Alternative Energy – Finals/ 5/4/2006 17:08 Page 305
Wind Energy
INTRODUCTION: WHAT IS WIND ENERGY?
The word ‘‘windmill’’ for many people brings to mind the
Netherlands, whose countryside for centuries has been dotted with
thousands of windmills. Windmills represent an early technical
skill or ingenuity (inventiveness) that seemed to be lost during
the industrial revolution, when fossil fuels replaced wind and
running water as the most widely used energy sources. Some
people of the twenty-first century support a return to greater
reliance on the wind that powers windmills, chiefly because wind
power is clean and endlessly renewable.
Historical overview
The first written record of a windmill is in a Hindu book from
about 400 BCE (before the common era). About four hundred years
later, the Greek inventor Hero of Alexandria devised a wind-driven
motor he used to provide air pressure to operate an organ. From
about 400 CE (common era), there are references to prayer wheels
driven by wind and water in the Buddhist countries of central Asia.
These devices were handheld windmills that contained prayers and
religious texts on rolls of thin paper wound around an axle. Individuals could access the prayers whenever they wanted (the thought was
increasing the speed of the spinning prayer wheels strengthened the
prayers). Early devices used the power of the wind, but it was not
until much later that wind power was developed as a way to do work.
Some historians believe that the earliest true windmills—that is,
windmills built to do work—were built in China two thousand
years ago, but no records exist. The first recorded references to
true windmills date from seventh-century Persia, later called Iran,
particularly the province of Sijistan, which became Afghanistan.
Alternative Energy
305
Alternative Energy – Finals/ 5/4/2006 17:08 Page 306
WIND ENERGY
Words to Know
Anemometer
wind speed.
A device used to measure
Coriolis force The movement of air
currents to the right or left caused by
Earth’s rotation.
Drag The slowing force of the wind as it
strikes an object.
Kilowatt-hour One kilowatt of electricity
consumed over a one-hour period.
Kinetic energy The energy contained in a
mass in motion.
Lift The aerodynamic force that operates
perpendicular to the wind, owing to differ-
ences in air pressure on either side of a
turbine blade.
Nacelle The part of a wind turbine that
houses the gearbox, generator, and other
components.
Rotor The hub to which the blades of a
wind turbine are connected; sometimes
used to refer to the rotor itself and the
blades as a single unit.
Stall The loss of lift that occurs when a
wing presents too steep an angle to the
wind and low pressure along the upper
surface of the wing decreases.
Wind farm A group of wind turbines that
provides electricity for commercial uses.
During the reign of the Muslim caliph ‘Umar I (633–44), windmills
were constructed primarily to obtain water for irrigating crops and
grinding grain. These working windmills may have been imported
into China from the Middle East by Genghis Khan (1162–1227),
the Mongol conqueror of much of what is now Iran and Iraq
(1216–23). The first reference to a Chinese windmill dates from
the year 1219, when a statesman named Yehlu Chhu-Tshai documented construction of one. Windmills became widely used along
the coasts of China during this period.
The design of these seventh-century windmills, some of which
survive in Iran and Afghanistan, was the reverse of modern windmills. In modern windmills the axle is horizontal and is positioned
at the top of the windmill. In early Middle Eastern windmills the
blades that turned in the wind were enclosed in a chamber at the
bottom of the windmill. The blades were attached to a vertical axle,
which was attached to a millstone above. The early windmills,
which are still used, could grind a ton of grain per day and
generate about one-half the power of a small car.
Windmills in Europe
During the Crusades, which took place over a two-hundred-year
period beginning in 1095, European conquerors of Palestine probably
became familiar with Middle Eastern windmills and imported the
technology back to Europe. The first documented reference to a
306
Alternative Energy
Alternative Energy – Finals/ 5/4/2006 17:08 Page 307
WIND ENERGY
This windmill, seen in the
Netherlands, is typical of
what many people envision
for windmills. ª Royalty-Free/
Corbis.
