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2.2.3.1. Darrieus Wind Turbine
Darrieus rotor, named after its inventor Georges Jeans Darrieus, works due to the lift force generated from a set of airfoils Fig. 2.3. The original design blades are shaped like
egg beaters or troposkein turning rope and are under pure tension while in operation. This typical blade helps to reduce bending stress experienced by the blades. There are several
variations in the Darrieus design of which some are with straight vertical blades, usually called Giromills. Darrieus rotor usually works at high tip speed ratio which makes it
attractive for wind electric generators. However, they are not self-starting and require external „excitation‟ to cut-in. Moreover, the rotor produces peak torque only twice per
revolution.
2.2.3.2. Savonius Wind Turbine
The Savonius wind turbine, invented by S.J. Savonius, is a vertical axis machine consisting of two half cylindrical or elliptical blad
es arranged in „S‟ shape Fig.2.4. Convex of one side and the concave side of the other are facing the wind at a time as shown
in Fig.2.5. The basic driving force of Savonius rotor is drag. The drag coefficient of a concave surface is more than the convex surface. Hence, the concave half cylinder that facing
the wind will experience more drag force than the convex one, thus force the rotor to rotate. Sometimes two or more rotors fixed one over the other at 90° offset may be used to reduce
the torque fluctuations during rotation. Another way to get its efficiency is adding deflector augmenters with the rotor. The augmenter shades the convex half facing the wind and directs
the flow to the concave half thus enhancing the performance.
Figure 2.4 Savonious Wind Turbine Models perpustakaan.uns.ac.id
commit to user
11 Being drag machines, Savonius rotors have lower power coefficient. However, some
experimental rotors have shown power coefficient up to 35 percent. These rotors have high solidity and thus high starting torque. They work at low tip speed ratios, with the maximum
of about one. They have simple construction and it even can be made from oil barrels cut in two halves lengthwise. Hence they are preferred for high torque-low speed applications like
water pumping.
Figure 2.5 Principle of Savonius RotorSathyajith 2006
2.2.3.3. Advantages of Savonius Wind Turbine
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