This thesis deals with a system for operation of directly driven offshore wave energy converters. The work that has been carried out includes laboratory testing of a permanent magnet linear generator, wave energy converter mechanical design and offshore testing, and finally design, implementation, and offshore testing of an underwater collector substation. Long-term testing of a single point absorber, which was installed in March 2006, has been performed in real ocean waves in linear and in non-linear damping mode. The two different damping modes were realized by, first, a resistive load, and second, a rectifier with voltage smoothing capacitors and a resistive load in the DC-link. The loads are placed on land about 2 km east of the Lysekil wave energy research site, where the offshore experiments have been conducted. In the spring of 2009, another two wave energy converter prototypes were installed. Records of array operation were taken with two and three devices in the array. With two units, non-linear damping was used, and with three units, linear damping was employed. The point absorbers in the array are connected to the underwater substation, which is based on a 3 m3 pressure vessel standing on the seabed. In the substation, rectification of the frequency and amplitude modulated voltages from the linear generators is made. The DC voltage is smoothened by capacitors and inverted to 50 Hz electrical frequency, transformed and finally transmitted to the on-shore measuring station. Results show that the absorption is heavily dependent on the damping. It has also been shown that by increasing the damping, the standard deviation of electrical power can be reduced. The standard deviation of electrical power is reduced by array operation compared to single unit operation. Ongoing and future work include the construction and installation of a second underwater substation, which will connect the first substation and seven new WECs.

The objective of this thesis is to develop a model that will integrate multiple buoys to a power take-off hub. The model will be derived using a time domain analysis and will consider the hydraulic coupling of the buoys and the power take-off. The derived model is reproduced in MATLAB in order to run simulations. This will give possibility to conduct a parameter study and evaluate the performance of the system. The buoy simulation model is provided by Wave4Power (W4P). It consists of a floater that is rigidly connected to a fully submerged vertical (acceleration) tube open at both ends. The tube contains a piston whose motion relative to the floater-tube system drives a power take-off mechanism. The power take-off model is provided by Ocean Harvesting Technologies AB (OHT). It comprises a mechanical gearbox and a gravity accumulator. The system is utilized to transform the irregular wave energy into a smooth electrical power output. OHT's simulation model needs to be extended with a hydraulic motor at the input shaft. There are control features in both systems, that need to be connected and synchronized with each other. Another major goal within the thesis is to test different online control techniques. A simple control strategy to optimize power capture is called sea-state tuning and it can be achieved by using a mechanical gearbox with several discrete gear ratios or with a variable displacement pump. The gear ratio of the gear box can be regulated according to a 2D look up table based on the average wave amplitude and frequency over a defined time frame. The OHT power take-off utilizes a control strategy, called spill function, to limit the excess power capture and keep the weight accumulator within a span by disengaging the input shaft from the power take-off. This is to be modified to implement power limitation with regulation of the gear ratio of the gearbox. / +46736290781

A small-scale wave energy converter was designed and built for teaching and academic purposes to be used at The Division for Electricity, in the Ångström Laboratory at Uppsala University. The design of the power take-off (PTO) makes use of magnets passing through a copper coil for electricity generation. The magnets are attached by a string to the floating buoy in the small-scale wave tank which leads to a joint oscillation. Design parameters are executed using COMSOL Multiphysics which illustrates the total voltage output generated as well as the total magnetic field. Simulations and calculations in MATLAB were performed to extract the expected damping coefficient and plots of the buoy position compared to the wave amplitude. Lastly, a PTO prototype were built and compared with the simulations. The PTO shows electricity generation with the aid of a voltmeter showcasing the voltage. Additional information on future improvements to further aid teaching and academic understanding of wave energy converter are mentioned in the final section of this study.

For the west coast of North America, from northern California to southern Washington, a habitat suitability prediction framework was developed to support wave energy device siting. Concern that wave energy devices may impact the seafloor and benthos has renewed research interest in the distribution of marine benthic invertebrates and factors influencing their distribution. A Bayesian belief network approach was employed for learning species-habitat associations for Rhabdus rectius, a tusk-shaped marine infaunal Mollusk. Environmental variables describing surficial geology and water depth were found to be most influential to the distribution of R. rectius. Water property variables, such as temperature and salinity, were less influential as distribution predictors. Species-habitat associations were used to predict habitat suitability probabilities for R. rectius, which were then mapped over an area of interest along the south-central Oregon coast. Habitat suitability prediction models tested well against data withheld for crossvalidation supporting our conclusion that Bayesian learning extracts useful information available in very small, incomplete data sets and identifies which variables drive habitat suitability for R. rectius. Additionally, Bayesian belief networks are easily updated with new information, quantitative or qualitative, which provides a flexible mechanism for multiple scenario analyses. The prediction framework presented here is a practical tool informing marine spatial planning assessment through visualization of habitat suitability. / Graduation date: 2012

