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Wind power

From Academic Kids

Wind power is the kinetic energy of wind, or the extraction of this energy by wind turbines. This article deals mainly with the intricacies of large-scale deployment of wind turbines to generate electricity.

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Map of available wind power over the United States. This type of undetailed map is most useful for scouting for offshore locations.
Contents

Wind energy

Main article: Wind

An estimated 1 to 3 percent of the energy from the Sun is converted into wind energy. This is about 50 to 100 times more energy than is converted into biomass by all the plants on earth through photosynthesis. Most of this wind energy can be found at high altitudes where continuous wind speeds of over 160 km/h (100 mph) are common. Eventually, the wind energy is converted through friction into diffuse heat all through the earth's surface and atmosphere.

While the exact kinetics of wind are extremely complicated and relatively little understood, the basics of its origins are relatively simple. The earth is not heated evenly by the sun. Not only do the poles receive less energy from the sun than the equator does, but dry land heats up (and cools down) more quickly than the seas do. This powers a global atmospheric convection system reaching from the earth's surface to the stratosphere which acts as a virtual ceiling.

The change of seasons, change of day and night, the Coriolis effect, the irregular albedo (reflectivity) of land and water, humidity, and the friction of wind over different terrain are some of the many factors which complicate the flow of wind over the surface.

Energy extraction calculations

The power in the wind can be extracted by having it act on moving wings, connected to a rotor, which converts some of that power into torque on the rotor. The amount of power transferred depends on the wind speed (cubed), the swept area (linearly), and the density of the air (linearly).

The mass flow of air that travels through the swept area of a wind turbine varies with the wind speed and air density. As an example, on a cool 15 degrees C (59 degrees F) day at sea level, air density is about 1.22 kilograms per cubic metre (it gets less dense with higher humidity). An 8 m/s breeze blowing through a 100 meter diameter rotor would move about 76,000 kilograms of air per second through the swept area.

The kinetic energy of a given mass varies with the square of its velocity. Because the mass flow increases with the wind speed, the wind energy available to a wind turbine increases as the cube of the wind speed. The power of the example breeze above through the example rotor would be about 2.5 megawatts.

As the wind turbine extracts energy from the air flow, the air is slowed down, which causes it to spread out, causing it to divert around the wind turbine to some extent. A German physicist, Albert Betz, determined in 1919 that a wind turbine can extract at most 59% of the energy that would otherwise flow through the turbine's cross section. The Betz limit applies regardless of the design of the turbine. More recent work [1] (http://mystic.math.neu.edu/gorban/Gorlov2001.pdf) by Gorlov shows a theoretical limit of about 30% for propeller-type turbines. Actual efficiencies range from 10% to 20% for propeller-type turbines, and are as high as 35% for three-dimensional vertical-axis turbines like Darrieus or Gorlov turbines (see below).

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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 Raleigh model distribution for the same average wind speed. Energy is the Betz limit through a 100 meter diameter circle facing directly into the wind. Total energy for the year through that circle was 15.4 gigawatt-hours.

Windiness varies, and an average value for a given location is not in itself a clear indication of the amount of energy a wind turbine could yield there. The distribution model most frequently used is the Raleigh model, an example of which is plotted to the right against an actual measured dataset.

Because available power rises with the cube of wind speed, much of the average power available to a windmill comes in short bursts. The 2002 Lee Ranch sample is telling: half of the energy available arrived in just 15% of the operating time.

Since wind speed is not constant, a wind generator's annual energy production is never as much as its nameplate rating multiplied by the total hours in a year. The ratio of actual productivity in a year to this theoretical maximum is called the annual load factor. A well-sited wind generator will have an annual load factor of as much as 35%. When comparing the size of wind turbine plants to fueled power plants, it is important to note that 1000 kW of wind-turbine potential power would be expected to produce as much energy in a year as approximately 350 kW of fuel-fired generation.

