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Hydrodynamic Modelling for a Point Absorbing Wave Energy Converter
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
2011 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Surface gravity waves in the world’s oceans contain a renewable source of free power on the order of terawatts that has to this date not been commercially utilized. The division of Electricity at Uppsala University is developing a technology to harvest this energy. The technology is a point absorber type wave energy converter based on a direct-driven linear generator placed on the sea bed connected via a line to a buoy on the surface.

The work in this thesis is focused mainly on the energy transport of ocean waves and on increasing the transfer of energy from the waves to the generator and load. Potential linear wave theory is used to describe the ocean waves and to derive the hydrodynamic forces that are exerted on the buoy. Expressions for the energy transport in polychromatic waves travelling over waters of finite depth are derived and extracted from measured time series of wave elevation collected at the Lysekil test site. The results are compared to existing solutions that uses the simpler deep water approximation. A Two-Body system wave energy converter model tuned to resonance in Swedish west coast sea states is developed based on the Lysekil project concept. The first indicative results are derived by using a linear resistive load. The concept is further extended by a coupled hydrodynamic and electromagnetic model with two more realistic non-linear load conditions.

Results show that the use of the deep water approximation gives a too low energy transport in the time averaged as well as in the total instantaneous energy transport. Around the resonance frequency, a Two-Body System gives a power capture ratio of up to 80 percent. For more energetic sea states the power capture ratio decreases rapidly, indicating a smoother power output. The currents in the generator when using the Two-Body system is shown to be more evenly distributed compared to the conventional system, indicating a better utilization of the electrical equipment. Although the resonant nature of the system makes it sensitive to the shape of the wave spectrum, results indicate a threefold increase in annual power production compared to the conventional system.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis , 2011. , p. 91
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 878
Keyword [en]
Ocean wave energy, Point absorber, Wave energy converter, Wave energy transport, Polychromatic wave, Linear generator, Resonance, Finite depth, Modelling
National Category
Electrical Engineering, Electronic Engineering, Information Engineering Marine Engineering Oceanography, Hydrology and Water Resources
Research subject
Engineering Science with specialization in Electronics
Identifiers
URN: urn:nbn:se:uu:diva-160319ISBN: 978-91-554-8214-5 (print)OAI: oai:DiVA.org:uu-160319DiVA: diva2:450881
Public defence
2011-12-09, Häggsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 13:15 (English)
Opponent
Supervisors
Available from: 2011-11-17 Created: 2011-10-21 Last updated: 2018-01-12Bibliographically approved
List of papers
1. Offshore experiments on a direct-driven Wave Energy Converter
Open this publication in new window or tab >>Offshore experiments on a direct-driven Wave Energy Converter
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2007 (English)Conference paper, Published paper (Refereed)
National Category
Engineering and Technology
Research subject
Engineering Science with specialization in Science of Electricity
Identifiers
urn:nbn:se:uu:diva-14172 (URN)
Conference
Proceedings of the 7th European Wave and Tidal Energy Conference, 11-13 September 2007, Porto, Portugal.
Available from: 2008-04-25 Created: 2008-04-25 Last updated: 2016-04-22
2. Wave Climate off the Swedish West Coast
Open this publication in new window or tab >>Wave Climate off the Swedish West Coast
2009 (English)In: Renewable energy, ISSN 0960-1481, E-ISSN 1879-0682, Vol. 34, no 6, p. 1600-1606Article in journal (Refereed) Published
Abstract [en]

This paper presents and discusses the wave climate off the Swedish west coast It is based on 8 years (1997-2004) of wave data from 13 sites, nearshore and offshore, in the Skagerrak and Kattegat. The data is a product of the WAM and SWAN wave models calibrated at one site by a wave measurement buoy. It is found that the average energy flux is approximately 5.2 kW/m in the offshore Skagerrak, 2.8 kW/m in the nearshore Skagerrak, and 2.4 kW/m in the Kattegat. One of the studied sites, i.e. site 9, is the location of a wave energy research site run by the Centre for Renewable Electric Energy Conversion at Uppsala University. This site has had a wave power plant installed since the spring of 2006, and another seven are planned to be installed during 2008. Wave energy as a renewable energy source was the driving interest that led to this study and the results are briefly discussed from this perspective.

