The following was written under contract to Microsoft for inclusion in their 1999 edition of Encarta, Microsoft’s digital encyclopedia.
I. Introduction
Wind power, or more correctly wind energy, is the ability to use the power
in the winds blowing across the surface of the earth in order to perform
work, such as pumping water, grinding grain, or generating electricity.
Wind is the result of the differential solar heating of the earth’s
surface. Warm air rises over land heated by the sun and cooler air rushes
in to replace it. The air moving across the land surface is what we call
the “wind”.
Weary Egyptians may have been the first to use the energy in the wind when
they sailed up the Nile against the current. Sailing vessels powered by the
wind plied the world’s seas for centuries and were the principal form of
commercial transport until the late 19th century.
Wind energy has long been used to pump water from the lowlands of northern Europe and to grind grain. The term “windmill” derives from its use in grinding or “milling” grain. The oldest known windmills were used by
Persians in the 7th century.
Windmills first appeared in Europe during the 12th century in southern
England and northwestern France. From this cradle of technology, the
windmill spread into nearby Flanders (1190), then on into Germany (1222),
and subsequently north to Denmark (1259). By the 17th century, windmills
were such a commonplace feature of the landscape that the fictional Don
Quixote was tilting at them on the plains of La Mancha in central Spain and
the famed artist Rembrandt van Rijn was painting them in scenes typifying
the Dutch landscape.
Only by tapping the wind could Jan Leegwater, and the Dutch engineers that
followed him, drain the polders and make the Netherlands what it is today.
But the wind could and did do much more than merely pump water.
European windmills were also used to saw timber, shred tobacco, grind stone
for coloring paint, press flax seed for oil, and make paper. The 700
windmills in the Zaan district north of Amsterdam formed the core of what
would become the center of Dutch manufacturing, and launch what would later be known as the Industrial Revolution. In 1850, 90% of the power used in Dutch industry came from the wind. Only in the late 19th century did the
use of wind energy wane.
During the 17th through early 19th centuries, there were some 3,000 stately
“Dutch” or European windmills in Denmark; 8,000 in Great Britain; 9,000 in
the Netherlands; 18,000 in northern Germany; and as many as 20,000 in
France.
Wind energy boomed again in the late 1800s, this time in the New World,
with the development of the American farm windmill. By the late 19th
century, 77 firms, most in the area of Chicago, Illinois, were assembling
them in a myriad of styles. The farm windmill was ideally suited for
pumping water from relatively deep wells, and one historian has given it
credit, along with the Colt 45 revolver and the barbed wire fence, for
enabling European settlement of the American Great Plains. The farm
windmill industry peaked in the early part of the 20th century, but more
than 1 million water-pumping farm windmills are still in use worldwide, and
several firms continue to build them.
Today the wind is at work again, driving modern wind-electric turbines in
North America, Europe, and Asia.
II. Evolution of the Windmill into the Modern Wind Turbine
The early Persian windmills were simple devices using bundles of reeds tied
to paddles that moved about a vertical axis. The crude mud-brick structures
supporting the windmill directed the wind onto the paddles. Such machines
can be used only where the wind is unidirectional. They are unsuited for
places where the wind can come from any point on the compass.
Historians are unsure why Europeans, unlike their Persian counterparts,
developed windmills that spun their rotors around a horizontal axis.
European windmills typically used four “wings” or blades, though some used
five and occasionally as many as six. In early European windmills, the
entire windmill was turned to face the wind about a stout vertical post.
These short and squat “postmills” can still be seen throughout Northern
Europe. Later, the tower became fixed and the rotor was pointed into the
wind on a movable cap. This enabled the tower to reach much greater heights and sizes than possible using a single post for support. Some “tower
windmills” reached heights of three stories and literally towered over
other buildings on the landscape.
