Blades

The energy crisis caused a considerable growth of interest in alternative sources of energy in the past few years. Among the several energy sources being explored, wind energy which is a form of solar energy, become a significant energy source. If the efficiency of a windmill can be increased, then the dependency on expensive, polluting power generators will be reduced. By doing this, we would be helping the environment and economy at the same time. In order to increase the efficiency, we must analyze the components. The most important components of a windmill are windmill blades.windmill blades before instalation
Factors affecting the performance of windmill blades:
Shape of Blade

This is important because if an optimum blade shape is discovered, then the overall productivity of a windmill can be increased. The shape of windmill blades highly influences their rotation. Few different shapes of windmill blades available are: the flat, rectangular ones, the wing shaped ones and the ones with edges tapered to a thin line or edges rounded similar to the ones of an airplane wing. In the case of the horizontal axis windmills, the wing shape has proved to be the most efficient one. The vertical axis windmills works very well with all the shapes of the windmill’s blades, however the best performance is given by the flat ones as well as the ones with both edges rounded.
Material for Windmill Blades:

Decades ago wood was used to make the windmill blades. Nowadays wood is still used but the design is however different and a lighter wood material is used, to carve the blade and faster speed rotation.
The latest blade design is made of fiber-glass and epoxy resin. Although this blade is in its first stages and not yet marketed, what makes it unique is its curvature like tip which allows it to catch low wind speeds.

However some turbine blades have even been made with the raw material made for PVC piping which have been found to break in strong lengthy winds, but are inexpensive to replace.

The turbine blades made of carbon fiber are light weight, and has a razor sharp edge which allows it to literally cut through the wind and makes it almost silent. This material is preferred and is used in most wind turbine machines sold today.
Length of windmill blades

The length is also an important factor. If a blade is longer, it covers a larger surface while it rotates, hence they can catch more wind with every rotation. This may show the way to more torque. These blades are growing longer from the 30 to 40 meters up to 60 meters.
Number of Blades:

The most important thing to take into consideration when talking about windmill blades is their number. The old ones normally had 4 very heavy blades. Majority of them were use in grain grinding. After that three blades became the most popular. More blades seemed to raise noise and slow down the rotation of the turbine’s propeller formation. After more research was done the perception of using more than three blades was no longer a factor. Currently, the most recent windmills are equipped with two or three blades.
Surface Treatment

This is important since if an optimum surface treatment can be determined, the blades would not only be protected from the elements but also be more productive.
Tip Speed Ratio

The tip speed ratio is very important. The ratio of the speed of the wind and the speed of the blade tips is called Tip Speed Ratio. High efficiency 3-blade-turbines have tip speed of 6 to 7. The tip speed ratio is directly proportional to the windmill’s productivity.

If the efficiency of a wind turbine is improved, then more power can be generated thus decreasing the need for costly power generators that cause pollution. This would also lessen the cost of power for the common people. The wind is factually there for the taking and doesn’t cost any money. Power can be produced and stored by a wind turbine with little or no pollution. If the efficiency of the common wind turbine is enhanced, the common people can cut back on their power costs enormously. Wind energy contributes very little pollution, toxic by-products or greenhouse gasses; it is still a sufficient supplement for non-renewable fuels

