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This dissertation aims to present a detailed analysis of the grid voltage disturbance in frequency domain for the current control design in the grid-tie inverter applications and to propose current control techniques in order to minimize its impact and maximize feasibility of the power conditioning system in distributed generations. Because the grid voltage is constantly changing, the inverter must be able to response to it. If the inverter is unable to respond properly, then the grid voltage power comes back to the system and damages the fuel cell power conditioning systems. A closed-loop dynamic model for the current control loop of the grid-tie inverter has been developed. The model explains the structure of the inverter admittance terms. The disturbance of the grid voltages has been analyzed in frequency domain. The admittance compensator has been proposed to prevent the grid voltage effect. The proposed lead-lag current control with admittance compensator transfers current properly without system failure. In order to get rid of the steady-state error of the feedback current, a proportional-resonant controller (PR) has been adopted. A PR control with admittance compensation provides great performance from zero power to full power operation. In addition, active and reactive power flow controller has been proposed based on the PR controller with admittance compensation. The proposed active and reactive power flow control scheme shows a wide range power flow control from pure leading power to pure lagging power. Finally, the proposed controller scheme has been verified its feasibility in three phase grid-tie inverter applications. First of all, a half-bridge grid-tie inverter has been designed with PR controller and admittance compensation. Then three individual grid-tie inverters has been combined and produced three phase current to the three phase grid in either balanced condition or unbalanced condition. The proposed control scheme can be applied not only single phase grid-tie inverter application, but also three phase grid-tie inverter application. This research can be applicable to the photovoltaic PCS as well. This technology makes renewable energy source more plausible for distributed generations. / Ph. D.

Widespread use of solid oxide fuel cells is hindered by a lack of long-term durability of seals between metallic and ceramic components caused by thermal expansion mismatch induced cracking. A novel gas seal design incorporating an engineered thermal expansion gradient in a SrO-La₂O₃-A₂O₃-B₂O₃-SiO₂ glass matrix with a TiNiHf shape memory alloy mesh for active stress relief and crack healing is being developed. Coefficient of thermal expansion (CTE) measurements of the seal and fuel cell components shows the possibility for a thermal expansion gradient. Differential scanning calorimetry and microscopy have shown that the TiNiHf alloy has a shape memory transition in the desired range of 200-250ºC. The oxide glass partially crystallizes during thermal cycling which has been observed through X-ray diffraction and dilatometry. The CTE decreases from 9.3Ã 10-6/°C to 6.6Ã 10-6/°C after thermal cycling. Neutron diffraction data from TiNiHf /glass composite samples reveals that the TiNiHf alloy has the ability of absorbing residual stresses from a glass matrix during martensitic phase transition. There is evidence from microscopy that the glass composition is important in determining if reaction will occur with the TiNiHf alloy. The TiNiHf alloy mesh structures can be created using the 3D printing process. This process has been adapted to allow for printing of very thin wire mesh structures of Ni and NiTi powders with a more suitable binder solution. A bi-layer test fixture has been developed which will be useful for assessing leak rate through seal materials. / Master of Science

Comprendre, contrôler et optimiser le mécanisme de la réaction de réduction de l’oxygène à la cathode, cette démarche devient une nécessité pour améliorer les dispositifs de conversion d'énergie de haute performance tels que les piles à combustible à oxyde électrolyte solide (PAC). Des films poreux à conduction mixte, ionique et électronique (MIEC) et leurs composites comprenant un conducteur ionique offrent des propriétés uniques. Cependant, la corrélation des propriétés intrinsèques des composants d'électrodes aux caractéristiques microstructurales reste une tâche difficile. Dans le cadre de cette thèse, la couche fonctionnelle de cathode de La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF) pure et du composite LSCF/Ce0.9Gd0.1O2-δ (CGO) a été élaborée par la technique d’atomisation électrostatique. Une microstructure à porosité hiérarchique a été obtenue dans un domaine nanométrique à micrométrique. Les films ont été recouverts d’un collecteur de courant (CCL), LSCF, par sérigraphie. Une étude paramétrique a été réalisée expérimentalement pour optimiser la double couche en termes de taille de particules, de composition et d'épaisseur des couches de CFL et CCL. En se basant sur ces résultats, un modèle éléments finis 3D a été développé en utilisant les paramètres microstructuraux déterminés par tomographie de FIB-SEM dans une géométrie simple, similaire à des caractéristiques colonnaires. Dans ce travail, un guide de conception du matériau d’électrode a été proposé reliant des performances électrochimiques optimisées à la microstructure et aux propriétés du massif en combinant une étude expérimentale et une étude théorique de modélisation. Une cellule complète de PAC intégrant la cathode optimisée double couche de LSCF a été testée dans des conditions réelles d'exploitation. / Understanding, controlling and optimizing the mechanism of oxygen reduction reaction at the cathode need to be addressed for high performance energy conversion devices such as solid oxide fuel cells (SOFCs). Structured porous films of mixed ionic electronic conductors (MIECs) and their composites with addition of a pure ionic conductor offer unique properties. However, correlating the intrinsic properties of electrode components to microstructural features remains a challenging task. In this PhD thesis, cathode functional layers (CFL) of La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF) and LSCF/Ce0.9Gd0.1O2-δ (CGO) composite cathodes with hierarchical porosity from nano- to micro-range are fabricated by electrostatic spray deposition technique. The films were topped with LSCF as a current collecting layer (CCL) by screen printing technique. A parametric optimization study was conducted experimentally in terms of particle size, composition, and thickness of CFL and CCL layers. The experimental results were supported by a numerical 3D Finite Element Model (FEM). Microstructural parameters determined by FIB-SEM tomography were used in a simple geometry similar to experimentally observed columnar features. In this work, experimental results and modelling were combined to provide design guidelines relating optimized electrochemical performances to the microstructure and bulk material properties. A complete fuel cell with optimized cathode film was tested in real SOFC operational conditions.

