Wind Energy and Wind Power
Wind is a form of solar
energy. Winds are caused by the uneven heating of the atmosphere by the
sun, the irregularities of the earth's surface, and rotation of the earth. Wind
flow patterns are modified by the earth's terrain, bodies of water, and
vegetative cover. This wind flow, or motion energy, when "harvested"
by modern wind turbines, can be used to generate electricity.
Causes The Wind?
Wind is simply the air in
motion. Usually when we are talking about the wind it is the horizontal
motion we are concerned about. If you hear a forecast of west winds of 10
to 20 mph that means the horizontal winds will be 10 to 20 mph FROM
Although we cannot actually
see the air moving we can measure its motion by the force that it applies
on objects. For example, on a windy day leaves rustling or trees swaying
indicate that the wind is blowing. Officially, a wind vane measures the
wind direction and an anemometer measures the wind speed.
The vertical component of
the wind is typically very small (except in thunderstorm updrafts)
compared to the horizontal component, but is very important for
determining the day to day weather. Rising air will cool, often to
saturation, and can lead to clouds and precipitation. Sinking air warms
causing evaporation of clouds and thus fair weather.
You have probably seen a
surface map marked with H's and L's which indicate high and low pressure
centers. Surrounding these "highs" and "lows" are
lines called isobars. "Iso" means "equal" and a
"bar" is a unit of pressure so an isobar means equal pressure.
We connect these areas or equal pressure with a line. Everywhere along
each line is constant pressure. The closer the isobars are packed together
the stronger the pressure gradient is.
Pressure gradient is the difference
in pressure between high and low pressure areas. Wind speed is directly
proportional to the pressure gradient. This means the strongest winds
are in the areas where the pressure gradient is the greatest.
Also, notice that the wind
direction (yellow arrows) is clockwise around the high pressure system and
counter-clockwise around the low pressure system. In addition, the
direction of the wind is across the isobars slightly, away from the center
of the high pressure system and toward the center of the low pressure
system. Why does this happen? To understand we need to examine the forces
that govern the wind.
There are three forces that
cause the wind to move as it does. All three forces work together at the
gradient force (Pgf) is a force that tries to equalize pressure
differences. This is the force that causes high pressure to push air
toward low pressure. Thus air would flow from high to low pressure if the
pressure gradient force was the only force acting on it.
However, because of the
earth's rotation, there is second force, the Coriolis force
that affects the direction of wind flow. Named after Gustav-Gaspard
Coriolis, the French scientist who described it mathematically
Wind is the movement of
air over the surface of the Earth, from areas of high pressure to low
pressure. But what causes the changes in pressure? There are a few
concepts that we will have to explore to find exactly how this works,
but ultimately all the energy on our planet comes from the Sun.
The Sun gives out all
sorts of radiation, including heat and light energy, and is so powerful
that it radiates 170,000,000 GW of energy to the Earth! When this
energy reaches the Earth, the ground and other surfaces absorb it, and
heat the surrounding air. It's these differences in temperature,
together with the rotation of our planet, that create the wind.
About 1 to 2 per cent of
the energy coming from the sun is converted into wind energy, which is
enough to meet the electricity needs of the world three times over, and
is a source of power that will never run out.
The density of air
Air, like all substances
around us, has a certain density.
mass (kilograms) /volume (meters cubed)
The density of air is
small but not zero. If air didn't weigh anything, the atmosphere would
float off into space, which would be bad!
There is only a thin
layer of air surrounding the earth, what we know as our atmosphere. This
extends upwards more than 50 kilometres above ground level. At this
height the density is less than 1% of the ground level value. If the
earth were the size of a football, the atmosphere would be equivalent to
a 1mm thick layer on the surface of the football.