Keyword
Wave climate, Wave power, Sea state, Extreme waves, Skagerrak, Kattegat
National Category
Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-97856 (URN)10.1016/j.renene.2008.11.016 (DOI)000264306500024 ()
Available from: 2008-11-21 Created: 2008-11-21 Last updated: 2017-12-14Bibliographically approved
3. 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
4. Artificial reef effect and fouling impacts on offshore wave power foundations and buoys: a pilot study
Open this publication in new window or tab >>Artificial reef effect and fouling impacts on offshore wave power foundations and buoys: a pilot study
2009 (English)In: Estuarine, Coastal and Shelf Science, ISSN 0272-7714, E-ISSN 1096-0015, Vol. 82, no 3, p. 426-432Article in journal (Refereed) Published
Abstract [en]

Little is known about the effects of offshore energy installations on the marine environment, and further research could assist in minimizing environmental risks as well as in enhancing potential positive effects on the marine environment. While biofouling on marine energy conversion devices on one hand has the potential to be an engineering concern, these structures can also affect biodiversity by functioning as artificial reefs. The Lysekil Project is a test park for wave power located at the Swedish west coast. Here, buoys acting as point absorbers on the surface are connected to generators anchored on concrete foundations on the seabed. In this study we investigated the colonisation of foundations by invertebrates and fish, as well as fouling assemblages on buoys. We examined the influence of surface orientation of the wave power foundations on epibenthic colonisation, and made observations of habitat use by fish and crustaceans during three years of submergence. We also examined fouling assemblages on buoys and calculated the effects of biofouling on the energy absorption of the wave power buoys. On foundations we demonstrated a succession in colonisation over time with a higher degree of coverage on vertical surfaces. Buoys were dominated by the blue mussel Mytilus edulis. Calculations indicated that biofouling have no significant effect in the energy absorption on a buoy working as a point absorber. This study is the first structured investigation on marine organisms associated with wave power devices

Keyword
biodiversity, benthos, fish, shellfish, renewable energy, wave power, Sweden
National Category
Biological Sciences Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-107217 (URN)10.1016/j.ecss.2009.02.009 (DOI)000265573400007 ()
Available from: 2009-07-29 Created: 2009-07-29 Last updated: 2017-12-13Bibliographically 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, p. 50-54Article 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. Wave energy converter with enhanced amplitude response at frequencies coinciding with Swedish west coast sea states by use of a supplementary submerged body
Open this publication in new window or tab >>Wave energy converter with enhanced amplitude response at frequencies coinciding with Swedish west coast sea states by use of a supplementary submerged body
2009 (English)In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 106, no 6, article id 064512Article in journal (Refereed) Published
Abstract [en]

The full-scale direct-driven wave energy converter developed at Uppsala University has been in offshore operation at the Swedish west coast since 2006. Earlier simulations have now been validated by full-scale experiment with good agreement. Based on that, a theoretical model for a passive system having optimum amplitude response at frequencies coinciding with Swedish west coast conditions has been developed. The amplitude response is increased by adding supplementary inertia by use of the additional mass from a submerged body. A sphere with neutral buoyancy is chosen as the submerged body and modeled as being below the motion of the waves. The model is based on potential linear wave theory and the power capture ratio is studied for real ocean wave data collected at the research test site. It is found that the power capture ratio for the two body system can be increased from 30% to 60% compared to a single body system. Increased velocity in the system also decreases the value for optimal load damping from the generator, opening up the possibility to design smaller units.

Place, publisher, year, edition, pages
USA: American Institute of Physics, 2009
Keyword
data acquisition, ocean waves
National Category
Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-113142 (URN)10.1063/1.3233656 (DOI)000270378100147 ()0021-8979 (ISBN)
Note

10907471 Wave Energy Converter enhanced amplitude response Swedish west coast sea states supplementary submerged body Uppsala University offshore operation passive system optimum amplitude response neutral buoyancy waves motion potential linear wave theory power capture ratio ocean wave data collection research test site Two Body System single body system

Available from: 2010-01-25 Created: 2010-01-25 Last updated: 2017-12-12Bibliographically approved
7. Depth variation of energy transport in fluid gravity waves
Open this publication in new window or tab >>Depth variation of energy transport in fluid gravity waves
2010 (English)In: Journal of Renewable and Sustainable Energy, ISSN 1941-7012, Vol. 2, no 2, p. 023104-Article in journal (Refereed) Published
Abstract [en]

We calculate the distribution of energy flux as a function of the distance below the surface for propagating polychromatic gravity fluid surface waves. Linear theory has been used to derive closed-form expressions for the energy flux as a function of depth. In this context we discuss the power distribution for real ocean waves measured off the west coast of Sweden and compare this to the energy flux distribution for waves with Pierson-Moskowitz and Bretschneider spectral distributions. This is done in order to get an improved understanding of how to improve the power absorption in wave energy converters, and this is also discussed in this paper.

