Electrochemical impedance spectroscopy (EIS) is a powerful technique that can be applied in-situ to deconvolute the various loss mechanisms in the polymer electrolyte fuel cell (PEFC) that occur at different rates. The frequency response of a PEFC that results from EIS is in essence characterised by energy dissipating and energy storing elements of the cell. It can be represented by an equivalent circuit that is composed of resistors and capacitors respectively. By understanding the arrangement and magnitude of the electrical components in the equivalent electrical circuit, it is possible to generate a deeper understanding of how and where the electrical energy that is generated due to the redox reaction is being dissipated and retained within the real physical system. Although the use of equivalent circuits is often an adequate approach, some electrochemical processes are not adequately described by electrical components. In which case, it is necessary to adopt a more rigorous approach of describing processes through the use of differential equations to describe the physics of the electrochemical system at the frequency domain. Studies in the literature have attempted to construct mathematical models to describe the impedance response of the cathode catalyst layer (CCL) based on conservation equations describing the electrochemical and diffusion processes. However this has resulted in a complicated mathematical analysis which in turn results in complicated solutions. The resulting equations cannot be easily validated against real-world EIS measurements and only analytical results have been reported. In this thesis a mathematical model to describe the impedance response of the CCL has been developed. This model is derived from fundamental electrochemical theory describing the physics of the CCL. The mathematical treatment is simplified by taking into account some considerations based on the EIS theory. The resulting model can be easily applied to real-world EIS measurements of PEFCs and presents parameters commonly known in the electrochemical area. The scientific contribution of this doctoral thesis is mainly divided in two sections: Modelling and Application. The first step of the modelling section develops an equation describing charge conservation in the CCL and together with Ohm s Law equation accounting for ionic conduction, predicts the impedance response of the CCL at low currents. The second step includes the change of oxygen concentration during the oxygen reduction reaction (ORR) into the equation accounting for CCL low current operation. The study of mass transport in the CCL is very complex; the literature has treated it with simplifications and approximations. The finite diffusion distance for oxygen to reach the reaction sites in the CCL forms a complicated network of multi-phase parallel and serial paths and can change in dimension at different operating conditions (flooding, drying). In the mathematical treatment of this doctoral thesis the finite diffusion distance and surface concentration of oxygen in the CCL are considered to be independent of the thickness of the CCL. EIS reflects only bulk measurements based on the total CCL thickness. Even though this results in an over-simplification for the oxygen diffusion in the total CCL, this approach simplifies the mathematical treatment to predict the impedance response of the CCL at high current operation, and as result it can be successfully validated against real-world EIS measurements. In the application section the model is applied with real-world EIS measurements of PEFCs. First the model is applied with EIS measurements presenting inductive effects at high frequencies. The model reveals mechanisms masked at high frequencies of the impedance spectrum by inductance effects. The results demonstrate that the practice of using the real part of the Nyquist plot where the imaginary part is equal to zero to quantify the ohmic resistance in PEFCs can be subject to an erroneous interpretation due to inductive effects at high frequencies. Secondly the model is applied to cathode impedance data obtained through a three-electrode configuration in the measurement system and gives an insight into the mechanisms represented at low frequencies of the impedance complex-plot. The model predicts that the low frequency semicircle in PEFC measurements is attributed to low equilibrium oxygen concentration in the CCL-gas diffusion layer (GDL) interface and low diffusivity of oxygen through the CCL. In addition the model is applied with simultaneous EIS measurements in an Open-Cathode PEFC stack. The factors that limit the performance of the PEFC stack are evaluated with simultaneous EIS measurements and the model. The results show that the change in impedance response of individual cells within the stack is attributed to oxygen limitations, degradation in membrane electrode assemblies (MEAs) and temperature distribution. This EIS knowledge enables an assessment of the state of health in operational fuel cell stacks. In the last section of the application section, the mathematical model translated in the time domain via reverse Laplace Transform predicts the current distribution through the CCL. This provides information to improve the performance of the CCL as well as determine the uptake of product water in the membrane. Finally the conclusions and future work are presented. This doctoral thesis has established a backbone understanding of how the electrochemical and diffusion mechanisms relate to the electrochemical impedance spectra of PEFCs. The goal of a future work is to develop this EIS knowledge into a real-time EIS system for non-intrusive diagnostics of degradation in operational PEFCs. This implies a modification of the model to consider oxygen transport through the CCL thickness as part of a multi-species mixture using mass transport theory including concentrated solution theory to fuel cell engineering.

