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Modelling and Experimental Verification of Direct Drive Wave Energy Conversion: Buoy-Generator Dynamics
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
2007 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis is focused on development of models and modelling of a wave energy converter in operation. Through the thesis linear potential wave theory has been used to describe the wave-buoy interaction. The differences lie in the generator models, in the simplest model the generator is a mechanical damper characterized by a damping factor. In the most advanced generator model the magnetic fields is calculated the by a FE-method, which gives detailed description of the electric properties and the effect it has on the buoy dynamics. Moreover, an equivalent circuit description of the generator has been tested. It has the same accuracy as the field based model but with a strongly enhanced CPU time. All models are verified against full scale experiments. The models are intended to be used for design of the next generation wave energy converters. Further, the developed models have also been used to study what effect buoy geometry and generator damping have on the ability to energy absorption.

In the spring 2006 a full scale wave energy converter was installed at the west coast of Sweden. It was in operation and collected data during three months. During that period the load resistance was varied in order to study the effect on the energy absorption. These collected data was then used in the verification of the developed models.

In the year 2002 a wave energy project started at Uppsala University; this work is a part of that larger project which intendeds to develop a viable wave energy conversion concept.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis , 2007. , p. 76
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 287
Keywords [en]
Engineering physics, Wave energy, Potential wave theory, Linear generator, Simulations, Experiments, Point absorber, FEM, Equivalent circuit theory
Keywords [sv]
Teknisk fysik
National Category
Engineering and Technology
Identifiers
URN: urn:nbn:se:uu:diva-7785ISBN: 978-91-554-6850-7 (print)OAI: oai:DiVA.org:uu-7785DiVA, id: diva2:169996
Public defence
2007-04-13, Polhemsalen, Ångströmlaboratoriet, Uppsala, 10:00 (English)
Opponent
Supervisors
Available from: 2007-03-23 Created: 2007-03-23 Last updated: 2010-11-10Bibliographically approved
List of papers
1. Study of a Longitudinal Flux Permanent Magnet Linear Generator for Wave Energy Converters
Open this publication in new window or tab >>Study of a Longitudinal Flux Permanent Magnet Linear Generator for Wave Energy Converters
2006 (English)In: International journal of energy research (Print), ISSN 0363-907X, E-ISSN 1099-114X, Vol. 30, no 14, p. 1130-1145Article in journal (Refereed) Published
Abstract [en]

A directly coupled linear permanent magnet generator of longitudinal flux-type is investigated. The generator will be used for power take-off in a wave energy converter. A combined field- and circuit model, solved by a time stepping finite element technique, is used to model and analyse the electromagnetic behaviour of the machine. A large number of simulations form the basis of a design study where the influence of armature current level, number of cables per slot, and pole width is investigated with respect to efficiency, generator size, and the load angle. A case study is performed for a chosen generator design. The electromagnetic behaviour is examined both for nominal load and for overloads. The generator has a nominal output power of 10 kW for a constant piston speed of 0.7 m s(-1). The electromagnetic efficiency at nominal load is 86.0%, the load angle 6.6 degrees, and the power fluctuation 1.3%. At 300% overload the load angle barely exceeds 12 degrees and the cable temperature is below 25 degrees C provided that the stator back is thermally connected to the sea water. The numerical calculations have been verified for small speeds by experiments.

Keywords
design methodology, finite element, linear synchronous generators, permanent magnet, wave energy converter
National Category
Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-95676 (URN)10.1002/er.1209 (DOI)000242188900002 ()
Available from: 2007-03-23 Created: 2007-03-23 Last updated: 2017-12-14Bibliographically approved
2. An electrical approach to wave energy conversion
Open this publication in new window or tab >>An electrical approach to wave energy conversion
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2006 (English)In: Renewable energy, ISSN 0960-1481, E-ISSN 1879-0682, no 31, p. 1309-1319Article in journal (Refereed) Published
Abstract [en]

