# Wind power

Burbo Bank Offshore Wind Farm, at the entrance to the River Mersey in North West England.
The Shepherds Flat Wind Farm is a 845 megawatt (MW) wind farm in the U.S. state of Oregon.

Wind power is the conversion of wind energy into a useful form of energy, such as using wind turbines to make electrical power, windmills for mechanical power, windpumps for water pumping or drainage, or sails to propel ships.

Large wind farms consist of hundreds of individual wind turbines which are connected to the electric power transmission network. For new constructions, onshore wind is an inexpensive source of electricity, competitive with or in many places cheaper than fossil fuel plants.[1][2] Small onshore wind farms provide electricity to isolated locations. Utility companies increasingly buy surplus electricity produced by small domestic wind turbines.[3] Offshore wind is steadier and stronger than on land, and offshore farms have less visual impact, but construction and maintenance costs are considerably higher.

Wind power, as an alternative to fossil fuels, is plentiful, renewable, widely distributed, clean, produces no greenhouse gas emissions during operation and uses little land.[4] The effects on the environment are generally less problematic than those from other power sources. As of 2011, Denmark is generating more than a quarter of its electricity from wind and 83 countries around the world are using wind power to supply the electricity grid.[5] In 2010 wind energy production was over 2.5% of total worldwide electricity usage, and growing rapidly at more than 25% per annum.

Wind power is very consistent from year to year but has significant variation over shorter time scales. As the proportion of windpower in a region increases, a need to upgrade the grid, and a lowered ability to supplant conventional production can occur.[6][7] Power management techniques such as having excess capacity storage, geographically distributed turbines, dispatchable backing sources, storage such as pumped-storage hydroelectricity, exporting and importing power to neighboring areas or reducing demand when wind production is low, can greatly mitigate these problems.[8] In addition, weather forecasting permits the electricity network to be readied for the predictable variations in production that occur.[9][10]

## History

### Mechanical power

Medieval depiction of a wind mill

Sailboats and sailing ships have been using wind power for thousands of years, and architects have used wind-driven natural ventilation in buildings since similarly ancient times. The use of wind to provide mechanical power came somewhat later in antiquity. The windwheel of the Greek engineer Heron of Alexandria in the 1st century AD is the earliest known instance of using a wind-driven wheel to power a machine.[11][12][13]

The first windmills were in use in Persia at least by the 9th century and possibly as early as the 7th century.[14] The use of windmills became widespread across the Middle East and Central Asia, and later spread to China and India.[11] By 1000 AD, windmills were used to pump seawater for salt-making in China and Sicily.[15] Windmills were used extensively in Northwestern Europe to grind flour from the 1180s,[12] and windpumps were used to drain land for agriculture and for building.[16] Early immigrants to the New World brought the technology with them from Europe.[16]

In the US, the development of the water-pumping windmill was the major factor in allowing the farming and ranching of vast areas otherwise devoid of readily accessible water.[17] Windpumps contributed to the expansion of rail transport systems throughout the world, by pumping water from water wells for steam locomotives. The multi-bladed wind turbine atop a lattice tower made of wood or steel was a century a fixture of the landscape throughout rural America.[18]

In 1881, Lord Kelvin proposed using wind power when coal ran out, as "so little of it is left".[19] Solar power was also proposed, at about the same time.[20]

### Electrical power

Trends in the top five countries generating electricity from wind, 1980-2012 (US EIA)
Blyth's "windmill" at his cottage in Marykirk in 1891

In July 1887, a Scottish academic, Professor James Blyth, built a cloth-sailed wind turbine in the garden of his holiday cottage in Marykirk and used the electricity it produced to charge accumulators which he used to power the lights in his cottage.[21] His experiments culminated in a UK patent in 1891.[22] In the winter of 1887–1888 US inventor Charles F. Brush produced electricity using a wind powered generator which powered his home and laboratory until about 1900. In the 1890s, the Danish scientist and inventor Poul la Cour constructed wind turbines to generate electricity, which was used to produce hydrogen and Oxygen by electrolysis and a mixture of the two gases was stored for use as a fuel.[22] La Cour was the first to discover that fast rotating wind turbines with fewer rotor blades were the most efficient in generating electricity and in 1904 he founded the Society of Wind Electricians.[23]

By the mid-1920s, 1 to 3-kilowatt wind generators developed by companies such as Parris-Dunn and Jacobs Wind-electric[19] found widespread use in the rural areas of the mid-western Great Plains of the US but by the 1940s the demand for more power and the coming of the electrical grid throughout those areas made these small generators obsolete.[24]

IN 1931 the French aeronautical engineer, George Darrieus was granted a patent for the Darrieus wind turbine which uses vertical-axis airfoils to create rotation[25] and a 100 kW precursor to the modern horizontal wind generator was used in Yalta, in the USSR. In 1956 Johannes Juul, a former student of la Cour, built a 200 kW, three-bladed turbine at Gedser in Denmark, which influenced the design of many later turbines.[23]

In 1975 the United States Department of Energy funded a project to develop utility-scale wind turbines. The NASA wind turbines project built thirteen experimental turbines which paved the way for much of the technology used today.[23] Since then, turbines have increased greatly in size with the Enercon E-126 capable of delivering up to 7.5 Megawatts (MW).[nb 1][26] Wind turbine production has expanded to many countries and wind power is expected to grow worldwide in the twenty-first century.

