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Experimental results on power absorption from a wave energy converter at the Lysekil wave energy research site
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
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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. 

Place, publisher, year, edition, pages
2015. Vol. 77, 9-14 p.
National Category
Engineering and Technology
Research subject
Engineering Science with specialization in Science of Electricity
Identifiers
URN: urn:nbn:se:uu:diva-238267DOI: 10.1016/j.renene.2014.11.050ISI: 000349504800002OAI: oai:DiVA.org:uu-238267DiVA: diva2:770676
Available from: 2014-12-11 Created: 2014-12-11 Last updated: 2017-12-05Bibliographically approved
In thesis
1. 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
2. Hydrodynamic Modelling of Wave Power using Electrical Equivalent Circuit Theory
Open this publication in new window or tab >>Hydrodynamic Modelling of Wave Power using Electrical Equivalent Circuit Theory
2014 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Ocean wave energy can be harnessed and converted into electric energy nowadays. This provides a possibility for populations that live on islands or along coastlines to utilize the renewable and safe power produced by ocean waves. Point absorbing wave energy converter (WEC) is one example of such devices for electrical power production from ocean waves. It is composed of a floating buoy on the water surface, and a linear generator that sits on seabed and is connected with the buoy via a line. Electricity is generated when the buoy moves up and down in the waves.

The geometry and dimensions of the floating buoy have dominant influences on the energy absorption. This thesis introduces an equivalent electric circuit for modelling the  hydrodynamic interaction between the wave and a cylindrical buoy. The model allows a rapid assessment of the velocity, force in the connection line and output power, by which the system design and optimization can be performed faster and easier.

The electric circuit model is based on the WEC's dynamic force analysis, and the electric components' parameters are determined from analytical approximations of the hydrodynamic coefficients. The simulation results of the equivalent circuit for one typical wave climate in Lysekil has been presented, and the results indicate a good fitting with former experimental results.

The thesis also includes a hydrodynamic study for a torus shaped buoy, which aims at  applying a theoretical background for a force measurement experiment. A comparison has been conducted between the torus buoy and two similar cylindrical buoys. Preliminary WAMIT simulation results demonstrate that the force in the connection line will be 5% bigger by using the torus buoy. It is also found that the torus buoy is advantageous for its larger excitation force and smaller added mass. A brief introduction of the 500kN force measurement system and the communication test have been introduced as well.

Place, publisher, year, edition, pages
Uppsala universitet, 2014. 50 p.
Series
UURIE / Uppsala University, Department of Engineering Sciences, ISSN 0349-8352
Keyword
wave energy, hydrodynamic modelling, electric circuit, force and power analysis
National Category
Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-238264 (URN)
Presentation
2014-04-16, Å4001, 14:15 (English)
Opponent
Supervisors
Funder
Swedish Research CouncilSwedish Energy Agency
Available from: 2014-12-15 Created: 2014-12-11 Last updated: 2015-10-26Bibliographically approved
3. Modelling Wave Power by Equivalent Circuit Theory
Open this publication in new window or tab >>Modelling Wave Power by Equivalent Circuit Theory
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The motion of ocean waves can be captured and converted into usable electricity. This indicates that wave power has the potential to supply electricity to grids like wind or solar power. A point absorbing wave energy converter (WEC) system has been developed for power production at Uppsala University. This system contains a semi-submerged buoy on the water surface driving a linear synchronous generator placed on the seabed. The concept is to connect many small units together, to form a wave farm for large-scale electricity generation.

A lot of effort has gone into researching how to enhance the power absorption from each WEC unit. These improvements are normally done separately for the buoy, the generator or the electrical system, due to the fact that modelling the dynamic behavior of the entire WEC system is complicated and time consuming. Therefore, a quick, yet simple, assessment tool is needed. 

This thesis focuses on studying the use of the equivalent circuit as a WEC system modelling tool. Based on the force analysis, the physical elements in an actual WEC system can be converted into electrical components. The interactions between the regular waves, the buoy, and the Power Take-off mechanism can be simulated together in one circuit network. WEC performance indicators like the velocity, the force, and the power can be simulated directly from the circuit model. Furthermore, the annual absorbed electric energy can be estimated if the wave data statistics are known.

The linear and non-linear equivalent circuit models developed in this thesis have been validated with full scale offshore experimental results. Comparisons indicate that the simplest linear circuit can predict the absorbed power reasonably well, while it is not so accurate in estimating the peak force in the connection line. The non-linear circuit model generates better estimations in both cases. To encourage researchers from different backgrounds to adapt and apply the circuit model, an instruction on how to establish a non-linear equivalent circuit model is supplied, as well as on how to apply the model to accelerate the decision making process when planning a WEC system.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2015. 75 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1309
Keyword
Wave energy, hydrodynamics, electric circuit, electrical analogy, energy absorption, force, system modelling, Simulink, engineering science, renewable energy
National Category
Engineering and Technology
Research subject
Engineering Science with specialization in Science of Electricity
Identifiers
urn:nbn:se:uu:diva-265270 (URN)978-91-554-9390-5 (ISBN)
Public defence
2015-12-11, Häggsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 13:15 (English)
Opponent
Supervisors
Funder
Swedish Energy AgencyStandUpSwedish Research Council, KOF11 2011-6312Swedish Research Council, 621-2009-3417
Available from: 2015-11-19 Created: 2015-10-26 Last updated: 2016-01-13
4. Theoretical and Experimental Analysis of Operational Wave Energy Converters
Open this publication in new window or tab >>Theoretical and Experimental Analysis of Operational Wave Energy Converters
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis studies wave energy converters developed at Uppsala University. The wave energy converters are of point absorbing type with direct driven linear generators. The aim has been to study generator design with closed stator slots as well as offshore experimental studies.

By closing the stator slots, the harmonic content in the magnetic flux density is reduced and as a result the cogging forces in the generator are reduced as well. By reducing these forces, the noise and vibrations from the generator can be lowered. The studies have shown a significant reduction in the cogging forces in the generator. Moreover, by closing the slots, the magnetic flux finds a short-cut through the closed slots and will lower the magnetic flux linking the windings.

The experimental studies have focused on the motion of the translator. The weight of the translator has a significant impact on the power absorption, especially in the downward motion. Two different experiments have been studied with two different translator weights. The results show that with a higher translator weight the power absorption is more evenly produced between the upward and downward motion as was expected from the simulation models. Furthermore, studies on the influence of the changing active area have been conducted which show some benefits with a changing active area during the downward motion. The experimental results also indicate snatch-loads for the wave energy converter with a lower translator weight.

Within this thesis results from a comparative study between two WECs with almost identical properties have been presented. The generators electrical properties and the buoy volumes are the same, but with different buoy heights and diameters. Moreover, experimental studies including the conversion from AC to DC have been achieved.

The work in this thesis is part of a larger wave power project at Uppsala University. The project studies the whole process from the energy absorption from the waves to the connection to the electrical grid. The project has a test-site at the west coast of Sweden near the town of Lysekil, where wave energy systems have been studied since 2004.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2016. 67 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1339
Keyword
ocean wave energy, WEC, permanent magnet, linear generator, closed stator slots, offshore experiments
National Category
Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-274635 (URN)978-91-554-9460-5 (ISBN)
Public defence
2016-03-11, Häggsalen, Ångström, Lägerhyddsvägen 1, Uppsala, 13:00 (English)
Opponent
Supervisors
Available from: 2016-02-18 Created: 2016-01-24 Last updated: 2016-03-09

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Output format
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