Self-Reacting Point Absorber Wave Energy Converters

Date

2015-08-31

Authors

Beatty, Scott J.

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Abstract

A comprehensive set of experimental and numerical comparisons of the performance of two self-reacting point absorber wave energy converter (WEC) designs is undertaken in typical operating conditions. The designs are either currently, or have recently been, under development for commercialization. The experiments consist of a series of 1:25 scale model tests to quantify hydrodynamic parameters, motion dynamics, and power conversion. Each WEC is given a uniquely optimized power take off damping level. For hydrodynamic parameter identification, an optimization based method to simultaneously extract Morison drag and Coulomb friction coefficients from decay tests of under-damped, floating bodies is developed. The physical model features a re-configurable reacting body shape, a feedback controlled power take-off, a heave motion constraint system, and a mooring apparatus. A theoretical upper bound on power conversion for single body WECs, called Budal's upper bound, is extended to two body WECs. The numerical analyses are done in three phases. In the first phase, the WECs are constrained to heave motion and subjected to monochromatic waves. Quantitative comparisons are made of the WEC designs in terms of heave motion dynamics and power conversion with reference to theoretical upper bounds. Design implications of a reactive power take-off control scheme and relative motion constraints on the wave energy converters are investigated using an experimentally validated, frequency domain, numerical dynamics model. In the second phase, the WECs are constrained to heave motion and subjected to panchromatic waves. A time domain numerical model, validated by the experimental results, is used to compare the WECs in terms of power matrices, capture width matrices, and mean annual energy production. Results indicate that the second WEC design can convert 30% more energy, on average, than the first design given the conditions at a representative location near the West coast of Vancouver Island, British Columbia, Canada. In the last phase, the WECs are held with three legged, horizontal, moorings and subjected to monochromatic waves. Numerical simulations using panelized body geometries for calculations of Froude-Krylov, Morison drag, and hydrostatic loads are developed in ProteusDS. The simulation results---mechanical power, mooring forces, and dynamic motions---are compared to model test results. The moored WEC designs exhibit power conversion consistent with heave motion constrained results in some wave conditions. However, large pitch and roll motions severely degrade the power conversion of each WEC at wave frequencies equal to twice the pitch natural frequency. Using simulations, vertical stabilizing strakes, attached to the reacting bodies of the WECs are shown to increase the average power conversion up to 190% compared to the average power conversion of the WECs without strakes.

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Keywords

Wave Energy Conversion, Renewable Energy, Power take-off, Self-reacting, Model testing, Point absorbers

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