European windmill dates to 1105 in France, the home of most of the
early crusaders. A similar reference is made to a windmill in England in
1180. Both of these windmills were built to pump water to drain land.
For reasons that are unknown, the Europeans mounted the windmill blade on a horizontal axle rather than a vertical one. They may
have adopted the design from water wheels, which by this time were
being mounted on horizontal axles (poles around which an object
rotates). Some of the windmills from this period were able to lift more
than 16,000 gallons (60,566 liters) of water per hour, using augers (a
type of screw) that raised the water from lower levels to higher levels,
where the water could be sent into channels. The augers acted like
spiral staircases that carried the water up as the windmills turned.
Alternative Energy
307
Alternative Energy – Finals/ 5/4/2006 17:08 Page 308
WIND ENERGY
Al-Dimashqi Describes a Windmill
In the thirteenth century, the Arab historian al-Dimashqi
(1256–1327), described a windmill:
When building mills that rotate by the wind, they proceed
as follows. They erect a high building, like a minaret, or
they take the top of a high mountain or hill or a tower of a
castle. They build one building on top of another. The upper
structure contains the mill that turns and grinds, the lower
one contains a wheel rotated by the enclosed wind. When
the lower wheel turns, the mill stone above also turns. . . .
Such mills are suitable on high castles and in regions
which have no water, but have a lively movement of the air.
These windmills were often arranged in what were called gangs,
meaning that they were arranged in rows so that water could be
drained in stages, especially from lower to higher levels.
Because much of the Netherlands is below sea level, the Dutch
made extensive use of windmills to drain land and to grind grain.
By the fourteenth century the Dutch had introduced or adopted a
number of technologies, such as post mills and tower mills. The
post mill consisted of a four-bladed mill mounted on a central
vertical post or shaft. Wooden gears transferred the power of the
shaft to a grindstone. The grindstone turned to make grain into
flour. The tower mill, which originated along the Mediterranean
seacoast in the thirteenth century, consisted of a post mill mounted
on top of a multistory tower. This tower housed the grinding
machinery and had rooms for grain storage and other milling
functions as well as living quarters in the bottom story. The tower
mill is the type most often seen in pictures of Dutch windmills.
A major concern of windmill operators was to make sure that
the mill was positioned correctly in relation to the wind. This task
was done with a large lever at the back of the windmill that was
pushed to move the windmill blades toward the wind. The blades
were made of lattice frames over which canvas sails were stretched.
By 1600, windmills were in such widespread use in Holland that
the bishop of Holland, seeing a chance to increase funds for the
church, declared an annual tax on windmill owners.
Also by that time the basic technology of windmills was in place.
It remained for engineers and inventors to find ways to increase
efficiency, primarily by coming up with new designs for windmill
blades. Some of these designs included improvements in the blade’s
308
Alternative Energy
Alternative Energy – Finals/ 5/4/2006 17:08 Page 309
WIND ENERGY
camber, or the outward curve of the blade from its leading edge (the
edge first struck by the wind) to its trailing edge. Other experiments
were conducted to find the best location for the blades spar, or the
long piece of a blade; its center of gravity; and the correct amount of
twist in the blade. One of the most prominent millwrights (mill
builders) during the period, Jan Adriaanzoon Leeghwater (1575–
1650), experimented with these matters. Largely through his efforts,
about twenty-six lakes in the Netherlands were drained.
By the end of the nineteenth century, at least 30,000 windmills
were operating in Europe. These windmills were used not only to
pump water and grind grain but also to power sawmills and for
other industrial uses, including processing agricultural products
such as spices, cocoa, dyes, paints, and tobacco.
Interior of a windmill, in
Spain, showing the wooden
gears that were powered by
the wind. ª Corbis.
Windmills in North America
In the seventeenth and eighteenth centuries, the Dutch migrated
to the American colonies in large numbers. They brought with them
the technology for constructing windmills, and many Dutch-style
windmills were built throughout New York and New England,
where they worked well in the relatively gentle eastern winds.
Alternative Energy
309