Renewable marine energy has attracted considerable interest in recent years, especially in the UK due to its excellent location to take advantage of this sustainable energy source. Dierent types of device have been developed over several decades to capture the energy of sea waves but they all need to be able to convert this mechanical energy into electrical energy. The success of wave energy converters (WECs) depends on their eciency, reliability and their ability to react to the variable wave conditions. Although a number of simulation studies have been undertaken, these have used signicantly simplied models and any experimental data is scarce. This work considers a heaving point absorber with a hydraulic power takeo unit. It employs a common hydraulic power take-o design, which uses the heaving motion of the buoy to drive an actuator that behaves like a linear pump. Energy storage is used to provide power smoothing in an attempt to give a constant power output from a hydraulic motor coupled to a generator. Although this design has been presented before, the ineciencies and dynamics of the components have not been investigated in detail. The aim of this work is to create an understanding of the non-linear dynamics of a hydraulic power take-o unit and how these aect the hydrodynamic behaviour of the WEC. A further aim is to predict the eciency of the power take-o unit and determine tuning and control methods which will improve the power generation. In order to do this and test the device in dierent wave conditions, a full hydrodynamic and hydraulic model is developed using the Simulink and SimHydraulics software package. The model is initially tested with regular waves to determine the behaviour of the power take-o unit and a method for adjusting the hydraulic motor displacement depending on the frequency of the incoming wave is investigated. The optimal eective PTO damping to maximise power generation is found to be dependent on the signicant wave frequency and the values of PTO damping are signicantly dierent to previous work using a linear power take-o model which emphasises the importance of including the ineciencies of the hydraulic components. The model is then analysed with irregular waves to predict the behaviour and power levels in realistic wave conditions. Power generation reduces in comparison to regular waves but a similar tuning method to maximise power generation still exists. A hydraulic motor speed control method is shown to increase power generation in irregular waves by maintaining the motor speed within an acceptable working range. Wave data from the Atlantic Ocean is then used to investigate the benets of an adaptive tuning method which uses estimated wave parameters for a number of dierent sea conditions. Results show only minimal gains from using active tuning methods over a passive method. However, results revealed signicant power losses in both calm and rough sea conditions with the PTO most ecient, at approximately 60%, in an average sea power. A scaled experimental power take-o unit is developed to help validate the simulation results. The power take-o unit is tested using a hardware-in-the-loop system in which the hydrodynamic behaviour of the WEC is predicted by a realtime simulation model. The experimental results show good agreement to the simulation with the PTO showing similar characteristics and tuning trends for maximising power generation.

The aim of this study is to develop a seawater desalination system that uses sea-wave energy as the sole energy source for system operation. This system is composed of a sea-wave energy acquisition system, a reverse-osmosis device, and a proposed mechanism linking the system to function synchronously. The relationships between various system parameters and system characteristics are analyzed. The limitations and constraints of system operations are then suggested. For the purpose of comparison, another system, which indirectly drives the reverse-osmosis system through an additional stage of energy storage, is introduced. To analyze the system dynamic properties, the following steps are implemented. First, a mathematical model than can properly describe the system characteristics is derived. This model is found to be a nonlinear one, which increase the difficulties of system analysis enormously. However, it is also noted through a preliminary examination that the effect of system nonlinearity becomes insignificantly if the system parameters are properly adjusted. Under these parameters, the linearied model is analyzed. The effects of different system parameters on the amount of energy acquisition and desalinated water are investigated. The analysis indicates that the amount of energy acquisition or desalinated water is closely related to both the selected energy acquisition system and the desalination system. For a given energy acquisition system and sea wave condition, an improper system parameter selection of desalination system will either make the whole system operation inefficient or devastate the functioning of acquisition system. This suggests that certain parameters of the desalination system must be adjustable in a real operation. The study also shows that the linearied system can be approximated by a model with two degrees of freedom. This model may offer the convenience for the optimization of system parameters.