Keyword
gravity waves, hydroelectric power, ocean waves, surface waves (fluid)
National Category
Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-140346 (URN)10.1063/1.3424711 (DOI)000287922200005 ()
Note

11431539 energy flux distribution polychromatic gravity fluid surface waves energy transport linear theory closed-form expressions power distribution ocean waves Sweden Pierson-Moskowitz distribution Bretschneider spectral distributions power absorption wave energy converters

Available from: 2011-01-05 Created: 2011-01-05 Last updated: 2016-04-18Bibliographically approved
8. 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, p. 377-385Article 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
9. Optimization of the Dimensions of a Gravity-based Wave Energy Converter Foundation Based on Heave and Surge Forces
Open this publication in new window or tab >>Optimization of the Dimensions of a Gravity-based Wave Energy Converter Foundation Based on Heave and Surge Forces
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2011 (English)In: 9th European Wave and Tidak 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-160035 (URN)
Conference
9th European Wave and Tidak Energy Conference, Southampton, UK, 2011
Available from: 2011-10-13 Created: 2011-10-13 Last updated: 2017-01-25
10. 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
11. Total instantaneous energy transport in polychromatic fluid gravity waves at finite depth
Open this publication in new window or tab >>Total instantaneous energy transport in polychromatic fluid gravity waves at finite depth
2012 (English)In: Journal of Renewable and Sustainable Energy, ISSN 1941-7012, Vol. 4, no 3, p. 033108-1-033108-8Article in journal (Refereed) Published
Abstract [en]

The total instantaneous energy transport can be found for polychromatic waves when using the deep water approximation. Expanding this theory to waves in waters of finite depth, we derive an expression for the total instantaneous energy transport for polychromatic fluid gravity waves based on potential theory with linearized free surface boundary conditions. We present the results for time series of wave elevation measured at the Uppsala University wave energy research test site. We show that a significant proportion of the total instantaneous energy transport is not accounted for when using the deep water theory. This is important since many wave energy conversion devices under development will operate in waters that do not fulfil the deep water criteria.

National Category
Engineering and Technology
Research subject
Engineering Science with specialization in Science of Electricity
Identifiers
urn:nbn:se:uu:diva-160028 (URN)10.1063/1.4719678 (DOI)000305882200019 ()
Available from: 2011-10-13 Created: 2011-10-13 Last updated: 2017-01-18
12. Properties of the energy transport for plane-parallel polychromatic surface gravity waves in waters of arbitrary depth
Open this publication in new window or tab >>Properties of the energy transport for plane-parallel polychromatic surface gravity waves in waters of arbitrary depth
(English)In: IEEE Journal of Oceanic Engineering, ISSN 0364-9059, E-ISSN 1558-1691Article in journal (Other academic) Submitted
National Category
Fluid Mechanics and Acoustics Oceanography, Hydrology and Water Resources
Identifiers
urn:nbn:se:uu:diva-160026 (URN)
Available from: 2011-10-13 Created: 2011-10-13 Last updated: 2018-01-12Bibliographically approved
13. Modelling and Simulation of Linear Wave Energy Converter
Open this publication in new window or tab >>Modelling and Simulation of Linear Wave Energy Converter
Show others...
(English)In: IET Renewable Power GenerationArticle in journal (Other academic) Submitted
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:uu:diva-160304 (URN)
Available from: 2011-10-20 Created: 2011-10-20 Last updated: 2011-11-23Bibliographically approved
14. A resonant Two Body System for a point absorbing Wave Energy Converter with direct-driven linear generator
Open this publication in new window or tab >>A resonant Two Body System for a point absorbing Wave Energy Converter with direct-driven linear generator
2011 (English)In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 110, no 12, p. 124904-Article in journal (Refereed) Published
Abstract [en]

Based on an earlier conceptual model of a two body system point absorbing wave energy converter tuned to resonance in Swedish west coast sea states, an extended coupled hydrodynamic, mechanic, and electromagnetic model has been developed. The hydrodynamic characteristics of the two body system are studied in the frequency and time domain, while its response to real Swedish west coast sea states are studied in the time domain, by using a wave energy converter model with two independently moving bodies connected to a direct driven linear generator with non-linear damping. The two body system wave energy converter gives nearly 80% power capture ratio in irregular waves. The resonant behaviour is shown to be sensitive to the shape of the spectrum, and the distance between the two bodies is shown to have a large effect on the power absorption.

Keyword
direct energy conversion, linear machines, resonant power convertors, wave power generation
National Category
Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-160032 (URN)10.1063/1.3664855 (DOI)000298639800142 ()
Available from: 2011-10-13 Created: 2011-10-13 Last updated: 2017-12-08

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