Thesis (MScEng (Mechanical and Mechatronic Engineering))--University of Stellenbosch, 2010.

The search for new solutions for the generation of energy is becoming more and more important for our future. Big arguments and disagreements on e.g. the questions of gas transport or the dependence on energy supplied by other countries raise demands on the development of new forms of alternative energy resources. Wave power is one of the main sources of renewable energy due to the high power density stored in ocean waves. Nevertheless, the dynamic forces of waves are so large that serious questions popped up on how to design a system which could work even in an unfavourable wave climate or could at least retain working capabilities after big storms without significant damages. This thesis studies the reliability of the mechanical parts of a linear direct driven permanent magnet generator. The results of offshore experiment where strain gauge sensors instrumented on the capsule and the inner framework structure are presented. Stress estimation analyses using strain gauges are carried out. A method for measuring forces and moments in the mechanical structure of the WEC is developed. Evaluation of the lateral force acting on the outer structure is a key factor for the design and construction of the WEC. A method for the measurement of the lateral force acting on the capsule has been developed. A study of the inclination angle between the Wave Energy Converter and the floating buoy has been carried out. The aim of this work is to contribute to the development of wave energy conversion system, and especially to the estimation of structural loads which are important for the survivability of the system under hard sea states. This work is a step that may influence future design of wave energy devices in terms of material aspect, survivability in a hard wave climate and cost-effective renewable energies.

This PhD is dedicated to the development of an advanced numerical model for simulating interactions between free surface waves of arbitrary steepness and rigid bodies in high amplitude motions. Based on potential theory, it solves the coupled dynamics of waves and structure with the implicit method by Van Daalen (1993), also named the acceleration potential method by Tanizawa (1995). The precision of this two-dimensional model is tested on a wide range of applications involving the forced motion or free motion of a submerged horizontal cylinder of circular cross-section : diffraction by a fixed cylinder, radiation by a cylinder in specified high amplitude motions, wave absorption by the Bristol cylinder. In each of these applications, numerical results are compared to experimental data or analytical solutions based on the linear wave theory, with a good agreement especially for small amplitude motions of the cylinder and small wave steepnesses. The irregular wave generation by a paddle and the possibility to add an extra circular cylinder are integrated in the model and illustrated on practical applications with simple wave energy converters. The model is finally extended to three dimensions, with preliminary results for a sphere in large amplitude heaving oscillations

Surface gravity waves in the world’s oceans contain a renewable source of free power on the order of terawatts that has to this date not been commercially utilized. The division of Electricity at Uppsala University is developing a technology to harvest this energy. The technology is a point absorber type wave energy converter based on a direct-driven linear generator placed on the sea bed connected via a line to a buoy on the surface. The work in this thesis is focused mainly on the energy transport of ocean waves and on increasing the transfer of energy from the waves to the generator and load. Potential linear wave theory is used to describe the ocean waves and to derive the hydrodynamic forces that are exerted on the buoy. Expressions for the energy transport in polychromatic waves travelling over waters of finite depth are derived and extracted from measured time series of wave elevation collected at the Lysekil test site. The results are compared to existing solutions that uses the simpler deep water approximation. A Two-Body system wave energy converter model tuned to resonance in Swedish west coast sea states is developed based on the Lysekil project concept. The first indicative results are derived by using a linear resistive load. The concept is further extended by a coupled hydrodynamic and electromagnetic model with two more realistic non-linear load conditions. Results show that the use of the deep water approximation gives a too low energy transport in the time averaged as well as in the total instantaneous energy transport. Around the resonance frequency, a Two-Body System gives a power capture ratio of up to 80 percent. For more energetic sea states the power capture ratio decreases rapidly, indicating a smoother power output. The currents in the generator when using the Two-Body system is shown to be more evenly distributed compared to the conventional system, indicating a better utilization of the electrical equipment. Although the resonant nature of the system makes it sensitive to the shape of the wave spectrum, results indicate a threefold increase in annual power production compared to the conventional system.

