Numerical Study on a Multibuoy-Type Wave Energy Converter With Hydraulic PTO System Under Real Sea Conditions

This study aims to investigate the power takeoff (PTO) performance of a two-buoy-type wave energy converter (WEC) with a PTO system under real sea conditions. The two-buoy-type WEC is composed of two hemispherical heaving buoys with a hydraulic PTO system. The hydrodynamic analysis considering two-body interaction is carried out based on the wave green function and Cummins equation. In addition, a hydraulic PTO system is described as an approximate Coulomb damping force and applied to the equation of motion for a WEC. First, by comparing the extraction power on various irregular wave conditions, the PTO performance of an isolated-buoy WEC, and the two-buoy-type WEC is estimated. Through this, it is found that the maximum extraction power occurred when the wave peak period is 1.2 times longer than the heave natural period of a WEC. By comparing the external force and wave elevations near the WEC, three incident wave directions are applied for the maximum or minimum extraction power. Based on these results, the two-buoy-type WEC is applied to the Korean coastal area (Uljin). The PTO force is calculated from the polynomial regression analysis to produce maximum extraction power on various irregular wave conditions. Finally, the annual extraction power and capture width of the WEC in Uljin are calculated. The relation between the design wave period and heave natural period of the WEC is investigated. The annual extraction power is maximum when the design wave period is 20% longer than the WEC's heave natural period and the incident wave is 90°.