Modeling the Efficiency of a Semi-Submerged Ocean Wave Energy Converter

Ocean waves contain tremendous power. They represent a fascinating and virtually untapped source of renewable energy. Wave energy converter technology is a very young field compared with other renewable energy sources like wind and solar power. The technologies that exist are widely varied, with no clear convergence towards the best design. In such a diverse market there is great potential for new technologies to achieve success. In this work, a new method for harnessing energy from ocean waves is explored. Developed by Rohrer Technologies, Inc., the RTI G2 wave energy converter proposes new solutions to the two greatest challenges faced by wave energy conversion technologies: the ability to efficiently capture power across a broad spectrum of waves and the ability to survive in heavy seas. It is well known among ocean engineers and scientists that the vast majority of the energy transported by ocean waves exists near the surface. Therefore the RTI G2 concept proposes to survive heavy seas by submerging to escape the most violent and energetic wave forces. The RTI G2 is a terminator-type wave energy converter. The system consists of an air-filled chamber, whose compression and expansion is driven by incident wave forces acting on an aluminum plate mounted on one end of the chamber. The motion of the compression chamber may be in the horizontal direction or angled downward. By adjusting the orientation angle, the behavior of hydrostatic forces, especially the hydrostatic stiffness, is controlled. This thesis develops an analytical mathematical model of the absorbed power of the RTI G2 ocean wave energy converter concept. The results are calibrated with previously collected experimental data, showing good agreement. The mathematical model is also extended to compare the relative performance of a fixed-foundation system and a floating system. An experimental prototype is constructed and tested to calibrate this extended, floating model. In these experiments the floating behavior of the frame is simulated by fixing the RTI G2 model to a wheeled carriage that is mounted on tracks running along the wave flume. Results are studied using two metrics. The power efficiency of the system, defined as the ratio of the absorbed power to the available power in an ocean wave, is the first such metric. However ocean waves represent free energy, and so it can be argued that even a system with low efficiency can be of economic value. Therefore a second metric, defined as the ratio of absorbed power to necessary volume of the system, is also measured. In offshore systems volume is a good indicator of cost, and so this metric provides a gauge of economic efficiency. The results show that the mathematical model agrees well with experimental results. High friction damping in the experimental models limit the efficiency, but high economic efficiency may be achieved