- Wind turbines -
A wind turbine is a machine for converting kinetic energy in wind into mechanical energy. If mechanical energy is used directly by machinery, such as a pump or grinding stones, the machine is called a windmill. If the mechanical energy is converted to electricity, the machine is called a wind generator. Wind generators are typically placed in large numbers, creating a wind farm. This article discusses the conversion machinery.
Today's wind energy plants developed from windmill technology and knowledge of aerodynamics. In 1920 Albert Betz showed physically conditionally at the most 59.3 % of the energy of the wind is usable. Its theory for the shaping of the rotor blades is also today still the basis for the interpretation of the plants. At the beginning of the 1980s the Danish concept with wind energy plants grew. In Denmark the basis for modern wind power utilization was developed at that time.
Windmills
Windmills, generally associated with the Netherlands, were historically used
to grind grain or pump water from low-lying land. They greatly accelerated shipbuilding
in the Netherlands, and were instrumental in keeping its polders dry.
American-style farm windmills
These windmills were used by American prairie farmers in generating electricity
and to pump water. They typically had many blades, operated at tip speed ratios
not better than one, and had good starting torque.
Common modern wind turbines
Pointed into the wind by computer controlled motors. This is the type of turbine
that is used commercially to produce electricity. They are usually white in
color.
Ducted rotor
Still something of a research project, the ducted rotor consists of a turbine
inside a duct which flares outwards at the back. The main advantage of the ducted
rotor is that it can operate in a wide range of winds. Another advantage is
that the generator operates at a high rotation rate, so it doesn't require a
bulky gearbox, so the mechanical portion can be smaller and lighter. A disadvantage
is that (apart from the gearbox) it is more complicated than the unducted rotor
and the duct is usually quite heavy, which puts an added load on the tower.
The wind blows faster at higher altitudes because of the drag of the surface (sea or land) and the viscosity of the air. The variation in velocity with altitude, called wind shear is most dramatic near the surface. Typically, in daytime 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%. Doubling the tower height generally requires doubling the diameter as well, increasing the amount of material by a factor of eight.
Installing a wind turbine on house.
It is said that Typically, a household sees a reduction of between a quarter and a third in its annual electricity bill Solar panels can supplement wind to boost a home's renewable sources but some households do claim to make a profit purely from wind, by generating so much electricity that the amount they sell back is greater than the amount they buy.
That would require a very energy efficient house and living in a particularly windy part of the UK.
"You may get the 4-5,000 units a year to run a household but not every single hour of every day so you would need to have standard electricity grid connection to get electricity from the grid.
Talk to neighbours and the council about planning issues
Check local windspeeds
Shop around - do you want a roof-mounted or garden one?
Chat to manufacturers before buying
By 2005 7,000 households were given grants to get the turbines installed in Britain.
How long does it take for a turbine to pay back the energy used to manufacture
it?
The comparison of energy used in manufacture with the energy produced by a power
station is known as the 'energy balance'. It can be expressed in terms of energy
'pay back' time, i.e. as the time needed to generate the equivalent amount of
energy used in manufacturing the wind turbine or power station.
The average wind farm in the UK will pay back the energy used in its manufacture within six to eight months, this compares favourably with coal or nuclear power stations, which take about six months. This figure may be different for house turbines.
How strong does the wind have to blow for the wind turbines to work?
Wind turbines start operating at wind speeds of 4 to 5 metres per second (around
10 miles an hour) and reach maximum power output at 15 meters/second (around
33 miles per hour). At very high wind speeds, i.e. gale force winds, (25 metres/second,
50+ miles/hour) wind turbines shut down.
Unfortunately, whilst the wind is free, the means to extract the power from it is not. Buying and installing turbines a turbine costs, and operation and maintenance costs. The output from a wind turbine is sensitive to wind speed. It is essential turbines should be sited from obstructions, with clear exposure or fetch for the prevailing wind. Wind speed also increases with height so it is best to have the turbine high up, and most small turbines have towers much higher relative to their diameter than large ones.
An ideal position for a wind turbine generator is a smooth hill top, with a flat clear fetch, at least in the prevailing wind direction. The wind speeds up significantly near the top of the hill and the air flow should be reasonably smooth and free from excessive turbulence. Excessive turbulence causes fatigue damage and shortens a turbine's working life.
In practice, especially for very small machines which need to be located near to the user, ideal siting is not be easy. As far as possible, keep away from local obstructions such as large trees and houses, or use a taller tower to ensure that the turbine is well above the obstructions.
Before considering installation of a wind turbine the potential site should be assessed. Initial indications of wind strength and direction can be obtained by observing the deformation of vegetation and trees, and in many cases the user may already have a good feel for the winds in the locality. Impressions can be deceptive.
A more reliable way to evaluate the wind resource, which is strongly recommended when there is doubt over whether the wind is strong enough, is to take regular measurements over a period of several months, preferably a year.
Clearly the expenditure on site assessment should be in relation to the proposed size and cost of the installation. Very little or no cost is justified in evaluating a site for a 50W battery charger, whereas a 60kW wind turbine, for example, would in general merit site measurements.
