Hydrodynamic Modeling of the Energy Conversion from Ocean Waves to Electricity

Sammanfattning: Research within wave energy conversion has seen a large increase during the last years. Several concepts are now taking the final steps towards commercialization. Conversion from wave energy for grid connection is a multi disciplinary task involving many challenges; one being the fact that the device has to be dimensioned for a nominal power and still be able to withstand the extreme powers in some of the harshest conditions on earth. The research and development of a concept based on a point absorber converting wave energy to a direct driven linear generator were initiated in 2002 at Uppsala University. Theoretical simulations have now been compared to offshore full scale experiments at the research test site. This thesis focuses on the hydrodynamic wave/device interaction for a Wave Energy Converter subjected to Swedish west coast wave climate. First conclusions on the Wave Energy Converters dynamics and energy absorption have been drawn based on earlier results. A thorough study of the sea state of Skagerrak and Kattegat has been made to asses the potential and get design parameters for the Wave Energy Converter. Based on that, a Wave Energy Converter model using potential linear wave theory and with the generator as a linear damper, has been developed to simulate the dynamics and power capture ratio. The model has also been used to simulate how biomass accumulated on the buoy will affect the dynamics. As a next step, the model has been further developed to simulate a two component system having optimum amplitude response at frequencies coinciding with Swedish west coast conditions. Results are presented for time series of real ocean wave data collected at the research test site as well as for empirical spectra. An enhanced amplitude response for the two component system is achieved by adding supplementary inertia by use of the added mass from a submerged body. Simulations show that the increased velocity in the two component system gives an increased power capture ratio by a factor of two compared to the regular system of a point absorber. The increased velocity also leads to a decrease in optimal damping for energy absorption with 60 %. The main weakness of the hydrodynamical modeling is thought to be the neglected viscosity that might lead to unpredicted energy losses, pointing out the direction for improvement and future work.