Explain the principles of operation of the wind turbine Savonius and Darrieus?

A savonius is a type of vertical axis wind turbine and it is a drag-type V A w T   which operates in the same way as  a cup anemometer.With  a savonius wind turbine it does not  matter from which direction the wind is blowing,since there will always be more force exerted on whichever cup has its open face into the wind and this will push the rotor around, so this type is ideal for areas with very troubled t winds.

However it is15:% of the wind energy hitting the rotor is turned into rotational mechanical energy and this is much less than could achieve with a Darrieus  wind turbine.Savonius turns slowly but generate high torque,therefore they are not ideal for electricity generation.

Savonius wind turbine is best  for pumping water and grinding grain.

 A Darrieus wind can spin at many times the speed of the wind hitting.

Hence adarrieus wind turbine generates less torque than a  savonius but it rotates much faster, so this makes Darrieus wind much better suited to electricity generation rather than water pumping and similar activities.

Darrieus wind turbines are not self starting, therefore a small powered motor is required to start off the rotation and then when it has enough speed the  wind passing across the aerofoils starts  to generate and  the rotor is driven around by the wind.