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As a growing segment of the population becomes interested in making the switch to renewable energy, wind power generators are rapidly becoming more popular than ever. In just the last year alone, the power generated worldwide by wind turbines grew by more than 30% to a total output of nearly 160,000 megawatts. Unlike the wind turbines that were put in place during the 70s and 80s, today’s wind turbines are remarkably efficient and are accounting for a growing percentage of the electrical power available on the grid. Given the fact that energy analysts believe that wind power generators will become even more important to our energy supply in the years to come, it is important to understand the benefits posed by this technology, as well as the significant hurdles that face the adoption of wind turbines in many parts of the world.
While there a number of different wind turbines in use today, they all work on the same basic principle. Wind turbines take the basic energy of wind and transfer the energy into kinetic energy through the use of mechanical machinery. Unlike some types of renewable energy, wind turbines actually have a long history of human use that date back as far as 200 BC. For example, windmills were considered essential to farming in many parts of the world until very recent history. The first automatic windmill appeared neared the end of the 19th century and were able to generate an electrical output of 12 kilowatts. During the 30s and 40s, the use of windmills to generate electricity was quite common in areas of the United States that had yet be granted access to the growing electrical grid of the country.
Today’s wind power generators have come a long way and have an electrical output that dwarfs that of the early the windmills used for electricity on 20th century farms. Most modern wind turbines are based on the basic design of horizontal-axis wind turbines, also known as HAWTs. These are the turbines that many citizens see when driving through more remote areas of the country and are the models that will be used in the offshore wind farms of the future.
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Since a tower produces turbulence behind it, the turbine is usually positioned upwind of its supporting tower. Turbine blades are made stiff to prevent the blades from being pushed into the tower by high winds. Additionally, the blades are placed a considerable distance in front of the tower and are sometimes tilted forward into the wind a small amount.
Downwind machines have been built, despite the problem of turbulence (mast wake), because they don't need an additional mechanism for keeping them in line with the wind, and because in high winds the blades can be allowed to bend which reduces their swept area and thus their wind resistance. Since cyclical (that is repetitive) turbulence may lead to fatigue failures, most HAWTs are of upwind design.
Turbines used in wind farms for commercial production of electric power are usually three-bladed and pointed into the wind by computer-controlled motors. These have high tip speeds of over 320 km/h (200 mph), high efficiency, and low torque ripple, which contribute to good reliability. The blades are usually colored white for daytime visibility by aircraft and range in length from 20 to 40 metres (66 to 130 ft) or more. The tubular steel towers range from 60 to 90 metres (200 to 300 ft) tall. The blades rotate at 10 to 22 revolutions per minute. At 22 rotations per minute the tip speed exceeds 90 metre per second (300 ft/s). A gearbox is commonly used for stepping up the speed of the generator, although designs may also use direct drive of an annular generator. Some models operate at constant speed, but more energy can be collected by variable-speed turbines which use a solid-state power converter to interface to the transmission system. All turbines are equipped with protective features to avoid damage at high wind speeds, by feathering the blades into the wind which ceases their rotation, supplemented by brakes.
Vertical axis designVertical-axis wind turbines (or VAWTs) have the main rotor shaft arranged vertically. Key advantages of this arrangement are that the turbine does not need to be pointed into the wind to be effective. This is an advantage on sites where the wind direction is highly variable, for example when integrated into buildings. The key disadvantages include the low rotational speed with the consequential higher torque and and hence higher cost of the drive train, the inherently lower power coefficient, the 360 degree rotation of the aerofoil within the wind flow during each cycle and hence the highly dynamic loading on the blade, the pulsating torque generated by some rotor designs on the drive train, and the difficulty of modelling the wind flow accurately and hence the challenges of analysing and designing the rotor prior to fabricating a prototype.
With a vertical axis, the generator and gearbox can be placed near the ground, using a direct drive from the rotor assembly to the ground-based gearbox, hence improving accessibility for maintenance.
When a turbine is mounted on a rooftop, the building generally redirects wind over the roof and this can double the wind speed at the turbine. If the height of the rooftop mounted turbine tower is approximately 50% of the building height, this is near the optimum for maximum wind energy and minimum wind turbulence. It should be borne in mind that wind speeds within the built environment are generally much lower than at exposed rural sites, noise may be a concern and an existing structure may not adequately resist the additional stress.
Another type of vertical axis is the Parallel turbine similar to the cross flow fan or centrifugal fan it uses the ground effect. Vertical axis turbines of this type have been tried for many years: a large unit producing up to 10 kW was built by Israeli wind pioneer Bruce Brill in 1980s: the device is mentioned in Dr. Moshe Dan Hirsch's 1990 report, which decided the Israeli energy department investments and support in the next 20 years. The Magenn Wind Kite blimp blimp uses this configuration as well, chosen because of the ease of running.
Unconventional designsOne E-66 wind turbine at Windpark Holtriem, Germany, carries an observation deck, open for visitors. Another turbine of the same type, with an observation deck, is located in Swaffham, England. Airborne wind turbines have been investigated many times but have yet to produce significant energy. Conceptually, wind turbines may also be used in conjunction with a large vertical solar updraft tower to extract the energy due to air heated by the sun.Wind turbines which utilise the Magnus effect have been developed.The ram air turbine is a specialist form of small turbine that is fitted to some aircraft. When deployed, the RAT is spun by the airstream going past the aircraft and can provide power for the most essential systems if there is a loss of all on-board electrical power.
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