Design, Analysis and Testing of a Self-reactive Wave Energy Point Absorber with Mechanical Power Take-off

Ocean wave as a renewable energy source possesses great potential for solving the world energy crisis and benefit human beings. The total theoretical potential wave power on the ocean-facing coastlines of the world is around 30,000 TWh, although cannot all be adopted for generating electricity, the amount of the power can be absorbed still can occupy a large portion of the world's total energy consumption. However, multiple reasons have stopped the ocean wave energy from being widely adopted, and among those reasons, the most important one is immature of the Power Take-off (PTO) technology.

In this dissertation, a self-reactive two-body wave energy point absorber that is embedded with a novel PTO using the unique mechanism of Mechanical Motion Rectifier (MMR) is investigated through design, analysis and testing to improve the energy harvesting efficiency and the reliability of the PTO. The MMR mechanism can transfer the reciprocated bi-directional movement of the ocean wave into unidirectional rotation of the generator. As a result, this mechanism brings in two advantages towards the PTO. The first advantage it possess is that the alternating stress of the PTO is changed into normal stress, hence the reliability of the components are expected to be improved significantly. The other advantage it brings in is a unique phenomenon of engagement and disengagement during the operation, which lead to a piecewise nonlinear dynamic property of the PTO. This nonlinearity of the PTO can contribute to an expanded frequency domain bandwidth and better efficiency, which are verified through both numerical simulation and in-lab experiment. During the in-lab test, the prototyped PTO achieved energy transfer efficiency as high as 81.2%, and over 40% of efficiency improvement compared with the traditional non-MMR PTO under low-speed condition, proving the previously proposed advantage.

Through a more comprehensive study, the MMR PTO is further characterized and a refined dynamic model. The refined model can accurately predict the dynamic response of the PTO. The major factors that can influence the performance of the MMR PTO, which are the inertia of the PTO, the damping coefficient, and the excitation frequency, are explored through analysis and experiment comprehensively. The results show that the increase on the inertia of the PTO and excitation frequency, and decrease on the damping coefficient can lead to a longer disengagement of the PTO and can be expressed analytically.

Besides the research on the PTO, the body structure of the point absorber is analyzed. Due to the low-frequency of the ocean wave excitation, usually a very large body dimension for the floating buoy of the point absorber is desired to match with that frequency. To solve this issue, a self-reactive two-body structure is designed where an additional frequency between the two interactive bodies are added to match the ocean wave frequency by adopting an additional reactive submerged body. The self-reactive two-body structure is tested in a wave to compare with the single body design. The results show that the two-body structure can successfully achieve the frequency matching function, and it can improve more than 50% of total power absorption compared with the single body design.