## Wind energy

Wind energy is the kinetic energy of air in motion, also called wind. Total wind energy flowing through an imaginary area A during the time t is:

$E = \frac{1}{2}mv^2 = \frac{1}{2}(Avt\rho)v^2 = \frac{1}{2}At\rho v^3,$[27]

where ρ is the density of air; v is the wind speed; Avt is the volume of air passing through A (which is considered perpendicular to the direction of the wind); Avtρ is therefore the mass m passing per unit time. Note that ½ ρv2 is the kinetic energy of the moving air per unit volume.

Power is energy per unit time, so the wind power incident on A (e.g. equal to the rotor area of a wind turbine) is:

$P = \frac{E}{t} = \frac{1}{2}A\rho v^3.$[27]

Wind power in an open air stream is thus proportional to the third power of the wind speed; the available power increases eightfold when the wind speed doubles. Wind turbines for grid electricity therefore need to be especially efficient at greater wind speeds.

Map of available wind power for the United States. Color codes indicate wind power density class. (click to see larger)

Wind is the movement of air across the surface of the Earth, affected by areas of high pressure and of low pressure.[28] The surface of the Earth is heated unevenly by the Sun, depending on factors such as the angle of incidence of the sun's rays at the surface (which differs with latitude and time of day) and whether the land is open or covered with vegetation. Also, large bodies of water, such as the oceans, heat up and cool down slower than the land. The heat energy absorbed at the Earth's surface is transferred to the air directly above it and, as warmer air is less dense than cooler air, it rises above the cool air to form areas of high pressure and thus pressure differentials. The rotation of the Earth drags the atmosphere around with it causing turbulence. These effects combine to cause a constantly varying pattern of winds across the surface of the Earth.[28]

The total amount of economically extractable power available from the wind is considerably more than present human power use from all sources.[29] Axel Kleidon of the Max Planck Institute in Germany, carried out a "top down" calculation on how much wind energy there is, starting with the incoming solar radiation that drives the winds by creating temperature differences in the atmosphere. He concluded that somewhere between 18 TW and 68 TW could be extracted.[30] Cristina Archer and Mark Z. Jacobson presented a "bottom-up" estimate, which unlike Kleidon's are based on actual measurements of wind speeds, and found that there is 1700 TW of wind power at an altitude of 100 metres over land and sea. Of this, "between 72 and 170 TW could be extracted in a practical and cost-competitive manner".[30] They later estimated 80 TW.[31] However research at Harvard University estimates 1 Watt/m2 on average and 2–10 MW/km2 capacity for large scale wind farms, suggesting that these estimates of total global wind resources are too high by a factor of about 4.[32]

### 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.

The strength of wind 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 Weibull model closely mirrors the actual distribution of hourly/ten-minute wind speeds at many locations. The Weibull factor is often close to 2 and therefore a Rayleigh distribution can be used as a less accurate, but simpler model.[33]

### High altitude winds

Power generation from winds usually comes from winds very close to the surface of the earth. Winds at higher altitudes are stronger and more consistent, and may have a global capacity of 380 TW.[31] Recent years have seen significant advances in technologies meant to generate electricity from high altitude winds.[34]

## Wind farms

Wind turbines at the Jepirachí Eolian Park in La Guajira, Colombia.

A wind farm is a group of wind turbines in the same location used for production of electricity. A large wind farm may consist of several hundred individual wind turbines distributed over an extended area, but the land between the turbines may be used for agricultural or other purposes. A wind farm may also be located offshore.

Almost all large wind turbines have the same design — a horizontal axis wind turbine having an upwind rotor with three blades, attached to a nacelle on top of a tall tubular tower. In a wind farm, individual turbines are interconnected with a medium voltage (often 34.5 kV), power collection system and communications network. At a substation, this medium-voltage electric current is increased in voltage with a transformer for connection to the high voltage electric power transmission system.

Many of the largest operational onshore wind farms are located in the US. As of 2012, the Alta Wind Energy Center is the largest onshore wind farm in the world at 1020 MW, followed by the Shepherds Flat Wind Farm (845 MW), and the Roscoe Wind Farm (781.5 MW). As of September 2012, the Sheringham Shoal Offshore Wind Farm and the Thanet Wind Farm in the UK are the largest offshore wind farms in the world at 317 MW and 300 MW, followed by Horns Rev II (209 MW) in Denmark.

There are many large wind farms under construction including; The London Array (offshore) (630 MW), BARD Offshore 1 (400 MW), Sheringham Shoal Offshore Wind Farm (317 MW), Lincs Wind Farm (offshore), Clyde Wind Farm (548 MW), Greater Gabbard wind farm (500 MW), Macarthur Wind Farm (420 MW), Lower Snake River Wind Project (343 MW) and Walney Wind Farm (367 MW).

A panoramic view of the Whitelee Wind Farm with Lochgoin Reservoir in the foreground.

