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Design, production and characterization of glass-ceramic based sealants for solid oxide fuel cells applications
No full textPlanar solid oxide fuel cells (SOFCs) are capable of achieving higher power density than tubular SOFC, but hermetic seals are required to prevent mixing of the fuel and oxidant, It is still a challenge to develop sealing materials that retain desirable physical properties, are chemically compatible with other fuel cell components at high temperature (e.g. 800 °C) in a wide range of oxygen partial pressure, and remain operational over thousands of hours. In most planar SOFCs stacks designs, the interconnect is sealed to the cell components. The seal between the metal interconnect and the ceramic SOFCs components presents a challenge. Design, development and implementation of reliable sealants may contribute to the destiny of SOFC electrical power-generation technology. Glass-ceramics, which can be prepared by controlled sintering and crystallization of glasses, possess superior mechanical properties and higher viscosity at the SOFC operating temperature than glasses. The application of a protective coating on the alloy surface has been proven as a practical and effective method to reduce corrosion rates and/or inhibit Cr volatilization and thus cathode poisoning. Though some coatings can be highly effective in reducing corrosion rates and reducing area specific resistance of metallic interconnects, their properties for blocking chromium diffusion are limited and need more research on advanced materials and new processing methods. Furthermore, in the complex contest of the SOFC stack, the interconnect/sealant interface plays a key role in the stack reliability, efficiency and durability that depends also on the gas tightness provided by seals during SOFC operation for thousands of hours. It is desirable that reactions between the sealant and the coating or the metallic interconnect are limited during SOFC relevant operating conditions, otherwise spallation and detachments at the interfaces can occur and determine leakage and SOFC degradation. Finally, different approaches are used in this work for the integration (i. e. joining) of ceramic and metallic components in solid oxide fuel cells (SOFCs) stacks, where dissimilar materials have to be joined and sealed for a reliable long-term operation. In particular, the thermo-mechanical compatibility of sealants with other stack components critically influence the reliability and the robustness of SOFCs device
Probabilistic generation of time-coupled aggregate residential demand patterns
No full textFor distribution system studies, a relevant aspect is the characterisation of the aggregate demand in a feeder. The probabilistic model of the aggregate demand is very useful for system operators or aggregators to extract information about the demand side behaviour in the operation of smart grids and microgrids. The time step used to scan the aggregate demand pattern is very important to preserve the information about the consumers' behaviour and the related uncertainty. The conventional models of aggregate electrical demand consider an average value for a specific time step (e.g. 30 minutes to 1 hour). In this paper, a faster time step (1 minute) is considered to construct a time-coupled probabilistic model of the aggregate residential demand based on Beta distributions. For a given number of aggregate loads, the parameters of the Beta distributions are found by taking into account the aggregate demand pattern variations at two successive time steps. The probabilistic model is then used to generate a number of aggregate demand scenarios. The effectiveness of the proposed scenario generation method is evaluated by using goodness of fit tests like the Kolmogorov-Smirnov test and the average mean absolute percentage erro
Stochastic Modelling of Multi-Energy Carrier Dependencies in Smart Local Networks with Distributed Energy Resources
No full textIn a multi-energy system, there are different types of dependencies among the energy carriers. Internal dependencies refer to possible changes in the energy source in the presence of energy converters and storage and are managed by the system operator through the control strategies applied to the equipment. External dependencies are due to the choice of the energy supply according to customer preferences, when alternative solutions are available. This paper introduces a new model of the external dependencies within a multi-generation representation based on energy hubs. The external dependencies are addressed through a stochastic model, in order to take into account the possible uncertainty in the customers' decisions. This model is then used to introduce carrier-based demand response, in which the user participates in demand response programmes aimed at promoting the shifting among different energy sources by preserving the service provided to the end users. The results obtained from the new model in deterministic and stochastic cases indicate the appropriateness and usefulness of the proposed approac
Miniaturized antenna design & Radiation pattern corrections in Fresnel region
