Abstract
Wave energy converters (WECs) show promising potential to significantly contribute to global renewable energy goals. Numerous WEC designs have been proposed and investigated, but wave energy conversion technology has yet to reach convergence in the same way wind or solar has. Of the designs currently in existence, surface-piercing WECs are the most prevalent, and subsurface WECs are only in the beginning stages of gaining a foothold in the space. However, there are distinct advantages of subsurface WECs that make them a compelling pursuit, including increased resiliency to extreme wave conditions and the ability to submerge them within the water column so as to create no surface expression or to deconflict with marine traffic. Several subsurface point absorber WEC projects have been initiated by AWS Ocean Energy, Symphony Wave Power, Carnegie Clean Energy, and CalWave Power Technologies, and have reached later stages of development. However, even within this one WEC archetype, there is non-convergence in their design in regard to their choice of power take-off (PTO) and mooring system configuration. Current research shows that the number of tethers and their configuration in a mooring system affects the dynamics of the WEC and its ability to capture energy in modes of heave, surge, and pitch that can be converted into usable energy by the PTO system. This study explores different PTO and mooring system configurations in regular and irregular wave conditions to investigate the effect of increasing the number of tethers and PTOs on power performance. Additionally, the rate of energy generation decay of different configurations is analyzed as they are submerged deeper within the water column. This investigation numerically analyzes a single-tether, three-tether, four-tether, and five-tether configuration in a singular cohesive study utilizing hydrodynamic modeling in ProteusDS, a time-domain dynamics analysis software. Results suggest that a three-tether system can provide better broadband performance, slightly improved power quality, and reduced PTO damping and tether tension loads over a single-tether configuration but at the cost of adding expenditures and complexity associated with two additional PTOs and supplementary mooring components. Findings also suggest that a three-tether configuration most efficiently utilizes its energy capture modes, and adding additional tethers and PTOs does not result in a significant benefit and may actually lead to less energy generation. All configurations are shown to have similar energy generation decay that follows the exponential decaying trends of the wave orbital trajectories.