Windmill performance increased greatly between the 12th and 19th centuries
By the time the European windmill began to fall out of favor, the typical
machine used a four-blade, cloth-covered rotor spanning 25 meters (80 feet)
in diameter and capable of developing 25 to 30 kilowatts of mechanical
power. Technical innovations included automatic fan tails for pointing the
rotor into the wind, automatic movable louvers instead of cloth sails, air
brakes, and airfoil-shaped leading edges of the blades that anticipated the
wings of modern aircraft and the blades of modern wind turbines.
At their zenith, there were some 1,500 megawatts of European windmills in
use, a level not seen again until 1988. And it was only in the late 1980s
that wind turbines of equivalent size were once again plentiful.
Modern airfoils and materials enable today’s wind turbine to extract 10
times more power from the wind than the European windmill. Today a modern wind turbine similar in size to Dutch windmills, for example, can generate 250 to 300 kW of power.
There were similar improvements during the 100-year reign of the American
water-pumping windmill. Early farm windmills used a number of simple wooden slats. In the late 1800s, engineer Thomas Perry conducted the first
scientific tests on windmill designs. By using a steam-driven test stand,
Perry designed a rotor nearly twice as efficient as those then in use.
Perry took his improved windmill design to LaVerne Noyes who used it to
create the world’s most successful farm windmill, the Aermotor.
Although it was not the first to use metal blades, Aermotor’s stamped
sheet-metal “sails” revolutionized the farm windmill. Perry’s design has
worked so reliably that it is still widely used.
Today’s farm windmill, a product of Perry’s experiments more than 100 years
ago, produces about one-tenth the power of an equivalent size modern wind
turbine. This helps explain why the multi-blade farm windmill was never
successfully adapted to the generation of electricity.
During the 1930s, there was another resurgence of wind power. Interest in
using electric lighting and appliances on the part of homesteaders on the
North American Great Plains led to the development of small,
battery-charging wind turbines. These “windchargers” were the forerunners
of the small two- and three-bladed wind turbines used today for remote
telecommunication stations and off-the-grid homes in the industrial world,
and village electrification in developing countries.
The world again had its eye turned toward wind energy during the oil crises
of the 1970s. Governments in many countries launched programs to develop
modern wind turbines and to stimulate a market for wind and other forms of
renewable energy. Many of these programs failed. But some succeeded,
especially those in Denmark and California.
III. Capturing the Energy in the Wind
The energy in the wind is a cubic function of wind speed. For every
doubling of wind speed, there is a corresponding eight-fold increase in the
energy available, that is 2 X 2 X 2 = 8. As a result, wind turbines are
located in regions of the world where it is most windy.
Sites with good wind energy are often located along coastlines, on
ridgetops, and in mountain passes. Many of the wind turbines in Denmark,
Germany, and the Netherlands, for example, are installed near their long
coastlines. In contrast, most of the wind turbines in Great Britain are
installed in the mountains of Wales and the Pennine chain. Nearly all of
the thousands of wind turbines operating in California are found in three
passes: the Altamont Pass east of San Francisco, the Tehachapi Pass north
of Los Angeles, and the San Gorgonio Pass near Palm Springs.
For most continental locations, the winds are strongest during the winter
and spring and weakest during summer and fall. However, regional weather
patterns and local topographic conditions can create wind patterns
different from these. For example, the wind is strongest in the Altamont
Pass during the summer months, when temperature differences between the hot Central Valley and the cold waters of the Pacific Ocean are greatest.
Because the strength of the wind varies by time of day, season, and even
from one year to the next, wind energy is an intermittent resource: it is
sometimes unavailable. At windy sites it is common for a wind turbine to
operate 60% of the time, but it typically will not produce at full capacity
during the entire period it operates.
No power plant operates at full capacity 100% of the time. Coal-fired power
plants on average operate at 75% to 85% of their full capacity. Wind
turbines on average operate 25% to 35% of their full capacity at windy
sites.
Despite their intermittency, the use of wind turbines requires no storage
for windless days when interconnected with large electricity networks such
as those found in North America, Europe, and parts of Asia. The effect of
days with little or no wind is minimized by the geographic dispersal of
wind turbines in most countries where they are now used.