### Feeding into grid

Induction generators, often used for wind power, require reactive power for excitation so substations used in wind-power collection systems include substantial capacitor banks for power factor correction.[35] 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 (see: Low voltage ride through). In particular, induction generators cannot support the system voltage during faults, unlike steam or hydro turbine-driven synchronous generators. Doubly fed machines generally have more desirable properties for grid interconnection.[36][37] 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 behavior of the wind farm turbines during a system fault.[38][39]

### Offshore wind power

Offshore wind power refers to the construction of wind farms in large bodies of water to generate electricity. These installations can utilise the more frequent and powerful winds that are available in these locations and have less aesthetic impact on the landscape than land based projects. However, the construction and the maintenance costs are considerably higher.[40][41] As of 2011, offshore wind farms were at least 3 times more expensive than onshore wind farms of the same nominal power[42] but these costs are expected to fall as the industry matures.[43]

Siemens and Vestas are the leading turbine suppliers for offshore wind power. DONG Energy, Vattenfall and E.ON are the leading offshore operators.[44] As of October 2010, 3.16 GW of offshore wind power capacity was operational, mainly in Northern Europe. According to BTM Consult, more than 16 GW of additional capacity will be installed before the end of 2014 and the UK and Germany will become the two leading markets. Offshore wind power capacity is expected to reach a total of 75 GW worldwide by 2020, with significant contributions from China and the US.[44]

## Wind power capacity and production

Worldwide wind generation up to 2010

Worldwide there are now over two hundred thousand wind turbines operating, with a total nameplate capacity of 282,482 MW as of end 2012.[45] The European Union alone passed some 100,000 MW nameplate capacity in September 2012,[46] while the United States surpassed 50,000 MW in August 2012 and China's grid connected capacity passed 50,000 MW the same month.[47][48]

World wind generation capacity more than quadrupled between 2000 and 2006, doubling about every three years. The United States pioneered wind farms and led the world in installed capacity in the 1980s and into the 1990s. In 1997 German installed capacity surpassed the U.S. and led until once again overtaken by the U.S. in 2008. China has been rapidly expanding its wind installations in the late 2000s and passed the U.S. in 2010 to become the world leader.

At the end of 2012, worldwide nameplate capacity of wind-powered generators was 282 gigawatts (GW), growing by 44 GW over the preceding year.[45] According to the World Wind Energy Association, an industry organization, in 2010 wind power generated 430 TWh or about 2.5% of worldwide electricity usage,[49] up from 1.5% in 2008 and 0.1% in 1997.[50] Between 2005 and 2010 the average annual growth in new installations was 27.6%.[51] Wind power market penetration is expected to reach 3.35% by 2013 and 8% by 2018.[51][52]

Several countries have already achieved relatively high levels of penetration, such as 28% of stationary (grid) electricity production in Denmark (2011),[53] 19% in Portugal (2011),[54] 16% in Spain (2011),[55] 16% in Ireland (2012)[56] and 8% in Germany (2011).[57] As of 2011, 83 countries around the world were using wind power on a commercial basis.[5]

Europe accounted for 48% of the world total wind power generation capacity in 2009. In 2010, Spain became Europe's leading producer of wind energy, achieving 42,976 GWh. Germany held the top spot in Europe in terms of installed capacity, with a total of 27,215 MW as of 31 December 2010.[58]

Top 10 countries
by nameplate windpower capacity
(2012 year-end)[45]
Country New 2012
capacity (MW)
Windpower total capacity
(MW)
% world total
China 12,960 75,324 26.7
United States 13,124 60,007 21.2
Germany 2,145 31,308 11.1
Spain 1,122 22,796 8.1
India 2,336 18,421 6.5
UK 1,897 8,845 3.0
Italy 1,273 8,144 2.9
France 757 7,564 2.7
Portugal 145 4,525 1.6
(rest of world) 6,737 39,853 14.1
World total 44,799 MW 282,587 MW 100%
Top 10 countries
by windpower electricity production
(2011 totals)[59]
Country Windpower production
(TWh)
% world total
United States 120.5 26.2
China 88.6 19.3
Germany 48.9 10.6
Spain 42.4 9.2
India 24.9 5.4
UK 15.5 3.4
France 12.2 2.7
Italy 9.9 2.1
Denmark 9.8 2.1
(rest of world) 67.7 14.7
World total 459.9 TWh 100%

### Growth trends

Worldwide installed capacity 1997–2020 [MW], developments and prognosis. Data source: WWEA[60]
Worldwide installed wind power capacity forecast (Source: Global Wind Energy Council)[61][62]

In 2010, more than half of all new wind power was added outside of the traditional markets in Europe and North America. This was largely from new construction in China, which accounted for nearly half the new wind installations (16.5 GW).[63]

