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Buoy Geometry, Size and Hydrodynamics for Power Take Off Device for Point Absorber Linear Wave Energy Converter
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
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 [en]
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: urn:nbn:se:uu:diva-220344ISBN: 978-91-554-8902-1 (print)OAI: oai:DiVA.org:uu-220344DiVA: diva2:704990
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
List of papers
1. The Lysekil Wave Power Project: Status Update
Open this publication in new window or tab >>The Lysekil Wave Power Project: Status Update
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2008 (English)Conference paper, Published paper (Refereed)
National Category
Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-97846 (URN)
Available from: 2008-11-21 Created: 2008-11-21 Last updated: 2014-04-29Bibliographically approved
2. Tracking a Wave Power Buoy Using a Network Camera: System Analysis and First Results
Open this publication in new window or tab >>Tracking a Wave Power Buoy Using a Network Camera: System Analysis and First Results
2009 (English)In: Volume 4: Ocean Engineering; Ocean Renewable Energy; Ocean Space Utilization, Parts A and B, Honolulu, Hawaii, 2009, 799-807 p.Conference paper, Published paper (Refereed)
Abstract [en]

Anobservation system has been set up on a small isleton the Swedish west coast. The purpose of the systemis to monitor the wave buoys in The Lysekil Project.The project is an attempt to harvest wave energy usinglinear generators and point absorbing buoys. The observation system isself-sufficient and uses a network camera to follow the buoymotions. The first results from the camera, which has beenoperating since July 2008, have been analyzed to examine themotion tracking capabilities of the system. The motion tracking wouldwork as a complement to the other measurements that arebeing done on the buoy. The method for extracting motiondata from the two-dimensional pictures is presented. The results aregraphs of translative buoy motion in two dimensions, and rotationalmotion about two different axes. The vertical buoy motion forthe studied sequence is in the range of ±0.5 m.

Place, publisher, year, edition, pages
Honolulu, Hawaii: , 2009
National Category
Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-113302 (URN)10.1115/OMAE2009-79121 (DOI)
Conference
ASME 2009 28th International Conference on Ocean, Offshore and Arctic Engineering (OMAE2009) May 31–June 5, 2009 , Honolulu, Hawaii, USA
Available from: 2010-01-26 Created: 2010-01-26 Last updated: 2014-04-29Bibliographically approved
3. Design proposal of electrical system for linear generator wave power plants
Open this publication in new window or tab >>Design proposal of electrical system for linear generator wave power plants
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2009 (English)In: 35TH ANNUAL CONFERENCE OF IEEE INDUSTRIAL ELECTRONICS, IEEE , 2009, 4180-4185 p.Conference paper, Published paper (Refereed)
Abstract [en]

This paper describes an electrical system layout for a wave power plant connecting linear generators to the grid. The electrical power out from the wave energy converters must be converted before they can be connected to the grid. The conversion is carried out in marine substations that will be placed on the seabed.

The paper presents experimental power data from a wave energy converter that has been in operation at the Lysekil research site since March 2006. Moreover, results and analyses from experiments and simulations from tests with the generator connected to a rectifier and filter are presented. A simulation is made to show the difference between having the generator connected to a linear load and a nonlinear load, which would be the case when the generator is connected to the grid.

Place, publisher, year, edition, pages
IEEE, 2009
Keyword
electrical system layout, linear generator wave power plants, marine substations, wave energy converters
National Category
Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-112956 (URN)10.1109/IECON.2009.5414903 (DOI)000280762001321 ()978-1-4244-4648-3 (ISBN)978-1-4244-4650-6 (ISBN)
Conference
35th Annual Conference of the IEEE-Industrial-Electronics-Society (IECON 2009), Porto, PORTUGAL, NOV 03-05, 2009
Available from: 2010-01-22 Created: 2010-01-22 Last updated: 2016-04-14Bibliographically approved
4. Description of a torus shaped buoy for wave energy point absorber
Open this publication in new window or tab >>Description of a torus shaped buoy for wave energy point absorber
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2010 (English)Conference paper, Published paper (Refereed)
National Category
Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-142504 (URN)
Conference
Renewable Energy 2010, 27 June - 2 July, Pacifico Yokohama, Japan
Available from: 2011-01-14 Created: 2011-01-14 Last updated: 2015-01-07Bibliographically approved
5. Catch the wave to electricity: The Conversion of Wave Motions to Electricity Using a Grid-Oriented Approach
Open this publication in new window or tab >>Catch the wave to electricity: The Conversion of Wave Motions to Electricity Using a Grid-Oriented Approach
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2009 (English)In: IEEE Power and Energy Magazine, ISSN 1540-7977, Vol. 7, no 1, 50-54 p.Article in journal (Refereed) Published
Abstract [en]

The ocean are largely an untapped source of energy. However, compared to other energies, power fluctuations for ocean waves are small over longer periods of time. This paper present a grid-oriented approach to electricity production from ocean waves, utilizing a minimal amount of mechanical components.

