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Linear wave energy converter: Study of electromagnetic design
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity. (Wave Energy)ORCID iD: 0000-0002-4882-4151
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 [en]
Wave energy, permanent magnets, linear generator, closed stator slots, offshore experiments
National Category
Engineering and Technology
Identifiers
URN: urn:nbn:se:uu:diva-237522OAI: oai:DiVA.org:uu-237522DiVA: diva2:768226
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
List of papers
1. 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
2. Experimental results of rectification and filtration from an offshore wave energy system
Open this publication in new window or tab >>Experimental results of rectification and filtration from an offshore wave energy system
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2009 (English)In: Renewable energy, ISSN 0960-1481, E-ISSN 1879-0682, Vol. 34, no 5, 1381-1387 p.Article in journal (Refereed) Published
Abstract [en]

The present paper presents results from a wave energy conversion that is based on a direct drive linear generator. The linear generator is placed on the seabed and connected to a buoy via a rope. Thereby, the natural wave motion is transferred to the translator by the buoy motion. When using direct drive generators, voltage and current output will have varying frequency and varying amplitude and the power must be converted before a grid connection. The electrical system is therefore an important part to study in the complete conversion system from wave energy to grid connected power. This paper will bring up the first steps in the conversion: rectification and filtration of the power. Both simulation studies and offshore experiments have been made. The results indicate that this kind of system works in a satisfactory way and a smooth DC power can be achieved with one linear generator.

Keyword
Wave energy conversion, Electric rectifiers, Energy conversion, Experiments, Porous materials, Power takeoffs, Takeoff
National Category
Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-112941 (URN)10.1016/j.renene.2008.09.010 (DOI)000263608500026 ()
Available from: 2010-01-22 Created: 2010-01-22 Last updated: 2017-12-12Bibliographically approved
3. Study of aWave Energy Converter Connected to a Nonlinear Load
Open this publication in new window or tab >>Study of aWave Energy Converter Connected to a Nonlinear Load
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2009 (English)In: IEEE Journal of Oceanic Engineering, ISSN 0364-9059, E-ISSN 1558-1691, Vol. 34, no 2, 123-127 p.Article in journal (Refereed) Published
Abstract [en]

This paper presents experimental results from a wave energy converter (WEC) that is based on a linear generator connected to a rectifier and filter components. The converter-filter system is installed onshore, while the linear wave generator operates offshore a few kilometers from the Swedish west coast. The power from the generator has been rectified with a diode bridge and then filtered using a capacitive filter. Performance of the whole conversion system was studied using resistive loads connected across the filter. The aim was to investigate the operational characteristics of the generator while supplying a nonlinear load. By changing the value of the resistive component of the load, the speed of the translator can be changed and so also the damping of the generator. The power absorbed by the generator was studied at different sea states as well. The observations presented in this paper could be beneficial for the design of efficient wave energy conversion systems.

Keyword
ocean waves
National Category
Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-112947 (URN)10.1109/JOE.2009.2015021 (DOI)000266245600004 ()
Available from: 2010-01-22 Created: 2010-01-22 Last updated: 2017-12-12Bibliographically approved
4. Experimental Results From an Offshore Wave Energy Converter
Open this publication in new window or tab >>Experimental Results From an Offshore Wave Energy Converter
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2010 (English)In: Journal of Offshore Mechanics and Arctic Engineering-Transactions of The Asme, ISSN 0892-7219, E-ISSN 1528-896X, Vol. 132, no 4, 041103- p.Article in journal (Refereed) Published
Abstract [en]

An offshore wave energy converter (WEC) was successfully launched at the Swedish west coast in the middle of March 2006. The WEC is based on a permanent magnet linear generator located on the sea floor driven by a point absorber. A measuring station has been installed on a nearby island where all measurements and experiments on the WEC have been carried out. The output voltage from the generator fluctuates both in amplitude and frequency and must therefore be converted to enable grid connection. In order to study the voltage conversion, the measuring station was fitted with a six pulse diode rectifier and a capacitive filter during the autumn of 2006. The object of this paper is to present a detailed description of the Lysekil research site. Special attention will be given to the power absorption by the generator when it is connected to a nonlinear load.

