This master thesis is the result after two weeks of persistent effort in a wave basin in Aalborg, Denmark, and later analysis in Uppsala. The client was a start-up wave energy company called WaveTube from Gothenburg. The assignment was to verify their newly built prototype with their mathematical model and with the result advise, give suggestions for improvements to a new prototype. In Aalborg a program called Wavelab were used to simulate the different wave states, when all data were collected, the data were analysed in MATLAB. The results shows that the prototype can deliver an output around 40kW in a significant wave height of 3.25 m, this can be compared with the theoretical model which resulted in a output about 150kw. Suggestions for improvement are given, and also specific areas that would require more research. Although the experimental result did not reach the theoretical, the experiments deemed successful since the main task was proof of concept. WaveTube has been selected for additional tests through the EU project Marinet, that will take place during the fall of 2014.

Since 2002, the department of electricity at Uppsala university has pushed the Lysekil project. The project has a number of wave energy converters installed in the sea southwest of Lysekil. The purpose of this work is to design, build and test a DC-DC converter, which will later be used as a necessary part of the grid connection of a wave energy converter. Since a wave energy converter does not generate electricity at a constant frequency, it is not possible to use a gearbox. Instead, power is rectified and, if there are several wave power energy converters, are put together with the others before it is inverted and transformed to the correct voltage level, and finally connected to the grid [1]. The designed DC-DC converter is a converter of the type "inverting buck-boost", i.e. a converter that can both lower and raise the voltage, and inverts the polarity of the output. Although the voltage in normal circumstances will only be increased, the simulations showed that the efficiency and cost of components did not differ much between a "boost" and "buck-boost" converter, thus considered flexibility to be able to lower the voltage if needed. The project also includes a small part to the construction of a bridge rectifier, but as the most difficult moment in the project is the DC-DC converter, the greatest focus will be there.

It is not a coincidence that over half the world‟s population live in coastal areas using the sea as a mean to develop its industry, thus the sea is present in most aspects of daily life. Because of the vital relationship with the marine environment, for many years mankind is aware of the high energy potential contained in waves. During the last hundred years, thousands patents of devices for the extraction of the energy from waves have been published. However, the researching still faces the challenge of develop the optimal wave energy converter that matches robustness, to withstand extreme marine conditions, and sensitivity, to respond the different sea states. In this thesis a scale model of a wave overtopping converter has been designed, built and tested. In this type of wave electricity converter the waves ascend a ramp, filling a reservoir located at a certain height above sea level. The stored water in the reservoir is discharged back into the sea, powering a turbine, thus generating electricity. The system is composed of a wave energy converter, at a scale of 1:100 without turbine, a test channel and a plunger type wave maker. Different sea conditions have been simulated, to assess how the different configurations of the device influence the obtained hydraulic power and flow. It has been concluded that there is an appropriate configuration of the wave electricity converter for each wave period and height. The simulated sea conditions were composed of wave periods of around a second and wave heights of about two centimeters. Finally by applying scale transformations, an estimation of the hydraulic power that the wave electricity converter would extract with this configuration in the deep waters of Tenerife South has been calculated. Summarizing, in this thesis the methodology of testing and the comparison with real conditions has been developed.

This research investigates the effects that geometry and control have on the absorption characteristics of active wavemakers and looks at the feasibility of modelling these wavemakers in commercial computational fluid dynamic software. This thesis presents the hydrodynamic coefficients for four different types of wavemakers. The absorption characteristics of these wavemakers are analysed using different combinations of control impedance coefficients. The effect of combining both geometry and control is then investigated. Results, quantifying the absorption characteristics are then presented. It is shown that the amount of absorption for a given paddle differs greatly depending on the choice of control coefficients used to implement complex conjugate control. Increased absorption can be achieved over a broader bandwidth of frequencies when the geometry of the wavemaker is optimised for one specific frequency and the control impedance is optimised for an alternate frequency. In conjunction to this theoretical study, a numerical investigation is conducted in order to verify and validate two commercial computational fluid dynamic codes' suitability to model the previously discussed absorbing wavemakers. ANSYS CFX and FLOW3D are used to model a physical wavemaker. Both are rigorously verified for discretisation errors and CFX is validated against linear wavemaker theory. Results show good agreement and prediction of the free surface close to the oscillating wavemaker, but problems with wave height attenuation and excessive run times were encountered.

