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Wind power is the conversion of wind energy into useful form, such as electricity, using wind turbines. In windmills, wind energy is directly used to crush grain or to pump water. At the end of 2006, worldwide capacity of wind-powered generators was 73.9 gigawatts. Although wind currently produces just over 1% of world-wide electricity use, it accounts for approximately 19% of electricity production in Denmark, 9% in Spain and Portugal, and 6% in Germany and the Republic of Ireland (2007 data). Globally, wind power generation more than quadrupled between 2000 and 2006.
Wind power is produced in large scale wind farms connected to electrical grids, as well as in individual turbines for providing electricity to isolated locations. Wind energy is plentiful, renewable, widely distributed, clean, and reduces greenhouse gas emissions when it displaces fossil-fuel-derived electricity. The intermittency of wind seldom creates insurmountable problems when using wind power to supply up to roughly 10% of total electrical demand (low to moderate penetration), but it presents challenges that are not yet fully solved when wind is to be used for a larger fraction of demand.
Since antiquity, mankind has been using wind energy; it is thus not a new idea. For centuries, windmills and watermills were the only source of motive power for a number of mechanical applications, some of which are even still used today.
Humans have been using wind energy in their daily work for some 4,000 years. Sails revolutionized seafaring, which no longer had to make do with muscle power. In 1700 B.C., King Hammurabi of Babylon used wind powered scoops to irrigate Mesopotamia.
Aside from pumps for irrigation or drainage, windmills were mostly used to ground grain. Thus, we still speak of "windmills" today, even when we are talking about machines that do not actually grind, such as sawmills and hammer mills
But the wind turbines that generate electricity today are new and innovative. Their success story began with a few technical innovations, such as the use of synthetics to make rotor blades. Developments in the field of aerodynamics, mechanical/electrical engineering, control technology, and electronics provide the technical basis for wind turbines commonly used today.Since 1980, wind turbines have been becoming larger and more efficient at rates otherwise only seen in computer technology.
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Power is available from the kinetic energy of the mass of air moving in wind. The amount of energy that wind carries increases by a factor of two as its speed increases and is proportional to the mass of air that passes through the plane of the area swept by the rotors. As power is the product of energy (work) within a given time frame, the power of wind increases by a factor of three as the speed of wind increases. Because of the low density of air (Pair=1.25 kg / m3), the power density of wind is much lower than that of water power (Pwater=1000 kg / m3), for instance. The power that can be harvested from wind is calculated in terms of the swept area -- for a horizontal axis wind turbine (HAWT), the area through which the rotor blades pass. As a result, if the diameter of the rotor blades is doubled, the power increases by a factor of four. If the wind speed then doubles, power increases by a factor of eight.
This optimum performance cP is attained when a wind turbine's rotors slow the wind down by one third. Current wind turbines convert up to 50% of energy in wind into electricity, thus coming very close to the theoretical limit.
The Earth is unevenly heated by the sun resulting in the poles receiving less energy from the sun than the equator does. Also the dry land heats up (and cools down) more quickly than the seas do. The differential heating drives a global atmospheric convection system reaching from the Earth's surface to the stratosphere which acts as a virtual ceiling. Most of the energy stored in these wind movements can be found at high altitudes where continuous wind speeds of over 160 km/h (100 mph) occur. Eventually, the wind energy is converted through friction into diffuse heat throughout the Earth's surface and the atmosphere.
There are many thousands of wind turbines operating, with a total capacity of 73,904 MW of which wind power in Europe accounts for 65% (2006). Wind power was the most rapidly growing means of alternative electricity generation at the turn of the 21st century. World wind generation capacity more than quadrupled between 2000 and 2006. 81% of wind power installations are in the US and Europe, but the share of the top five countries in terms of new installations fell from 71% in 2004 to 62% in 2006. By 2010, the World Wind Energy Association expects 160GW of capacity to be installed worldwide, up from 73.9 GW at 2006, implying an anticipated net growth rate of more than 21% per year.
