Performance Enhancements and Validations of a Generic Ocean-Wave Energy Extractor

Abstract

This paper evaluates two aspects of enhancements made to a generic ocean-wave energy extraction device, developed recently at University of California (UC)-Berkeley with features reported in Yeung et al. (2010, “Design, Analysis, and Evaluation of the UC-Berkeley Wave-Energy Extractor,” ASME J. Offshore Mech. Arct. Eng., 134(2), p. 021902). First, the differences in hydrodynamic performance between flat- and hemispherical bottom floaters were investigated theoretically using the UC Berkeley 2D viscous-flow solver: FSRVM (Seah and Yeung, 2008, “Vortical-Flow Modeling for Ship Hulls in Forward and Lateral Motion,” Proceedings of the 27th Symposium on Naval Hydrodynamics, Seoul, Korea). The predicted enhancement was compared with experimental results, demonstrating that an increase in motion of over 50% was realizable. Second, important modifications to the design, fabrication, and material of the rotor and stator of the permanent magnet linear generator (PMLG) were made with the aim to increase both power output and mechanical-to-electrical conversion efficiency, ηel. Increased power extraction and efficiency were achieved, doubling what had been previously reported. The nonlinear relationship between the generator damping and the magnet-coil gap width was also investigated to verify that the conditions for optimal power extraction presented in Yeung et al. (2010, “Design, Analysis, and Evaluation of the UC-Berkeley Wave-Energy Extractor,” ASME J. Offshore Mech. Arct. Eng., 134(2), p. 021902) were achievable with the PMLG. Experimental results, obtained from testing the coupled floater and PMLG systems in a wave tank, revealed that realized capture widths were more than double those from the previous design. These results further confirmed that matching of the generator and floater damping significantly increased the global efficiency of the extraction process.