Savonius Rotor Properties Relevant to SavRot Applications
The relationships between rotation rate, torque and efficiency of the Savonius rotor are very well known. Numerous tests were done during patent development work by Savonius in the mid-1920s. His initial focus was on rotor applications as a replacement for the American windmill and sails on ships. He also used it in the surf zone to drive a pump that delivered seawater to the aquarium at Monaco. Among his many publications is a paper delivered in 1931 at a meeting of mechanical engineers in the United States.
The Savonius rotor is simple in form and easy to construct. The basic configuration consists of a pair of half-cylinder blades offset so as to provide an S-shaped channel for flow transverse to the axis. The ends of the rotor are capped with circular flat plates of slightly larger diameter than the “wing spread” of the rotor. The axis of rotation is through the center of the end plates.
The physical simplicity of the rotor does not extend to the dynamical properties governing its rotation in a moving fluid. During the last nine decades there have been many studies and experiments to examine fundamental properties and potential applications. Under the SAVROT REFERENCES TAB is a somewhat random selection of publications that illustrate the nature of the problems posed by the rotor. Included is a paper by the principal inventor of SavRot that was done as a part of the development of a technique for measuring water movement in the deep ocean.
The Savonius rotor, under optimum operating conditions for energy conversion, has an efficiency of at least 30%. The produced power is proportional to orthogonal flow across the projected area, i.e., the wing spread times the length of the rotor. The equivalent projected area of horizontal axis wind turbines is the circular swept area of the three blades. The efficiency curves for the Savonius rotor and the American windmill were erroneously swapped in an early publication that gave the rotor an unearned reputation for 20%.
Virtually all of the published tests of Savonius rotors have been performed on vertical axis two-tier rotors with a ratio of length to wing spread in the range of 1.5 to 2.5. The SavRot application for ocean wave energy conversion has been centered on horizontal axis single tier rotors with a larger ratio of length to wing spread.
Torque of the Savonius rotor is very high at low fluid flow speeds. Therefore, the rotor does not stall at slow current speeds. This characteristic is in sharp contrast to wind systems. The static torque variation for each of two tiers (“steps”) of “conventional” Savonius rotors has a small rotational interval that is slightly negative. Altering the circular configuration of the blades generally increases the rotational angle and intensity of the negative torque. Arranging the rotor in two tiers produces a resultant torque which is positive at all angles of rotation. The SavRot concept for operation in oscillatory flow of ocean surface waves circumvents the impact of the momentary torque reversal.