Combined electrical and structural models of five types of permanent magnet linear electrical machines suitable for direct-drive power take-off on wave energy applications are presented. Electromagnetic models were developed using polynomial approximation to finite element analysis results. The structural models are based on simple beam theory, other classical techniques, and automated finite element analysis formulations. The machine models have been integrated with a time-domain model of a wave energy converter based on a heaving buoy. They have then been optimised using a genetic algorithm approach, using a score based primarily on the amortised cost per unit of energy production. The optimised designs have then been used for a comparison of the economic performance of the generator types.

Most designs for wave energy converters include a hydraulic (or pneumatic) interface between the wave device and the generator to smooth electricity production, but a direct drive power take-off system is a possible way of increasing the power transfer efficiency and the reliability, which was first adopted by Archimedes Wave Swing. Direct drive wave energy systems normally include a low speed linear generator directly coupled with the wave device. With no mechanical interface, the mechanical energy loss and maintenance requirements can, in theory, be significantly reduced. To maximize the energy capture, the motion of the wave energy converter must be controlled to achieve mechanical resonance so that the velocity is in phase with the incoming waves. So far, a number of control methods have been proposed, but few of them have been tested experimentally. For direct drive linear generators in real sea conditions, reaction force control is shown to be an effective way to achieve control where knowledge of future wave could not be required. Different reaction force control methodologies are suggested where the force is provided directly from the linear generator. Among these methodologies, complex conjugate control is regarded as the optimal control and can be used to achieve mechanical resonance. When resonance occurs, some system parameters such as the system excursion and required power take-off force become extremely large, and may exceed the design parameters. In this thesis, the system is modelled under reaction force control taking into account practical considerations which are based on design parameters. A novel control scheme for a direct drive linear generator to achieve such reaction force control in irregular waves is proposed, where a voltage-source rectifier is employed as the bridge between the linear generator and the dc bus. The application of linear generator in real wave conditions not only has inherent advantages, but also present a big challenge for controller design in order to obtain maximum power production. For a linear generator in real sea states, reaction force control idea can be implemented to adjust the velocity of motion, hence to maximize the power production, where the required currents in the generator coils to provide the desired force are constantly varying in frequency and amplitude. The control strategy of the active rectifier is developed based on the derived three-phase currents and the dynamic response of the system to determine varying modulation indices. The unknown situations and some unmeasurable parameters in the system degrade the performance of the control system, hence the current feedback and PI controller are both adopted to reject the effect of the disturbance. Simulation verifications are included for the proposed control idea.

This document presents a novel active magnetic levitation system. In the pursued of this endeavour different topics related with wave energy were explore. Climate change and energy security are the main motivation to pursued new options for non-fossil fuels energy generation. An overview of renewable energy and specifically of wave energy was presented. The potential for wave energy in The United Kingdom turn out to be 75 TWh/year from wave energy, 3 times more of what wind energy has produced in 2013. This means a massive impact on the energy market and emission reduction. In order to achieve this, improvements on wave energy devices have to be done. An overview of wave energy converters was covered selecting the C-Gen as the generator topology this document will base its studies. Linear generator bearings are desired to have long lifespan with long maintenance intervals. The objective is to come with an active magnetic levitation design that can replace traditional bearings augmenting the reliability of the system. Therefore magnetic bearings option have been reviewed and simulation experimentations has resulted in a novel active magnetic levitation system using an air-cored coils Halbach array acting over a levitation track. The configuration would generate bi directional repulsion forces with respect of the levitating body. Different software were used to analyse the magnetic field and forces generation. Additionally a prototype was built and tested to corroborate the results. As part of the modelling a mathematical model was explored and robust control implementation was also realised. Finally a scalability study of the device as well as a reliability analysis was done. Although the reliability studies shows an increase of ten times of the mean time to failure, the concept is not able to endure the loads acting on the generator unless the magnetic bearings became bigger than the generator and therefore economically unfeasible.

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.

Wave energy is a renewable energy source with a large potential to contribute to the world's electricity production. There exist several technologies on how to convert the energy in the ocean waves into electric energy. The wave energy converter (WEC) presented in this thesis is based on a linear synchronous generator. The generator is placed on the seabed and driven by a point absorbing buoy on the ocean surface. Instead of having one large unit, several smaller units are interconnected to increase the total installed power. To convert and interconnect the power from the generators, marine substations are used. The marine substations are placed on the seabed and convert the fluctuating AC from the generators into an AC suitable for grid connection. The work presented in the thesis focuses on the first steps in the electric energy conversion, converting the voltage out from the generators into DC, which have an impact on the WEC's ability to absorb and produce power. The purpose has been to investigate how the generator will operate when it is subjected to different load cases and to obtain guidelines on how future systems could be improved. Offshore experiments and simulations have been done on full scale generators connected to four different loads, i.e. one linear resistive load and three different non-linear loads representing different cases for grid connected WECs. The results show that the power can be controlled and optimized by choosing a suitable system for the WEC. It is not obvious which kind of system is the most preferable, since there are many different parameters that have an impact on the system performance, such as the size of the buoy, how the generator is designed, the number of WECs, the highest allowed complexity of the system, costs and so on. Therefore, the design of the electrical system should preferably be carried out in parallel with the design of the WEC in order to achieve an efficient system. / <p>Felaktigt tryckt som Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology 727</p>