Global Wind Energy Council (GWEC) figures show that 2007 recorded an increase of installed capacity of 20 GW, taking the total installed wind energy capacity to 94 GW, up from 74 GW in 2006. Despite constraints facing supply chains for wind turbines, the annual market for wind continued to increase at an estimated rate of 37%, following 32% growth in 2006. In terms of economic value, the wind energy sector has become one of the important players in the energy markets, with the total value of new generating equipment installed in 2007 reaching €25 billion, or US$36 billion.[64] Although the wind power industry was affected by the global financial crisis in 2009 and 2010, a BTM Consult five-year forecast up to 2013 projects substantial growth. Over the past five years the average growth in new installations has been 27.6% each year. In the forecast to 2013 the expected average annual growth rate is 15.7%.[51][52] More than 200 GW of new wind power capacity could come on line before the end of 2013. Wind power market penetration is expected to reach 3.35% by 2013 and 8% by 2018.[51][52] ### 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 15–50%; values at the upper end of the range are achieved in favourable sites and are due to wind turbine design improvements.[65][66][nb 2] Online data is available for some locations, and the capacity factor can be calculated from the yearly output.[67][68] For example, the German nation-wide average wind power capacity factor over all of 2012 was just under 17.5% (45867 GW·h/yr / (29.9 GW × 24 × 366) = 0.1746),[69] and the capacity factor for Scottish wind farms averaged 24% between 2008 and 2010.[70] Unlike fueled generating plants, the capacity factor is affected by several parameters, including the variability of the wind at the site and the size of the generator relative to the turbine's swept area. A small generator would be cheaper and achieve a higher capacity factor but would produce less electricity (and thus less profit) in high winds. Conversely, a large generator would cost more but generate little extra power and, depending on the type, may stall out at low wind speed. Thus an optimum capacity factor of around 40–50% would be aimed for.[66][71] In a 2008 study released by the U.S. Department of Energy's Office of Energy Efficiency and Renewable Energy, the capacity factor achieved by the U.S. wind turbine fleet is shown to be increasing as the technology improves. The capacity factor achieved by new wind turbines in 2010 reached almost 40%.[72][73] ### 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 energy storage, 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 compensate for the varying power generation produced by wind plants. Studies have indicated that 20% of the total annual electrical energy consumption may be incorporated with minimal difficulty.[74] These studies have been for locations with geographically dispersed wind farms, some degree of dispatchable energy or hydropower with storage capacity, demand management, and interconnected to a large grid area enabling the export of electricity when needed. Beyond the 20% level, there are few technical limits, but the economic implications become more significant. Electrical utilities continue to study the effects of large scale penetration of wind generation on system stability and economics.[75][76][77][78] A wind energy penetration figure can be specified for different durations of time. On an annual basis, as of 2011, few grid systems have penetration levels above 5%: Denmark – 29%, Portugal – 19%, Spain – 19%, Ireland – 18%, and Germany – 11%. For the U.S. in 2011, the penetration level was estimated at 3.3%.[79] To obtain 100% from wind annually requires substantial long term storage. On a monthly, weekly, daily, or hourly basis—or less—wind can supply as much as or more than 100% of current use, with the rest stored or exported. Seasonal industry can take advantage of high wind and low usage times such as at night when wind output can exceed normal demand. Such industry can include production of silicon, aluminum, steel, or of natural gas, and hydrogen, which allow long term storage, facilitating 100% energy from variable renewable energy.[80][81] Homes can also be programmed to accept extra electricity on demand, for example by remotely turning up water heater thermostats.[82] ### Variability Windmills are typically installed in favourable windy locations. In the image, wind power generators in Spain, near an Osborne bull. Electricity generated from wind power can be highly variable at several different timescales: hourly, daily, or 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 an increase in the already existing energy demand management, load shedding, storage solutions or system interconnection with HVDC cables. Fluctuations in load and allowance for failure of large fossil-fuel generating units require reserve capacity that can also compensate for variability of wind generation. Increase in system operation costs, Euros per MWh, for 10% & 20% wind share[6] Country 10% 20% Germany 2.5 3.2 Denmark 0.4 0.8 Finland 0.3 1.5 Norway 0.1 0.3 Sweden 0.3 0.7 Wind power is however, variable, but during low wind periods it can be replaced by other power sources. Transmission networks presently cope with outages of other generation plants and daily changes in electrical demand, but the variability of intermittent power sources such as wind power, are unlike those of conventional power generation plants which, when scheduled to be operating, may be able to deliver their nameplate capacity around 95% of the time. Presently, grid systems with large wind penetration require a small increase in the frequency of usage of natural gas spinning reserve power plants to prevent a loss of electricity in the event that conditions are not favorable for power production from the wind. At lower wind power grid penetration, this is less of an issue.[83][84][85] GE has installed a prototype wind turbine with onboard battery similar to that of an electric car, equivalent of 1 minute of production. Despite the small capacity, it is enough to guarantee that power output complies with forecast for 15 minutes, as the battery is used to eliminate the difference rather than provide full output. The increased predictability can be used to take wind power penetration from 20 to 30 or 40 per cent. The battery cost can be retrieved by selling burst power on demand and reducing backup needs from gas plants.[86] A report on Denmark's wind power noted that their wind power network provided less than 1% of average demand on 54 days during the year 2002.[87] Wind power advocates argue that these periods of low wind can be dealt with by simply restarting existing power stations that have been held in readiness, or interlinking with HVDC.[88] Electrical grids with slow-responding thermal power plants and without ties to networks with hydroelectric generation may have to limit the use of wind power.[87] According to a 2007 Stanford University study published in the Journal of Applied Meteorology and Climatology, interconnecting ten or more wind farms can allow an average of 33% of the total energy produced (i.e. about 8% of total nameplate capacity) to be used as reliable, baseload electric power which can be relied on to handle peak loads, as long as minimum criteria are met for wind speed and turbine height.[89][90] Conversely, on particularly windy days, even with penetration levels of 16%, wind power generation can surpass all other electricity sources in a country. In Spain, on 16 April 2012 wind power production reached the highest percentage of electricity production till then, with wind farms covering 60.46% of the total demand.[91] In Denmark, which had power market penetration of 30% in 2013, over 90 hours, wind power generated 100% of the countries power, peaking at 122% of the countries demand at 2am on the 28th October.[92] A 2006 International Energy Agency forum presented costs for managing intermittency as a function of wind-energy's share of total capacity for several countries, as shown in the table on the right. Three reports on the wind variability in the UK issued in 2009, generally agree that variability of wind needs to be taken into account, but it does not make the grid unmanageable. The additional costs, which are modest, can be quantified.[7] Solar power tends to be complementary to wind.[93][94] On daily to weekly timescales, high pressure areas tend to bring clear skies and low surface winds, whereas low pressure areas tend to be windier and cloudier. On seasonal timescales, solar energy peaks in summer, whereas in many areas wind energy is lower in summer and higher in winter.[nb 3][95] Thus the intermittencies of wind and solar power tend to cancel each other somewhat. In 2007 the Institute for Solar Energy Supply Technology of the University of Kassel pilot-tested a combined power plant linking solar, wind, biogas and hydrostorage to provide load-following power around the clock and throughout the year, entirely from renewable sources.[96] ### Predictability Wind power forecasting methods are used, but predictability of any particular wind farm is low for short-term operation. For any particular generator there is an 80% chance that wind output will change less than 10% in an hour and a 40% chance that it will change 10% or more in 5 hours.[97] However, studies by Graham Sinden (2009) suggest that, in practice, the variations in thousands of wind turbines, spread out over several different sites and wind regimes, are smoothed. As the distance between sites increases, the correlation between wind speeds measured at those sites, decreases.[98] Thus, while the output from a single turbine can vary greatly and rapidly as local wind speeds vary, as more turbines are connected over larger and larger areas the average power output becomes less variable and more predictable.[99] Wind speeds can be accurately forecast over large areas, and hence wind is a predictable source of power for feeding into an electrical grid. However, due to the variability, although predictable, wind energy availability must be scheduled.[citation needed] ### Reliability Wind power hardly ever suffers major technical failures, since failures of individual wind turbines have hardly any effect on overall power, so that the distributed wind power is highly reliable and predictable,[100] whereas conventional generators, while far less variable, can suffer major unpredictable outages. ### Integration with other sources The combination of diversifying variable renewables by type and location, forecasting their variation, and integrating them with dispatchable renewables, flexible fueled generators, and demand response can create a power system that has the potential to meet power supply needs reliably. Integrating ever-higher levels of renewables is being successfully demonstrated in the real world:[101] In 2009, eight American and three European authorities, writing in the leading electrical engineers' professional journal, didn't find "a credible and firm technical limit to the amount of wind energy that can be accommodated by electricity grids". In fact, not one of more than 200 international studies, nor official studies for the eastern and western U.S. regions, nor the International Energy Agency, has found major costs or technical barriers to reliably integrating up to 30% variable renewable supplies into the grid, and in some studies much more. – Reinventing Fire[101] ### Energy storage Typically, conventional hydroelectricity complements wind power very well. When the wind is blowing strongly, nearby hydroelectric plants can temporarily hold back their water. When the wind drops they can, provided they have the generation capacity, rapidly increase production to compensate. This gives a very even overall power supply and virtually no loss of energy and uses no more water. Alternatively, where a suitable head of water is not available, pumped-storage hydroelectricity or other forms of grid energy storage can store energy developed by high-wind periods and release it when needed.[102] The type of storage needed depends on the wind penetration level – low penetration requires daily storage, and high penetration requires both short and long term storage – as long as a month or more. 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 any wind energy stored but it is not envisaged that this would apply to a large proportion of wind energy generated. For example, in the UK, the 1.7 GW Dinorwig pumped storage plant evens out electrical demand peaks, and allows base-load suppliers to run their plants more efficiently. Although pumped storage power systems are only about 75% efficient, and have high installation costs, their low running costs and ability to reduce the required electrical base-load can save both fuel and total electrical generation costs.[103][104] In particular geographic regions, peak wind speeds may not coincide with peak demand for electrical power. In the US states of California and Texas, for example, hot days in summer may have low wind speed and high electrical demand due to the use of air conditioning. Some utilities subsidize the purchase of geothermal heat pumps by their customers, to reduce electricity demand during the summer months by making air conditioning up to 70% more efficient;[105] widespread adoption of this technology would better match electricity demand to wind availability in areas with hot summers and low summer winds. Another option is to interconnect widely dispersed geographic areas with an HVDC "Super grid". In the U.S. it is estimated that to upgrade the transmission system to take in planned or potential renewables would cost at least$60 billion.[106]