A numerical model of the steady state and alternating current behaviour of a solid-oxide fuel cell is presented to explore the possibilities to diagnose and identify degradation mechanisms in a minimally invasive way using impedance spectroscopy. This is the first report of an SOFC impedance model to incorporate degradation, as well as the first one to include the ribbed interconnect geometry, using a 2-D approximation. Simulated degradation modes include: electrode/electrolyte delamination, interconnect oxidation, interconnect/electrode interface detachment, and anode sulfur poisoning. Detailed electrode-level simulation replaces the traditional equivalent circuit approach, allowing the simulation of degradation mechanisms that alter the shape of the current path. The SOFC impedance results from calculating the cell response to a small oscillatory perturbation in potential. Starting from the general equations for mass and charge transport, and assuming isothermal and isobaric conditions, the system variables are decomposed into a steady-state component and a small perturbation around the operating point. On account of the small size of the imposed perturbation, the time dependence is eliminated, and the original equations are converted to a new linear, time independent, complex-valued system, which is very convenient from a numerical viewpoint. Geometrical and physical modifications of the model simulate the aforementioned degradation modes, causing variations in the impedance. The possibility to detect unique impedance signatures is discussed, along with a study of the impact of input parameter inaccuracies and parameter interaction on the presented results. Finally, a study of pairs of concurrent degradation modes reveals the method’s strengths and limitations in terms of its diagnosis capabilities.

The objective of this dissertation is to carry out dynamic modeling, analysis and control for Voltage-Source Converters (VSC). Two major applications of VSC will be investigated in this dissertation: microgrid application and High Voltage Direct Current (HVDC) application. In microgrid applications, VSC is used to integrate distributed energy sources such as battery and provide system functions: such as real and reactive power regulation, voltage and frequency support during islanding condition, and abnormal system condition mitigation. In HVDC applications, VSC is used to interconnect dc systems with ac systems. The functions supplied by VSC are similar to that in microgrids. However, the transfer capability and stability in such kind of system are of major interests. Therefore, Part I of this dissertation focuses on VSC's applications in microgrids. A battery's inverter can be operated in both grid-connected PQ regulation mode and voltage and frequency support mode during islanding condition. Transition scheme between these two control modes is firstly investigated to guarantee a smooth dynamic performance. Secondly, a coordinated control strategy between battery's and PV station's VSCs is developed to improve microgrid's power flow. Thirdly, power quality improvement through the battery's inverter is investigated. VSC's control and capability for microgrid operation at normal, transient, and abnormal conditions will be modeled and analyzed. Part II of this dissertation focuses on VSC's applications in HVDC. The following topics are investigated in this dissertation: (i) how to design VSC-HVDC's controller using system identification method? (ii) How to coordinate VSCs in multi-terminal HVDC scenarios? And (iii) how to determine VSC-HVDC system's transfer capability based on stability limits? High-fidelity simulation technology is employed to tackle control validation while frequency domain impedance modeling technique is employed to develop analytical models for the systems. With linear system analysis tools such as Nyquist plots and Bode plots, stability limits and impacting factors of VSC-HVDC systems can be identified. This dissertation led to four journal papers (two accepted, one request of revision, one to submit) and five conference papers. The major contributions of this dissertation include: 1) Developed VSC and microgrid models in high-fidelity simulation environment. Developed and validated VSC control schemes for variety of microgrid operations: normal, abnormal, and transient. The developed technologies can facilitate a battery to make up solar power, improve system dynamic performance during transients, and improve power quality. 2) Developed VSC-HVDC simulation models, including two-terminal HVDC and multi-terminal HVDC. Developed VSC-HVDC control schemes for two-terminal and multi-terminal systems. Developed analytical impedance models for VSC-HVDC systems and successfully carried out stability limit identification.

A numerical model of the steady state and alternating current behaviour of a solid-oxide fuel cell is presented to explore the possibilities to diagnose and identify degradation mechanisms in a minimally invasive way using impedance spectroscopy. This is the first report of an SOFC impedance model to incorporate degradation, as well as the first one to include the ribbed interconnect geometry, using a 2-D approximation. Simulated degradation modes include: electrode/electrolyte delamination, interconnect oxidation, interconnect/electrode interface detachment, and anode sulfur poisoning. Detailed electrode-level simulation replaces the traditional equivalent circuit approach, allowing the simulation of degradation mechanisms that alter the shape of the current path. The SOFC impedance results from calculating the cell response to a small oscillatory perturbation in potential. Starting from the general equations for mass and charge transport, and assuming isothermal and isobaric conditions, the system variables are decomposed into a steady-state component and a small perturbation around the operating point. On account of the small size of the imposed perturbation, the time dependence is eliminated, and the original equations are converted to a new linear, time independent, complex-valued system, which is very convenient from a numerical viewpoint. Geometrical and physical modifications of the model simulate the aforementioned degradation modes, causing variations in the impedance. The possibility to detect unique impedance signatures is discussed, along with a study of the impact of input parameter inaccuracies and parameter interaction on the presented results. Finally, a study of pairs of concurrent degradation modes reveals the method’s strengths and limitations in terms of its diagnosis capabilities.