Within a wave energy converter's operational bandwidth, device operation tends to be optimal in converting mechanical energy into a more useful form at an incident wave period that is proximal to that of a power-producing mode of motion. Point absorbers, a particular classification of wave energy converters, tend to have a relative narrow optimal bandwidth. When not operating within the narrow optimal bandwidth, a point absorber's response and efficiency is attenuated. Given the wide range of sea-states that can be expected during a point absorber's operational life, these devices require a means to adjust, or control, their natural response to maximize the amount of energy absorbed in the large population of non-optimal conditions. In the field of wave energy research, there is considerable interest in the use of non-linear control techniques to this end. Non-linear control techniques introduce time-varying and state dependent control parameters into the point absorber motion equations, which usually motivates a computationally expensive numerical integration to determine the response of the device - important metrics such as gross converted power and relative travels of the device's pieces are extracted through post processing of the time series data. As an alternative, the work presented in this thesis was based on a closed form perturbation based approach for analysis of the response of a device with periodically-varying control parameters, subject to regular wave forcing, in the frequency domain. The proposed perturbation based method provides significant savings in computational time and enables the device's response to be represented in a closed form manner with a relatively small number of solution components - each component is comprised of a complex amplitude and oscillation frequency. This representation of the solution was found to be very concise and descriptive, and to lend itself to the calculation of gross absorbed power and travel constraint violations, making it extremely useful in the automated design optimization process; the methodology allows large number of design iterations, including both physical design and control variables, to be evaluated and conclusively compared. In the development of the perturbation method, it was discovered that the device's motion response can be calculated from an in nite series of second order ordinary differential equations that can be truncated without destroying the solution accuracy. It was found that the response amplitude operator for the generic form of a solution component provides a means to gauge the device's response to a given wave input and control parameter variation, including a gauge of the solution process stability. It is unclear as of yet if this is physical, a result of the solution process, or both. However, for a given control parameter set resulting in an unstable solution, the instability was shown to be, at least in part, a result of the device's dynamics. If the stability concerns can be addressed through additional constraints and updates to the wave energy converter hydrodynamic parameters, the methodology will expand on the commonly accepted boundaries for wave energy converter frequency domain analysis methods and be of much practical importance in the evaluation of control techniques in the field of wave energy converter technology. / Graduate

No description available.

Ocean Data Acquisition System (ODAS) buoys are deployed in many seas around the world, a subset of these are wave monitoring buoys. Most are powered by solar panels. Many of these buoys are subjected to movement from waves, and could benefit from a wave energy converter specifically designed for ODAS buoys (WECO). A particular buoy that could benefit from this technology is the TriAXYS wave buoy [1]. This thesis discusses the development of a self-contained WECO that would replace one of the buoys four on board batteries, and harvest energy from the buoy motion to charge the remaining three batteries. A major constraint on the WECO is that it can’t affect buoy motion and jeopardize wave data that is derived from the motion. Rather than follow a traditional approach to simulating the motion of the buoy / WECO system, using hydrodynamic modelling and theoretical wave profiles, existing motion data from a buoy installation was analyzed to find the loads that were applied to the buoy to cause the motion. The complete set of mass properties of the TriAXYS buoy were derived from the 3D model provided by AXYS Technologies. These mass properties were compared to the linear and rotational accelerations to find the loads that were applied at the buoy center of gravity (CG) to cause the recorded motion. An installation off the coast of Ucluelet, BC was selected for this investigation because it is subjected to open ocean swells, and data from the winter months of November to March of 2014 to 2016 is available. Winter data was used since there is more wave action to power the WECO during the winter months, and there is sufficient solar irradiation to power the buoy in summer months. Accurate buoy motion data at a 4 Hz sampling rate was available from three rotational rate gyros and three linear accelerometers installed in the buoy. Each dataset of samples represented a 20 minute window that was recorded once every hour. Five conceptual WECO designs were developed, each of which focused on the extraction of power from a different degree of freedom (DOF) of buoy motion (surge, sway, heave, roll, and iv pitch). Three designs used a sliding (linear) oscillating mass, and one was aligned with each of the surge, sway, and heave axis of the buoy. Two designs used a rotating oscillating mass, and the axis of rotation of each device was aligned with either the roll or pitch axis of the buoy. All proposed WECO configurations were modeled as articulated mass, spring, and damper systems in MATLAB using the Lagrange method. Each WECO/buoy assembly formed an articulated body. Mass properties for each configuration were derived from the 3D models. The equations of motion for the original buoy no longer applied, but the environmental forces applied to the hull would still be valid as long as the WECO didn’t alter motion significantly. The power take off (PTO) was modeled following standard convention as a viscous dashpot. The damping effect of the dashpot was included in the models using Rayleigh’s dissipation function that estimated the energy dissipated by the PTO. A subset of load datasets was selected for evaluating the maximum power potential of each WECO. Each WECO was tuned to each dataset of loads using the spring rate, and the damping coefficient was optimized to find the maximum power while avoiding end stop collisions. A second subset of data was selected to evaluate the average power that would be generated throughout the winter months for the two most promising designs. This evaluation was performed for static spring and damping coefficients, and the coefficients that resulted in the highest power output were discovered. The motion of the WECO oscillating mass with respect to the buoy was used in conjunction with the damping ratio to form an estimate of the ideal (i.e. with no mechanical or electrical losses) power generation potential of each WECO configuration during the winter months. The two leading WECO designs both had sliding (linear) oscillating masses, one was aligned with the surge axis and produced theoretical average of just over 0.5 W, the other was aligned with the heave axis and produced theoretical average of just under 0.5 W. / Graduate

