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Energies 2013, 6(8), 4001-4044; doi:10.3390/en6084001

Discrete Displacement Hydraulic Power Take-Off System for the Wavestar Wave Energy Converter

1,2,* , 2
1 Department of Energy Technology, Aalborg University, Pontoppidanstrade 101, Aalborg DK-9220, Denmark 2 Wavestar, Park Alle 350A, Broendby DK-2605, Denmark
* Author to whom correspondence should be addressed.
Received: 14 May 2013 / Revised: 28 June 2013 / Accepted: 15 July 2013 / Published: 7 August 2013
(This article belongs to the Special Issue Energy from the Ocean - Wave and Tidal Energy)


The Wavestar Wave Energy Converter (WEC) is a multiple absorber concept, consisting of 20 hemisphere shaped floats attached to a single platform. The heart of the Wavestar WEC is the Power Take-Off (PTO) system, converting the wave induced motion of the floats into a steady power output to the grid. In the present work, a PTO based on a novel discrete displacement fluid power technology is explored for the Wavestar WEC. Absorption of power from the floats is performed by hydraulic cylinders, supplying power to a common fixed pressure system with accumulators for energy smoothing. The stored pressure energy is converted into electricity at a steady pace by hydraulic motors and generators. The storage, thereby, decouples the complicated process of wave power absorption from power generation. The core for enabling this PTO technology is implementing a near loss-free force control of the energy absorbing cylinders. This is achieved by using special multi-chambered cylinders, where the different chambers may be connected to the available system pressures using fast on/off valves. Resultantly, a Discrete Displacement Cylinder (DDC) is created, allowing near loss free discrete force control. This paper presents a complete PTO system for a 20 float Wavestar based on the DDC. The WEC and PTO is rigorously modeled from incident waves to the electric output to the grid. The resulting model of +600 states is simulated in different irregular seas, showing that power conversion efficiencies above 70% from input power to electrical power is achievable for all relevant sea conditions.
Keywords: wave energy; PTO; Wavestar; hydraulics; WEC wave energy; PTO; Wavestar; hydraulics; WEC
This is an open access article distributed under the Creative Commons Attribution License (CC BY) which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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Hansen, R.H.; Kramer, M.M.; Vidal, E. Discrete Displacement Hydraulic Power Take-Off System for the Wavestar Wave Energy Converter. Energies 2013, 6, 4001-4044.

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