Germany has an installed capacity of wind and solar that exceeds daily demand, and has been exporting peak power to neighboring countries.[citation needed] A more practical solution is the installation of thirty days storage capacity able to supply 80% of demand, which will become necessary when most of Europe's energy is obtained from wind power and solar power. Just as the EU requires member countries to maintain 90 days strategic reserves of oil it can be expected that countries will provide electricity storage, instead of expecting to use their neighbors for net metering.[107]

### Capacity credit and fuel savings

The capacity credit of wind is estimated by determining the capacity of conventional plants displaced by wind power, whilst maintaining the same degree of system security,.[108] However, the precise value is irrelevant since the main value of wind is its fuel and CO2 savings,[citation needed] and wind is not expected to be constantly available.[109]

## Economics

Wind turbines reached grid parity (the point at which the cost of wind power matches traditional sources) in some areas of Europe in the mid-2000s, and in the US around the same time. Falling prices continue to drive the levelized cost down and it has been suggested that it has reached general grid parity in Europe in 2010, and will reach the same point in the US around 2016 due to an expected reduction in capital costs of about 12%.[1] Nevertheless, a significant amount of the wind power resource in North America remains above grid parity due to the long transmission distances involved.

### Cost trends

Estimated cost per MWh for wind power in Denmark
The National Renewable Energy Laboratory projects that the levelized cost of wind power in the U.S. will decline about 25% from 2012 to 2030.[110]
A turbine blade convoy passing through Edenfield in the U.K. (2008). Even longer two-piece blades are now manufactured, and then assembled on-site to reduce difficulties in transportion.

