Academic literature on the topic 'OWC'

Accurate predictions of the annual energy yield from wave energy converters are essential to the development of the wave industry. The current method based on power matrices uses only a small part of the data available from sea state estimations and it is consequently prone to inaccuracies. The research presented in this work investigates the issue of energy yield prediction and questions the power matrix method. This is accomplished by quantifying the influence of several directional sea states parameters on the performances of wave energy converters. The approach taken was to test several wave energy converters in the Edinburgh Curved tank with a large set of sea states. The selected wave energy converters are a fix OWC, a set of two OWCs acting as a weak directional device and the desalination duck model. Uni-modal and bi-modal sea states were used. For the uni-modal sea states, parameters related to the wave system shape were considered. For the bi-modal sea states, the relative position of the wave system peaks was investigated and the uni-modality index was introduced to quantify the degree to which sea states could be considered bi-modal. For all sea states, the significant wave height was kept constant. The experimental work required good spectral estimates. The MLM and MMLM were adapted to deterministic waves to improve their stability and accuracy. A routine to isolate wave systems was also developed in order to estimate parameters with respect to each wave systems. For uni-modal spectra, parametric models of the observed performances of the devices could be devised. The frequency spreading and its interaction with the energy period proved to be as important as the energy period itself, which suggests that the frequency spreading should be used for energy production prediction. For bi-modal spectra, evidence of the duck sensitivity to directionality was found while the OWCs were not affected.

Ces travaux de recherche s'inscrivent dans le cadre des études de faisabilité du stockage du dioxyde de carbone en couches géologiques profondes, et portent plus particulièrement sur l'évolution des propriétés de confinement de roches-couvertures de type argilite, soumises à des fluides enrichis en CO2. Pour ce faire, les argilites de Tournemire (Aveyron, France) ont été utilisées comme roches analogues, après avoir identifié leurs possible points de faiblesse vis-à-vis du stockage, à savoir leur minéralogie, la présence de fissures naturelles remplies de calcite et la présence d'interfaces ciment/argilite attendues au niveau des puits d'injection rebouchés. Le montage expérimental de type " diffusion à travers " a été adapté pour permettre d'estimer (i) la possible modification des paramètres de transport diffusif enregistrés, avant et après attaque acide, pour les différents traceurs radioactifs (tritium et chlore-36) et non-radioactifs (deutérium et bromure) utilisés pour caractériser les échantillons d'argilite de Tournemire et de pâte de ciment et (ii) l'évolution des compositions chimiques des solutions contenues dans les réservoirs amont et aval des cellules de diffusion au cours des attaques acides. Enfin, l'analyse des solides a été réalisée pour partie par MEB-EDS, µTomographie-X et DRX. Tout d'abord, pour l'ensemble des échantillons étudiés, les valeurs des paramètres de transport déterminées avant attaque acide (coefficient de diffusion effectif et porosité) sont cohérentes avec celles de la littérature. En outre, il apparaît que tous les matériaux ont fortement réagi aux attaques acides. Ainsi, les argilites ont vu leurs paramètres de diffusion augmenter jusqu'à un facteur deux, notamment pour les traceurs anioniques, et ce, quelque soit la proportion de minéraux carbonatées présents initialement dans les échantillons d'argilite. Les observations post-mortem ont permis l'identification d'une zone de dissolution des minéraux carbonatés en leur au sein, mais dont l'extension (400 µm au maximum) ne peut à elle seule expliquer cette importante dégradation des propriétés de confinement. Seuls des phénomènes non observables à l'échelle d'investigation, tels des effets de digitation au sein du réseau poreux pourraient en être à l'origine. Par ailleurs, les échantillons d'argilites recoupés par des fissures naturelles ont systématiquement développé des courts-circuits hydrauliques, qui s'expliquent par la mise en place d'un processus de chenalisation préférentielle au sein même de la fissure, observé au MEB-EDS et par µTomographie X. De plus, les matériaux cimentaires ont vu leurs propriétés de confinement améliorées, et ce, plus le fluide d'attaque était proche de la neutralité. Ceci est lié à l'apparition d'une croûte de calcite d'environ 30 µm d'épaisseur, situé à l'avant d'une zone de dissolution, dont l'épaisseur est proportionnelle à la quantité d'acide en solution. Cette croûte imperméable, obstruant la porosité, serait responsable de la diminution des paramètres de diffusion, observée à l'échelle de l'échantillon, ainsi que le reproduisent les simulations numériques. Enfin, les interfaces ciment/argilites n'ont pas révélé de changement de leurs propriétés de confinement à l'échelle globale, la dégradation des argilites étant compensé par l'aggradation du ciment via la formation de cette croûte de calcite.