Motions in nature, for example ocean waves, can play a significant role in tomorrow's electricity production, but the constructions require adaptations to its media. Engineers planning hydropower plants have always taken natural conditions, such as fall height, speed of flow, and geometry, as basic design parameters and constraints in the design. The present paper describes a novel approach for electric power conversion of the vast ocean wave energy. The suggested linear electric energy converter is adapted to the natural wave motion using straightforward technology. Extensive simulations of the wave energy concept are presented, along with results from the experimental setup of a multisided permanent magnet linear generator. The prototype is designed through systematic electromagnetic field calculations. The experimental results are used for the verification of measurements in the design process of future full-scale direct wave energy converters. The present paper, describes the energy conversion concept from a system perspective, and also discusses the economical and some environmental considerations for the project.

National Category
Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-95677 (URN)10.1016/j.renene.2005.07.009 (DOI)
Available from: 2007-03-23 Created: 2007-03-23 Last updated: 2017-12-14Bibliographically approved
3. Time stepping finite element analysis of variable speed synchronous generator with rectifier
Open this publication in new window or tab >>Time stepping finite element analysis of variable speed synchronous generator with rectifier
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2006 (English)In: Applied Energy, ISSN 0306-2619, E-ISSN 1872-9118, Vol. 83, no 4, p. 371-386Article in journal (Refereed) Published
Abstract [en]

Several applications for renewable energy conversion make use of variable speed generators. A conversion from variable frequency to grid frequency is therefore essential. One part of the converter is a rectifier. A rectifier model is presented, which is integrated in a time stepping finite element simulation environment where the generator and circuit equations are solved simultaneously. The model handles bidirectional alternator speeds as the application is a linear generator for ocean wave energy conversion. The rectifier model is extended with a load model, consisting of R, L and E, and simulations show what impact the rectifier has on the generator’s behaviour.

National Category
Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-95678 (URN)10.1016/j.apenergy.2004.10.016 (DOI)
Available from: 2007-03-23 Created: 2007-03-23 Last updated: 2017-12-14Bibliographically approved
4. Dynamics of a Linear Generator for Wave Energy Conversion
Open this publication in new window or tab >>Dynamics of a Linear Generator for Wave Energy Conversion
2004 (English)Conference paper, Published paper (Refereed)
National Category
Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-95679 (URN)
Conference
23rd International Conference on Offshore Mechanics and Arctic Engineering, Vancouver, Canada, 20-25 June
Available from: 2007-03-23 Created: 2007-03-23 Last updated: 2013-07-26Bibliographically approved
5. Hydrodynamic Modelling of a Direct Drive Wave Energy Converter
Open this publication in new window or tab >>Hydrodynamic Modelling of a Direct Drive Wave Energy Converter
2005 (English)In: International Journal of Engineering Science, no 43, p. 1377-1387Article in journal (Refereed) Published
National Category
Engineering and Technology
Research subject
Engineering Science with specialization in Science of Electricity
Identifiers
urn:nbn:se:uu:diva-95680 (URN)
Available from: 2007-03-23 Created: 2007-03-23 Last updated: 2016-04-22
6. Theory and Experiment on an Elastically Moored Cylindrical Buoy
Open this publication in new window or tab >>Theory and Experiment on an Elastically Moored Cylindrical Buoy
2006 (English)In: IEEE Journal of Oceanic Engineering, ISSN 0364-9059, E-ISSN 1558-1691, Vol. 31, no 4, p. 959-963Article in journal (Refereed) Published
Abstract [en]

In this paper, we compare simulated forces and accelerations for a moored floating buoy with full-scale experimental results in real ocean waves. The buoy is moored with a wire connected by springs to a concrete foundation situated at the seafloor. This study aims to develop a computer model using potential theory with a linearized free-surface boundary condition to describe the motion of such a system. The intention is to use the model for future study of wave-energy absorption and design of converters. Another objective is to see how complex a model is required to get accurate results. The method used is computationally fast and makes it possible to couple linear buoy wave interaction with nonlinear generator models, so that different loads and latching can be studied. A computationally fast method is required to model farms of wave-energy converters.

