Passively-tuned roll-based wave energy converter for enhanced efficiency and frequency adaptability

Wave Energy Converters (WECs) typically exhibit natural oscillation frequencies that are significantly higher than the dominant frequencies of ocean waves, limiting their energy capture efficiency. Unlike conventional designs that rely on complex active control systems to address this mismatch, this study investigates a passive alternative based on inverted cone-shaped submerged structures that entrap seawater during upward motion, thereby increasing the effective added mass, lowering the natural frequency, and enabling resonance tuning of a roll-based WEC. Building on previous numerical validation, we present results from tests on a 1:40-scale model in regular and irregular waves. Five configurations with varying cone size and suspension distance were evaluated under regular wave excitation. The configuration achieving the highest performance reached a maximum Capture Width Ratio (CWR) of 52%, exceeding the 20%–40% range typical of conventional WECs. To assess robustness under realistic conditions, that configuration was further tested in irregular wave spectra representative of swell-dominated seas. Even under random excitation, the tuned device maintained efficiencies above 20%, demonstrating robustness against spectral variability. The experimental results show close agreement with predictions from a linear analytical model and confirm that passive tuning via cone-shaped structures effectively broadens the resonance bandwidth of roll-harvesting WECs. By combining high efficiency, robustness, and structural simplicity, this low-cost, scalable approach addresses a long-standing limitation of WECs and provides a viable pathway toward full-scale deployment with integrated power take-off damping and adaptation to diverse wave climates.