Submitted by Maria Aparecida (cidazen@gmail.com) on 2017-08-25T13:13:40Z No. of bitstreams: 1 Pedro Bezerra.pdf: 8233591 bytes, checksum: 5f7ac936ebaea088e770ad09f5751e48 (MD5) / Made available in DSpace on 2017-08-25T13:13:40Z (GMT). No. of bitstreams: 1 Pedro Bezerra.pdf: 8233591 bytes, checksum: 5f7ac936ebaea088e770ad09f5751e48 (MD5) Previous issue date: 2012-11-23 / CNPQ, CAPES / The challenges on the world’s energy sector, such as environmental questions and the rising on demand, has been stimulated the use of several kind of energy sources, such as tidal energy. These challenges associated to development of the technologies used in electromechanical devices enable the tidal exploitation attractive under technological and economic aspects. In this work, a methodology to estimating the energetic performance of a tidal power plant on the estuary of Bacanga is presented. Moreover, a dispatch strategy based on Genetic Algorithms was developed to maximize the total energy generated by the plant through an optimal scheduling of the turbines during the generation process. By modeling components and tidal cycle, several generation scenarios were simulated and analyzed considering single-effect and double-effect operation over one year of operation. From results obtained, it can be concluded that, although some constraints to the full exploitation of the potential of the estuary, it is still possible to exploit a significant amount of energy. This is due to improvements in efficiency of electromechanical devices currently available that allow the exploitation of energy in very low heads, as is the case of Bacanga. / Os grandes desafios do setor energético mundial tais como questões ambientais e a crescente demanda por energia, têm estimulado a utilização de diversas fontes alternativas de energia, inclusive as marés. Tais desafios associados ao desenvolvimento das tecnologias utilizadas em dispositivos eletromecânicos tornaram a exploração da energia das marés atrativa sob os aspectos tecnológicos e econômicos. Neste trabalho, é apresentada uma metodologia para a estimação de indicadores energéticos de uma usina maremotriz. Foi desenvolvida uma estratégia de despacho baseada em Algoritmos Genéticos visando-se maximizar a energia total gerada pela usina através do despacho ótimo das turbinas durante o processo de geração. Através da modelagem de componentes e do ciclo de mares, foram simulados e analisados cenários de geração em efeito-simples e efeito-duplo para um ano de operação. Como estudo de caso foi considerado o estuário do Bacanga, localizado na área urbana de São Luís, Maranhão. Dos resultados obtidos pode-se concluir que, embora algumas restrições impeçam o pleno aproveitamento do potencial do estuário, ainda é possível um aproveitamento significativo do mesmo. Isto se deve em grande parte às melhorias, em termos de eficiência, dos equipamentos eletromecânicos disponíveis atualmente, que possibilitam a exploração energética de baixíssimas quedas, como é o caso do Bacanga.

Current hydro-turbines aim to capture the immense energy available in tidal movements, however commonly applied technologies rely on principles more applicable in hydroelectric dams. Tidal stream currents, such as in Coastal Georgia, are not strong enough to make such turbines both efficient and economically viable. This research proposes a novel low-energy vortex shedding vertical axis turbine (VOSTURB) to combat the inefficiencies and challenges of hydro-turbines in low velocity free tidal streams. Some of the energy in tidal streams is extracted naturally from vortex shedding; as water streams past a bluff body, such as pier, low pressure vortices form alternatively on each side, inducing a rhythm of pressure differentials on the bluff body and anything in its wake. VOSTURB aims to capture this energy of the vortices by installing a hydrofoil subsequent to the bluff body. This foil, free to oscillate, translates the vortex energy into oscillatory motion, which can be converted into a form of potential energy. The presented research will act as a 'proof of concept.' It aims to assess such foil motion, or the ability of VOSTURB to capture vortex energy, and begin to assess the amount of tidal energy that can be theoretically harnessed. In this study a small scale model of VOSTURB, a cylindrical bluff body with a hammer shaped hydrofoil, was tested in a hydraulic flume for various mean flow speeds. Tangential accelerations of the foil's center of gravity were obtained through the use of an accelerometer. The acceleration data was analyzed utilizing Fourier analysis to determine the fundamental frequency of the wing oscillations. The available power to be harnessed from the oscillatory motion was then estimated utilizing