Wind power is the conversion of wind energy into a useful
form of energy, such as electricity, using wind turbines. At the end of 2008,
worldwide nameplate capacity of wind-powered generators was 121.2 gigawatts
(GW). In 2008, wind power produced about 1.5% of worldwide electricity
usage and is growing rapidly, having doubled in the three years between 2005 and
2008. Several countries have achieved relatively high levels of wind power
penetration, such as 19% of stationary electricity production in Denmark, 11% in
Spain and Portugal, and 7% in Germany and the Republic of Ireland in 2008. As of
May 2009, eighty countries around the world are using wind power on a commercial
Large-scale wind farms are connected to the electric power
transmission network; smaller facilities are used to provide electricity to
isolated locations. Utility companies increasingly buy back surplus electricity
produced by small domestic turbines. Wind energy as a power source is attractive
as an alternative to fossil fuels, because it is plentiful, renewable, widely
distributed, clean, and produces no greenhouse gas emissions. However, the
construction of wind farms is not universally welcomed because of their visual
impact and other effects on the environment.
Wind power is
non-dispatchable, meaning that for economic operation, all of the available
output must be taken when it is available. Other resources, such as hydropower,
and standard load management techniques must be used to match supply with
demand. The intermittency of wind seldom creates problems when using wind power
to supply a low proportion of total demand.
Small-scale wind power is the name given to wind
generation systems with the capacity to produce up to 50 kW of electrical power.
Isolated communities, that may otherwise rely on diesel generators may use wind
turbines to displace diesel fuel consumption. Individuals may purchase these
systems to reduce or eliminate their dependence on grid electricity for economic
or other reasons, or to reduce their carbon footprint. Wind turbines have
been used for household electricity generation in conjunction with battery
storage over many decades in remote areas. Grid-connected wind
turbines may use grid energy storage, displacing purchased energy with local
production when available. Off-grid system users can either adapt to
intermittent power or use batteries, photovoltaic or diesel systems to
supplement the wind turbine. In urban locations, where it is difficult to obtain
predictable or large amounts of wind energy (little is known about the actual
wind resource of towns and cities), smaller systems may still be used to run
low-power equipment. Equipment such as parking meters or wireless Internet
gateways may be powered by a wind turbine that charges a small battery,
replacing the need for a connection to the power grid.
Diagram of a Grid-tied Wind Power System
Wind energy in many jurisdictions receives some financial
or other support to encourage its development. Wind energy benefits from
subsidies in many jurisdictions, either to increase its attractiveness, or to
compensate for subsidies received by other forms of production which have
significant negative externalities.
In the United States, wind power receives a tax credit for
each kWh produced; at 1.9 cents per kWh in 2006, the credit has a yearly
inflationary adjustment. Another tax benefit is accelerated depreciation. Many
American states also provide incentives, such as exemption from property tax,
mandated purchases, and additional markets for "green credits." Countries such
as Canada and Germany also provide incentives for wind turbine construction,
such as tax credits or minimum purchase prices for wind generation, with assured
grid access (sometimes referred to as feed-in tariffs). These feed-in tariffs
are typically set well above average electricity prices. The Energy Improvement
and Extension Act of 2008 contains extensions of credits for wind, including
Secondary market forces also provide incentives for
businesses to use wind-generated power, even if there is a premium price for the
electricity. For example, socially responsible manufacturers pay utility
companies a premium that goes to subsidize and build new wind power
infrastructure. Companies use wind-generated power, and in return they can claim
that they are making a powerful "green" effort. In the USA the organization
Green-e monitors business compliance with these renewable energy credits.
Source: Adapted from Wikipedia®
and used under the
Creative Commons Attribution-ShareAlike License
American Wind Energy Association, adapted.
In reality, wind energy is a converted form of
solar energy. The sun's radiation heats different parts of the earth at
different rates-most notably during the day and night, but also when different
surfaces (for example, water and land) absorb or reflect at different rates.
This in turn causes portions of the atmosphere to warm differently. Hot air
rises, reducing the atmospheric pressure at the earth's surface, and cooler air
is drawn in to replace it. The result is wind.
Air has mass, and when it is in motion, it contains the
energy of that motion ("kinetic energy"). Some portion of that energy can
converted into other forms mechanical force or electricity that we can use to
What is a wind turbine and how does it work?
A wind energy system transforms the kinetic energy of the
wind into mechanical or electrical energy that can be harnessed for practical
use. Mechanical energy is most commonly used for pumping water in rural or
remote locations- the "farm windmill" still seen in many rural areas of the U.S.
is a mechanical wind pumper - but it can also be used for many other purposes
(grinding grain, sawing, pushing a sailboat, etc.). Wind electric turbines
generate electricity for homes and businesses and for sale to utilities.
There are two basic designs of wind electric turbines:
vertical-axis, or "egg-beater" style, and horizontal-axis (propeller-style)
machines. Horizontal-axis wind turbines are most common today, constituting
nearly all of the "utility-scale" (100 kilowatts, kW, capacity and larger)
turbines in the global market.
Turbine subsystems include:
|a rotor, or blades, which convert the wind's energy
into rotational shaft energy;|
|a nacelle (enclosure) containing a drive train,
usually including a gearbox* and a generator;|
|a tower, to support the rotor and drive train; and|
|electronic equipment such as controls, electrical
cables, ground support equipment, and interconnection equipment.|
*Some turbines do not require a gearbox
Wind turbines vary in size. This chart depicts a variety
of historical turbine sizes and the amount of electricity they are each capable
of generating (the turbine's capacity, or power rating).