Wind power is capital intensive, but has no fuel costs.[111] The marginal cost of wind energy once a plant is constructed is usually less than 1-cent per kW·h.[112] This cost has reduced as wind turbine technology improved. There are now longer and lighter wind turbine blades, improvements in turbine performance and increased power generation efficiency. Also, wind project capital and maintenance costs have continued to decline.[113]

The estimated average cost per unit incorporates the cost of construction of the turbine and transmission facilities, borrowed funds, return to investors (including cost of risk), estimated annual production, and other components, averaged over the projected useful life of the equipment, which may be in excess of twenty years. Energy cost estimates are highly dependent on these assumptions so published cost figures can differ substantially. In 2004, wind energy cost a fifth of what it did in the 1980s, and some expected that downward trend to continue as larger multi-megawatt turbines were mass-produced.[114] As of 2012 capital costs for wind turbines are substantially lower than 2008–2010 but are still above 2002 levels.[115] A 2011 report from the American Wind Energy Association stated, "Wind's costs have dropped over the past two years, in the range of 5 to 6 cents per kilowatt-hour recently.... about 2 cents cheaper than coal-fired electricity, and more projects were financed through debt arrangements than tax equity structures last year.... winning more mainstream acceptance from Wall Street's banks.... Equipment makers can also deliver products in the same year that they are ordered instead of waiting up to three years as was the case in previous cycles.... 5,600 MW of new installed capacity is under construction in the United States, more than double the number at this point in 2010. Thirty-five percent of all new power generation built in the United States since 2005 has come from wind, more than new gas and coal plants combined, as power providers are increasingly enticed to wind as a convenient hedge against unpredictable commodity price moves."[116]

A British Wind Energy Association report gives an average generation cost of onshore wind power of around 3.2 pence (between US 5 and 6 cents) per kW·h (2005).[117] Cost per unit of energy produced was estimated in 2006 to be comparable to the cost of new generating capacity in the US for coal and natural gas: wind cost was estimated at $55.80 per MW·h, coal at$53.10/MW·h and natural gas at $52.50.[118] Similar comparative results with natural gas were obtained in a governmental study in the UK in 2011.[119] The presence of wind energy, even when subsidised, can reduce costs for consumers (€5 billion/yr in Germany) by reducing the marginal price, by minimising the use of expensive peaking power plants.[120] In February 2013 Bloomberg New Energy Finance reported that the cost of generating electricity from new wind farms is cheaper than new coal or new baseload gas plants. When including the current Australian federal government carbon pricing scheme their modeling gives costs (in Australian dollars) of$80/MWh for new wind farms, $143/MWh for new coal plants and$116/MWh for new baseload gas plants. The modeling also shows that "even without a carbon price (the most efficient way to reduce economy-wide emissions) wind energy is 14% cheaper than new coal and 18% cheaper than new gas."[121] Part of the higher costs for new coal plants is due to high financial lending costs because of "the reputational damage of emissions-intensive investments". The expense of gas fired plants is partly due to "export market" effects on local prices. Costs of production from coal fired plants built in "the 1970s and 1980s" are cheaper than renewable energy sources because of depreciation.[121]

A 2009 study on wind power in Spain by Gabriel Calzada Alvarez of King Juan Carlos University concluded that each installed MW of wind power led to the loss of 4.27 jobs, by raising energy costs and driving away electricity-intensive businesses.[122] However, the U.S. Department of Energy found the study to be seriously flawed, and the conclusion unsupported.[123]

Theoretical analysis suggests that at the current technology level airborne wind energy systems can be as much as 10 times less expensive than conventional wind turbines for the same average power output.[124][125]

### Incentives and community benefits

U.S. landowners typically receive $3,000–$5,000 annual rental income per wind turbine, while farmers continue to grow crops or graze cattle up to the foot of the turbines.[126] Shown: the Brazos Wind Farm in Texas.
Some of the 6,000 wind turbines in California's Altamont Pass Wind Farm aided by tax incentives during the 1980s.[127]

The U.S. wind industry generates tens of thousands of jobs and billions of dollars of economic activity.[128] Wind projects provide local taxes, or payments in lieu of taxes and strengthen the economy of rural communities by providing income to farmers with wind turbines on their land.[126][129] Wind energy in many jurisdictions receives 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 US, wind power receives a production tax credit (PTC) of 1.5¢/kWh in 1993 dollars for each kW·h produced, for the first ten years; at 2.2 cents per kW·h in 2012, the credit was renewed on 2 January 2012, to include construction begun in 2013.[130] A 30% tax credit can be applied instead of receiving the PTC.[131][132] 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".[133] The Energy Improvement and Extension Act of 2008 contains extensions of credits for wind, including microturbines. 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.[134][135] As of December 2013, US Senator Lamar Alexander and other senators are arguing that the "wind energy production tax credit should be allowed to expire at the end of 2013."[136]

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 undertaking strong "green" efforts. In the US the organization Green-e monitors business compliance with these renewable energy credits.[137]

## Environmental effects

According to the manager of this wind farm, livestock ignore wind turbines, [138] and continue to graze as they did before wind turbines were installed.