Multiple wave energy devices supported by a common structure represent one possible method of efficiently converting ocean wave energy into electricity. In this study, experimental measurements of multiple small-scale wave energy devices are reported to assist the development and validation of numerical models. Through observation and measurement, the response of two float geometries subjected to a range of wave conditions and device settings were determined. A range of regular wave conditions were identified that caused a linear relationship to occur between the heave displacement amplitude of the float and the incident wave amplitude. These test cases will enable comparisons to be made with linear simulations of response. Tests conducted in various wave conditions have highlighted the capability of altering the device response by changing the equilibrium draft of one float geometry. Additional damping on the upper surface of the float, due to wave overtopping, could be exploited as a method of limiting the heave response of the device in large amplitude waves. The influence of hydrodynamic interactions on arrays of closely spaced devices has been experimentally investigated for devices subjected to regular and irregular wave conditions. The magnitude and occurrence of interactions and their affect on the individual device response is demonstrably dependent on the incident wave frequency and device separation distance. Compared to an isolated device, positive interactions result in higher average power outputs for an array of devices at certain wave frequencies. Positive interactions occuring at particular wave frequencies are balanced by negative interactions at other wave frequencies, in agreement with published numerical studies of array performance. Varying the level of mechanical damping applied to the float through the power take-off system results in a frequency shift of the calculated power transfer function and alters the motion path of the float. This finding implies that the level of generator torque could be used as an alternative method to tune the response of the device based on the measured incident wave-field. Several time-averaged and time-varying approaches to simulating the response of a wave energy device subjected to wave-field forcing and undergoing free response have been studied. By comparing the simulated and measured responses, the feasibility of using linear and non-linear force terms in a time-varying model has been assessed. In general, single degree-of-freedom simulations based on linear hydrodynamic parameters tend to over-predict device response amplitudes, requiring the application of additional damping. The simulation approach which resulted in the closest agreement with measured responses required the combination of linear diffraction force and radiation added mass terms with non-linear drag and buoyancy force terms, as well as body inertia and gravity forces. This approach goes part way to simulating the complex time-varying hydrodynamics associated with a wave energy device subjected to wave-field forcing.

The research conducted in this study sought to appraise and validate the IEC-TC-114 technical specification for wave energy resource assessment in terms of its methodologies, requirements, and sources of error. This assessment was conducted through the pilot application of the IEC-TS to a project site located on the west coast of Vancouver Island in British Columbia. The objectives of this research were to (1) establish a firm scientific rational for the technical specification, (2) suggest improvements for future iterations of the technical specification, (3) conduct a sensitivity analysis to determine sources or error and uncertainty within the specification, (4) develop a user guidance , and (5) to complete an assessment of the wave energy resource for all three classes of assessment at the pilot project site. At the time of this research there has been limited assessment and validation of the technical specification and its applicability to real world studies. This study is also limited in its scope to a singular site within Canada and so specific observations made may not be applicable to all locations. It was observed over the course of this study that the IEC-TS could be applied to the assessment of wave energy with reasonable computational effort. Additionally, the IEC-TS is overall relatively easy to understand and apply, with the only exception to this being the validation procedure which lacked clarity. However, it was noted that in many cases the IEC-TS would not provide defined requirements, but only recommendations which would allow researchers to potentially disregard the recommendations provided within. While this is likely due to the fact that different sites may have different requirements, it does call into question the need for a standard if it is not going to have strict requirements to ensure that all resource assessments are completed using state of the art knowledge and procedures.

