A wind farm is a group of wind turbines in the same location used for production of electric power. Individual wind turbines are interconnected with a medium voltage power collection system and communications network. At a substation, this medium-voltage electrical current is increased in voltage with a transformer for connection to the high voltage transmission system.
Spain, Denmark, and Germany are Europe's main wind energy producers. A large wind farm may consist of a few dozen to several hundred individual wind turbines, and cover an extended area of hundreds of square miles, but the land between the turbines may be used for agricultural or other purposes. A wind farm may be located off-shore to take advantage of strong winds blowing over the surface of an ocean or lake.
 Location planning
A quantity called Wind Power Density (WPD) is used to select locations for wind energy development. The WPD is a calculation relating to the effective force of the wind at a particular location, frequently expressed in term of the elevation above ground level over a period of time. It takes into account velocity and mass. Color coded maps are prepared for a particular area describing, for example, "Mean Annual Power Density, at 50 Meters." The results of the above calculation are used in an index developed by the National Renewable Energy Lab and referred to as "NREL CLASS." The larger the WPD calculation the higher it is rated by class.
Wind farm siting can be highly controversial, particularly when sites are picturesque or environmentally sensitive, such as having substantial bird life, or requiring roads to be built through pristine areas. These areas are generally non-residential due to the noise concerns and setback requirements.
Access to the power grid must be taken into mind. The further from the power grid, there will be need for more transmission lines to span from the farm directly to the power grid or transformers will have to be built on the premises depending upon the types of turbines being used.
 Wind speed
As a general rule, wind generators are practical if windspeed is 10 mph (16 km/h or 4.5 m/s) or greater. An ideal location would have a near constant flow of non-turbulent wind throughout the year with a minimum likelihood of sudden powerful bursts of wind. An important factor of turbine siting is also access to local demand or transmission capacity.
Usually sites are preselected on basis of a wind atlas, and validated with wind measurements. Meteorological wind data alone is usually not sufficient for accurate siting of a large wind power project. Collection of site specific data for wind speed and direction is crucial to determining site potential. Local winds are often monitored for a year or more, and detailed wind maps constructed before wind generators are installed.
To collect wind data a meteorological tower is installed with instruments at various heights along the tower. All towers include anemometers to determine the wind speed and wind vanes to determine the direction. The towers generally vary in height from 30 to 60 meters. The towers primarily are guyed steel-pipe structures which are left to collect data for one to two years and then disassembled. Data is collected by a data logging device which stores and transmits data for analysis. Great attention must be paid to the exact positions of the turbines (a process known as micro-siting) because a difference of 30 m can nearly double energy production.
For smaller installations where such data collection is too expensive or time consuming, the normal way of prospecting for wind-power sites is to directly look for trees or vegetation that are permanently "cast" or deformed by the prevailing winds. Another way is to use a wind-speed survey map, or historical data from a nearby meteorological station, although these methods are less reliable.
The wind blows faster at higher altitudes because of the reduced influence of drag. The increase in velocity with altitude is most dramatic near the surface and is affected by topography, surface roughness, and upwind obstacles such as trees or buildings. Typically, the increase of wind speeds with increasing height follows a wind profile power law, which predicts that wind speed rises proportionally to the seventh root of altitude. Doubling the altitude of a turbine, then, increases the expected wind speeds by 10% and the expected power by 34%.
 Wind park effect
The "wind park effect" refers to the loss of output due to mutual interference between turbines. Wind farms have many turbines and each extracts some of the energy of the wind. Where land area is sufficient, turbines are spaced three to five rotor diameters apart perpendicular to the prevailing wind, and five to ten rotor diameters apart in the direction of the prevailing wind, to minimize efficiency loss. The loss can be as low as 2% of the combined nameplate rating of the turbines.
In a large wind park, due to "multifractal" effects between individual rotors, the behaviour deviates significantly from Kolmogorov's turbulence scaling for individual turbines.
 Environmental and aesthetic impacts
Compared to the environmental effects of traditional energy sources, the environmental effects of wind power are relatively minor. Wind power consumes no fuel, and emits no air pollution, unlike fossil fuel power sources. The energy consumed to manufacture and transport the materials used to build a wind power plant is equal to the new energy produced by the plant within a few months of operation. Garrett Gross, a scientist from UMKC in Kansas City, Missouri states, "The impact made on the environment is very little when compared to what is gained." While a wind farm may cover a large area of land, many land uses such as agriculture are compatible.
