Abstract:
Investment in renewable energy technology, such as wave power, is increasingly seen as a beneficial and economically-viable alternative to existing fossil-based power plants. New Zealand has particularly energetic ocean waves and is well positioned to harness wave energy as a source of power. This thesis presents an in-depth analysis of scale-model experiments conducted on a novel WEC (Wave Energy Converter) device called the ‘spar fork’. The model was built using Froude scaling, with a dimensional scale ratio of 1:25 between model and prototype.
Tests were performed on the device in a 20m long wave flume, with waves being created and measured in the flume using a specially designed control system created in LabVIEW 8.0. Wave flume operation was validated by showing strong correlation between experimental results and dispersion theory. Sub-surface velocity profiles were shown to compare well with Stokes’ Second Order theory. Wave reflection analysis showed that reflected wave heights for the newly-installed flume beach were approximately 5% of the total wave height, which is well inside the acceptable range for wave flume modelling - that is, less than 10%.
Friction-torque for the spar fork model (representing the generator resistance of the prototype) was evaluated using pendulum decay methods. With software developed in MATLAB, angular velocities of the moving parts of the spar fork were measured using image processing and particle tracking methods. From the measurements taken, the power generated could be calculated for a number of wave states, also providing the opportunity to test a number of spar fork configurations.
It was found that the optimum spar fork design will include a significantly buoyant spar section tightly moored to the sea floor by no less than two mooring lines. The arm of the spar fork was optimum when its length was equal to the wave height. It is recommended that active float size is also maximised, as both these variables increase the amount of torque that can be generated. Designing the spar to be of natural heave frequency less than, but approaching the frequency of the design wave climate is particularly advantageous for power generation (although challenging physically). Further device recommendations are given in the conclusions.
For the expected wave climate on the west coast of New Zealand (using significant wave properties), the total power output of the tested device is shown to increase from 2.2kW for the original design to 5.4kW after optimisation.