Facts

   1. Wind energy is one of the lowest-priced renewable sources of energy.
   2. Wind energy is the fastest growing section of all renewable energy sources.
   3. Wind energy is incredibly exploited in Germany where Germany leads the way with 8750 MW of electrical energy produced from wind energy.[#1]
   4. Wind energy is more used in Europe than in America, because of favorable climate conditions and because of USA traditional relying on fossil fuels.[#1]
   5. Wind energy is mainly transformed form of the Sun’s energy. Wind energy is airflows created by different air temperatures in different locations. The sun does all these Earthly dynamics. That’s why wind energy is solar energy.
   6. If properly developed, wind power could successfully reduce carbon emissions in the US by at least one third every year and help realize a global carbon dioxide reduction of 4% yearly.
   7. In 2006, seven wind turbines off the coast of Dublin, Ireland, represent the largest wind turbines in the world with a capacity of 3.6 MW each. By 2009, the largest wind farm is positioned in the US. It is the Florida Power and Light’s Horse Hollow Wind Energy Center, in Taylor County, Texas. It has 421 wind turbines having a capacity of 735 megawatts.
   8. In 2005 wind energy generated less than 1% of global energy and generated 58,982 MW worldwide. By end 2008 it measured 27,051 MW. 8% of Europe electricity is derived from wind, well ahead of coal and natural gas. The US is the country with the largest wind energy capacity and China exceeded its 2010 target of 10,000 to reach 12,200 MW.[#3]
   9. At the 2004 Wind Energy expo in Hamburg, Germany Danish company LM Glasfiber aired out the world’s longest windmill blade. The blade was 61.5 meters long and weighed in at just less than 18 tons, which as far as 61.5 meter windmill blades go, is extremely light. When three blades are positioned on a windmill they have a rotary diameter of 126 meters. [#4]
  10. Wind energy theory was discovered in 1919 by the German physicist Albert Betz and published in his book “Wind-Energie”.[#1]
  11. In 1995, the cost of electricity generated from gas and coal was between 3 and 4 US cents per kilowatt-hour, nuclear power cost 10 to 14 cents, wind power was 5 to 7 cents and solar photovoltaic power was 25 to 40 cents. But the price gap between non-renewable and renewable energy is closing. By 2030, wind, solar and biomass power may cost less than fossil or nuclear fuels.[#2]