No full textIn this thesis, two research problems have been addressed. Electronic devices require an ever increasing reduction of size and hence the important communication component i.e., the antenna needs to be miniaturized as well to cope up with other technologies. Addressing this problem, in first part of the thesis, a miniaturized antenna was designed and fabricated after analysis for its proper positioning on the PCB. Also, an analysis was done to see the effect on the resonance of the radiating element due to the proximity of other components present on the same PCB. In in this part of the thesis, the analysis and the optimization for size and positioning, of a miniaturized antenna, along with the effects of the proximity to other components has been performed, and a compact antenna has been designed, realized and tested. The measured results were found to be in good agreement with the simulated ones. The second part of the thesis consists of the analysis of the radiation pattern of microwave antennas. The test ranges for Far Field radiation pattern need a certain separation distance between antenna under test and the test probe; this separation requirement increases with frequency and if this separation requirement is not met then some corrections need to be performed on the measured radiation pattern, in particular for the error induced in side lobe level. When far field test site minimum distance requirements between antenna under test and, test probe are not fulfilled. This problem was analyzed for microwave aperture antennas. The side lobe level error due to this problem and its correction has been discussed. On the basis of above analysis Far Field side lobe level corrections factors were obtained in the form of graphs and a closed form formula was suggested on the basis of analysis performe
Thermoelectric Metrology
No full textMetrology, the science of measure, is in constant evolution and development in every field. Recently, due to the energy problem, some physical effect and related device are receiving new attention, in particular: piezoelectricity and thermoelectricity. We think that the time is ripe to expand the science of measure in a new may:Thermoelectric Metrology. This work represent a new beginning in this field: we study and build a new method and instrument to measure the figure of merit zT and Resistance R, also we describe a new way to use thermoelectric device (in particular Peltier cells), to apply linear system theory and frequency analysis to measure thermo-physical property such as thermal effusivity and thermal diffusivit
AUTOMATIC ON-LINE FAULT DETECTIONALGORITHM FOR HYDRAULIC TURBINES
No full textPower plants are one of the most important parts of electric networks. The most important parts in the power plants are turbines, generators and power transformers. These devices are very expensive and their healthy is vitally important. Therefore diagnosis and monitoring systems for preventing catastrophic faults and also for detecting the on-going damages are really necessary and valuable. According to the expense of maintenance of turbines and transformers and sometimes spending a lot of time to inspect them inside, it is necessary to predict the fault in them before happening and without inspecting inside (nonintrusive). With on-line predictive maintenance we would be able to decrease the cost of maintenance and surly improve the performance, and also detecting the incipient faults and finally forecasting the remained life age of these devices. According to my research, the best methods for predicting faults in power transformers are on-line DGA, on-line SFRA and on-line PD method. Also one of the best on-line methods for diagnosis in turbines is vibration analysis. By mounting some acceleration transducers on the bearings of turbines, we can measure the vibration of the system and after that by doing some post processing; we can decide that the turbine's condition is normal or no. Also sometimes we can discover the origin of the vibration, for example unbalance, misalignment and so on. In this thesis, first of all I describe the main methods that I have used for writing the fault detection algorithm and then I apply the fault detection algorithm on RTR simulation model in MATLAB environment to check the validity of algorithm. After that I apply the written algorithm on a real test-bench called "5assi" that it is a rotor with 3 active magnetic bearings. I measure the vibration signals of 5assi by using some displacement sensors and I check the performance of the algorithm in many cases includes normal condition and unbalance condition with different levels of unbalance. The results show that the algorithm is able to detect the unbalance problem and warn operators perfectly. At the last step I apply the algorithm on a real time monitoring system on a turbine in the power plant "Pont St. Martin" in the province "Aosta" in the north of Italy to check the performance of the algorithm in a real situation. We use eight accelerometers to measure the vibration signals and finally we generate one index for each sensor that shows the condition of the correspondent signal. With trending these indexes during the time and according to some defined alarm and trip levels, we would be able to detect almost any kind of faults in turbines before happening and warn the operators in a proper time. With using this on-line monitoring system, we would be able to save a lot of money and time and surly we can increase the performance of the power plant