Small wind turbines used with photovoltaic panels for providing electricity
to remote sites often require the use of batteries and sometimes a backup
generator for extended periods without sufficient sun or wind.
Modern wind turbines are as reliable as conventional power plants. Most
wind turbines used for commercial power generation are available to
generate electricity more than 97% of the time. There are many modern wind
turbines in Denmark and California that have been in use since the early
1980s. Some small wind turbines at remote sites have been used for nearly
as long. On North America’s Great Plains the farm windmill has been in
continuous use at some homesteads for generations. And there are examples
of European windmills that have been in use at the same site for nearly 300
years.
IV. Modern Wind Power Technology
The modern wind turbine is the result of steady, continuous refinements of
designs introduced during the late 1970s and early 1980s. There have been
no technological breakthroughs that have radically changed the way wind
turbines work, and none are expected.
Today’s wind-electric turbine typically uses a rotor with two to three
blades spinning upwind of the tower. Most designs use three blades, though
rotors with two blades are found on some very small wind turbines and on
some giant machines and there is one manufacturer of one-bladed wind
turbines. The rotor captures the kinetic energy in the wind and converts it
into mechanical energy. The spinning rotor drives an electrical generator
either directly or indirectly through a transmission. The generator
converts the mechanical energy of the spinning rotor shaft into
electricity. Cables carry the electricity produced by the generator down
the tower.
Wind turbines can be arbitrarily divided into three size classes: small,
medium, and large. The small wind turbine category includes micro turbines.
These are wind generators so small they can be carried by hand or
transported by Mongolian nomads on horseback. Wind turbines in this
sub-class use rotors less than 1 meter (3.3 feet) in diameter and generate
from 50 watts to 500 watts. Small wind turbines include micro turbines and
wind machines up to 15 meters (50 feet) in diameter capable of generating
50 to 60 kilowatts. Small wind turbines are used primarily for remote sites
where conventional, central-station power is either too expensive or too
unreliable.
Medium-sized wind turbines use rotors spanning 15 meters (50 feet) to 60
meters (200 feet) in diameter. These turbines vary in generator capacity
from 50 kilowatts to 1,500 kilowatts. During the late 1990s most commercial
medium-sized wind turbines were in the 500-kilowatt to 750-kilowatt class.
Large wind turbines are behemoths with rotors from 60 meters (200 feet) to
100 meters (330 feet) in diameter. Machines of this size are capable of
generating 2 to 3 megawatts. As of the late 1990s there were no commercial
wind turbines of this size, though various governments have attempted to
develop machines of this size and a few examples are still standing.
Because conventional coal-fired and oil-fired power plants increase in cost
effectiveness with size, giant wind turbines were once thought to be more
economic than smaller machines. However, multi-megawatt wind turbines
proved less economic and less reliable than medium-sized turbines.
Most medium-sized wind turbines use a combination of gearbox and generator to convert the mechanical energy contained in the rotor to electricity. Some medium-sized wind turbines spin the generator directly without using a transmission.
Many small wind turbines also drive the generator directly with the rotor.
These designs eliminate the need for a gearbox or transmission.
There are two principle wind turbine configurations: Those, such as nearly
all conventional wind turbines, whose rotors spin about a horizontal axis,
and those where the rotor spins about a vertical axis.
Conventional horizontal-axis wind turbines must orient the rotor with
respect to changes in wind direction. Most small wind turbines use a tail
vane to direct the rotor into the wind. Most medium-sized wind turbines
mechanically aim the rotor into the wind with an electric motor controlled
by a signal from a wind vane mounted on the nacelle. There are a few small
wind turbines and an even smaller number of medium-sized wind turbine
designs that place the rotor downwind of the tower. These wind turbines use
aerodynamic forces on the spinning rotor to passively orient the rotor
downwind of the tower.
Vertical-axis wind turbines, such as the Darrieus or “eggbeater” design,
are omni-directional. They can accept the wind from any direction. Though
there are some Darrieus wind turbines still operating in California, none
are being manufactured today.