National Category
Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-112949 (URN)10.1109/MPE.2008.930658 (DOI)000262015100004 ()
Available from: 2010-01-22 Created: 2010-01-22 Last updated: 2017-01-25Bibliographically approved
6. Lysekil Research Site, Sweden: A status update
Open this publication in new window or tab >>Lysekil Research Site, Sweden: A status update
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2011 (English)In: 9th European Wave and Tidal Energy Conference, Southampton, UK, 2011, 2011Conference paper, Published paper (Refereed)
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Research subject
Engineering Science with specialization in Science of Electricity
Identifiers
urn:nbn:se:uu:diva-160039 (URN)
Conference
9th European Wave and Tidal Energy Conference, Southampton, UK, 5-9 September 2011
Available from: 2011-10-13 Created: 2011-10-13 Last updated: 2017-01-25
7. Theory and Simulations of an End Stop Solution in a Linear Wave Power Generator
Open this publication in new window or tab >>Theory and Simulations of an End Stop Solution in a Linear Wave Power Generator
(English)Manuscript (preprint) (Other academic)
National Category
Other Civil Engineering
Identifiers
urn:nbn:se:uu:diva-207205 (URN)
Available from: 2013-09-10 Created: 2013-09-10 Last updated: 2014-09-18Bibliographically approved
8. Toroidal Buoy for Point Absorver WEC with Focus on Added Mass and Overloads
Open this publication in new window or tab >>Toroidal Buoy for Point Absorver WEC with Focus on Added Mass and Overloads
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(English)In: Article in journal (Refereed) Submitted
Abstract [en]

In order for a wave energy converter (WEC) to produce electricity at competitive prices, the generator must not be over dimensioned in order to save money on production, transport and installation, but the WEC, in this case a point absorber, must also be dimensioned to withstand harsh sea states. High added mass will in some cases create severe inertia forces on the generator and might shorten the service life of the system. The magnitude of the overload forces can be reduced by choosing a buoy geometry with small added mass. Measurements of snap loads produced by a toroidal buoy and calculations of buoy intertia forces causing overloads on a generator with a toroidal buoy and an equal generator with a cylindrical buoy are investigated in order to compare the two shapes numerically. A toroidal buoy was found to have less added mass than a vertical cylindrical buoy with similar excitation force, which caused reduced overloads at short wave periods, thus reducing the overloads on the generator. Snap loads were found to not cause much greater loads than other forces found during operation.

Keyword
WEC, point absorber, toroidal, torus; buoy, overload, survivability, wave, energy, converter
National Category
Energy Engineering
Identifiers
urn:nbn:se:uu:diva-220341 (URN)
Available from: 2014-03-13 Created: 2014-03-13 Last updated: 2014-09-18
9. Power Production by Linear Wave Energy Converters of Point Absorber Type with Vertical Cylindrical Buoys during Various Sea States
Open this publication in new window or tab >>Power Production by Linear Wave Energy Converters of Point Absorber Type with Vertical Cylindrical Buoys during Various Sea States
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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
wave power, point absorber, WEC, experiment, efficiency, energy, buoy
National Category
Energy Engineering
Identifiers
urn:nbn:se:uu:diva-220342 (URN)
Available from: 2014-03-13 Created: 2014-03-13 Last updated: 2014-12-11
10. Measurements of Extreme Forces on a Wave Energy Converter of Point Absorber Type and Estimation of Added Mass of Cylindrical Buoy
Open this publication in new window or tab >>Measurements of Extreme Forces on a Wave Energy Converter of Point Absorber Type and Estimation of Added Mass of Cylindrical Buoy
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2014 (English)In: 33Rd International Conference On Ocean, Offshore And Arctic Engineering, 2014, Vol 9B: Ocean Renewable Energy, ASME Press, 2014Conference paper, Published paper (Refereed)
Abstract [en]

A wave energy converter (WEC) of point absorber type is tested at the west coast of Sweden. The buoy is a vertical cylinder. The linear generator on the seabed has limited stroke length. Large waves cause the generator to reach its maximum stroke length. As this happen, a spring in the generator is compressed, causing the buoy to instantly come to rest. During this process the force between the buoy and the generator is measured. Also the acceleration of the buoy is measured. This process and the extreme forces on the generator hull is described and the study shows that the magnitude of this force is greatly influenced by the added mass of the buoy and thus the buoy geometry. The ratio between the extreme forces on the hull and the forces during normal operation will affect the dimensioning and economy of the WEC. Force acting between generator and buoy were measured during various events as the WEC was operating. Added mass was estimated from the measurements

Place, publisher, year, edition, pages
ASME Press, 2014
Keyword
added mass, buoy, wave, energy, converter, point, absorber, cylindrical, heave
National Category
Energy Engineering
Identifiers
urn:nbn:se:uu:diva-220343 (URN)000363499100037 (DOI)978-0-7918-4554-7 (ISBN)
Conference
33rd International Conference on Ocean, Offshore and Arctic Engineering
Available from: 2014-03-13 Created: 2014-03-13 Last updated: 2016-01-12Bibliographically approved

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