Danger to birds and bats has been a concern in many locations. Some dismiss the number of birds killed by wind turbines as negligible when compared to the number that die as a result of other human activities, and especially the environmental impacts of using non-clean power sources. Others are in very strong disagreement with the placement of wind farms. New evidence suggests that the critically endangered California Condor is being killed at the Tehachapi Pass wind farm in Southern California. Bat species appear to be at risk during key movement periods. Almost nothing is known about current populations of these species and the impact on bat numbers as a result of mortality at windpower locations. Offshore wind sites 10 km or more from shore do not interact with bat populations but their placement is of great concern if there are nearby bird colonies.
Aesthetics have also been an issue in some areas. In the USA, the Massachusetts Cape Wind project was delayed for years mainly because of aesthetic concerns. In the UK, repeated opinion surveys have shown that more than 70% of people either like, or do not mind, the visual impact. According to a town councillor in Ardrossan, Scotland, the overwhelming majority of locals believe that the Ardrossan Wind Farm has enhanced the area, saying that the turbines are impressive looking and bring a calming effect to the town.
 Effect on power grid
Utility-scale wind farms must have access to transmission lines to transport energy. The wind farm developer may be obligated to install extra equipment or control systems in the wind farm to meet the technical standards set by the operator of a transmission line. The company or person that develops the wind farm can then sell the power on the grid through the transmission lines and ultimately chooses whether to hold on to the rights or sell the farm or parts of it to big business like GE, for example.
Onshore turbine installations in hilly or mountainous regions tend to be on ridgelines generally three kilometers or more inland from the nearest shoreline. This is done to exploit the so-called topographic acceleration as the wind accelerates over a ridge. The additional wind speeds gained in this way make a significant difference to the amount of energy that is produced. Great attention must be paid to the exact positions of the turbines (a process known as micro-siting) because a difference of 30 m can sometimes mean a doubling in output.
Nearshore turbine installations are on land within three kilometers of a shoreline or on water within ten kilometers of land. These areas are good sites for turbine installation, because of wind produced by convection due to differential heating of land and sea each day. Wind speeds in these zones share the characteristics of both onshore and offshore wind,depending on the prevailing wind direction.
Offshore wind development zones are generally considered to be ten kilometers or more from land. Offshore wind turbines are less obtrusive than turbines on land, as their apparent size and noise is mitigated by distance. Because water has less surface roughness than land (especially deeper water), the average wind speed is usually considerably higher over open water. Capacity factors (utilisation rates) are considerably higher than for onshore and nearshore locations.
Transporting large wind turbine components (tower sections, nacelles, and blades) is much easier over water than on land, because ships and barges can handle large loads more easily than trucks/lorries or trains. On land, large goods vehicles must negotiate bends on roadways, which fixes the maximum length of a wind turbine blade that can move from point to point on the road network; no such limitation exists for transport on open water.
Offshore wind turbines will probably continue to be the largest turbines in operation, since the high fixed costs of the installation are spread over more energy production, reducing the average cost. Turbine components (rotor blades, tower sections) can be transported by barge, making large parts easier to transport offshore than on land, where turn clearances and underpass clearances of available roads limit the size of turbine components that can be moved by truck. Similarly, large construction cranes are difficult to move to remote wind farms on land, but crane vessels easily move over water. Offshore wind farms tend to be quite large, often involving over 100 turbines.
In areas with extended shallow continental shelves, water not deeper than 40 m (130 feet), windy but without Category 4 or higher storms, fixed-bottom turbines are now available and practical to install.
Offshore installation monopile wind turbines are generally more expensive than onshore installations but this depends on the attributes of the site. Offshore fixed-bottom towers are generally taller than onshore towers once the submerged height is included. Offshore foundations may be more expensive to build. Power transmission from offshore turbines is through undersea cable, often using high voltage direct current operation if significant distance is to be covered. Offshore saltwater environments also raise maintenance costs by corroding the towers, but fresh-water locations such as the Great Lakes do not. Repairs and maintenance are usually more costly than on onshore turbines, motivating operators to reduce the number of wind turbines for a given total power by installing the largest available units. An example is Belgium's Thorntonbank Wind Farm with construction underway in 2008, featuring 5 MW wind turbines from REpower, which were among the largest wind turbines in the world at the time. Offshore saltwater wind turbines are outfitted with extensive corrosion protection measures including coatings and cathodic protection, which may not be required in fresh water locations.
