wind power
Wind Power and Windmills
Aeration Windmill
Aermotor Windmill The Wind Turbine
Important Factors to Consider When Building A Windmill
Location
Wind Characteristics
Terrain Characteristics Windmill Efficiency
The Purpose of Windmill Shrouds Electricity and Storage of Energy
Brakes Windmills Today
Bibliography
Excellent offsite link from our links
page Wind Power
(from
Denmark) very detailed
source of information
These styles of windmills will not produce electricity! Many people think you can buy a regular windmill to run your air conditioner, freezer, washer, dryer, etc. and you can not. This is all 240 volts A.C.
Many have seen ads "sell electricity back to your utility company." Do you think your local electric company would let you run a line from a windmill, or even a generator, and send electricity to their line? Dream on...
You can make a windmill produce 12 volt D.C. to charge a battery, but you can charge your batteries with solar cells today much cheaper than using a windmill. The big windmills you see in Ravenshoe , Queensland will produce electricity but they cost thousands and thousands of dollars.
Have you ever seen a windmill at someone's home that produces 240 volt AC? The technology is not here today to produce electricity for homes.
The Wind Turbine
The wind turbine, also called a windmill, is a means of harnessing the kinetic energy of the wind and converting it into electrical energy. This is accomplished by turning blades called aerofoils, which drive a shaft, which drive a motor (turbine) and ar e connected to a generator. "It is estimated that the total power capacity of winds surrounding the earth is 1 x 1011 Gigawatts" (Cheremisinoff 6). The total energy of the winds fluctuates from year to year. Windmill expert Richard Hills said that the wind really is a fickle source of power, with wind speeds to low or inconsistent for the windmill to be of practical use. However, that hasn't stopped windmill engineers from trying. Today, there are many kinds of windmills, some of which serve different functions. They are a complex alternative energy source.
example. The horizontal-axis was invented in Egypt and Greece in 300 BCE. "It had 8 to 10 wooden beams rigged with sails, and a rotor which turned perpendicular to the wind direction" (Naar 5). This specific type of windmill became
popular in Portugal and Greece. In the 1200's, the crusaders built and developed the post-mill, which where used to mill grain. It was first used to produce electricity in Denmark i n the late 1800's and spread soon after to the U.S. In America, windmills made the great plains. They were used to pump water and irrigate crops. During World War I, farmers rigged
windmills to generate 1 kW of DC current. They mounted their devices on the tops of buildings and towers. On western farms and railroad stations, the pumping windmill was 6-16m high with a 2-3m wheel diameter"
(45)]. With 15kmh wind speed, a 2m-diameter wheel, a 60cm diameter pump cylinder, a windmill-pump could lift 200L per hour to a height of 12m. A 4m in diameter wheel could lift 250L per hour to a height of 38m. (Naar, p. 46).
The growth of wind-electricity in Australia was greatly peaked in the 1930's and 40's . However, in the 1970's, due to oil shortages, earlier prototypes of high-speed horizontal-axis windmills were developed. High-speed horizontal-axis types are used for many purposes, come in many sizes. These include the typical windmills on windmill farm and any other wind turbines in which the shaft turned by the aerofoils is horizontal. High-speed horizontal types may have 1, 2, 3, 4, or many aerofoils.
Low-speed types such as European ones have much larger aerofoils in relation to their height above the ground. Low speed types such as western Quensland ones are usually a pinwheel, with many small blades encircled with an outer frame like a wheel.
Vertical-axis windmills
were
first developed in the Persians in 1500 BCE to mill corn and were still
in use in the 1970's in the Zahedan region. Sails
were
mounted
on a boom, which was attached to a shaft that turned
vertically.
By 500 BCE, the technology had spread to Northern Africa and
Spain.
Low-speed ve rtical-axis windmills are popular in Finland.
They
are
about 150 years old. They consist of a 200L oil drum split in
half.
They are used to pump water and aerate land. They are
inefficient.
High-speed vertical-axis
windmills include the Darrieus models.
These have long, thin, curved outer blades, which rotate at 3 to 4
times
the wind speed. They have a low starting torque and a high
tip-speed
ratio. They are inexpensive and are used for electricity
generation
and irrigation. There are three types, the delta, chi, and
gamma
models. All models are built on a tripod. The
advantages to
a Darrieus-windmill are that it can deliver mechanical power at ground
level. The generator, gearbox, and turbine components are on
the
ground, instead of at the top of a tower as in horizontal-axis
windmills.
They cost much less to construct, because there is less material, and
the
pitch
of the blades does not have to be adjusted.