Germany, Spain, the United States, India, and Denmark have made the largest investments in wind-generated electricity. Denmark is prominent in the manufacturing and use of wind turbines, with a commitment made in the 1970s to eventually produce half of the country's power by wind. Denmark generates over 20% of its electricity with wind turbines -- the highest percentage of any country -- and is fifth in the world in total wind power generation.
Germany is the leading producer of wind power, with 28% of the total world capacity in 2006 and a total output of 38.5 TWh in 2007 (6.3% of German electricity); the official target is for renewable energy to meet 12.5% of German electricity needs by 2010 — this target may be reached ahead of schedule. Germany has 18,600 wind turbines, mostly in the north of the country — including three of the biggest in the world, constructed by the companies Enercon (6 MW), Multibrid (5 MW) and Repower (5 MW). Germany's Schleswig-Holstein province generates 36% of its power with wind turbines.
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In 2005, the government of Spain approved a new national goal for installed wind power capacity of 20,000 MW in 2010. With a record installation of 3515 MW in 2007 (for a total figure of 15,145 MW), this target will probably be reached ahead of schedule. A significant acceleration of the bureaucratic proceedings and connections to grid, and the legislative change occurred during 2007 (with Royal Decree 661/2007), have accelerated the developing of many wind parks, so that they could still run under the previous more favourable conditions.
In recent years, the United States has added more wind energy to its grid than any other country; U.S. wind power capacity grew by 45% to 16.8 gigawatts in 2007. Texas has become the largest wind energy producing state, surpassing California. In 2007, the state expects to add 2 gigawatts to its existing capacity of approximately 4.5 gigawatts. Iowa and Minnesota are expected to each produce 1 gigawatt by late-2007. Wind power generation in the U.S. was up 31.8% in February, 2007 from February, 2006. The average output of one megawatt of wind power is equivalent to the average electricity consumption of about 250 American households. According to the American Wind Energy Association, wind will generate enough electricity in 2008 to power just over 1% (4.5 million households) of total electricity in US, up from less than 0.1% in 1999. US Department of Energy studies have concluded wind harvested in just three of the fifty U.S. states could provide enough electricity to power the entire nation, and that offshore wind farms could do the same job.
India ranks 4th in the world with a total wind power capacity of 6,270 MW in 2006, or 3% of all electricity produced in India. The World Wind Energy Conference in New Delhi in November 2006 has given additional impetus to the Indian wind industry. The windfarm near Muppandal, Tamil Nadu, India, provides an impoverished village with energy. India-based Suzlon Energy is one of the world's largest wind turbine manufacturers.
In December 2003, General Electric installed the world's largest offshore wind turbines in Ireland, and plans are being made for more such installations on the west coast, including the possible use of floating turbines.
In 2005, China announced it would build a 1000-megawatt wind farm in Hebei for completion in 2020. China reportedly has set a generating target of 20,000 MW by 2020 from renewable energy sources — it says indigenous wind power could generate up to 253,000 MW. Following the World Wind Energy Conference in November 2004, organised by the Chinese and the World Wind Energy Association, a Chinese renewable energy law was adopted. In late 2005, the Chinese government increased the official wind energy target for the year 2020 from 20 GW to 30 GW.
Mexico recently opened La Venta II wind power project as an important step in reducing Mexico's consumption of fossil fuels. The project (88MW) the first of its kind in Mexico, will provide 13 percent of the electricity needs of the state of Oaxaca and by 2012 will have a capacity of 3500 MW.
Another growing market is Brazil, with a wind potential of 143 GW. The federal government has created an incentive program, called Proinfa, to build production capacity of 3300 MW of renewable energy for 2008, of which 1422 MW through wind energy. The program seeks to produce 10% of Brazilian electricity through renewable sources. France recently announced a very ambitious target of 12 500 MW installed by 2010.
The power coefficient of a wind turbine's rotor blade is calculated according to the laws of airfoil theory. As with the wing of an airplane, air passing over a rotor blade creates an aerodynamic profile with low pressure above the wing, pulling the wing up, and overpressure below, pushing it up.
