Savonius wind turbines are a type ofvertical-axis wind turbine (VAWT), used for converting the force of thewind intotorque on a rotatingshaft. 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 inairborne systems.

The Savonius wind turbine was invented by theFinnish engineerSigurd Johannes Savonius in 1922 and patented in 1926.^[1][2]Europeans had earlier experimented with curved blades on vertical wind turbines for many decades. The earliest mention is by the Bishop ofCsanád County,Fausto Veranzio, who was also an engineer. He wrote in his 1616 bookMachinae 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 achieved by Savonius. In his biography, there is mention of his intention to develop a turbine-type rotor similar to theFlettner rotor, but self-rotating. He experimented with his rotor on various small rowing craft on lakes in Finland. No results of his investigations are known, but theMagnus effect is confirmed^{[clarification needed]} by Felix van König (1978).^[3] Two Savonius wind turbine patents were filed in the U.S.: one in 1925^[4] and one in 1928,^[5] by Savonius.

The Savonius turbine is one of the simplest turbines.Aerodynamically, it is adrag-type device, consisting of two or three scoops.^[6] Looking down on the rotor from above, a two-scoop machine might resemble the letter "S" incross section. Because of thecurvature, 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'spower than other similarly-sized lift-type turbines. In practice, much of the swept area of a Savonius rotor may be near the ground if it has a short mount without an extended post, making the overall energy extraction less effective due to the lowerwind speeds found at lower heights. They have several advantages over horizontal axis wind turbines, notably, low noise levels, the ability to operate with low wind speeds and relative independence on the wind direction.^[7]

According toBetz's law, the maximum power that is possible to extract from a theoretical ideal rotor is${\displaystyle P_{\mathrm{m}\mathrm{a}\mathrm{x}}=\frac{16}{27}\frac{1}{2}\rho \cdot h\cdot d\cdot v^3}$, where${\displaystyle \rho }$ is thedensity of air,${\displaystyle h}$ and${\displaystyle d}$ are the height and diameter of the rotor and${\displaystyle v}$ is the wind speed. However, in practice the extractable power is about half that^[8] (one can argue that only one half of the rotor — the scoop co-moving with the wind — works at each instant of time) and depends also on the efficiency of the given rotor. Thus, for the theoretical ideal rotor, one gets${\displaystyle P_{\mathrm{m}\mathrm{a}\mathrm{x}}\approx 0.18\mathrm{k}\mathrm{g}{\mathrm{m}}^{-3}\cdot h\cdot d\cdot v^3}$, but the average maximum efficiency${\displaystyle Cp}$ of the Savonius wind turbine is around 20% (${\displaystyle Cp=0.2}$),^[9] making the real extractable power of the typical Savonius${\displaystyle P_{\mathrm{m}\mathrm{a}\mathrm{x}}\approx 0.12\mathrm{k}\mathrm{g}{\mathrm{m}}^{-3}\cdot h\cdot d\cdot v^3}$.

Theangular frequency of a rotor is given by${\displaystyle \omega =\frac{\lambda \cdot v}{r}}$, where${\displaystyle r}$ is the radius and${\displaystyle \lambda }$ is a dimensionless factor called thetip-speed ratio.λ is a characteristic of each specific windmill, and for a Savonius rotorλ is typically around unity.

For example, an oil-barrel sized Savonius rotor withh=1 m andr=0.5 m under a wind ofv=10 m/s, will generate a maximum power of120 W and a maximum angular speed of20 rad/s (190 revolutions per minute).

Savonius turbines are used whenever cost orreliability is much more important thanefficiency.

Mostanemometers 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 generateelectric power on deep-waterbuoys, which need small amounts of power and get very little maintenance. Design is simplified because, unlike with horizontal axis wind turbines (HAWTs), no pointing mechanism is required to allow for shifting wind direction and the turbine is self-starting. Savonius and other vertical-axis machines are suited to pumping water and other high torque, low rpm applications, and are not usually connected to electric power grids. In the early 1980s, Risto Joutsiniemi developed ahelical rotor (wiki:fi) version that does not require end plates, has a smoother torque profile and is self-starting in the same manner as is a crossed pair of straight rotors.^{[citation needed]}

The most ubiquitous application of the Savonius wind turbine is theFlettner ventilator, which is commonly seen on the roofs of vans and buses^[where?] and is used as a cooling device. This rotor was developed for ventilation by the German aircraft engineerAnton Flettner in the 1920s.^[10] It uses the Savonius wind turbine to drive an extractor fan. The vents are still manufactured in the UK by Flettner Ventilator Limited.^[11]

Specifically constructed Savonius wind turbines have been used to provide power to autonomousneutrino detector stations of theARIANNA experiment on theRoss-Ice Shelf inAntarctica.^[12]

In Europe, small Savonius wind turbines can sometimes be seen used as "animated" advertising signs in which the rotational movement helps to draw attention to the item advertised. They sometimes feature a simple two-frameanimation.^{[citation needed]}

• Airborne wind turbines
• Kite types
• When the Savonius rotor axis is set horizontally and tethered, then kiting^{[clarification needed]} results. There are scores of patents and products that use the net liftMagnus effect that occurs in theautorotation of the Savonius rotor. The spin may be mined for some of its energy for making sound, heat, or electricity.

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  Operation of a Savonius turbine
• 
  A Savonius rotor bladed WECS

Wikimedia Commons has media related toSavonius wind turbines.

• Build your own model Savonius wind turbine
• Windside Savonius Type Wind Turbines for use in Extreme Environments
• Savonius turbines for dynamic environments www.spinpower.org
• How to build a Savonius wind generator

• Notions of Wind Energy for the Complete Idiot

• Vertical axis wind turbines
• Finnish inventions
• 20th-century inventions

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• Wikipedia articles needing clarification from June 2021
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