Three-Tether Wave Energy Converter: Hydrodynamic Modelling, Performance Assessment and Control

Abstract

Hydro, wind and solar power have become major contributors to the global renewable energy market. However, ocean wave power is emerging as a strong contender in the renewable energy mix due to its high power density and minimal environmental impact. Wave energy has the potential to provide an off-grid electricity solution to remote island communities, and fulfil offshore power needs of small industrial projects. One of the best wave energy resources in the world is concentrated along the southern margin of Australia, and if harnessed, wave power could contribute up to 27 per cent of the country’s electricity demand by 2050. Over the past few decades, a large number of concepts and designs have been suggested to convert wave energy into electricity. Despite a huge effort made by industry and the scientific community, the technology for extracting power from ocean waves still remains at a pre-commercial stage of development. The main challenge is to design an economically viable wave energy converter (WEC) where its life-cycle costs (investments, operation and maintenance) can be justified by the amount of generated electricity. This thesis focuses on the performance improvement of a particular class of wave energy converters, namely, a bottom-referenced fully submerged point absorber, by means of the three-tether mooring configuration. The main contribution is made towards the design, optimisation and control of the converter in order to answer three research questions: (i) what distinctive features of the fully submerged WECs can be utilised to increase their power absorption efficiency; (ii) how geometric parameters of the converter, such as the tether arrangement, shape, and aspect ratio affect the system performance; and (iii) what factors influence the practical implementation of the optimal control strategies on the three-tether WEC. To explore these questions, numerical frequency- and time-domain models have been developed using state-of-the-art techniques based on linear hydrodynamic theory. In order to gain background knowledge and build a core understanding of the submerged systems, the difference between floating and fully submerged point absorbers is investigated. Attention is given to the distinctive features observed in the hydrodynamic properties, power production limits, and control performance. Recommendations are provided on the choice of the buoy size and shape, depending on the wave climate of the deployment site. The advantages of employing multiple degrees of freedom in energy harvesting, especially for submerged converters, are demonstrated. The design considerations of the three-tether WEC are investigated from a number of perspectives including the tether arrangement, mass, shape, and aspect ratio of the buoy. A clear correlation between an optimal tether inclination angle and the buoy aspect ratio is identified. The comparison of three-tether WECs with different buoy geometries is performed not only based on their power output, but also taking into account a range of cost-related performance metrics. Moreover, the benefits of the three-tether converter over its single-tether counterpart are demonstrated through the detailed techno-economic analysis of both prototypes. The final aspect of this dissertation is devoted to the development of the advanced control system for the three-tether WEC. The causal velocity tracking controller is taken as a basis and extended to the multivariable control problem. It is demonstrated that the designed controller is able to improve the power absorption of the three-tether WEC as compared to a quasi-standard control approach while imposing a series of technical requirements on the power take-off machinery.