Laboratory scale modelling of a point absorber wave energy converter

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Flannery, Brian
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University College Cork
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Finding an effective Power Take Off (PTO) system, that can deal with the low power levels (< 1W), associated with laboratory scale testing of Wave Energy Converts (WEC) is a notoriously difficult problem. This thesis investigates the use a magnetic particle brake to address the issue. The work describes 1:50 scale tests of a generic point absorber device, restricted to heave motions only. The model set-up is not intended to replicate any existing WEC device but rather to provide a good case study for examining performance characteristics such as exciting forces, response amplitudes and power absorption for both regular and irregular wave conditions. The PTO was found to be an effective low cost solution for the scale of operations considered. The brake adequately met the requirements of supplying resistive damping at the typically low speeds and could supply enough force to hold device at the stillwater level for wave force excitation tests. The characteristics of the PTO proved to be ineffective to study the advanced control technique known as latching. A new control method based on Wavelet analysis, utilising the PTOs ability to vary the damping in real time is described. The method yielded marginally improved energy capture. Three different shapes of float have been tested: simple right cylinder, a right cylinder with conical and hemispherical ends. Experimental testing was carried out using the (25m long, 18m wide and 1m deep) wave basin at the Hydraulics & Maritime Research Centre. The cone shape was found to be the most efficient with the use of mechanically induced control forces. A numerical model predicts the theoretical response of the buoy using linear theory, allowing for a validation study between the experimental and numerical results. A time domain model has been developed using Simulink and the boundary element method package WAMIT enabling non-linearity’s in the set-up to be examined. It was found that using a linear hydrostatic restoring force underestimated the resonance frequency of the cone and hemisphere. The numerical models yielded poor prediction results due the increased device motions, violating the linear assumptions.
Wave energy
Flannery, B. 2018. Laboratory scale modelling of a point absorber wave energy converter. PhD Thesis, University College Cork.