds are forecast - it probably lacks
a shutdown system.
Wind Turbine Types
you are considering buying or building a wind turbine for making
electricity, you'll almost certainly be comparison shopping for a
modern, electricity producing, lift-based horizontal axis machine. But
by taking a look at some historical wind turbine designs, it gets
easier to explain the physics concepts involved.
Drag vs. Lift
turbines are divided into two types, drag machines and lift machines,
based on the aerodynamic principles they utilize, and two more types -
Horizontal Axis and Vertical Axis machines - depending on their
that use drag to make them spin are the oldest way to harvest wind
power, and the easiest to understand. The blades or cups push against
the wind, and the wind pushes against the blades. The resulting
rotation is very slow. And the blades or cups that are swinging back
around after making power are hurting power output because they are
the wrong direction, against the wind. The earliest examples of
drag-based wind power design are grain grinding and water pumping
machines from Persia and China, with records dating back to 500-1500
Note the wall that's erected around the half of the machine that is hurting performance by moving against the wind.
In any drag-based design, the blades can never move faster than the wind.
This turns out to be a critical concept for both efficiency and the ease of generating electrical power.
wind turbines are the standard now, but lift concepts have been in use
for thousands of years. Mariners as early as 3200 BC used lift whenever
they took a boat with sails out onto the water and turned the sails to
give the boat maximum speed. An airfoil shape (just like the cross
section of an airplane wing) gives lift, and has a curved surface on
top. Air moves over the curved top of the airfoil faster than it does
under the flat side on the bottom, which makes a lower pressure area on
top, and therefore an upward force - that's lift. The key concept of lift
and wind power is that
lift forces allow the blade tips of a wind turbine to move faster than the wind is moving.
Horizontal Axis Wind Turbines (HAWTs)
Vertical Axis Wind Turbines (VAWTs)
HAWTs are what most people first think of when someone says "windmill" - blades moving perpendicular to the ground.
a VAWT, the blades move parallel to the ground. Both HAWTs and VAWTs
can be either drag or lift based, though only lift designs are commonly
used as they are reasonably efficient for electricity generation. Below
are some commonly seen wind power designs, and explanations of the
principles on which they work.
not exclusively Dutch in origin, these machines were built all over
Europe for grinding grain, and the earliest ones were drag-based.
Dutch made major improvements circa 1390 AD by incorporating lift into
the blade design. The machine was pointed into the wind manually by the
American Waterpumping HAWTs
Over 6 million of these were installed on farms and ranches across America, starting in the mid 1800s.
typical Aermotor water pumping windmill, still common and in operation
all over the American West. Photo courtesy of DeanBennett.com, Denver,
CO. This company sells all the replacement parts to keep these
beautiful old machines running, and also sells new waterpumping
were used purely for mechanical power to drive a pump shaft in a well,
and point into the wind via a tail vane. Many of these antiques are
still in use, and some are still manufactured new! Companies like
Dean Bennett in Denver, CO, USA
still sell all the
replacement parts to restore and maintain waterpumper mills, and also
sell brand new machines. These designs are mostly drag-based, providing
high torque for the pump shaft, but low blade speed. This makes them
difficult to use for electricity production, but excellent for moving
that heavy pump shaft.
Electricity-Generating HAWTs: They come in sizes ranging from tiny (4
foot diameter, to mount on a sailboat or remote cabin) to huge (300
foot diameter, multi-megaWatt, utility-scale machines). These machines
can be designed for either "upwind" or "downwind" operation. In upwind
turbines, the blades are in front of the tower toward the oncoming
wind, and point into the wind using a tail vane or (in giant turbines)
electronic controls. Downwind turbines don't have a vane, and the
blades are behind the tower relative to the wind. While upwind designs
are the most common, there are excellent downwind machines commercially
All modern electricity-producing HAWTs are lift-based, so the blade tips can travel faster than the wind.
resulting high RPMs are ideal for producing electricity, and these
machines can be highly efficient. Small machines are approaching 35%
efficiency (Cp=35%), while utility-scale machines are rapidly
approaching the Betz Limit (Cp<59.26%, see Part 1 of this series,
ESSN July 2005).
ancient Persian design shown before, the Panemone, is one example.
Other designs include the Savonious Rotor which can be easily built
using coffee cans, plastic buckets, or metal barrels.