The difference in pressures exerts a lift on the wing vertical to the direction in which the wind is blowing and creates resistance in the direction of the wind (incident flow). For a wind turbine's rotor blade rotating around the rotor axis, the incident flow is the result of the geometric addition of wind velocity v and the circumferential speed u, which increases in linear fashion the longer the blade is. In other words, the lift exerted on the rotor blade is not only the result of wind velocity, but mostly out of the blade's own rotation. Speeds at the tip of the blade are thus very great. Current wind turbines have rotor tips travelling at velocities six times faster than the speed of the wind. The tip speed ratio is thus lambda = 6. The rotor tip can then be traveling at velocities of 60 m/s to 80 m/s.
The energy that the rotor harvests is equivalent to the lifting force in the swept area minus the resistance force in the swept area. The forces applied in the direction of the axis drive the rotor, which then not only harvests the energy of the wind, but also exerts a load on the tower and the foundation.
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Wind turbines are categorized according to a number of criteria: The position of the axis (horizontal or vertical) is obvious. Horizontal axis wind turbines (HAWTs) can be further divided into those with rotors rotating in front of the tower (windward) and those rotating behind the tower (leeward) vis-à-vis the direction of the wind. The tip speed ratio and the number of blades determine the response of the drive, and hence how the wind turbine can be used. In modern wind turbines that generate electricity, there are different types of nacelles that turn on top of the tower to face the wind. There are turbines with gearboxes and without and nacelles whose components (bearings, gears, generator) are positioned separately or have multiple functions integrated in one component (bedding of rotor shaft in the gearbox).
Poles (generally guyed) are usually only used for small wind turbines (up to 10 kW). Free-standing towers are either steel or concrete tubular towers or pylons.
Modern wind turbines are complex technical systems that combine the theoretical basics of a number of fields:
Aerodynamics, lightweight construction >> rotor blades, dynamics, overall system)
Mechanical and plant engineering >> machines with shafts, gearboxes, bearings, brakes, and tower.
Electrical engineering >> generator, frequency converter, mains connection, electrical lines.
Electronics, instrumentation and controls, and computer science >> system controls, remote monitoring, sensors.
Construction engineering >> foundation, access roads.
Meteorology >> design, yield.
At present, three concepts for the feed of electricity to the power grid dominate the market. The following table provides an overview of the differences and common ground between these types.
- The "Danish concept"
- The pitch concept with a synchronous generator
- The pitch concept with a doubly fed asynchronous generator
In the Danish concept, which completely dominated the market up to the mid-1990s, the asynchronous generator "naturally" limits power production in strong wind or gusts. It restricts the speed of the system to the frequency of the power grid, so that the rotor cannot turn faster when the wind blows stronger.
In this concept, the rotor blades are designed to create turbulence at a certain wind velocity, preventing the lift from accelerating rotation any further even though the blades are not themselves pitched. Johannes Juul developed this concept.
The use of an asynchronous generator also eliminates the synchronization needed for a synchronous generator. In other words, the system is simple and robust.
The pitched concepts developed from 1990 to 2000 turn the rotor blades in and out of the wind along their axis. Depending on the wind velocity, the machines run at various speeds. The blades are turned out of the wind to limit power generation when the wind becomes too strong (above 12 m/s). The blades are only turned into the wind to start the system. Under normal conditions, the turbines are run at a set optimal angle for the best power generation, with the speed of rotation increasing until nominal output is attained. From then on, the pitch of the blades is activated to keep power production constant.
Wind turbines only appear to be simple constructions. There are many steps from the draft to construction before the turbine can begin generating environmentally friendly energy in the field.
Wind is not constant, so wind turbines do not always run at nominal output. The amount of energy generated is below the amount theoretically possible. One speaks of a capacity factor, which is the yearly yield in kilowatt-hours divided by the product of the wind turbine's nominal output and the 8,760 hours in a year.. Depending on the location, the capacity factor can range from 30% in coastal areas with great wind to around 18% at inland locations with less wind.
It is true that wind energy is not available at all times. However, the wind energy fed to the power grid does make up part of the baseload. The large number of wind turbines already installed in Germany (17,500 as of December 31, 2005) ensures that wind power is always being fed to the grid somewhere. Over large areas, some 10% of the nominal power of all wind turbines can be expected to be fed to the grid as constant output.
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