Dissertations / Theses on the topic 'Wave energy absorption'

In this thesis some theoretical problems related to the propagation and absorption of Electron Cyclotron Gaussian beams in tokamak plasmas of interest for nuclear fusion applications are investigated. To account for diffraction effects, beam propagation is analyzed in the framework of the complex eikonal method, a generalization of geometrical optics in which the phase function is assumed to be complex valued, with the non-negative imaginary part accounting for the finite width of the beam cross section. Within this framework, the solution at the dominant order in the expansion parameter is well-known, and the wave beam is modeled as a bundle of “extended rays”. The derivation of the transport equation for the field amplitude is much more complicated with respect to the standard geometrical optics one, hampering the derivation of the wave energy flux. In this work, an argument is proposed that greatly simplifies the analysis of the transport equation allowing us to derive the wave energy flux. This result, not available in the literature in the case of beam propagation in anisotropic media like magnetized plasmas, has been obtained in collaboration with O. Maj (IPP, Garching, Germany), and published on Physics of Plasmas. The effects of the finite beam width on the Electron Cyclotron resonant interaction have been described with a model that takes into account the transverse wave vector spectrum width and the non-uniformity of the equilibrium magnetic field. The model has been implemented in a modified version of the GRAY code [D. Farina, Fusion Sci. Technol. 52, 154 (2007)]. The differences between the power absorption profi les obtained using this model and the “plane wave” one are illustrated numerically in ITER conditions and are found to be small for realistic cases, thus justifying the use of the usual model for practical purposes.

The motion of ocean waves can be captured and converted into usable electricity. This indicates that wave power has the potential to supply electricity to grids like wind or solar power. A point absorbing wave energy converter (WEC) system has been developed for power production at Uppsala University. This system contains a semi-submerged buoy on the water surface driving a linear synchronous generator placed on the seabed. The concept is to connect many small units together, to form a wave farm for large-scale electricity generation. A lot of effort has gone into researching how to enhance the power absorption from each WEC unit. These improvements are normally done separately for the buoy, the generator or the electrical system, due to the fact that modelling the dynamic behavior of the entire WEC system is complicated and time consuming. Therefore, a quick, yet simple, assessment tool is needed.  This thesis focuses on studying the use of the equivalent circuit as a WEC system modelling tool. Based on the force analysis, the physical elements in an actual WEC system can be converted into electrical components. The interactions between the regular waves, the buoy, and the Power Take-off mechanism can be simulated together in one circuit network. WEC performance indicators like the velocity, the force, and the power can be simulated directly from the circuit model. Furthermore, the annual absorbed electric energy can be estimated if the wave data statistics are known. The linear and non-linear equivalent circuit models developed in this thesis have been validated with full scale offshore experimental results. Comparisons indicate that the simplest linear circuit can predict the absorbed power reasonably well, while it is not so accurate in estimating the peak force in the connection line. The non-linear circuit model generates better estimations in both cases. To encourage researchers from different backgrounds to adapt and apply the circuit model, an instruction on how to establish a non-linear equivalent circuit model is supplied, as well as on how to apply the model to accelerate the decision making process when planning a WEC system.

The wave energy converter developed at Uppsala University consists of a linear generator at the seabed driven by the motion of a buoy on the water surface. The energy absorbed by the generator is negatively affected by variations of the mean sea level caused by tides, changes in barometric pressure, strong winds, and storm surges. The work presented in this doctoral thesis aims to investigate the losses in energy absorption for the present generation wave energy converter due to the effect of sea level variations, mainly caused by tides. This goal is achieved through the modeling of the interaction between the waves and the point absorber. An estimation of the economic cost that these losses imply is also made. Moreover, solutions on how to reduce the negative effect of sea level variations are discussed. To this end, two compensation systems which adjust the length of the connection line between the floater and the generator are designed, and the first prototype is built and tested near the Lysekil research site. The theoretical study assesses the energy loss at about 400 coastal points all over the world and for one generator design. The results highlight critical locations where the need for a compensation system appears compelling. The same hydro-mechanic model is applied to a specific site, the Wave Hub on the west coast of Cornwall, United Kingdom, where the energy loss is calculated to be about 53 %. The experimental work led to the construction of a buoy equipped with a screw jack together with its control, measurement and communication systems. The prototype, suitable for sea level variations of small range, is tested and its performance evaluated. A second prototype, suitable for high range variations, is also designed and is currently under construction. One main conclusion is that including the compensation systems in the design of the wave energy converter will increase the competitiveness of the technology from an economic point of view by decreasing its cost per kWh. The need for a cost-effective wave energy converter with increased survivability emphasizes the importance of the presented research and its future development.