[Photos courtesy of the
American Wind Energy Association
simple anemometer is another drag-based VAWT design. While fun for
experimenters and students to build and test, these designs are
extremely inefficient, and give only low torque since the blades or
cups can never travel faster than the wind. Yes, I know ... anemometers
often spin quite fast, but as they usually have a very small arm
length, they have very little torque.
Giromill, and H-rotor designs are big improvements over drag-based
machines, since the blades have airfoils and utilize lift to move
faster than the wind. However, there are inherent difficulties with any
VAWT design, and these problems are why VAWTs have never been very
successful in the commercial market, on either small or large scales.
[Photos courtesy of the
American Wind Energy Association
Should I choose a HAWT or a VAWT for my installation?
If you are looking for a fun wind power experiment or science fair project, a VAWT might suffice.
You can find some good ideas here.
need to make some serious electricity to power an on- or off-grid home,
a lift-based HAWT is the best choice. Plus, you'll have a very hard
time even finding a commercial VAWT from a reputable manufacturer for
sale in any size.
The disadvantages of VAWTs are numerous:
VAWTs must be built at least
twice as big
as HAWTs to make the same amount of power, since half of the machine is
moving in the wrong direction (towards the oncoming wind) at any given
time (remember the Panemone?)
of this, VAWTs go through a fatigue cycle on every rotation. This means
the design must be very strong and sturdy - which also translates to
higher cost and more weight.
More weight also mean that the tower must be more sturdy, another added expense.
wind turbines must be flown high in the air to get above obstructions.
Near the ground, on a rooftop, or in any direction from obstructions
such as buildings, slopes, or trees, turbulence steals large amounts of
power and causes unnecessary fatigue in both HAWTs and VAWTs.
general, VAWTs are also lower in efficiency than HAWTs. Drag-based
designs of any kind are the worst because maximum possible efficiency
(Cp) is directly related to how much faster than the wind the blade
tips are moving. This ratio of blade speed to wind speed is called the
Tip Speed Ratio (TSR), and the best possible Cp is obtained around TSR
= 5-6. Only lift-based VAWT designs can even approach this TSR, and are
still limited by the other factors listed above.
[Courtesy of windturbine-analysis.com]
This diagram shows the maximum Cp theoretically possible, versus Tip Speed Ratio,
for different wind turbine designs.
Wind turbine blade and rotor design
people are surprised the first time they look at wind turbine blades
and rotors close up. The flat sides of the blades face the wind, and
they have a distinctive twist to them, from a steep pitch at the root
to a very shallow pitch at the tip. Why is this, and why do some
turbines have more blades than others?
HAWTs, the blades are inclined to the oncoming wind and constrained to
move around a horizontal axis of rotation. They start to move by
deflecting the wind, just like a rudder inclined to the flow of water
forces a boat to change direction. However, like an aircraft wing,
their airfoil cross section adds lift to the blades as they speed up,
and greatly increases the rotational force. The blades are wide at
their base and taper as they go out because the tips move faster
than the base. They are also twisted so that the angle off
BLADE TWIST attack decreases from where the air is moving relatively slowly.
the angle of attack is wrong for the apparent wind, the airfoil stalls
and ceases to produce lift - the same thing that happens when an airplane
tries to climb too steeply for its speed and begins to fall. When a
wind turbine rotor begins to start spinning from a full stop, it is
always stalling. As the wind increases and the blades pick up speed,
the angle of attack gets better and better, and the turbine accelerates
dramatically from the added lift force. It's fun to watch this happen!
And a turbine can stall at higher windspeeds too - it will no longer pick
up RPMs as the wind increases. This is not frequently observed, as the
turbine has usually furled by that point to reduce wind input.
the number of blades and the amount of the swept area that's taken up
with their surface area increase (this ratio of blade surface area to
swept area is called "solidity"), more torque and less RPM are
produced, the tip speed ratio is lower, and the blades must be
proportionally narrower. The typical 3-bladed rotor is the best
compromise for physical strength and rotation speed.
many myths going around about wind turbines, especially VAWTs.
Unfortunately for VAWT enthusiasts (some of whom are probably already
drafting irate emails to me), almost every wind turbine investment scam
(ranging from small scale to utility scale) in the last 50 years
involved VAWTs. The reason for this abundance of scams is simply that
look new and different,
and are intriguing to the public.
