Application of a Passive Control Technique to the ISWEC

The ISWEC is a floating, slack-moored, gyroscopic Wave Energy Converter. The system consists of a floating hull carrying a gyroscope inside. The full-scale prototype installed in Pantelleria in 2015 has an electric-mechanical PTO composed by a gearbox and a brushless torque motor. The system is torque controlled to keep the gyroscope in the desired position range and to obtain maximum productivity. At present, the proportional-derivative (PD) law regulating the torque on the PTO includes a stiffness term to recall the gyroscope in the vertical position and a damping term to extract power. The PTO performs the recall effect, resulting in an increase of the root-mean-squared torque load and inducing a flux of reactive power not directly involved in the power extraction. To overcome such problems, this paper analyses the use of an eccentric mass to provide the stiffness term. In order to optimize the power extraction, the recall stiffness and therefore the mass quantity and distance with respect to the precession axis have to be designed. A mass with a variable distance from the precession axis and another one with a fixed position are tested to evaluate if a stiffness term tunable with respect to different sea states carries a relevant gain in system productivity. Numerical simulations with the time domain, non-linear ISWEC model demonstrate a high reduction of the PTO torque levels. Annual productivity and PTO torque in relation to different flywheel speeds are compared for the two different control solutions.