Because there are miles
of air above us and it is all pushing down, the air at the bottom gets
squeezed creating a pressure, like the pressure you feel at the bottom
of a swimming pool. The size of this pushing force over each unit of
area is called the air pressure, or atmospheric pressure.
force (Newtons) /area (m2)
(The unit of pressure is
called the Pascal or Pa for short, 1 Pascal = 1 Newton per m2)
The pressure on the
earth's surface due to the air above us is about 100000 Pa - 101,325 Pa
on average. That's 1Kg pushing on every square cm! 101,325 Pa is also
commonly referred to as 'one atmosphere'. The weight of a column of
water 10 meters high would be needed to increase the air pressure at the
base of the column by 1 atmosphere.
A barometer measures air
pressure. If you took a barometer up in a hot air balloon you would see
the pressure reading fall the higher the balloon goes. This happens
because there is less air above the balloon the higher up into the
atmosphere it goes. If you went too high the air pressure would become
so low that you would not be able to breathe properly. This is why
modern passenger jets have 'pressurised cabins' to keep the conditions
similar to that at the earth's surface so the passengers are more
There is another unit of
pressure called the "milli-bar" or mbar for short. There are
exactly 100 Pascals per milli-bar, so 1000 Mb is about one atmosphere.
If you watch the weather
forecast on TV you may see a map showing atmospheric pressure. This is
referred to as an isobar chart.
Isobars are similar to
contour lines. Instead of the lines showing areas where the ground is
the same height above sea level, the lines show areas where the
atmospheric pressure is the same. The closer the lines are together the
more rapidly the pressure changes from one place to another. This is
similar to contour lines on a map, the closer they are together the more
steep the slope.
Why does the pressure
vary from place to place and from day to day?
There are two causes:
1) the rotation
of the earth
As the earth spins on its axis it drags the atmosphere round with it.
However, the air higher up in the atmosphere is less affected by this
dragging/stirring effect. The difference in the air speed at different
levels in the atmosphere causes the air to mix, forming turbulence,
which causes wind at the earth's surface.
The rotation of the earth
causes another related phenomenon, the Coriolis force.This
is best demonstrated by example. Take a piece of paper and pin it onto
something which will not get damaged, e.g. a carpet. Rotate the paper
anti clockwise (to represent the movement of the earth), whilst at the
same time trying to draw a straight line. The line you draw will appear
A similar effect occurs
when air is moving over the surface of the earth as it rotates. Instead
of travelling in a straight line, the path of the moving air veers to
the right. As a result instead of the air (or wind) moving in a straight
line from areas of higher pressure to areas of lower pressure, it moves
almost parallel to the isobars. The result is that the wind circles in a
clockwise direction towards the area of low pressure. In the Southern
hemisphere, the wind will circle in an anti-clockwise direction and
clockwise in the Northern hemisphere.
2) the heating
effect of the sun
The warming effect of the sun varies with latitude and with the time of
day. Warmer air is less dense than cooler air, and rises above it, so
the pressure above the equator is lower than the pressure above the
The warming effect is
greater over the equator as the sun is directly overhead. Nearer the
earth's poles the angle at which the suns rays hit the earth is more
acute, so the same amount of energy is spread over a greater area.
Differences Drive Air Circulation
Satellite: Terra Sensor: MODIS
regions around equator, at 0° latitude are heated more by the sun than the rest
of the globe. These hot areas are indicated in the warm colors, red, orange and
yellow in this infrared picture of sea surface temperatures . Hot air is lighter
than cold air and will rise into the sky until it reaches approximately 10 km (6
miles) altitude and will spread to the North and the South. If the globe did not
rotate, the air would simply arrive at the North Pole and the South Pole, sink
down, and return to the equator.
