Vertical Axis Wind Turbine - Wikipedia, The Free Encyclopedia

4/6/2014 Vertical axis wind turbine - Wikipedia, the free encyclopedia

http://en.wikipedia.org/wiki/Vertical_axis_wind_turbine 1/5

The world's tallest vertical-axis

wind turbine, in Cap-Chat,

Quebec

Vertical axis wind turbineFrom Wikipedia, the free encyclopedia

Vertical-axis wind turbines (VAWTs) are a type of windturbine where the main rotor shaft is set vertically and the maincomponents are located at the base of the turbine. Thisarrangement allows the generator and gearbox to be locatedclose to the ground, facilitating service and repair. VAWTs do

not need to be pointed into the wind,[1] which removes the needfor wind-sensing and orientation mechanisms. Major drawbacksfor the early designs (Savonius, Darrieus and giromill) includedthe significant torque variation during each revolution, and thehuge bending moments on the blades. Later designs solved thetorque issue by providing helical twist in the blades similar toGorlov's water turbines.

A VAWT tipped sideways, with the axis perpendicular to thewind streamlines, functions similarly. A more general term thatincludes this option is "transverse axis wind turbine". Forexample, the original Darrieus patent, US Patent 1835018,includes both options.

Drag-type VAWTs such as the Savonius rotor typically operateat lower tipspeed ratios than lift-based VAWTs such asDarrieus rotors and cycloturbines.

Contents

1 General aerodynamics2 Advantages of vertical axis wind turbines3 Disadvantages of vertical axis wind turbines4 Applications5 References6 External links

General aerodynamics

The forces and the velocities acting in a Darrieus turbine are depicted in figure 1. The resultantvelocity vector, , is the vectorial sum of the undisturbed upstream air velocity, , and the velocity

vector of the advancing blade, .



Fig1: Forces and velocities acting

in a Darrieus turbine for various

azimuthal positions

Five-kilowatt vertical axis wind

turbine

Thus the oncoming fluid velocity varies during each cycle.Maximum velocity is found for and the minimum is foundfor , where is the azimuthal or orbital blade position.The angle of attack, , is the angle between the oncoming airspeed, W, and the blade's chord. The resultant airflow creates avarying, positive angle of attack to the blade in the upstreamzone of the machine, switching sign in the downstream zone ofthe machine.

From geometrical considerations, the resultant airspeed flowand the angle of attack are calculated as follows:

[2]

where is the tip speed ratio parameter.

The resultant aerodynamic force is resolved either into lift (F_L)- drag (D) components or normal (N) - tangential (T)components. The forces are considered acting at the quarter-chord point, and the pitching moment is determined to resolvethe aerodynamic forces. The aeronautical terms "lift" and "drag"refer to the forces across (lift) and along (drag) the approachingnet relative airflow. The tangential force acts along the blade'svelocity, pulling the blade around, and the normal force actsradially, pushing against the shaft bearings. The lift and the dragforce are useful when dealing with the aerodynamic forcesaround the blade such as dynamic stall, boundary layer etc.;while when dealing with global performance, fatigue loads, etc.,it is more convenient to have a normal-tangential frame. The liftand the drag coefficients are usually normalised by the dynamicpressure of the relative airflow, while the normal and tangential coefficients are usually normalised bythe dynamic pressure of undisturbed upstream fluid velocity.

A = Surface Area

The amount of power, P, that can be absorbed by a wind turbine:



Where is the power coefficient, is air density, is the swept area of the turbine, and is the

wind speed.[3]

Advantages of vertical axis wind turbines

VAWTs offer a number of advantages over traditional horizontal-axis wind turbines (HAWTs). Theycan be packed closer together in wind farms, allowing more in a given space. They are quiet, omni-directional, and they produce lower forces on the support structure. They do not require as much windto generate power, thus allowing them to be closer to the ground where windspeed is lower. By beingcloser to the ground they are easily maintained and can be installed on chimneys and similar tall

structures.[4]

Research at Caltech has also shown that carefully designing wind farms using VAWTs can result in

power output ten times as great as a HAWT wind farm the same size.[5]

VAWTs are much quieter than HAWTs, with dB levels at ground level ten meters from the tower

measured at around 95 dB for a HAWT[citation needed] the sound of passing traffic on a busy

highway measured from the road shoulder versus 38dB for a VAWT[citation needed] the sound ofwhispered conversation. This is due to several factors, starting with the much lower tip speed ofVAWTs.

However most of the above claims are considered debatable by people with experience of windengineering. There is very little history of successful VAWT operation, despite their long history.During this history they have repeatedly been portrayed as innovative, more efficient, more bird-

friendly, more cost-effective etc. whereas there is no factual evidence for any of these claims.[6]

Disadvantages of vertical axis wind turbines

VAWTs have a tendency to stall under gusty winds. They produce very low starting torque, and exhibitdynamic stability problems. The VAWTs are sensitive to off-design conditions and have a low

installation height limiting operation to lower wind speed environments.[7]

The blades of a VAWT are prone to fatigue due to the wide variation in applied forces during eachrotation. The vertically oriented blades used in early models twisted and bent during each turn,causing them to crack. Over time the blades broke apart, sometimes leading to catastrophic failure.

