Wind Turbine

A wind turbine is a machine for converting the kinetic energy in wind into mechanical energy.   If the mechanical energy is used directly by machinery, such as a pump or grinding stones, the machine is usually called a windmill.   If the mechanical energy is then converted to electricity, the machine is called a wind generator.   Wind turbines can be separated into two types based on the axis about which the turbine rotates.   A horizontal-axis turbine (HAWT) rotates around a horizontal rotor axis and is much more common.   A vertical-axis turbine (VAWT) rotates around a vertical rotor axis and is much less frequently used.   Wind turbines can also be classified by the location in which they are to be used.   Onshore, offshore, or even aerial wind turbines have unique design characteristics.   Wind turbines may also be used in conjunction with a solar collector to extract the energy due to air heated by the sun and rising through a large vertical solar updraft tower. ^{Wiki n.p.}

Horizontal-axis wind turbines (HAWT) have the main rotor shaft and electrical generator at the top of a tower, and must be pointed into the wind.   Small turbines are pointed by a simple wind vane, while large turbines generally use a wind sensor coupled with a servo motor.   Most have a gearbox to turn the slow rotation of the blades into a quicker rotation that is more suitable for generating electricity.   Since a tower produces turbulence behind it, the turbine is usually pointed upwind of the 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 up a small amount.   Downwind machines have been built, despite the problem of turbulence, 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.   Because turbulence leads to fatigue failures and reliability is so important, most HAWTs are upwind machines. ^{Wiki n.p.}

There are several types of HAWT:^{Wiki n.p.}

Windmills, usually with wooden shutters or fabric sails, were developed in Europe.   These windmills were pointed into the wind manually or via a tail-fan and were typically used to grind grain.   In the Netherlands they were also used to pump water from low-lying land, and were instrumental in keeping its polders dry.   Windmills were also located throughout the USA, especially in the Northeastern region.

Rural windmills, invented in 1876, were used by Australian and later American farmers to pump water and to generate electricity.   They typically had many blades, operated at tip speed ratios not better than one, and had good starting torque.   Some had small direct-current generators used to charge storage batteries, to provide a few lights, or to operate a radio receiver.   The American rural electrification connected many farms to centrally-generated power and replaced individual windmills as a primary source of farm power in the 1930's.   Such devices are still used in locations where it is too costly to bring in commercial power.

Common modern wind turbines and ( 2 ) are usually three-bladed, sometimes two-bladed or even one-bladed (and counterbalanced), and pointed into the wind by computer-controlled motors.   They have high tip speeds of up to 6x wind speed, high efficiency, and low torque ripple which contributes to good reliability.   This is the type of turbine that is used commercially to produce electricity.   The blades are usually colored light gray to blend in with the clouds and range in length from 20 to 40 metres (60 to 120 feet) or more.

VAWT:   Vertical-axis wind turbines (VAWTs) have the main rotor shaft running vertically.   Key advantages of this arrangement are that the generator and/or gearbox can be placed at the bottom, near the ground, so the tower doesn't need to support it, and that the turbine doesn't need to be pointed into the wind.   Drawbacks are usually the pulsating torque that can be produced during each revolution and the drag created when the blade rotates into the wind.   It is also difficult to mount vertical-axis turbines on towers, meaning they must operate in the often slower, more turbulent air flow near the ground, with resulting lower energy extraction efficiency. ^{Wiki n.p.}

Offshore wind turbines are less obtrusive than turbines on land, as their apparent size and noise can be mitigated by distance.   Because water has less surface roughness than land (especially deeper water), the average wind speed is usually considerably higher over open water.   Capacity factors (utilisation rates) are considerably higher than for onshore and near-shore locations which allows offshore turbines to use shorter towers, making them less visible.   In stormy areas with extended shallow continental shelves (such as Denmark), turbines are practical to install.   Locations have begun to be developed in the North American Great Lakes, with one project by Trillium Power approximately 20 km from shore and over 700 MW in size.   Ontario, Canada is aggressively pursuing wind power development and has many onshore wind farms and several proposed near-shore locations but presently only one offshore development. ^{Wiki n.p.}

In most cases offshore environment is more expensive than onshore.   Offshore towers are generally taller than onshore towers once the submerged height is included, and offshore foundations are more difficult to build and more expensive.   Power transmission from offshore turbines is generally through undersea cable, which is more expensive to install than cables on land, and may use high voltage direct current operation if significant distance is to be covered — which then requires yet more equipment.   The offshore environment can also be corrosive and abrasive in salt water locations, but locations such as the Great Lakes are in fresh water and do not have many of the issues found in the ocean or sea.   Repairs and maintenance are usually much more difficult, and generally more costly, than on onshore turbines.   Offshore wind turbines are outfitted with extensive corrosion protection measures like coatings and cathodic protection however some of these measures may not be required in fresh water locations.   While there is a significant market for small land-based windmills, offshore wind turbines have recently been and will probably continue to be the largest wind turbines in operation because larger turbines allow for the spread of the high fixed costs involved in offshore operation over a greater quantity of generation, reducing the average cost.   For similar reasons, offshore wind farms tend to be quite large—often involving over 100 turbine as opposed to onshore wind farms which can operate competitively even with much smaller installations. ^{Wiki n.p.}