The Variable Inertia System Wave Energy Converter (VISWEC) is a self-reacting point absorber (SRPA) type wave energy converter (WEC) capable of changing its mechanical impedance using an internal reaction mass system. The reaction mass is coupled to a rotating assembly capable of varying its inertia and this changing inertia has the effect of creating an added inertial resistance, or effective mass, to oscillations of the reaction mass. An SRPA has two main bodies, designated Float and Spar, capable of utilizing the relative motion between the two bodies to create power through a power take-off (PTO). The implementation of the reaction mass, a 3rd body, and the variable inertial system (VIS) is designed to change the response of the Spar in order to create larger relative velocities between the two bodies and thus more power. It is also possible to lock the VIS within the Spar, and when this is done the system is reduced to a conventional 2-body SRPA configuration. To better understand the effects of the implementation of the VIS on the overall stability of the VISWEC and the power conversion performance, a numerical model simulation within ProteusDS, a time-domain modelling software, was created. Power production and parametric excitation are the metrics of comparison between the two systems. Parametric excitation is a phenomenon that correlates wave excitation frequency to roll stability and has been shown to negatively affect power production in SRPAs. Simulations of the 2 and 3-body provide a basis of comparison between the two systems and allow the assessment of parametric excitation prohibited or exacerbated by the implementation of the VIS as well as power production. The simulation executed within the commercial software ProteusDS incorporates articulated bodies defined with physical parameters connected through connections allowing kinematic constraints and relations and hydrodynamics of the hull geometries as they are exposed to regular waves. ProteusDS also has the ability to apply kinematic constrains on the entire system allowing the analysis of isolated modes of motion. The implementation of the VIS demonstrates a generally higher power production and stabilization of the system with regards to parametric excitation. While the 3-body system is more stable, the bandwidth at which rolling motion is induced increased in comparison to the 2-body system. Rolling motions in both the 2 and 3-body systems are characteristic of parametric excitation and show a direct correlation to reduced power production. Overall the 3-body VISWEC outperforms the typical 2-body SRPA representation but more research is required to refine the settings of the geometric and PTO control. / Graduate

A prototype point absorber style wave energy converter has been proposed for deployment off the West coast of Vancouver Island near the remote village of Hotsprings Cove in Hesquiaht Sound; a site identified as having significant wave energy potential. The proposed design consists of two components, a long unique cylindrical spar and a concentric toroid float. To serve ongoing wave energy converter (WEC) dynamics modelling and control research in support of that project, an experimental facility for small scale physical model testing is desired at UVIC. In the immediate term, the facility could be used to determine the hydrodynamic coefficients over a range of wave frequencies. Refined estimates of the hydrodynamic coefficients would be exploited in the optimisation of the WEC geometry. To date, WEC research at UVIC has neglected the frequency dependence of the hydrodynamic coefficients, relying on limited experimental results to provide a single frequency invariant set of coefficient estimates. / Graduate / 0791 / 0547 / 0548 / mercedes.baylis@hotmail.com

Abstract     In this project, I present a design of a scale model of a linear generator (LG) similar to a full size Wave Energy Converter (WEC) being developed at Uppsala University since 2002 and commercialized by Seabased AB. The purpose of a WEC is to convert the energy from ocean waves into electrical energy. In order to implement the behaviour of the prototype design, a preliminary study has been done to further build it for use in education, laboratory tests and research. The challenge with this project is to scale down the WEC but maintain the shape, appearance and characteristics of the generator for educational purposes. A miniature version of a WEC, previously developed by Uppsala University in collaboration with Seabased Industry AB, has been designed with scaling rate 1:14 of the linear dimensions. In this case, the value of the output power is not important- it has simply been calculated. The electrical rated parameters of the three phase generator are power  26 W,  peak line-line voltage  13 V and  rated armature current  2 A. The mechanical parameters utilized in the design are the total length and the diameter of the miniature WEC, 50 cm and 25 cm, respectively. The simulated prototype model (described in Section 5.4) has been validated with an experimental setup comprising translator and stator (described in Section 5.1), where the translator is moved by a programmed industrial robot. The experimental results have shown good agreement with the simulations.