this fundamental frequency. Ultimately it was found that the frequency of the VOSTURB foil oscillations corresponded highly with the theoretical frequency of vortex shedding for all moderate to high flow speeds. Low speeds were found to produce inconsistent and intermittent small oscillations. This signifies at moderate to high flow speeds, VOSTURB was able to transform some vortical energy into kinetic. The maximum average power obtained 8.4 mW corresponded to the highest flow velocity 0.27 m/s. Scaled to Rose Dhu prototype conditions this represented 50 W at a flow velocity of 0.95m/s, the maximum available at Rose Dhu. Although it was ascertained that VOSTURB could consistently capture some of the vortical energy; the percentage of which could not be calculated with certainty. Thus, the average kinetic power assessments of the foil were compared to the available power of the mean flow for each flow speed calculated by two methods: (1) over the foil's swept area; (2) the area of fluid displaced by the bluff body immediately in front of the foil. The maximum efficiency of the foil, found for the fastest flow speed was at 18% and 45% respectively. It was found that both average foil power, available flow power, and efficiency all decreased with a decrease in flow velocity. This study can serve as only a preliminary study for the effectiveness of VOSTURB as a hydro-turbine for tidal power. In the experiments, the foil was allowed to oscillate freely with little resistance. Future testing of VOSTURB needs to observe whether the vortex energy can overcome the resistive torque introduced by a generator to induce oscillatory motion as well as further optimize the foil design. While the testing in this project assesses the kinetic energy or power of the vortex shedding, this future testing will provide insight into the actual work that can actually be converted into potential energy or power. Complementing this research, both a Harmonic Analysis of Least Squares (HAMELS) and a Complex Empirical Orthogonal Function (CEOF) Analysis was conducted on available surface height and current velocity data separately from an available Regional Ocean Modeling System (ROMS) model of Coastal Georgia. Such analysis were conducted to observe spatial and temporal tidal patterns advantageous to a possible prototype installation of a tidal turbine such as VOSTURB. The more conventional HAMELS analysis, which isolates components of a signal with a certain frequency, identified temporal and spatial patterns attributed to tidal constituents. CEOF analysis, where major patterns of variance are identified not according to prescribed frequencies, was employed to identify any patterns possible not attributed to the tidal constituents. This study was also in part to observe whether the CEOF analysis could identify any patterns of tidal propagation that could not be resolved by the HAMELS analysis. The CEOF and HAMELS analysis of the surface height output produced very similar results: major modes of surface height variation due to the diurnal and semidiurnal tidal constituents propagating up the estuary. The CEOF results did not produce any additional information that could not be found through the HAMELS analysis of the constituents and presented such results in an arguably more convoluted manner. In addition, the surface height analysis provided no direct insight into areas more advantageous to tidal power. The CEOF analysis of the vector current velocity data however did provide some insight. The CEOF of the current data was able to isolate patterns of variance corresponding to the tidal constituents. However, the CEOF was also able to identify local 'hotspots' of high current magnitudes not resolved by HAMELS. These local areas of high current magnitudes, most likely due to changes in hydrodynamic conditions such as channel constrictions, are advantageous for tidal power applications. These general areas could serve as a starting point for the location selection process for a possible prototype installation of VOSTURB if the area was refined more. Ultimately for a prototype installation of VOSTURB, further experimentation and analysis is required for both the turbine design and placement, such as a power conversion methodology for the turbine and a more spatially resolute set of data to perform a CEOF analysis on. With these tasks completed, the prototype installation will be part of a larger effort between the Georgia Institute of Technology and the Girl Scouts of America to create completely sustainable "Eco-Village" on Rose Dhu Island, GA. With an extensive community outreach planned to educate the public, Rose Dhu, along with championing hydrokinetic energy, will serve as a paradigm for sustainable design and energy.

