Year

2013

Degree Name

Doctor of Philosophy

Department

School of Mathematics and Applied Sciences

Abstract

The last two decades have seen increasing interest in renewable energy technologies in response to the pollution effects from extensive use of fossil fuels and global warming. It is claimed by some that ocean wave energy alone could potentially supply the worldwide need for electricity, making it a significant clean energy resource. However, wave energy is still at an early stage of research and development and only a few companies are at pre-commercial stage in the development of their technologies.

With recent increases in computer performance, well-developed mathematical knowledge in hydrodynamics and advances in the development of numerical methods, numerical models have become an accurate, low-cost and fast tool in the research and development of hydrodynamic systems at sea. In the case of ocean wave energy systems, numerical analysis may be used in the design and optimisation of such devices.

The focus of the present body of work is on the analysis of the performance Oscillating Water Column (OWC) devices. An OWC device is essentially a surfacepiercing chamber with a submerged opening in which the free surface moves as a result of the interaction between the incident wave field and the device. The free surface movement induces a flow of air through an air turbine connecting the chamber to the outside ambient air. A generator is then attached to the air turbine allowing the production of electricity.

Even under linear water wave theory, the wave diffraction and radiation situation presented by a fixed OWC system has a number of unique features. The free surface within the OWC represents a complex boundary condition compared to the remainder of the free surface, which is simply at atmospheric pressure. This boundary condition includes the influences of a dynamic pressure which is a function of the flow of air induced by the oscillations of the water free surface, the compressibility of the air contained in the chamber, and the characteristics of the turbine system. These turbine characteristics may also be changed in real time so as to optimise the energy output of the system. Moreover, the maximum power output, resonant frequency and power bandwidth of the system are all highly dependent on the physical properties of the device. Using a new Finite Element Method (FEM) model developed by the present author, the optimum turbine parameters have been determined through the computation of the hydrodynamic properties of the system, and the effects of the OWC physical attributes have been studied in order to develop the most efficient design.

Multi-chamber OWC devices or ‘farms’ of OWCs are more likely to be deployed than a single device in order to harness the maximum available energy in a region and to facilitate installation and electrical power transmission. Analysis of this situation involves additional complicating factors since the diffraction, radiation and interaction between the devices need to be accurately modelled. In the course of the present study the energy-capture behaviour of OWC devices within an array has been found to differ significantly to that of a single isolated system. This behaviour is notably dependent on the spacing between devices relative to the incident wavelength. Moreover, optimum turbine parameters change depending on the position of the device in the array and power-capture optimisation of the overall system needs to take into account each of the device characteristics and all the interactions between devices.

OWCs may also be configured as floating devices and may be located a significant distance off-shore to access greater wave power availability. The motion of the floating structure then becomes an important factor in the power extraction of the system, where the air flow through the turbines and, therefore, the power extraction depend on the relative motion between the water and the body. Moreover, each motion is coupled; higher amplitudes in the water column induce an increase in the hydrodynamic forces and influence the motions of the body, whereas larger body motions will increase the generation of radiation waves influencing the motion of the water inside the chamber. The present study of heaving OWC devices also shows that mooring properties can have a significance influence on the energetic characteristic of the system and become an important element in the design of such systems.

Most of the present WEC studies were performed using linear water wave theory. The validity of this theory is dependent on the assumption that wave amplitudes are small in comparison to wavelength. This is not always a valid assumption in practice. One of the main focuses of the research has been to extend the model up to the second-order Stokes’ wave theory in order to determine, for the first time, non-linear effects on the behaviour of fixed and heaving OWC devices. It is found that second-order terms become especially important around the natural resonance frequencies of the system. As a consequence, these effects could induce an important decrease in the mean power output of the device relative to the incident wave power.

The development of the new 3D FEM model for the analysis of OWC devices by the present author represents a contribution to our theoretical knowledge and understanding of these OWC systems in an area that has received scant attention in the past. While, the results presented in the thesis are focused on relatively simple OWC geometries, the model has also been applied to assist in the development of more complex and practical systems for industry.

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