The interaction between water waves and a floating or fixed body is bi-directional: wave forces act on and cause motion in the body, and the body alters the wave field. The impact of the body on its wave field is important to understand because: 1) it may have positive or negative consequences on the natural or built environment; 2) multiple bodies in proximity interact via the waves that are scattered and radiated by them; and 3) in ocean wave energy conversion, by conservation of energy, as a device absorbs energy, so too must the energy be removed from the wave field. Herein, the cylindrical solutions to the linear wave boundary-value problem are used to analyze the floating body wave field. These solutions describe small-amplitude, harmonic, potential-flow waves in the form of a Fourier summation of incoming and outgoing, partial, cylindrical, wave components. For a given geometry and mode of motion, the scattered or radiated waves are characterized by a particular set of complex cylindrical coefficients. A novel method is developed for finding the cylindrical coefficients of a scattered or radiated wave field by making measurements, either computationally or experimentally, over a circular-cylindrical surface that circumscribes the body and taking a Fourier transform as a function of spatial direction. To isolate evanescent modes, measurements are made on the free-surface and as a function of depth. The technique is demonstrated computationally with the boundary-element method software, WAMIT. The resulting analytical wave fields are compared with those computed directly by WAMIT and the match is found to be within 0.1%. A similar measurement and comparisons are made with experimental results. Because of the difficulty in making depth-dependent measurements, only free-surface measurements were made with a circular wave gauge array, where the gauges were positioned far from the body in order to neglect evanescent modes. The experimental results are also very good. However, both high-order harmonics and wave reflections led to difficulties. To compute efficiently the wave interactions between multiple bodies, a well-known multiple-scattering theory is employed, in which waves that are scattered and radiated by one body are considered incident to another body, which in turn radiates and scatters waves, sending energy back to the first. Wave fields are given by their cylindrical representations and unknown scattered wave amplitudes are formulated into a linear system to solve the problem. Critical to the approach is the characterization of, for each unique geometry, the cylindrical forces, the radiated wave coefficients, and the scattered waves in the form of the diffraction transfer matrix. The method developed herein for determining cylindrical coefficients is extended to new methods for finding the quantities necessary to solve the interaction problem. The approach is demonstrated computationally with WAMIT for a simple cylinder and a more complex wave energy converter (WEC). Multiple-scattering computations are verified against direct computations from WAMIT and are performed for spectral seas and a very large array of 101 WECs. The multiple-scattering computation is 1,000- 10,000 times faster than a direct computation because each body is represented by 10s of wave coefficients, rather than 100s to 1,000s of panels. A new expression for wave energy absorption using cylindrical coefficients is derived, leading to a formulation of wave energy absorption efficiency, which is extended to a nondimensional parameter that relates to efficiency, capture width and gain. Cylindrical wave energy absorption analysis allows classical results of heaving and surging point absorbers to be easily reproduced and enables interesting computations of a WEC in three-dimensions. A Bristol Cylinder type WEC is examined and it is found that its performance can be improved by flaring its ends to reduce "end effects". Finally, a computation of 100% wave absorption is demonstrated using a generalized incident wave. Cylindrical representations of linear water waves are shown to be effective for the computations of wave-body wave fields, multi-body interactions, and wave power absorption, and novel methods are presented for determining cylindrical quantities. One of the approach's greatest attributes is that once the cylindrical coefficients are found, complex representations of waves in three dimensions are stored in vectors and matrices and are manipulated with linear algebra. Further research in cylindrical water waves will likely yield useful applications such as: efficient computations of bodies interacting with short-crested seas, and continued progress in the understanding of wave energy absorption efficiency.