Because of these problems, VAWTs have proven less reliable than HAWTs.[8]

Research programmes (in 2011) sought to overcome the inefficiencies associated with VAWTs byoptimising turbine placement within wind farms. It is thought that, despite the lower wind-speedenvironment at low elevations, "the scaling of the physical forces involved predicts that [VAWT] windfarms can be built using less expensive materials, manufacturing processes, and maintenance than

is possible with current wind turbines".[9]

Applications



The Windspire, a small VAWT intended for individual (home or office) use was developed in the early2000s by US company Mariah Power. The company reported that several units had been installed

across the US by June 2008.[10]

Arborwind, an Ann-Arbor (Michigan, US) based company, produces a patented small VAWT which

has been installed at several US locations as of 2013.[11]

In 2011, Sandia National Laboratories wind-energy researchers began a five-year study of applying

VAWT design technology to offshore wind farms.[12] The researchers stated: "The economics ofoffshore windpower are different from land-based turbines, due to installation and operationalchallenges. VAWTs offer three big advantages that could reduce the cost of wind energy: a lowerturbine center of gravity; reduced machine complexity; and better scalability to very large sizes. Alower center of gravity means improved stability afloat and lower gravitational fatigue loads.Additionally, the drivetrain on a VAWT is at or near the surface, potentially making maintenanceeasier and less time-consuming. Fewer parts, lower fatigue loads and simpler maintenance all leadto reduced maintenance costs."

A 24-unit VAWT demonstration plot was installed in southern California in the early 2010s by Caltechaeronautical professor John Dabiri. His design was incorporated in a 10-unit generating farm

installed in 2013 in the Alaskan village of Igiugig.[13]

Dulas, Anglesey received permission in March 2014 to install a prototype VAWT on the breakwaterat Port Talbot waterside. The turbine is a new design, supplied by Wales-based C-FEC

(Swansea),[14] and will be operated for a two-year trial.[15] This VAWT incorporates a wind shieldwhich blocks the wind from the advancing blades, and thus requires a wind-direction sensor and a

positioning mechanism, as opposed to the "egg-beater" types of VAWTs discussed above.[16]

References

1. ^ Jha, Ph.D., A.R. (2010). Wind turbine technology. Boca Raton, FL: CRC Press2. ^ Amina El Kasmi, Christian Masson, An extended k-epsilon model for turbulent flow through horizontal-

axis wind turbines, Journal of Wind Engineering and Industrial Aerodynamics, Volume 96, Issue 1,January 2008, Pages 103-122 (http://www.sciencedirect.com/science/article/B6V3M-4NT24P6-1/2/2441e0d12f1c52c23c3ead5e874baf11), retrieved 2010-04-26

3. ^ Sandra Eriksson, Hans Bernhoff, Mats Leijon, (June 2008), "Evaluation of different turbine conceptsfor wind power" (http://www.sciencedirect.com/science/article/B6VMY-4N68049-1/2/cd2b3f0dc193eb9c6f9260b18e460eb4), Renewable and Sustainable Energy Reviews 12 (5): 14191434, doi:10.1016/j.rser.2006.05.01 (http://dx.doi.org/10.1016%2Fj.rser.2006.05.01), ISSN 1364-0321(//www.worldcat.org/issn/1364-0321), retrieved 2010-04-26

4. ^ Steven Peace, Another Approach to Wind(http://www.memagazine.org/backissues/membersonly/jun04/features/apptowind/apptowind.html),retrieved 2010-04-26

5. ^ Kathy Svitil, Wind-turbine placement produces tenfold power increase, researchers say(http://phys.org/news/2011-07-wind-turbine-placement-tenfold-power.html), retrieved 2012-07-31

6. ^ http://www.wind-works.org/cms/index.php?id=1167. ^ Jha, Ph.D., A.R. (2010). Wind turbine technology. Boca Raton, FL: CRC Press8. ^ Chiras, D. (2010). Wind power basics: a green energy guide. Gabriola Island, BC, Canada: New

Society Pub.9. ^ http://www.sciencedaily.com/releases/2011/07/110713131644.htm



10. ^ [1] (http://news.cnet.com/8301-11128_3-9956965-54.html) C-NET, "Vertical-axis wind turbine spinsinto business" (2 June 2008)

11. ^ [2] (http://www.arborwind.com/about-us/history/) Arborwind (website), "History"12. ^ [3] (http://www.renewableenergyworld.com/rea/news/article/2012/08/offshore-use-of-vertical-axis-wind-

turbines-gets-closer-look) Renewable Energy World, August 2012, "Offshore Use of Vertical-Axis WindTurbines Gets A Closer Look"

13. ^ [4] (http://www.technologyreview.com/news/513266/will-vertical-turbines-make-more-of-the-wind/)Technology Review, "Will Vertical Turbines Make More of the Wind?"

14. ^ [5] (http://www.c-fec.com/) C-FEC (website)15. ^ [6] (http://www.renewableenergyfocus.com/view/37298/dulas-secures-consent-for-prototype-vertical-

axis-wind-turbine/) Renewable Energy Focus (website), 05 March 201416. ^ [7] (http://www.c-fec.com/turbine/) C-FEC (website), "Turbine"

External links

Cellar Image of the Day (http://cellar.org/iotd.php?t=9878) Shows a VAWT transverse to thewind, yet with the axis horizontal, but such does not allow the machine to be called a HAWT.

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Vertical Axis Wind Turbine - Wikipedia, The Free Encyclopedia