A numerical model of the steady state and alternating current behaviour of a solid-oxide fuel cell is presented to explore the possibilities to diagnose and identify degradation mechanisms in a minimally invasive way using impedance spectroscopy. This is the first report of an SOFC impedance model to incorporate degradation, as well as the first one to include the ribbed interconnect geometry, using a 2-D approximation. Simulated degradation modes include: electrode/electrolyte delamination, interconnect oxidation, interconnect/electrode interface detachment, and anode sulfur poisoning. Detailed electrode-level simulation replaces the traditional equivalent circuit approach, allowing the simulation of degradation mechanisms that alter the shape of the current path. The SOFC impedance results from calculating the cell response to a small oscillatory perturbation in potential. Starting from the general equations for mass and charge transport, and assuming isothermal and isobaric conditions, the system variables are decomposed into a steady-state component and a small perturbation around the operating point. On account of the small size of the imposed perturbation, the time dependence is eliminated, and the original equations are converted to a new linear, time independent, complex-valued system, which is very convenient from a numerical viewpoint. Geometrical and physical modifications of the model simulate the aforementioned degradation modes, causing variations in the impedance. The possibility to detect unique impedance signatures is discussed, along with a study of the impact of input parameter inaccuracies and parameter interaction on the presented results. Finally, a study of pairs of concurrent degradation modes reveals the method’s strengths and limitations in terms of its diagnosis capabilities. / Applied Science, Faculty of / Mechanical Engineering, Department of / Graduate

The objective of this dissertation is to carry out dynamic modeling, analysis and control of power systems with Renewable Energy Sources (RES) such as: Photovoltaic (PV) power sources and wind farms. The dissertation work is mainly focused on microgrid since it plays a major role in modern power systems and tend to have higher renewable power penetration. Two main theoretical concepts, dynamic phasor and impedance modeling have been adopted to model and analyze the power systems/mocrogrids with RES. The initial state calculation which is essential for small signal analysis of a system is carried out as the first step of the dissertation work. Dynamic phasor and impedance modeling techniques have been utilized to model and analyze power systems/micogrids as the second phase of the work. This part consists of two main studies. First case investigates the impedance modeling of Thyristor Controller Series Capacitor (TCSC) for sub-synchronous resonance (SSR) analysis where a wind farm is connected to a power system through series compensated line. Second case utilizes the dynamic phasor concept to model a microgrid in unbalanced condition. Here the unbalance is caused by a single phase PV connected to the microgrid. Third Phase of the dissertation work includes upper level control of the microgrid. Here prediction and optimization control for a microgrid with a wind farm, a PV system, an energy storage system and loads is evaluated. The last part of the dissertation work focuses on real time modeling and hardware in loop simulation test bed for microgrid applications. This dissertation has led to four journal papers (three accepted, one submitted) and five conference papers.

The master’s thesis is focused on the analysis and modeling of power line communication with aim to implement power line channel simulator in the programming environment Matlab. At the beginning of the thesis are summarized basic information about the PLC communication, its basic distribution in terms of frequency bands and are also mentioned advantages and disadvantages. The emphasis on the meaning and importance of modeling with the aim to analyze PLC communication channel is specified further. The following is a detailed description of the current power lines communication models. Power line channel simulator was created on the basis of mathematical analysis for low-voltage (LV) distribution networks. The created simulator uses a model of cascaded two-ports and works in random or fixed mode. Input parameters are real topologies, different types of loads and cables. RLC resonant circuit as a frequency selective impedance is also included in impedance models. Based on the obtained results was performed the analysis, determined the critical parameters and extreme values. Comparison of effectiveness of created simulator was processed on the basis of available simulators from other authors. Two versions of simulator were created using the results obtained under comparison of effectiveness. For comparison was also performed real measurement on a simple topology. The experimental measurements results have been implemented in the simulator.

