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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 2007, worldwide capacity of wind-powered generators was 94.1 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 increased more than fivefold between 2000 and 2007.

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 a low proportion of total demand, but it presents extra costs when wind is to be used for a large fraction of demand.

History

The earliest historical reference describes a windmill used to power an organ in the 1st century AD. Windmills were used extensively in Northwestern Europe to grind flour beginning in the 1180s, and many Dutch windmills still exist. In the United States, the development of the "water-pumping windmill" was the major factor in allowing the farming and ranching of vast areas of North America, which were otherwise devoid of readily accessible water. They contributed to the expansion of rail transport systems throughout the world, by pumping water from wells to supply the needs of the steam locomotives of those early times.The multi-bladed wind turbine atop a lattice tower made of wood or steel was, for many years, a fixture of the landscape throughout rural America.The modern wind turbine was developed beginning in the 1980s, although designs are still under development.

Wind energy

The origin of wind is complex. 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 is an estimated 72 TW of wind energy on the Earth that potentially can be commercially viable.  Not all the energy of the wind flowing past a given point can be recovered (see Betz' law).  

Distribution of wind speed

Distribution of wind speed (red) and energy (blue) for all of 2002 at the Lee Ranch facility in Colorado. The histogram shows measured data, while the curve is the Rayleigh model distribution for the same average wind speed. Energy is the Betz limit through a 100 meter diameter circle facing directly into the wind. Total energy for the year through that circle was 15.4 gigawatt-hours.

Windiness varies, and an average value for a given location does not alone indicate the amount of energy a wind turbine could produce there. To assess the frequency of wind speeds at a particular location, a probability distribution function is often fit to the observed data. Different locations will have different wind speed distributions.

The Rayleigh model closely mirrors the actual distribution of hourly wind speeds at many locations.Because so much power is generated by higher windspeed, much of the energy comes in short bursts. The 2002 Lee Ranch sample is telling; half of the energy available arrived in just 15% of the operating time. The consequence is that wind energy does not have as consistent an output as fuel-fired power plants; utilities that use wind power must provide backup generation for times that the wind is weak. Making wind power more consistent requires that storage technologies must be used to retain the large amount of power generated in the bursts for later use.

Worldwide installed capacity in 2006 and prediction 1997-2010,  Grid management Induction generators often used for wind power projects require reactive power for excitation, so substations used in wind-power collection systems include substantial capacitor banks for power factor correction. Different types of wind turbine generators behave differently during transmission grid disturbances, so extensive modelling of the dynamic electromechanical characteristics of a new wind farm is required by transmission system operators to ensure predictable stable behaviour during system faults.

In particular, induction generators cannot support the system voltage during faults, unlike steam or hydro turbine-driven synchronous generators (however properly matched power factor correction capacitors along with electronic control of resonance can support induction generation without grid). Doubly-fed machines, or wind turbines with solid-state converters between the turbine generator and the collector system, have generally more desirable properties for grid interconnection.

Transmission systems operators will supply a wind farm developer with a grid code to specify the requirements for interconnection to the transmission grid. This will include power factor, constancy of frequency and dynamic behaviour of the wind farm turbines during a system fault.

Capacity factor

Since wind speed is not constant, a wind farm's annual energy production is never as much as the sum of the generator nameplate ratings multiplied by the total hours in a year. The ratio of actual productivity in a year to this theoretical maximum is called the capacity factor.

Typical capacity factors are 20-40%, with values at the upper end of the range in particularly favourable sites. For example, a 1 megawatt turbine with a capacity factor of 35% will not produce 8,760 megawatt-hours in a year, but only 0.35x24x365 = 3,066 MWh, averaging to 0.35 MW.

Online data is available for some locations and the capacity factor can be calculated from the yearly output.Unlike fueled generating plants, the capacity factor is limited by the inherent properties of wind. Capacity factors of other types of power plant are based mostly on fuel cost, with a small amount of downtime for maintenance.

Nuclear plants have low incremental fuel cost, and so are run at full output and achieve a 90% capacity factor. Plants with higher fuel cost are throttled back to follow load. Gas turbine plants using natural gas as fuel may be very expensive to operate and may be run only to meet peak power demand. A gas turbine plant may have an annual capacity factor of 5-25% due to relatively high energy production cost.

According to a 2007 Stanford University study published in the Journal of Applied Meteorology and Climatology, interconnecting ten or more wind farms allows 33 to 47% of the total energy produced to be used as reliable, baseload electric power, as long as minimum criteria are met for wind speed and turbine height. Intermittency and penetration limits

Electricity generated from wind power can be highly variable at several different timescales: from hour to hour, daily, and seasonally. Annual variation also exists, but is not as significant. Because instantaneous electrical generation and consumption must remain in balance to maintain grid stability, this variability can present substantial challenges to incorporating large amounts of wind power into a grid system. Intermittency and the non-dispatchable nature of wind energy production can raise costs for regulation, incremental operating reserve, and (at high penetration levels) could require energy demand management, load shedding, or storage solutions.