Read more

Anemometers

Anemometer
A hemispherical cup anemometer of the type
invented in 1846 by John Thomas Romney
Robinson
Cup-type anemometer with vertical axis, a sensor
on a remote meteorological station deployed on
Skagit Bay, Washington July–August, 2009.
An anemometer is a device for measuring wind speed, and is a
common weather station instrument. The term is derived from the
Greek word anemos, meaning wind. The first known description of an
anemometer was given by Leon Battista Alberti around 1450.[1] They
are also very easy to make as a project.
Anemometers can be divided into two classes: those that measure the
wind's speed, and those that measure the wind's pressure; but as there
is a close connection between the pressure and the speed, an
anemometer designed for one will give information about both.
Velocity anemometers                                    
Cup anemometers
A simple type of anemometer, invented (1846) by Dr. John Thomas
Romney Robinson, of Armagh Observatory. It consisted of four
hemispherical cups each mounted on one end of four horizontal arms,
which in turn were mounted at equal angles to each other on a vertical
shaft. The air flow past the cups in any horizontal direction turned the
cups in a manner that was proportional to the wind speed. Therefore,
counting the turns of the cups over a set time period produced the
average wind speed for a wide range of speeds. On an anemometer
with four cups it is easy to see that since the cups are arranged
symmetrically on the end of the arms, the wind always has the hollow
of one cup presented to it and is blowing on the back of the cup on the
opposite end of the cross.
When Robinson first designed his anemometer, he asserted that the
cups moved one-third of the speed of the wind, unaffected by the cup
size or arm length. This was apparently confirmed by some early
independent experiments, but it was incorrect. Instead, the ratio of the
speed of the wind and that of the cups, the anemometer factor, depends
on the dimensions of the cups and arms, and may have a value between two and a little over three. Every experiment
involving an anemometer had to be repeated.
The three cup anemometer developed by the Canadian John Patterson in 1926 and subsequent cup improvements by
Brevoort & Joiner of the USA in 1935 led to a cupwheel design which was linear and had an error of less than 3% up
to 60 mph (97 km/h). Patterson found that each cup produced maximum torque when it was at 45 degrees to the
wind flow. The three cup anemometer also had a more constant torque and responded more quickly to gusts than the
four cup anemometer.
The three cup anemometer was further modified by the Australian Derek Weston in 1991 to measure both wind
direction and wind speed. Weston added a tag to one cup, which causes the cupwheel speed to increase and decrease
as the tag moves alternately with and against the wind. Wind direction is calculated from these cyclical changes in
cupwheel speed, while wind speed is as usual determined from the average cupwheel speed.
Three cup anemometers are currently used as the industry standard for wind resource assessment studies.
Anemometer 2
A windmill style of anemometer
Windmill anemometers
The other forms of mechanical velocity anemometer may be described
as belonging to the windmill type or propeller anemometer. In the
Robinson anemometer the axis of rotation is vertical, but with this
subdivision the axis of rotation must be parallel to the direction of the
wind and therefore horizontal. Furthermore, since the wind varies in
direction and the axis has to follow its changes, a wind vane or some
other contrivance to fulfil the same purpose must be employed. An
aerovane combines a propeller and a tail on the same axis to obtain
accurate and precise wind speed and direction measurements from the same instrument. In cases where the direction
of the air motion is always the same, as in the ventilating shafts of mines and buildings for instance, wind vanes,
known as air meters are employed, and give most satisfactory results.
Hot-wire anemometers
Hot-wire sensor
Hot wire anemometers use a very fine wire (on the order of several
micrometres) electrically heated up to some temperature above the
ambient. Air flowing past the wire has a cooling effect on the wire. As the
electrical resistance of most metals is dependent upon the temperature of
the metal (tungsten is a popular choice for hot-wires), a relationship can be
obtained between the resistance of the wire and the flow speed.[2]
Several ways of implementing this exist, and hot-wire devices can be
further classified as CCA (Constant-Current Anemometer), CVA
(Constant-Voltage Anemometer) and CTA (Constant-Temperature
Anemometer). The voltage output from these anemometers is thus the
result of some sort of circuit within the device trying to maintain the specific variable (current, voltage or
temperature) constant.
Additionally, PWM (pulse-width modulation) anemometers are also used, wherein the velocity is inferred by the
time length of a repeating pulse of current that brings the wire up to a specified resistance and then stops until a
threshold "floor" is reached, at which time the pulse is sent again.
Hot-wire anemometers, while extremely delicate, have extremely high frequency-response and fine spatial resolution
compared to other measurement methods, and as such are almost universally employed for the detailed study of
turbulent flows, or any flow in which rapid velocity fluctuations are of interest.
Anemometer 3
Laser Doppler anemometers
Drawing of a laser anemometer. The laser is emitted (1) through the front lens (6) of the
anemometer and is backscattered off the air molecules (7). The backscattered radiation
(dots) re-enter the device and are reflected and directed into a detector (12).
Laser Doppler anemometers use a
beam of light from a laser that is
divided into two beams, with one
propagated out of the anemometer.
Particulates (or deliberately introduced
seed material) flowing along with air
molecules near where the beam exits
reflect, or backscatter, the light back
into a detector, where it is measured
relative to the original laser beam.
When the particles are in great motion,
they produce a Doppler shift for
measuring wind speed in the laser
light, which is used to calculate the
speed of the particles, and therefore the air around the anemometer.[3]
Sonic anemometers
3D ultrasonic anemometer
Sonic anemometers, first developed in the 1970s, use ultrasonic sound
waves to measure wind velocity. They measure wind speed based on
the time of flight of sonic pulses between pairs of transducers.
Measurements from pairs of transducers can be combined to yield a
measurement of velocity in 1-, 2-, or 3-dimensional flow. The spatial
resolution is given by the path length between transducers, which is
typically 10 to 20 cm. Sonic anemometers can take measurements with
