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Wave Power Absorption as a Function of Water Level and Wave Height: Theory and Experiment
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.
2010 (English)In: IEEE Journal of Oceanic Engineering, ISSN 0364-9059, E-ISSN 1558-1691, Vol. 35, no 3, p. 558-564Article 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.

Place, publisher, year, edition, pages
2010. Vol. 35, no 3, p. 558-564
Keyword [en]
Energy capture, experimental results, linear generator, power absorption, wave power
National Category
Engineering and Technology
Identifiers
URN: urn:nbn:se:uu:diva-97852DOI: 10.1109/JOE.2010.2052692ISI: 000283226500008OAI: oai:DiVA.org:uu-97852DiVA, id: diva2:172938
Available from: 2012-01-09 Created: 2008-11-21 Last updated: 2017-12-14Bibliographically approved
In thesis
1. Energy from Ocean Waves: Full Scale Experimental Verification of a Wave Energy Converter
Open this publication in new window or tab >>Energy from Ocean Waves: Full Scale Experimental Verification of a Wave Energy Converter
2008 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

A wave energy converter has been constructed and its function and operational characteristics have been thoroughly investigated and published. The wave energy converter was installed in March of 2006 approximately two kilometers off the Swedish west coast in the proximity of the town Lysekil. Since then the converter has been submerged at the research site for over two and a half years and in operation during three time periods for a total of 12 months, the latest being during five months of 2008. Throughout this time the generated electricity has been transmitted to shore and operational data has been recorded. The wave energy converter and its connected electrical system has been continually upgraded and each of the three operational periods have investigated more advanced stages in the progression toward grid connection. The wave energy system has faced the challenges of the ocean and initial results and insights have been reached, most important being that the overall wave energy concept has been verified. Experiments have shown that slowly varying power generation from ocean waves is possible.

Apart from the wave energy converter, three shorter studies have been performed. A sensor was designed for measuring the air gap width of the linear generator used in the wave energy converter. The sensor consists of an etched coil, a search coil, that functions passively through induction. Theory and experiment showed good agreement.

The Swedish west coast wave climate has been studied in detail. The study used eight years of wave data from 13 sites in the Skagerrak and Kattegatt, and data from a wave measurement buoy located at the wave energy research site. The study resulted in scatter diagrams, hundred year extreme wave estimations, and a mapping of the energy flux in the area. The average energy flux was found to be approximately 5.2 kW/m in the offshore Skagerrak, 2.8 kW/m in the near shore Skagerrak, and 2.4 kW/m in the Kattegat.

A method for evaluating renewable energy technologies in terms of economy and engineering solutions has been investigated. The match between the technologies and the fundamental physics of renewable energy sources can be given in terms of the technology’s utilization. It is argued that engineers should strive for a high utilization if competitive technologies are to be developed.

Place, publisher, year, edition, pages
Uppsala: Universitetsbiblioteket, 2008. p. 130
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 580
Keyword
wave power, wave energy converter, sea trials, ocean energy, linear generator, point absorber, search coil, wave climate, utilization
National Category
Other Engineering and Technologies
Identifiers
urn:nbn:se:uu:diva-9404 (URN)978-91-554-7354-9 (ISBN)
Public defence
2008-12-12, Polacksbackens aula, Lägerhyddsv. 2, Uppsala, 13:00 (English)
Opponent
Supervisors
Available from: 2008-11-21 Created: 2008-11-21 Last updated: 2012-11-09Bibliographically approved
2. Buoy and Generator Interaction with Ocean Waves: Studies of a Wave Energy Conversion System
Open this publication in new window or tab >>Buoy and Generator Interaction with Ocean Waves: Studies of a Wave Energy Conversion System
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. p. 52
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 872
Keyword
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:nbn:se:uu:diva-160085 (URN)978-91-554-8192-6 (ISBN)
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

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