Compared to the environmental impact of traditional energy sources, the environmental impact of wind power is relatively minor in terms of pollution. 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. While a wind farm may cover a large area of land, many land uses such as agriculture are compatible, with only small areas of turbine foundations and infrastructure made unavailable for use.[139][140]

There are reports of bird and bat mortality at wind turbines as there are around other artificial structures. The scale of the ecological impact may[141] or may not[142] be significant, depending on specific circumstances. Although many artificial structures can kill birds, wind power has a disproportionate effect on certain endangered bird species.[143] An especially vulnerable group are raptors, which are slow to reproduce and favor the high wind speed corridors that wind turbine companies build turbines in, to maximize energy production.[143] Although they have a negligible effect on most birds, in some locations there is a disproportionate effects on some birds of conservation concern, such as the golden eagle and raptor species.[143]

However, a large meta-analysis of 616 individual studies on electricity production and its effects on avian mortality concluded that the most visible impacts of wind technology are not necessarily the most flagrant ones, as:[144]

 “ Wind turbines seem to present a significant threat as all their negative externalities are concentrated in one place, while those from conventional and nuclear fuel cycles are spread out across space and time. Avian mortality and wind energy has consequently received far more attention and research than the avian deaths associated with coal, oil, natural gas and nuclear power generators [although] study suggests that wind energy may be the least harmful to birds. ”

Prevention and mitigation of wildlife fatalities, and protection of peat bogs,[145] affect the siting and operation of wind turbines.

There are anecdotal reports of negative effects from noise on people who live very close to wind turbines. Peer-reviewed research has generally not supported these statements.[146]

## Politics

### Central government

Fossil fuels are subsidized by many governments,[citation needed] and wind power and other forms of renewable energy are also often subsidized. For example a 2009 study by the Environmental Law Institute[147] assessed the size and structure of U.S. energy subsidies over the 2002–2008 period. The study estimated that subsidies to fossil-fuel based sources amounted to approximately $72 billion over this period and subsidies to renewable fuel sources totalled$29 billion. In the United States, the federal government has paid US$74 billion for energy subsidies to support R&D for nuclear power ($50 billion) and fossil fuels ($24 billion) from 1973 to 2003. During this same time frame, renewable energy technologies and energy efficiency received a total of US$26 billion. It has been suggested that a subsidy shift would help to level the playing field and support growing energy sectors, namely solar power, wind power, and biofuels.[148] History shows that no energy sector was developed without subsidies.[148]

According to the International Energy Agency (IEA) (2011), energy subsidies artificially lower the price of energy paid by consumers, raise the price received by producers or lower the cost of production. "Fossil fuels subsidies costs generally outweigh the benefits. Subsidies to renewables and low-carbon energy technologies can bring long-term economic and environmental benefits".[149] In November 2011, an IEA report entitled Deploying Renewables 2011 said "subsidies in green energy technologies that were not yet competitive are justified in order to give an incentive to investing into technologies with clear environmental and energy security benefits". The IEA's report disagreed with claims that renewable energy technologies are only viable through costly subsidies and not able to produce energy reliably to meet demand.

In the US, the wind power industry has recently increased its lobbying efforts considerably, spending about $5 million in 2009 after years of relative obscurity in Washington.[150] By comparison, the US nuclear industry alone spent over$650 million on its lobbying efforts and campaign contributions during a single ten-year period ending in 2008.[151][152][153]

Following the 2011 Japanese nuclear accidents, Germany's federal government is working on a new plan for increasing energy efficiency and renewable energy commercialization, with a particular focus on offshore wind farms. Under the plan, large wind turbines will be erected far away from the coastlines, where the wind blows more consistently than it does on land, and where the enormous turbines won't bother the inhabitants. The plan aims to decrease Germany's dependence on energy derived from coal and nuclear power plants.[154]

Commenting on the EU's 2020 renewable energy target, economist Professor Dieter Helm is critical of how the costs of wind power are cited by lobbyists. Helm also says that the problem of intermittent supply will probably lead to another dash for gas or dash for coal in Europe, possibly with a negative impact on energy security.[155] A House of Lords Select Committee report (2008) on renewable energy in the UK reported a "concern over the prospective role of wind generated and other intermittent sources of electricity in the UK, in the absence of a break-through in electricity storage technology or the integration of the UK grid with that of continental Europe".[156]

### Public opinion

Environmental group members are both more in favor of wind power (74%) as well as more opposed (24%). Few are undecided.

Surveys of public attitudes across Europe and in many other countries show strong public support for wind power.[157][158][159] About 80% of EU citizens support wind power.[160] In Germany, where wind power has gained very high social acceptance, hundreds of thousands of people have invested in citizens' wind farms across the country and thousands of small and medium sized enterprises are running successful businesses in a new sector that in 2008 employed 90,000 people and generated 8% of Germany's electricity.[161][162] Although wind power is a popular form of energy generation, the construction of wind farms is not universally welcomed, often for aesthetic reasons.[139][157][158][159][160][163][164]

In Spain, with some exceptions, there has been little opposition to the installation of inland wind parks. However, the projects to build offshore parks have been more controversial.[165] In particular, the proposal of building the biggest offshore wind power production facility in the world in southwestern Spain in the coast of Cádiz, on the spot of the 1805 Battle of Trafalgar.[166] has been met with strong opposition who fear for tourism and fisheries in the area,[167] and because the area is a war grave.[166]

In a survey conducted by Angus Reid Strategies in October 2007, 89 per cent of respondents said that using renewable energy sources like wind or solar power was positive for Canada, because these sources were better for the environment. Only 4 per cent considered using renewable sources as negative since they can be unreliable and expensive.[169] According to a Saint Consulting survey in April 2007, wind power was the alternative energy source most likely to gain public support for future development in Canada, with only 16% opposed to this type of energy. By contrast, 3 out of 4 Canadians opposed nuclear power developments.[170]