wind rises from the equator and moves north and south in the higher layers of
the atmosphere. Around 30° latitude in both hemispheres the Coriolis force
prevents the air from moving much farther. At this latitude there is a high
pressure area, as the air begins sinking down again. As the wind rises from the
equator there will be a low pressure area close to ground level attracting winds
from the North and South. At the Poles, there will be high pressure due to the
cooling of the air. Keeping in mind the bending force of the Coriolis force, we
thus have the following general results for the prevailing wind direction:
NASA JPL Satellite: QuikSCAT Sensor: SeaWinds
terms "wind energy" or "wind power" describe the process by
which the wind is used to generate mechanical power or electricity. Wind
turbines convert the kinetic energy in the wind into mechanical power. This
mechanical power can be used for specific tasks (such as grinding grain or
pumping water) or a generator can convert this mechanical power into electricity
to power homes, businesses, schools, and the like.
maps of average wind speed help researchers determine where to develop wind
energy. Wind turbines (high-tech windmills) can generate power in places far
from power plants and without an electricity grid - but planners need to know
where there is sufficient wind for the turbines to operate efficiently. A team
at NASA's Langley Research Center developed these maps, and maps of solar
insolation, and provide them free of charge. Private companies are using these
data to design, build, and market new technologies for harnessing this energy.
How Wind Power Is Generated
The terms "wind
energy" or "wind power" describe the
process by which the wind is used to generate mechanical power or
electricity. Wind turbines convert the kinetic energy in the wind into
mechanical power. This mechanical power can be used for specific tasks (such as
grinding grain or pumping water) or a generator can convert this mechanical
power into electricity to power homes, businesses, schools, and the like.
Wind turbines, like aircraft
propeller blades, turn in the moving air and power an electric generator
that supplies an electric current. Simply stated, a wind turbine is the opposite
of a fan. Instead of using electricity to make wind, like a fan, wind turbines
use wind to make electricity. The wind turns the blades, which spin a shaft,
which connects to a generator and makes electricity.
Wind Turbine Types
Modern wind turbines fall into
two basic groups; the horizontal-axis variety, like the
traditional farm windmills used for pumping water, and the vertical-axis
design, like the eggbeater-style Darrieus model, named after its French
inventor. Most large modern wind turbines are horizontal-axis turbines.
- Anemometer: Measures
the wind speed and transmits wind speed data to the controller.
- Blades: Most turbines
have either two or three blades. Wind blowing over the blades causes the
blades to "lift" and rotate.
- Brake: A disc brake,
which can be applied mechanically, electrically, or hydraulically to stop
the rotor in emergencies.
- Controller: The
controller starts up the machine at wind speeds of about 8 to 16 miles per
hour (mph) and shuts off the machine at about 55 mph. Turbines do not
operate at wind speeds above about 55 mph because they might be damaged by
the high winds.
- Gear box: Gears connect
the low-speed shaft to the high-speed shaft and increase the rotational
speeds from about 30 to 60 rotations per minute (rpm) to about 1000 to 1800
rpm, the rotational speed required by most generators to produce
electricity. The gear box is a costly (and heavy) part of the wind turbine
and engineers are exploring "direct-drive" generators that operate
at lower rotational speeds and don't need gear boxes.
- Generator: Usually an
off-the-shelf induction generator that produces 60-cycle AC electricity.
- High-speed shaft:
Drives the generator.
- Low-speed shaft: The
rotor turns the low-speed shaft at about 30 to 60 rotations per minute.
- Nacelle: The nacelle
sits atop the tower and contains the gear box, low- and high-speed shafts,
generator, controller, and brake. Some nacelles are large enough for a
helicopter to land on.
- Pitch: Blades are
turned, or pitched, out of the wind to control the rotor speed and keep the
rotor from turning in winds that are too high or too low to produce
- Rotor: The blades and
the hub together are called the rotor.
- Tower: Towers are made
from tubular steel (shown here), concrete, or steel lattice. Because wind
speed increases with height, taller towers enable turbines to capture more
energy and generate more electricity.
- Wind direction: This is
an "upwind" turbine, so-called because it operates facing into the
wind. Other turbines are designed to run "downwind," facing away
from the wind.