Some examples (fictional, but similar to actual advertising claims):
now in this unobtrusive, world-changing, previously suppressed, new
technology that will put a (insert company name) wind turbine on every
rooftop in America, solving our energy crisis and oil shortage
First of all, VAWTs are not new technology - see the ancient Persian Panemone VAWT pictured before. The technology has
been suppressed - The US Government NREL, DOE and Sandia laboratories
have extensive tested and computer modelled VAWT performance.
Furthermore, rooftop installations are not practical - turbulence affects
both HAWTs and VAWTs, and they
must fly in smooth air, well
above any obstructions. Now take the average energy usage for an
average home - about 9000 kW/h per year in the US, and 5000 kW/h per
year in Europe. Take the yearly power production estimates from a
reputable wind turbine manufacturer at a reasonable average wind speed
(say 5 m/s, 11 mph). For a Bergey XL.1 (8.2 foot diameter rotor),
Bergey estimates 1800 kW/h per year. So you'd need 5 of these flying on
tall towers and above all obstructions to possibly power your 9000 kW/h
per year house. Now remember that VAWTs must be twice as large as HAWTs
to make the same power. The 5 Bergeys would sweep 284 square feet. The
VAWT would have to sweep 567 square feet to get the same power
output - that means a machine at least 24 feet high by 24 feet wide,
mounted at least 20 feet above the nearest tree or building. That
doesn't sound either practical or unobtrusive. Hold on tightly to your
wallet, and consult an investment counsellor before spending money.
turbines with only 2 or 3 blades let too much wind slip through and be
wasted - my Savonious VAWT (or multi-blade waterpumper-type) design will
capture ALL the wind.
is a myth! The Betz limit of Cp<59.26% applies to both HAWTs and
VAWTs. Behind and in front of every operating wind turbine the air is
moving slower, and the wind tends to go around the machine instead of
through it. Plus, both the Savonious and American Multiblade designs
are mostly drag machines, and therefore very limited in efficiency
because of their low Tip Speed Ratio (see chart on previous page).
Modern utility-scale wind turbines are coming close to the Betz limit,
but drag designs have little chance of ever coming near even half of
blades means you get more power! Replace your existing 3-blade rotor
with our 6/8/12/16 bladed rotor and outperform all 3-blade designs.
Not a good idea, you'll actually get
power from your existing machine! Wind turbine alternators and
generators are designed to work in a specific RPM range, and lowering
their RPM and TSR by adding more blades means you get less power, not
more. The torque will increase, but that doesn't help your electrical
generation at all. For the same reason, it's not practical to convert
an American Multiblade Waterpumper windmill to make electricity - the
RPMs are much too low, and adding any kind of gearing to increase shaft
speed seriously hurts power output, especially in (the most common and
most important) low wind speeds. 3 blades are the best compromise of
RPM vs. torque. Any design with high solidity won't be suitable for
producing electricity efficiently.
a wind turbine close to the ground is very much like installing solar
panels in the shade - a large part of your investment is wasted.
General guidelines are that wind turbines should fly 30 feet above any
obstruction within 300 feet. Windspeeds increase as you go higher, and
turbulence decreases. Turbulence affects all wind turbine designs, and
besides dramatically reducing potential power, turbulent winds near the
ground buffet any turbine, causing serious stress on all components and
sometimes premature failure.
general, urban and suburban wind turbines are possible only on a very
small scale because of local building codes and impact on neighbors.
And "small scale" means a very small turbine, which won't do much to
offset the typical power use of a typical suburban home. (see parts 1
and 2). Most building codes require that any tower have a "fall zone"
that does not impact any neighboring property, and also restrict the
height. Locations near airports have tower codes even more strict, and
your relations with neighbors regarding aesthetic and noise concerns
must be addressed. If you are on a small urban or suburban lot with
many houses nearby, your best bet may to be to invest the money you
considered for a wind turbine into refitting your home with
energy-efficient lighting, appliances, and heat, with the future option
of a photovoltaic system in mind too.
you are in a more rural area and can comply with local restrictions,
it's important to put your tower as high as you can at the best
possible location. This is expensive - in general a good tower will cost
as much as the wind turbine that will fly on it! Local geography,
property lines, and cost may limit your tower options. Up here in our
remote mountainous area, many of us are forced to cheat the 30-foot
-above-300-foot rule all the time. The key factor is, how much are you
investing in your wind turbine and tower? If it's homebrewed and
inexpensive, just fly it as high as you possibly can - if you built it,
you probably know how to fix it. If turbulence from being flown too
near the ground causes a failure, your loss is only some repairs and
engineering lessons learned. If you just dropped big money on a
commercial turbine, you'd be crazy to fly it too low - and no commercial
installer would even touch the job! Seek expert opinions, they often
cost only a couple beers.
