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Buoy and Generator Interaction with Ocean Waves

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Buoy and Generator Interaction with Ocean Waves To Magdalena Front cover: The buoy of the wave energy converter L3. In the background, Klam- merskär and the observation tower. June 2009. Back cover: The rubber dinghy Bullen, which has made many trips to and from Klammerskär, with and without the author. In the background, the southern marker buoy for the Lysekil research site can be seen. July 2011. List of Papers This thesis is based on the following papers, which are referred to in the text by their Roman numerals. I Leijon, M., Boström, C., Danielsson, O., Gustafsson, S., Haikonen, K., Langhamer, O., Strömstedt, E., Stålberg, M., Sundberg, J., Svensson, O., Tyrberg, S., and Waters, R. “Wave Energy from the North Sea: Experiences from the Lysekil Research Site”. Surveys in Geophysics, Vol. 29, Issue 3, pp. 221–240, May 2008. II Tyrberg, S., Stålberg, M., Boström, C., Waters, R., Svensson, O., Strömstedt, E., Savin, A., Engström, J., Langhamer, O., Gravråkmo, H., Haikonen, K., Tedelid, J., Sundberg, J., and Leijon, M. “The Lysekil Wave Power Project: Status Update”. Proceedings of the 10th World Renewable Energy Congress, WREC, Glasgow, UK, pp. 1061–1066, July 2008. III Leijon, M., Waters, R., Stålberg, M., Svensson, O., Boström, C., Ström- stedt, E., Engström, J., Tyrberg, S., Savin, A., Gravråkmo, H., Bern- hoff, H., Sundberg, J., Isberg, J., Ågren, O., Danielsson, O., Eriksson, M., Lejerskog, E., Bolund, B., Gustafsson, S., and Thorburn, K. “Catch the Wave to Electricity: The Conversion of Wave Motions to Electric- ity Using a Grid-Oriented Approach”. IEEE Power & Energy Magazine Vol. 7, Issue 1, pp. 50–54, January/February 2009. IV Tyrberg, S., Gravråkmo, H., Leijon, M. “Tracking a Wave Power Buoy Using a Network Camera — System Analysis and First Results”. Pro- ceedings of the 28th International Conference on Offshore Mechanics and Arctic Engineering (peer-reviewed), OMAE, Honolulu, USA, June 2009. V Tyrberg, S., Waters, R., and Leijon, M. “Wave Power Absorption as a Function of Water Level and Wave Height: Theory and Experiment”. IEEE Journal of Oceanic Engineering, Vol. 35, Issue 3, pp. 558–564, July 2010. VI Boström, C., Lejerskog, E., Tyrberg, S., Svensson, O., Waters, R., Savin, A., Bolund, B., Eriksson, M., and Leijon, M. “Experimental Results From an Offshore Wave Energy Converter”. Journal of Offshore Mechanics and Arctic Engineering, Vol. 132, Issue 4, pp. 041103-1–041103-5, November 2010. VII Tyrberg, S., Svensson, O., Kurupath, V., Engström, J., Strömstedt, E., and Leijon, M. “Wave Buoy and Translator Motions — On-site Mea- surements and Simulations”. IEEE Journal of Oceanic Engineering, Vol. 36, Issue 3, pp. 377–385, July 2011. VIII Lejerskog, E., Gravråkmo, A., Savin, A., Strömstedt, E., Tyrberg, S., Haikonen, K., Krishna, R., Boström, C., Rahm, M., Ekström, R., Svens- son, O., Engström, J., Ekergård, B., Baudoin, A., Kurupath, V., Hai, L., Li, W., Sundberg, J., Waters, R., and Leijon, M. “Lysekil Research Site, Sweden: A Status Update”. Proceedings of the 9th European Wave and Tidal Energy Conference (peer-reviewed), EWTEC11, Southamp- ton, UK, September 2011. IX Lindroth, S. and Leijon, M., “Offshore Wave Power Measurements - a Review”, Renewable & Sustainable Energy Reviews, in press, available online, DOI:10.1016/j.rser.2011.07.123, September 2011. X Lindroth, S. and Leijon, M., “Spectral Parameters and Wave Energy Converter Performance”, Manuscript, October 2011. Reprints were made with permission from the publishers. The author has also contributed to the following papers, not included in the thesis. Paper A is a conference version of Paper VI. For Paper B the contribution of the author was minor, and the scope differs somewhat from the rest of the papers. A Boström, C., Lejerskog, E., Tyrberg, S., Svensson, O., Waters, R., Savin, A., Bolund, B., Eriksson, M., and Leijon, M. “Experimental results from an offshore wave energy converter”. Proceedings of the 27th International Conference on Offshore Mechanics and Arctic Engineering (peer-reviewed), OMAE, Estoril, Portugal, June 2008. B Gravråkmo, H., Leijon, M., Strömstedt, E., Engström, J., Tyrberg, S., Savin, A., Svensson, O., Waters, R., “Description of a Torus Shaped Buoy for Wave Energy Point Absorber”. Proceedings of Renewable Energy 2010, Yokohama, Japan, June/July 2010. Received Best Paper Award. Contents 1 Introduction . 11 1.1 Wave energy and ways to capture it . 11 1.2 Working in engineering science . 12 1.3 Aims of the thesis . 13 2 Background . 15 2.1 The Lysekil Project . 15 2.2 Timeline . 15 2.3 Theory . 18 2.3.1 Describing waves . 18 2.3.2 Linear generators . 21 3 Ocean Measurements . 23 3.1 History and development of wave power measurements . 23 3.2 The fundamental difficulties . 