Theoretical analysis of a cyclic pitch turbine

Theoretical analysis of a drag based vertical axis wind turbine is presented. This builds on previous work where we introduced a novel turbine (Cyclic pitch turbine, CPT) with a tilting mechanism. It allows for cyclic pitch variations of the blades while they rotate about an axis perpendicular to the fluid flow direction. The pitching mechanism changes the angle of attack of the blades from 90 degrees while in the drive stroke to 0 degrees while in the recovery stroke and then back to 90 degrees so that the blades are active during the drive stroke and passive during the recovery stroke. The tilting of the blades takes place while the blades are transitioning from one stroke to the other stroke. In this configuration, the blades present maximum frontal area during the drive stroke and minimum frontal area during the recovery stroke. The drag force on a simple shaped bodies is directly proportional to the effective frontal area with respect to the flow direction of the fluid and hence it is expected that the drag force is maximum during the drive stroke while the blades are perpendicular to the flow and that it is minimum during the recovery stroke as the blades are parallel to the fluid flow direction. Comparison of static torque coefficients of a flat plate to that of a bucket blade typical in a Savonius turbine is performed. It is shown that this new turbine has higher and more consistent starting torque. It is also shown that the positive difference in the torque coefficient would only increase as the turbine starts to rotate and increase in rpm. As a consequence, the relative velocity between the blades and the fluid flow negatively affects the Savonius configuration more than a CPT turbine. As a result, this turbine is expected to be more efficient than the Savonius turbine. Optimization has been performed for this formulation for tip speed ratio (TSR) and active blade angle. To perform the tilting motion of the blades, the turbine uses a novel hybrid mechanism combining the workings of cams and swashplates. A prototype has been developed which integrates ball bearings into the mechanism instead of sliding components used in our first generation turbine that change sliding motion into lower friction rolling motion. This increased the efficiency of the turbine. The mathematical formulations developed compares the static characteristics of the developed CPT turbine to Savonius turbines and help understand its dynamic characteristics.