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Author Topic: Different types of wind turbines  (Read 23724 times)
james
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« on: August 11, 2009, 03:31:38 PM »

Types of wind turbines

Wind turbines can be separated into two types based by the axis in which the turbine rotates. Turbines that rotate around a horizontal axis are more common. Vertical-axis turbines are less frequently used.

Horizontal axis

Horizontal-axis wind turbines (HAWT) have the main rotor shaft and electrical generator at the top of a tower, and are usually pointed into the wind. Most small turbines are pointed by a simple wind vane, although there are now a number of more modern designs which are classed as down wind machines and which require no tail vane. Large turbines generally use a wind sensor coupled with a servo motor. Most have a gearbox, which turns the slow rotation of the blades into a quicker rotation that is more suitable to drive an electrical generator.[9]
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 (mast wake), because they don't need an additional mechanism to keep 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 cyclic (that is repetitive) turbulence may lead to fatigue failures most large HAWTs are upwind machines.

HAWT Subtypes

12th-century windmills

These squat structures, typically (at least) four bladed, 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.
In Schiedam, the Netherlands, a traditional style windmill (the Noletmolen) was built in 2005 to generate electricity.[10] The mill is one of the tallest Tower mills in the world, being some 42.5 metres (139 ft) tall.

19th-century windmills

The Eclipse windmill factory was set up around 1866 in Beloit, Wisconsin and soon became successful building mills for pumping water on farms and for filling railroad tanks. Other firms like Star, Dempster, and Aeromotor also entered the market. Hundreds of thousands of these mills were produced before rural electrification and small numbers continue to be made.[6] 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 power to 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 by the 1950s. They were also produced in other countries like South Africa and Australia (where an American design was copied in 1876[11]). Such devices are still used in locations where it is too costly to bring in commercial power.

Modern wind turbines

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 up to six times the wind speed, high efficiency, and low torque ripple, which contribute to good reliability. The blades are usually colored light gray to blend in with the clouds and range in length from 20 to 40 metres (65 to 130 ft) or more. The tubular steel towers range from 200 to 300 feet (60 to 90 metres) tall. The blades rotate at 10-22 revolutions per minute.[12][13] A gear box is commonly used to step 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 control systems. These systems employ anemometers and wind vanes to determine wind speed and direction. Based on this information, the turbine yaw drive will turn the blade face into the wind, and the blade pitch can be altered to maximize output.
In very high wind speed conditions, the control system will shut the turbine down to avoid damage.

HAWT advantages

Variable blade pitch, which gives the turbine blades the optimum 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.

The tall tower base 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%.

High efficiency, since the blades always move perpendicularly to the wind, receiving power through the whole rotation. In contrast, all vertical axis wind turbines, and most proposed airborne wind turbine designs, involve various types of reciprocating actions, requiring airfoil surfaces to backtrack against the wind for part of the cycle. Backtracking against the wind leads to inherently lower efficiency.

HAWT disadvantages

The tall towers and blades up to 90 meters long are difficult to transport. Transportation can reach 20% of equipment costs.

Tall HAWTs are difficult to install, needing very tall and expensive cranes and skilled operators.

Massive tower construction is required to support the heavy blades, gearbox, and generator.

Reflections from tall HAWTs may affect side lobes of radar installations creating signal clutter, although filtering can suppress it.

Their height makes them obtrusively visible across large areas, disrupting the appearance of the landscape and sometimes creating local opposition.

Downwind variants suffer from fatigue and structural failure caused by turbulence when a blade passes through the tower's wind shadow (for this reason, the majority of HAWTs use an upwind design, with the rotor facing the wind in front of the tower).

HAWTs require an additional Yaw drive control mechanism to turn the blades toward the wind.





Vertical axis

Vertical-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. VAWTs can utilize winds from varying directions.

It is difficult to mount vertical-axis turbines on towers, meaning they are often installed nearer to the base on which they rest, such as the ground or a building rooftop. This can provide the advantage of easy accessibility to mechanical components. However, wind speed is slower at a lower altitude, so less wind energy is available for a given size turbine. Air flow near the ground and other objects can create turbulent flow, which can introduce issues of vibration, including noise and bearing wear which may increase the maintenance or shorten the service life. In designs that do not have helical rotors significant torque variation will occur.

Extensive research on wind turbines carried out in the late 1970s and 1980s by the Department of Energy included some vertical axis designs; none of these designs succeeded in the marketplace due to inherent downfalls of this design. "...when it came down to cost of electricity as a result of efficiency, reliability, and economy of materials, verticals could not compete with horizontals" [14] In addition, many of the claims of current vertical axis wind turbine manufacturers are unsubstantiated or are incorrect.