Turbine siting

As a general rule, wind generators are practical where the average wind speed is greater than 20 km/h (5.5 m/s or 12.5 miles/hour). Obviously, meteorology plays an important part in determining possible locations for wind parks, though it has great accuracy limitations. Meterological wind data is not usually sufficient for accurate siting of a large wind power project. An 'ideal' location would have a near constant flow of unturbulent wind throughout the year, and wouldn't suffer too many sudden powerful bursts of wind.

The wind blows faster at higher altitudes because of the reduced influence of drag of the surface (sea or land) and the reduced viscosity of the air. The variation in velocity with altitude, called wind shear is most dramatic near the surface. Typically, the variation follows the 1/7th 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 farms or wind parks often have many turbines installed. Since each turbine extracts some of the energy of the wind, it is important to provide adequate spacing betweeen turbines to avoid excess energy loss. Where land area is sufficient, turbines are spaced 3 to 5 rotor diameters apart perpendicular to the prevailing wind, and 5 to 9 rotor diameters apart in the direction of the prevailing wind, to minimize efficiency loss. The "wind park effect" loss can be as low as 2% of the combined nameplate rating of the turbines.

Utility-scale wind turbine generators have low temperature operating limits which restrict the application in areas that routinely experience temperatures less than -20 C. Wind turbines must be protected from ice accumulation, which can make anemometer readings inaccurate and which can cause high structure loads and damage. Some turbine manufacturers offer low-temperature packages,at a cost of a few percent of the turbine cost, which include internal heaters, different lubricants, and different alloys for structural elements, to make it possible to operate the turbines at lower temperatures. If the low-temperature interval is combined with a low-wind condition, the wind turbine will require station service power, equivalent to a few percent of its output rating, to maintain internal temperatures during the cold snap. For example, the St. Leon, Manitoba project has a total rating of 99 MW and is estimated to need up to 3 MW (around 3% of capacity) of station service power a few days a year for temperatures down to -30 C. This factor affects the economics of wind turbine operation in cold climates. [1]

Onshore

In areas with dramatic topography, the precise location of wind turbines can greatly affect their productivity. A difference of 30 m can sometimes mean a doubling in output. Often the local winds are precisely monitored for a year or more with anemometers and modelled before wind generators are installed. 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 is 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 this is less reliable.

Offshore

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Wind often flows briskly and smoothly over water since there are no obstructions. The large and slow turning turbines of this offshore wind farm near Copenhagen take advantage of the moderate yet constant breezes at this location.

Offshore wind turbines are considered to be less unsightly (they can be invisible from shore), and because the winds are usually more potent offshore, such turbines don?t need to reach quite as high into the air. Offshore conditions are harsh though, abrasive and corrosive, and it?s often impossible or near-impossible to repair a broken down turbine in open waters.

In stormy areas with extended shallow continental shelves and sand banks (such as Denmark), turbines are reasonably easy to install, and give good service - Denmark's offshore wind generation provides about 12-15% of total electricity demand in the country. At the site shown, the wind is not especially strong but is very consistent. The largest offshore wind turbine park in the world, off the coast of Wicklow in Ireland, is also located on a sand bank.

Airborne

Schemes have been dreamed up in which windpower generators would be lifted by kites or balloons to altitudes and regions of powerful high altitude winds. A recent concept would have turbines working like the blades of a helicopter generating lift and electricity from constant, high altitude winds. According to its designers, while some of the energy in the wind would be 'lost' on lift, the constant and potent winds allow it generate constant electricity. Since the winds usually blow horizontally, the turbines would be at an angle from the horizontal, catching winds while still generating lift. Deployment could be done by feeding electricity to the turbines, which would turn them into electromotors, lifting the structure into the sky.

These systems would have the advantage of tapping an almost constant wind and doing so without a set of slip rings or yaw mechanism. The main disadvantage is that kites and 'helicopters' come down when there is insufficient wind. Balloons can be added to the mix to keep the contraption up without wind; but, balloons leak slowly and have to be at least resupplied with lifting gas, possibly patched as well. Also this scheme requires a very long power cable and an aircraft exclusion zone.

Utilization

Large scale

There are now many thousands of wind turbines operating in various parts of the world, with a total capacity of over 47,317 MW of which Europe accounts for 72% (2005). It was the most rapidly-growing means of alternative electricity generation at the turn of the century and provides a valuable complement to large-scale base-load power stations. World wind generation capacity quadrupled between 1997 and 2002. 90% of wind power installations are in the US and Europe.