The aim of this study is to present, compare and improve the options of power transmission for tidal current arrays. The potential to generate low or zero carbon power from the world’s tides is increasing as technology moves forward. The technically available tidal current energy resource, the resource that can be captured using existing technology, in the United Kingdom can supply a significant amount of the UK electricity demand. Even though tidal current devices have similarities to offshore wind turbines in many aspects, a number of characteristics differentiate the approach needed regarding power transmission and drive-train design. Some of these characteristics are: predictable direction and speed of the tidal current, predetermined available area in a tidal channel, less swept area due higher density of water, continuous underwater operation and smaller distances to shore. This thesis is based on the hypothesis that tidal current energy can be harnessed using today’s technology in an efficient manner. Technology progression never stops and as new materials and methods become available the cost of utilising tidal current energy will drop in the years to come. However, the research question that has to be asked is whether using today’s technology tidal arrays can be an alternative source of electrical power. In order to respond to this research question electromechanical models of tidal current devices have been developed in detail, from resource to the grid connection, using mathematical linear and non-linear programming in MATLAB/Simulink. The tidal models developed include the tidal resource, the tidal turbine with pitch control, geared induction and synchronous generators, the power electronics with the generator controller, the grid side controller, the cables for power transmission, the filters and the grid connection. All the modelling aspects of this study are presented in Chapter 3. Single tidal current devices were compared using different generator technologies, squirrel cage induction generator or permanent magnet synchronous generator, and different location of the power converters, in the nacelle near the generator or many kilometres apart from the generator. Regarding the generator technology, results showed that even though differences are minor, the permanent magnet synchronous generators are more efficient. Regarding the location of the power converters results showed that positioning the power converters in the nacelle always yields fewer electrical losses but component accessibility is minimised due to the underwater operation of the tidal current device. A key focus aspect of the study is the power transmission option with onshore converters which is presented in detail. Using this concept it is possible to generate electricity from tidal current devices but at the same time keep the highest possible system reliability despite the continuous underwater operation. This concept has been used in the first demonstration tidal current arrays developed by Andritz Hydro Hammerfest. What is more, data provided by Andritz Hydro Hammerfest were utilised in order to validate the simulation models. In this study a step forward is taken regarding the concept of keeping the converter dry and controlling the tidal current generator from afar. An algorithm is developed to design power harmonic filters for systems that use long distance controls. Power harmonic filters allow the long distance control system to operate reliably under all conditions but generate significant electrical losses. The power harmonic filter design algorithm presented in this thesis estimates the exact filter parameters so that the filter ensures maximum system reliability and generate minimum possible losses. In addition tidal array topologies using this concept are developed. The final part of this thesis compares a number of different tidal array topologies based on resource to grid efficiency and component accessibility for maintenance. Results showed that when tidal current devices are clustered per four turbines on offshore platforms it is efficient to use as many clusters as possible connected to a single cable whose both ends are connected to the grid. Locating the power converters in the nacelle yields fewer electrical losses compared to locating the power converters on the offshore platform. However, the difference is minimised because the distance between the tidal current device and the offshore platform is the least possible. Having the power converters on an offshore platform is beneficial in terms of accessibility for maintenance and operation because they are not underwater. The results and the methodology from this thesis can be extended to other offshore renewable energy systems such as the wind and wave. In addition, this study can be used as a stepping stone for decision making by tidal current developers.

Tidal power, or the use of tides for electricity production, exists in many forms including tidal barrages, which exploit tidal head differentials, and turbines placed directly in regions with large tidal current velocities. The latter is actively being investigated in many countries around the world as a means of providing renewable and wholly predictable electricity (cf. wind, solar and wave power). The expansion of the in-stream tidal industry is hindered however by several factors including: turbine durability, deployment and maintenance costs, and the lack of abundant locations which meet the necessary current velocities for turbine start-up and economic power production. A new novel type of augmentation device, entitled the ‘Tidal Acceleration Structure’ or TAS (Canadian patent pending 2644792), has been proposed as a solution to the limited number of coastal regions which exhibit fast tidal currents. In preliminary investigations, the TAS, a simple Venturi section consisting of walls extending from the seafloor to above the high water mark in an hourglass shape, was found as able to more than double current velocities entering the device. The results indicated a significant advantage over other current channeling technologies and thus the need for more in-depth investigations. The main objective of the present study was to optimise the design of the TAS and to predict the power that a turbine placed within it could extract from flow. To do this, two principal methods were employed. Firstly, a 1:50 scale model of the TAS was tested and its shape optimised in a 1.5 m wide flume. Secondly, a 3D numerical model (ANSYS Fluent) was used for comparison with the experimental results. During the tests, a TAS configuration was found that could accelerate upstream velocities by a factor of 2.12. In separate tests, turbines were simulated using Actuator Disc Theory and porous plates. The TAS-plate combination was found to be able to extract up to 4.2 times more power from flow than the stand-alone plate, demonstrating that the TAS could provide turbines with a significant advantage in slower currents. Though further research is needed, including the testing of a larger TAS model in conjunction with a small in-stream turbine, the results of this thesis clearly demonstrate the potential of the TAS concept to unlock vast new areas for tidal energy development.

