Design of a Floating-moored OWC Wave Energy Converter Equipped with a Cross-flow Air Turbine

Abstract

Extensive investigation on ocean wave energy converters (WECs) have been carried out to improve competitiveness of wave energy. The Oscillating Water Column (OWC) is considered as one of the most promising ones among various wave energy converters due to its mechanical and structural design simplicity. Numerous large-scale OWC WECs have been developed but most of them are not commercialized yet owing to its high costs in construction, installation, operation and maintenance. Thus, the small-sized floating-moored OWC WEC, a cylinder-type, has been proposed and investigated herein. The hydrodynamic behavior and interactions between oceans waves and OWC WEC should be well-understood for improving the device performance. The hydrodynamic performance of the 3D floating OWC WEC was studied under regular waves of different heights and periods with nonlinear power-take-off (PTO) damping condition by an orifice plate. An incompressible Computational Fluid Dynamics (CFD) simulation of the OWC based on the RANS equations and VOF surface capturing scheme was conducted through numerical wave tank (NWT) tests to obtain the optimized geometric design of the floating OWC chamber. The cylinder-type OWC chamber having GM value of 0.17 m from free surface and buoyancy ratio of 1.57 was determined.xiii The taut-type mooring system was adopted for the floating OWC device. The mooring system was designed to restrain the motions of the floating OWC by 8 mooring lines having high stiffness for improvement in wave energy conversion. Nonlinear motion and mooringline response of the floating-moored OWC model were numerically analyzed in regular waves using ANSYS AQWA and Orcaflex. In addition, the hydrodynamic performance of the floating-moored OWC with an orifice on top of it was investigated by CFD simulations. A cross-flow air turbine, which is an air-driven and self-rectifying turbine, was proposed for the power take-off (PTO) system of the OWC wave energy converter. The complicated non-linear behavior of airflows through the turbine under variety flow conditions was predicted by numerical and experimental investigations. The geometries of a nozzle and rotor of the air turbine were optimized first under constant flow conditions, and it was found that the nozzle geometry is the most effective factor for higher turbine performance among the selected design parameters. In addition, the performance of the optimized model in reciprocating airflows was numerically tested, and the higher efficiency with more improved operating range was observed. The numerical model was then validated through the experimental results with an averaged difference of 3.5%. The performance analysis of the floating-moored OWC equipped with the cross-flow air turbine was conducted through the wave tank test. Dynamic response and energy conversion performance of a 1:4 scale OWC model were investigated together in regular waves. The maximum hydrodynamic performance of the OWC device was observed nearby the peak heave RAO due to the moonpool effect, and the peak turbine efficiency was obtained at the highest wave height and lowest wave period.