Denmark, Ireland and Germany have made considerable investments in wind generated electricity. Denmark is especially a leader in the production and use of turbines, with a commitment made in the 1970s to eventually produce half of the country's power by wind. The Irish government recently constructed the world's largest offshore wind turbine park and plans are being made for more such installations on the west coast, including the possible the use of floating turbines. Germany already produces 40% of the entire world's wind power, and the hope is that by 2010, wind will meet 12.5% of German electricity needs. Germany has 16,000 wind turbines, mostly concentrated in the north of the country, near the border with Denmark - including the biggest in the world, owned by the Repower company. While the United States government lost interest when the price of oil dropped after the 1970s oil crisis, the Danes and Germans continued their efforts and now are a leading exporter of large turbines (each generating 0.66 to 5.0 megawatt)..

Wind accounts for 0.4% of the total electricity production on a global scale (2002). Germany is the leading producer of wind power with 35% of the total world capacity in 2005 (10% of German electricity). The United States and Spain are next in terms of installed capacity. According to the American Wind Energy Association, wind generated enough electricity to power 0.4% (1.6 million households) of total electricity in US, up from less than 0.1% in 1999. Germany's Schleswig-Holstein province generates 25% of its power with wind turbines. Denmark generates over 20% of its electricity with wind turbines, the highest percentage of any country and is fourth in the world in total power generation. Today (2005) Germany produces more electricity from wind power than from hydropower plants.

After Denmark, Germany, the US and Spain, India ranks 5th in the world with a total wind power capacity of 3500 MW. Almost half of this capacity (1600 MW) was added in the last two years, and of new electricity capacity additions in the country, wind power accounted for over 20% of the total in that period. Currently wind power generates 3% of all electricity produced in India. Unlike the others in the top 5 though, India's estimated wind power potential is pretty low at just 45 gigawatts, while world wide potential is estimated at 72 terawatts, with the US and Northern Europe among the regions with the maximum potential.

Small scale

Wind turbines have been used for household electricity generation in conjunction with battery storage over many decades in remote areas. Household generator units of more than 1 kW are now functioning in several countries.

To compensate for the varying power output, grid-connected wind turbines utilise some sort of grid energy storage. Off-grid systems either adapt to intermittent power or use photovoltaic or diesel systems to supplement the wind turbine.

Wind turbines range from small four hundred watt generators for residential use to several megawatt machines for wind farms and offshore. The small ones have direct drive generators, direct current output, aeroelastic blades, lifetime bearings and use a vane to point into the wind; while the larger ones generally have geared power trains, alternating current output, flaps and are actively pointed into the wind. As technology progresses, large generators are becoming as simple as small generators. Direct drive generators and aeroelastic blades for large wind turbines are being researched and direct current generators are sometimes used.

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This rooftop-mounted urban wind turbine charges a 12 volt battery and runs various 12 volt appliances within the building on which it is installed.

In urban locations, where it is difficult to obtain large amounts of wind energy, smaller systems may still be used to run low power equipment. Distributed power from rooftop mounted wind turbines can also alleviate power distribution problems, as well as provide resilience to power failures. Equipment such as wireless internet gateways may be powered by a wind turbine that charges a small battery, replacing the need for a connection to the power grid and/or maintaining service despite possible power grid failures.

The Lakota turbine by Aeromax is approximately 7 feet (2 m) in diameter and produces 900 watts of three phase power. It uses a three phase rectifier and charge controller so that it is free to spin at whatever speed is optimal for a given wind condition. Lightweight materials (the entire turbine weighs only 16kg (35 pounds)) allow it to respond quickly to the gusts of wind typical of urban settings. It attaches to a size 9 structural pipe (similar to a TV antenna mast). The Lakota is very quiet. Even when standing up on the roof right next to the mast it is inaudible. Climbing up the mast, it is still inaudible from just a few feet under the turbine. A dynamic braking system regulates the speed by dumping excess energy, so that the turbine continues to produce electricity even in high winds. The dynamic braking resistor may be installed inside the building, so that the 'heat loss' will heat the inside of the building (i.e. during high winds when more heat is lost by the building, more heat is also produced by the braking resistor). The proximal location makes low voltage (12 volt, or the like) energy distribution practical, e.g. in a typical installation the braking resistor can be located just inside to where the mast is attached to the building. Such small-scale renewable energy sources also impart a beneficial psychological effect on building owners, so that they begin to take on a keen awareness of electricity consumption, possibly reducing their consumption down to the average level that the turbine can produce.


