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.
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. 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%.
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
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 donīt need to reach quite as high into the air. Offshore conditions are harsh though, abrasive and corrosive, and itīs often impossible or near-impossibe to repair a broken down turbine in open waters. In stormy areas with extended shallow continental shelves (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.
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 and Germany have made conciderable 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. 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 only 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 1080 MW. It has established 1025 MW of power in commercial projects.
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 1,000 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.
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 electicity 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
Yield
- Most critics support the goals of renewable energy to reduce reliance on fossil and nuclear fuels, reduce the emission of greenhouse gases and other pollution (such as that causing acid rain), and establish a sustainable source of energy, but they question wind energy's ability to significantly move society towards these goals. They point out that 30% average output is considered high for wind facilities, that besides low output they provide electricity in response to the wind rather than consumer demand, and that this intermittency ensures that no "conventional" power plants can be shut down, particularly less efficient plants that are able to switch on and off in a matter of seconds.
- Another charge is that output figures, such as "Denmark produces over 20% of its electricity from wind," do not account for the electricity used by the plants themselves or electricity that is simply absorbed by the international grid because it is produced when demand is already being met by other sources. It is also noted that because electricity production uses only part (about a third) of society's energy, wind power does nothing to mitigate the effects of most of our 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.
- It is often pointed out, by groups such as the UN's Intergovernmental Panel on Climate Change, that the desired mitigation goals can be achieved at much less cost and to a much 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 benign in previously undeveloped locations. It requires wide straight flat roads, a large hole filled with tons of steel and concrete to secure each giant assembly, clearing of trees in wooded areas, a transformer for each turbine, and 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. [1]
- Siting them offshore can address these objections in some cases, while raising other issues, 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.
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.
- 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
Supporters of wind energy state that:
Yield
- Land-based wind energy has the potential of covering six times the world's electricity consumption, or 100% of the world's total energy consumption.
Coping with intermittent energy
- 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 have the capacity to 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 per year are killed by cars alone.
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/Morality
- It is possible to hold a conversation directly underneath a modern wind turbine without any difficulty whatever and without raising one's voice. The modern turbine is quieter than its predecessors owing to improvements in the blade design. It makes a gentle "swish swish swish" sound that could be quite pleasant and soothing. In addition, relatively, the background noise of rustling trees, passing cars, etc., exceeds turbine noise.
- More recent wind farms have their turbines spaced further apart, due to the higher capacity of the individual wind turbines. They no longer have the cluttered look of the early wind farms.
- Wind turbines are beautiful, graceful machines that symbolise humans in harmony with the natural world. As a form of sculpture, wind turbines are a dynamic (moving) art form. Urban wind turbines like the 750 kW Lagerwey in Toronto can actually be popular gathering places.
- Wind turbines make the process of producing electricity visible.
Sources
Technical
Political
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wind-farm -- A website for news, articles, and discussion of wind facilities.
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ExternE -- The European research project about the external costs of energy production in general.
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Wind systems - On the site of the Australian Greenhouse Office.
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Windpower Monthly -- Wind Energy Magazine.
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Enertrag.de -- A German energy company investing in wind turbines. Ownes 13 wind parks as of October 2004.