Cette thèse se focalise sur un système récupérateur de l’énergie des vagues qui est constitué d’un flotteur contenant des cuves partiellement remplis d’eau. Lorsque les vagues mettent en mouvement le flotteur, un tourbillon de type vidange apparaît dans une des cuve. Pour extraire l’énergie, une turbine, reliée à une génératrice, est plongée dans ce tourbillon. Tout d’abord, le tourbillon de vidange est étudié expérimentalement dans un contenant fixe. Les hauteurs d’eau et les vitesses du liquide sont mesurées. Ces vitesses sont estimées par vélocimétrie laser (LaserDoppler Velocimetry, LDV). Cet écoulement est modélisé numériquement en résolvant les équations de Navier-Stokes dans les deux phases (eau et air) par la méthode des volumes finis (avec le logiciel OpenFOAM). L’interface entre les deux phases est déterminée par la méthode des Volume of Fluid (VoF). Des comparaisons entre les résultats de ces deux approches sont menées. Ensuite, l’écoulement à l’intérieur du système houlomoteur est étudié en plaçant une maquette du dispositif sur un Hexapode (machine capable d’imposer des mouvements à la maquette à la manière d’un flotteur en mer). Les hauteurs d’eau et les efforts hydrodynamiques sur la maquette et, le cas échéant, la puissance électrique produite sont mesurés. Ces données sont comparées aux résultats d’un modèle numérique similaire à celui utilisé pour la première campagne expérimentale mais appliqué à ce dispositif. Enfin, l’influence de la turbine sur le reste du système est étudiée et son comportement en puissance est évalué pour différents mouvements imposés. Un premier modèle numérique de cette turbine est comparé aux données expérimentales. / This thesis focuses on the physical and numerical model of a wave energy converter (WEC). This device is made up of a buoy with compartments aboard partially filled with water. When the waves move the buoy, a bathtub vortex appears in one of these compartments. The energy is harvested with a turbine placed at the vortex’s center. First, the bathtub vortex is studied numerically and experimentally in a fixed compartment. Water levels are measured using acoustics sensors and water velocities are measured by Laser Doppler Velocimetry (LDV). This flow is modeled solving the Navier-Stokes equations in the two phases (air and water) with a finite volume method (with the software OpenFOAM). The interface is determined using the volume of fluid (VoF) method. Comparisons between experimental data and numerical data are presented. Afterwards, a second experimental campaign is conducted to study the complete flow inside the WEC. Therefore a model of the inside part of the WEC is fixed at the top of a Hexapod. This device can translate and rotate the model in the same way the waves would move a buoy. Water levels and hydrodynamic forces on the model are measured. When the turbine is there, the tension delivered by its generator is measured. This experimental device is modeled numerically. This model is closed to the first one. The results are compared with experimental data. Finally, a preliminary study of the turbine shows its influence on the general flow in the WEC and the evolution of the turbine power with the imposed motion. A first model of the turbine in a fixed compartment is presented and compared with experimental data.

Wave Energy Converters (WEC) are used to transform energy stored in ocean waves into electrical energy. One type of WECs consists of buoyant bodies. To extract energy from their motion, hydraulic cylinders can be used to generate hydraulic power. For conversion into electric power various systems have been analysed in literature. However, the focus was put on efficiency and rigorous analyses of the system behaviour are still missing. In this paper an exemplary system consisting of two hydraulic cylinders, switchable check valves, accumulators and three motor-generator sets is analysed with help of simulation and measurement. This exemplary system is called WavePOD and was installed at the Institute for Fluid Power Drives and Controls (IFAS) of RWTH Aachen University together with Aquamarine Power and Bosch Rexroth for testing. In this paper the data collected during various test phases is used for system analysis and for validating the simulation. The simulation model is presented. The system’s response to various switching operations is investigated. Comparing the simulation with measurements validates the system`s dynamic model.

Obtaining the perfect renewable energy source is one of the most important questions of our lifetime. One renewable energy source that could be of interest in this question because of the characteristics of the power that could be extracted from it is wave energy. There has however not been enough research done to reach a technology viable enough for large scale adoption. This study was made to investigate how to formulate the optimum design guidelines and criteria for a chain and shackle in the connection line between buoy and wave energy converter (WEC). Firstly, by conducting a literature study the material and design of the system was chosen. A main goal of the report was to make it have value in the industry, because of this the choice of design and material was based on industry standards. The material choice became the austenitic stainless steel with the grade R4, and the design of choice became the stud less chain link and the forelock shackle. A value of the expected force in the buoy WEC connection line (buoy line) was extracted from sixteen different data sets given from a wave tank test done in COAST Laboratory of Plymouth University, UK. These tests were done with two different buoys, one with a cylindrical shape and one with a torus shape. They were also done with and without dampening (the dampening was equal to 59 kN). Each of these four configurations had four different tests conducted on them resulting in a total of sixteen different data sets. The force value that occurs in the buoy line from the sixteen different wave tank tests was then scaled up and used in calculating the final diameter of the chain link. A safety factor of 1.35 was used to account for the statistical uncertainties in the characteristic properties of the specific part. These calculations were based on the fact that all chains have to be proof loaded at 70 % of their minimum theoretical breaking load and that a chain should at maximum undergo a force that is equal to 25 % of its minimum breaking load. Extra material was also added to accommodate for the corrosion that will occur in submerged environments. Finally, a finite element analysis was done on one of the chains links. The results showed that the biggest amount of von Mises stress and equivalent plastic strain occur in the inner corner of the chain link. All the contact area and the “crown” were however also shown to have plastically deformed. The plastic deformation in the contact area does not discredit the design because it is a local plasticity in a small region which leads to work hardening which in turn means that the new yield strength is higher at the deformed points, this in turn means that the wave climate will only elastically deform the system under its cyclic load and that the system will not plastically deform more than the results from the proof loading. This is very positive and will give the system a prolonged lifetime. However plastic deformation in the “crown” contributes to crack initiation which with time may lead to fatigue failure and should be considered in future studies.