In the BDAS project one of the goals is to create a new solution for monitoring bone healing to complement current techniques. Data have been collected in clinical trials from infants treated for Craniosynostosis by a craniotomic surgery. The data are collected with a biomedical sensor based in microwave technology. This sensor could be able to sense changes in the composition of the different tissues in the upper hemisphere of the head, by noticing a difference in the propagation of the microwaves, as the bone injury from the craniectomy heals over time. In this thesis I analyze the validity of a proposed analytical model for the biosensor and extend the clinical data analysis in BDAS project. The validity of the model is analyzed by comparing its outcomes to available measurements from phantoms mimicking living tissues and to numerical simulations. In the data analysis two hypotheses are formulated and tested regarding the location of the measurement points with respect to a positioning grid and the healing over time too. By deriving a set of parameters for each collected dataset in the clinical trials, a distinct pattern was found which shows visible changes over the course of the healing process with this technique.

Les batteries à électrolyte circulant ou redox flow batteries (RFB) représentent une technologie prometteuse pour répondre aux besoins grandissants de stockage d'énergie. Elles seraient particulièrement adaptées aux réseaux électriques qui comptent une part grandissante d'énergie d'origine renouvelable, produite en intermittence. L'objet de ce travail est l'étude d'un nouveau type de RFB, actuellement développé par l'entreprise Kemiwatt. Il repose sur l'utilisation de molécules organiques, qui sont abondantes et recyclables. Le but de cette étude est d'améliorer la compréhension fondamentale de la batterie grâce à l'utilisation d'outils d'analyse précis et innovants. Chaque composant du système a d'abord été analysé via des moyens expérimentaux ex-situ. Les deux électrolytes composant la batterie ont ensuite été étudiés séparément en conditions réelles de circulation dans une cellule symétrique. Couplées à un modèle d'électrode volumique, les données ont été analysées pour identifier les facteurs limitants de chaque solution. La batterie entière a ensuite été étudiée dans un dispositif segmenté, permettant l'accès à la distribution interne du courant. Une étude paramétrique, réalisée avec la cellule segmentée a permis d'observer les effets du courant, du débit et de la température sur le fonctionnement de la cellule, puis d'établir une cartographie des conditions de fonctionnement idéales, suivant la puissance et l'état de charge de la batterie. L'aspect hydrodynamique du système a finalement été abordé en développant un modèle fluidique ainsi qu'une maquette expérimentale de cellule transparente pour visualiser l'écoulement. / Redox Flow Batteries (RFBs) are a promising solution for large-scale and low-cost energy storage necessary to foster the use of intermittent renewable sources. This work investigates a novel RFB chemistry under development at the company Kemiwatt. Based on abundant organic/organo-metallic compounds, this new technology promises the deployment of sustainable and long-lived systems. The study undertakes the building of a thorough knowledge base of the system by developing innovative reliable analytical tools. The investigation started from the evaluation of the main factors influencing the battery performance, which could be conducted ex-situ on each material composing the cell. The two electrolytes were then examined independently under representative operating conditions, by building a symmetric flow cell. Cycling coupled with EIS measurements were performed in this set-up and then analyzed with a porous electrode model. This combined modeling-experimental approach revealed unlike limiting processes in each electrolyte along with precautions to take in the subsequent steps (such as membrane pretreatment and electrolyte protection from light). A segmented cell was built and validated to extend the study to the full cell system. It provided a mapping of the internal currents, which showed high irregularity during cycling. A thorough parameter study could be conducted with the segmented platform, by varying successively the current density, the flow rate, and the temperature. The outcome of this set of experiments would be the construction of an operational map that guides the flow rate adjustment, depending on the power load and the state of charge of the battery. This strategy of flow rate optimization showed promising outcomes at the lab-cell level. It can be easily adapted to real-size systems. Ultimately, an overview of the hydrodynamic behavior at the industrial-cell level was completed by developing a hydraulic modeling and a clear cell as an efficient diagnostic tool.

PLC - power line communication is not new. It has been known for many years. But It never be used in massive scale. There were only sporadic applications, for example ripple control system HDO used in the Czechoslovakia. PLC currently experiencing a renaissance thanks to the advent of Smart Grid. PLC offering relatively low bit rates and relatively unreliable transmission, but these disadvantages compensates very low costs to build a communication infrastructure and it offers specific functionalities for Smart Grid. The question is whether the declared parameters will be met in the real world. This thesis tries to find an answer.