Controversy

The debate around wind energy is heated and often emotional. Arguments of both parties are listed below.

Arguments of opponents

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Some of the over 4000 wind turbines at Altamont Pass, in California. Developed during a period of tax incentives in the 1980s, this wind farm has more turbines than any other. These units are likely Enertech E44-40kWs.

Economics

  • Wind power depends on operational subsidies. In the United States, wind power receives a subsidy of 1.9 cents per kilowatt-hour produced. 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." Countries such as Canada and Germany also provide tax credits and other incentives for wind turbine construction.

Yield

  • The goals of renewable energy development are reduction of reliance on fossil and nuclear fuels, reduction of greenhouse gas and other emissions, and establishment of a sustainable source of energy. Some critics question wind energy's ability to significantly move society towards these goals. They point out that 30% annual load factor is considered typical for wind facilities. The intermittent and un-schedulable nature of wind turbine power requires that "spinning reserves" are maintained for supply security. This may mean that "conventional" peaking power power plants may not be shut down; such plants operate at lower efficiency than base-load generating units.
  • Another charge is that output figures, such as "Denmark produces over 20% of its electricity from wind," do not account for electricity that is simply absorbed by the international grid because it is produced when demand is already being met by other sources.
  • Since electric power production is only part (about a third) of society's energy use, wind power does nothing to mitigate the larger part of the effects of energy use. For example, despite aggressive installation of wind facilities in the U.K., that country's CO2 emissions continued to rise in 2002 and 2003.
  • Groups such as the UN's Intergovernmental Panel on Climate Change, state that the desired mitigation goals can be achieved at lower cost and to a greater degree by continued improvements in general efficiency — in building, manufacturing, and transport — than by wind power.

Ecological footprint

  • The construction of a large facility is also far from ecologically neutral if the location has no previous development. It requires roads, foundations, clearing of trees, and construction of power lines. The clearing of trees is necessary since obstructions within a distance ten times the height of the turbine reduce yield dramatically. A distance of twenty times is preferred.
  • A wind farm that produces the energy equivalent of a conventional power plant would have to cover an area of approximately 300 square miles. [2] (http://wired-vig.wired.com/wired/archive/13.02/nuclear.html?pg=2&topic=nuclear&topic_set=)
  • Offshore sites eliminate some of these objections, bur raise others such as dangers to navigation and the possible adverse effect of low-frequency vibration on aquatic mammals.
  • Another important complaint is that windmills kill too many birds, especially birds of prey, and bats. Siting generally takes into account bird flight patterns, but most paths of bird migration, particularly for birds that fly by night, are unknown. A survey at Altamont Pass, California conducted by a California Energy Commission in 2004 showed that turbines killed 4,700 birds annually (1,300 of which are birds of prey). The numbers of bats killed by existing facilities has surprised even industry personnel [3] (http://vawind.org/Assets/Docs/BCI_ridgetop_advisory.pdf)

Scalability

  • The large number of turbines required for a viable wind plant, and the huge number of plants required to meet the ambitious goals of the wind industry and governments, ensures that more people will be affected by them.

Aesthetics

  • There is resistance to the establishment of land based wind farms owing initially to perceptions they are noisy and contribute to "visual pollution," i.e., they are considered to be eyesores. Moving the turbines offshore mitigates the problem, but offshore wind farms are more expensive to maintain and there is an increase in transmission loss due to longer distances of power lines.
  • The large installations of a modern wind facility are typically 100 m high to the tip of the rotor blade, and, besides the continuous motion of the 35-m-long rotor blades through the air, each time the blade passes the tower a deep subsonic thump is produced, which is a form of noise pollution.
  • Residents near windmills complain of "shadow flicker," which is the alternating pattern of sun and shade caused by a windmill casting a shadow over residences.