very fine temporal resolution, 20 Hz or better, which makes them well
suited for turbulence measurements. The lack of moving parts makes
them appropriate for long term use in exposed automated weather
stations and weather buoys where the accuracy and reliability of
traditional cup-and-vane anemometers is adversely affected by salty air
or large amounts of dust. Their main disadvantage is the distortion of
the flow itself by the structure supporting the transducers, which
requires a correction based upon wind tunnel measurements to
minimize the effect. An international standard for this process, ISO
16622 Meteorology—Sonic anemometers/thermometers—Acceptance
test methods for mean wind measurements is in general circulation. Another disadvantage is lower accuracy due to
precipitation, where rain drops may vary the speed of sound.
Since the speed of sound varies with temperature, and is virtually stable with pressure change, sonic anomometers
are also used as thermometers.
Two-dimensional (wind speed and wind direction) sonic anemometers are used in applications such as weather
stations, ship navigation, wind turbines, aviation and weather buoys.
Anemometer 4
Ping-pong ball anemometers
A common anemometer for basic use is constructed from a ping-pong ball attached to a string. When the wind blows
horizontally, it presses on and moves the ball; because ping-pong balls are very lightweight, they move easily in light
winds. Measuring the angle between the string-ball apparatus and the line normal to the ground gives an estimate of
the wind speed.
This type of anemometer is mostly used for middle-school level instruction which most students make themselves,
but a similar device was also flown on Phoenix Mars Lander .
Pressure anemometers
The first designs of anemometers which measure the pressure were divided into plate and tube classes.
Plate anemometers
These are the earliest anemometers and are simply a flat plate suspended from the top so that the wind deflects the
plate. In 1450, the Italian art architect Leon Battista Alberti invented the first mechanical anemometer; in 1664 it was
re-invented by Robert Hooke (who is often mistakenly considered the inventor of the first anemometer). Later
versions of this form consisted of a flat plate, either square or circular, which is kept normal to the wind by a wind
vane. The pressure of the wind on its face is balanced by a spring. The compression of the spring determines the
actual force which the wind is exerting on the plate, and this is either read off on a suitable gauge, or on a recorder.
Instruments of this kind do not respond to light winds, are inaccurate for high wind readings, and are slow at
responding to variable winds. Plate anemometers have been used to trigger high wind alarms on bridges.
Tube anemometers
Helicoid propeller anemometer incorporating a
wind vane for orientation.
James Lind's anemometer of 1775 consisted simply of a glass U tube
containing liquid, a manometer, with one end bent in a horizontal
direction to face the wind and the other vertical end remains parallel to
the wind flow. Though the Lind was not the first it was the most
practical and best known anemometer of this type. If the wind blows
into the mouth of a tube it causes an increase of pressure on one side of
the manometer. The wind over the open end of a vertical tube causes
little change in pressure on the other side of the manometer. The
resulting liquid change in the U tube is an indication of the wind speed.
Small departures from the true direction of the wind causes large
variations in the magnitude.
The highly successful metal pressure tube anemometer of William
Henry Dines in 1892 utilized the same pressure difference between the
open mouth of a straight tube facing the wind and a ring of small holes
in a vertical tube which is closed at the upper end. Both are mounted at
the same height. The pressure differences on which the action depends
are very small, and special means are required to register them. The
recorder consists of a float in a sealed chamber partially filled with water. The pipe from the straight tube is
connected to the top of the sealed chamber and the pipe from the small tubes is directed into the bottom inside the
float. Since the pressure difference determines the vertical position of the float this is a measure of the wind speed.
The great advantage of the tube anemometer lies in the fact that the exposed part can be mounted on a high pole, and
requires no oiling or attention for years; and the registering part can be placed in any convenient position. Two
connecting tubes are required. It might appear at first sight as though one connection would serve, but the differences  in pressure on which these instruments depend are so minute, that the pressure of the air in the room where the
recording part is placed has to be considered. Thus if the instrument depends on the pressure or suction effect alone,
and this pressure or suction is measured against the air pressure in an ordinary room, in which the doors and
windows are carefully closed and a newspaper is then burnt up the chimney, an effect may be produced equal to a
wind of 10 mi/h (16 km/h); and the opening of a window in rough weather, or the opening of a door, may entirely
alter the registration.
While the Dines anemometer had an error of only 1% at 10 mph (16 km/h) it did not respond very well to low winds
due to the poor response of the flat plate vane required to turn the head into the wind. In 1918 an aerodynamic vane
with eight times the torque of the flat plate overcame this problem.
Effect of density on measurements
In the tube anemometer the pressure is measured, although the scale is usually graduated as a velocity scale. In cases
where the density of the air is significantly different from the calibration value (as on a high mountain, or with an
exceptionally low barometer) an allowance must be made. Approximately 1½% should be added to the velocity
recorded by a tube anemometer for each 1000 ft (5% for each kilometer) above sea-level.
Notes
[1] Invention of the Meteorological Instruments, W.E. Knowles Middleton, Johns Hopkins Press, Baltimore, 1969
[2] "Hot-wire Anemometer explanation" (http:/ / www. efunda. com/ designstandards/ sensors/ hot_wires/ hot_wires_intro. cfm). eFunda. .
Retrieved September 18, 2006.
[3] Iten, Paul D. (June 29, 1976). "Laser doppler anemometer" (http:/ / patft. uspto. gov/ netacgi/ nph-Parser?patentnumber=3966324). United
States Patent and Trademark Office. . Retrieved September 18, 2006.
References
• Dines, William Henry. Anemometer. 1911 Encyclopædia Britannica.
• Meteorological Instruments, W.E. Knowles Middleton and Athelstan F. Spilhaus, Third Edition revised,
University of Toronto Press, Toronto, 1953
• Invention of the Meteorological Instruments, W.E. Knowles Middleton, The Johns Hopkins Press, Baltimore,
1969