The province of Ontario in Canada is pursuing several proposed nearshore locations in the Great Lakes, including a project by Trillium Power approximately 20 km from shore and over 700 MW in size. Other Canadian projects include one on the Pacific west coast.
As of 2008, Europe leads the world in development of fixed-bottom offshore wind power, due to strong wind resources and shallow water in the North Sea and the Baltic Sea, and limitations on suitable locations on land due to dense populations and existing developments. Denmark installed the first offshore wind farms, and for years was the world leader in offshore wind power until the United Kingdom gained the lead in October, 2008, with 590 MW of nameplate capacity installed. The United Kingdom planned to build much more extensive offshore wind farms by 2020. Other large markets for wind power, including the United States and China focused first on developing their on-land wind resources where construction costs are lower (such as in the Great Plains of the U.S., and the similarly wind-swept steppes of Xinjiang and Inner Mongolia in China), but population centers along coastlines in many parts of the world are close to offshore wind resources, which would reduce transmission costs.
On 21 December 2007, Q7 (later renamed as Princess Amalia Wind Farm) exported first power to the Dutch grid, which was a milestone for the offshore wind industry. The 120 MW offshore wind farm with a construction budget of €383 million was the first to be financed by a nonrecourse loan (project finance). The project comprises 60 Vestas V80-2MW wind turbines. Each turbine's tower rests on a monopile foundation to a depth of between 18–23 meters at a distance of about 23 km off the Dutch coast.
 Deep-water, floating turbine technologies
In 2009, the first deep-water, large-capacity, floating wind turbine is being built by StatoilHydro. The 2.3 MW turbine can be anchored in water 120–700 m deep. It will be tested off the coast of Norway for two years. The 120-meter-tall tower was towed 10 km offshore into the Amoy Fjord, in 220-meter-deep water, off of Stavanger, Norway on 2009-06-06 for a two year test run. The unit "is expected to start feeding power into the mainland grid by mid-July."
Through 2003, existing offshore wind turbine technology deployments had been limited to water depths of 30-meters utilizing fixed-bottom technology which necessarily limits deployments to the near-coastal sea surface.
Worldwide deep-water wind resources are extremely abundant in deep-water areas with depths up to 600 meters, which are thought to best facilitate transmission of the generated electric power to shore communities. The U.S. deep-water wind resource is second only to China. Although limited early conceptual work on deep-water floating turbine technologies was done in 1972, it was not until the mid 1990’s, after the onshore, foundation-tower, commercial wind industry was well established, that design of deep-water technologies was taken up again by the mainstream research community.
New deep-water, floating-turbine technologies are only recently beginning to be deployed. The first large-capacity floating wind turbine is the Hywind, a 2.3 MW turbine in 220-meter deep water in the North Sea, 10 km southwest of Karmøy, Norway. The unit was assembled and tested in the summer of 2009 and became operational in September, 2009.
Airborne wind turbines would eliminate the cost of towers and might also be flown in high speed winds at high altitude. No such systems are in commercial operation.
 World's Wind farm capacity
In 2007, there were 42 wind farms operating in Australia. Some of the largest wind farms in Australia are:
- Lake Bonney Wind Farm (SA) - 239.5 MW
- Woolnorth Wind Farm (TAS) - 140 MW
- Brown Hill Range Wind Farm (Hallett, SA) - 94.5 MW
- Wattle Point (SA) - 90.75 MW
- Alinta/Walkaway (WA) - 90 MW
- Emu Downs Wind Farm (WA) - 80 MW
- Mount Millar Wind Farm (SA) - 70 MW
During the 1980s the country of Barbados experimented with the construction of a wind turbine at the Lamberts, St. Lucy area of Barbados. A lone tower was built for testing purposes after it was determined that this part of the island had the best potential for the usage of wind power. The Barbados Light and Power Company (BL&P) Co. met opposition due to concerns by local residents about noise concerns. Attempts have been made to replace the current abandoned wind turbine, but opposition continues to mount against the development of the 11 additional turbines for the site which could provide an estimated roughly 10 MW of energy. The Government of Barbados has also reiterated its commitment to developing wind power but has been unsuccessful to date in the last five years.