The electricity generated by a utility-scale wind turbine
is normally collected and fed into utility power lines, where it is mixed with
electricity from other power plants and delivered to utility customers. Today
(August 2005), turbines with capacities as large as 5,000 kW (5 MW) are being
wind turbines made of?
The towers are mostly tubular and made of steel. The
blades are made of fiberglass-reinforced polyester or wood-epoxy.
How big is a wind
Utility-scale wind turbines for land-based wind farms
come in various sizes, with rotor diameters ranging from about 50 meters to
about 90 meters, and with towers of roughly the same size. A 90-meter machine,
definitely at the large end of the scale at this writing (2005), with a 90-meter
tower would have a total height from the tower base to the tip of the rotor of
approximately 135 meters (442 feet).
Offshore turbine designs now under development will have
larger rotors—at the moment, the largest has a 110-meter rotor diameter—because
it is easier to transport large rotor blades by ship than by land.
Small wind turbines intended for residential or small
business use are much smaller. Most have rotor diameters of 8 meters or less and
would be mounted on towers of 40 meters in height or less.
How much electricity can one wind turbine generate?
The ability to generate electricity is measured in watts.
Watts are very small units, so the terms kilowatt (kW, 1,000 watts), megawatt
(MW, 1 million watts), and gigawatt (pronounced "jig-a-watt," GW, 1 billion
watts) are most commonly used to describe the capacity of generating units like
wind turbines or other power plants.
Electricity production and consumption are most
commonly measured in kilowatt-hours (kWh). A kilowatt-hour means one kilowatt
(1,000 watts) of electricity produced or consumed for one hour. One 50-watt
light bulb left on for 20 hours consumes one kilowatt-hour of electricity (50
watts x 20 hours = 1,000 watt-hours = 1 kilowatt-hour).
The output of a wind turbine depends on the turbine's
size and the wind's speed through the rotor. Wind turbines being manufactured
now have power ratings ranging from 250 watts to 5 megawatts (MW).
Example: A 10-kW wind turbine can generate about 10,000
kWh annually at a site with wind speeds averaging 12 miles per hour, or about
enough to power a typical household. A 5-MW turbine can produce more than 15
million kWh in a year--enough to power more than 1, 400 households. The average
U.S. household consumes about 10,000 kWh of electricity each year.
Example: A 250-kW turbine installed at the
elementary school in Spirit Lake, Iowa, provides an average of 350,000 kWh of
electricity per year, more than is necessary for the 53,000-square-foot school.
Excess electricity fed into the local utility system earned the school $25,000
in its first five years of operation. The school uses electricity from the
utility at times when the wind does not blow. This project has been so
successful that the Spirit Lake school district has since installed a second
turbine with a capacity of 750 kW.
Wind speed is a crucial element in projecting turbine
performance, and a site's wind speed is measured through wind resource
assessment prior to a wind system's construction. Generally, an annual average
wind speed greater than four meters per second (m/s) (9 mph) is required for
small wind electric turbines (less wind is required for water-pumping
operations). Utility-scale wind power plants require minimum average wind speeds
of 6 m/s (13 mph).
The power available in the wind is proportional to the
cube of its speed, which means that doubling the wind speed increases the
available power by a factor of eight. Thus, a turbine operating at a site with
an average wind speed of 12 mph could in theory generate about 33% more
electricity than one at an 11-mph site, because the cube of 12 (1,768) is 33%
larger than the cube of 11 (1,331). (In the real world, the turbine will not
produce quite that much more electricity, but it will still generate much more
than the 9% difference in wind speed.) The important thing to understand is that
what seems like a small difference in wind speed can mean a large difference in
available energy and in electricity produced, and therefore, a large difference
in the cost of the electricity generated. Also, there is little energy to be
harvested at very low wind speeds (6-mph winds contain less than one-eighth the
energy of 12-mph winds).
How many turbines does it take to make one megawatt (MW)?
Most manufacturers of utility-scale turbines offer
machines in the 700-kW to 2.5-MW range. Ten 700-kW units would make a 7-MW wind
plant, while 10 2.5-MW machines would make a 25-MW facility. In the future,
machines of larger size will be available, although they will probably be
installed offshore, where larger transportation and construction equipment can
be used. Units up to 5 MW in capacity are now under development.
How many homes can one megawatt of wind energy supply?
An average U.S. household uses about 10,655
kilowatt-hours (kWh) of electricity each year. One megawatt of wind energy can
generate from 2.4 to more than 3 million kWh annually. Therefore, a megawatt of
wind generates about as much electricity as 225 to 300 households use. It is
important to note that since the wind does not blow all of the time, it cannot
be the only power source for that many households without some form of storage
system. The "number of homes served" is just a convenient way to translate a
quantity of electricity into a familiar term that people can understand.
(Typically, storage is not needed, because wind generators are only part of the
power plants on a utility system, and other fuel sources are used when the wind
is not blowing. According to the U.S. Department of Energy , "When wind is added
to a utility system, no new backup is required to maintain system reliability."
Wind Energy Myths, Wind Powering America Fact Sheet Series,
What is a wind
The most economical application of wind electric turbines
is in groups of large machines (660 kW and up), called "wind power plants" or
"wind farms." For example, a 107-MW wind farm near the community of Lake Benton,
Minn., consists of turbines sited far apart on farmland along windy Buffalo
Ridge. The wind farm generates electricity while agricultural use continues
Wind plants can range in size from a few megawatts to
hundreds of megawatts in capacity. Wind power plants are "modular," which means
they consist of small individual modules (the turbines) and can easily be made
larger or smaller as needed. Turbines can be added as electricity demand grows.
Today, a 50-MW wind farm can be completed in 18 months to two years. Most of
that time is needed for measuring the wind and obtaining construction
permits—the wind farm itself can be built in less than six months.