A fresh look at the savonius rotor

Wind turbine experts the world over seem to rate turbines according to one criteria.....POWER COEFFICIENT....

Power coefficient is simply the percentage of the wind's kinetic energy that is harvested. In other words it is the electrical output power of the turbine divided by the total kinetic energy of the wind. (Typically HAWT's can get around 35-40% and Savonius rotors get about 15-25% ....meaning that they generally don't even get on the shortlist!!)

The total kinetic energy of the wind is calculated by multiplying:

• air density (1.208kg/m^3 at sea level)


• 0.5


• wind velocity cubed (in m/s)


• cross sectional area of the air mass being considered (m^2)

Answer will be in Watts (W)

BUT ......there is (in my opinion) more to consider than just the projected area of the wind turbine and based on this I do believe that Savonius Rotors definitely deserve another look!!! More about this later...

So I persuaded a local company (Energy Solutions Africa) that they should look at a Savonius for their hybrid system. What followed was another year of tinkering (a nice word for development on a beer budget!). One of the biggest problems was finding a suitable generator that could work across the wind speed spectrum and that would be efficient and affordable. Initial attempts involved small (cheaper), fast turning generators. These were geared up by various means. In an attempt to minimise losses one such attempt involved a 2m diameter sprocket which we manufactured from a number of laser cut pieces. The most elegant soloution turned out in the end to be an enormous directly driven generator that could cope with the power requirements across the whole speed spectrum purely by virtue of its size. Price was high but with all things considered it was the best solution. ESA is finalising a few prototype machines at present which will after further testing hopefully lead to many installations in Africa.

The bottom line is that this turbine is:

• totally quiet....you could install it in a suburban neighbourhood without your neighbour even hearing it
• It is highly visible to our feathered friends....meaning that eveyone is happy (us greenies included!)
• It accepts wind from any direction (without the need of a tail and/or a swivelling head as on a HAWT)
• It occupies a rectangular piece of sky somewhat larger than the circular piece of sky needed by the equivalent power HAWT .....but hey...the sky is still free!!
• The blade tips are moving at about 1 to 1.2 times the speed of the wind where the HAWT'S samurai swords are slicing through the air at between 5 and 7 times the speed of the wind!!