VAWT subtypes

Darrieus wind turbine
"Eggbeater" turbines. They have good efficiency, but produce large torque ripple and cyclic stress on the tower, which contributes to poor reliability. Also, they generally require some external power source, or an additional Savonius rotor, to start turning, because the starting torque is very low. The torque ripple is reduced by using three or more blades which results in a higher solidity for the rotor. Solidity is measured by blade area over the rotor area. Newer Darrieus type turbines are not held up by guy-wires but have an external superstructure connected to the top bearing.

Giromill
A subtype of Darrieus turbine with straight, as opposed to curved, blades. The cycloturbine variety has variable pitch to reduce the torque pulsation and is self-starting.[15] The advantages of variable pitch are: high starting torque; a wide, relatively flat torque curve; a lower blade speed ratio; a higher coefficient of performance; more efficient operation in turbulent winds; and a lower blade speed ratio which lowers blade bending stresses. Straight, V, or curved blades may be used.

Savonius wind turbine
These are drag-type devices with two (or more) scoops that are used in anemometers, Flettner vents (commonly seen on bus and van roofs), and in some high-reliability low-efficiency power turbines. They are always self-starting if there are at least three scoops. They sometimes have long helical scoops to give a smooth torque.

VAWT advantages

A massive tower structure is less frequently used, as they are more frequently mounted with the lower bearing mounted near the ground, making it easier to maintain the moving parts.

Designs without yaw mechanisms are possible with fixed pitch rotor designs.

They have lower wind startup speeds than HAWTs. Typically, they start creating electricity at 6 m.p.h. (10 km/h).

They may be built at locations where taller structures are prohibited.

VAWTs situated close to the ground can take advantage of locations where mesas, hilltops, ridgelines, and passes funnel the wind and increase wind velocity.

They may have a lower noise signature.

VAWT disadvantages

Most 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.

A VAWT that uses guy-wires to hold it in place puts stress on the bottom bearing as all the weight of the rotor is on the bearing. Guy 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 guy wired models.

While the parts are located on the ground, they are also located under the weight of the structure above it, which can make changing out parts nearly impossible without dismantling the structure if not designed properly.

Having rotors located close to the ground where wind speeds are lower due to wind shear, they may not produce as much energy at a given site as a HAWT with the same footprint or height.

Because they are not commonly deployed due mainly to the serious disadvantages mentioned above, they appear novel to those not familiar with the wind industry. This has often made them the subject of wild claims and investment scams over the last 50 years




Unconventional wind turbines

As of 2009, the most common type of wind turbine is the three-bladed horizontal-axis wind turbine (HAWT), but there are various types of wind turbine that differ from the standard type.

Modified HAWT

Ducted Rotor

Still something of a research project, the ducted rotor consists of a turbine inside a duct which flares outwards at the back. They are also referred as Diffuser-Augmented Wind Turbines (i.e. DAWT). The main advantage of the ducted rotor is that it can operate in a wide range of winds and generate a higher power per unit of rotor area. 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 Éolienne Bollée is an example of a DAWT.

Maglev wind turbine

Magnetic levitation turbines are an experiment in adapting maglev bearings for wind turbines. If successful they are likely to substantially reduce minimum wind speeds necessary for power generating and increase operating efficiency.

Co-axial, multi-rotor horizontal-axis turbines

Two or more rotors may be mounted to the same driveshaft, with their combined co-rotation together turning the same generator — fresh wind is brought to each rotor by sufficient spacing between rotors combined with an offset angle alpha from the wind direction. Wake vorticity is recovered as the top of a wake hits the bottom of the next rotor. Power has been multiplied several times using co-axial, multiple rotors in testing conducted by inventor and researcher Douglas Selsam, for the California Energy Commission in 2004. The first commercially available co-axial multi-rotor turbine is the patented dual-rotor American Twin Superturbine from Selsam Innovations in California, with 2 propellers separated by 12 feet. It is the most powerful 7-foot diameter turbine available, due to this extra rotor.

Counter-rotating horizontal-axis turbines

Counter rotating turbines can be used to increase the rotation speed of the electrical generator. As of 2005, no large practical counter-rotating HAWTs are commercially sold. When the counter rotating turbines are on the same side of the tower, the blades in front are angled forwards slightly so as to avoid hitting the rear ones. If the turbine blades are on opposite sides of the tower, it is best that the blades at the back be smaller than the blades at the front and set to stall at a higher wind speed. This allows the generator to function at a wider wind speed range than a single-turbine generator for a given tower. To reduce sympathetic vibrations, the two turbines should turn at speeds with few common multiples, for example 7:3 speed ratio. Overall, this is a more complicated design than the single-turbine wind generator, but it taps more of the wind's energy at a wider range of wind speeds.