A 2003 survey of residents living around Scotland's 10 existing wind farms found high levels of community acceptance and strong support for wind power, with much support from those who lived closest to the wind farms. The results of this survey support those of an earlier Scottish Executive survey 'Public attitudes to the Environment in Scotland 2002', which found that the Scottish public would prefer the majority of their electricity to come from renewables, and which rated wind power as the cleanest source of renewable energy.[171] A survey conducted in 2005 showed that 74% of people in Scotland agree that wind farms are necessary to meet current and future energy needs. When people were asked the same question in a Scottish renewables study conducted in 2010, 78% agreed. The increase is significant as there were twice as many wind farms in 2010 as there were in 2005. The 2010 survey also showed that 52% disagreed with the statement that wind farms are "ugly and a blot on the landscape". 59% agreed that wind farms were necessary and that how they looked was unimportant.[172] Scotland is planning to obtain 100% of electricity from renewable sources by 2020.[173]

### Community

Wind turbines such as these, in Cumbria, England, have been opposed for a number of reasons, including aesthetics, by some sectors of the population.[174][175]

Many wind power companies work with local communities to reduce environmental and other concerns associated with particular wind farms.[176][177][178] In other cases there is direct community ownership of wind farm projects. Appropriate government consultation, planning and approval procedures also help to minimize environmental risks.[157][179][180] Some may still object to wind farms[181] but, according to The Australia Institute, their concerns should be weighed against the need to address the threats posed by climate change and the opinions of the broader community.[182]

In America, wind projects are reported to boost local tax bases, helping to pay for schools, roads and hospitals. Wind projects also revitalize the economy of rural communities by providing steady income to farmers and other landowners.[126]

In the UK, both the National Trust and the Campaign to Protect Rural England have expressed concerns about the effects on the rural landscape caused by inappropriately sited wind turbines and wind farms.[183][184]

Some wind farms have become tourist attractions. The Whitelee Wind Farm Visitor Centre has an exhibition room, a learning hub, a café with a viewing deck and also a shop. It is run by the Glasgow Science Centre.[185]

In Denmark, a loss-of-value scheme gives people the right to claim compensation for loss of value of their property if it is caused by proximity to a wind turbine. The loss must be at least 1% of the property's value.[186]

Despite this general support for the concept of wind power in the public at large, local opposition often exists and has delayed or aborted a number of projects.[187][188][189]

While aesthetic issues are subjective and some find wind farms pleasant and optimistic, or symbols of energy independence and local prosperity, protest groups are often formed to attempt to block new wind power sites for various reasons.[181][190][191]

This type of opposition is often described as NIMBYism,[192] but research carried out in 2009 found that there is little evidence to support the belief that residents only object to renewable power facilities such as wind turbines as a result of a "Not in my Back Yard" attitude.[193]

## Small-scale wind power

Small-scale wind power is the name given to wind generation systems with the capacity to produce up to 50 kW of electrical power.[194] Isolated communities, that may otherwise rely on diesel generators, may use wind turbines as an alternative. Individuals may purchase these systems to reduce or eliminate their dependence on grid electricity for economic 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.[24]

Grid-connected domestic wind turbines may use grid energy storage, thus replacing purchased electricity with locally produced power when available. The surplus power produced by domestic microgenerators can, in some jurisdictions, be fed into the network and sold to the utility company, producing a retail credit for the microgenerators' owners to offset their energy costs.[195][196]

Off-grid system users can either adapt to intermittent power or use batteries, photovoltaic or diesel systems to supplement the wind turbine. Equipment such as parking meters, traffic warning signs, street lighting, or wireless Internet gateways may be powered by a small wind turbine, possibly combined with a photovoltaic system, that charges a small battery replacing the need for a connection to the power grid.[197]

In locations near or around a group of high-rise buildings, wind shear generates areas of intense turbulence, especially at street-level.[198] The risks associated with mechanical or catastrophic failure have thus plagued urban wind development in densely populated areas, rendering the costs of insuring urban wind systems prohibitive.[199] Moreover, quantifying the amount of wind in urban areas has been difficult, as little is known about the actual wind resources of towns and cities.[200]

A Carbon Trust study into the potential of small-scale wind energy in the UK, published in 2010, found that small wind turbines could provide up to 1.5 terawatt hours (TW·h) per year of electricity (0.4% of total UK electricity consumption), saving 0.6 million tonnes of carbon dioxide (Mt CO2) emission savings. This is based on the assumption that 10% of households would install turbines at costs competitive with grid electricity, around 12 pence (US 19 cents) a kW·h.[201] A report prepared for the UK's government-sponsored Energy Saving Trust in 2006, found that home power generators of various kinds could provide 30 to 40% of the country's electricity needs by 2050.[202]

Distributed generation from renewable resources is increasing as a consequence of the increased awareness of climate change. The electronic interfaces required to connect renewable generation units with the utility system can include additional functions, such as the active filtering to enhance the power quality.[203]

## Notes

1. ^ Wind turbine power output is measured in kilowatts (kW) or megawatts (MW), and energy output in megawatt-hours (MWh), gigawatt-hours (GWh) or terawatt-hours or (TWh). One gigawatt-hour is one million kilowatt-hours, and one terawatt-hour is a billion kilowatt hours.
2. ^ For example, a 1 MW turbine with a capacity factor of 35% will not produce 8,760 MW·h in a year (1 × 24 × 365), but only 1 × 0.35 × 24 × 365 = 3,066 MW·h, averaging to 0.35 MW.
3. ^ California and Minnesota are exceptions.

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