Wave energy is one of the most promising renewable energy sources for future exploitation. This thesis focuses on thermodynamic effects within Oscillating Water Column (OWC) devices equipped withWells turbines, particularly humidity effects. Previous theoretical studies of the operation of OWCs have resulted in expressions for the oscillation of the water surface in the chamber of an OWC based on linear wave theory, and the air expansion{compression cycle inside the air chamber based on ideal gas theory. Although in practice high humidity levels occur in OWC devices open to the sea, the influence of atmospheric conditions such as temperature and moisture on the performance of Wells turbines has not yet been studied in the field of ocean energy. Researchers have reported substantial differences between predicted and measured power output, and performance rates of OWCs presently coming into operation. The effect of moisture in the air chamber of the OWC causes variations on the atmospheric conditions near the turbine, modifying its performance and efficiency. Discrepancies in available power to the turbine are believed to be due to the humid air conditions, which had not been modelled previously. This thesis presents a study of the influence of humid air on the performance of an idealised Wells turbine in the chamber of an OWC using a real gas model. A new formulation is presented, including a modified adiabatic index, and subsequent modified thermodynamic state variables such as enthalpy, entropy and specific heat. The formulation is validated against experimental data, and found to exhibit better agreement than the ideal approach. The analysis indicates that the real gas behaviour can be explained by a non{dimensional number which depends on the local pressure and temperature in the OWC chamber. A first approach to the OWC formulation through the calculation of real air flow in the OWC is given, which predicts a 6% decrease in efficiency with respect to the ideal case when it is tested with a hypothetical pulse of pressure. This is important because accurate prediction of efficiency is essential for the optimal design and management of OWC converters. A numerical model has also been developed using computational fluid dynamics (CFD) to simulate the OWC characteristics in open sea. The performance of an OWC turbine is studied through the implementation of an actuator disk model in Fluent®. A set of different regular wave tests is developed in a 2D numerical wave flume. The model is tested using information obtained from experimental tests on a Wells{type turbine located in a wind tunnel. Linear response is achieved in terms of pressure drop and air flow in all cases, proving effectively the applicability of the actuator disk model to OWC devices. The numerical model is applied first to an OWC chamber containing dry air, and then to an OWC chamber containing humid air. Results from both cases are compared, and it is found that the results are sensitive to the degree of humidity of the air. Power decreases when humidity increases. Finally, results from the analytical real gas and numerical ideal gas models are compared. Very satisfactory agreement is obtained between the analytical and the numerical models when humidity is inserted in the gaseous phase. Both analytical and numerical models with humid air show considerable differences with the numerical model when dry air is considered. However, at the resonance frequency, results are independent of the gas model used. At every other frequency analysed, the real gas model predicts reduced values of power that can fall to 50% of the ideal power value when coupled to the radiation-diffraction model for regular waves. It is recommended that real gas should be considered in future analyses of Wells turbines in order to calculate accurately the efficiency and expected power of OWC devices.

La tesi fa una panoramica delle tecnologie di comunicazione ottica, i principi di funzionamento, tecniche e strumenti utilizzati e possibili campi di applicazione. Nello specifico si esamina FSO, una tecnologia che opera nella banda delle frequenze a Infrarossi. VLC che opera nello spettro della luce visibile e UVC che opera con la luce Ultravioletta. Il terzo capitolo anlaizza LiFi, una tecnlogia che rappresenta l'evoluzione di VLC.

