jmex, welcome to the forum.
jmex said:
Why it should be restricted for pumping purpose? I have gone through some papers and they say savonius is best for pumping and darrius for power generation.
Because that is the polite way of putting it. Pumping a small amount of water up a few metres to maintain a header tank can be done by a Savonius rotor when a convenient wind blows. That is all the work a Savonius rotor can really be expected to do.
The Savonius rotor is a “mind virus” that infects unwary beginners to windmill technology. Many never recover.
It is so elegant and conceptually simple that, if it worked efficiently, you would expect to see them everywhere, which you don't. Experimenters, ignoring engineering wisdom, have naturally selected the Savonius in the same way that unproductive weeds with bright flowers in a field, are grown and bred for decorative purposes in a flower garden.
Books and articles that focus on the Savonius rotor are not written by engineers. They are written by the infected victims.
A Savonius rotor cannot be automatically feathered or turned into the wind during a wind storm. It must be unloaded to spin at a very high speed so as to reduce the load on the tower. But any unbalance at storm speeds shakes the tower apart, then the entire system is lost. The rotational energy has to go somewhere so everything gets bent. They are not worth rebuilding, which is why you do not see them everywhere.
The relative inefficiency of the Savonius rotor requires a stand or tower that is twice the weight and so, for the same power extraction, costs twice as much as other turbine types. The oil drum may be cheap, but the mounting is very expensive.
A Savonius will not always self start, especially if there is an initial load and a gently increasing wind. It will rotate to the position of zero power and sit there. To start reliably, it is necessary to mount at least two Savonius rotors at 90°, one above the other on the same shaft.
Apart from pumping small amounts of water the Savonius can be used to start a more efficient turbine such as the Darrieus rotor. In that application the elegance of the Savonius is actually being used to market the more efficient Darrieus turbine.
Probably the best use for the Savonius rotor is to drive many small Tibetan Buddhist prayer wheels.