Another type of HSVAW's are the
Madaras and Flettner types, revolving cylinder
s which sit on a tracked carriage. "The motion of a spinning
cylinder
causes the carriage to move over a circular track and the carriage
wheels
to drive an electric generator" (Justus). The Savonius
model, which originated in Finland in the 1920's, is a n S-shaped
blade, which rotates and turns a vertical shaft. Today, these
types
of windmills are very popular with scientists
and their technology is being developed.
Important
Factors to Consider When Building A Windmill
2) Available wind energy is generally high in the central Austrsalia because of the wide expanses of level (low surface roughness) terrain.
3) Available wind energy is generally low throughout eastern Australia with the exception of mountain passes but higher on the southern and western coasts
Also important to consider the wind characteristics where you are going to build: 1) the mean wind speed (calculated my cubing the averages and taking the mean of the cubes) and its seasonal variations.2) The probability distribution of wind speed and of extreme winds. The mean wind speed must be high enough, and the distribution must be so that all the data points are very similar.
3) The height variation of wind speed and wind direction. Wind cannot be too high or too low in relation to the ground or it is too difficult to harness.
4) The gustiness of the wind field in both speed and direction. Gusty winds greatly affect the power output of the windmills and are usually harmful.
5) The wind direction
distribution and probability of sudden large shifts in di
rection.
The wind must be
unlikely to suddenly shift direction.
It must blow in the same general direction.
6) the seasonal density of the air, and variations of density of the air with height. The denser the air, the worse it will be for windmills.
7) Hazard conditions such as sandstorms, humidity, and salt-spray, which are bad for windmills.
8) Trade winds in the subtropics, and the channeled wind through mountain passes are especially beneficial to windmills.
Terrain CharacteristicsOnce a suitable location is found, the wind is analysed extensively, and the criteria is met, there are still more requisites. 1) The terrain upon which the windmills are built must be relatively flat. The elevation
difference between the turbine site and the terrain is no larger than 60 meters over a 12-km radius. You may have seen windmills such as those in Ravenshoe, Far North Queensland on little hills, but this is because the requirement is met. The hill may be the only one in a mountain pass
2) All hills must have small height to width ratios: h:l must be < 0.016.
3) The elevation difference
between
the highest and lowest point must be 1/3 or less of the
height difference between the bottom
of the rotor disk and the lowest
point in the terrain strip. The
surface roughness of the terrain upon
which the windmill is to be built must
be low. If it varies by more than
10%, this is no good. The terrain
must be smooth, and consistently so. A
rough surface has more of a negative
effect on the wind than a smooth
surface. There is a value n,
called, which is assigned to the terrain in
terms of its roughness. This
value is used to calculate the height of the
windmill. For instance, over
the sea, the index location, n is 0.14.
Over rough inland country, n is 0.34.
Some generators are connected to heating coils. Generators that produce AC can be hooked up to AC motors such as water pumps. Windmills are NOT efficient. At the very most, a windmill can extract only 16/27ths of the kinetic energy from the wind. This is called the Betz Limit and it can be mathematically proven through calculus. Most of today's windmills extract about 30 perc ent of the wind's energy.
The Southern Cross farm windmill can only extract 10%.
An important equation used to find the wind power density, how much power is available per square meter is the equation
, where P is the wind power density in W/m2, p is the density of the air, and u³ is the cube of the wind velocity.
An equation for the power available is :
, where p is the kinetic energy density J/m³, V is the velocity of the wind, A is the cross sectional area of the wind on the turbine.
less than the final power output, since gear trains and generators cause
power to be lost) is as follows:
Where Cp is the power coefficient
(Power of shaft), p is the air density,
D is the rotor diameter, V is the velocity
of the wind and P is the net
power output.
Also Cp = P available/P turbine
The power available is a
function of
elevation. At ground level, 100% of
the power is available. At 30m, 97%
is available. At 1500m , 86% is available.
The
Purpose of Windmill Shrouds
a way to channel the wind.
An equation for the power harnessed by a shrouded wind turbine is:
P(Pe) = ( QT ((p + (k) where P is the power,
Pe is the power extracted, ( is
the
turbine efficiency, QT is the
volumetric flow rate of air on the
turbine, (V/A), ((p + (k) is the change
in pressure energy between the inlet
and the exit of the wind turbine, and
k is the cane in kinetic energy of
a unit volume of air that passes
through the machine.
Shrouds concentrate and diffuse the
wind as it passes through a
horizontal access wind-turbine.
They reduce the turbulence of the wind
and "direct it".
b) the shroud can greatly reduce tip-losses, and
c) the aerofoils would not have
to be
rotated in a direction parallel to the wind if the wind-direction
changed.
