New concepts for the direct conversion of the movement of a vertical heaving buoy point absorber to electricity are presented. The low loss direct conversion and generation system without intermediate hydraulic system permits hereto unprecedented speed and force control flexibility of the buoy movement.
Three solutions are compared: an induction generator with a rack-pinion linear to rotary motion conversion and a step-up gear, a high pole application-specific permanent magnet (PM) generator with a belt-pulley linear to rotary conversion, and a linear PM generator mounted directly on the guide of the buoy. The linear generator is the least robust and most challenging in design, while the induction generator solution is found best due to its cost and maintenance advantages. The PM rotating generator solution shows the highest efficiency.
Control strategies considering practical limitations (motor size, inverter rating) are explored and compared to linear damping and latching control. The peak power handling capability is optimized while diminishing the annual energy take-off only marginally.
Simulations for irregular wave traces and the efficiency consequences are discussed. A 1:5 scaled test system using this direct conversion was installed at shore in real sea waves. Experimentally measured energy capture widths for wave-to-electricity in the range of 25% are reported.