Most wind turbines, both large and small, use fiberglass blades, though
some use composite wood instead. No contemporary wind turbines use steel or aluminum blades. Steel is too heavy and aluminum is too prone to break
after the repeated flexures common in wind turbine blades.
Medium-sized wind turbines predominately use induction or “asynchronous”
generators. These generators are widely available and inexpensive. They
produce utility-compatible electricity directly without the need for
complex electronics.
Many medium-sized wind turbines use two induction generators: one for use
in low winds, and a second, much larger generator, for use in strong winds.
For example, a typical wind turbine using two generators will operate the
small generator in winds from four meters per second (nine miles per hour)
to seven meters per second (15 miles per hour). The wind turbine will
operate the larger generator in winds from seven meters per second (15
miles per hour) until the cut-out wind speed is reached, typically 25-30
meters per second (55-65 miles per hour), when the wind turbine
automatically turns itself off. The large generator is often four or more
times the size of the small generator.
An increasing number of medium-sized wind turbines use a single generator
with dual windings. These turbines operate in the same way as those with
dual generators. Instead of switching from a small generator to a larger
generator as wind speeds increase, the turbine switches from one set of
windings to another within the same generator. The winding for low winds
energizes only four of the six magnetic poles inside the generator. In
strong winds the turbine magnetizes all six poles to take full advantage of
the generator’s electrical capacity.
Use of either dual generators or dual windings increases both the
electrical and aerodynamic efficiency of the wind turbine over that of
designs using a single induction generator.
Some companies have introduced multi-pole, direct-drive generators for use
on wind turbines of 500 kilowatt and larger. Though the concept has long
been used in small wind turbines, only recently has it been adapted to much
larger wind turbines.
In a conventional medium-sized wind turbine, a transmission or gearbox
steps up the speed of the slowly spinning rotor, for example 30 revolutions
per minute, to the 1500 to 1800 revolutions per minute required by
induction generators. By spinning the armatures of generators at these
speeds, the generator can be constructed compactly with a minimal use of
costly materials. Because conventional generators are relatively slender,
they can be housed inside the wind turbine’s nacelle.
The new multi-pole generators are connected directly to the slowly turning
wind turbine rotor. There is no transmission. As a consequence, the rotor
of the generator spins at the same speed as the rotor of the wind turbine.
To produce the same amount of power as that of a conventional high-speed
generator, the direct-drive generator must be considerably larger in
diameter and use more magnetic poles. The diameter of these so-called “ring
generators” is typically 10% of that of the wind turbine’s rotor. For
example, a 500-kilowatt wind turbine using a rotor 40 meters (130 feet) in
diameter will use a ring generator nearly 4 meters (13 feet) in diameter.
Due to its sheer size, it is difficult to shelter a ring generator within a
streamlined nacelle. This gives direct-drive wind turbines a characteristic
shape visible for some distance.
Designers of multi-pole, direct-drive generators believe that this approach
enables the wind turbine to capture more energy in low winds than
conventional wind turbines driving induction generators through a step-up
transmission. In high winds, they believe this design also enables the wind
turbine to better absorb gusts than conventional designs. The ability to
absorb gusts may give increase the turbine’s lifespan. However, these
advantages are offset by the sophisticated electronics necessary to convert
the multi-pole generator’s output to utility-compatible electricity.
The smallest of the small wind turbines, those from 50 watts to 1,000
watts, are installed on simple guyed poles 10 meters (33 feet) to 20 meters
(66 feet) tall. Larger small wind turbines, those from 1 kilowatt to 30
kilowatts are installed on towers of guyed tubes or lattice masts and on
free-standing lattice or tubular towers from 20 meters (66 feet) to 40
meters (130 feet) in height. Medium-sized wind turbines are today
predominately installed on tubular steel towers 25 meters (80 feet) to 50
meters (160 feet) tall, though there are several thousand older wind
turbines installed in California and India on lattice towers.
