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Research Papers

Influence of Generator Damping on Peak Power and Variance of Power for a Direct Drive Wave Energy Converter

[+] Author and Article Information
Magnus Stålberg

Swedish Center for Renewable Electric Energy Conversion, Department of Engineering Sciences, Division for Electricity,  Uppsala University, Box 534, SE-75121 Uppsala, Swedenmagnus.stalberg@angstrom.uu.se

Rafael Waters, Mats Leijon

Swedish Center for Renewable Electric Energy Conversion, Department of Engineering Sciences, Division for Electricity,  Uppsala University, Box 534, SE-75121 Uppsala, Sweden

Oskar Danielsson

 Seabased Industry AB, Dag Hammarskjölds väg 52B, SE-75183 Uppsala, Sweden

J. Offshore Mech. Arct. Eng 130(3), 031003 (Jul 11, 2008) (4 pages) doi:10.1115/1.2905032 History: Received September 05, 2007; Revised February 05, 2008; Published July 11, 2008

The first offshore prototype of a wave energy converter system has been launched off the Swedish west coast. The concept is based on a point absorber directly coupled to a linear generator located on the ocean floor. The wave energy converter is part of a research project that will study the electric system of ten units forming a small farm of wave power plants as they are linked and connected to an electric grid. A full scale farm will consist of a large number of interconnected units. The chosen direct drive system reduces the mechanical complexity of the converter but has repercussions on the electric system. The output from the generator will vary with the speed of the point absorber, leading to large fluctuations of power on the second scale. This has implications on both the individual generator and on the system as a whole. The hydrodynamic behavior of the point absorber depends, to a large extent, on the damping of the generator. The damping, in turn, can be remotely controlled by changing the load resistance. It has previously been shown that this has a large influence on the power absorbed by the wave energy converter. This paper investigates the peak power, the translator speed, and the variance of the power at different sea states and for different levels of damping. The peak power has an impact on the design of the generator and the required ability, for a single unit, to handle electric overloads. The momentum of the translator is directly proportional to its speed. The speed is thus important for the design of the end stop. The variance of the power of one unit will have an impact on the farm system behavior. The study is based on two and a half months of experimental measurements on the prototype wave energy converter and a wave measurement buoy. The aim is to analyze whether load control strategies may influence the dimensioning criteria for the electric system and the generator. The results are compared to previously investigated relationships between the absorbed mean power and the load resistance as a function of sea state. In the study, it was found that the maximum power is approximately proportional to the average power, while the maximum translator speed and standard deviation decrease as the damping factor is increased.

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Copyright © 2008 by American Society of Mechanical Engineers
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Figures

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Figure 1

The WEC prototype in the harbor of Lysekil on March 13, 2006 (date of submersion). The yellow buoy is visible in the lower right hand corner of the picture.

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Figure 2

The Uppsala University wave energy test site is situated outside of the town Lysekil on the Swedish west coast

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Figure 3

The one-phase circuit model of the installed equipment on the research test site

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Figure 4

Maximum instantaneous electric power per half hour as depending on damping factor and sea state. The red, black, and blue lines are least squares approximations of the 12.7kNs∕m, 8.53kNs∕m, and 5.23kNs∕m damping factors.

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Figure 5

Maximum translator speeds as a function of sea state. The red, black, and blue lines are least squares approximations of the 12.7kNs∕m, 8.53kNs∕m, and 5.23kNs∕m damping factors.

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Figure 6

Standard deviation of absorbed active electric power normalized by average absorbed active electric power as a function of sea state. The red, black and blue lines are least squares approximations of the 12.7kNs∕m, 8.53kNs∕m, and 5.23kNs∕m damping factors. The standard deviation for the weakest sea states has been excluded to clarify the trend of the more powerful sea states, which are of significance for the survivability of the WEC and for the dimensioning of the electric system.

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