At low levels of wind penetration, fluctuations in load and allowance for failure of large generating units requires reserve capacity that can also regulate for variability of wind generation.Pumped-storage hydroelectricity or other forms of grid energy storage can store energy developed by high-wind periods and release it when needed. Stored energy increases the economic value of wind energy since it can be shifted to displace higher cost generation during peak demand periods.

 The potential revenue from this arbitrage can offset the cost and losses of storage; the cost of storage may add 25% to the cost of wind energy.Peak wind speeds may not coincide with peak demand for electrical power. In California and Texas, for example, hot days in summer may have low wind speed and high electrical demand due to air conditioning. In the UK, however, winter demand is higher than summer demand, and so are wind speeds.

Solar power 

Massachusetts Maritime Academy's shows the effect. A combined power plant linking solar, wind, bio-gas and hydrostorage is proposed as a way to provide100% renewable power. The 2006 Energy in Scotland Inquiry report expressed concern that wind power cannot be a sole source of supply, and recommends diverse sources of electric energy. A report from Denmark noted that their wind power network was without power for 54 days during 2002. tends to be complementary to wind; on most days with no wind there is sun and on most days with no sun there is wind. A demonstration project at the

Penetration

Wind energy "penetration" refers to the fraction of energy produced by wind compared with the total available generation capacity. There is no generally accepted "maximum" level of wind penetration. The limit for a particular grid will depend on the existing generating plants, pricing mechanisms, capacity for storage or demand management, and other factors. An interconnected electricity grid will already include reserve generating and transmission capacity to allow for equipment failures; this reserve capacity can also serve to regulate for the varying power generation by wind plants.

Studies have indicated that 20% of the total electrical energy consumption may be incorporated with minimal difficulty. These studies have been for locations with geographically dispersed wind farms, some degree of dispatchable energy, or hydropower with storage capacity, demand management, and interconnection to a large grid area export of electricity when needed.

Beyond this level, there are few technical limits, but the economic implications become more significant.At present, few grid systems have penetration of wind energy above 5%: Denmark (values over 18%), Spain and Portugal (values over 9%), Germany and the Republic of Ireland (values over 6%). The Danish grid is heavily interconnected to the European electrical grid, and it has solved grid management problems by exporting almost half of its wind power to Norway.

The correlation between electricity export and wind power production is very strong.A study commissioned by the state of Minnesota considered penetration of up to 25%, and concluded that integration issues would be manageable and have incremental costs of less than one-half cent ($0.0045) per kWh.

Predictability

Related to variability is the short-term (hourly or daily) predictability of wind plant output. Like other electricity sources, wind energy must be "scheduled". The nature of this energy source makes it inherently variable. Wind power forecasting methods are used, but predictability of wind plant output remains low for short-term operation.

Turbine placement

Good selection of a wind power site and positioning of turbines within the site is critical to economic development of wind power. Aside from the availability of wind itself, other significant factors include the availability of transmission lines, cost of land acquisition, land use considerations, and environmental impact of construction and operations. Off-shore locations may offset their higher construction cost with higher annual load factors, thereby reducing cost of energy produced.

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 the end of 2006, implying an anticipated net growth rate of more than 21% per year.Denmark generates nearly one-fifth of its electricity with wind turbines -- the highest percentage of any country -- and is fifth in the world in total wind power generation.

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 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.In 2005, the government of Spain approved a new national goal for installed wind power capacity of 20,000 MW in 2010. With 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 areexpected 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 U.S., up from less than 0.1% in 1999. U.S. 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.

MexicoLa Venta II wind power project as an important step in reducing Mexico's consumption of fossil fuels. The 88 MW project is the first of its kind in Mexico, and will provide 13 percent of the electricity needs of the state of Oaxaca. By 2012 the project 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.South Africa has a proposed station situated on the West Coast north of the Olifants River mouth near the town of Koekenaap, east of Vredendal in the Western Cape province. The station is proposed to have a total output of 100MW although there are negotiations to double this capacity.

The plant could be operational by 2010.France has announced a target of 12,500 MW installed by 2010.Canada experienced rapid growth of wind capacity between 2000 and 2006, with total installed capacity increasing from 137 MW to 1,451 MW, and showing an annual growth rate of 38%. Particularly rapid growth was seen in 2006, with total capacity doubling from the 684 MW at end-2005. This growth was fed by measures including installation targets, economic incentives and political support.

For example, the Ontario government announced that it will introduce a feed-in tariff for wind power, referred to as 'Standard Offer Contracts', which may boost the wind industry across the province. In Quebec, the provincially-owned electric utility plans to purchase an additional 2000 MW by 2013.

Small wind generation systems with capacities of 100 kW or less are usually used to power homes, farms, and small businesses. Isolated communities that otherwise rely on diesel generators may use wind turbines to displace diesel fuel consumption. Individuals purchase these systems to reduce or eliminate their electricity bills, or simply to generate their own clean power.

Wind turbines have been used for household electricity generation in conjunction with battery storage over many decades in remote areas. Increasingly, U.S. consumers are choosing to purchase grid-connected turbines in the 1 to 10 kilowatt range to power their whole homes. Household generator units of more than 1 kW are now functioning in several countries, and in every state in the U.S.Grid-connected wind turbines may use grid energy storage, displacing purchased energy with local production when available.

Off-grid system users 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, 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.

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