Renewable energy sources are a hot topic, both when it comes to climate change and the constant increase in demand of electricity due to population growth and a more electrified society. One such energy source is wave energy - an energy source with great potential but still relatively new with the need for further development. Blekinge Institute of Technology (BTH) together with Ocean Harvesting Technologies (OHT) have made a collaboration to build a scaled Hardware-In-the-Loop (HIL) system of a power take-off (PTO) based on OHTs wave energy converter, InfinityWEC. The purpose is to teach the students at BTH about hydrodynamic and HIL simulations. A manual will also be written to help students perform the lab activities. A model of the HIL system will first be implemented in Matlab/Simulink, both with and without the WEC-Sim hydrodynamic simulation toolbox and simulations will be run to predict the system's behaviour. To parametrize the hydrodynamic model, the open-source Boundary Element Method (BEM) code, NEMOH, is used. The HIL system consists of electric motors, connected mechanically to each other with a coupling. One of the motors is the actuator, which applies torque to the second motor according to the simulated hydrodynamic loads on the buoy. The second motor on the other hand applies a torque according to the load connected to it or torque-controlled according to a selected control strategy. In this thesis two different types of loading is used: 1) resistive load without control of the generator drive, 2) resistive and capacitive load with reactive control of the generator drive. The load resistance can be changed within a limited range as well as the sea state. Data that can be collected are the position and angular velocity of the motors, the currents to and from the two motors and the voltage over the load capacitance. The project concluded that the compensation needed for the motors to get the true hydrodynamic force has little effect when using reactive control and that a protective capacitor is be needed between the actuator motor and the power supply to protect it from reverse current. Finally, this work demonstrated the effectiveness of HIL systems to execute simulations to test and validate PTO systems in wave energy converters. The advantages are that one can create representative wave loading without the presence of water and with ease test different sea states. / Förnybara energikällor är ett hett ämne, både när det gäller klimatförändringar och den ständiga ökningen av efterfrågan av el på grund av befolkningsökning och ett mer elektrifierat samhälle. En sådan energikälla är vågenergi - en energikälla med stor potential men fortfarande relativt ny med behov av vidareutveckling. Blekinge Tekniska Högskola (BTH) vill tillsammans med Ocean Harvesting Technologies (OHT) konstruera ett skalat Hardware-In-The-Loop (HIL) system av power take-off (PTO) baserat på OHT:s vågenergiomvandlare, InfinityWEC. Syftet är att lära eleverna på BTH om hydrodynamik och HIL-simuleringar. En manual kommer också att skrivas för att hjälpa eleverna att utföra labbaktiviteterna. En modell av HIL-systemet kommer först att konstrueras i Matlab/Simulink, både med och utan WEC-Sim hydrodynamisk simuleringsverktygslåda och simuleringar kommer att köras för att förutsäga systemens beteende. För att bestämma de nödvändiga parametrarna för hydrodynamiska modellen används Boundary Element Method koden NEMOH. HIL-systemet består av elmotorer, som är mekaniskt anslutna till varandra med en koppling. En av motorerna är ställdonet, som tillämpar vridmoment på den andra motorn enligt de simulerade hydrodynamiska belastningarna på bojen. Den andra motorn tillämpar ett vridmoment enligt belastningen som är kopplad till den eller är moment reglerad enligt en vald kontrollstrategi. I denna avhandling används två olika typ av belastning: 1) resistiv belastning utan kontroll av generatorndrivdonet, 2) resistiv och kapacitive belastning med reaktiv kontroll av generatorndrivdonet. Belastningsmotståndet kan ändras inom ett visst intervall och lika så havstillståndet. Data som kan samlas in är motorernas position och vinkelhastighet, strömmen till och från de två motorerna och spänningen över last kapasitatorn. I projektet drogs slutsatsen att den kompensation som behövs för motorerna för att få den riktiga hydrodynamiska kraften har liten påverkan reaktiv kontroll används och att det behövs en skyddande kondensator mellan ställdonsmotorn och strömförsörjningen för att skydda den mot bakström. Slutligen visade detta arbete hur effektiva HIL-system är för att utföra simuleringar för att testa och validera PTO-system i vågenergiomvandlare. Fördelarna är att man kan skapa representativ vågbelastning utan närvaro av vatten och med lätthet testa olika havstillstånd.

