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Research Papers: Ocean Engineering

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

[+] Author and Article Information
Nathan Tom

Department of Mechanical Engineering,
University of California at Berkeley,
Berkeley, CA 94720
e-mail: nathan.m.tom@gmail.com

Ronald W. Yeung

American Bureau of Shipping Inaugural
Chair in Ocean Engineering
Director, Computational Marine Mechanics
Laboratory (CMML)
Department of Mechanical Engineering,
University of California at Berkeley,
Berkeley, CA 94720
e-mail: rwyeung@berkeley.edu

From [16]: $X¯3=X3/πρga2$, $m¯0=kH$, $d¯=d/a$, $μ¯33=μ33/πρa3$, $λ¯33=λ33/πρσa3$, $σ¯=σa/g=ka$.

$w/D$ was incorrectly stated in [10].

1Ph.D. Candidate, Ocean Engineering Group, University of California at Berkeley.

2Corresponding author.

Contributed by the Ocean, Offshore, and Arctic Engineering Division of ASME for publication in the JOURNAL OF OFFSHORE MECHANICS AND ARCTIC ENGINEERING. Manuscript received April 23, 2012; final manuscript received February 18, 2013; published online September 4, 2013. Assoc. Editor: Hideyuki Suzuki. Paper presented initially at the 2012 ASME 31st International Conference on Offshore Mechanics and Arctic Engineering (OMAE2012), Rio de Janeiro, Brazil, July 1–6, Paper No. OMAE2012-83736.

J. Offshore Mech. Arct. Eng 135(4), 041101 (Sep 04, 2013) (10 pages) Paper No: OMAE-12-1040; doi: 10.1115/1.4024150 History: Received April 23, 2012; Revised February 18, 2013

## 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.

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## Figures

Fig. 1

Vortex-blob patterns from FSRVM [18] for prescribed heave oscillation: a comparison between 2D RB and FB shapes: red and green crosses represent clockwise and counterclock-wise vorticity, respectively. Simulation parameters are comparable to the 3D geometry, (see Fig. 2): (d/B) = 2.24, A3 = 6 in, σ=3.65 rad/s. The hydrodynamic coefficients μ33 and λT for each shape are shown in the upper left corner of each plot. Note: B is the two-dimensional beam, which is the same value as the three-dimensional diameter, D.

Fig. 2

Profile view of FB and RB floaters

Fig. 3

Heave RAO of the floater as a function of angular frequency σ for both floaters with Bg = 0. The resonance frequencies for both are kept at σ = 3.7  rad/s or T = 1.7 s.

Fig. 4

Heave-displacement time histories of free-decay tests for the FB and RB floaters

Fig. 5

Comparison of the normalized heave wave-exciting force X¯3 as a function ka

Fig. 6

Schematic of the pole-slot configuration for one side of PMLG

Fig. 7

Comparison of Pel and Pme against length of magnet array

Fig. 8

Top and front views of one phase of one stator side

Fig. 9

Comparison of Pme: LSS design versus SSS design

Fig. 10

Pel and ηel for a wgap of 0.405 in: LSS versus SSS design

Fig. 11

Pel and ηel for a wgap of 0.48 in: LSS versus SSS design

Fig. 12

Power-loss breakdown for solid steel stator

Fig. 13

Power-loss breakdown for laminated steel stator

Fig. 14

Effect of MCGW on Pel for varying load resistance R

Fig. 15

Effect of MCGW on ηel for varying load resistance R

Fig. 16

Effect of MCGW on Bg for varying load resistance R

Fig. 17

Comparison of Pel and Bg against maximum rotor speed

Fig. 18

Photo showing the assembled coupled system in wave-tank tests. A Solidworks schematic can be found in [10].

Fig. 19

Comparison of the C¯w as a function of σ

Fig. 20

Comparison of C¯w for varying load resistance R

Fig. 21

Comparison of C¯w as a function of σ for SSS

Fig. 22

Comparison of C¯w as a function of σ for LSS

Fig. 23

Comparison of Pel as a function of σ

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