Integrating multi-type Wave Energy Converters (WECs) enables wave energy to be extracted from multiple harvesting manners simultaneously, which presents a more competitive energy conversion technology. In this study, an Oscillating Water Column (OWC) and an Oscillating Buoy (OB) are combined to devise a hybrid WEC system that serves as a floating breakwater. The two devices are aligned in line with the incident wave direction, where the OB is deployed upstream from the OWC. A series of physical experiments are conducted to understand the working principle of the hybrid WEC system. After a fully non-linear time-domain model is validated by the measured data, the comparisons of the hybrid system and its respective isolated devicesare emphasised. Then, the hydrodynamic dependence of the hybrid system is comprehensively tested. The results indicated that the hybrid system design provides higher energy conversion as well as better wave attenuation performance compared to the isolated OWC and OB devices. Additionally, the OB outperforms the OWC in terms of wave energy conversion. The OWC-OB gap resonance has a negative impact on wave energy extraction but has a negligible effect on wave attenuation. The OWC with a deeper draft and symmetric wall performs better both in terms of wave energy extraction and wave attenuation in long-period waves. The overall conversion efficiency decreases with the increase of the OB draft, but the transmission coefficient follows an opposite trend. The PTO dampings of the OB and the OWC dominate the overall conversion efficiency and the transmission coefficient, respectively. • Hydrodynamics of a OWC-OB WEC-breakwater system is investigated experimentally. • A fully nonlinear numerical model is developed to cross-check the measured data. • Higher energy conversion and wave attenuation are realized by the hybrid system. • Wave resonance in the gap leads to the sudden reduction of energy conversion. • Two PTO systems play different roles in the hydrodynamic performance. 

Experimental and numerical analysis of a hybrid WEC-breakwater system combining an oscillating water column and an oscillating buoy

A theoretical model based on the linear potential theory is presented for two heaving oscillating water column (OWC) devices separated by a gap. The model includes relative motion and phase control between the devices and trapped water columns, and the hydrodynamic performance of the dual-OWC system thence evaluated. Matching conditions are employed along the common interfaces, and the power take-off model and motion equations of the OWC devices are incorporated into the solution procedure. At the top of each chamber, a Wells turbine is installed to extract wave power. To achieve the optimal overall power extraction performance, a numerical strategy of successive approximation is utilized to seek the optimal turbine damping combinations for the separated units. The effects of lip-wall draft and chamber breadth on the performance of a fully-free heaving dual-OWC system are explored. In view of the deficiency of a fully-free heaving system, two alternative optimization strategies are proposed, one focusing on the control of relative motion and phase between the water columns and the heaving devices, the other on utilizing resonance phenomenon inside the gap, achieved by tuning imposed linear spring constants and gap distance, respectively. It is shown that the control between heave motion of devices and water columns inside the chambers is beneficial for extracting more power over a broader range of wave frequencies. Moreover, enhanced extraction is likely over a wider range of wave conditions when the gap distance to wavelength triggers a sloshing mode inside the gap.

A heaving system with two separated oscillating water column units for wave energy conversion

Semi-analytical study on an integrated-system with separated heaving OWC and breakwater: Structure size optimization and gap resonance utilization

An immersed finite element material point (IFEMP) method for free surface fluid–structure interaction problems

Nonlinear wave loads on an offshore oscillating-water-column wave energy converter array

The hydrodynamic performance of a floating cylindrical oscillating water column (OWC) wave energy converter is investigated experimentally and numerically. The physical experiment is carried out in a wave flume at Dalian University of Technology. The floating cylindrical OWC device is constrained by springs and only moves vertically. A second-order time-domain Higher-Order Boundary Element Method, based on the perturbation expansion technique, is used to simulate the nonlinear wave interaction with the floating OWC device. The nonlinear terms concerning the pneumatic and viscous damping are introduced to the free surface boundary conditions inside the OWC chamber. The chamber surface elevation and air pressure, the hydrodynamic efficiency, and the vertical displacement of the OWC device are examined in detail. Good agreements are obtained between experimental data and numerical results. Then, the effects of opening ratio, wave steepness, mooring stiffness and chamber draft on the hydrodynamic performance are then investigated. It is found that the optimal opening ratio is between 0.02 and 0.03. The mooring stiffness plays an important role on the hydrodynamic response of the OWC device. The hydrodynamic efficiency and effective frequency bandwidth increase with the mooring stiffness.

Shuqun Cai

Papers

Nonlinear hydrodynamic analysis of an offshore oscillating water column wave energy converter