Application of wave-to-wire modelling to optimise oscillating water column wave energy converters for small installations in the Mediterranean Sea

Sea waves are potentially a relevant renewable resource for their availability and the power conveyed. Oscillating water column (OWC) systems, operating with Wells or impulse turbines, are one of the most suited solutions for the effective conversion of this potential. In these devices, the chamber structure (primary converter) turns sea wave motion into an oscillating airflow driving an air turbine (secondary converter) connected with an electric generator (tertiary converter) through a control strategy. Until now, the analysis of the three converters has been mainly performed individually, while a holistic approach is required. An analytical wave-to-wire model was developed to evaluate the energy conversion process from the sea waves to the electric wire. A rigid piston model was applied to determine the hydrodynamics, thermodynamics, and aerodynamics of the caisson. The aerodynamics and dynamics of either a monoplane isolated Wells turbine and an axial impulse turbine were modelled with two fast and reliable codes based on the blade element momentum theory. A control algorithm was implemented to regulate the instantaneous torque and angular velocity of the rotor and electric generator. The three models were jointly solved to simulate the mutual interaction of the three converters. Scatter matrices were utilised to impose the wave conditions of a selected Mediterranean site located in Tuscany (Italy) for a distinctive caisson, previously optimised with experiments. The operating maps of the system configurations were obtained to vary the geometry and functioning conditions. The maps were used to select the optimal turbines of both the typologies for the specific location. Finally, the operating curves, the performance parameters, and the energy harnessed were achieved, providing an interesting overview of the annual and seasonal functioning. The selected systems with the Wells and impulse turbines convert 14.67 and 14.00 MWh/year and operate with a global efficiency of 5.32% and 5.08%, respectively.