V. Wind Farms and Wind Power Plants
Wind turbines can be used singly, in clusters, and in wind farms. Clusters
are small groups containing two to ten wind turbines. Wind farms or, more
correctly, wind power plants, can contain any number of wind turbines. One
wind power plant in California uses more than 1,000 wind turbines and
places such as California’s Tehachapi Pass contain several wind farms of
similar size.
Wind farms take different shapes in different regions. On flat terrain,
wind power plants often comprise rectilinear arrays of wind turbines
standing in formation like a phalanx of marching soldiers. One of the
world’s most visually pleasing wind farms is Taendpibe-Velling Maersk on
the west coast of Denmark’s Jutland peninsula.
In hilly or mountainous terrain transverse to the prevailing winds, such as
in California’s Altamont Pass, designers lace the ridges with long lines of
wind turbines.
Wind turbines can also be placed in a long single row on other linear
features of the landscape such as dikes or breakwaters. Linear arrays can
be found throughout the Netherlands where the wind turbines are placed
parallel to the country’s many dikes and drainage canals. Wind turbines
have also been successfully installed in lines on harbor breakwaters at
Ebeltoft in Denmark, Zeebrugge in Belgium, and Blyth Harbor in England.
Wind plants are similar to conventional power plants. They are simply an
assembly of multiple independent generators. Though the wind turbines in a
wind plant are often connected to a central monitoring system, each wind
turbine operates independently. In either a cluster or a wind farm, the
generated electricity is aggregated, whether from two turbines or two
thousand, and delivered to the utility network.
The world’s largest arrays of wind turbines are found in California. In the
Altamont Pass there are more than 6,000 wind turbines in several different
wind plants. In Southern California, the Tehachapi Pass contains nearly
5,000 wind turbines in several wind farms, and wind plants near Palm
Springs contain some 3,000 wind turbines.
Wind power plants are thought to generate electricity more economically
than single wind turbines or small clusters. Proponents argue that it is
more cost-effective to operate and maintain hundreds of wind turbines in
one location, rather than dispersed turbines. However, concentrating wind
turbines, especially in giant wind plants of thousands of wind turbines,
can reduce overall productivity, as one wind turbine can interfere with the
next. These large arrays may also increase the visual intrusion of the wind
turbines on the landscape and increase impacts on wildlife.
Unlike California, the success of wind energy in Denmark and Germany is not
the result of large wind farms. By the late 1990s Denmark and Germany
combined had installed one-third more wind generating capacity than the
entire United States, and in both countries two-thirds of this capacity was
installed as single wind turbines or in small clusters.
California’s wind power plants provide 1% of the state’s electricity. Wind
turbines in the German state of Schleswig-Holstein produce 10% of total
electricity consumption. Wind turbines delivered 6% of Denmark’s
electricity during the late 1990s and by the turn of the century will
produce 10%. On the west coast of Denmark, the municipality of Ringkobing
meets nearly 40% of its electricity with nearby wind turbines. [Editor.
Please note that the “o” in Ringkobing is the Danish symbol for “oe” in
English, that is, o with a slash.] And the Danish municipality of Sydthy in
the far northwest corner of Jutland, is a net exporter of wind-generated
electricity (115% of total consumption) and nearly all of the of the more
than 130 wind turbines were installed by individuals and cooperatives as
single turbines.
VI. The Future of Wind Energy
During the late 1990s, nearly 1,500 megawatts of new wind-generating
capacity were being installed per year, most of this in Europe and Asia. By
the year 2000 there will be some 40,000 wind turbines representing 12,000
megawatts of generating capacity installed worldwide. These wind turbines
will produce 20 terawatt-hours (20 billion kilowatt-hours) annually, an
amount of electricity equivalent to that generated by three to four large
nuclear power plants.
Concern about the emission of global warming gases and other forms of air
pollution from fossil-fueled power plants and the perceived hazards of
nuclear power will spur continued growth of wind energy in the years ahead.