Bidirectional air turbines are used in oscillating water column (OWC) power plants for harnessing ocean wave energy. This paper describes the bidirectional aerodynamic and aero-acoustic facility at the University of Siegen for model air turbines performance testing. At least nine test facilities are known worldwide, but their layout, the performance testing procedure and the presentation of performance data are not standardized to this day. The layout of the facility at the University of Siegen follows ideas in ISO 5801 for fan performance testing. The pressurized air supply is bidirectional but steady-state. Achievable values of Reynolds and Mach number of the test turbines are 1,000,000 and 0.5, respectively. In addition, the facility is equipped with acoustic attenuators in the air supply for allowing synchronous determination of aerodynamic and acoustic characteristics of a turbine. A good practice guideline for turbine performance testing and presentation is proposed by showing full sets of non-dimensional aerodynamic and acoustic performance characteristics from two sample model turbines. Eventually, a comparison of in situ data from a full-scale turbine in transient operation with scaled up steady-state model performance measurements underlines the usefulness of steady-state model performance testing.

This work is divided in to two distinct parts. In the first part a model is developed to assess the redistribution of wave energy about an offshore array of overtopping type wave energy converters. The model is based on a classical analytical solution for diffraction about a breakwater which is modified to consider an array of dissipating, reflecting and transmitting breakwater segments, which are used to approximate an overtopping type WEC array. The model is computationally efficient and phase resolving which allows the effect of wave scattering to be investigated for large domains with high resolution irregular wave distributions. It was found that the radial waves generated by the diffraction effect spreads and defocus wave energy away from the geometrical shadow of the array. This counteracts the rate of recovery of wave energy deficit from wave directional spreading. In the second part, short-term wave forecasting for pneumatic power regulation through relief valve control is investigated at the Pico oscillating water column power plant, located in the Azores. Operational data from the Pico OWC is used to develop and critically assess a number of univariate and multivariate short-term wave forecast modelling approaches. A number of relief valve control strategies, which utilise a short-term wave forecast, are also developed and assessed using a numerical time-domain wave to wire system model. A system model for the Pico OWC is developed and validated using operational data from the Pico plant. The absolute performance potential resulting from control utilising a perfect forecast is considered, in addition to the realistic potential where a forecast, realisable in real-time, is used to drive control actions. One of the proposed relief valve control strategies is within the mechanical limitations of the existing relief valve adjustment system at Pico and this strategy was deployed in real field tests. Field test results of the plant’s performance under this strategy closely matched the simulated performance and power enhancements of up to 29% were achieved in certain sea states and the expected annual power enhancement was projected to be around 10%. Simulations of the long term plant performance under the more advanced relief valve control strategies project far greater potential for enhanced power production although these could not be tested in the field due to the project limitations.

Although oscillating water column type wave energy devices are nearing the stage of commercial exploitation, there is still much to be learnt about many facets of their hydrodynamic performance. This research uses the commercially available FLUENT computational fluid dynamics flow solver to model a complete OWC system in a two dimensional numerical wave tank. A key feature of the numerical modelling is the focus on the influence of the front wall geometry and in particular the effect of the front wall aperture shape on the hydrodynamic conversion efficiency. In order to validate the numerical modelling, a 1:12.5 scale experimental model has been tested in a wave tank under regular wave conditions. The effects of the front lip shape on the hydrodynamic efficiency are investigated both numerically and experimentally and the results compared. The results obtained show that with careful consideration of key modelling parameters as well as ensuring sufficient data resolution, there is good agreement between the two methods. The results of the testing have also illustrated that simple changes to the front wall aperture shape can provide marked improvements in the efficiency of energy capture for OWC type devices.