Read more

The Parts of a Windmill

The Parts of a Windmill

    * Anemometer. This is essentially a speedometer for the windmill - a gauge to tell how fast the blades are rotating. Some more sophisticated systems have a feedback loop so the anemometer will "talk" to the control computer, regulating the blades' speed so damage won't occur.
    * Blades. Most turbines have at least two blades, similar to the halves of a airplane's propeller. Many windmills have more than two blades. Wind impacts these blades, causing "lift," making the windmill rotate.
    * Brake. More expensive windmills have a brake. The brake can be applied to stop the windmill in an emergency.
    * Controller. Again, more expensive and sophisticated wind turbines (proper name for a windmill that generates electrical power) have a computerized controller. One function of the controller is to start and stop the windmill. Normally, winds less than 8 miles per hour don't generate any electricity, and winds greater that 55 mph can damage to the windmill. The computer lets the windmill operate between these velocities.
    * Gear Box. Some larger windmills have gearboxes, usually to increase the shaft speed from 60 rpm to a maximum of 1800 revolutions per minute. As these gearboxes can cost a lot, and can have high rates of failure, windmill engineers are always looking for ways to go "gearless" - meaning direct-drive. That saves money and makes the windmills much simpler.
    * Generator. This is the part that actually makes the electricity. Similar to the alternator (or generator) in your car, the generator makes the electrical power.
    * Nacelle. This is a fancy French name for the cover that protects the generator and gearbox (if included).
    * Pitch. Pitch is the "tilt" of the blades, or the angle that the blades are at to "capture" the wind so the generator will rotate. Too much pitch can make the generator turn too fast and damage the windmill.
    * Rotor. The rotor is attached to the generator shaft and also holds the blades and hub.
    * Tower. The tower can be made of wood on the smaller windmills, and is usually made of steel or aluminum on the larger units. This is what holds the windmill up into the airflow.
    * Wind Vane. This is the "tail" that helps keep the windmill pointed into the wind. Very important for maximum power generation.

Read more

Basic AC Generator

Basic AC Generator

The basic components and operation of an AC (Alternating Current) generator are shown here. Its operation applies the principle of electromagnetic induction as previously explained.

In this case the moving permanent magnet is the armature and the stationary non-permanent magnet is the stator.

In the graph the red curve indicates strength if the field induced by the stator. Note how the induced field strength changes in both magnitude and polarity as the armature magnet rotates. This is illustrated by the changing size of the N and S.

The blue curve indicates the output voltage which is proportional to the rate of change of the field strength.

Note how the output voltage is related to the rotation of the armature magnet. As either pole of the armature magnet swings nearest a pole of the stator (points A/E and C) the rate of change of the strength of the induced magnetic field in the stator is smallest and the resulting output voltage is passing through zero.   On the other hand, when the swinging armature magnet is at right angles to the poles of the stator (points C and D), the induced flux is changing most rapidly and the voltage across the coil is at its highest value (positive or negative).

As the armature completes one revolution after another, the two curves on the graph repeat themselves. The form of these curves in known as a sine curve (or sine wave). One complete cycle of the sine curve relates to one revolution of the armature or 360 degrees of rotation. We can see that the voltage curve is a quarter of a cycle behind the field strength curve. In other words, the two curves are out of phase by 90 degrees.

AC generators with permanent magnet armatures are generally small such as bicycle generators (in the pre-LED era). Large AC generators, such as those used for power generation, do not have permanent magnet armatures. They have an electromagnet powered by a small DC generator (called an exciter) usually located on the drive shaft.

