The best option for the serious home power generation
installation is a combination of solar and wind. There will be
days of full sunshine and not a slightest hint of a breeze, or
days with heavy rain clouds and strong winds. If
you have the space and a location with good yearly winds, then
a home made windmill could be a viable alternative to solar cells.
Building your own windmill requires no special
abilities or equipment, all you need are some basic handyman skills,
a few tools, and the time to assemble and experiment.
Using the wind.
There is a lot of energy in the wind. Power is
the cube of speed, so a 40kmh wind has 8 times the power of a
20kmh wind. As an example a perfectly efficient windmill may produce
200 watts of power in a 20kmh breeze, 800 watts in a 40kmh wind,
and 6400 watts in a 80kmh storm gust.
But what sort of windmills are we talking about.
First up a few simple rules about windmills. Windmills behave
in a way very similar to your average car engine. They have a
power and torque curve, with different speeds for maximum power
or torque. For electrical power generation ideally you need to
operate your windmill in its peak power output.
1. More blades = less speed, less power but more
torque, perfect for pumping water.
2. Less blades = more speed.
3. Larger propeller diameter = less speed but
more power.
Two other factors to consider are turbulence and
wind-shadow. Turbulence can be caused by the disrupted wind from
one blade to the next, or anything up-wind of the windmill and
will have a big effect on efficiency. Wind shadow is the effect
the windmill mast has on the propeller as each blade passes the
mast. This shadow causes a sudden pressure change behind the propeller
blade and results in vibration.
As
a rule 3 blades is the best compromise between power, torque and
speed. A 2 blade propeller will run faster, but there are dramatic
vibration problems with 2 blade windmills during wind direction
changes and therefore not recommended. A typical home made wind
generator with a 3 blade propeller diameter of 2 meters will spin
from 100 rpm to 600 rpm and is capable of supplying over 500watt.
The old farm style of windmills ( Southern Cross,
Comet ) used for water pumping have lots of blades, giving them
a lot of torque, however many blades means low RPM, and they are
not suited to power generation. It is possible to use a gear box
to speed up the output, but this creates its own problems and
is best avoided.
Windmill blades can be made from just about anything.
Wood, steel, fiberglass, carbon fibre, etc. I've even seen a windmill
using 8 wheelbarrows! Wood is the most common material for the
DIY handyman, its cheap, easily formed, strong and flexible, remember
trees are very good at bending in the wind without breaking
The
profile of a well designed windmill blade resembles a aircraft
wind, giving lift on the trailing edge. The blade has a angle
of attack to the wind to give its best lift, and this lift drives
the blade forward. There also needs to be a slight twist along
the length of the blade, the blade tip is traveling much faster
than the part of the blade closest to the center of the propeller,
so the blade needs to be tilted to give the same angle of attack.
You
also need to consider tip speed, or TSR (Tip Speed Ratio). The
tips of the windmill blade travel much faster than the wind speed,
so a TSR of 7 means the tips travel at 7 times the wind speed.
A typical 2 meter diameter turbine at 500rpm could have a tip
speed of over 200kmh, and any airborne dust or unfortunate insects
will be very abrasive at this speed. For windmill blades made
from a soft material, such as timber, a layer of wear resistant
aluminum tape or fiberglass is usually applied to the leading
edge.
The Power Generator
So what can we use for a generator on a windmill.
There are several options.
Most beginners to home made wind generation try
to use the common car alternator. While cheap, readily available
and capable of producing over 600 watts, car alternators need
to be spun at over 2000 rpm before it will generate any useful
power. A windmill large enough to drive an alternator to its full
power capacity would not spin fast enough ( the larger and more
powerful the windmill, the slower it will spin ), so you would
need a mechanism to step up the windmill speed to over 2000 rpm.
You could use a gear set, chain or belt drive, but these have
disadvantages, including noise, unreliability, poor starting and
reduced efficiency.
You need a generator that will produce power at
low speed, which is why most successful home made windmills use
permanent magnet alternators.
Large Permanent Magnet DC Motors, like those used in electric
wheel chairs, golf buggies are one option. Unfortunately these
are expensive, have relatively low output when used as a generator,
and need to have the brushes replaced from time to time.
Modified Car Alternators. Some success can be
achieved by replacing the standard armature with a new machined
armature containing permanent magnets. While results are better
at low speed than a standard alternator, output is still not good.
Modified
Induction Motors. Single or 3 phase induction motors can be modified
by fitting permanent magnets to the armature. This is done by
machining the armature, in a lathe, down to a size that will allow
you to glue magnets to the armature. By using strong Neo magnets,
a typical 1hp induction motor can supply over 10 amps. The available
current is limited by the gauge of the copper wire in the windings,
so some hobbyists rewind the motor with heavier wire with good
results in excess of 30 amps. A disadvantage of modified induction
motors is the power curve, peak power will be reached at a low
rpm, any faster and the power drops off dramatically. This is
due to the high number of laminations in the stator, giving a
poor high frequency response.
Switched
DC motors, ie Stepper Motors, such as used in most computer equipment,
make fantastic generators! They have good frequency response over
a wide operating speed, are readily available and cheap. Larger
motors can be searched from industrial equipment, motion systems
and some domestic appliances, including washing machines ( more
on this later ). One disadvantage of these motors is the cogging
effect. Cogging is the vibration you can feel when you try to
rotate a stepper motor, and is caused by the interaction of the
magnets and laminations/poles. In a windmill application this
can make the windmill hard to start in light winds, as it needs
to overcome the first "cog" to get going. Once motion
has started the cogging effect has little impact on performance,
other than a slight vibration and whirring noise. Most motors
can be modified to reduce cogging, but I believe if the wind isn't
strong enough to start the windmill, then its not really windy
enough to generate any useful power anyway.