Arguments of supporters

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Erection of an Enercon E-70

Supporters of wind energy state that:

Yield

  • Wind power is a renewable resource, which means using it will not deplete the earth's supply of fossil fuels. It also is a clean energy souce, and produces no carbon dioxide sulfur dioxide, particulates, or any other type of air pollution, as do conventional fossil fuel power sources.
  • Winds long-term technical potential is believed 5 times current global energy consumption or 40 times current electricity demand. This requires 12.7% of all land area, or that land area with Class 3 or greater potential at a height of 80 meters. It assumes that the land is covered with 6 large wind turbines per square kilometer. Offshore resources experience mean wind speeds ~90% greater than that of land, so offshore resources could contribute substantially more energy.[4] (http://www.stanford.edu/group/efmh/winds/global_winds.html)[5] (http://www.ens-newswire.com/ens/may2005/2005-05-17-09.asp#anchor6). This number could also increase with higher altitude ground based or airborne wind turbines [6] (http://www.wired.com/news/planet/0,2782,67121,00.html?tw=wn_tophead_2).

Coping with intermittent energy

  • Electricity demand is very variable, conventional powerplants can drop off the grid within a few seconds, and the output of coal and nuclear powerplants is constant and cannot follow demand. As a consequence the production system is already equiped with substantial, quickly adjustable back-up. Experts agree that at least 20% of wind electricity can be integrated in the existing production system with no need for any additional back-up or storage facilities.
  • The creation of a "burst electricity" industry, where excess electricity can be used extremely cheaply on windy days for opportunistic production would greatly improve the efficiency of wind turbines. Applications such as electrolysis for hydrogen fuel, and other processes that are efficient with intermittent electricity usage can use the intermittent energy provided by wind turbines, preventing windmills from being forced to idle during days of excess power availability.
  • Existing European hydroelectric power plants can store enough energy to supply one month's worth of European energy consumption. Improvement of the international grid would allow using this at relatively short term at low cost, supplementing wind power. Excess wind power could even be used to pump water up into collection basins for later use.
  • Geographically-spread wind turbine parks used together produce power much more constantly.

Ecological footprint

  • The energy consumption for production, installation, operation and decommission of a wind turbine is usually earned back within 3 months of operation.
  • After decommissioning wind turbines, even the foundations are removed.
  • Studies show that the number of birds and bats killed by wind turbines is negligible compared to the amount that die as a result of other human activities such as traffic, hunting, power lines and high-rise buildings and especially the environmental impacts of using non-clean power sources. For example, in the UK, where there are a few hundred turbines, about one bird is killed per turbine per year; 10 million (http://www.bwea.org/media/news/birds.html) per year are killed by cars alone.
  • Clearing of wooded areas is often completely unnecessary for grounded turbines, as the practice of farmers leasing their land out to companies building wind farms is becoming ever more common. The annual royalties farmers receive are often up to four thousand dollars. What's more, the land can still be used for farming and cattle grazing.

Economic feasibility

  • Conventional and nuclear power plants receive massive amounts of direct and indirect governmental subsidies. If a comparison is made on real production costs, wind energy is competitive in many cases. If the full costs (environmental, health, etc.) are taken into account, wind energy is competitive in most cases. Furthermore, wind energy costs are continuously decreasing due to technology development and scale enlargement.
  • Conventional and nuclear plants also have sudden unpredictable outages. Statistical analysis shows that 1000 MW of wind power can replace 300 MW of conventional power.

Aesthetics

  • It is possible to hold a conversation directly underneath a modern wind turbine without any difficulty whatsoever and without raising one's voice. The modern turbine is quieter than its predecessors owing to improvements in the blade design.
  • Newer wind farms have their turbines spaced further apart, due to the greater power of the individual wind turbines. They no longer have the cluttered look of the early wind farms.
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