The impact of hydrodynamic coupling on the performance of multi-mode wave energy converters

Abstract

Recently, research interest has deepened in developing technologies that are capable of generating electricity from renewable sources. Ocean wave energy is one such source, and is gaining attention due to its high energy density and favourable variability properties compared to other sources such as solar and wind. Although many Wave Energy Converter (WEC) prototypes have been proposed over the years, there is still no convergence on the best design. An emerging subset of WEC designs are ‘multi-mode converters’, which are capable of absorbing power from multiple hydrodynamic modes. This allows them to generate more energy from incoming waves compared to most other WECs, which typically use only one Degree-of-Freedom (DOF) for power absorption. However, one of the key challenges in the design and control of multi-mode WECs is the strong coupling between hydrodynamic modes, which can potentially lead to sub-optimal performance. The effect of this coupling on the device performance may also be further exacerbated when nonlinear hydrodynamic effects are considered. This thesis is dedicated to building an understanding of the impact of nonlinear coupling between hydrodynamic modes on the power absorption efficacy of a submerged, multi-mode, point absorber WEC with a flat cylindrical geometry. From this, the project also intends to provide general recommendations regarding the control and design of multi-mode WECs for increased performance. Three specific research questions were investigated: (i) what is the effect of nonlinear hydrodynamic coupling forces, caused by the change in projected surface area with large pitch motions, on the performance of multi-mode WECs, (ii) how should the surge, heave and pitch hydrodynamic modes be tuned to enhance the performance of WECs subjected to nonlinear coupling forces and (iii) what design parameters can be implemented to passively tune the hydrodynamic modes in a nonlinear, under-actuated WEC device. To address these questions, various numerical models were developed and compared, ranging from low fidelity models in the frequency-domain based on linear hydrodynamic models, to a weakly nonlinear hydrodynamic code based on the weak-scatterer approximation. Initially, it was necessary to gain a fundamental understanding of the nonlinear hydrodynamic forces acting on a device forced to undergo large pitch motions and oscillate in multiple hydrodynamic modes simultaneously. To this end, initial investigations assumed a simple WEC system with fully idealised kinematic control, wherein the pitch and surge motions could be explicitly defined. It was found that simultaneous surge and pitch motions changed the radiation forces acting on the WEC, resulting in significant reductions to the maximum power that could be absorbed by the device. Different approaches for adjusting the dynamics and resonance behaviour of the multi-mode WEC through tuning of the hydrodynamic modes were then investigated. Under the effects of nonlinear coupling between hydrodynamic modes, tuning the surge, heave and pitch modes to the same natural frequency was demonstrated to result in significant reductions in power absorbed, especially when the pitch amplitude was high. Recommendations were therefore made to decouple these modes when developing multi-mode WECs in the case where the design does not limit large pitch amplitudes. From the models investigated, this tuning approach also demonstrated a potential for improving the broadband power absorption efficacy of the device in irregular waves. In the final stage of this project, the impact of nonlinear coupling in an under-actuated system was investigated. A sensitivity study was conducted to investigate the effect of adjusting the geometric design of a three-tethered WEC on the resonance behaviour of each hydrodynamic mode. It was concluded that for maximum power absorption, two out of three of the device’s planar rigid body modes should be utilised to harvest energy from incident waves. Furthermore, for this WEC geometry and design, these rigid body modes should contain predominantly surge and heave motions. Subharmonic excitations caused by nonlinear forces arising from the tether arrangement and hydrodynamic interactions were also found to significantly reduce the performance of the device compared to the predictions from linear theory. It was determined that the power absorbed by the device was most sensitive to the arrangement of the tethers, while adjusting parameters related to the mass distribution resulted in little benefit to the overall device performance.