Electromagnetic Induction
Moving Magnet
Field Strength Curve Detail
The animated illustration above demonstrates the principles of electromagnetic induction on which electric generators are based.
The moving magnet is a permanent magnet.
The stationary magnet is a non-permanent magnet made of soft iron. It becomes magnetized only when immersed in the magnetic field that surrounds the passing permanent magnet.  The strength of this induced magnetic field rises from a relatively low value to a maximum density and then falls back to the low value again. This is indicated by the N and S that temporarily appear while the moving permanent magnet is at its nearest.
In the graph the red curve shows the strength of the induced magnetic field. The blue curve shows the electric potential (voltage) that appears across the coil. This voltage is proportional to the rate of change of the induced field strength. The generation of an electric potential by an induced and changing magnetic field is known as electromagnetic induction.
Consider the strength of the induced magnetic field as the moving permanent magnet passes from point A to point E. The field strength first rises from nearly zero slowly at first. But, as point B is reached, the field strength rises at a faster and faster rate. This rate of increase is highest at point B as is shown by the slope of the tangent line b. This is where the output voltage is at its maximum positive value.
From point B to C the rate of increase if the induced magnetic field decreases until it reaches zero at point C. The slope of the tangent line, c, at point C is zero. The corresponding output voltage is zero at point C also.
From point C to point E the situation is reversed - the field strength falls back to nearly zero at point E. Its maximum rate of fall is at point D where the voltage is at its maximum negative value as indicated by the negative slope of the tangent line d.
Note that the magnetic field strength curve (red curve) as shown here is generalised - the actual shape will depend on: (1) the amount of separation between the moving permanent magnet and the stationary soft iron core magnet, (2) the shape and size of both magnets and (3) the rate of travel of the moving magnet.

Read more

Basic Generator Theory

Basic Generator Theory

The generator creates electricity by a series of fine wire windings inside a magnetic field, called an armature. As the armature is spun inside this magnetic field by the generator's motor, current and voltage gets generated in those windings of wire and electricity is transferred. That current and voltage will be directly proportional to the speed that the armature spins and to the strength of the magnetic field.  Each complete revolution, one complete cycle of alternating current (AC) is developed. This is called a rotating armature.
In a stationary armature,  the magnetic field rotates around the armature. The advantage of having a stationary armature winding is that the generated voltage can be connected directly to the load.
The frequency of the generated voltage is dependent on the number of field poles and the speed at which the generator is operated. Frequency, measured in Hertz (Hz), is the number of complete cycles per second in alternating current direction. As current flows through the armature, there is some amount of resistance and inductive reactance. The combination of these make up what is known as the internal resistance.. When the load current flows, a voltage drop is developed. When a Direct Current (DC) voltage is applied to the field windings of a dc generator, current flows through the windings and sets up a steady magnetic field. This is called Field Excitation.
An exciter is part of the generator package supplying direct current to the alternator field windings to magnetize the rotating poles. The exciter output may be controlled by a voltage regulator. Types of exciters include brush type with rotating commutator, static excitation or brush less generator and exciter. A regulator is an important option to consider if there are frequency or voltage sensitive equipment such as computers.

Read more

History of Windmills

History of Windmills

--------------------------------------------------------------------------------
Introduction
Since ancient times, man has harnessed the power of the wind to provide motive power for transportation. Likewise, the technique of grinding grain between stones to produce flour is similarly ancient, and widespread. Quite where and when these two came together in the first windmill is unknown, but a likely scenario suggests a Persian origin, from where (tradition has it) the knowledge spread back into Northern Europe as a result of the Crusades. However, since the Persian mills were quite unlike the early European designs it seems just as likely that the adaptation of wind as a power source was independently discovered in Europe, albeit at a later date. (Of course wind was not the first non-human power source applied to the task of grinding corn - it was preceeded by both animal power, and in all probability by water power).
European millwrights became highly skilled craftsmen, developing the technology tremendously, and as Europeans set off colonizing the rest of the globe, windmills spread throughout the world.