Strategic choices in safety systems of complex public institutions: from setting the risk analysis techniques to conservative and ameliorative quality management
No full textOS&H appears to be particularly complex in the case of large public facilities (Ministries, public offices, ...) open to the patrons, due to a number of typical characteristics, such as the often very large number of workers and patrons and the wide range of patrons' typologies, the age and historical/artistic value of many Italian settlements, the common possibility of internal/external interference of functional volumes. As a consequence, the usual approaches to the OH&S problems in industrial activities or secondary educational institutions can prove quite inadequate in many cases. The task of Occupational Risk Assessment and Management analysts becomes even more demanding where the attention is focused on activities which are institutionally involved in both high degree university education and in advanced research, the latter carried on in laboratories where also special devices, often of original or somehow modified design, are used. On the basis of literature findings, no very helpful suggestion based on real case histories are available, and in many cases the approach is limited to general purpose references on a limited number of Hazard Factors, integrated with spots on demand interventions, whilst both Prevention through Design general approaches to the OS&H problems, and quality approaches (e.g. according to OHSAS 18000) in the Residual Risk Assessment and Management are not common. The present thesis discusses with particular reference to Universities in general and to the Italian situation in particular, the results achieved in a multidisciplinary work -made exhaustive thanks to the presence of highly qualified experts among the staff- carried out in cooperation by Politecnico di Torino and Università degli Studi di Torino. The most important outcome was the definition of a Guideline for the Occupational Risk Assessment and Management of workers and users of large public facilities, specially conceived to provide for each involved person an effective evaluation of the actual residual exposure to the total number of Hazard Factors, systematically identified by means of a consistent Job Analysis. The Guideline, focalized in particular on the universities, stresses the paramount importance of a detailed and unambiguous definition, since the very first step of the study, of the Line and Staff Organization, upon which to base the discussion of the three basic Occupational Safety topics in the following hierarchic order: a) the workplace general safety characteristics, b) the workers OS&H along their activity in terms of involved residual risks, taking into account the available equipment (tools, machines and fittings), and c) the quality management of the OS&H system. The technique, carefully tested both in its general approach and in a number of sub-applications for special aspects, can moreover play an effective role in the continuous process of assisted training of the subjects operating in the various Areas (Departments, Services, ... ) who, thanks to the detailed knowledge of their area, will be able to provide a capillary contribution in the phases of Hazards Factors Identification, and of suitable OH&S conditions -as defined in the Risk Management process- conservation and improvement. This PhD thesis, analyzes in depth the applicative phases and sub-phases of the proposed model with some examples of real cases. In particular, the thesis stresses the essential need of following the hierarchical phases of approach to provide an exhaustive management tool for Occupational Risk Assessment and Managemen
Free Vibration Analysis of Laminated Beam by Polynomial, Trigonometric, Exponential and Zig-Zag Theories
No full textA number of refined beam theories are discussed in this paper to trace the free vibration response of laminated beams, including thin-walled boxes. By expanding the unknown displacement variables over the beam section axes using Taylor type expansions, trigonometric series, exponential, hyperbolic and zig-zag functions, many new displacement fields were obtained and, for the first time, evaluated for the dynamic analyses of composite structures. The finite element method is used to derive governing equations in weak form. These equations are written using the unified formulation introduced by the first author, in terms of fundamental nuclei, whose forms do not depend on the expansions used. The natural frequencies are compared with results available in the literature or with those obtained by the finite element models related to commercial software. A number of analyses were conducted to compare various theories, including Euler- Bernoulli and Timoshenko models. The advantages/disadvantages of using the different theories are discussed for significant problems related to laminated beams as well as thin-walled boxes. It is shown that refined kinematic theories are able to yield a very accurate evaluation of fundamental as well as higher mode frequencies in a way comparable to threedimensional analysis, but it is obtained with a strong reduction of computational costs