Appa designed and demonstrated a contra rotor wind turbine in FY 2000–2002 funded by California Energy Commission. This study showed 30 to 40% more power extraction than a comparable single rotor system. Further it was observed that the slower the rotor speed, the better the performance. Consequently Megawatt machines benefit most. This also cancells the gyroscopic forces. There will be an improvement in overall efficiency.

Furling tail and twisting blades turbines

In addition to variable pitch blades, furling tails and twisting blades are other improvements on wind turbines. Similar to the variable pitch blades, they may also greatly increase the efficiency of the turbine and be used in diy construction.

Telescopic blades

The next step in making improvements to wind turbines is the use of telescopic blades. Telescopic blades can change the blades length thus increasing or decreasing the turbines swept area. Telescopic blades make a turbine more productive by increasing the turbines rotor diameter during low wind conditions. In high wind conditions when the turbine is in need of reducing loads the blades can be retracted to make the rotor smaller.



Modified VAWT

Aerogenerator

The Aerogenerator is a special design of vertical axis wind turbine which could allow greater energy outputs. [3]

Savonius wind turbine

The Savonius wind turbine are a type of vertical-axis wind turbine (VAWT), used for converting the power of the wind into torque on a rotating shaft. They were invented by the Finnish engineer Sigurd J. Savonius in 1922
Savonius turbines are one of the simplest turbines. Aerodynamically, they are drag-type devices, consisting of two or three scoops. Looking down on the rotor from above, a two-scoop machine would look like an "S" shape in cross section. Because of the curvature, the scoops experience less drag when moving against the wind than when moving with the wind. The differential drag causes the Savonius turbine to spin. Because they are drag-type devices, Savonius turbines extract much less of the wind's power than other similarly-sized lift-type turbines. Much of the swept area of a Savonius rotor is near the ground, making the overall energy extraction less effective due to lower wind speed at lower heights.

Augmented "G" model VAWT: "G" Model Wind Turbine (GMWT)

The "G" Model VAWT Turbine is equipped with three -self directioning- "Augmentation And Directioning Wings=AADW" placed outer section of classical Darrieus blades. The GMWT is capable to increase almost fivefold the efficiency of classical Darrieus Blades: [4] AADW adjust itself to the wind direction without any external power. The result combination ("G" Model Wind Turbine) need very low cut-in wind speed, has self starting ability togetherwith high capacity factor.



Aerial

It has been suggested that wind turbines could be flown in high speed winds at high altitude taking advantage of the steadier winds at high altitudes. No such systems currently exist in the marketplace. The idea of airborne wind turbines reappears in the industry every few years, and seldom (if ever) gets off the drawing board.



H-rotor

Vertical axis wind turbine that has two vertical blades mounted on horizontal arms and looks like a large letter H atop a tower. They are mathematically not as efficient as some other designs because as one blade is pushed by the wind the other is facing in its direction.



Wind belt

The Windbelt is a device for converting wind power to electricity. A windbelt is essentially an aeolian harp except that it exploits the motion of the string produced by the the aeroelastic flutter effect to move a magnet closer and farther from one or more electromagnetic coil(s) and thus inducing current in the wires that make up the coil.



Vaneless ion wind generator

A vaneless ion wind generator or power fence is a proposed wind power device that produces electrical energy directly by using the wind to pump electric charge from one electrode to another, with no moving parts.



Piezoelectric wind turbines

The present research suggests that it is possible to generate power using the wind through Piezoelectric wind turbines which use the flow of wind over a body, such as a small leaf-shaped piezoelectric material like PVDF and take the power from the flapping movement of that leaf. This could be very useful for generating energy by having a building covered with these leaves, similar to an ivy-clad house, which then take and store wind energy as it blows around the building.



Solar Updraft Towers

The solar updraft tower is a proposed type of renewable-energy power plant. It combines three old and proven technologies: the chimney effect, the greenhouse effect, and the wind turbine. Air is heated by sunshine and contained in a very large greenhouse-like structure around the base of a tall chimney, and the resulting convection causes rising airflow to rise through the updraft tower. The air current from the greenhouse up the chimney drives turbines, which produce electricity. A successful research prototype operated in Spain in the 1980s, and many modelling studies have been published as to optimization, scale, and economic feasibility.



















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