CONTENTS

  • 1 INTRODUCTION
  • HISTORY
  • MECHANICAL POWER
  • ELECTRICAL POWER
  • 2WIND ENERGY
  • DISTRIBUTION OF WIND SPEED
  • HIGH ALTITUDE WINDS
  • 3WIND FARMS
  • FEEDING INTO GRID
  • OFFSHORE WIND POWER
  • 4WIND POWER CAPACITY AND PRODUCTION
  • GROWTH TRENDS
  • CAPACITY FACTOR
  • PENETRATION
  • VARIABILITY
  • PREDICTABILITY
  • RELIABILITY
  • INTEGRATION WITH OTHER SOURCES
  • ENERGY STORAGE
  • CAPACITY CREDIT AND FUEL SAVINGS
  • 5ECONOMICS
  • COST TRENDS
  • INCENTIVES AND COMMUNITY BENEFITS
  • 6ENVIRONMENTAL EFFECTS
  • 7POLITICS
  • CENTRAL GOVERNMENT
  • PUBLIC OPINION
  • COMMUNITY
  • 8SMALL-SCALE WIND POWER

WIND ENERGY

INTRODUCTION

Wind poweris the conversion ofwindenergyinto a useful form of energy, such as usingwind turbinesto makeelectrical power, windmillsfor mechanical power, wind pumps forwater pumpingordrainage, orsailsto propelships.

Largewind farmsconsist of hundreds of individualwind turbineswhich are connected to theelectric power transmissionnetwork. Offshore wind is steadier and stronger than on land, and offshore farms have less visual impact, but construction and maintenance costs are considerably higher. Small onshore wind farms provide electricity to isolated locations. Utility companies increasinglybuy surplus electricityproduced by small domestic wind turbines.

Wind power, as an alternative tofossil fuels, is plentiful,renewable, widely distributed,clean, produces nogreenhouse gasemissions during operation and uses little land.[2]Theeffects on the environmentare 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 on a commercial basis.[3]In 2010 wind energy production was over 2.5% of total worldwide electricity usage, and growing rapidly at more than 25% per annum. The monetary cost per unit of energy produced is similar to the cost for new coal and natural gas installations.

Wind power is very consistent from year to year but has significant variation over shorter time scales. Theintermittencyof wind seldom creates problems when used to supply up to 20% of total electricity demand,but as the proportion increases, a need to upgrade the grid, and a lowered ability to supplant conventional production can occur.[6]Power management techniques such as having excess capacity storage, geographically distributed turbines, dispatchable backing sources, storage such aspumped-storage hydroelectricity, exporting and importing power to neighboring areas or reducing demand when wind production is low, can greatly mitigate these problems.[7]In addition,weather forecastingpermits the electricity network to be readied for the predictable variations in production that occur.

History

Mechanical power

Medieval depiction of awind mill

Sailboatsandsailing shipshave been using wind power for thousands of years, and architects have used wind-drivennatural ventilationin buildings since similarly ancient times. The use of wind to provide mechanical power came somewhat later in antiquity. The windwheel of the Greek engineerHeron of Alexandriain the 1st century AD is the earliest known instance of using a wind-driven wheel to power a machine.

The first windmills were in use in Iran at least by the 9th century and possibly as early as the 7th century.The use of windmills became widespread across the Middle East and Central Asia, and later spread to China and India.By 1000 AD, windmills were used to pump seawater for salt-making in China and Sicily.Windmills were used extensively in Northwestern Europe to grind flour from the 1180s,andwindpumpswere used to drain land for agriculture and for building.Early immigrants to theNew Worldbrought the technology with them from Europe.

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.[16]Windpumps contributed to the expansion of rail transport systems throughout the world, by pumping water from water wells forsteam 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.

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

Electrical power

Blyth's "windmill" at his cottage in Marykirk in 1891

In July 1887, a Scottish academic, ProfessorJames Blyth, built a cloth-sailed wind turbine in the garden of his holiday cottage in Marykirk and used the electricity it produced to chargeaccumulatorswhich he used to power the lights in his cottage.[20]His experiments culminated in a UK patent in 1891.[21]In the winter of 1887/8 US inventorCharles F. Brushproduced electricity using a wind powered generator which powered his home and laboratory until about 1900. In the 1890s, the Danish scientist and inventorPoul la Courconstructed wind turbines to generate electricity, which was used to producehydrogenandOxygenbyelectrolysisand a mixture of the two gases was stored for use as a fuel.[21]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.

By the mid-1920s, 1 to 3-kilowatt wind generators developed by companies such asParris-Dunnand Jacobs Wind-electric[18]found widespread use in the rural areas of the midwestern 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.

IN 1931 the French aeronautical engineer,George Darrieuswas granted a patent for theDarrieus wind turbinewhich usedairfoilsto create rotation[24]and a 100kW precursor to the modern horizontal wind generator was used in Yalta, in the USSR. In 1956Johannes Juul, a former student of la Cour, built a 200kW, three-bladedturbine at Gedserin Denmark, which influenced the design of many later turbines.

