A passive control strategy applied to the iswec device: numerical modelling and experimental tests

The ISWEC is a floating, slack-moored, wave energy converter, absorbing energy through an electric-mechanical power take-off (PTO), moved by a gyroscopic, activated by the pitching motion of the floater. The system is torque-controlled in order to keep the gyroscope in the desired position range and to maximise productivity. At present, a proportional-derivative (PD) control law regulates the torque on the PTO, comprising a linear stiffness term to recall the gyroscope in the vertical position and a linear damping term to extract power. However, the recall task demands high torques to the PTO, inducing an undesirable flux of reactive power. This paper discusses a technological innovation to address such issues, consisting of an additional eccentric mass to provide the restoring action instead of the stiffness term. The mass and distance from the precession axis should to be optimally designed. Two configurations are considered, one with fixed distance, and one with distance tuneable according to the incoming sea state. Time-domain nonlinear numerical simulations inform the optimization and design of the eccentric mass. Simulations demonstrate the effectiveness of the strategy, inducing a high reduction of the PTO torque levels while maintaining similar power conversion efficiencies. Similar results are obtained via experimental tests performed on a Hardware-in-the-loop (HIL) test bench.