Here in Australia we are fortunate that Fisher
& Paykel sell a washing machine that uses a large permanent
magnet stepper like motor. These motors ( referred to as a F&P
) are easy to come by and make excellent wind generators. You
will notice most of the windmills and projects on this site revolve
around the F&P. The F&P do have a problem with startup
cog, but we have found a few tricks to eliminate the problem.
Air
Core Generators. One way to reduce the cogging effect is to remove
any magnetic material, ie laminations, from within the coils of
your generator. Of course this will reduce the overall efficiency
of the generator as the magnetic path isn't concentrated around
the coils. There have been some clever ways to overcome this problem.
One common design is to embed the coils in an epoxy disc which
is held stationary, with magnets rotating on each side of the
disc. Good results have been achieved, but there are disadvantages.
Winding and gluing your own coils into a non-metallic core stator
is very time consuming. And as the coils are embedded in epoxy,
heat dissipation is poor making it very easy to cook a set of
coils. And as the design has poor efficiency, large and expensive
magnets must be used.
Location, Location, Location.
An effective windmill needs clean air, that is
a breeze with no obstacles. Obstacles can include trees, buildings
and even the contour of the land. As a rule, the higher the better,
but be practical while a 20 meter mast would be ideal, you need
to get the windmill up there, and may need to bring it down for
maintenance. 5 meters above the nearest obstacle is a good start.
Do not mount the windmill on the roof of your house! Windmills
produce some vibration, and while this is almost inaudible, the
vibration will travel down the mast and into the surrounding ground.
If you were to mount it on a roof the vibration would be VERY
audible.
Also consider safety. Things can go wrong, blades
can come off, towers can fall down. So please be locate the windmill
in a people free area.
My personal preference for tower design is a 7
meter tall folding tower made from 75mm od 6mm wall galvanized
pipe. The tower is supported by eight 5mm stainless steel guy
wires, each connected to a steel post embedded in a 700mm deep
hole filled with concrete. You may need to talk to your local
council before erecting a windmill tower, and they may require
a structural engineers plans before proceeding. There is nothing
more worrying that watching your windmill buck around in a violent
storm, and nothing more rewarding than having it survive with
no damage.
Using the power
Regulating windmill power is a little more tricky
than solar cells. Solar cells can be open circuited when the storage
battery has reached full charge. This is not a good idea for windmills.
Say your windmill is pumping out 20 amps into a 12 volt battery
in a strong wind, and spinning at about 400 RPM. If your regulator
circuit suddenly open circuits the windmill, it has no load and
will speed up to, say, 600 RPM. At this high speed the windmill
output voltage could reach over 50 volts unloaded. If you regulator
suddenly reconnects the battery the windmill output current will
be excessive and can cause damage. Remember the windmill is acting
like a big flywheel and will take some time to slow down to a
safe speed. Excessive speed can also increase the wear and tear
on the windmill especially bearings and blade tips.
So a suitable regulator will need to switch the
windmill output from the battery bank to a dummy load, therefore
keeping the windmill RPM down to a reasonable speed. I use 4 100
watt 12 volt spotlamps, wired in parallel, but you could use several
electric heater elements. I have heard of installations where
heating elements are mounted into the hot water system , so once
the batteries are charged the windmill is helping to provide hot
water.
Building your own windmill can be a very satisfying
experience, I have built several over the last few years, learning
from the failures and improving with each new design.
The Fisher and Paykel Smartdrive
As already mentioned above, on this web site I
often refer to something called a F&P. F&P, or Fisher
and Paykel, is a white goods manufacturer based in New Zealand.
In the 1990's F&P introduced the "Smartdrive" motor
into their range of washing machines.
The Smartdrive motor was a revolution in domestic washing machine
drives at the time and has since then been copied by other manufacturers.
The motor was designed to be efficient, easy to repair and cheap
to make. If you want to find out more about the development of
the Smartdrive, there is an interesting article worth reading
here.
The Smartdrive washing machine has been a big
seller for F&P in Australia and New Zealand, and it was only
as matter of time before someone discovered the motors made excellent
alternators. One of the pioneers in Smartdrive use as a alternator
was Michael from EcoInovation in New Zealand ( www.ecoinn.co.nz
), who has used the Smartdrive in hydro and wind installations.
Over the last few years there has been a growth
in the use and development of the Smartdrive by windmill enthusiast's,
with many of the early problems now solved ( cogging or bad startup
was the biggest problem, but is now a thing of the past with decogging
techniques like pole reshaping and 7 phase
conversions )
So how do you get a F&P Smartdrive motor?
The Smartdrives motors parts can be bought new from any F&P
agent, but the parts can be expensive, up to $200 for the stator,
shaft and hub, depending on how much markup the agent puts on
the parts. A better option is to pick up the parts 2nd hand. I
have sourced my motors from my local white goods repair center,
I can usually pick up a couple of dead washing machines for a
carton of beer and with any luck the motors are in good enough
condition to use. I've also bought motors from ebay sellers, just
search for Smartdrive or windmill and they pop up every couple
of weeks or so. Another option is to post a message in the forums
section of this web site and your pretty much guaranteed to get
a complete motor for a good price. In New Zealand they can be
sourced from www.ecoinn.co.nz,
and in the USA try www.randysworkshop.com.