Wave energy technology is currently growing and gaining popularity. With around 100 separate technologies researched globally in over 25 countries wave energy are believed to soon be able to compete with other renewable sources such as wind energy. One of the new technologies is WaveTube; a wave energy converter currently under development and in need of technical verification. The basic idea of WaveTube is a partially submerged container with an enclosed fresh water volume. The kinetic energy of the ocean waves are transferred onto the floating container, creating an inner flow in the structure and electricity is generated as the fresh water flows through turbines. Previous small-scale model tests have confirmed the basic idea of WaveTube and an inherent continuation is visualizing and evaluating the inner flow using Computational Fluid Dynamics. A simplified 2D simulation where the WaveTube structure is subject to a pure sinusoidal, rotational motion was believed to be able to give useful information about the inner flow field. However, this Master Thesis project shows that a simulation using ANSYS Fluent of this case is not a successful approach. With inner moving parts a so called dynamic mesh was required, which updates the mesh as the boundaries move. In order for this method to be successful the mesh needs to be of high quality. However, for the complex geometry that WaveTube is no mesh was found to meet the requirements and the calculations using the Volume of Fluid method were not able to proceed.

This research revolves around the concept design and theoretical validation of a new type of wave energy converter (WEC), comprising a pitching floater integrated with a resonant U-tank (RUT) and a Wells turbine as power take-off (PTO). Theoretical formulation of a fully coupled multi-body dynamic system, incorporating the thermodynamic processes of the RUT air chamber, its interaction with the PTO dynamics and their coupling with the floater is presented. Inaccuracies of the dynamic modeling of RUT based on Lloyd's low order model, which assumes constant hydrodynamic parameters irrespective of the frequency, are demonstrated by a series of high fidelity CFD simulations. These simulations are a systematic series of fully viscous turbulent simulations, using unsteady RANSE solvers, of the water sloshing at different frequencies of oscillation. Calibration of Lloyd’s model with CFD results evidenced that the RUT hydrodynamic parameters are not invariant to frequency. A numerical model was developed based on Simulink WEC-Sim libraries to solve the non-linear thermo-hydrodynamic equations of the device in time domain. For power assessment, parametric investigations are conducted by varying the main dimensions of the RUT and power RAOs were computed for each iteration. Performance in irregular sea state are assessed using a statistical approach with the assumption of linear wave theory. By superimposing spectrum energy density from two resource sites with RAO, mean annual energy production (MEAP) are computed. The predicted MEAP favorably compares with other existing devices, confirming the superior efficiency of the new proposed device over a larger range of incident wave frequency. / M.S. / This study present results of an investigation into a new type of wave energy converter which can be deployed in ocean and by its pitch response motion, it can harvest wave energy and convert it to electrical energy. This device consist of a floater, a U-tank (resonant U-tank) with sloshing water free to oscillate in response to the floater motion and a pneumatic turbine which produces power as air is forced to travel across it. The pneumatic turbine is used as the power take-off (PTO) device. A medium fidelity approach was taken to carry out this study by applying Lloyd’s model which describes the motion of the sloshing water in a resonant U-tank. Computational fluid dynamics (CFD) studies were carried out to calibrate the hydrodynamic parameters of the resonant U-tank as described by Lloyd and it was discovered that these parameters are frequency dependent, therefore Lloyd’s model was modelled to be frequency dependent. The mathematical formulation coupling the thermodynamic evolution of air in the resonant U-tank chamber, modified Lloyd’s sloshing water equation, floater dynamics and PTO were presented for the integrated system. These set of thermo-hydrodynamic equations were solved with a numerical model developed using MATLAB/Simulink WEC-Sim Libraries in time domain in other to capture the non-linearity arising from the coupled dynamics. To assess the annual energy productivity of the device, wave statistical data from two resource sites, Western Hawaii and Eel River were selected and used to carrying out computations on different iterations of the device by varying the tank’s main dimensions. This results were promising with the most performing device iteration yielding mean annual energy production of 579 MWh for Western Hawaii.