Application of a Passive Control Technique to the ISWEC_Experimental Tests on a 1to8 HIL Test Rig

In this work, we address the use of Hardware In the Loop (HIL) test rig for renewable energy application (which details are described in [4]), focusing on examination of different system configuration. Such test rig is designed to evaluate the performances of the wave harvesting system called ISWEC. The ISWEC is a floating, slack-moored, gyroscopic Wave Energy Converter (WEC). The full-scale prototype installed in Pantelleria in 2015 has an electric-mechanical Power take-off (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. In order to obtain the optimal system configuration, two different control method are under study: a proportional-derivative (PD) law (actually used on the full-scale prototype deployed in Pantelleria) and a passive control method. The PD control law regulates the torque on the electric-mechanical PTO providing a stiffness term to recall the gyroscope in the vertical position and a damping term to extract power. In this configuration, 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 (passive control method) basing on experimental tests on a HIL test rig. The main purpose is to evaluate the system performances focusing on the comparison of the two different control method. The experimental tests both demonstrate the reduction of the PTO torque and justify the gap in the annual system productivity provided by the passive control respect to the PD law as assessed with the numerical model.