The cut in speed is the lowest wind speed below which no usable power
can
be produced by a wind turbine. This means that the wind must
be
fast
enough to move the aerofoils to drive the shaft
to create enough power, after much
is lost, so that the end amount of power is greater than zero.
turbine, which is dependent on a number of factors, especially the
generator. In calculating the height of the windmill, it is important to
keep in mind that the windmill must be high enough to be above
obstructions. The wind velocity decreases as one approaches the surface.
That means that the higher you build, the better chance
there will be that the wind speed is higher, however, you must find the
perfect medium--there are often more variables as you increase in
altitude. In calculating how high a windmill should be the following
equation is used:
Where V1 is the wind speed at
the highest
point of the highest blade, V2 is the wind speed at the lowest
point of the lowest blade, H1 is the
height of the highest point, and H2 is the height of the lowest
point.
n is the index location of the site, a va lue that measures the
roughness
of the terrain.
The support of the windmill is generally made out of steel. The windshaft is the shaft which carries the windwheel or aerofoils. It is turned as the aerofoils turn. It is made of steel or wood.
Aerofoils are the blades
on a windmill. They can be made out of any
material. They
were
first made of wood or wood composites. Steel was used after
that.
Aluminum is used in the Darrieus windmills because it is
much stronger. Unfortunately, Aluminum
fatigues quicker. Some windmills use fiberglass
blades. New
materials such as strong alloys are being used in today's windmills
experimentally.
It is important that the blades have a large lift force and a small drag force. The lift force is the force needed to bend the flow of the (fluid) air. It is the force perpendicular to the stream of the air. The drag force is the force parallel to the stream. The aerofoil must be able to develop a lift force at least 50 times greater than the drag. Torque acts on the aerofoil with a vector from the center of rotation away.
Other forces that act on the blades of windmills are wind shears, wind gusts, which push on the aerofoils, gravity, a pull towards the earth, and shifts in the direction of the wind. Shifts in the direction of the wind are often accounted for by having a small blade, called a tailvane, on the backside of a windmill. The wind blows on a flat side of the tail, which is oriented differently from the aerofoils. Then, the aerofoils can be rotated to face into the wind. If the wind is blowing in the directi on of this tail instead of the direction of the aerofoils, the tail rotates a shaft, which rotates the whole windmill in the proper direction so as to orient it towards the wind.
Wind gusts can greatly affect a windmill. A turbulent gust is a gust greater than two minutes with a certain mean wind speed. Gusts are analyzed extensively, with magnitudes, one fo r the lull speed, which is the wind speed of a negative gust amplitude, and the peak speed, which is the wind speed for a positive gust amplitude.
The gust amplitude is the difference between the largest speed in the gust and the mean speed. The gust du ration is the time from the beginning to the end of a gust. The gust frequency is the number of positive gusts, which occur per unit time. The gust formation time is the time it takes from the beginning of a gust to the time it attains the peak gust spe ed. The gust decay time is the time it takes for the gust the end after it reaches its highest amplitude.
There is quite a bit of
terminology
with aerofoils. The angle of the surface to the fluid flow
is the angle of attack, alpha.
The angle of attack must be just right. If it is too great,
the
lift
will dramatically decrease and the drag will increase, st alling the
windmill.
At rest, (when the windmill is not in operation), the angle of attack
is
85°. When in motion, the angle of attack is anywhere
from
2-10
degrees. Newer and more advanced windmills have an angle of
attack
in the upper end of this range. The pitch angle, ß
is the angle between the chord of the aerofoil
and its plane of rotation. The
pitch
angle can be adjusted. Solidity is the
ratio of the blade width (at widest point) to the distance between the
centers of the blades. A typical "pinwheel Southern Cross
windmill"
might have
a ratio of about 1:1, because the blades
are very narrow and very close together, whereas a new two-bladed
aerofoil
would have a ratio of about 0.03. There is a transfer of work
between
the wind stream and the moving
blade. In order for this transfer
to be efficient, a typical blade is usually 1/4 the width of its
length.
(If the blade is 3m long, it will be 750mm wide at its widest point).
Aerofoils come in many
shapes. Some blades are made a little wider than this ratio,
because it is easier to start such a windmill. However,
blades
like
this aren't as efficient. No matter what the shape, "most
have a
blunt nose and a finely tapering tab le" (Calvert). A flow
must
be
able to follow the curved surfaces of the aerofoil without being
separated.