A three dimensional computational model of a full scale axial flow tidal turbine has been used to investigate the effects of a range of realistic environmental conditions on turbine performance. The model, which is based on the Reynolds averaged Navier-Stokes equations, has been developed using the commercial flow solver ANSYS Fluent. A 1:30 scale tidal turbine is simulated in an open channel for comparison to existing experimental data. The rotor blades are directly resolved using a body-fitted, unstructured computational grid. Rotor motion is enabled through a sliding mesh interface between the rotor and the channel boundaries. Reasonably good agreement in thrust and power is observed. The computed performance curves are offset from the measured performance curves by a small increment in rotor speed. Subsequently, a full scale axial flow turbine is modelled in a variety of conditions representative of tidal channel flows. A parametric study is carried out to investigate the effects of flow shear, confinement and alignment on turbine performance, structural loading, and wake recovery. Mean power and thrust are found to be higher in sheared flow, relative to uniform flow of equivalent volumetric flow rate. Large fluctuations in blade thrust and torque occur in sheared flow as the blade passes through the high velocity freestream flow in the upper portion of the profile and the lower velocity flow near the channel bed. A stronger shear layer is formed around the upper portion of the wake in sheared flow, leading to enhanced wake mixing. Mean power and thrust are reduced when the turbine is simulated at a lower position in a sheared velocity profile. However, fluctuations in blade loading are increased due to the higher velocity gradient. The opposite effects are observed when the turbine operates at greater heights in sheared flow. Flow misalignment has a negative impact on mean rotor thrust and power, as well as on unsteady blade loading. Although the range of unsteady loading is not increased significantly, additional perturbations are introduced due to interactions between the blade and the nacelle. A deforming surface is introduced using the volume-of-fluid method. Linear wave theory is combined with the existing free surface model to develop an unsteady inflow boundary condition prescribing combined sheared flow and free surface waves. The relative effects of the sheared profile and wave-induced velocities on turbine loading are identified through frequency analysis. Rotor and blade load fluctuations are found to increase with wave height and wave length. In a separate study, the performance of bi-directional ducted tidal turbines is investigated through a parametric study of a range of duct profiles. A two dimensional axi-symmetric computational model is developed to compare the ducted geometries with an unducted device under consistent blockage conditions. The best-performing ducted device achieves a peak power coefficient of approximately 45% of that of the unducted device. Comparisons of streamtube area, velocity and pressure for the flow through the ducted device shows that the duct limits the pressure drop across the rotor and the mass flow through the rotor, resulting in lower device power.

As the demands for energy increased with the global increase in population, there is a need to create and invest in more clean and renewable energy sources. Energy derived from the movement of the tides is an ancient concept that is currently being harnessed in a handful of large tidal range locations. However, the need to move from fossil fuel driven energy sources to those that are clean and non-polluting is a priority for a sustainable future. Globally, hydropower potential is estimated to be more than 16,400-Terawatt hours annually. Given that the electricity consumption worldwide was at 15,068-Terawatt hours in 2016, if properly utilized, hydropower could supply a substantial percentage of current demand. Most of the current hydropower supply is drawn from well-established dams and tidal barrage systems. However, tidal power plants that harness the change in water height and flow along the coast (i.e. using tidal energy) have the potential to push these figures even higher. Although there is no exact number for lengths of global coastlines, there are estimates that put that number between 220,000 and 880,000 miles of coasts. These opportunities in tidal energy technologies that harness energy from the sea may one day be the key to solving our energy crises. This research explored in detail a closed, convergent system for optimal extraction of head-driven tidal energy with minimal adverse environmental effects. The long-term goal of this project is to create a system that is viable in low tidal range locations traditionally not considered for locations of tidal energy systems, therefore increasing the overall global tidal energy portfolio. By implementing a closed system of ‘bladders’ and convergent nozzles to optimize the flow rate of the contained fluid, the proposed system can 1) derive tidal energy in low tidal range geographies 5 2) avoid typical hazards like system biofouling, marine life propeller impacts, and 3) allow for ease of installation, operation, and maintenance.