Thesis (PhD (Chemical Engineering))--Cape Peninsula University of Technology, 2019<br>The South African wine industry is constantly facing several challenges which affect the quality of wine, the local/global demand and consequently the revenue generated. These challenges include the ongoing drought, bush fires, climate change and several liquor amendment bills aimed at reducing alcohol consumption and alcohol outlets in South Africa. It is therefore critical for the wine industry to expand and find alternative ways in which sub-standard or surplus wine grapes can be used to prevent income losses and increase employment opportunities. Traditional Balsamic Vinegar (TBV) is a geographically and legislative protected product produced only in a small region in Italy. However, the methodology can be used to produce similar vinegars in other regions. Balsamic-styled vinegar (BSV), as defined in this thesis, is a vinegar produced by partially following the methods of TBV while applying process augmentation techniques. Balsamic-styled vinegar is proposed to be a suitable product of sub-standard quality or surplus wine grapes in South Africa. However, the production of BSV necessitates the use of cooked (high sugar) grape must which is a less favourable environment to the microorganisms used during fermentation. Factors that negatively affect the survival of the microorganisms include low water activity due to the cooking, high osmotic pressure and high acidity. To counteract these effects, methods to improve the survival of the non-Saccharomyces yeasts and acetic acid bacteria used are essential. The primary aim of this study was to investigate several BSV process augmentation techniques such as, aeration, agitation, cell immobilization, immobilized cell reusability and oxygen mass transfer kinetics in order to improve the performance of the microbial consortium used during BSV production. The work for this study was divided into four (4) phases. For all the phases a microbial consortium consisting of non-Saccharomyces yeasts (n=5) and acetic acid bacteria (n=5) was used. Inoculation of the yeast and bacteria occurred simultaneously. The 1st phase of the study entailed evaluating the effect of cells immobilized by gel entrapment in Ca-alginate beads alongside with free-floating cells (FFC) during the production of BSV. Two Ca-alginate bead sizes were tested i.e. small (4.5 mm) and large (8.5 mm) beads to evaluate the effects of surface area or bead size on the overall acetification rates. Ca-alginate beads and FFC fermentations were also evaluated under static and agitated (135 rpm) conditions. The 2nd phase of the study involved studying the cell adsorption technique for cell immobilization which was carried-out using corncobs (CC) and oak wood chips (OWC), while comparing to FFC fermentations. At this phase of the study, other vinegar bioreactor parameters such as agitation and aeration were studied in contrast to static fermentations. One agitation setting (135 rpm) and two aeration settings were tested i.e. high (0.3 vvm min−1) and low (0.15 vvm min−1) aeration conditions. Furthermore, to assess the variations in cell adsorption capabilities among individual yeast and AAB cells, the quantification of cells adsorbed on CC and OWC prior- and post-fermentation was conducted using the dry cell weight method. The 3rd phase of the study entailed evaluating the reusability abilities of all the matrices (small Ca-alginate beads, CC and OWC) for successive fermentations. The immobilized cells were evaluated for reusability on two cycles of fermentation under static conditions. Furthermore, the matrices used for cell immobilization were further analysed for structure integrity by scanning electron microscopy (SEM) before and after the 1st cycle of fermentations. The 3rd phase of the study also involved the sensorial (aroma and taste) evaluations of the BSV’s obtained from the 1st cycle of fermentation in order to understand the sensorial effects of the Ca-alginate beads, CC and OWC on the final BSV. The 4th phase of the study investigated oxygen mass transfer kinetics during non-aerated and aerated BSV fermentation. The dynamic method was used to generate several dissolved oxygen profiles at different stages of the fermentation. Consequently, the data obtained from the dynamic method was used to compute several oxygen mass transfer parameters, these include oxygen uptake rate ( 𝑟𝑟𝑂𝑂2 ), the stoichiometric coefficient of oxygen consumption vs acid yield (𝑌𝑌𝑂𝑂/𝐴𝐴), the oxygen transfer rate (𝑁𝑁𝑂𝑂2 ), and the volumetric mass transfer coefficients (𝐾𝐾𝐿𝐿𝑎𝑎). During all the phases of the study samples