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Windmills - Windmill Farms
A windmill is an engine powered by the energy of wind to mill grain, often
contained in a large building as in traditional post mills, smock mills
and tower mills. It also refers to small tower mounted wind engines used
to pump water on farms. The modern wind power machines used for generating
electricity are more properly called wind turbines.
Common applications of windmills are grain milling, water pumping, threshing,
and saw mills. Over the ages, windmills have evolved into more sophisticated
and efficient wind-powered water pumps and electric power generators.
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.
There are a number of types of windmills. They are divided into Horizontal-Axis
and Vertical-Axis types. Low speed horizontal-axis windmills are used
for water pumping and air compressing. Southern Cros windmills are an
example. Earlier windmills such as the ones in England and Holland build
a couple hundred years ago are another
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.
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