- Wind vane: Measures
wind direction and communicates with the yaw drive to orient the turbine
properly with respect to the wind.
- Yaw drive: Upwind
turbines face into the wind; the yaw drive is used to keep the rotor facing
into the wind as the wind direction changes. Downwind turbines don't require
a yaw drive, the wind blows the rotor downwind.
- Yaw motor:
Powers the yaw drive.
- other equipment, including
controls, electrical cables, ground support equipment, and interconnection
Wind turbines are often grouped
together into a single wind power plant, also known as a wind farm,
and generate bulk electrical power. Electricity from these turbines is fed into
a utility grid and distributed to customers, just as with conventional power
Wind Turbine Size and Power
Wind turbines are available in a
variety of sizes, and therefore power ratings. The largest machine has blades
that span more than the length of a football field, stands 20 building stories
high, and produces enough electricity to power 1,400 homes. A small home-sized
wind machine has rotors between 8 and 25 feet in diameter and stands upwards of
30 feet and can supply the power needs of an all-electric home or small
business. Utility-scale turbines range in size from 50 to 750
kilowatts. Single small turbines, below 50 kilowatts, are used for homes,
telecommunications dishes, or water pumping.
Disadvantages of Wind-Generated Electricity
A Renewable Non-Polluting
Wind energy is a free,
renewable resource, so no matter how much is used today, there will
still be the same supply in the future. Wind energy is also a source of clean,
non-polluting, electricity. Unlike conventional power plants, wind
plants emit no air pollutants or greenhouse gases. According to the U.S.
Department of Energy, in 1990, California's wind power plants offset the
emission of more than 2.5 billion pounds of carbon dioxide, and 15 million
pounds of other pollutants that would have otherwise been produced. It would
take a forest of 90 million to 175 million trees to provide the same air
Even though the cost of wind
power has decreased dramatically in the past 10 years, the technology requires a
higher initial investment than fossil-fueled generators.
Roughly 80% of the cost is the machinery, with the balance being site
preparation and installation. If wind generating systems are compared with
fossil-fueled systems on a "life-cycle" cost basis (counting fuel and
operating expenses for the life of the generator), however, wind costs are much
more competitive with other generating technologies because there is no fuel to
purchase and minimal operating expenses.
Although wind power plants have
relatively little impact on the environment compared to fossil fuel power
plants, there is some concern over the noise produced by the
rotor blades, aesthetic (visual) impacts, and birds and bats
having been killed (avian/bat mortality) by flying into the
rotors. Most of these problems have been resolved or greatly reduced through
technological development or by properly siting wind plants.
Supply and Transport Issues
The major challenge to using wind
as a source of power is that it is intermittent and does not
always blow when electricity is needed. Wind cannot be stored (although
wind-generated electricity can be stored, if batteries are used), and not all
winds can be harnessed to meet the timing of electricity demands. Further, good
wind sites are often located in remote locations far from areas
of electric power demand (such as cities). Finally, wind resource development
may compete with other uses for the land, and those alternative uses
may be more highly valued than electricity generation. However, wind turbines
can be located on land that is also used for grazing or even farming.
renewable energy (except tidal and geothermal power), and even the energy in
fossil fuels, ultimately comes from the sun. The sun radiates
100,000,000,000,000 kilowatt hours of energy to the earth per hour. In other
words, the earth receives 10 to the 18th power of watts of power. About 1 to 2
per cent of the energy coming from the sun is converted into wind energy. That
is about 50 to 100 times more than the energy converted into biomass by all
plants on earth.
Energy Projects Throughout the United States of America
Installed MW for each
Total Power Capacities (MW)
reaching 1,000 MW of wind energy in 1985, it took more than a decade for wind to
reach the 2,000-MW mark in 1999. Since then, installed capacity has grown
fivefold. Today, U.S. wind energy installations produce enough electricity on a
typical day to power the equivalent of over 2.5 million homes.
continued its growth in 2008 at an increased rate of 29 %.