Basic requirements
Get a reliable estimate of the winds to be expected at the proposed site. There
is no substitute for actual measurements. The turbine manufacturers should be
prepared to help. You can get an estimate of the annual mean wind speed for
a given location by entering the OS grid reference into the UK wind speed database
Mount the turbine on as high a tower as possible and well clear of obstructions,
but do not go to extremes. Easy access will be required for erection and foundations
for the tower may be needed depending on the size and tower type. It is also
important to ensure that the wind turbine can be easily lowered for inspection
and maintenance.
Try to have a clear, smooth fetch to the prevailing wind, e.g. over open water
or smooth ground. If possible site the turbine on a smooth hill.
Use cable of adequate current carrying capacity. Cable costs can be substantial.
Consult your local council as to whether you need planning permission. You should
try to minimise the environmental impact of the turbine, and it will be helpful
to inform your neighbours of your plans at early stage.
If the machine is intended for connection to the grid you must discuss it with
your local electricity company.
History
Wind machines were used for grinding grain in Persia as early as 200 B.C. The machine spread throughout the Islamic world and were introduced by Crusaders into Europe in the 13 th century. By the 14 th century Dutch windmills were in use to drain the Rhine River delta. In Denmark by 1900 there were about 2500 windmills for mechanical loads such as pumps and mills, producing a combined peak power of about 30 MW. The first windmill for electricity production was built in Denmark in 1890, and in 1908 there were 72 wind-driven electric generators from 5 kW to 25 kW.
By the 1930s windmills were mainly used to generate electricity on farms, mostly in the US where distribution systems had not been installed. In this period, high tensile steel was cheap, and windmills were placed atop prefabricated open steel lattice towers. A forerunner of modern horizontal-axis wind generators was in service at Yalta, USSR in 1931. This was a 100 kW generator on a 30 m tower, connected to the local 6.3 kV distribution system. It was reported to have an annual load factor of 32 per cent, not much different from current wind machines.
In 1941 the world's first megawatt size wind turbine was connected to the local electrical distribution system in Vermont. This 1.25 MW turbine operated for 1100 hours. In the 1940s, the U.S. had a rural electrification project that killed the natural market for wind-generated power, since network power distribution provided a farm with more dependable usable energy for a given amount of capital investment.
In the 1970s many began to desire a self sufficient life style. Solar cells were too expensive for smallscale electrical generation, so practical people turned to windmills. At first they built adhoc designs using wood and automobile parts. Most discovereda reliable wind generator is a moderately complex engineering project. Practical people began to search for and rebuild farm windgenerators from the 1930s. Jacobs wind generators were especially sought after.
Iin the 80s, California provided tax rebates for ecologically harmless power. These rebates funded the use of wind power for utility electricity. These machines, gathered in large wind parks such as at Altamont Pass would be considered small and uneconomic by modern wind power development standards.
In the 90s, as aesthetics and durability became important, turbines were placed on steel or reinforced concrete towers. Small generators are connected to the tower on the ground, then the tower is raised into position. Larger generators are hoisted into position on the tower and there is a ladder or staircase inside the tower to allow technicians to reach and maintain the generator. Originally wind generators were built right next to where their power was needed. With the availability of long distance electric power transmission, wind generators are often on wind farms in windy locations and huge ones are being built offshore, sometimes transmitting power back to land using high voltage submarine cable. Since wind turbines are a renewable means of generating electricity, they are being widely deployed, but their cost is often subsidised by taxpayers, either directly or through renewable energy credits. Much depends on cost of alternative sources of electricity. Wind generator cost per unit power has been decreasing by about four percent per year.
Noise
There is always low frequency noise present in any ambient quiet background
and it can be produced by a variety of sources, including machinery, transport
and natural sources such as the sea, wind and thunder. It has been repeatedly
shown by measurements of wind turbine noise undertaken in the UK, Denmark, Germany
and the USA over the past decade, and accepted by experienced professionals,
the levels of low frequency noise and vibration radiated from modern, upwind
configuration wind turbines are at a very low level so low that they lie below
the threshold of perception.
Trivia
Foreign names for wind turbine
Windenergieanlage = German
Aerogenerador = Spain
Éolienne = France
Vindkraftverk = Swedish
Energia eólica = Português
Vindmølle = Dansk (Danish)
Tory leader Cameron has put a wind turbine on his house. While Labour party Energy Minister Malcolm Wicks also wants to install his own mini-windmill.
In glasgow a housing association wants to install mini wind turbines in homes in the north of Glasgow.
Scots based celebrity champions including Franz Ferdinand lead singer Alex Kapranos, top actor Brian Cox, celebrity chef Nick Nairn, legendary broadcaster Magnus Magnusson and Changing Rooms designer Anna Ryder Richardson - have voiced their support for wind power.
The popular support for wind was unveiled at the launch of the Scottish Embrace the Revolution campaign, which gives a voice to the large majority of Scots who back wind power.
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