There is currently a high demand for electric power from renewablesources. One source that remains relatively untapped is the motionof ocean waves. Anders Hagnestål has been developing a uniquelyefficient and simplified design for a point-absorb buoy generator byconverting its linear motion directly into alternating electric power usinga linear PM engine. To test this method, a smaller prototype isbuilt. Its characteristics present some unusual challenges in the designand construction of its winding.Devices of this type typically use relatively low voltage (690V typicallyfor a wind turbine, compared to the 10kV range of traditionalpower plants). To achieve high power, they need high current, whichin turn requires splitting the conductors in the winding into isolatedparallel strands to avoid losses due to eddy currents and current crowding.However, new losses from circulating currents can then arise. Inorder to reduce said losses, the parallel conductors should be transposedin such a way that the aggregate electromotive force the circuitsthat each pair of them forms is minimized.This research and prototyping was performed in absence of advancedindustrial means of construction, with limited space, budget,materials, manpower, know-how, and technology. Manual ingenuityand empirical experimentation were required to find a practical implementationfor: laying the cables, fixing them in place, transferringthem to the machine, stripping their coating at the ends and establishinga reliable connection to the current source.Using theoretical derivations and FEM simulation, a sufficientlygood transposition scheme is proposed for the specific machine thatthe winding is built for. A bobbin replicating the shape of the enginecore is built to lay down the strands.The parallel strands are then organized each into their respectivebobbin, with a bobbin rack and conductor funneling device being designedand constructed to gather them together into a strictly-organizedbundle. An adhesive is found to set the cables in place.Problems with maintaining the orientation and configuration of thecables in the face of repeated torsion are met and solved. A chemicalsolution is used to strip the ends of the conductors, and a reliableconnection is established by crimping the conductors into a bi-metalCu-Al lug.ivIn conclusion, the ideal transposition schemes required to cancelout circulating currents due to magnetic flux leakage are impossibleto put in practice without appropriate technological means. The feasibletransposition scheme turns out to be a simple mirroring of conductors’positions, implemented by building each half of the windingseparately around replicas of the core and then connecting them usingcrimping lugs. / Efterfrågan på el från förnybara källor är hög och inget tyder på att det kommer ändras den närmsta tiden. En källa till förnybar el som än idag står relativt orörd är den där man använder energin från havsvå- gor. Det är denna förnybara källa Anders Hagnestål haft i åtanke när han nu bygger en unikt effektiv generator med syftet att i ett senare skede utvinna el med hjälp av flytande punktabsorberande vågkraft- system. Generatorn är av den linjära typen och omvandlar det punk- tabsorberande systemet rörelse till el. För att testa denna generator- modell så påbörjades bygget av två fullskaliga prototyper 2017. Denna uppsats behandlar specifikt arbetet med generatorlindningen till pro- totyperna och innefattar processen från design till själva byggnatio- nen. Lindingen består av flertalet mindre och isolerade lindningsleda- re med uppgift att bland annat minska skinneffekt och virvelströms- förluster. När man använder denna metod så uppkommer dock ett nytt problem vilket härstammar från att lindningsledarna är samman- kopplade i vardera ända och bildar på så sätt n slutna strömkretsar. Konsekvensen kan vara stora förluster från cirkulerande strömmar på grund av det magnetiska ströflöde som finns runt järnkärnan som lindningen omsluter. Utgångspunkten för att minimera dessa cirkule- rande strömmar är att transponera alla lindningsledare på ett sätt så att den resulterande elektromotoriska spänningen för varje strömkrets blir så liten som möjligt. Med hjälp av förenklade modeller samt FEM simuleringar så bestämdes ett lämpligt sätt att transponera lindningstrådarna utifrån oli- ka kriterier. Lösningen blev att lindningstrådarna endast transponera- des en gång med en så kallad 180 grader transponering. Detta ger en tillräckligt god minimering av de cirkulerande ström- marna, men den stora fördelen med denna lösning är att det är möjligt att linda maskinen med de små resurser projektet hade tillgång till, dock var detta till en stor nackdel då väldigt mycket tid gick till att hitta egna tillvägagångsätt för att utföra byggandet av lindningen på ibland okonventionella sätt.