Ocean wave energy is considered to be one of the important potential renewable energy resources for sustainable development. Various wave energy converter technologies have been proposed to harvest the energy from ocean waves. This thesis is based on the linear generator wave energy converter developed at Uppsala University. The research in this thesis focuses on the foundation optimization and the power absorption optimization of the wave energy converters and on the wave climate modelling at the Lysekil wave converter test site. The foundation optimization study of the gravity-based foundation of the linear wave energy converter is based on statistical analysis of wave climate data measured at the Lysekil test site. The 25 years return extreme significant wave height and its associated mean zero-crossing period are chosen as the maximum wave for the maximum heave and surge forces evaluation. The power absorption optimization study on the linear generator wave energy converter is based on the wave climate at the Lysekil test site. A frequency-domain simplified numerical model is used with the power take-off damping coefficient chosen as the control parameter for optimizing the power absorption. The results show a large improvement with an optimized power take-off damping coefficient adjusted to the characteristics of the wave climate at the test site. The wave climate modelling studies are based on the wave climate data measured at the Lysekil test site. A new mixed distribution method is proposed for modelling the significant wave height. This method gives impressive goodness of fit with the measured wave data. A copula method is applied to the bivariate joint distribution of the significant wave height and the wave period. The results show an excellent goodness of fit for the Gumbel model. The general applicability of the proposed mixed-distribution method and the copula method are illustrated with wave climate data from four other sites. The results confirm the good performance of the mixed-distribution and the Gumbel copula model for the modelling of significant wave height and bivariate wave climate.

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.

Résolution de problème extérieur pour l'équation des ondes. Etude du cas des ondes harmoniques. Méthode de Schenk, Méthode variationnelle de Nedelec, proposition d'un nouveau système d'équations intégrales pour la résolution du problème du mur absorbant. Analyse de diverses fonctions intégrales pouvant être utilisées pour des calculs d'ondes transitoires (emploi de formules de potentiels retardés). Proposition d'un cadre fonctionnel lié aux formules d'énergie des ondes. Formulations variationnelles espace temps et shémas de type Galerkin basés sur ces mêmes formules d'énergie. Analyse des schémas de type collocation utiles pour discrétiser l'équation intégrale de Kirchoff.

Doutoramento em Engenharia Mecânica
A incorporação de materiais absorsores de energia (AE) em sistemas de protecção é uma clara possibilidade de melhoraria do seu desempenho, devido à elevada relação entre a sua resistência e o seu peso, e a excelente capacidade para absorverem energia quando solicitados dinamicamente. As propriedades mecânicas da cortiça (e.g. a baixa densidade e a elevada rigidez e resistência específicas) sugerem que este material — assim como os seus derivados — podem apresentar propriedades excelentes quando aplicados como núcleos em sistemas AE do tipo estrutura sanduíche. Esta dissertação engloba trabalho experimental e numérico. O primeiro conjunto de testes experimentais consistiu na caracterização experimental dinâmica (ondas de choque de explosivos) do comportamento de dois micra aglomerados de cortiça (MAC), NL20 e TB40. Um pendulo balístico de 4 cabos foi usado para a medição do impulso transmitido a uma amostra de MAC impactada por uma onda de choque com origem na detonação de um explosivo energético. Foi registado o movimento do pêndulo e os valores de força resultantes. Um modelo numérico do problema recorrendo ao método dos elementos finitos (MEF) foi também desenvolvido, apresentando uma elevada correlação com a análise experimental, permitindo assim o desenvolvimento de um modelo constitutivo adequado à modelação do comportamento dinâmico dos MAC neste tipo de solicitações. Na segunda fase de testes experimentais, os MAC testados anteriormente são incorporados como núcleos em estruturas sanduíche com faces de alumínio (liga 5754-H22). Foram medidos os valores de defleção e o impulso transmitido ao pêndulo através do movimento oscilatório. São determinados os efeitos da densidade e da espessura dos núcleos na resposta estrutural do sistema. Também neste caso foi desenvolvido um modelo recorrendo ao MEF e posteriormente validado com resultados experimentais.
Structures incorporating energy absorbing materials (EA) are highly relevant in the design of blast protection systems due o their high strength-to-weight ratio and excellent energy absorption capacity under dynamic loading. The mechanical properties of cork (e.g. low density and high specific stiffness and strength) suggest that this material — and its compounds — may have excellent properties when acting as core in energy absorbing sandwich structures. This thesis focuses on experimental and numerical work. The first experimental set of tests presented consisted in the experimental characterisation of the dynamic (blast) behaviour of two micro-agglomerated cork compounds (MAC), NL20 and TB40. A 4-cable ballistic pendulum was used to measure the impulse transmitted to the specimen subjected to a shock-wave originated from the detonation of a high explosive. The displacement/movement of the pendulum and load values were measured. A numerical study of the problem using the finite element method (FEM) led to a good correlation with the experimental analysis leading to the development of a material constitutive model that suited the dynamic behaviour of the studied MAC compounds along the analysed loading regimes. On the second stage of the experimental work the two micro-agglomerated cork compounds tested previously were incorporated as cores in sandwich structures with 5754-H22 aluminium alloy face sheets. The deflection of the front and back face sheets was measured as well as the transmitted impulse from the oscillatory movement of the pendulum. The effects of the core thickness and density on the structural response were determined. A numerical model of the problem, again using the FEM, was developed which was then validated with the experimental results.

