The Savonius is a simple turbine design, which was coined in its modern form by Finnish engineer Sigurd Johannes Savonius in 1922. It is commonly used for applications that demand high reliability rather than high efficiency (e.g. ventilation, anemometers, domestic microgeneration).
Savonius turbines are much less efficient than HAWT and the Darrieus VAWT (around 15%, see below Calculating wind power), but contrary to the former, they perform well in turbulent wind conditions and, contrary to the latter, they are self starting. Structurally they are resilient, resist strong winds well without being damaged and are quiet compared to other types.
As opposed to the Darrieus which uses lift, the Savonius is a ‘drag’ device. It consists of 2-3 “scoops”: curved structures that experience less drag when moving against the wind than when moving with the wind because of the scoops’ curved shape. From an aerodynamic perspective it is this differential drag that causes the Savonius turbine to spin