Thesis (MScEng)--Stellenbosch University, 2012. / ENGLISH ABSTRACT: As a renewable energy source, wave energy has the potential to contribute to the increasing global demand for power. In South Africa specifically, the country’s energy needs may easily be satisfied by the abundance of wave energy at the South-West coast of the country. Commercially developing and utilizing wave energy devices is not without its challenges, however. The ability of these devices to survive extreme weather conditions and the need to achieve cost-efficacy while achieving high capacity factors are but some of the concerns. Constant changes in wave heights, lengths and directions as well as high energy levels and large forces during storm conditions often lead to difficulties in keeping the complexity of the device down, avoiding over-dimensioning and reaching high capacity factors. The point absorber device developed as part of this research is based on an innovation addressing the abovementioned issues. An approach is followed whereby standard "offthe- shelf" components of a proven hydraulics technology are used. The size of the device is furthermore adaptable to different wave climates, and the need for a control system is not necessary if the design parameters are chosen correctly. These characteristics enable low complexity of the device, excellent survivability and an exceptionally high capacity factor. This may lead to low capital as well as low operationand maintenance costs. In this paper the working principle of this concept is presented to illustrate how it utilises the available wave energy in oceans. The results obtained from theoretical tests correlate well with the experimental results, and it is proven that the device has the ability to achieve high capacity factors. As the device makes use of existing, "off-the-shelf" components, cost-efficient energy conversion is therefore made feasible through this research. / AFRIKAANSE OPSOMMING: As ’n hernubare/ herwinbare energiebron bied golfenergie die potensiaal om by te dra tot die bevrediging van die stygende globale energie-navraag. In spesifiek Suid-Afrika kan die oorvloed van beskikbare golfenergie aan die Suid-Weskus van die land gebruik word om aan die land se energiebehoeftes te voldoen. Betroubaarheid en oorlewing in erge weerstoestande, koste-effektiwiteit en die behaal van hoë kapasiteitsfaktore is beduidende struikelblokke wat oorkom moet word in die poging om ’n golfenergie-omsetter wat kommersieël vervaardig kan word, te ontwikkel. Daarby dra voortdurende veranderings in golfhoogtes, -lengtes en -rigtings sowel as hoë energievlakke en groot kragte tydens storms by to die feit dat dit moeilik is om die kompleksiteit van die stelsel laag te hou. Dit terwyl daar voorkom moet word dat die toestel oorontwerp en verhoed word dat hoë kapsiteitsfaktore bereik word. Die puntabsorbeerder-toestel wat in hierdie navorsing ontwikkel is, bestaan uit ’n ontwerp wat spesifiek ontwikkel is om die bogenoemde probleme aanspreek. ’n Unieke benadering is gevolg waardeur standaard, maklik-bekombare komponente gebruik is en die komponent-groottes ook aangepas kan word volgens golfgroottes. Indien die ontwerpsdimensies akkuraat gekies word, is die moontlikheid verder goed dat ’n beheerstelsel nie geïmplementeer hoef te word nie. Hierdie eienskappe verseker lae stelselkompleksiteit, uitstekende oorlewingsvermoë en ’n uitstaande kapasiteitsfaktor. Lae kapitaal- sowel as onderhoudskostes is dus moontlik. Die doel van hierdie dokument is om die werking van die konsep voor te stel en teoreties sowel as prakties te evalueer. Die resultate van teoretiese toetse stem goed ooreen met eksperimentele resultate, en dit is duidelik dat die toestel hoë kapasiteitsfaktore kan behaal. Aangesien die toestel verder gebruik maak van bestaande komponente wat alledaags beskikbaar is, word die koste-effektiewe omsetting van golfenergie dus moontlik gemaak deur hierdie navorsing.