The predictable nature of the tides offers a regular, reliable source of renewable energy that can be harnessed using tidal stream turbines (TSTs). The UK's practically extractable tidal stream energy resource has the potential to supply around 7 % of the country's annual electricity demand. As of 2016, the world's first commercial scale arrays have been deployed around the UK and France. The harsh nature of the marine operating environment poses a number of engineering challenges, where the optimal turbine design solution remains under investigation. In this thesis, a numerical model is developed to assess the power production and hydrodynamic behaviour of horizontal axis tidal turbines. The developed model builds upon well established and computationally efficient Blade Element Momentum Theory (BEMT) method for modern three-bladed wind turbines. The main novel contribution of this thesis is extending the application to an alternative design of a ducted, high solidity and open centre TST. A validation study using measurements from multiple different scale model experimental tank tests has proven the applicability of the model and suitability of the imposed correction factors. The analytical modifications to account for ducted flow were subsequently indirectly verified, where predictions of turbine power and axial thrust forces under optimal operating speeds were within 2 % of those using more advanced computational fluid dynamics (CFD) methods. This thesis presents a commercial application case of two turbines designed by OpenHydro, examining the BEMT performance with a sophisticated blade resolved CFD study. A comparison of results finds that the model is capable of predicting the average peak power to within 12 %, however it under predicts thrust levels by an average of 35 %. This study concludes that the model is applicable to ducted turbine configurations, but is limited in capturing the complex flow interactions towards the open centre, which requires further investigation. The computational efficiency of the newly developed model allowed a structural analysis of the composite blades, thus demonstrating it is suitable to effectively evaluate engineering applications. Stresses are seen to be dominated by flap-wise bending moments, which peak at the mid-length of the blade. This tool will further enable EDF to perform third party assessments of the different turbine designs, to aid decision making for future projects.

Co-location of offshore wind turbines at sites being developed for tidal stream arrays has been proposed as a method to increase capacity and potentially reduce the cost of electricity compared to operating either technology independently. This research evaluates the cost of energy based on capital expenditure and energy yield. It is found that, within the space required around a single 3 MW wind turbine, co-location provides a 10-16% cost saving compared to operating the same size tidal-only array without a wind turbine. Furthermore, for the same cost of electricity, a co-located farm could generate 20% more yield than a tidal-only array. These results are based on analysis of a case-study site in the Pentland Firth. Wind energy is assessed using an eddy viscosity wake model in OpenWind, with a 3 MW rated power curve and thrust coefficient from a Vestas V90 turbine. Three years of wind resource data is from the UK Met Office UK Variable (UKV) 1.5 km numerical model and corrected against a 400 m Weather Research and Forecasting (WRF) model run over the site. Tidal stream energy is modelled using a semi-empirical superposition of self-similar plane wakes, with a generic 1 MW rated power curve and thrust based on a full-scale, fixed-pitch turbine. Coincident tidal resource data is from the Forecasting Ocean Assimilation Model (FOAM) at 7.5 km resolution and correlated with a 150 m ADvanced CIRCulation model (ADCIRC). Wave parameters are corrected from ERA-Interim data with six months of wave buoy data. Multiple tidal turbine array layouts are considered, with maximum tidal energy generated for a staggered array with spacing of 20 tidal turbine diameters, Dt , streamwise and 1.5Dt cross-stream. However, cheapest cost of electricity from the tidal-only array, was found for a single row of turbines, due to minimal wake effects. Laboratory experiments were undertaken to validate the superposition wake model for use with large, shared support structures. Two rotors mounted either side of a central tower generate a peak wake velocity deficit 70% greater than predicted by superposition. This was due to high local blockage and a complex near-wake structure, with a corresponding increase in tower drag of 9%. Further experiments evaluated the impact of oblique inflow on turbines yawed at +/-15 degrees. These results validated a theoretical cosine correction for thrust coefficient and characterised the centreline wake drift with downstream distance. Extreme environmental loads for a shared support structure, compared to structures for wind-only and tidal-only, have also been modelled. A non-linear wave model was used to represent a single wave form with 1% occurrence for each hour of time-series data. Overturning moment about the base of a shared support, with one wind and two tidal turbines, was found to be 4.5% larger than for a wind-only turbine in strong current and with turbines in different operational states. Peak loads across the tidal array were found to vary by 2.5% and so little load reduction benefit could be gained by locating a shared support in a more sheltered area of the array.

