A study of radiated surface waves in the context of the capture of ocean wave energy
Thesis DisciplineMechanical Engineering
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
The current drive to find new low-carbon forms of electricity generation has intensified interest in capturing energy from ocean waves. The large range of devices currently proposed to fulfil this drive raises the question "What are the characteristics of an effective wave energy converter?". This thesis explores the design of a particular type of wave energy converter (WEC), namely the point absorber, by examining the properties and interactions of different types of surface waves. Current theory suggests that in order for a WEC to effectively absorb wave energy, it must also be an efficient wave radiator. The solutions to the Laplace equation in circular-cylindrical coordinates (known as circular waves) are used as point sources for the surface waves radiated from an oscillating body. The properties of circular waves provide guidelines with regard to the shape, movement and radiation properties required to make an effective WEC. The interactions of circular waves with plane waves also determines the theoretical optimum performance of a WEC in threedimensional wave fields. Limitations on the theoretical size of radiated waves are identified and linked to the size of the oscillating body. This establishes the connection between a body's displaced volume and its performance as a WEC. Practical experiments aimed at determining if the circular waves correspond to waves radiated by different oscillating bodies were completed. A method of consistently generating and measuring radial waves was developed and data recorded for a cylinder, sphere and flat plate oscillating vertically or horizontally. The results show that the circular-cylindrical models are accurate, providing the body is axisymmetric and oscillates either vertically, or horizontally with a small amplitude. The experiments also demonstrate the practical implications of radiating surface waves. This thesis advances the understanding of point absorber wave energy converters by identifying simple design guidelines that embody the underlying physics of surface waves. It also develops new methods for generating, measuring and analysing the waves radiated from an oscillating body.