Photo Comparison of Sites
let's have a look at a few pictures taken at real sites to see what
makes them suitable, or not. We'll start with a couple of good ones.
Homebrew 1.5kW turbine flying high in Colorado, USA Photo by Dan Fink
This could perhaps be a "little" higher?!
Homebrew turbines flying at pretty good sites, Colorado, USA.
Both could use another 10-20 feet of tower
(notice local tree heights)
[photos by Dan Fink]
vary on whether detailed wind data acquisition should be used for a
siting decision, or simply general wind data for the area. My opinion
is that the cost of your pre-purchase data acquisition should directly
correlate to how much you are planning to spend on the installation.
Are you looking at spending US$30,000 for a 12 kW Bergey to offset your
home or business power use on the grid? Darn right, erect a
meteorological (MET) anemometer tower and collect data for a year or
two, then take it to an expert analyst. Planning to spend US$3000 on a
small commercial or mid-size homebuilt wind turbine for an off-grid
home? Put up some inexpensive
An alternative car alternator - or is it a swamp buggy?
anemometers on PVC pipe masts, and watch your input during windy months
or for a whole year. US$800 on a tiny wind turbine or homebuilt
mid-size model for your vacation cabin? Just do it! Keep in mind how
much power you can expect from different sizes of turbine in your area,
and the importance of flying it as high as possible.
data on wind power potential at a given site can be gleaned by simply
observing the wind first hand. Ridge tops are usually the best turbine
locations, so how tall are the trees on your ridge? How big a tower do
you need to get 30 feet above them? About how fast is the wind blowing
each time you visit? Keep a log. And how fast are the trees growing in
height each year? Do the trees on your ridge have more branches on one
side than the other? Such tree "flagging" can also be a rough indicator
of wind potential at a site. Also, talk to neighbors that have lived in
the area a long time.
GP Index - Courtesy NREL
data logging can be as simple and cheap or as complicated and expensive
as you choose to make it. If you are planning a substantial investment,
choose your logging equipment accordingly. Some government and
non-profit organizations are making advanced wind data logging systems
available on loan for zero or small cost - search Google for
"anemometer loan program" to find details for your area.
A professional-grade, ready-to-fly logging MET anemometer system (including 30-meter tilt-up tower) can be had from
for a cool
US$3000 - intimidating, but not if you are looking at spending US$30K on
a turbine. US$600 to NRG systems will get you an excellent professional
instrument, even capable of plotting the Weibull distribution of your
wind speeds (see part 1) - you build your own tower or tree mount for it,
and be sure to fly it high or else turbulence will compromise your
data. Home weather stations by Davis Instruments, Oregon Scientific or
LaCrosse come in at around US$300. For bottom feeders like me, US$40
will get you the parts to build a homebrew
built from a bicycle speedometer, or an even cheaper
Easter egg anemometer.
And check out the
from Inspeed.com for a
US$60 commercial versions of the bike speedometer anemometer, and a
US$99 anemometer with computer interface and software that give an
on-screen display and also puts your wind data directly into a
Chasing a storm with a Vortex anemometer
There are two basic types of towers for wind turbines:
Fixed - The kind you have to climb. If you (or a friend, or a professional
wind turbine installer) don't mind high climbing and know the necessary
safety precautions, harnesses, and knots, a lattice tower with 3 guy
wires can be very cost effective. The tower sections, and finally the
wind turbine, are raised one at a time using a gin pole and davit, or
the whole thing at once with a crane. Lattice tower sections are often
available used or as surplus. Freestanding, fixed towers are more
expensive: wider at the base and narrow up towards the top, but they
need no guy wires. At very steep, craggy or tree-filled sites, fixed
and freestanding towers have the advantage that a much smaller
"footprint" of level ground free of trees is needed.