Direct drive Permanent Magnet Linear Generators (PMLGs) are used in energy converters for energy harvesting from marine waves. Greater reliability and simplicity can be achieved for Wave Energy Converters (WECs), by using direct drive machines linked to the power take-off device, in comparison with WECs using rotational generators combined with hydraulic or mechanical interfaces to convert linear to rotational torque. However, owing to the relatively low velocities of marine waves and the desire for significant energy harvesting by each individual unit, direct drive PMLGs share large permanent magnet volumes and hence, high magnetic forces. Such forces can generate vibrations and reduce the lifetime of the bearings significantly, which is leading to an increase in maintenance costs of WECs. Additionally, a power electronics converter is required to integrate the generator‘s electrical output to meet the requirements for connection to the national grid. This thesis is concerned mainly with the fundamental investigation into the use PMLGs for direct drive WECs. Attention is focused on developing several new designs based on tubular long stator windings topologies and optimisation for flat PMLGs. The designs are simulated as air- and iron-cored machines by means of Finite Element Analysis (FEA). Furthermore, a new power electronics control system is proposed to convert the electrical output of the long stator generators. Various wave energy-harvesting technologies have been reviewed and it has been found that permanent magnet linear machines demonstrate great potential for integration in WECs. The main reason is the strong exaltation flux provided by the high number of permanent magnets. Such flux, combined with design simplicity, can deliver high induced voltage as well as structural integrity. In the thesis, a flat single and double structured iron-cored PMLG is studied and optimised. Several magnetic force mitigation techniques are investigated and an optimisation is conducted. The optimisation is concerned mainly with increasing electrical output power and reducing the magnetic forces in the generators. As a result, an optimal design introducing the idea of separated magnetic cores has been proposed. The FEA simulations reveal that magnetic separation in the yoke can increase significantly the energy-harvesting capability of PMLGs. Furthermore, the concept of the design of long stator windings for tubular PMLGs is studied. Two long stator generators having different magnetisation topologies and similar sizes to existing machine are modelled and compared to the existing machine. The similar-sized existing and proposed PMLGs are simulated by FEA. In this way, settings such as different boundary conditions, symmetry boundaries and material properties are used to gain confidence in the simulated results of the proposed machines. Moreover, the simulated results for the existing PMLG are verified against previously performed numerical simulations and practical tests delivered and published as part of other research. The outcome for the proposed PMLGs reveals several advantages for the long stator design, such as lower cogging forces and higher energy harvesting and a lower price of the raw structural materials. Additionally, the thesis proposes and simulates a new design for an air-cored PMLG. To boost the output power, the proposed design is based on a long stator topology adopting two sets of permanent magnet rings sandwiching copper windings in a tubular structure. The design is compared with a current machine in FEA and the results show significant reduction in radial forces and an increase in energy harvesting. Finally, a novel power electronics control system, bypassing inactive coils is suggested and simulated as part of the grid integration system for the long stator PMLGs. The new system achieves a reduction in the thermal losses in the power electronics switches in comparison with existing systems. The power electronics system and the generator have been simulated in Matlab coupled externally with FEA (JMAG Designer).

On March 13th, 2006, the Division of Electricity at Uppsala University deployed its first wave energy converter, L1, in the ocean southwest of Lysekil. L1 consisted of a buoy at the surface, connected through a line to a linear generator on the seabed. Since the deployment, continuous investigations of how L1 works in the waves have been conducted, and several additional wave energy converters have been deployed. This thesis is based on ten publications, which focus on different aspects of the interaction between wave, buoy, and generator. In order to evaluate different measurement systems, the motion of the buoy was measured optically and using accelerometers, and compared to measurements of the motion of the movable part of the generator - the translator. These measurements were found to correlate well. Simulations of buoy and translator motion were found to match the measured values. The variation of performance of L1 with changing water levels, wave heights, and spectral shapes was also investigated. Performance is here defined as the ratio of absorbed power to incoming power. It was found that the performance decreases for large wave heights. This is in accordance with the theoretical predictions, since the area for which the stator and the translator overlap decreases for large translator motions. Shifting water levels were predicted to have the same effect, but this could not be seen as clearly. The width of the wave energy spectrum has been proposed by some as a factor that also affects the performance of a wave energy converter, for a set wave height and period. Therefore the relation between performance and several different parameters for spectral width was investigated. It was found that some of the parameters were in fact correlated to performance, but that the correlation was not very strong. As a background on ocean measurements in wave energy, a thorough literature review was conducted. It turns out that the Lysekil project is one of quite few projects that have published descriptions of on-site wave energy measurements.