- São Gonçalo do Amarante/CE (10 Turbines)
- Prainha de Aquiraz-CE (20 Turbines)
- Mucuripe-CE (4 Turbines)
- Fernando de Noronha Island-PE 1&2 (2 Turbines)
- Olinda-PE 1&2 (2 Turbines)
- Morro do Camelinho-MG (4 Turbines)
- Palmas-PR (5 Turbines)
- Osório-RS (75 Turbines)
- Rio do Fogo - RN (61 turbines)
The total capacity of all wind farms in Canada is 2,369 MW as of January, 2009. There are currently no operating wind farms in Nunavut (territory) or the Northwest Territories.
The largest wind farms in Canada are:
- Melancthon EcoPower Centre - Shelburne, Ontario, 199.5 MW
- Wolfe Island Wind Project - Kingston, Ontario, 197.8 MW
- Prince Project — Phase I & II - Sault Ste. Marie, Ontario, 189 MW
- Enbridge Ontario Wind Farm - Bruce County, Ontario, 181 MW
- Murdochville Project; Phase I & II & III - Murdochville, Quebec, 162 MW
- Centennial Wind Power Facility - Swift Current, Saskatchewan, 149.4 MW
- Carleton Wind Farm - Carleton, Quebec, 109.5 MW
- Port Alma Wind Farm - Chatham-Kent, Ontario 101 MW
- Anse-à-Valleau Wind Farm - Gaspé, Quebec, 100.5 MW
- Erie Shores - Port Burwell, Ontario, 99 MW
- St. Leon Wind Farm - St. Leon, Manitoba, 99 MW
- West Cape Wind Park - West Cape, PEI, 99 MW
- Kent Hills - Moncton, New Brunswick, 96 MW
Having more than doubled its installed wind power capacity each year from 2005-2009, China grew its wind power faster on a percentage basis than any other large country. With wind power investment of US$600 million in 2006 and total installed capacity of 2300 MW, China was the eighth largest wind-power producer in the world. At the end of 2007, China had increased its installed capacity to just over 6000 MW to move into fifth place globally. The Chinese wind industry reached the official target of 5 GW for the year 2010 three years early, so policymakers doubled the target to 10 GW; however, by the end of 2009 China had already reached 25 GW, having installed more new wind power generating capacity in 2009 than any other country. Chinese analysts estimate that the total potential wind power generating capacity in China exceeds 1000 GW. Large wind resources are in the northern part of the country, including Xinjiang and Inner Mongolia, with vast windswept plains constituting China's "wind belt" similar to the Great Plains of the United States and Canada. Wind power development is increasing incomes and tourism in these formerly remote regions.
 European Union
Germany has the second largest number of wind farms in the world after the United States. Its installed capacity was 20,622 MW as of December 2006. The second country in capacity was Spain with 11,615 MW although by the end of 2010 the United Kingdom will have the second highest with a total of 12,277 MW. The third was Denmark with 3,136 MW. Italy was in the fourth position, with 2,123 MW.
In May 2006, operational wind farms in the UK comprised an installed capacity of 1,693 MW, in Portugal 1188 MW, in France 918 MW and in Ireland 1255 MW as of the 1st March 2009. The planned 322 MW wind farm south of Glasgow will be the biggest wind farm in Europe. The €350 million farm is ordered by Scottish Power and the 140 wind turbines are to be delivered by Siemens.
In 2006, the British government gave planning consent for the world's largest offshore wind farm, the 'London Array'. It is to be built 12 miles off of the Kent coast and will include 341 turbines. A small farm of eight turbines has been erected at North Pickenham run by Enertrag UK Ltd with two smaller units at nearby Swaffham run by Ecotricity.
