Savonius wind turbines are a type of vertical-axis wind turbine (VAWT), used for converting the force of the

wind Wind is the natural movement of atmosphere of Earth, air or other gases relative to a planetary surface, planet's surface. Winds occur on a range of scales, from thunderstorm flows lasting tens of minutes, to local breezes generated by heatin ...

into

torque In physics and mechanics, torque is the rotational analogue of linear force. It is also referred to as the moment of force (also abbreviated to moment). The symbol for torque is typically \boldsymbol\tau, the lowercase Greek letter ''tau''. Wh ...

on a rotating 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 systems.

Origin

The Savonius wind turbine was invented by the Finnish engineer Sigurd Johannes Savonius in 1922 and patented in 1926.

Europe Europe is a continent located entirely in the Northern Hemisphere and mostly in the Eastern Hemisphere. It is bordered by the Arctic Ocean to the north, the Atlantic Ocean to the west, the Mediterranean Sea to the south, and Asia to the east ...

ans had earlier experimented with curved blades on vertical wind turbines for many decades. The earliest mention is by the Bishop of Csanád County,

Fausto Veranzio Fausto Veranzio (; ; Hungarian language, Hungarian and Latin regional pronunciation, Vernacular Latin: ''Verancsics Faustus'';Andrew L. SimonMade in Hungary: Hungarian contributions to universal culture/ref>Flettner 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 the

Magnus effect The Magnus effect is a phenomenon that occurs when a spin (geometry), spinning Object (physics), object is moving through a fluid. A lift (force), lift force acts on the spinning object and its path may be deflected in a manner not present when ...

is confirmed by Felix van König (1978). Two Savonius wind turbine patents were filed in the U.S.: one in 1925 and one in 1928, by Savonius.

Operation

The Savonius turbine is one of the simplest turbines.

Aerodynamic Aerodynamics () is the study of the motion of atmosphere of Earth, air, particularly when affected by a solid object, such as an airplane wing. It involves topics covered in the field of fluid dynamics and its subfield of gas dynamics, and is an ...

ally, it is a drag-type device, consisting of two or three scoops. Looking down on the rotor from above, a two-scoop machine might resemble the letter "S" in cross section. Because of the

curvature In mathematics, curvature is any of several strongly related concepts in geometry that intuitively measure the amount by which a curve deviates from being a straight line or by which a surface deviates from being a plane. If a curve or su ...

, 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. 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 lower

wind speed In meteorology, wind speed, or wind flow speed, is a fundamental atmospheric quantity caused by air moving from high to low pressure, usually due to changes in temperature. Wind speed is now commonly measured with an anemometer. Wind spe ...

s 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.

Power and rotational speed

According to

Betz's law In aerodynamics, Betz's law indicates the maximum Power (physics), power that can be extracted from the wind, independent of the design of a wind turbine in open flow. It was published in 1919 by the German physicist Albert Betz.Betz, A. (1966) ' ...

, the maximum power that is possible to extract from a theoretical ideal rotor is

P_ = \frac \frac \rho \cdot h \cdot d \cdot v^3

, where

\rho

is the density of air,

h

and

d

are the height and diameter of the rotor and

v

is the wind speed. However, in practice the extractable power is about half that (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

P_ \approx 0.18\, \mathrm \cdot h \cdot d \cdot v^3

, but the average maximum efficiency

Cp

of the Savonius wind turbine is around 20% (

Cp = 0.2

), making the real extractable power of the typical Savonius

P_ \approx 0.12\, \mathrm \cdot h \cdot d \cdot v^3

. The

angular frequency In physics, angular frequency (symbol ''ω''), also called angular speed and angular rate, is a scalar measure of the angle rate (the angle per unit time) or the temporal rate of change of the phase argument of a sinusoidal waveform or sine ...

of a rotor is given by

\omega = \frac

, where

r

is the radius and

\lambda

is a dimensionless factor called the tip-speed ratio. ''λ'' is a characteristic of each specific windmill, and for a Savonius rotor ''λ'' is typically around unity. For example, an oil-barrel sized Savonius rotor with ''h''= and ''r''= under a wind of ''v''=, will generate a maximum power of and a maximum angular speed of (190 revolutions per minute).

Use

Savonius turbines are used whenever cost or reliability is much more important than

efficiency Efficiency is the often measurable ability to avoid making mistakes or wasting materials, energy, efforts, money, and time while performing a task. In a more general sense, it is the ability to do things well, successfully, and without waste. ...

. Most anemometers 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 Electricity is the set of physical phenomena associated with the presence and motion of matter possessing an electric charge. Electricity is related to magnetism, both being part of the phenomenon of electromagnetism, as described by Maxwel ...

power on deep-water

buoy A buoy (; ) is a buoyancy, floating device that can have many purposes. It can be anchored (stationary) or allowed to drift with ocean currents. History The ultimate origin of buoys is unknown, but by 1295 a seaman's manual referred to navig ...

s, 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 a 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. 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. This rotor was developed for ventilation by the German aircraft engineer

Anton Flettner Anton Flettner (1 November 1885 – 29 December 1961) was a German Aerospace engineering, aviation engineer and inventor. Born in Hattersheim am Main#Eddersheim, Eddersheim (today a district of Hattersheim am Main), Flettner made important contri ...

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. Specifically constructed Savonius wind turbines have been used to provide power to autonomous

neutrino A neutrino ( ; denoted by the Greek letter ) is an elementary particle that interacts via the weak interaction and gravity. The neutrino is so named because it is electrically neutral and because its rest mass is so small ('' -ino'') that i ...

detector stations of the ARIANNA experiment on the Ross-Ice Shelf in

Antarctica Antarctica () is Earth's southernmost and least-populated continent. Situated almost entirely south of the Antarctic Circle and surrounded by the Southern Ocean (also known as the Antarctic Ocean), it contains the geographic South Pole. ...

. 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-frame

animation Animation is a filmmaking technique whereby still images are manipulated to create moving images. In traditional animation, images are drawn or painted by hand on transparent celluloid sheets to be photographed and exhibited on film. Animati ...

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

that occurs in the autorotation of the Savonius rotor. The spin may be mined for some of its energy for making sound, heat, or electricity.

Gallery

Image:Savonius-rotor_en.svg, Operation of a Savonius turbine Image:Savonius_bladed_rotor_WEH.JPG, A Savonius rotor bladed WECS

References

External links

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
{{Wind power
Notions of Wind Energy for the Complete Idiot
Vertical axis wind turbines Finnish inventions 20th-century inventions