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 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. Europeans 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, 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 achieved by Savonius. In his biography, there is mention of his intention to develop a turbine-type rotor similar to the 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 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. Aerodynamically, 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, 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 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.
Power and rotational speed According to Betz's law, the maximum power that is possible to extract from a theoretical ideal rotor is P m a x = 16 27 1 2 ρ ⋅ h ⋅ d ⋅ v 3 {\displaystyle P_{\mathrm {max} }={\frac {16}{27}}{\frac {1}{2}}\rho \cdot h\cdot d\cdot v^{3}} , where ρ {\displaystyle \rho } is the density of air, h {\displaystyle h} and d {\displaystyle d} are the height and diameter of the rotor and v {\displaystyle 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 m a x ≈ 0.18 k g m − 3 ⋅ h ⋅ d ⋅ v 3 {\displaystyle P_{\mathrm {max} }\approx 0.18\,\mathrm {kg\,m^{-3}} \cdot h\cdot d\cdot v^{3}} , but the average maximum efficiency C p {\displaystyle Cp} of the Savonius wind turbine is around 20% ( C p = 0.2 {\displaystyle Cp=0.2} ), making the real extractable power of the typical Savonius P m a x ≈ 0.12 k g m − 3 ⋅ h ⋅ d ⋅ v 3 {\displaystyle P_{\mathrm {max} }\approx 0.12\,\mathrm {kg\,m^{-3}} \cdot h\cdot d\cdot v^{3}} . The angular frequency of a rotor is given by ω = λ ⋅ v r {\displaystyle \omega ={\frac {\lambda \cdot v}{r}}} , where r {\displaystyle r} is the radius and λ {\displaystyle \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=1 m and r=0.5 m under a wind of v=10 m/s, will generate a maximum power of 120 W and a maximum angular speed of 20 rad/s (190 revolutions per minute).
Use
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