Professional wind turbine dealer, installer and mechanic Victor Creazzi of
Aerofire Wind Power on a routine service call.
Seriously acrophobic Dan Bartmann of Otherpower.com
is assisting. Victor wisely refrained from any "tower-top humor" after he got DanB up there!
The turbine shown is an Atlantic Orient located at the NREL,
NWTC, Golden, Colorado, USA
[Photo by Dan Fink]
Typical freestanding tower with no guy wires.
Requires a large concrete base.
Tilt-up - This is a great alternative for non-climbers, though it is often more
expensive. A tilt-up tower has the big advantage that you can do all
your adjustment and maintenance to the turbine while it's on the
ground, instead of while you are hanging from the tower top. It also
uses a "gin pole" to raise it, but in this case the gin pole is a lever
arm that stands straight up when the tower is down, and lays along the
ground when the tower is up. Tilt up towers use 4 guy wire locations
instead of 3, and the guy anchors must be perfectly square to the tower
base. Tilt-up towers are generally made of steel pipe, which is heavier
and more expensive than lattice. The prospect of high climbing a fixed
tower might convince you that the added expense (and required large
level area, clear of trees and obstructions) of a tilt-up is justified.
A 40-foot tilt-up tower being raised via pickup truck and gin pole. Colorado, USA
[Photo by Dan Fink]
tower base actually does not receive much stress compared to the guy
anchors - once the tower is erected, all of the force on the base is
straight down, from the weight of the turbine and tower. Fixed towers
require a chunk of concrete at the base, and the manufacturer's specs
will tell you how much mud to pour. A metal plate is bolted into the
concrete and the lattice sections attached to the plate. Freestanding
towers need even more concrete - with no guy wires, the weight and
depth of the base are critical. Again, follow the tower manufacturer's
towers benefit from a concrete base because of the side forces when
raising or lowering the tower, but some perfectly good commercial
designs use a flat metal base that's simply spiked directly into the
dirt. What's most important is the strength of the guy wire anchors,
which are usually either concrete or (when digging is impossible) large
metal pitons sunk into holes drilled into the rock, and further secured
with masonry epoxy. Fixed towers give you a little leeway on exactly
where the 3 guy anchors are placed, but tilt-ups don't. If the 4 guy
anchors on a tilt-up are not perfectly square and level with the base
and each other, the side guy wires may alternately tighten and loosen
during raising and lowering. This requires diligence and very slow
action by the owner during erection, because wire rope guys don't give
any visible indication of how much tension they are under - they look the
same whether there's 100 pounds of force on them or 1000. A broken guy
wire can be a disaster for the turbine, tower, and could even kill the
owner. Seek expert advice if you've never raised a tilt-up tower
before! It would be wise to do the same if you're thinking of a fixed
Lightning Protection and Wiring
turbine towers must be properly grounded or induced current from a
nearby lightning strike could fry the rest of the components of your
power system. Generally, the metal components of the tower and turbine
are grounded to one or more ground rods near the tower base. The
electrical wires from the turbine are NOT bonded to this ground - they
run into the house, and are grounded by the power system's main ground.
Very tall towers may have a ground rod at the base plus an additional
ground rod at each guy anchor, with all guy wires that connect to that
anchor bonded together.
wires from the turbine to the house must also be properly sized to
avoid too much resistance heating loss. Follow the manufacturer's
recommendations, and be sure to factor in
tower height when making your calculations. Many turbine manufacturers
no longer make their mid-size models in 12V configurations, it's now
24V or 48V only. Low voltage 12V systems will need thick, expensive
wire, and internal losses in the turbine's generator are also a problem
at 12V. If you are pricing wire for installing a 12V turbine a few
hundred feet from your house, you'll quickly see the advantage of
upgrading to a 24V or 48V power system!
all small off-grid wind turbines use what's called a "dump load
controller" to regulate their power input to the system. The raw output
of a small wind turbine is "wild AC", meaning it varies in both
frequency and voltage and is not directly useful for much of anything.
The controller converts this to DC for charging the home's battery
bank, and the batteries themselves regulate the incoming voltage level.
