Development and analysis of vertical-axis wind turbines - Wind Power Generation and Wind Turbine Design - page 281

Figure 3 : The principal embodiment of the Savonius VAWT patent [6 ].
The Savonius rotor is primarily a drag device with some inherent augmentation
of the rotor performance available due to the air fl ow across each vane and mutual
coupling of the two halves of the rotor. Like all drag machines it has a low operat-
ing tip speed ratio. This makes it less suitable for electricity generation than devices
with higher tip speeds, since a high shaft speed is generally preferred to minimise
the step-up ratio requirement of the gearbox coupling a rotor to a conventional
electrical generator. Several new versions of the Savonius have been manufactured
in recent years including devices with spiral blades of relatively short span mounted
on a wide rotor hub.