Experimental and Numerical Study of Spar Buoy-magnet/spring Oscillators Used as Wave Energy Absorbers

We study free-floating point absorption wave generators, consisting of an assemblage of one or a few (mostly heaving) spar buoys, housing at least one short-stroke linear generator (SSLG), made of a magnet, suspended to a spring, and oscillating within a coil. This system is aimed at producing low and renewable wave power (up to kW) for marine coastal surveillance systems. Both scale model experiments and numerical modeling are performed in order to tune the system’s parameters and maximize its response for a target sea-state (i.e., operate near resonance in heave and magnet motion). We find that, for such buoy systems, viscous friction is the dominant damping mechanism near resonance and, hence, the buoy’s wet extremities must also be properly streamlined, and rolling must be minimized as it may significantly increase such damping. This can be achieved with a so-called trispar system, in which 3 spars buoys of identical diameter are mounted in an equilateral triangle configuration, one diameter apart from each other. Since the heave resonance period of a spar buoy is primarily a function of its draft, to lower this period and better match the resonance period of the SSLG, the draft of each buoy in the trispar is varied (in the scale model, to 25, 50 and 100 cm), with the longest spar buoy housing the SSLG, while simultaneously adjusting their dead weight. Experimental results in periodic waves, well supported by numerical modeling, show a significantly improved performance of the trispar vs. single spar design, both with respect to parasitic roll oscillations (almost none observed for the tripar) and power generation. The good performance of the trispar, particularly in terms of “Capture Width Ratio”, is confirmed by preliminary numerical simulations in irregular waves. Future work will test the trispar in irregular waves and explore dynamic tuning strategies (e.g., latching) of the SSLG, in order to further improve power generation.