Abstract
Wave energy converters (WECs) continue to be explored as part of a potential solution to meet global renewable energy goals. While surface-piercing WECs currently dominate the field, subsurface WECs are just starting to establish their presence. Subsurface WECs have several distinct advantages that make them worthwhile to examine, including their low visual profile and ability to survive extreme sea states. However, due to the decreasing orbital velocity and wave energy flux found as you submerge deeper in the water column design of a power take off system that increases the band of power capture becomes increasingly important. Numerical modeling of a full-scale device has shown that the both the number of tethers along with their configuration significantly affects the dynamics of the WEC system as well as its ability to capture power in multiple hydrodynamic modes such as heave, surge, and pitch. The experimental campaign, currently underway at the O.H Hinsdale wave flume at Oregon State University looks to assess the effect of increasing the number of coupled PTO-mooring tethers from one to three in both regular and irregular wave conditions on both power stability and production. Additionally, device hydrodynamics will be characterized by comparing response amplitude operators (RAO’s) between both the single and three tether cases, allowing for comparison of the dominant hydrodynamic mode or modes at each period. Furthermore, energy capture as a function of submergence depth is investigated to identify practical deployment depths. Preliminary results show that the dominant hydrodynamic modes near the resonant frequency for the experimental scale device in both the single and three tether configuration are heave and pitch. With a larger peak in pitch motions in the three tether configurations. Confirming numerical modeling results that suggested adding additional tethers can increase the band of energy capture across period and provide a more stable continuous power output. Findings also show that the optimal damping for the single tether case is much higher than in the three-tether case indicating that a three tether physical system may be easier to control and build due to lower tether tensions and motor requirements. Both configurations show that energy generation capability decays exponentially following the trend of the wave orbitals independent of the number of coupled PTO-mooring lines. Finally, results confirm that most of the energy capture potential for a submerged device exists in the top 1/4th of the water column.