Keyword
ocean wave power, linear generators, conversion systems, experimental results
National Category
Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-133599 (URN)10.1115/1.4001443 (DOI)000283325300003 ()
Available from: 2010-11-15 Created: 2010-11-11 Last updated: 2017-12-12Bibliographically approved
5. 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
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. Detailed Study of Closed Stator Slots for a Direct-Driven Synchronous Permanent Magnet Linear Wave Energy Converter
Open this publication in new window or tab >>Detailed Study of Closed Stator Slots for a Direct-Driven Synchronous Permanent Magnet Linear Wave Energy Converter
2014 (English)In: Machines, Vol. 2, no 1, 73-86 p.Article in journal (Refereed) Published
Abstract [en]

The aim of this paper is to analyze how a permanent magnet linear generator for wave power behaves when the stator slots are closed. The usual design of stator geometry is to use open slots to maintain a low magnetic leakage flux between the stator teeth. By doing this, harmonics are induced in the magnetic flux density in the air-gap due to slotting. The closed slots are designed to cause saturation, to keep the permeability low. This reduces the slot harmonics in the magnetic flux density, but will also increase the flux leakage between the stator teeth. An analytical model has been created to study the flux through the closed slots and the result compared with finite element simulations. The outcome shows a reduction of the cogging force and a reduction of the harmonics of the magnetic flux density in the air-gap. It also shows a small increase of the total magnetic flux entering the stator and an increased magnetic flux leakage through the closed slots.

National Category
Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-237484 (URN)10.3390/machines2010073 (DOI)
Available from: 2014-12-03 Created: 2014-12-03 Last updated: 2016-03-09
8. Experimental results on power absorption from a wave energy converter at the Lysekil wave energy research site
Open this publication in new window or tab >>Experimental results on power absorption from a wave energy converter at the Lysekil wave energy research site
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2015 (English)In: Renewable energy, ISSN 0960-1481, E-ISSN 1879-0682, Vol. 77, 9-14 p.Article in journal (Refereed) Published
Abstract [en]

Power generation from wave power has a large potential to contribute to our electric energy production, and today, many wave power projects are close to be commercialized. However, one key issue to solve for many projects is to decrease the cost per installed kW. One way to do this is to investigate which parameters that have a significant impact on the wave energy converters (WEC) performance. In this paper, experimental results on power absorption from a directly driven point absorbing WEC are presented. The experiments have been carried out at the Lysekil research site in Sweden. To investigate the performance of the WEC, the absorbed power and the speed of the translator are compared. The result confirms that the buoy size and the translator weight have a large impact on the power absorption from the generator. By optimizing the buoy size and translator weight, the WEC is believed to produce power more evenly over the upward and downward cycle. Moreover, to predict the maximum power limit during normal operation, a simulation model has been derived. The results correlates well with experimental data during normal operation. 

National Category
Engineering and Technology
Research subject
Engineering Science with specialization in Science of Electricity
Identifiers
urn:nbn:se:uu:diva-238267 (URN)10.1016/j.renene.2014.11.050 (DOI)000349504800002 ()
Available from: 2014-12-11 Created: 2014-12-11 Last updated: 2017-12-05Bibliographically approved
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. A wave power unit
Open this publication in new window or tab >>A wave power unit
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2009 (English)Patent (Other (popular science, discussion, etc.))
National Category
Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-140112 (URN)
Patent
WO 2010/085188 (2010-07-29)
Available from: 2011-01-04 Created: 2011-01-04 Last updated: 2016-04-14Bibliographically approved
11. A Switchgear
Open this publication in new window or tab >>A Switchgear
2011 (English)Patent (Other (popular science, discussion, etc.))
National Category
Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-238262 (URN)
Patent
WO2011/031198
Available from: 2014-12-11 Created: 2014-12-11 Last updated: 2016-04-19

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