were extracted on weekly intervals to evaluate pH, sugar, salinity, alcohol and total acidity using several analytical instruments. The 4th phase of the study involved additional analytical tools, i.e. an oxygen µsensor to evaluate dissolved oxygen and the ‘Speedy breedy’ to measure the respiratory activity of the microbial consortium used during fermentation. The data obtained from the 1st phase of the study demonstrated that smaller Ca-alginate beads resulted in higher (4.0 g L-1 day−1) acetification rates compared to larger (3.0 g L-1 day−1) beads, while freely suspended cells resulted in the lowest (0.6 g L-1 day−1) acetification rates. The results showed that the surface area of the beads had a substantial impact on the acetification rates when gel entrapped cells were used for BSV fermentation. The 2nd phase results showed high acetification rates (2.7 g L-1 day−1) for cells immobilized on CC in contrast to cells immobilized on OWC and FFC, which resulted in similar and lower acetification rates. Agitated fermentations were unsuccessful for all the treatments (CC, OWC and FFC) studied. Agitation was therefore assumed to have promoted cell shear stress causing insufficient acetification during fermentations. Low aerated fermentations resulted in better acetification rates between 1.45–1.56 g L-1 day−1 for CC, OWC and FFC. At a higher aeration setting, only free-floating cells were able to complete fermentations with an acetification rate of 1.2 g L-1 day−1. Furthermore, the adsorption competence data showed successful adsorption on CC and OWC for both yeasts and AAB with variations in adsorption efficiencies, whereby OWC displayed a lower cell adsorption capability compared to CC. On the other hand, OWC were less efficient adsorbents due to their smooth surface, while the rough surface and porosity of CC led to improved adsorption and, therefore, enhanced acetification rates. The 3rd phase results showed a substantial decline in acetification rates on the 2nd cycle of fermentations when cells immobilized on CC and OWC were reused. While cells entrapped in Ca-alginate beads were able to complete the 2nd cycle of fermentations at reduced acetification rates compared to the 1st cycle of fermentations. The sensory results showed positive ratings for BSV’s produced using cells immobilized in Ca-alginate beads and CC. However, BSV’s produced using OWC treatments were neither ‘liked nor disliked’ by the judges. The SEM imaging results further showed a substantial loss of structural integrity for Ca-alginate beads after the 1st cycle fermentations, with minor changes in structural integrity of CC being observed after the 1st cycle fermentations. OWC displayed the same morphological structure before and after the 1st cycle fermentations which was attributed to their robustness. Although Ca-alginate beads showed a loss in structural integrity, it was still assumed that Ca-alginate beads provided better protection against the harsh environmental conditions in contrast to CC and OWC adsorbents due to the acetification rates obtained on both cycles. The 4th phase data obtained from the computations showed that non-aerated fermentations had a higher 𝑌𝑌𝑂𝑂/𝐴𝐴, 𝑟𝑟𝑂𝑂2 , 𝑁𝑁𝑂𝑂2 and a higher 𝐾𝐾𝐿𝐿𝑎𝑎 . It was clear that aerated fermentations had a lower aeration capacity due to an inappropriate aeration system design and an inappropriate fermentor. Consequently, aeration led to several detrimental biochemical changes in the fermentation medium thus affecting 𝐾𝐾𝐿𝐿𝑎𝑎 and several oxygen mass transfer parameters which serve as a driving force. Overall, it was concluded that the best method for BSV production is the use of cells entrapped in small alginate beads or cells adsorbed on CC under static and non-aerated fermentations. This conclusion was based on several factors such as cell affinity/cell protection, acetification rates, fermentation period and sensorial contributions. However, cells entrapped in Ca-alginate beads had the highest acetification rates. The oxygen mass transfer computations demonstrated a high 𝐾𝐾𝐿𝐿𝑎𝑎 when Ca-alginate beads were used under static-non-aerated conditions compared to fermentations treated with CC. Therefore, a fermentor with a high aeration capacity needs to be designed to best suit the two BSV production systems (Ca-alginate beads and CC). It is also crucial to develop methods which can increase the robustness of Ca-alginate beads in order to improve cell retention and reduce the loss of structural integrity for subsequent cycles of fermentation. Studies to define parameters used for upscaling the BSV production process for large scale productions are also crucial.