An important limiting factor of wind power is variable power generated by wind farms. In most locations the wind blows only part of the time, which means that there has to be back-up capacity of conventional generating capacity to cover periods that the wind is not blowing. To address this issue it has been proposed to create a "supergrid" to connect national grids together across western Europe, ranging from Denmark across the southern North Sea to England and the Celtic Sea to Ireland, and further south to France and Spain especially in Higueruela which was considered for some time the biggest wind farm in the world. The idea is that by the time a low pressure area has moved away from Denmark to the Baltic Sea the next low appears of the coast of Ireland. Therefore, while it is true that the wind is not blowing everywhere all of the time, it will always be blowing somewhere. Such a supergrid would therefore reduce the need for backup capacity.
At the end of September 2007, India had 7660 MW of wind generating capacity and is the fourth largest market in the world. Indian Wind Energy Association has estimated that with the current level of technology, the ‘on-shore’ potential for utilization of wind energy for electricity generation is of the order of 65,000 MW. There are about a dozen wind pumps of various designs providing water for agriculture, afforestation, and domestic purposes, all scattered over the country. The wind farms are predominantly present in the states of Tamil Nadu, Maharashtra, Karnataka and Gujarat. Other states like Andhra Pradesh, Rajasthan, Kerala and Madhya Pradesh have a very good potential.
There is no particular controversy about the sightliness or otherwise of the Wakamatsu ward Hibikinada Wind Farm in Kitakyushu, as there is in some other countries. It is far from the scenic areas of Wakamatsu, and on windy reclaimed land. Asahi Shimbun reported on May 18, 2005, that many utilities have put limits on the amount of wind power they will allow, because of lack of confidence in their ability to deal with the variable output. It should be noted that several European countries are successfully accommodating significantly higher shares of wind energy in to their networks and that the Japanese grid is capable of coping with large conventional power stations disconnecting unexpectedly due to faults; on the other hand, it is true that integrating wind power or unreliable conventional power stations into island grids is more difficult than into continent-wide inter-connected grids.
A partial list of wind farms in Japan include:
- Hibikinada Wind Farm (10 turbines)
- Aoyama Plateau Wind Farm (32 turbines)
- Nunobiki Plateau Wind Farm (33 turbines)
- Seto Wind Farm (11 turbines)
- Tarfaya Wind Farm (200 MW) "Under construction"
- Tangier Wind Farm (140 MW - 165 turbines) "Under construction"
- Amogdoul Farm (60 MW)
- Touahar Farm (60 MW) "Under construction"
- Koudia Al Baida Farm (50 MW - 84 turbines)
 New Zealand
New Zealand is located in the northern latitudes of the 'roaring 40s' — an abundant wind energy resource. The Brooklyn Wind Turbine was installed on the top of a hill in Brooklyn, Wellington in March 1993 as part of a research project commissioned by the now defunct Electricity Corporation of New Zealand. Later in 1996, Wairarapa Electricity (became part of Genesis Energy in 1999) built the Hau Nui Wind Farm, New Zealand's first wind farm, south east of Martinborough on the coastal road to White Rock. Meridian Energy recently applied for, and obtained with conditions, resource consent to build a consignment of wind farms in the rural Makara Hill area west of Wellington. Meridian Energy have finished the Te Apiti Wind Farm on the Ruahine Ranges. It can be seen clearly at Ashhurst near Palmerston North. The Te Rere Hau Wind Farm is under construction nearby. Meridian Energy's White Hill wind farm at Mossburn in the South Island, reached full capacity in 2007. TrustPower purchased the Tararua wind farm, located on the Tararua Ranges behind Palmerston North, from Tararua Wind Power Limited. As of September 2007 this was New Zealand's largest wind farm, and the largest in the southern hemisphere, with an installed capacity of 161MW, half of the country's total installed capacity. Applications for resource consent have been submitted for several new wind farms, with a total potential capacity of 1900MW as of late 2007.
Antarctica‘s largest wind farm located on Ross Island at New Zealand’s Scott Base, the almost 1 MW wind farm is powered by three 333 kW Enercon wind turbines and will provide up to 11% of the power needed by the base, which will cut down on diesel use by 120,000 gallons and reduce carbon dioxide output by 1,370 tons annually.