In the pursuit of low-carbon, renewable energy sources one option with great potential in the UK is tidal energy. Specifically the proposed construction of the Swansea Bay Tidal Lagoon (SBTL) in South Wales has become one such discussed option. With a potential net annual output of 400 GWh and a 120-year lifetime the scheme represents a long-term and large-scale electricity production option. An assessment of carbon emissions and embodied energy (EE) of the lagoon’s life cycle was carried out. Total lifetime carbon emissions for the SBTL are in the region of 470,000 tCO2e and EE was found to be around 7,800 TJ. The assessment shows that the SBTL has significantly lower emissions per year than the existing National Grid mix and with emissions of around 0.01 kgCO2e/KWh is significantly lower than the UK emissions target of 0.07 kgCO2e/KWh. Energy payback of the SBTL was found to be in the region of 5.5 years. The use of dredged ballast infill sourced from within the area of the lagoon plays an important role in keeping emissions and energy use low; and is a key consideration when planning future tidal lagoon structures.

L'amélioration des performances des chaînes de conversion dédiées à la récupération d'énergie par les hydroliennes est un point particulièrement important pour rendre cette ressource économiquement attractive. La minimisation du coût de l'énergie produite passe nécessairement par une amélioration des performances de la chaîne de conversion électromécanique et une réduction des coûts de maintenance et de production des éléments la constituant. Dans ce contexte particulier, les génératrices à aimants permanents apparaissent particulièrement intéressantes dans la mesure où elles sont bien adaptées à un fonctionnement à basse vitesse et à fort couple. Ceci permet d'éliminer des systèmes mécaniques très complexes, encombrants et exigeants en maintenance, tels que le multiplicateur de vitesse et/ou le système d'orientation des pales. L'objectif de cette thèse est d’explorer un certain nombre de pistes concernant les outils, les concepts et les règles de conception à mettre en oeuvre pour dimensionner une génératrice associée en entraînement direct à une turbine hydrolienne à pas fixe. Les outils mis au point dans ces travaux englobent des modèles multi-physiques intégrés dans une démarche de conception qui se veut la plus globale possible. Cette méthodologie tient compte de la caractéristique de la ressource (courants de marées), de celle de la turbine (hélice), des spécifications de la génératrice à aimants permanents, de la mise en oeuvre d’une stratégie de pilotage associant MPPT et limitation de puissance par défluxage à fort courants de marées, en plus des contraintes liées au convertisseur. L'environnement de conception développé est basé sur un couplage des modèles dans une procédure d'optimisation. Les résultats obtenus mettent en lumière les points clés associés au développement d’une telle génératrice pour un contexte hydrolien. / The improvements of marine current turbines drive train are key features to ensure safe operation and to make tidal energy resource cost-attractive. In this context, eliminating mechanical systems that demand high-level of maintenance can be an interesting way to improve the global behavior of tidal turbines. For that purposes, the presented studies focus on design methodologies and concepts of direct-driven generators associated with fixed-pitch turbines. The proposed designs are based on multiphysics models of the generator that are integrated in an optimization process taking into account the drive train environment. For these reasons, several models have been integrated into a global design strategy in order to find solutions that improve marine current turbines performances. This strategy is based on the use of an optimization process that combines electromagnetic model, thermal model, turbine performances model, and tidal resource velocity profile. This methodology integrates also an efficient control strategy based on a maximum power point tracking (MPPT) approach at low tidal speed and a flux-weakening power limitation control at high tidal speed. This control at high tidal velocities is in this work achieved by considering only the generator electrical control without using blade pitching systems. The obtained results highlight trends that could lead to an improvement of the design and they help designers to set relevant technological choices in order to ensure significant cost reduction and highly improve the reliability of marine current turbines.