This thesis is concerned with the development of linear generators for use as the power take off mechanism in marine renewable energy converters. Delivering significant power at the low velocities demanded by wave and tidal stream energy converters requires a large force, which must be reacted by an electrical machine in a direct drive system. Attention is focused on the development of two novel topology linear permanent magnet machines suitable for use in this application. For each topology, models are presented that are capable of predicting the force characteristics and dynamic generator performance. The models, which are verified experimentally, reveal significant behavioural differences between the two topologies. The designer is thus provided with an interesting choice when considering a direct drive power take off strategy. In short, a variable reluctance machine is shown to develop a high shear force in its airgap, offering the potential of a compact generator, yet its performance is hindered by a poor power factor and the presence of significant airgap closure forces. The second machine, an air cored stator encompassing a permanent magnet translator, is shown to lend itself favourably as a generator, but only at the expense of requiring a large quantity of magnetic material and developing a significantly lower shear stress. Mechanical issues involved in the direct integration of linear electrical machines into the marine environment are examined. Details of two existing marine renewable energy devices are used to hypothesise about the characteristics of realistic sized generators of both the topologies investigated. Direct drive power take off is shown to represent a feasible alternative to the complex systems frequently proposed in these applications.

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Wave energy conversion remains a promising technology with substantial renewable resources to be exploited in many parts of the world. However to be commercially attractive more effective conversion is desirable. There is scope for increasing power capture by use of several bodies responding with several modes, some or all of which may undergo resonance for frequencies within a wave climate. This theme is explored here with a floating moored line absorber system where the relative motion generates power by incorporation of a damper to represent the power take off. To be most effective the bodies should be responding in anti-phase requiring spacing between adjacent bodies of half a wavelength. First a converter design including two bodies is investigated experimentally and numerically responding solely in heave. The bodies have drafts to provide resonant frequencies within a wave spectrum, the stern diameter is as large as possible within the inertia regime and the bow diameter is optimised to provide maximum power. Experiments showed this system to be limited since the desirable anti phase heave modes were contaminated with other modes for off resonance response considerably reducing power generation. To stabilise motion in the desired modes another small float was introduced as the bow float rigidly connected by a beam to the mid float with the added benefit of adding forcing due to surge and pitch to some degree (following Prof Peter Stansby’s design). The sizes of the three floats increase from bow to stern, causing the line absorber to align with the wave direction. This system was optimised through experiments varying float spacing, diameter, draft and the hinge point above the mid float about which relative angular motion occurs. These experiments were undertaken at small scale in the wide Manchester University flume at about 1/40th scale. Regular and random (JONSWAP) waves were investigated including directionality and different spectral peakedness factor. Corresponding experiments were undertaken at five time larger scale (about 1/8th) in the wave basin at the COAST laboratory of Plymouth University. These tests were for a flat-based floats; the mechanical damping coefficient for larger scale was within the range for the smaller scale tests after appropriate (Froude) scaling. Tests at Manchester showed that the more rounded base floats (the mid float being hemi spherical) provided improved power capture. Device effectiveness is defined in terms of capture width ratio; that is the average power divided by the wave power per metre divided by the wavelength, defined by the energy period in the case of irregular waves. The experiments showed that capture width ratios were greater than 25% in regular waves and greater than 20% in irregular waves across a broad range of wave periods. With rounded base floats capture width ratios over 20% were achieved for a broad range of wave frequencies up to a maximum greater than 35%. Limited experiments at larger scale showed that increasing the damping coefficient could increase power capture by about 50%. Characterisation by capture width ratio is convenient for determining annual energy yield from scatter diagrams. This was undertaken for six sites of interest for wave energy conversion. It was assumed that the greatest power to weight ratio determines the most economic device; it was found that large devices could produce very large average power, for example average power of 2 MW, but the optimum power/weight ratio occurred at smaller scale, with average power typically 0.3 MW.

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.

Wave energy has the potential to be a major contributor to the global energy mix. It is estimated that commercial deployment of wave and tidal energy arrays could meet as much as 20% of the UK’s current electricity demand, with an installed capacity of 30-50 GW providing up to 16,000 jobs. However, the wave energy sector has not yet developed into a commercial industry due to several key challenges. One reason private investors have been reluctant to back the sector is that the uncertainty surrounding lifetime costs of wave energy arrays makes it difficult to obtain reliable estimates for overall cost of energy. In order to improve these estimates, a better understanding of the operations and maintenance (O&M) phase of wave energy arrays needs to be gained. This thesis presents an O&M simulation tool designed for wave energy arrays. The work presented uses the model to assess aspects of O&M strategies for two different types of wave energy converter. Uncertainty in the model inputs is also addressed by undertaking a series of sensitivity analyses. The methods and results presented in this thesis highlight the importance of using an O&M simulation tool to plan lifetime logistics for wave energy arrays and obtain realistic cost estimates. The work has also shown how an O&M tool can be used to identify critical components in wave energy converters, thereby helping to design the best device possible for the challenging marine environment. Understanding lifetime costs of wave energy arrays will drive the sector towards commercialisation, bringing wave energy a step closer to fulfilling its potential as a major contributor to global energy production.

