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Buoy and Generator Interaction with Ocean Waves: Studies of a Wave Energy Conversion System
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
2011 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

On March 13th, 2006, the Division of Electricity at Uppsala University deployed its first wave energy converter, L1, in the ocean southwest of Lysekil. L1 consisted of a buoy at the surface, connected through a line to a linear generator on the seabed. Since the deployment, continuous investigations of how L1 works in the waves have been conducted, and several additional wave energy converters have been deployed.

This thesis is based on ten publications, which focus on different aspects of the interaction between wave, buoy, and generator. In order to evaluate different measurement systems, the motion of the buoy was measured optically and using accelerometers, and compared to measurements of the motion of the movable part of the generator - the translator. These measurements were found to correlate well. Simulations of buoy and translator motion were found to match the measured values.

The variation of performance of L1 with changing water levels, wave heights, and spectral shapes was also investigated. Performance is here defined as the ratio of absorbed power to incoming power. It was found that the performance decreases for large wave heights. This is in accordance with the theoretical predictions, since the area for which the stator and the translator overlap decreases for large translator motions. Shifting water levels were predicted to have the same effect, but this could not be seen as clearly.

The width of the wave energy spectrum has been proposed by some as a factor that also affects the performance of a wave energy converter, for a set wave height and period. Therefore the relation between performance and several different parameters for spectral width was investigated. It was found that some of the parameters were in fact correlated to performance, but that the correlation was not very strong.

As a background on ocean measurements in wave energy, a thorough literature review was conducted. It turns out that the Lysekil project is one of quite few projects that have published descriptions of on-site wave energy measurements.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis , 2011. , 52 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 872
Keyword [en]
Wave power, Measurement systems, Marine technology, Energy conversion, Renewable energy, Energy absorption, Wave resource, Oceanic engineering, Linear generators, Point absorbers, Sea trials, Camera systems, Accelerometers, Offshore experiments
National Category
Energy Engineering Marine Engineering Energy Systems Ocean and River Engineering
Research subject
Engineering Science with specialization in Science of Electricity
Identifiers
URN: urn:nbn:se:uu:diva-160085ISBN: 978-91-554-8192-6 (print)OAI: oai:DiVA.org:uu-160085DiVA: diva2:448957
Public defence
2011-12-02, Häggsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 13:15 (English)
Opponent
Supervisors
Available from: 2011-11-11 Created: 2011-10-14 Last updated: 2012-01-09Bibliographically approved
List of papers
1. Wave Energy from the North Sea: Experiences from the Lysekil Research Site
Open this publication in new window or tab >>Wave Energy from the North Sea: Experiences from the Lysekil Research Site
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2008 (English)In: Surveys in geophysics, ISSN 0169-3298, E-ISSN 1573-0956, Vol. 29, no 3, 221-240 p.Article, review/survey (Refereed) Published
Abstract [en]

This paper provides a status update on the development of the Swedish wave energy research area located close to Lysekil on the Swedish West coast. The Lysekil project is run by the Centre for Renewable Electric Energy Conversion at Uppsala University. The project was started in 2004 and currently has permission to run until the end of 2013. During this time period 10 grid-connected wave energy converters, 30 buoys for studies on environmental impact, and a surveillance tower for monitoring the interaction between waves and converters will be installed and studied. To date the research area holds one complete wave energy converter connected to a measuring station on shore via a sea cable, a Wave Rider™ buoy for wave measurements, 25 buoys for studies on environmental impact, and a surveillance tower. The wave energy converter is based on a linear synchronous generator which is placed on the sea bed and driven by a heaving point absorber at the ocean surface. The converter is directly driven, i.e. it has no gearbox or other mechanical or hydraulic conversion system. This results in a simple and robust mechanical system, but also in a somewhat more complicated electrical system.

Keyword
Wave power, Renewable energy, Sea trial, Linear generator, Point absorber, Environmental impact
National Category
Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-107215 (URN)10.1007/s10712-008-9047-x (DOI)000260967900002 ()
Available from: 2009-07-29 Created: 2009-07-29 Last updated: 2015-02-03Bibliographically approved
2. 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
3. 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
4. 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
5. Wave Power Absorption as a Function of Water Level and Wave Height: Theory and Experiment
Open this publication in new window or tab >>Wave Power Absorption as a Function of Water Level and Wave Height: Theory and Experiment
2010 (English)In: IEEE Journal of Oceanic Engineering, ISSN 0364-9059, Vol. 35, no 3, 558-564 p.Article in journal (Refereed) Published
Abstract [en]

This paper investigates the sensitivity of a wave power system to variations in still water levels and significant wave heights. The system consists of a floating point absorber connected to a linear generator on the seabed. Changing still water levels are expected to affect the power absorption, since they will displace the equilibrium position for the generator translator. Similarly, changing significant wave heights will affect the rate at which the translator leaves the stator. Both these effects will in some cases result in a smaller active area of the stator. A theoretical expression to describe this effect is derived, and compared to measured experimental values for the wave energy converter at the Lysekil research site. During the time of measurements, the still water levels at the site were in the range of [-0.70 m, +0.46 m], and the significant wave heights in the range of [0 m, 2.7 m]. The experimental values exhibit characteristics similar to those of the theoretical expression, especially with changing significant wave heights.

