DESIGN OF AN OMNIDIRECTIONAL GYROSCOPIC WAVE ENERGY CONVERTER

Abstract The ISWEC, Inertial Sea Wave Energy Converter, is an offshore, single body,floating wave energy converter. The ISWEC research group so far has designed and developed a device of which the floater consists of a monolithic steel hull with a semi-elliptical side profile, but rectangular from the up view. The actual ISWEC device is directional: for its correct functioning the hull is required to self allign in the wave direction. The objective now is the design of an omnidirectional device capable to absorb energy from wave coming from all the directions. This configuration have been considered optimal for the advantages introduced in terms of the WEC installation (no mooring system ) and release time as well as obtaining, at the same time, an increase in safety. Such a device may be useful to provide energy to small island not connected to the national grid or to isuleted marin system as fish farms. The objective of my thesis has been the design of the an omidirectional ISWEC capable to produce 5-10kW, in particular the dimensioning of the three main linked subsystem; the hull, the gyrocopic group and the Power Take Off(PTO) unit. The hull of the omnidirectional device is composed by a tin cylinder containing the gyroscopic group, the component responsable of the conversion of the mechanical energy into the electric energy, such as the electric generator and the electronic drives and the battery for the energy storage. For this new system the flywheel is mounted horizontally and it is free to rotate with respect its axis. The gyroscopic effect are the result of the combination of the angular speed of the flywheel and the pith motion of the hull.The hull rotation is with respect a reference axis required to be perpendicular to the flywheel direction. For the Inertial effect(Corioli force) a torque is genereted on the precession axis of the PTO rigidly connected to the gyroscope. For improving the hydrodynamic property of the system the cylinder is coated with floaters,which are available commercialy and are assembled in polyethene modules filled with polyurethen foam. My studies have been focused on the upgrade of the mathematical model for better study this new solution. The design involved the dimensioning of the gyroscopic group:its mass and inertia, the PTO selection, the optimization of the hull shape and then of its hydrodynamic property, and the design of the control parameters (flywheel angular speed, the equivalent damping and the equivalent stifness of the PTO shaft). Power losses plays a very important role in this kind of application,for this reason a design tool has been studied for selecting bearings which guarantee their working of 20 years and at the same minimize the power losses. For such preliminary study a linear model has been used for simulating the system on MATLAB. For the computation of the hydrodynamic characteristics of the system NEMOH aproach has been used, It is an Open Souce Boundary element method (BEM) code working on Matlab.