They keep it down to their own voltage, UNTIL the batteries are full -
at that point, they can't regulate the voltage any more. Solar panels
can simply be disconnected from the system, but disconnecting a running
wind turbine (letting it "free wheel") can be a disaster. The turbine
will overspeed and possibly come apart. It must always be connected to
where the dump load comes in - when the batteries fill, the controller
starts diverting all or part of the incoming power to electric air or
water heating elements in your house to keep a load on the turbine
without wasting the incoming power. Commercial turbines come with this
system included, and homebrew builders usually make their own heating
element arrays and use a commercial controller (such as a Trace C-40 or
C-60) set in dump load mode to handle the diversion.
systems are regulated in a similar way. If the system includes
batteries, the only difference is that the grid itself is the dump
load, and any extra power beyond what your home is using runs your
electric meter backwards. In systems without batteries, a special
grid-tie inverter is used (one example is the WindyBoy) to convert the
turbine's wild AC directly into sync with the grid.
Noise, Birds and Bats
your wind turbine starts flying, you'll start getting questions from
friends and neighbors about noise, dead birds, and dead bats. There are
some pretty noisy commercial wind turbines on the market, but most are
extremely quiet. If noise is a concern for you, be sure to talk with
your dealer or installer and visit some working small turbine
installations to hear the noise for yourself. I usually describe the
sound of our homebrewed turbines up here as about the same as someone
riding by the house on a bicycle. In higher winds, the turbines make a
bit more noise, but the noise threshold is just barely above the sound
of the wind in the trees.
kills (and more recently, bat kills) are a big issue for the
under-informed, and those people who are against wind power
development. In reality the issue is minor and stems from early
utility-scale wind installations such as the wind farm at Altamont Pass
in California 30 years ago. This installation was inadvertently sited
in a bad area of the local ecosystem, full of raptors. Commercial wind
farms have been sited with birds in mind for many years now, and
derisive names like "Raptor-matic" and "Cuisinart in the sky" are just
anti-wind-power hype. The biggest modern killers of birds are power
lines, cell phone and other communications towers, pesticides, and
domestic cats. See
article on bird kills
for the AWEA for more details.
We have over a dozen wind turbines flying in our local off-grid area,
and no one has reported a single bird kill, ever.
bat kill issue has only cropped up recently and only at certain
installations, but the problem is currently being worked on by the best
and brightest ecologists and engineers. Just like the bird problems 30
years ago, the issue seems to stem from bad siting of the turbines in
bat flight paths instead of any intrinsic danger from the blades. Zero
dead bats reported up here. Though we recently got to witness a pair of
male Broadtail Hummingbirds engaged in an aerial duel in, around and
through the blades of a local turbine running during high winds - an
amazing display that caused no harm to the birds, and was like watching
the Ornithological Olympics for us!
Wind Turbine Watching
my wind turbine went up, I cursed the constant February winds that
blast our high mountain location. Now, I see the wind in a new
perspective. In fact, I find watching wind turbines to be much more
entertaining then watching television. Those first slow rotations in a
gentle breeze provide the anticipation, and the excitement kicks in
when the wind gets strong enough to give the correct angle of attack on
the blades to provide lift (see part 2 in the August 2005 edition of
the acceleration is dramatic. And then watching a turbine furling in
high winds is a lesson in physics beyond compare. Be sure to find a
spot in your house where you can sit and watch your turbine through a
window, and see both your anemometer and power input meter at the same
time. In addition to the wind power that you are harvesting, you'll
gain power by shutting off the TV too!
The article on Wind Power that you have just read was written by Dan Fink,
seen here top left playing the Washtub Bass and vocalizing in
enthusiastic harmony with his fellow members of the
Dog Mountain Band.
Dan Fink is not just a skilled musician, playing the Banjo, the Dobro
guitar and the Kazoo in addition to the Washtub Bass, but in his spare
time is the technical director for Forcefield in Fort Collins,
has lived off-grid, 12 miles from the nearest power or phone pole, for
14 years, and maintains Forcefield's websites and servers via a
satellite internet connection. His home is powered entirely by solar
and wind, so he knows what he is talking about!.
articles have appeared in such magazines as Back Home, Home Power, and
Zymurgy. In his spare time, Dan is a volunteer firefighter for the
Rist Canyon Volunteer Fire Dept.