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

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.