Analysis and optimization of a Dual Mass-Spring-Damper (DMSD) wave-energy convertor with variable resonance capability

To adapt to an ocean environment of changing wave period, a novel point-absorber concept is proposed: a fully-enclosed Dual Mass-Spring-Damper (DMSD) floater system that has an internal mass and spring so that its resonance frequency can be varied by changing the movable internal mass and spring. In a waterproof floater, this one-degree-of-freedom heaving energy converter can float or be submerged to evade excessive storm load. Optimization of the internal mass, spring and damping system in the frequency domain is first developed to demonstrate that favorable capture width is achievable for a range of incident-wave frequency. Next, power and motion response of this system in an ISSC wave spectrum are studied in time domain. With constant damping for power-takeoff, the DMSD system is found to have noticeably lower capture width in irregular waves than that in regular waves at a frequency that is the same as the peak wave frequency of the spectrum. Since excessive relative motion can occur in irregular-wave excitation, end-stops, modeled by a stiff spring and strong damper, are applied in the time-domain simulation. When properly designed, degradation of only 1–2% in the capture width in the irregular-wave environment is found when end-stops are applied.