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[[Image:Savonius wind turbine.jpg|thumb|upright|alt=Savonius wind turbine in front of tall building|Savonius wind turbine]]

'''Savonius wind turbines''' are a type of [[vertical-axis wind turbine]] (VAWT), used for converting the force of the [[wind]] into [[torque]] on a rotating [[Axle|shaft]]. The turbine consists of a number of aerofoils, usually—but not always—vertically mounted on a rotating shaft or framework, either ground stationed or tethered in [[Airborne wind turbine| airborne systems]].

== Origin ==
The Savonius wind turbine was invented by the [[Finland|Finnish]] engineer [[Sigurd Johannes Savonius]] in 1922. However, [[Europe|Europeans]] had been experimenting with curved blades on vertical wind turbines for many decades before this. The earliest mention is by the Italian Bishop of Czanad, who was also an engineer. He wrote in his 1616 book ''Machinae novae'' about several vertical axis wind turbines with curved or V-shaped blades. None of his or any other earlier examples reached the state of development made by Savonius. In his Finnish biography there is mention of his intention to develop a turbine-type similar to the Flettner-type, but autorotationary. He experimented with his rotor on small rowing vessels on lakes in his country. There are no results of his particular investigation known, but Magnus-Effect is confirmed by König.<ref>ISBN 3-7906-0077-6</ref>

==Operation==
[[Image:Savonius turbine.svg|thumb|left|upright|alt=Two-scoop Savonius turbine|Schematic drawing of a two-scoop Savonius turbine]]

The Savonius turbine is one of the simplest turbines. [[Aerodynamic]]ally, it is a [[Drag_(physics)|drag]]-type device, 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 [[wind power|power]] than other similarly-sized lift-type turbines. Much of the swept area of a Savonius rotor may be near the ground, if it has a small mount without an extended post, making the overall energy extraction less effective due to the lower wind speeds found at lower heights.
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==Power and rotational speed ==

The maximum power of a Savonius rotor is given by <math>P_{\mathrm{max}} = 0.36\, \mathrm{kg\, m^{-3}} \cdot h \cdot r \cdot v^3</math>, where <math>h</math> and <math>r</math> are the height and radius of the rotor and <math>v</math> is the wind speed.{{Citation needed|date=March 2013}}

The [[angular frequency]] of a rotor is given by <math>\omega = \frac{\lambda \cdot v}{r}</math>, where <math>\lambda</math> is a dimensionless factor called the tip-speed ratio. ''λ'' is characteristic of a specific windmill, and its value is typically between 0.5 and 14.{{Citation needed|date=March 2013}} In a Savonius rotor, ''λ''≈1.

For example, an oil-barrel sized Savonius rotor with ''h''={{val|1|u=m}} and ''r''={{val|0.5|u=m}} under a wind of ''v''={{val|10|u=m/s}}, will generate a maximum power of {{val|160|u=W}} and an angular speed of {{val|20|u=rad/s}} (190 revolutions per minute).

==Use==
[[Image:Taiwan 2009 JinGuaShi Historic Gold Mine Combined Darrieus Savonius Wind Turbines FRD 8638.jpg|thumb|right|150px|Combined Darrieus-Savonius generator in [[Taiwan]]]]

Savonius turbines are used whenever cost or [[Reliability engineering|reliability]] is much more important than [[Energy conversion efficiency|efficiency]].

Most [[anemometer]]s are Savonius turbines for this reason, as efficiency is irrelevant to the application of measuring wind speed. Much larger Savonius turbines have been used to generate [[electric]] power on deep-water [[buoy]]s, which need small amounts of power and get very little maintenance. Design is simplified because, unlike with horizontal axis wind turbines ([[Wind_turbine#Horizontal_axis|HAWT]]s), no pointing mechanism is required to allow for shifting wind direction and the turbine is self-starting. Savonius and other vertical-axis machines are good at pumping water and other high torque, low rpm applications and are not usually connected to electric power grids. They can sometimes have long [[helical]] scoops, to give smooth torque.

The most ubiquitous application of the Savonius wind turbine is the [[Flettner Ventilator]], which is commonly seen on the roofs of vans and buses and is used as a cooling device. The ventilator was developed by the German aircraft engineer [[Anton Flettner]] in the 1920s. It uses the Savonius wind turbine to drive an extractor fan. The vents are still manufactured in the UK by Flettner Ventilator Limited.<ref>[http://www.flettner.co.uk/home.htm Flettner]</ref>

Small Savonius wind turbines are sometimes seen used as advertising signs where the rotation helps to draw attention to the item advertised. They sometimes feature a simple two-frame [[animation]].

==Tethered airborne Savonius turbines==
* [[Airborne wind turbines]]
* [[Kite types]]
* When the Savonius rotor axis is set horizontally and tethered, then kiting results. There are scores of patents and products that use the net lift [[Magnus-effect]] that occurs in the [[Autorotation (helicopter)|autorotation]] of the Savonius rotor. The spin may be mined for some of its energy for making noise, heat, or electricity.

==Gallery==
<gallery>
Image:Savonius-rotor_en.svg|Operation of a Savonius turbine
Image:Savonius_bladed_rotor_WEH.JPG|A Savonius rotor bladed WECS
</gallery>

==References==
{{Reflist}}

==External links==
{{Commons category|Savonius wind turbines}}
* [http://www.re-energy.ca/wind-turbine Build your own model Savonius wind turbine]
* [http://www.windside.com/ Windside Savonius Type Wind Turbines for use in Extreme Environments]
* [http://www.seateach.com/Forgen.asp Forgen low power Savonius Type Wind Turbines for Marine Applications]
* [http://jason.wells.me/blog/?page_id=722 HelixWind Savonius Type Wind Turbine]
* [http://www.spinpower.org Savonius turbines for dynamic environments www.spinpower.org]
* [http://www.amics21.com/laveritat/savonius_generator_english.pdf How to build a Savonius wind generator]
{{Wind power}}
* [http://www.amics21.com/laveritat/notions_of_wind_energy_for_the_complete_idiot.pdf Notions of Wind Energy for the Complete Idiot]

[[Category:Vertical axis wind turbines]]
[[Category:Finnish inventions]]

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