This study evaluates two significant design modifications of a dual coaxial-cylinder system as a wave-energy extractor reported in Son and Yeung (2014, OMAE2014-#24582). First, a new and stronger power take-off (PTO) unit for a permanent magnet linear generator (PMLG) was built, along with an appropriate supporting structure, so as to match optimality conditions in terms of impedance matching and mechanical to electrical conversion efficiency. Based on a series of (dry-)bench tests, the properties of the PTO were obtained and the optimal operating conditions were determined. Second, the flat-bottom shape of the outer toroidal floater was modified according to “The Berkeley Wedge design” (Madhi et al, 2014, “The Berkeley Wedge: an asymmetrical energy-capturing floating breakwater of high performance,” Journal of Marine Systems and Ocean Technology, vol. 9(1), pp. 5–16). The new bottom shape led to reduction of the floater damping by almost 70%, which yielded a 3-fold increase in the floater motion response. Experiments in a wave-tank validated the response behavior of the dual-cylinder system with the use of the new PTO. The Berkeley-Wedge shape allowed more than 3 times more energy be extracted compared to the flat-bottom geometry, while the new generator also improved the energy conversion efficiency. As a result, the overall system efficiency of the device was enhanced remarkably five times over that of the previous design.

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