The Bangui Windmills are located in Bangui, Ilocos Norte, Philippines. The windmills, officially referred to as the NorthWind Bangui Bay Project, was built to use renewable energy sources, thus reducing the greenhouse gases that cause global warming. The project is the first Wind Farm in the Philippines consisting of wind turbines on-shore facing the South China Sea and considered to be the biggest in Southeast Asia. The project sells electricity to the Ilocos Norte Electric Cooperative (INEC) and provides 40% of the power requirements of Ilocos Norte via Transco Laoag.
 South Africa
The first commercial wind farm in South Africa was opened on the 23rd of May 2008, near Darling in the Western Cape. The first phase consists of four 1.3MW turbines supplied by Fuhrlander, Germany. The total power generated estimated at 5.2MW will be put into the national grid at 66kV. It has taken the developer Herman Oelsner 10 years to achieve his dream of being the first private wind farm in South Africa. There has been serious concerns regarding environmental and aviation matters some of which are still under investigation. Fuhrlaender will be responsible for the maintenance and upkeep of the wind farm until 2011 with the assistance of locally trained technicians.
Additionally, Klipheuwel wind farm, the first wind farm in sub-Saharan Africa, comprises three turbines – a Vestas V66 with 1.75 MW output, a Vestas V47 with 660 kW output and a Jeumont J48 with 750 kW output, giving a total output of almost 3.2 MW.
 United States
The United States was the second largest installed capacity of wind power, after Germany until 2008, when it surpassed Germany with the American Wind Energy Association stating that the United States had 21,000 MW of wind energy capacity at the end of 2008. A total of 8,538 MW were added in 2008. At the end of March 2008 the United States wind power capacity was 18,302 MW, which is enough to serve 4.9 million average households. Currently, the largest wind farm in the US – and the largest in the world – is Florida Power & Light's Horse Hollow Wind Energy Center, located in Taylor County, Texas. The Horse Hollow project operates 421 wind turbines and has a capacity of 735 megawatts. Prior to Horse Hollow's completion, the largest US wind farm was the Stateline Wind Project on the Oregon-Washington line, with a peak capacity of 300 megawatts.
Three California wind "farms" arguably have greater combined capacity but are actually collections of dozens of individual wind farms. The California farms have many different owners and turbine types and have been constructed, retrofitted and occasionally dismantled since they were first installed in late 1982. As of 2005, all three of these areas are seeing renewed growth. Primarily, the older and smaller wind turbines are being replaced with much larger, more efficient models. Some of the workhorses of the past were only 65 kilowatts (kW) in capacity or even smaller, though some were several hundred kW. Today, a few models approach 6,000 kW (6 MW). Secondarily, non-functional turbines are also being returned to service.
Northern California is home to one of the earliest large wind farms. An advantage of the Altamont Pass Wind Farm is that under hot inland (Central Valley) conditions, a thermal low is developed that brings in cool coastal marine air, driving the turbines at a time of maximum electricity demand. However, this phenomenon is not always reliable and with an inland high pressure condition the entire region can be both hot and windless. At this time additional power must be provided by natural gas-powered gas turbine peaker plants. From 2003 to 2006, dozens of state-of-the-art turbines were installed at the Montezuma Hills near the Sacramento River delta. Eight of the turbines, at 415 feet tall, are the largest in the United States—and are 110 feet taller than the Statue of Liberty. These 3-megawatt Vestas wind turbines each produce enough power to meet the annual needs of more than 1,000 households.
Even though California has some of the earliest and largest wind farms in the U.S., it does not have very many commercially viable wind farm sites, at least not onshore. Much of the Southwest is not much better, although there are some significant exceptions. The Great Plains states have an abundance of suitable sites for wind energy development and has become the major supplier of U.S. wind power. Texas (in the South) is the leading wind power state in the U.S. followed by Iowa in the Midwest. The Pacific Northwest and the Northeast also have many excellent sites as well. In contrast, the Southeast has very poor wind energy resources, though the Appalachian Mountains do provide a few good areas.
 Criticism and challenges
Wind farms are an expensive and inefficient way of generating sustainable energy, according to a study from Germany, the world's leading producer of wind energy. Critics of wind energy said it would be cheaper and more environmentally efficient to insulate old houses or to renew existing power stations. The main challenge with wind farms is that it is built in places where were electricity is not needed. The electricity then has to be moved somewhere else and that provide additional cost in transmission.