The mass flow rate is given by the equation:
m = p Vb A
, where p is the air density, Vb is the air speed at the blades and A is the area.
will rotate in relation to windmills with different numbers of blades:
Speed of windmill
= 1 / sq. root of number of blades
The aerofoils of a four bladed machine rotate 71% as fast as that of a 2 bladed machine. A six bladed machine rotates at 58% and an 8 bladed machine rotates at 56% as fast as a 2 bladed machine.
Electricity
and Storage of Energy
Storing the wind's energy
effectively
is the key to its long-term use. Windmills used as water
pumpers
or air-compressors can pump excess water, hydrogen or air into reserve
tanks. Today, there are a number of ways to store the wind's
energy.
Windmills are used to charge Electrolyte batteries. Lead-acid
or
Lead-cobalt car batteries are commonly used as well. However,
batteries
may be expensive and inefficient--they may lose 10-25% of the energy
stored
in them.
Nickel-Iron, Nickel-cadmium, and zinc-air
cells are often used as well. These tend to be more
efficient.
Some
windmills are now using organic electrolyte batteries such as CuCl2, Ni
Cl2, and NiF2 batteries as well as sodium-sulfur batteries, which
operate
at high temperature, are used.
Although uncommon and still in experimental phases, some energy is stored not by being converted directly into electrical energy, but rather by being stored as thermal or electromagnetic energy,
Sound Fluids are elastic. Pressure waves are constantly being created and propagated by the aerofoils and the turbine as a whole (entire components excepting the support). We can hear them in the sound given off. The sound intensity is directly proportional with the speed of the windmill.
The frequency of the waves is directly proportional to the angular speed of the blades on the rotor. The flutter you hear has aerodynamic and elastic properties. The higher speed the aerofoils are, the louder the sound a nd the louder the flutter they will make, as more pressure waves are being created and propagated. The generators are noisy. They often confuse birds and cause them to fly towards the turbine.
Windmills can be very
noisy. A
300 kW turbine at 1 mile away has a dB level equal to a traffic light
100
feet away (Gipe). Windmill sound levels are regulated.
The sound level must be kept under
46 dB in a residential area. Wind turbines can cause
interference,
disturbances with TV and radio reception (ghost images on TVs), affect
microwaves and disrupt satellite communication. These
problems
are
currently being resolved. Many have already been
fixed.
There
is also a .009 probability of a bird or insect being struck by the
blades.
Windmill makers must use artificial sound or florescent paint or scents
to scare away flying creatures.
Mechanical brakes are used to hold windmills at rest when they are not needed, are not functioning, or are under repair. Greek windmills used sticks or logs jammed into the ground to keep the windmill stopped, but modern brakes are more sophisticated. Many windmills today use airbrakes like those used in planes. Other windmills have rope brakes.
Ropes connected to the aerofoils are simply pulled and tethered to a post to keep the aerofoils from turning. The torque on a rope brake can be calculated by the equation :
(M-m)(R2 + r)g
Today, Sandia National
Laboratories,
Alcoa, GE, Boe ing, Grumman, UTC, Westinghouse, and other scientists
are
researching and developing Darrieuses and new types of
windmills.
Today, windmills are used to operate sawmills and oil mills in
Europe.
They are used in mining to extract minerals, to pump water , to
generate
electricity, and to charge
batteries. "Windmills have been
used on buoys moored far out in the ocean, the power being used for the
collection and transmission of oceanographic and weather
data.
They
also work in deserted places as an aid to radio and telephone
communications
and they are used to work navigation lights on isolated hazards"
(Calvert
77).
The Future will likely bring bigger and better things for the wind turbine. Many new wind turbine models are being built. The wind turbine holds much promise for energy production in the years to come.
BibliographyCalvert, N. G. Windpower Principles: Their application on the small scale. London: Charles Griffin and Co., Ltd., 1979.
Cheremisinoff, Nicholas P. Fundamentals of Wind Energy. Ann Arbor: Ann Arbor Science Publishers, Inc. 1978.
Gipe, Paul. Wind Energy Comes of Age. New York: John Wiley and Sons, Inc. 1995.
Hau, E., J. Langenbrinck, and W. Palz. Large Wind Turbines. Berlin: Springer-Verlag, 1993.
Hills, Richard L. Power From the Wind: A History of Windmill Technology. London: Cambridge University Press, 1994.
Justus, C. G. Winds and Wind System Performance. Philadelphia: The Franklin Institute Press, 1978.
Naar, Jon. The New Wind Power. New York: Penguin Books, 1982.
Taylor, R. H.
Alternative Energy
Sources for the Centralized Generation of Electricity.
Bristol,
England:
Adam Hilger, Ltd. 1983.