The great challenge of the 21st century to mitigate climate change requires generation of green electricity to be an achievable goal. One way of producing green electricity is through usage of wave energy converters which are devices that use the energy of the ocean waves to produce electricity.   W4P Waves4Power AB is a company from Sweden, devoted to commercializing their wave energy converter called the WaveEl buoy. The WaveEl buoy is a point absorber that produces electricity by using the energy of the waves to run a hydraulic motor connected to a generator. The working principle of the buoy is to let a water piston oscillate in a tube with a water column. The water column exerts flow resistance forces on the piston as it oscillates, and these forces create a frame of reference upon which the hydraulic motor system may operate. There are leakage clearances at the sides of the water piston that allow for flow of water past the piston and associated with this flow are parts of the flow resistance forces. The flow resistance forces that are present due to water flow in the leakage clearances are calculated with the use of a flow resistance parameter and, in the literature, there is little investigation conducted as to the importance of this parameter.  The goal of present thesis work was to investigate the effect on 4 parameters of the WaveEl buoy system (power captured from the waves, flow resistance force acting on the piston, mean piston position, and number of bumper hits) due to adoption of 3 different values of the flow resistance parameter. One of these values was the currently assigned value by Waves4Power at the time when this study was conducted. The value was the constant 0.75 and was a guess by Waves4Power. The other two values were received from a parallel thesis work done at Karlstad University by Linnéa Tebelius where, with the use of CFD, Tebelius calculated the flow resistance parameter with different levels of accuracy.  The results of present thesis work were generated from simulations in a MATLAB Simulink model describing the dynamics of the WaveEl buoy system. Simulated time varied from 5.5 to 8.5 minutes per simulation. Generated results were compared to the results from using 0.75 as the value for the flow resistance parameter and showed that the energy captured from the waves was, at most, overestimated by approximately 13 % and underestimated by 6 %, depending on applied level of accuracy for description of the dynamic flow resistance parameter and simulated wave state. Furthermore, it was found that number of bumper hits varied extensively, in some cases from 0 to 47, between simulations where the only difference was applied value of the flow resistance parameter – further indicating that assigning a more accurate value on the dynamic flow resistance parameter may be of great importance when modelling the dynamics of the WaveEl buoy system.

Ocean wave energy has the potential to play a crucial role in the shift to renewable energy. In order to improve wave energy conversion techniques, a recognition of the sub-optimal nature of traditional sequential design processes due to the interconnectedness of subsystems such as the geometry, power take-off, and controls is necessary. A codesign optimization in this paper seeks to include effects of all subsystems within one optimization loop in order to reach a fully optimal design for an oscillating-surge wave energy converter. A width and height sweep serves as a brute force geometry optimization while optimizing the power take-off components and controls using a pseudo-spectral method for each geometry. An investigation of electrical power and mechanical power maximization also outlines the contrasting nature of the two objectives to illustrate electrical power maximization's importance for identifying optimality. The codesign optimization leads to an optimal design with a width of 12 m and a height of 10 m. The power take-off and controls systems are also examined more in depth to identify important areas for increased focus during detailed design. Ultimately, the codesign optimization leads to a 61.4% increase in the objective function over the optimal design from a sequential design process while also requiring about half the power take-off torque. / Master of Science / Ocean wave energy has the potential to play a crucial role in the shift to renewable energy sources. The Earth's vast oceans have immense energy potentials throughout the world, which often follow the seasonal trends of electricity demand in temperate climates. Wave energy harvesting is a technology which has been studied significantly, but has not yet experienced commercial success, partially due to the lack of convergence on a type of wave energy converter. In order to improve wave energy conversion techniques and support the convergence on a particular type, a recognition of the sub-optimal nature of traditional sequential design processes due to the interconnectedness of subsystems is necessary. A codesign optimization in this paper seeks to include effects of all subsystems within one optimization loop in order to reach a fully optimal design for an oscillating-surge wave energy converter. A width and height sweep serves as a brute force geometry optimization while optimizing the power take-off and control components for each geometry. The codesign optimization leads to an optimal design with a width of 12 m and a height of 10 m. Ultimately, the codesign optimization leads to a 62% increase in performance over the result from a sequential design process.