Though wind energy is relatively benign, the installation and operation of
wind turbines does cause some environmental impacts. The most serious is
the potential visual intrusion of wind turbines on the landscape. But there
has also been complaints about noise from some wind turbines in California
and in Europe. Wind turbines are also known to kill some birds, including
some protected species. There is also concern that improper development on
steep slopes in California have led to increased soil erosion. Most of
these impacts can be avoided by sensitive siting and careful design of both
wind turbines and wind power plants.
Research will continue to increase the cost-effectiveness of wind energy by
improving the aerodynamic performance of wind turbine blades and such
mundane tasks as increasing the conversion efficiency of gearboxes and
generators. The focus of research at laboratories in North America, Europe,
and Asia is expected to shift towards how best to integrate wind turbines
with the communities where they will be placed, by reducing the aerodynamic and mechanical noise of wind turbines and by reducing wind energy’s impact on wildlife, especially birds, through more careful siting.
Though California once led the world in developing wind energy,
inconsistent energy policies at the federal and state governments have sent
conflicting signals to the North American wind industry that have led to
its steadily dwindling worldwide role. Though a burst of new wind turbine
installations are expected in California and the Upper Midwest from 1998
through 1999, the absence of a stable market suggests that the growth of
wind energy in North America will continue to significantly lag behind
Europe into the new millennia. However, commitments by the United States
and Canada to reduce global warming gases could spur expanded development of wind energy in North America once again.
The environmental benefits of wind energy have led several nations to set
ambitious targets for the use of wind energy. Wind energy is booming Europe most notably in Germany and Denmark. But even France, which generates 80% of its electricity with nuclear power, has launched an aggressive wind energy program as has Spain, Greece, Italy, Ireland and Great Britain. In the North German state of Schleswig-Holstein, wind turbines generate 10% of the region’s electricity and will reach the state’s goal of 20% shortly after the year 2000, nearly a decade ahead of schedule. Denmark, a nation of 5 million, produces 6% of its electricity with wind energy and will
generate 50% by the year 2030.
Wind energy has again come of age.
References:
Baker, T. Lindsay. “A Field Guide to American Windmills” (Norman:
University of Oklahoma Press, 1985) 528 pp, ISBN 0-806119012.
Gipe, Paul. “Wind Power for Home & Business” (White River Junction, VT:
Chelsea Green Publishing, June 1993) 414 pp, index, ISBN 0-930031-64-4.
Gipe, Paul. “Wind Energy Comes of Age” (New York: John Wiley & Sons, May
1995) 536 pp, notes, index, ISBN 0-471-10924-X.
Hills, R.L. “Power from the Wind: A History of Windmill Technology”
(Cambridge University Press, 1996) 334 pp, ISBN 0-52-141398-2 or ISBN
0-521-56686-X.
Robert Righter. “Wind Energy in America: A History” (Norman: University of
Oklahoma Press, 1996) 361 pp, notes, bibliographic note, index, ISBN
0-8061-2812-7.
Spera, David, ed. “Wind Turbine Technology: Fundamentals
Concepts of Wind Turbine Engineering” (New York: American Society of
Mechanical Engineers, 1994) 650 pp, 0-791812057.
Walker, John, and Jenkins, Nicholas. “Wind Energy Technology” (John Wiley &
Sons, 1997) 161 pp, 0-471-96044-6.
Woelfle, Gretchen. “The Wind at Work: An Activity Guide to Windmills”
(Chicago Rreview Press, 1997) 138 pp, bibliography, index, ISBN
1-55652-308-4.
World Wide Web URLs:
http://www.windpower.dk
Danish Wind Turbine Manufacturers Association (English, French, and
Danish). The site with the most comprehensive information on wind energy.
http://rotor.fb12.tu-berlin.de/engwindkraft.html
Aerospace Institute, Technical University of Berlin (English, French, and
German). Another very good site.
http://www.bwea.com/
British Wind Energy Association (English).
http://www.igc.org/awea/
American Wind Energy Association (English).