Células a combustível de óxido sólido suportadas no eletrólito de zircônia estabilizada com ítria (YSZ) foram fabricadas usando a técnica do recobrimento por rotação (spin-coating) para deposição de catodos de manganita de lantânio dopada com estrôncio (LSM) e anodos compósitos de níquel e YSZ (Ni-YSZ). Parâmetros microestruturais dos eletrodos, tais como espessura, tamanho médio de partículas e temperatura de sinterização foram otimizados, visando reduzir a resistência de polarização da célula e melhorar o seu desempenho. Estes estudos serviram de base para a fabricação de células com camada catalítica para uso com etanol direto. Sobre o anodo Ni-YSZ da célula foi depositada uma camada catalítica de céria dopada com gadolínia (CGO) com 0,1% em peso de irídio (Ir-CGO). A camada catalítica visa reformar o etanol antes do seu contato com o anodo Ni-YSZ, evitando o depósito de carbono na superfície do Ni que inviabiliza o uso de combustíveis primários contendo carbono nestas células a combustível. Inicialmente, a célula a combustível foi testada com etanol e as melhores condições de operação foram determinadas. Em seguida, as células unitárias foram testadas com etanol sem adição de água por períodos de tempo de até 390 horas. As células a combustível a etanol direto com camada catalítica operam no modo de reforma interna gradual, apresentando boa estabilidade e densidades de corrente similares às obtidas na operação com hidrogênio. Após a operação das células a combustível a etanol direto, análises de microscopia eletrônica de varredura mostraram que não houve formação significativa de depósitos de carbono na superfície do Ni, indicando que a camada catalítica de Ir-CGO foi efetiva para operação com o etanol. Testes de células a combustível a etanol direto sem a camada catalítica revelaram uma rápida degradação nas horas iniciais de operação com formação de grandes quantidades de depósitos de carbono identificados visualmente. Considerando-se a operação estável com etanol a seco por tempos relativamente longos de operação, os resultados alcançados representam um avanço significativo e apontam para o desenvolvimento de células a combustível a etanol direto usando-se os componentes tradicionais com a adição de uma camada catalítica. / Yttria-stabilized zirconia (YSZ) electrolyte supported solid oxide fuel cells were fabricated with spin-coated strontium-doped lanthanum manganite (LSM) cathodes and Ni-YSZ cermet anodes. The microstructural parameters of the electrodes such as thickness, average particle size, and sintering temperature were optimized to decrease the polarization resistance of the single cells and to improve their electrochemical performance. These preliminar studies provided the basis for the fabrication of single fuel cells with a catalytic layer of gadolinia-doped ceria (CGO) and 0.1 wt% iridium (Ir-CGO) deposited onto the anode. The catalytic layer aims at the stable operation with dry (direct) ethanol; it avoids the contact of the alcohol with the anode, preventing the anode degradation by carbon deposition. Initially, the single cells were tested with ethanol and optimized operating parameters were determined. Then, the single cells were operated with anhydrous ethanol for periods of time up to 390 hours. The single cells with catalytic layer operate by the gradual internal reforming of ethanol, with good stability and delivering similar electric current densities as the ones measured using hydrogen as fuel. After single cell operation on direct ethanol, scanning electron microscopy analyses identified no significant carbon deposition on the surface of Ni, indicating that the Ir-CGO catalytic layer was effective for the reforming of ethanol. Such results were compared to the ones of standard single cells operating on dry ethanol, which showed a fast degradation and the formation of large amounts of carbon deposits. Considering the rather stable performance of single cells running on dry ethanol for relatively long times, such results represent a significant advance towards the development of direct ethanol solid oxide fuel cells using the standard components and a catalytic layer.

