Numerical Modeling and 3D Investigation of INWAVE Device

In this article, numerical studies on a tightly moored point absorber type wave energy converter called INWAVE are presented. This system consists of a buoy, subsea pulleys, and a power take off (PTO) module. The buoy is moored by three ropes that pass through the subsea pulleys to the PTO module. Owing to the counterweight in the PTO module, a constant tension, which provides a horizontal restoring force to the buoy, is constantly applied to the rope. As waves pass by, the buoy is subjected to six degrees of freedom motion, consisting of surge, heave, sway, roll, pitch, and yaw, which causes reciprocating motion in the three mooring ropes. The PTO module converts the motion of the ropes into electric power. This process is expressed as a dynamic equation based on Newtonian mechanics and the performance of the device is analyzed using time domain simulation. We introduce the concept of virtual torsion spring in order to prevent the impact error in the ratchet gear modules which convert bidirectional motion of rope drum into unidirectional rotary motion. The three-dimensional geometrical relationship between the ropes and the buoy is investigated, and the effects of the angle of the mooring rope and the direction of wave propagation are addressed to determine the interaction between the tension of the rope and the buoy. Results have shown that the mooring rope angle has a large impact on the power extraction. The simulation results present a useful starting point for future experimental work.