Wind farm

A wind farm is a group of wind turbines in the same location used for production of electric power. Individual turbines are interconnected with a medium voltage (usually 34.5 kV) 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. A large wind farm may consist of a few dozen to about 100 individual wind turbines, and cover an extended area of hundreds of square miles (square kilometers), 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.

A proposed solution for the intermittency of wind power and other renewable power sources is to create a supergrid of interconnected wind farms across western Europe. This large-scale array of dispersed wind farms would be located in different wind regimes, reducing the overall variation in power output.

Wind speed


As a general rule, wind generators are practical where the average wind speed 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. A vitally 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 sometimes 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.

Wind farm siting can sometimes be highly controversial, particularly when sites are picturesque or environmentally sensitive (for instance, having substantial bird life).

Altitude
The wind blows faster at higher altitudes because of the reduced influence of drag of the surface and lower air viscosity. 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.

Environmental and aesthetic impacts
Nearshore and certain inland wind sites may have significant aesthetic impact, since the turbines are visible for great distances. However, both positive and negative impacts may occur. Some people find large wind turbines unsightly, whereas some wind farms have become tourist attractions. Wind farm siting must also consider impacts on wildlife, including migratory animals. Wind project proponents may face opposition from area residents concerned about sound level, light flicker, appearance, and the other impacts of wind turbine placement.

Development
To develop a wind farm, a suitable location is first identified. Good locations for wind farms should have fast steady winds and be near transmission lines. Land parcels on which wind turbines will be located then must be leased from the land owners. The wind resource must then be evaluated using data recorded by onsite meteorological towers. The wind farm project must then be financed and constructed.

Onshore
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
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.

Common issues that are shared within nearshore wind development zones are bird migration and nesting, aquatic habitat, transportation (including shipping and boating) and visual aesthetics. Residents near some sites have strongly opposed the installation of wind farms due to these concerns.

Offshore
List of offshore wind farms 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 can be 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.

In stormy areas with extended shallow continental shelves, turbines are practical to install.

Offshore installation is more expensive than onshore but this depends on the attributes of the site. Offshore 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. Offshore installations may use high voltage direct current operation if significant distance is to be covered. Offshore saltwater environments can 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. Offshore saltwater wind turbines are outfitted with extensive corrosion protection measures like coatings and cathodic protection, which may not be required in fresh water locations.

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 may limit the size of turbine components that can move by truck. Similarly, large construction cranes may be 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.

Denmark, for example, has many offshore windfarms.

The United Kingdom plans to use offshore wind turbines to generate enough power to light every home in the U.K. by 2020.

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.

Airborne
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.

Australia
In 2007, there were 42 wind farms operating in Australia. Some of the largest wind farms in Australia are:


 * 1) Lake Bonney Wind Farm (SA) - 239.5 MW
 * 2) Brown Hill Range Wind Farm (Hallett, SA) - 94.5 MW
 * 3) Wattle Point (SA) - 90.75 MW
 * 4) Alinta/Walkaway (WA) - 90 MW
 * 5) Emu Downs Wind Farm (WA) - 80 MW
 * 6) Mount Millar Wind Farm (SA) - 70 MW
 * 7) Cathedral Rocks (SA) - 66 MW

Brazil

 * 1) São Gonçalo do Amarante/CE (10 Turbines)
 * 2) Prainha de Aquiraz-CE (20 Turbines)
 * 3) Mucuripe-CE (4 Turbines)
 * 4) Fernando de Noronha Island-PE 1&2 (2 Turbines)
 * 5) Olinda-PE 1&2 (2 Turbines)
 * 6) Morro do Camelinho-MG (4 Turbines)
 * 7) Palmas-PR (5 Turbines)
 * 8) Osório-RS (75 Turbines)

Canada


The total capacity of all wind farms in Canada is approximately 1,856 MW as of January, 2008. There are currently no operating wind farms in British Columbia, New Brunswick, Nunavut (territory), or the Northwest Territories.

The five largest wind farms in Canada are:
 * 1) Prince Project — Phase I&II, (Sault Ste. Marie, Ontario), 189 MW
 * 2) Murdochville Project — Phase I&II&III (Murdochville, Quebec), 162 MW
 * 3) Centennial, (Swift Current, Saskatchewan), 149.4 MW
 * 4) Erie Shores (Port Burwell, Ontario), 99 MW
 * 5) Kent Hills, (Moncton, New Brunswick), 96 MW

China
Having more than doubled its installed wind power capacity each year from 2005-2007, China grew its wind power faster on a percentage basis than any other large country. With wind power investment of $600 million dollars U.S. 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; if current trends continue, they may double the target again to 20 GW by 2010. Chinese analysts estimate that the total potential wind power generating capacity in China exceeds 1 million MW. 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 largest number of wind farms in the world. Its installed capacity was 20,622 MW as of December 2006. The second country in capacity was Spain with 11,615 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 the Republic of Ireland 496 MW. 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" of interconnected wind farms 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.

India


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.

Japan


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 in to 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)

A number of smaller projects are run by the Japan Wind Development Company, LTD.

New Zealand
New Zealand is located in the northern latitudes of the 'roaring 40s' — an abundant wind energy resource. Genesis Energy built the Hau Nui wind farm in 1996 and is located south east of Martinborough on the coastal road to White Rock. 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. 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, with an installed capacity of 161MW, half of the country's total installed capacity. Applications for resource content have been submitted for several new wind farms, with a total potential capacity of 1900MW as of late 2007.

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 enormous concerns regarding environmental and aviation some of which still need to be resolved. DWP (Darling Wind Power) will be responsible for the maintenance and upkeep of the wind farm.

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 has the second largest installed capacity of wind power, after Germany. 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 than the Stateline farm, 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, the smallest utility-scale wind turbines are about 700 kW, with a few models approaching 5,000 kW (5 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. Solano County has one of the five major wind farms in California and has integrated the most advanced wind power technology anywhere in the United States. 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 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 however the region's potential is still largely untapped. Iowa and Minnesota are leading the Midwest in the development of wind energy with their combined capacities expected to reach 2,000 MW in 2007. The Pacific Northwest and the Northeast both have many excellent sites as well. In contrast, the Southeast has a very poor wind energy resource, though the Appalachian Mountains do provide a few good areas.