* All wind
turbines installed by the end of 2008 worldwide are generating 260 TWh per
annum, equalling more than 1,5 % of the global electricity consumption.
wind sector became a global job generator and has created 440'000 jobs
worldwide. * The wind sector represented in 2008 a turnover of 40
* For the
first time in more than a decade, the USA took over the number one
position from Germany in terms of total installations.
continues its role as the most dynamic wind market in the year 2008, more
than doubling the installations for the third time in a row, with today
more than 12 GW of wind turbines installed.
America and Asia catch up in terms of new installations with Europe which
* Based on
accelerated development and further improved policies, a global capacity
of more than 1'500'000 MW is possible by the year 2020.
market for new wind turbines showed a 42 % increase and reached an overall
size of 27'261 MW, after 19'776 MW in 2007 and 15'127 MW in the year
2006. Ten years ago, the market for new wind turbines had a size of 2'187
MW, less than one tenth of the size in 2008. In comparison, no new nuclear
reactor started operation in 2008, according to the International Atomic
wind markets 2008
USA and China took the lead, USA taking over the global number one
position from Germany and China getting ahead of India for the first time,
taking the lead in Asia. The USA and China accounted for 50,8 % of the
wind turbine sales in 2008 and the eight leading markets represented
almost 80 % of the market for new wind turbines - one year ago, still
only five markets represented 80 % of the global sales. The pioneer
country Denmark fell back to rank 9 in terms of total capacity, whilst
until four years ago it held the number 4 position during several years.
However, with a wind power share of around 20 % of the electricity supply,
Denmark is still a leading wind energy country worldwide.
The History of
Since ancient times, people have
harnessed the winds energy. Over 5,000 years ago, the ancient Egyptians used
wind to sail ships on the Nile River. By 200 B.C., simple windmills in China
were pumping water, while vertical-axis windmills with woven reed sails were
grinding grain in Persia and the Middle East The earliest known windmills were
in Persia (Iran).
These early windmills
looked like large paddle wheels.
model of a Persian windmill. Vertical-axis windmills were developed before 500 -
900 AD (some place their invention much earlier) to raise water and mill corn
and were still in use in the 1970's in the Zahedan region of Iran.
of Persian type windmills in Khorasan (a region that extends across Iran,
Turkmenistan and Afghanistan).
New ways of using the energy of
the wind eventually spread around the world. By the 11th century, people in the
Middle East were using windmills extensively for food production; returning
merchants and crusaders carried this idea back to Europe. The Dutch refined the
windmill and adapted it for draining lakes and marshes in the Rhine River Delta.
American colonists used windmills
to grind wheat and corn, to pump water, and to cut wood at sawmills.
Industrialization, first in
Europe and later in America, led to a gradual decline in the use of windmills.
The steam engine replaced European water-pumping windmills. In the 1930s, the
Rural Electrification Administration's programs brought inexpensive electric
power to most rural areas in the United States.
However, industrialization also
sparked the development of larger windmills to generate electricity. Commonly
called wind turbines, these machines appeared in Denmark as early as 1890. In
the 1940s the largest wind turbine of the time began operating on a Vermont
hilltop known as Grandpa's Knob. This turbine, rated at 1.25 megawatts in winds
of about 30 mph, fed electric power to the local utility network for several
months during World War II.
The popularity of using the
energy in the wind has always fluctuated with the price of fossil fuels. When
fuel prices fell after World War II, interest in wind turbines waned. But when
the price of oil skyrocketed in the 1970s, so did worldwide interest in wind
In the early 1980s wind
energy really took off in California, partly because of state policies that
encouraged renewable energy sources. Support for wind development has since
spread to other states.
Credit: NOAA, U.S. DOE, American Wind
Energy Association, Bureau of Land Management, Sandia National Labooratory, The
British Wind Energy Association, The World Wind Energy Association (WWEA),