Near-shore turbines are generally considered to be within a zone that is on land three kilometers of a shoreline and on water within ten kilometers of land.   Wind speeds in these zones share wind speed characteristics of both onshore wind and offshore wind.   Issues that are shared within near-shore wind development zones are ornithological (including bird migration and nesting), aquatic habitat, transportation (including shipping and boating) and visual aesthetics.   Sea shores also tend to be windy areas and good sites for turbine installation because a primary source of wind is convection from the differential heating and cooling of land and sea over the course of day and night.   Winds at sea level carry somewhat more energy than winds of the same speed in mountainous areas because the air at sea level is denser. ^{Wiki n.p.}

Onshore turbine installations in hilly or mountainous regions tend to be on ridgelines generally three kilometers or more inland from the nearest shoreline.   This is done to exploit the topographic acceleration where the hill or ridge causes the wind to accelerate as it is forced over it.   The additional wind speeds gained in this way make large differences to the amount of energy that is produced.   Great attention must be paid to the exact positions of the turbines (a process known as micro-siting) because a difference of 30 m can sometimes mean a doubling in output.   Local winds are often monitored for a year or more with anemometers and detailed wind maps constructed before wind generators are installed.   Wind farm siting can sometimes be controversial, particularly as the hilltop, often coastal sites preferred are often picturesque and environmentally sensitive (for instance, having substantial bird life). ^{Wiki n.p.}

Advantages and Disadvantages of HAWT and VAWT. ^{Wiki n.p.}

Advantages of vertical wind turbines.

• Easier to maintain because most of their moving parts are located near the ground. This is due to the vertical wind turbine’s shape. The airfoils or rotor blades are connected by arms to a shaft that sits on a bearing and drives a generator below, usually by first connecting to a gearbox.
• As the rotor blades are vertical, a yaw device is not needed, reducing the need for this bearing and its cost.
• Vertical wind turbines have a higher airfoil pitch angle, giving improved aerodynamics while decreasing drag at low and high pressures.
• Mesas, hilltops, ridgelines and passes can have higher and more powerful winds near the ground than up high because of the speed up effect of winds moving up a slope or funneling into a pass combining with the winds moving directly into the site. In these places, VAWTs placed close to the ground can produce more power than HAWTs placed higher up.
• Low height useful where laws do not permit structures to be placed high.
• Smaller VAWTs can be much easier to transport and install.
• Does not need a free standing tower so is much less expensive and stronger in high winds that are close to the ground.
• Usually have a lower Tip-Speed ratio so less likely to break in high winds.

Disadvantages of vertical wind turbines.

• Most VAWTs produce energy at only 50% of the efficiency of HAWTs in large part because of the additional drag that they have as their blades rotate into the wind. This can be overcome by using structures to funnel more and align the wind into the rotor or the "vortex" effect of placing straight bladed VAWTs closely togethe.
• There may be a height limitation to how tall a vertical wind turbine can be built and how much sweep area it can have.
• Most VAWTS need to be installed on a relatively flat piece of land and some sites could be too steep for them but are still usable by HAWTs.
• Most VAWT's have low starting torque.
• A VAWT that uses guyed wires to hold it in place puts stress on the bottom bearing as all the weight of the rotor is on the bearing. Guyed wires attached to the top bearing increase downward thrust in wind gusts. Solving this problem requires a superstructure to hold a top bearing in place to eliminate the downward thrusts of gust events in guyed wired models.

Advantages of horizontal wind turbines.

• Blades are to the side of the turbine's center of gravity, helping stability.
• Ability to wing warp, which gives the turbine blades the best angle of attack. Allowing the angle of attack to be remotely adjusted gives greater control, so the turbine collects the maximum amount of wind energy for the time of day and season.
• Ability to pitch the rotor blades in a storm, to minimize damage.
• Tall tower allows access to stronger wind in sites with wind shear. In some wind shear sites, every ten meters up, the wind speed can increase by 20% and the power output by 34%.
• Tall tower allows placement on uneven land or in offshore locations.
• Can be sited in forests above the treeline.
• Most are self-starting.
• Can be cheaper because of higher production volume, larger sizes and, in general higher capacity factors and efficiencies.

Diadvantages of horizontal wind turbines.

• HAWTs have difficulty operating in near ground, turbulent winds because their yaw and blade bearing need smoother, more laminar wind flows.
• The tall towers and long blades (up to 180 feet long) are difficult to transport on the sea and on land.
• Transportation can now cost 20% of equipment costs.
• Tall HAWTs are difficult to install, needing very tall and expensive cranes and skilled operators.
• Supply of HAWTs is less than demand and between 2004 and 2006, turbine prices increased up to 60%.
• The FAA has raised concerns about tall HAWTs effects on radar in proximity to air force bases.
• Their height can be create local opposition based on impacts to viewsheds.
• Offshore tower can be a navigation problem and must be installed in shallow seas. HAWTs can't be floated on barges.
• Downwind variants suffer from fatigue and structural failure caused by turbulence.

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