Si-SiC based materials obtained by infiltration of silicon: study and applications
No full textMany efforts and investments have been made in the last decades in order to obtain materials able to withstand extreme conditions in several technological fields such as future military, industrial and space based projects. The present PhD thesis addresses this problem dealing Ultra High Temperature Ceramics (UHTCs) and in particular focusing on Si-SiC-ZrB2 composites obtained with a technique named silicon reactive infiltration (SRI). Each major chapter of the present thesis relates to an article either published or in press on the journals indicated at the end in the seventh chapter (List of author's publications). The sequence of these chapters is obviously not random, but trying to follow the streamline that leads from the physico-chemical and technological properties of the material to its applications, in my case this is meant its use in industrial burners within an european project: CEREXPRO, that was financially supported by the European Commission in the FP7 (7th Framework Programme). The reader is introduced to the current state of the art of UHTC (Chapter 1) having a historical review of UHTC research carried out thus far. Since the most comprehensive research has been focused on compositions containing HfB2 or ZrB2, they are the primary materials discussed. The chapter concludes by illustrating a particular type of UHTC: the siliconized silicon carbide (Si-SiC) and the infiltration method used to obtain it. In the second chapter a methodology to produce Si-SiC-ZrB2 bulk ceramics, taking advantage of the reactive infiltration technique, is proposed. Preforms with different contents of SiC and ZrB2 were compacted with phenolic resin, pyrolysed and finally infiltrated with molten silicon at temperature above its melting point. A detailed study of the mutual interactions between the Si-SiC-ZrB2 constituent materials during their processing and of their chemical activity with oxygen at high temperatures in the prospect of using this material as a matrix of a continuous fiber reinforced composite is addressed in Chapter 3. The study of the early stage of oxidation is the first part of this chapter, while the second part is focused on the long-term oxidation of some selected samples at a high temperature. The aim of chapter four is to give an overview of the processing routes currently available for the preparation of macro-porous ceramics, with particular emphasis on the processing-microstructure-property relations inherent to each process. At the end, the chapter presents several high temperature applications in which Si-SiC foams are currently applied because of their superior thermo-mechanical properties. The Chapter 5, in fact, aims to track the changes of Si-SiC foam properties during the first part of a long time aging condition, typical of industrial burner applications. The project "CEREXPRO" (CERamic heat EXchangers with enhanced materials PROperties), discussed in the Chapter 6, was aimed to develop a new generation of ceramic heat exchangers for high temperature heat recovery with the target of significantly reducing the size and weight as well as also the price of such components by simplifying the manufacturing process and allowing a higher flexibility in the heat exchanger geometry. The use of precursors/template materials taken from the textile industries and a subsequent ceramic conversion, via liquid silicon infiltration, is proposed as the main technological path for reaching the above objectives. The main role of the Polytechnic within this project was the characterization process of material
Thermo-mechanical delamination analysis by a plate model with "adaptive" representation of displacements and temperature
No full textProgressive delamination under thermo-mechanical loading can lead either to a premature failure or a consistent loss of strength and stiffness of laminated and sandwich composites in service. Customary, three-dimensional finite element models are used to predict the stress fields with the highest accuracy, yet the behavior of the delamination crack is simulated by fracture mechanics or cohesive interface models, but these models may overwhelm the computational capacity. Refined plate models that accurately capture the stress fields with a lower computational effort are to date available, which may overcome the problem. Here a recently developed multilayered zig-zag model is applied to the analysis of delamination under impact and thermal loading. This model, which a priori fulfills the equilibrium of out-of-plane stresses and the heat conduction equation at the interfaces, has a hierarchic representation of displacements and temperature across the thickness, but just five displacement d.o.f. and two temperature d.o.f., in order to limit the memory storage occupation. The onset of delamination is predicted using stress based criteria, as customary, while a mesoscale model is used to accurately and efficiently accounts for the evolving damage. So, instead of guessing suited multiplication factors, the degraded properties of the failed regions are computed at each time step. The strain energy updating technique by the author is used to develop an efficient C finite element model. The numerical results show the good accuracy of the present modelling approac