Thesis (PhD)--Stellenbosch University, 2013. / ENGLISH ABSTRACT: In hierdie tesis word ’n nuwe lug kern permanent magnet (PM) lineêre generator (LG) vir toepassing tot direk aangedrewe (DA) oseaan golf energie omsetters (GEO) ontwikkel. Die nuwe LG word ontwikkel vanaf die lineêre dubbel-kant topologie in ’n poging om probleme met die huidige longitudinale vloed (LV) yster kern LGs, wat tot dusvêr oorheersend voorkom in eksperimentele DA-GEOs, te oorkom. Die grootste probleem met hierdie LGs is die masiewe aantrekkings kragte tussen hul yster stators en die PM transleerders. ’n Groot hoeveelheid strukturuele staal word benodig om die luggaping te handhaaf, terwyl die las op die laars ook ’n groot probleem is. Die nuwe LG gebruik ’n lug kern stator wat alle aantrekkings kragte tussen die stator en transleerder elimineer en dus die nodige strukturuele material verminder. Die topologie van die transleerder is ook van so ’n aard dat die netto aantrekkings kragte op enige spesifieke PM ideaal nul is; dit verminder die strukturuele materiaal selfs verder. Die transleerder het ook ’n nuwe transversale vloed pad wat die sogenaamde paarwyse vloed koppeling wat in LV-LGs voorkom, en die negatiewe effekte daarvan, verhoed. ’n Aantal nuwe bydraes tot die veld van LGs vir DA-GEO word in hierdie tesis gemaak. ’n Nuwe topologie lug kern PMLG is ontwikkel soos bespreek. Dit sluit in die ontwikkeling van analitiese en eindige element modelle en ’n optimerings prosedure wat vinnig optimale dimensies vir minimum aktiewe massa van die nuwe LG vind. In die ontwerp word dit ook gevind dat die drywingsdigtheid van LGs verbeter kan word deur zero oorvleuling tussen die die stator en transleerder by die slag endte toe te laat. ’n 1 kW prototipe van die nuwe LG word ontwerp en gebou; die uitvoerbaarheid van die konstruksie vir die nuwe topologie op ’n klein skaal word dus gedemonstreer. ’n Unieke toets opstelling word ook ontwerp en is gebasseer op bestaande toerusting in die vorm van ’n wind turbine generator en rug-aan-rug spannings bron omsetters. Met die toets opstelling word ’n enkel frekwensie golf ge-emuleer om die teorie en simulasies te verifieer en word ook ’n voorspellende beheer strategie geimplementeer, wat vir die eerste keer gedemonstreer word vir LGs vir DA-GEOs. Goeie ooreenstemming tussen die gemete en gesimuleerde data bevestig die voorgestelde modellerings en ontwerps metodes. / AFRIKAANSE OPSOMMING: In this thesis, a novel air-cored permanent magnet (PM) linear generator (LG) is developed with application to direct drive (DD) wave energy converters (WECs). The novel LG is developed from the linear double-sided topology in an attempt to overcome the problems with current longitudinal flux (LF) iron-cored LGs, which have so far been dominant in experimental DD-WECs. The biggest problem with these LGs is the massive attraction forces between their iron stators and PM translators. A large amount of structural steel is required to maintain the air gap, while the load on the bearings is also a large concern. The novel LG uses an air-cored stator which eliminates any attraction forces between the stator and translator and hence reduces the required structural material. Furthermore, the topology of the translator is such that the net attraction force on any particular PM is ideally zero, which even further reduces the structural material required for the translator. A new transverse circulating flux path is also introduced in the translator which prevents pair-wise flux coupling and its negative effects as observed in LF-LGs. A number of new contributions are made to the field of LGs for DD-WECs in this thesis. A novel topology aircored PMLG is developed as described. This includes the development of analytical and finite element models and an exhaustive optimisation procedure for quickly finding optimal dimensions for minimum active mass of the novel LG. In the design it is also found that the power density of LGs can be improved by allowing zero overlap between the stator and translator at the stroke ends. A 1 kW prototype of the novel LG is designed and built; the feasibility of constructing the novel LG on a small scale is as such demonstrated. A unique test rig is designed based on existing equipment in the form of a wind turbine generator and back-to-back voltage source converters. The test rig allows emulation of a monochromatic wave for verifying the theory and simulations and also allows for implementation of a predictive control strategy, which is for the first time demonstrated for LGs for DD-WECs. Good agreement between measured and simulated data confirms the presented modelling and design methods.

Thesis (MScEng)--Stellenbosch University, 2012. / ENGLISH ABSTRACT: See item for full text / AFRIKAANSE OPSOMMING: Sien item vir volteks