In 1975 theUnited States Department of Energyfunded a project to develop utility-scale wind turbines. TheNASA wind turbinesproject built thirteen experimental turbines which paved the way for much of the technology used today.[22]Since then, turbines have increased greatly in size with theEnercon E-126capable of delivering up to 7.5Megawatts (MW).[nb 1][25]Wind turbine production has expanded to many countries and wind power is expected to grow worldwide in the twenty-first century.

Wind energy

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

Wind energy is thekinetic energyof air in motion, also calledwind. Total wind energy flowing through an imaginary areaAduring the timetis:

[26]

whereρis thedensity of air;vis the windspeed;Avtis the volume of air passing throughA(which is considered perpendicular to the direction of the wind);Avtρis therefore the massmpassing per unit time. Note that ½ρv2is the kinetic energy of the moving air per unit volume.

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

[26]

Wind power in an open air stream is thusproportionalto thethird powerof 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.

Wind is the movement of air across the surface of the Earth, affected by areas of high pressure and of low pressure.[27]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.

The total amount of economically extractable power available from the wind is considerably more than present human power use from all sources.[28]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.Cristina Archer andMark Z. Jacobsonpresented 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".[29]They later estimated 80 TW.However research atHarvard Universityestimates 1 Watt/m2on average and 2-10 MW/km2capacity for large scale wind farms, suggesting that these estimates of total global wind resources are too high by a factor of about 4.

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

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.Recent years have seen significant advances in technologies meant togenerate electricity from high altitude winds.

Wind farms

Two of the wind turbines at theBlack Law Wind Farmin Scotland

A wind farm is a group ofwind turbinesin 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 awind 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 atransformerfor connection to the high voltageelectric power transmissionsystem.

Many of the largest operational onshore wind farms are located in the US. As of 2012, theAlta Wind Energy Centeris the largest onshore wind farm in the world at 1020 MW, followed by theShepherds Flat Wind Farm(845 MW), and theRoscoe Wind Farm(781.5 MW). As of September 2012, theSheringham Shoal Offshore Wind Farmand theThanet Wind Farmin the UK are the largest offshore wind farms in the world at 317 MW and 300 MW, followed byHorns Rev II(209 MW) in Denmark.

There are many large wind farms under construction including;The London Array (offshore)(1000 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) andWalney Wind Farm(367 MW).

A panoramic view of theWhitelee Wind Farmwith Lochgoin Reservoir in the foreground.

Feeding into grid

Induction generators, often used for wind power, requirereactive powerforexcitationsosubstationsused in wind-power collection systems include substantialcapacitorbanks forpower factor correction.Different types of wind turbine generators behave differently during transmission grid disturbances, soextensive modellingof 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 machinesgenerally have more desirable properties for grid interconnection.Transmission systems operators will supply a wind farm developer with agrid codeto specify the requirements for interconnection to the transmission grid. This will includepower factor, constancy offrequencyand dynamic behavior of the wind farm turbines during a system fault.

Offshore wind power

Aerial view ofLillgrund Wind Farm, Sweden

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.[39][40]As of 2011, offshore wind farms were at least 3 times more expensive than onshore wind farms of the same nominal powerbut these costs are expected to fall as the industry matures.

SiemensandVestasare the leading turbine suppliers for offshore wind power.DONG Energy,VattenfallandE.ONare the leading offshore operators.[43]As of October 2010, 3.16 GW of offshore wind power capacity was operational, mainly in Northern Europe. According toBTM 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.

As of September 2012, theGreater Gabbard Wind Farmin the United Kingdom is the largest offshore wind farm in the world at 504MW, followed byWalney Wind Farm(367 MW), also in the UK. TheLondon Array(630 MW) is the largest project under construction.

Wind power capacity and production

Worldwide there are now over two hundred thousand wind turbines operating, with a totalnameplate capacityof 282,482MW as of end 2012.TheEuropean Unionalone passed some 100,000 MW nameplate capacity in September 2012,while the United States surpassed 50,000 MW in August 2012 andChinapassed 50,000 MW the same month.

World wind generation capacity more than quadrupled between 2000 and 2006, doubling about every three years.The United States pioneered wind farmsand 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.

Capacity factor

Worldwide installed wind power capacity (Source:GWEC)

Since wind speed is not constant, a wind farm's annualenergyproduction 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 thecapacity factor. Typical capacity factors are 15–50%, with values at the upper end of the range in favourable sites and are due to wind turbine improvements.

Online data is available for some locations and the capacity factor can be calculated from the yearly output.For example, the German nation-wide average wind power capacity factor over all of 2012 was just under 17.5% (45867GW·h/yr/(29.9GW×24×366)=0.1746)and the capacity factor for Scottish wind farms averaged 24% between 2008 and 2010.

Unlike fueled generating plants the capacity factor is affected by several parameters, including the variability of the wind at the site but also thegeneratorsize. A small generator would be cheaper and achieve a higher capacity factor but would produce lesselectricity(and thus less profit) in high winds. Conversely, a large generator would cost more but generate little extra power and, depending on the type, maystallout at low wind speed. Thus an optimum capacity factor would be aimed for, of around 40–50%.

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%.

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 particulargridwill depend on the existing generating plants, pricing mechanisms, capacity forenergy storage, demand management and other factors. An interconnected electricity grid will already includereserve generatingandtransmission capacityto 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 ofdispatchable energyorhydropowerwith 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.