Design and hydrodynamic optimization of a wave energy harvester
Creators: name: AL SHAMI, Elie
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 400801
subject: Circuits and systems
classificationCode: 401702
subject: Dynamics, vibration and vibration control
classificationCode: 401703
subject: Energy generation, conversion and storage (excl. chemical and electrical)

description: The sun's thermal radiations cause a change in the air temperatures. This temperature difference in the atmosphere generates wind blowing over the surface of the ocean resulting in ocean waves and creating tremendous energy potential which can be converted to electrical energy. What is interesting about the thermal energy is that it increases ten times in its density going from one form to another, that is from the solar to the wind and to the wave. This ocean wave energy of a high density can be translated into electrical power with higher efficiency through ocean wave conversion systems than through other forms of renewable energy technologies. Ocean wave energy can be harvested by special devices which take advantage of the ocean surface movements and immense hydrodynamic forces to capture potential and kinetic energy and to convert the wave energy to electrical energy. These concepts have evolved since the late 1800s, but none of these harvesters was able to stand out as the definitive answer to ocean waves energy harvesting solution due to many obstacles such as the low periods of ocean waves, the complexity of the hydrodynamics and fluids interactions, and the intensive research and funding required for the PTO (Power Take-Off) mechanisms. This project will attempt to tackle these issues. The work presented in this thesis will handle the wave energy harvester design while focusing on the investigation and optimization of the effects of the regular and irregular wave loadings, hydrodynamic effects, nonlinear wave-floater fluid interactions on the WEC (Wave Energy Converter) performance, and the issues of the power take-off will be handled with a proposed novel wave energy harvester. First, a comprehensive linear mathematical model was formulated in the frequency domain, and the Taguchi method was implemented to investigate and optimize the design of two-body WECs. The submerged body was shown to have a large effect on the performance. Hence, a more comprehensive linearized mathematical model was formulated using algebraic solutions from the literature to calculate the performance of the two-body WEC based on the device dimensions and sea state without the need to resort to BEM (Boundary Element Method) Hydrodynamics simulations. The efficient mathematical model was put into use by implementing the Monte Carlo simulations to further and more accurately study the effect of different design parameters. Also, to further emphasize the importance of the submerged body on the performance of two-body WECs, the work was further expanded using a non-linear mathematical model in the time domain to compare different typical and novel shapes for the submerged body. Transient and three dimensional CFD (Computational Fluid Dynamics) simulations were utilized to derive the viscous drag coefficient of the different submerged bodies. Since the literature suggested the increase of degrees of freedom to two has many benefits on the performance of point absorber WECs, a study was conducted to showcase how the performance of WECs can be substantially increased with the increase of degrees of freedom of WECs from one to five while keeping the same totally mass and volume. These studies handled the analysis and optimization of point absorbers in conjunction with the high energetic far from shore locations around Australia. In contrast, a further study was implemented to design and optimize point absorbers for specific locations close to shore around all the Australian states. Regression analysis was implemented in order to derive analytical prediction equations for the performance of these WECs based on the sea-states and geometric parameters without the requirement of numerical simulations or complex analytical solutions. The prediction models were validated and utilized to optimize two-body point absorbers for close to shore Australian locations with both maximum power output and maximum power to weight ratios in mind. Finally, to address the issue concerning the PTO, a novel mechanism was proposed based on the scotch yoke to be used as a PTO in a WEC. A non-linear mathematical model was formulated and the device was designed and verified with physical testing. The novel PTO was showcased to have comparable performance to other renewable energy conversion technologies while being much smaller in dimensions compared to typical WECs.