National Category
Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-95681 (URN)10.1109/JOE.2006.880387 (DOI)000244319300022 ()
Available from: 2007-03-23 Created: 2007-03-23 Last updated: 2017-12-14Bibliographically approved
7. Instantaneous Energy Flux in Fluid Gravity Waves
Open this publication in new window or tab >>Instantaneous Energy Flux in Fluid Gravity Waves
(English)In: Physical Review EArticle in journal (Refereed) Submitted
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Research subject
Engineering Science with specialization in Science of Electricity
Identifiers
urn:nbn:se:uu:diva-95682 (URN)
Available from: 2007-03-23 Created: 2007-03-23 Last updated: 2016-04-22
8. Experimental results from sea trials of an offshore wave energy system
Open this publication in new window or tab >>Experimental results from sea trials of an offshore wave energy system
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2007 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 90, no 3, p. 034105-Article in journal (Refereed) Published
Abstract [en]

A full-scale prototype of a wave power plant has been installed off the Swedish west coast and the overall wave energy converter concept has been verified. Initial results have been collected and significant insights discovered. Energy absorption dependency on load as well as output voltage and power is demonstrated. It is shown that great overload capability of the directly driven linear generator is critical, and indicated that, for resistive loads, optimal load does not vary with wave climate. Future grid supplying energy production would necessitate parks of wave power plants in order to reduce power fluctuations.

Keywords
High-current and high-voltage technology: power systems; power transmission lines and cables, Electric motors
National Category
Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-95683 (URN)10.1063/1.2432168 (DOI)000243582400105 ()
Available from: 2007-03-23 Created: 2007-03-23 Last updated: 2017-12-14Bibliographically approved
9. Wave power absorption: Experiments in open sea and simulation
Open this publication in new window or tab >>Wave power absorption: Experiments in open sea and simulation
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2007 (English)In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 102, no 8, p. 084910-084910-5Article in journal (Refereed) Published
Abstract [en]

A full scale prototype of a wave power plant based on a direct drive linear generator driven by a point absorber has been installed at the west coast of Sweden. In this paper, experimentally collected data of energy absorption for different electrical loads are used to verify a model of the wave power plant including the interactions of wave, buoy, generator, and external load circuit. The wave-buoy interaction is modeled with linear potential wave theory. The generator is modeled as a nonlinear mechanical damping function that is dependent on piston velocity and electric load. The results show good agreement between experiments and simulations. Potential wave theory is well suited for the modeling of a point absorber in normal operation and for the design of future converters. Moreover, the simulations are fast, which opens up for simulations of wave farms.

Keywords
Hydroelectric, hydrothermal, geothermal and wind power, Ocean energy extraction, Ocean waves and oscillations
National Category
Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-95684 (URN)10.1063/1.2801002 (DOI)000250589300146 ()
Available from: 2007-03-23 Created: 2007-03-23 Last updated: 2017-12-14Bibliographically approved
10. Simulation of a Linear Generator for Wave Power Absorption -Part I: Modeling
Open this publication in new window or tab >>Simulation of a Linear Generator for Wave Power Absorption -Part I: Modeling
In: IEEE Transaction on Energy ConversionArticle in journal (Refereed) Submitted
Identifiers
urn:nbn:se:uu:diva-95685 (URN)
Available from: 2007-03-23 Created: 2007-03-23Bibliographically approved
11. Simulation of a Linear Generator for Wave Power Absorption -Part II: Verification
Open this publication in new window or tab >>Simulation of a Linear Generator for Wave Power Absorption -Part II: Verification
Show others...
In: IEEE Transaction on Energy ConversionArticle in journal (Refereed) Submitted
Identifiers
urn:nbn:se:uu:diva-95686 (URN)
Available from: 2007-03-23 Created: 2007-03-23Bibliographically approved

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