Preliminary Design of Variable-Pitch Systems for Darrieus Wind Turbine Using a Genetic Algorithm Based Optimization Procedure

Abstract

In recent years, the research in the field of Vertical Axis Wind Turbines (VAWT) has increased significantly because of the capability of these wind energy systems to operate with omni-directional wind with no yaw mechanism. However, VAWTs are commonly affected by a dynamic stall condition that is typically induced by a steep variation of the angle of attack of the blades during the turbine motion. This circumstance increases the noise and reduces the fatigue life of mechanical components. A consolidated strategy to delay the dynamic stall consists in a dynamic variation of the blade pitch angle. In the Literature, both active and passive variable-pitch systems have been presented to replicate an assigned variable-pitch law. An active variable-pitch system changes the blade pitch angle continuously during the turbine rotation by means of actuators that modify the blade aerodynamic layout, achieving the desired (optimal) angle of attach. A passive variable-pitch system modifies directly the pitch using a combination of aerodynamic forces and inertial loads acting on the blade. However, the set-up of passive variable-pitch mechanisms results quite complex and erratic in many cases.

In the present paper, two preliminary designs of variable-pitch systems, based on an active approach, are proposed. The first system is characterized by the following components: (i) an eccentric point with respect to the main rotational axis, which position varies dynamically during the turbine motion by means of two linear actuators; (ii) an additional linear actuator that induces a variation of the current distance between a specific hinge point on the airfoil chord and the eccentric point. An optimization procedure based on a binary genetic algorithm is performed to determine the optimal stroke variation of actuators during the turbine motion to reply an a priori known, effective variable-pitch law. The other active system is conceived to be mounted externally to the main VAWT structure (ground fixed) to allow the implementation also in small scale systems characterized by a limited airfoil thickness. In this case the blades are hinged at a quarter of the chord to allow a pitch variation; a pin mounted on a rear point along the chord at the bottom end of the blade is left free to slide into a curved (circular) slot cut out from the shield disk. The pin end is then constrained to follow the radial profile of an external loop during the turbine rotation. The conformation of the external loop has been determined using a genetic algorithm to replicate the kinematics of a previously achieved optimal pitch law.