24 3.3 The work at Klammerskär . 25 3.3.1 Successes and failures . 26 3.4 Interpreting measurements . 27 3.4.1 Comparing measurements with incoming waves . 29 4 WEC Performance . 31 4.1 Defining performance . 31 4.2 Controlling the variables . 32 4.2.1 AC/DC load and sea state . 33 4.2.2 Water level and wave height . 34 4.2.3 Spectral shape . 34 5 Summary of Papers . 37 6 Conclusions . 43 6.1 Measurements and buoy/generator motion . 43 6.2 WEC Performance . 44 7 Acknowledgements . 45 8 Svensk sammanfattning . 47 References . 49 Abbreviations and nomenclature Abbreviation Meaning AC Alternating Current DC Direct Current GPS Global Positioning System L1–9 Wave energy converter 1–9 LRS The Lysekil Research Site WEC Wave Energy Converter Symbol Unit Entity F Wb Magnetic flux r kg/m3 Density E V Electromotive force H m Wave height Hm0 m Significant wave height J W/m Energy transport N - Number of coil turns S( f ) m2/Hz Variance spectral density T s Period T−10 s Energy period Tp s Peak period f Hz Frequency f0 Hz Fundamental frequency f−10 Hz Energy frequency fp Hz Peak frequency g m/s2 Gravitational constant 2 k mk m Hz Spectral moment number k t s Time w m Water level 9 1. Introduction 1.1 Wave energy and ways to capture it The ever increasing need for energy in the world and the dangers of further use of fossil fuels have put a focus on renewable energy technologies. That there is a vast amount of energy in ocean waves has been known for a long time (see e.g. [1,2]), and this also becomes very evident to anyone who spends time out at sea. The exact amount of power available from waves is impossible to determine, since it depends on the performance of the technology chosen, but an estimation that about 1 TW continuously descends on the coastlines of the world was made in [3]. Regardless of the precision in that statement, it is clear that our oceans carry enough energy for wave power to be interesting to study further. Numerous projects have tried to harvest the energy in waves. There are quite a few projects still active around the world, the most famous of which is possibly the Pelamis [4, 5]. Some other concepts that are, or have been, active in recent years are the Wave Dragon [6], the Archimedes Wave Swing [7], Oceanlinx [8], the Ocean Energy Buoy [9], the Oregon State University project [10] and the Backward Bent Duct Buoy [11]. Further projects, past and present, can be found in a recent review by Falcão [12]. Historically, most attempts have failed in creating a technology that can survive the harsh ocean wave climates, or in designing a technology that is economically viable. There are some considerable advantages in using wave energy in compar- ison to other intermittent energy sources, such as wind and solar energy. But there are also some fundamental difficulties. On the positive side, wave en- ergy is a concentrated form of energy, since waves absorb the energy from winds blowing over large areas. This also means that waves, in comparison with winds, are more predictable and more evenly distributed in time. Even if the wind abates, waves will keep rolling in for some time. The major difficulty of tapping into the huge energy resource of ocean waves is the large variations of the powers and forces involved. For a sys- tem to be sustainable, it needs to produce electricity at average wave climates, but still survive harsh weathers and storms. There is therefore a risk of ending up with systems that are either too bulky to be economically viable, or con- versely, too fragile to survive in the long run. The Lysekil project, run by the Division of Electricity at Uppsala University, is an attempt to design a wave energy system which is sturdy enough, without becoming too expensive. 11 (b) (a) (c)f Figure 1.1: A classification of wave energy devices. (a) Wave activated bodies, (b) Overtopping devices, (c) Oscillating water columns [13]. Systems for wave energy conversion can be classified in many different ways. In Fig. 1.1, wave energy devices have been categorized based on their working principle. These are wave activated bodies, overtopping devices, and oscillating water columns. The first of these describes systems where the mo- tion of the waves is directly transferred to the Wave Energy Converter (WEC). Examples of this technology include the Pelamis mentioned above. Overtop- ping devices let water run up a slope into a reservoir, from where the potential energy is converted using a low-head turbine. The Wave Dragon is an example of such a technology. Oscillating water columns use a fluctuating air pressure above the ocean surface to drive a turbine. One way to achieve such a varying air pressure is to partially submerge a pipe into the water. The Backward Bent Duct Buoy is an example of a technology using an oscillating water column.
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