Submerged wave energy converter dynamics and the impact of PTO-mooring configuration on power performance

Wave energy and associated wave energy converters (WECs) show potential to significantly contribute to global renewable energy goals. Surface-piercing WECs are the most prevalent, while subsurface WECs are in nascent stages of gaining a foothold and development. However, there are distinct potential advantages to subsurface WECs that make them a compelling pursuit; including increased resiliency to extreme wave conditions and decreased visual impact. However, there is still significant variation in design regarding the power take-off (PTO) and mooring system configurations. Current research shows that the number of PTOs and their configuration in a mooring system affects the dynamics of the WEC and its ability to capture energy in heave, surge, and pitch. Primarily, this study explores the effect of increasing the number of PTOs on power performance in regular and irregular wave conditions. Secondly, the rate of energy generation decay of different PTO configurations as they are submerged deeper within the water column is analyzed. This study investigates a single, three, four, and five-tether PTO configuration utilizing ProteusDS, a time-domain hydrodynamics analysis software. Results suggest that a three-PTO system can provide better broadband performance, slightly improved power quality, and reduced PTO damping and tether tension over a single-tether PTO configuration. Without factoring in the effects of differing loads and component requirements on cost efficiency, a three-tether PTO configuration most efficiently captures energy from all degrees of freedom (DOF) as adding additional PTO-mooring lines does not result in a significant increase in energy generation. All configurations are shown to have similar energy generation decay that follows the exponential decaying trends of the wave orbital trajectories.