In the field of ocean wave energy converters (WECs), active areas of research are on a priori or in situ methods for power production estimates and on control system design. Linear potential flow theory modelling techniques often underpin these studies; however, such models rely upon small wave and body motion amplitude assumptions and therefore cannot be applied to all wave conditions. Nonlinear extensions can be applied to the fluid loads upon the structure to extend the range of wave conditions for which these models can provide accurate predictions. However, careful consideration of the thresholds of wave height and periods to which these models can be applied is still required. Experimental modelling in wave tank facilities can be used for this purpose by comparing experimental observations to numerical predictions using the experimental wave field as an input. This study establishes a recommended time domain numerical modeling approach for power production assessments of oscillating wave surge converters (OWSCs), a class of WEC designed to operate in shallow and intermediate water depths. Three candidate models were developed based on nonlinear numerical modelling techniques in literature, each with varying levels of complexity. Numerical predictions provided by each model were found to be very similar for small wave amplitudes, but divergence between the models was observed as wave height increased. Experimental data collected with a scale model OWSC for a variety of wave conditions was used to evaluate the accuracy of the candidate models. These experiments were conducted in a small-scale wave flume at the University of Victoria. A challenge with this experimental work was managing wave reflections from the boundaries of the tank, which were significant and impacted the dynamics of the scale model OWSC. To resolve this challenge, a modified reflection algorithm based upon the Mansard and Funke method was created to identify the incident and reflected wave amplitudes while the OWSC model is in the tank. Both incident and reflected wave amplitudes are then input to the candidate models to compare numerical predictions with experimental observations. The candidate models agreed reasonably well with the experimental data, and demonstrated the utility of the modified wave reflection algorithm for future experiments. However, the maximum wave height generated in the wave tank was found to be limited by the stroke length of the wavemaker. As a result, no significant divergence of the candidate model predictions from the experimental data could be observed for the limited range of wave conditions, and therefore a recommended model could not be selected based solely on the experimental/numerical model comparisons. Preliminary assessments of the annual power production (APP) for the OWSC were obtained for a potential deployment site on the west coast of Vancouver Island. Optimal power take-off (PTO) settings for the candidate models were identified using a least-squares optimization to maximize power production for a given set of wave conditions. The power production of the OWSC at full scale was then simulated for each bin of a wave histogram representing one year of sea states at the deployment site. Of the three candidate models, APP estimates were only obtained for Model 1, which has the lowest computational requirements, and Model 3, which implements the most accurate algorithm for computing the fluid loads upon the OWSC device. Model 2 was not considered as it provides neither advantages of Models 1 and 3. The APP estimates from Models 1 and 3 were 337 and 361 MWh per year. For future power production assessments, Model 3 is recommended due to its more accurate model of the fluid loads upon the OWSC. However, if the high computational requirements of Model 3 are problematic, then Model 1 can be used to obtain a slightly conservative estimate of APP with a much lower computational effort. / Graduate

This master thesis is done within the Energy Systems Engineering program at Uppsala University and performed for CorPower Ocean. Wave energy converters (WECs) are devices that utilize ocean waves for generation of electricity. The WEC developed by CorPower Ocean is small and intended to be deployed in an array. Placed in an array the different WECs will interact hydrodynamically and the combined power output is altered. The aim of this thesis is to model and investigate how the array configuration affects the electric power output. The goal is to target an optimal array layout for CorPower Ocean WECs, considering both average power and power smoothness in the optimization.   In this thesis multiple buoys have been implemented in the time-domain model at CorPower Ocean. The hydrodynamic interactions are computed using an analytical interactions theory together with a recently developed calibration method able of handling WEC bodies of complicated shapes. The array behavior in regular waves is analyzed and it is identified how the beneficial separation distances vary with wave length. It is observed that the best separation distances for high average power does not exactly correspond to the best for minimizing the peak-to-average power. Simulation results show that it is possible to obtain both high average array power as well as increased power smoothening in a regular wave. A genetic algorithm for optimizing the array configuration is designed and tested for two different array patterns. Initial simulations are conducted in realistic multi-directional irregular waves. The power smoothening capacity of the array remains even in these conditions but the exact extent of it is still uncertain.   This thesis delivers a WEC array simulation model as well as an initial view on the array characteristics of the phase controlled CorPower Ocean WEC. Additionally, it demonstrates an optimization algorithm taking both average power and power smoothness into account.

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.

Many wave energy convertors are designed to use either vertical (heave) or horizontal (surge) movements of waves. But the frequency response of small heaving buoys and oscillating water column devices shows that they are too stiff and so their resonance is at too short a period. A device moving in the horizontal (surge) direction has less restoring spring and so its resonance is at too long a period. It follows that a device that moved at some intermediate slope angle could have an intermediate value of hydrodynamic stiffness and so be resonant at a variable and desirable part of the wave spectrum. There have been two series of model tests in this work. The first used a simple free-floating model with no power take-off apparatus and with constraint achieved by means of a large inertia plate lying in the slope plane. The second used a rig that constrained the slope movement of the buoy head by means of hydrostatic bearings running on a guide rod set to the chosen slope angle. An external power take-off system was used to simulate a linear damper for absorbing the incident wave energy and control the motion of the model. This thesis firstly studies the potential of varying the slope angle as a way of tuning the natural period of the device to suit useful wave periods. Secondly, it studies the experimental and theoretical power capture ability of models with different slope angles in regular waves in the frequency domain. The hydrodynamic coefficients of the model were determined both experimentally and numerically based on linear hydrodynamic concepts. The power absorption of the models was calculated using the experimental data of the hydrodynamic coefficients and also measured directly. Some control of power take-off was also investigated. Some irregular wave tests were carried out for the 45 degrees slope angle case. The results show that it is feasible to alter the slope angle of the device as a way of tuning its natural period. However, in further studies of the power capture ability for different slope angles, the device shows a very wide bandwidth and high efficiency performance when it is set to 45 degrees slope angle. This suggests that to constrain the device to a 45 degrees slope angle is suitable for most of the sea states.