The multi-chamber Oscillating Water Column (OWC) device has recently become more attractive due to its potential high efficiency. In this paper, the hydrodynamic performance of a single-, dual- and triple-chamber OWC-breakwater are investigated experimentally. In the first instance, quantitative comparisons are implemented to understand the hydrodynamic performance of multi-chamber OWC-breakwaters. Specific attention has been dedicated to the hydrodynamic performance of capture width ratio (CWR), reflection coefficient, transmission coefficient, dissipation coefficient and effective frequency bandwidth. The investigation identified various findings that can be summarized as follows: i) hydrodynamic interactions between chambers in the multi-chamber OWC device has improved wave power extraction characteristics; ii) comparing with the conventional pontoon breakwater, the multi-chamber OWC-breakwater showed better wave attenuation performance in longer waves; iii) wave steepness is important for evaluating the performance of the multiple-chamber OWC-breakwater device; and iv) the implementation of the multi-chamber scheme broadens the effective frequency bandwidth (satisfied the condition of KT   0.2) of OWC-breakwater.

Experimental investigation on the hydrodynamic performance of a multi-chamber OWC-breakwater

Semi-analytical study on the hydrodynamic performance of an interconnected floating breakwater-WEC system in presence of the seawall

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.

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

In the present study, the hydrodynamic efficiency of a cylindrical wave energy converter (WEC) of vertical symmetry axis and arranged in front of a reflecting orthogonal breakwater is explored. The idea is based on exploiting the anticipated amplification of the scattered and the reflected wave fields originating from the presence of the vertical walls, towards increasing the WEC’s wave power absorption due to the walls’ wave reflections. Two types of converters are examined, namely the heaving device and the oscillating water column (OWC) device, assuming linear potential theory. The associated diffraction-, motion-, and pressure-radiation problems are solved using axisymmetric eigenfunction expansions for the velocity potential around the WECs by properly accounting for the wave field’s modification due to the walls’ presence. To this end, a theoretical formulation dealing with the evaluation of the converter’s performance is presented accounting for the coupling between the WEC and the reflecting vertical walls. The results depict that the amount of the harvested wave power by the WEC in front of an orthogonal wall is amplified compared to the absorbed wave power by the same WEC in the open sea.

https://www.mdpi.com/2077-1312/9/1/94/pdf

Hydrodynamic Efficiency of a Wave Energy Converter in Front of an Orthogonal Breakwater

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

Analytical investigation on the hydrodynamic performance of a multi-pontoon breakwater-WEC system

The invention belongs to the field of wave energy generation, and particularly relates to an oscillating water column type wave energy device with the large wave energy capture frequency width. Aimingat overcoming the defects of methods in the prior art, based on an existing technology, by adjusting the position of an intermediate cross wall in real time, the inherent frequency of a water body inan air chamber is changed, the wave energy capture effective width is increased, the wave energy extraction efficiency maintains the optimal state, and the generation efficiency of the oscillating water column type wave energy device is indirectly improved.

Xue Rong

Papers

Analytical investigation on the hydrodynamic performance of a multi-pontoon breakwater-WEC system

Oscillating water column type wave energy device with large wave energy capture frequency width