Optimizing ocean-wave energy extraction of a dual coaxial-cylinder WEC using nonlinear model predictive control

Active control based on nonlinear model predictive control (NMPC) methodology is applied to a candidate but realistic dual coaxial-cylinder wave-energy extractor system with the objective of maximizing energy capture over a range of incident ocean-wave conditions. The extractor consists of an outer cylinder (floater) moving relative to a tension-tethered inner cylinder, driving a permanent-magnet linear generator (PMLG) to acquire power take-off (PTO). The floater dynamics is solved in the time domain with the NMPC to constrain maximum floater motion and simultaneously incorporating the PMLG damping capability. The numerical model of the coupled floater and PTO dynamics is formulated in the state-space representation. The mechanical to electrical conversion efficiency of the PMLG, which was experimentally determined as a function of its power-extraction damping, is considered in the numerical simulation. The NMPC process yields a time-varying profile for the generator damping as the control strategy. Simulation results indicate strongly time-dependent and discontinuous generator-damping requirements for both regular and irregular incident-wave scenarios. However, the NMPC-controlled system significantly outperforms a passive-control, constant-damping system by exhibiting higher peak values of energy-extraction and a broader capturing bandwidth, thus confirming the feasibility and success of the control strategy.