Keyword
Energy capture, experimental results, linear generator, power absorption, wave power
National Category
Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-97852 (URN)10.1109/JOE.2010.2052692 (DOI)000283226500008 ()
Available from: 2012-01-09 Created: 2008-11-21 Last updated: 2012-01-09Bibliographically approved
6. 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
7. Wave Buoy and Translator Motions - On-Site Measurements and Simulations
Open this publication in new window or tab >>Wave Buoy and Translator Motions - On-Site Measurements and Simulations
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2011 (English)In: IEEE Journal of Oceanic Engineering, ISSN 0364-9059, E-ISSN 1558-1691, Vol. 36, no 3, 377-385 p.Article in journal (Refereed) Published
Abstract [en]

For a complete understanding of a wave energy conversion device, it is important to know how the proposed device moves in the water, how this motion can be measured, and to what extent the motion can be predicted or simulated. The magnitude and character of the motion has impacts on engineering issues and optimization of control parameters, as well as the theoretical understanding of the system. This paper presents real sea measurements of buoy motion and translator motion fora wave energy system using a linear generator. Buoy motion has been measured using two different systems: a land-based optical system and a buoy-based accelerometer system. The data have been compared to simulations from a Simulink model for the entire system. The two real sea measurements of buoy motion have been found to correlate well in the vertical direction, where the measured range of motion and the standard deviation of the position distributions differed with 3 and 4 cm, respectively. The difference in the horizontal direction ismore substantial. The main reason for this is that the buoy rotation about its axis of symmetry was not measured. However, used together the two systems give a good understanding of buoy motion. In a first comparison, the simulations show good agreement with the measured motion for both translator and buoy.

Keyword
Accelerometers, energy conversion, experimental results, image motion analysis, oceanic engineering, marine technology, wave power
National Category
Energy Engineering
Research subject
Engineering Science with specialization in Science of Electricity
Identifiers
urn:nbn:se:uu:diva-160072 (URN)10.1109/JOE.2011.2136970 (DOI)
Available from: 2012-01-09 Created: 2011-10-14 Last updated: 2017-12-08Bibliographically approved
8. 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
9. Offshore wave power measurements: a review
Open this publication in new window or tab >>Offshore wave power measurements: a review
2011 (English)In: Renewable & sustainable energy reviews, ISSN 1364-0321, E-ISSN 1879-0690, Vol. 15, no 9, 4274-4285 p.Article in journal (Refereed) Published
Abstract [en]

The first wave power patent was filed in 1799. Since then, hundreds of ideas for extraction of energy from ocean waves have surfaced. In the process of developing a concept, it is important to learn from previous successes and failures, and this is not least important when moving into the ocean. In this paper, a review has been made with the purpose of finding wave power projects that have made ocean trials, and that also have reported what has been measured during the trials, and how it has been measured.

In relation to how many projects have done work on wave power, surprisingly few have reported on such measurements. There can be many reasons for this, but one is likely the great difficulties in working with experiments in an ocean environment. Many of the projects have reported on sensor failures, unforeseen events, and other general problems in making measurements at sea.

The most common site measurement found in this review was wave height. Such measurements was almost universal, although the technologies used differed somewhat. The most common device measurements were electric voltages and/or currents and system pressures (air and water). Device motion and mooring forces were also commonly measured. The motion measurements differed the most between the projects, and many varying methods were used, such as accelerometers, wire sensors, GPS systems, optical systems and echo sounders.

Place, publisher, year, edition, pages
Elsevier, 2011
Keyword
Wave power, Measurement system, Offshore
National Category
Energy Engineering
Research subject
Engineering Science with specialization in Science of Electricity
Identifiers
urn:nbn:se:uu:diva-160081 (URN)10.1016/j.rser.2011.07.123 (DOI)000298764400006 ()
Available from: 2012-01-09 Created: 2011-10-14 Last updated: 2017-12-08Bibliographically approved
10. Spectral Parameters and Wave Energy Converter Performance
Open this publication in new window or tab >>Spectral Parameters and Wave Energy Converter Performance
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Wave energy converter performance is typically specified for given values of significant wave height Hm0 and energy period T-10, in a matrix. For such a representation to be valid, it must be assumed that two parameters determine the performance of the wave energy converter satisfactorily. However, studying performance data for the wave energy converter L1 at the Lysekil research site, it can be seen that for measurements with similar values of Hm0 and T-10, there are values of relative absorption that range from 5 % to 25 %. It therefore seems probable that the properties of the sea state that are not captured using Hm0 and T-10 have an effect on how a WEC performs. It has been proposed in the literature that the width of the wave spectrum is one such property. In this paper, six parameters that describe spectral width have been tested against performance data to look for correlations. In addition to this, the performance data was tested against peak period and standard deviation of peak frequency, as found through wavelet analysis. Out of the parameters tested, κ and ε1 displayed the strongest correlation with relative absorption. Even this correlation was not very strong however, and did only exhibit an r2-value of 0.39 in a linear fit for L1 connected to a 4.9 Ω load. It was also found that the lowest absorption values were connected to the lowest and highest values for peak period.

National Category
Energy Engineering
Research subject
Engineering Science with specialization in Science of Electricity
Identifiers
urn:nbn:se:uu:diva-160084 (URN)
Available from: 2011-10-14 Created: 2011-10-14 Last updated: 2012-01-04

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