Abstract
Among the wave energy converters (WECs), oscillating buoy is a promising type for wave energy development in offshore area. Conventional single-freedom oscillating buoy WECs with linear power take-off (PTO) system are less efficient under off-resonance conditions and have a narrow power capture bandwidth. Thus, a multi-freedom WEC with a nonlinear PTO system is proposed. This study examines a multi-freedom WEC with 3 degrees of freedom: surge, heave and pitch. Three different PTO systems (velocity-square, snap through, and constant PTO systems) and a traditional linear PTO system are applied to the WEC. A time-domain model is established using linear potential theory and Cummins equation. The kinematic equation is numerically calculated with the fourth-order Runge-Kutta method. The optimal average output power of the PTO systems in all degrees of freedom are obtained and compared. Other parameters of snap through PTO are also discussed in detail. Results show that according to the power capture performance, the order of the PTO systems from the best to worst is snap through PTO, constant PTO, linear PTO and velocity-square PTO. The resonant frequency of the WEC can be adjusted to the incident wave frequency by choosing specific parameters of the snap through PTO. Adding more DOFs can make the WEC get a better power performance in more wave frequencies. Both the above two methods can raise the WEC’s power capture performance significantly.
Highlights
Wave energy converters with various working principles have been proposed and studied within recent decades, including oscillating water column (OWC), overtopping and oscillating buoy
Comparation of the power take-off (PTO) systems To investigate which type of PTO has the best power capture performance, the optimal average output power is calculated in every degrees of freedom (DOF)
The wave frequency ranges with the average power are larger than the corresponding mean value of heave and 3-DOF buoys are listed
Summary
Wave energy converters with various working principles have been proposed and studied within recent decades, including oscillating water column (OWC), overtopping and oscillating buoy. The PTO damping of an oscillating buoy WEC is mainly related to parameters including the buoy’s velocity and displacement, or can be considered as a constant coefficient. Comparation of the PTO systems To investigate which type of PTO has the best power capture performance, the optimal average output power is calculated in every DOF. The average output power in surge peaks on wave frequency 2.2 rad/s and the maximum value is 2 times the maximum value of linear PTO. The wave frequency ranges with the average power are larger than the corresponding mean value of heave and 3-DOF buoys are listed. The output power can be increased significantly both by using nonlinear PTO
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