The pinnacles of windmill design include those built by the British, who developed many advanced "automatic control" mechanisms over the centuries, and the Dutch (who used windmills extensively to pump water and for industrial uses, as well as to grind grain).
As steam power developed, the uncertain power of the wind became less and less economic, and we are left today with a tiny fraction of the elegant structures that once extracted power from the wind. These remaining windmills, scattered throughout the world, are a historic, and certainly very photogenic, reminder of a past technological age. A number of mills have been restored, either visually, or in some cases back to full working order, where the trend for organic and non-manufactured foodstuffs has shifted the economics slightly back in their favour once again.
However the promise of widespread power from the wind lives on, both in the form of wind turbines producing electricity, and in the form of small scale windpumps (often largely low-tech "appropriate technology" installations) still used extensively in world agriculture.

--------------------------------------------------------------------------------

English WindmillsEvidence of windmills in England dates from the 12th century, with earlier references to "mills" (such as in the 11th century Doomsday Book) generally held to be talking about either animal or water powered mills.
The 14th and 15th centuries provide evidence of what the early mills looked like, with illustrations occuring in diverse media such as memorial brasses, stained glass, and wood carvings, as well as the expected manuscript records.

These early illustrations all show the simple, all wooden, post mill structure.
Post Mills
Post mills are so named because of the large upright post on which the mill's main structure (the "body" or "buck") is balanced. By mounting the body this way, the mill is able to rotate to face the (variable) wind direction.
To maintain the upright post, a structure consisting of horizontal crosstrees, and angled quarterbars is used. By far the most common arrangement was 2 cross bars at right angles to each other under the base of the post, together with 4 quarterbars. Occasionally however other arrangements did occur, such as 3 crosstrees, and consequently 6 quarterbars.
Initially the crosstrees would have rested directly on the ground, (or indeed were buried in the ground for extra stability) but since this makes them very succeptible to rotting, the crosstrees were soon being placed on brick piers, to raise them off the ground.
The body of the mill housed all the milling machinery - a large brake wheel on the same shaft as the sails (the "windshaft") transferred power to a smaller gear at right angles to it, called the wallower. The wallower shared a vertical shaft with the great spur wheel, and from this smaller wheel a "stone nut" was used to drive the millstone. As larger mill bodies were constructed, additional pairs of stones could be driven, by taking further power taps, each using an extra "stone nut" off the great spur wheel. In order to apply some level of control to the mill, the brake wheel could be slowed using a large wooden friction brake around its outer edge.
As already mentioned, the whole body rotated on the central post, in order to face the wind. To allow this to happen, a tailpole or tiller beam extended from the rear of the body. By pushing on this beam (or by using some form of winch or animal power) the miller rotated his mill. The tailpole also provides a useful attachment point for a ladder to provide access to the mill.
An obvious improvement on the early post mill, is to build a roundhouse up around the crosstrees and quarterbar structure. This makes this structure a lot more protected from the weather, and provides additional storage space.
Smock Mills
Smock mills (named after the dress like agricultural costume whose shape they vaguely resemble) are a fundamental improvement over the post mill design.
Instead of rotating the whole body of the mill to face the mill, the smock mill design consists of a fixed wooden body, holding the milling machinery, together with a rotatable cap, which holds just the roof, the sails, the windshaft and the brake wheel.
By rotating just the mill cap, the body of the mill can be made much larger than in a post mill, and hence able to house more pairs of stones, and more ancilliary machinery. In addition, the body can be made arbitrarily high, the extra height allowing the sails to catch more wind (and to a certain extent a taller body can allow longer sails to be employed, to the same end).
Smock mill bodies are theoretically roughly circular, though the use of straight timber means that most are actually eight sided. Other numbers of sides occur, including in England examples of six through to twelve sides. (In addition there are a number of small smock mills throughout the world which have square bodies). Many English smock mills are constructed above a more substantial brick built base, which may range from a few courses, up to several stories high.

Tower MillsTower mills take the smock mill design even further, by replacing the wooden body with a brick or stone built tower. Since the necessity of having straight sides (due to straight timber) is removed, true circular bodies are the common arrangement. However straight sided structures do occur, and even towers that start off with straight sides at the base, but change to a circular plan part way up (perhaps due to building the tower on what was originally a smock mill base).
By using brick or stone for the body, tower mills can be built even larger and taller than smock mills, and by being a more durable building material, the mills are more weatherproof, and more fireproof.

.

Read more