碩士
國立交通大學
土木工程系
89
Based on a two dimensional linear water wave theory, the boundary element method is developed and applied to study the effectiveness of a wave energy absorber in water to finite depth. The present study is concerned with wave energy which is defined as the incident wave power per unit crest length, efficiency of an absorber defined as the ratio of the wave energy absorbed by the floating structures per wave period to the available energy of the incident wave, and the best efficiency defined as wave energy absorbed by structures and the available energy of the incident wave are the same. In this thesis, the structures have three forms: a right wedge, a symmetric wedge, and a vertical flat plat; we assume that front two structures are constrained to oscillate in heave, and the third structures is constrained to oscillate in sway. The accuracy of the present numerical model is proved by comparing results of present numerical model, and laboratory experiment. When the boundary in back of the structures is radiation condition, the absorption of wave energy complete hardly by absorber, but exchange radiation boundary for vertical sidewall, the best efficiency was made in some relative depths. By changing the shape of the structures or the distance from structures to sidewall, the best efficiency relative to relative depths are different; in other words, the best efficiency of known wave conditions was got by changing the shape or location of the structures. Using the vertical flat plat to absorb wave energy, the arrange of the best efficiency relative to relative depths is wider than other two; considering above 90 percent of efficiency, using the vertical flat plat to absorb wave energy, the arrange of relative depths is also wider than other two. In summary, about three absorbers of the present study, the available of energy absorbed by vertical flat plat is better than others.

Technology advances continue to revolutionise military equipment. The development of new firepower induces an interest in the enhancement of protection gear, both for transportation vehicles and personnel. There has been a significant amount of research of methods to increase protection capabilities without increases in the weight of a given defence system. This dissertation seeks to develop an optimisation tool that results in light-weight armour plates without compromising protection capabilities. A thorough study on the propagation of elastic and plastic stress waves aims for a better understanding of how an armour system behaves upon ballistic impact. The first part of this dissertation focuses on the development of a Python script that provides an efficient approach to model generation in Abaqus. It enables the user to avoid time consuming actions when designing ballistic test models to later simulate through the software. This script is also used to validate the theory behind elastic and plastic stress wave propagation while also being able to access output databases and interpret obtained results. The importance of the script is relevant for the second part of the dissertation, which takes advantage of the Abaqus Python Application Programming interface (API) to perform optimisation procedures automatically. Focusing particularly on the application of the particle swarm optimisation algorithm, this work continuously improves the efficiency and accuracy of the mentioned algorithm by dividing three different optimisation problems into several experiments. Each one of the experiments is carefully defined to highlight the impact of a specific operating parameter of the algorithm. A validation of the stress wave propagation and how it is affected upon contact with layered media is carefully conducted through a series of different analysis approaches. It is shown that the plastic stress wave propagates slower than the elastic one and that plastic deformation affects the properties of the generated stress wave, such as wavelength. The implemented particle swarm optimisation algorithm proved to be an effective approach to problem solving, however, for complex problems the operational parameters must be carefully chosen.
Os avanços na tecnologia continuam a revolucionar equipamentos militares. O desenvolvimento de novas armas de fogo induz interesse no aprimoramento de equipamento de proteção, para veículos de transporte e pessoal. Tem havido uma quantidade significativa de investigação de métodos para aumentar as capacidades de proteção sem aumento de peso de um dado sistema de proteção. Esta dissertação tem como objetivo o desenvolvimento de uma ferramenta de otimização que resulta em placas de armadura de baixo peso sem comprometer capacidades de proteção. Um estudo cuidadoso acerca da propagação de ondas de tensão elásticas e plásticas procura compreender a forma como um sistema de armadura reage após um impacto balístico. A primeira parte desta dissertação foca-se no desenvolvimento de um código em Python que fornece uma abordagem eficiente à geração de modelos no Abaqus. Isto permite que o utilizador evite ações que consumam tempo ao criar modelos de teste balístico para simular mais tarde através do software. Este código é também usado para validar a teoria por detrás da propagação de ondas de tensão elásticas e plásticas e ao mesmo tempo habilitar o acesso a dados de saída do software e interpretar resultados obtidos. A importância do código é relevante para a segunda parte da dissertação, que tira vantagem da interface de aplicação e programação do Abaqus Python (API) para executar procedimentos de otimização de forma automática. Com foco em particular na aplicação do algoritmo de otimização por enxame de partículas, este trabalho melhora continuamente a eficácia e precisão do algoritmo mencionado através da divisão de três diferentes problemas de otimização em várias experiências. Cada uma das experiências é cuidadosamente definida para destacar o impacto de um parâmetro operacional específico do algoritmo. A validação da propagação da onda de tensão e como é afetada após contacto com um meio material de múltiplas camadas é cuidadosamente estudada através de séries de diferentes análises. É mostrado que a onda de tensão plástica se propaga mais lentamente que a elástica e que deformação plástica afeta as propriedades da onda de tensão gerada, tal como o comprimento de onda. O algoritmo de otimização por enxame de partículas implementado prova ser uma abordagem eficaz para a resolução de problemas, no entanto, para problemas complexos os parâmetros operacionais devem ser escolhidos com cuidado.
Mestrado em Engenharia Mecânica

