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Power Production by Linear Wave Energy Converters of Point Absorber Type with Vertical Cylindrical Buoys during Various Sea States
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
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(English)In: Article in journal (Refereed) Submitted
Abstract [en]

Two wave energy converters (WEC) were deployed near the city of Lysekil as part of Uppsala University's wave energy project. The converters were electrically identical and mechanically very similar. The converters were installed on the seabed at 25 m depth at the same site and believed to be exposed to very similar wave conditions. They utilized permanent magnets oscillating vertically due to surface buoys actuating the generators. The buoys were vertical cylinders excitated by ocean waves, with different diameters but with equal volume. The power production from the converters and the sea states were measured and comparison of power production between the two converters during various sea states was conducted. Also wind and tidal oscillations were considered and were found to influence the power production directly or indirectly, as did also significant wave height and energy period. The only difference between the two converters were the buoys, as the generators and electric loads were equal. During a 4 day period of various wave climates, the WEC with the buoy with 78 % larger water plane area than the WEC with the more narrow buoy with a water plane area of 7.07 m2 was found to produce 11 % more power.

Keyword [en]
wave power, point absorber, WEC, experiment, efficiency, energy, buoy
National Category
Energy Engineering
Identifiers
URN: urn:nbn:se:uu:diva-220342OAI: oai:DiVA.org:uu-220342DiVA: diva2:704665
Available from: 2014-03-13 Created: 2014-03-13 Last updated: 2014-12-11
In thesis
1. Buoy Geometry, Size and Hydrodynamics for Power Take Off Device for Point Absorber Linear Wave Energy Converter
Open this publication in new window or tab >>Buoy Geometry, Size and Hydrodynamics for Power Take Off Device for Point Absorber Linear Wave Energy Converter
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Wave energy converters of point absorber type have been developed and constructed. Full scale experiments have been carried out at sea and electricity has been successfully delivered. Linear permanent magnet generators together with a subsea substation and buoys of various geometric shapes have been investigated theoretically and experimentally. The design has in large extent an electronic approach, keeping the mechanical part of it as simple as possible, due to the long life span and reliability of electric components.

Because of the nature of a linear generator, the internal translator with permanent magnets has a limited stroke length which will be reached when the buoy is exposed to large wave heights. Internal springs at the top and bottom of the generator prevent the translator from hitting the generator hull. Inertial forces due to the mass and velocity of the translator and the buoy and its heave added mass compresses the spring. The added mass is a rather large part of the total moving mass. Simulations of a converter with a vertical cylindrical buoy and with a toroidal buoy were conducted, as well as real sea experiments with converters with cylindrical buoys of two different sizes and a toroidal buoy. The overloads are likely to affect the design and service life of the generator, the buoy and the wire which interconnects them.

Buoy shapes with as much excitation force as possible and as little heave added mass as possible were sought. A toroidal buoy caused less overloads on the generator at sea states with short wave periods and relatively large wave height, but for sea states with very long wave periods or extremely high waves, the magnitude of the overloads was mainly determined by the maximum displacement of the buoy.

Snap loads on the interconnecting wire, as the slack wire tensed up after a very deep wave trough, were found to be greater but of the same order of magnitude as forces during the rest of the wave cycle.

During a 4 day period at various wave conditions, two converters with cylindrical buoys proved efficiency between 11.1 % and 24.4 %. The larger buoy had 78 % larger water plane area than the other buoy which resulted in 11 % more power production. Short wave period was beneficial for the power production.

Infinite frequency heave added mass was measured for a cylindrical buoy at real sea and found to be greater than the linearly calculated theoretical added mass.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2014. 71 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1130
Keyword
point absorber, wave, energy, converter, ocean, wec, toroidal, buoy, torus, cylindrical, cylinder, experiment, full scale, trial, sea, energy, renewable
National Category
Energy Engineering Fluid Mechanics and Acoustics
Research subject
Engineering Science with specialization in Science of Electricity
Identifiers
urn:nbn:se:uu:diva-220344 (URN)978-91-554-8902-1 (ISBN)
Public defence
2014-04-28, Häggsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 13:15 (English)
Opponent
Supervisors
Available from: 2014-04-04 Created: 2014-03-13 Last updated: 2014-09-18Bibliographically approved
2. Linear wave energy converter: Study of electromagnetic design
Open this publication in new window or tab >>Linear wave energy converter: Study of electromagnetic design
2014 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

The thesis presents results from synchronous linear wave energy converters developed at Uppsala University. A study is done on closed stator slots and a study presenting offshore data focusing on the power absorption from a wave energy converter (WEC). The first step in studying the closed slots has been done during no-load, to study the magnetic flux path from the permanent magnets and to study the forces in the linear generator. The initial studies show a reduction in cogging force and a reduction in harmonics in the magnetic flux density in the air-gap. It also shows an increase of the total flux entering the stator and an increase in flux leakage. The study has been done with FEM simulations and compared with analytical calculations.

The second study was done to investigate the power absorption of a WEC in upward and downward motion in relation to the volume of the buoy and mass of the system. The experimental results were compared with a static model focusing on the limit in the absorption. As expected from the model, the WEC absorbs more energy in the upward direction. Also indications of snatch load were observed. Within this thesis, results from a comparison study between two WECs with almost identical electrical properties and the same volume of the buoy, but with different height and diameter have been presented. Moreover, experimental studies including the conversion step between AC to DC have been done.

The work done in this thesis is a part of a larger wave power project at Uppsala University. Where everything between the energy absorption from the waves to the connection to the electrical grid is studied. The project has a test-site at the west coast of Sweden near the town Lysekil, where wave energy research has been carried out since 2004.

Place, publisher, year, edition, pages
Uppsala: Uppsala universitet, 2014. 62 p.
Series
UURIE / Uppsala University, Department of Engineering Sciences, ISSN 0349-8352 ; 335-14L
Keyword
Wave energy, permanent magnets, linear generator, closed stator slots, offshore experiments
National Category
Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-237522 (URN)
Presentation
(English)
Opponent
Supervisors
Funder
Swedish Research Council, 2009-3417
Available from: 2014-12-11 Created: 2014-12-03 Last updated: 2014-12-11Bibliographically approved

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Gravråkmo, HalvarLejerskog, ErikStrömstedt, ErlandSavin, AndrejLeijon, Mats

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