Analysis and optimisation of the turbine-chamber coupling in an OWC wave energy converter by means of numerical modelling and laboratory tests cum particle imaging velocimetry

Zusammenfassung

In an oscillating water column (OWC) wave energy converter the damping that the turbine exerts on the movements of the water column is one of the main factors, if not the main, affecting the power output. The objective of this thesis is to investigate, and eventually to optimise, the turbine-chamber coupling in an OWC wave energy converter--in particular in the OWC projected to be installed in the breakwater at the port of A Guarda--with the final aim of maximising the capture factor of the device, i.e., the ratio of the pneumatic power absorbed by the chamber to the incident wave power. A methodology based on a combination of physical and numerical modelling is proposed. First, a comprehensive experimental campaign is carried out to determine how different parameters affect the performance of the OWC converter, among which the incident wave conditions, the tidal level and the turbine-induced damping. Second, the hydrodynamic efficiency of the converter is investigated by means of a novel approach. Particle imaging velocimetry (PIV) is used to evaluate the characteristics of the flow--including the velocity and vorticity fields, and the kinetic and turbulent kinetic energy--by means of a phase-averaging procedure. Finally, a 2D numerical model based on the Reynolds-averaged Navier-Stokes equations with the volume-of-fluid free surface tracking algorithm (RANS-VOF) is implemented and validated in order to determine the optimum turbine-chamber coupling for a given OWC. The results of the physical model tests show that the damping exerted by the turbine is the factor that most affects the chamber efficiency--even more than the wave conditions and the tidal level. The PIV measurements corroborate this finding: the turbine-induced damping affects the hydrodynamic behaviour of the chamber and thereby determines the amount of energy that the OWC is able to capture. Finally, the numerical model, after the validation process, allows the definition of the damping condition which maximizes the performance of the OWC, determining the characteristics that a turbine must meet to achieve the optimum coupling with the chamber. In conclusion, this thesis shows that a proper selection of the turbine-induced damping is the cornerstone of the performance of an OWC wave energy converter, which reflects the critical importance of the turbine-chamber coupling in OWC systems.