Studying Buoy Motion for Wave Power : Experiments at the Lysekil Research Site

Since 2002, the Division for Electricity at Uppsala University has been running the Lysekil project. The project is an attempt to construct and evaluate a technology for extracting electrical energy from the motion of ocean waves. The idea is to let this up-and-down motion drive a linear generator. A buoy moves thus in the waves, and is connected through a line to the generator at the sea floor. Three such wave energy converters, L1, L2, and L3, and a marine substation have been deployed in the ocean southwest of Lysekil on the Swedish west coast, at the Lysekil research site. Measuring equipment has also been deployed, together with a number of buoys for studying environmental impact. A measuring station has been installed on the nearby island of Hermanö, and an observation tower has been built on the islet of Klammerskär, south of the research site. This licentiate thesis describes the author's work on studying wave buoy motion and is based on five scientific papers, covering mainly two areas. Firstly, changes in water levels, and thereby changes in the equilibrium point for the buoy and generator, have been related to the ability of L1 to absorb energy. The results indicate that there is a correlation between water levels and energy absorption for L1 for the studied time period. When the water level deviates from average, the absorption values decrease. This is not unexpected, since the linear generator has a finite stroke. The effect is however noticeable primarily for water level deviations of more than 25 cm, and is only visible for those cases where either wave height or water level deviation is large. Secondly, the above mentioned observation tower has been designed and built. The tower is equipped with a network camera covering the research site, a wireless communication system and an energy system. The first acquired images of the buoy connected to L1, taken during the summer of 2008, have been analyzed, and buoy motion data has been extracted. The observation system has worked well, and the data on buoy motion (vertical motion in the range of +/-0.5 m correlates fairly well to measurements of significant wave height for the period (Hm0=0.82 m). A comparison with voltage data from the generator also indicates that the system has captured the dominating buoy motion. However, the system suffers from poor temporal resolution (about one image per second), and has not yet been synchronized with the other measurement systems at the site. Addressing these two problems is of high priority in the future