碩士
國立臺灣海洋大學
河海工程學系
102
Based on the experiment about hydraulic model,this research investigates the effect of reversing gear used in point absorption wave converter. Test regular waves and irregular waves in one direction, at a depth of 1.3 meters,discuss about RAO and power generation and power generation performance to understand the power of this wave energy generators capture and conversion system performance parameters as well as power generation, as a basic prototype testing physical prototypes in the sea. Test results show that period 1.8~2.2 seconds is the best wave wave power converter capture period, when external resistor in 36 ohm,wave energy generator capture system performance can capture more than 80%.Testing regular waves by these conditions,the performance of power generatiom is nearly 20%. Irregular wave tests, RAO almost all above 0.6, when significant period going longer the RAO even up to 0.8.

The increasing population density and the industrial expansion significantly affect the availability of land. In this context, the high modularity of Very Large Floating Structures (VLFSs) may indeed represent a promising alternative for multipurpose use. Furthermore, the interest for the sea as a source of renewable marine energy, particularly for wave energy, has tremendously increased in the last decade and years. Among the large diversity of Wave Energy Converters (WECs), the Oscillating Water Column (OWC) is one of the most promising concept. Moreover, when an OWC is incorporated in a VLFS, its efficiency in terms of wave energy absorption is not only increased, but also it has additionally the benefit of attenuating the heave motion of the floating structure. Hence, there is a growing interest in the development of an innovative VLFS equipped with OWC devices. In this scope, the mitigating effect of the OWC on the heave motion of the VLFS can be combined with an increased efficiency of the OWC, thus better contributing to supply energy for the facilities located on the floating system. The main goal of this PhD research is the investigation of a VLFS-OWC System conceived for a hypothetical installation in a Mediterranean area, characterized by a moderate wave climate. For this purpose, small-scale experiments have been carried out in the wave-current flume of the Maritime Engineering Laboratory (LABIMA) of Florence University. The laboratory tests focused on the effect of: (i) the OWC design parameters (i.e., OWC geometry); (ii) the incident wave conditions (i.e., regular and irregular wave trains); (iii) the damping induced by a non-linear air turbine (i.e., a self-rectifying impulse turbine) idealised by vents with different diameters in the OWC chamber roof; (iv) the length and the heave motion of the VLFS on the performance of the OWC, including the attenuating effect of the incorporated OWC on the heave motion of the VLFS-OWC system. The design of the fixed OWC, VLFS and VLFS-OWC models as well as the testing programme and laboratory procedures, are based on an extensive literature review of the available numerical and physical models on OWC devices and VLFS technologies. The main findings of this study may be summarized as follows: - the most dominant parameters affecting the performance of a fixed OWC are the chamber width (in wave propagation direction), the front wall draught and the damping induced by the air turbine; - the additional parameters affecting the efficiency of an OWC integrated in a VLFS are the length of the structure and the heave motion; - formulae are developed for predicting the heave motion of the VLFS-OWC system respectively, for regular waves and irregular waves; - formulae are developed for improving the prediction of the performance of a fixed OWC for a floating OWC (integrated in a VLFS) respectively, for regular irregular waves. These findings have contributed to improve the understanding of the functioning of the OWC device and the relative importance of the aforementioned parameters affecting the device under moderate wave climate.