Cost effective high volume manufacture of solid oxide fuel cells (SOFCs) is a major challenge for commercial success of these devices. More than fifteen processing methods have been reported in the literature, many of which could be used in various combinations to create the desired product characteristics. For some of these processes, high volume manufacturing experience is very limited or non-existent making traditional costing approaches inappropriate. Additionally, currently available cost models are limited by a lack of incorporation of device performance requirements. Therefore, additional modeling tools are needed to aid in the selection of the appropriate processing techniques prior to making expensive investment decisions. This project describes the development of a SOFC device performance model and a manufacturing process tolerance model. These models are then linked to a preliminary cost model; creating a true multi-process, performance based cost model that permits the comparison of manufacturing cost for different combinations of three processing methods. The three processing methods that are investigated are tape casting, screen printing, and sputtering. . This model is capable of considering production volume, process tolerance and process yield, in addition to the materials and process details. Initial comparisons were performed against processes used extensively within the solid oxide fuel cell industry and the cost results show good agreement with this experience base. Sensitivity of manufacturing costs to SOFC performance requirements such as maximum power density and operation temperature are also investigated.

"Planar Solid Oxide Fuel Cell manufacturing can be considered in the pilot plant stage with efforts driving towards large volume manufacturing. The science of the solid oxide fuel cell is advancing rapidly to expand the knowledge base and use of material combinations and layer forming methods for the unit cell. Few of the many processing methods, over 15, reported in literature for layer formation are used today in high volume manufacturing. It is difficult to establish future market demand and cost levels needed to plan a course of action today. The need to select amongst different designs, materials and processes will require a tool to aid in these decisions. A modeling tool is presented to robustly compare the various process combinations and manufacturing variable to make solid oxide fuel cells in order to identify key trends prior to making strategic investment decisions. The ability to accurately forecast investment requirements and manufacturing cost for a given high volume manufacturing (HVM) process based on expected volume is critical for strategic decisions, product placement and investor communications. This paper describes the use of an updated process based cost model that permits the comparison of manufacturing cost data for various process combinations, production volumes, and electrolyte layer thickness tolerances. The effect of process yield is addressed. Processing methods discussed include tape casting, screen printing and sputtering."

Solid Oxide Fuel Cells (SOFC) are electrochemical energy conversion devices which allow fuel gases, e.g. hydrogen or natural gas, to be converted to electricity and heat at much high efficiencies than combustion-based energy conversion technologies. SOFC are particularly suited to employment in stationary energy conversion applications, e.g. micro-combined heat and power (μ-CHP) and base load, which are certain to play a large role in worldwide decentralisation of power distribution and supply over the coming decades. Use of high-temperature SOFC technology within these systems is also a vital requirement in order to utilise fuel gases which are readily available in different areas of the world. Unfortunately, the limiting factor to the long-term commercialisation of SOFC systems is the redox instability, coking intolerance and sulphur poisoning of the state-of-the-art Ni-based cermet composite anode material. This research explores the ‘powder to power' development of alternative SOFC anode catalyst systems by impregnation of an A-site deficient La0.20Sr0.25Ca0.45TiO3 (LSCT[sub](A-)) anode ‘backbone' microstructure with coatings of ceria-based oxide ion conductors and metallic electrocatalyst particles, in order to create a SOFC anode which exhibits high redox stability, tolerance to sulphur poisoning and low voltage degradation rates under operating conditions. A 75 weight percent (wt. %) solids loading LSCT[sub](A-) ink, exhibiting ideal properties for screen printing of thick-film SOFC anode layers, was screen printed with 325 and 230 mesh counts (per inch) screens onto electrolyte supports. Sintering of anode layers between 1250 °C and 1350 °C for 1 to 2 hours indicated that microstructures printed with the 230 mesh screen provided a higher porosity and improved grain connectivity than those printed with the 325 mesh screen. Sintering anode layers at 1350 °C for 2 hours provided an anode microstructure with an advantageous combination of lateral grain connectivity and porosity, giving rise to an ‘effective' electrical conductivity of 17.5 S cm−1 at 850 °C. Impregnation of this optimised LSCT[sub](A-) anode scaffold with 13-16 wt. % (of the anode mass) Ce0.80Gd0.20O1.90 (CGO) and either Ni (5 wt. %), Pd, Pt, Rh or Ru (2-3 wt. %) and integration into SOFC resulted in achievement of Area Specific Resistances (ASR) of as low as 0.39 Ω cm−2, using thick (160 μm) 6ScSZ electrolytes. Durability testing of SOFC with Ni/CGO, Ni/CeO2, Pt/CGO and Rh/CGO impregnated LSCT[sub](A-) anodes was subsequently carried out in industrial button cell test rigs at HEXIS AG, Winterthur, Switzerland. Both Ni/CGO and Pt/CGO cells showed unacceptable levels of degradation (14.9% and 13.4%, respectively) during a ~960 hour period of operation, including redox/thermo/thermoredox cycling treatments. Significantly, by exchanging the CGO component for the CeO2 component in the SOFC containing Ni, the degradation over the same time period was almost halved. Most importantly, galvanostatic operation of the SOFC with a Rh/CGO impregnated anode for >3000 hours (without cycling treatments) resulted in an average voltage degradation rate of < 1.9% kh−1 which, to the author's knowledge, has not previously been reported for an alternative, SrTiO3-based anode material. Finally, transfer of the Rh/CGO impregnated LSCT[sub](A-) anode to industrial short stack (5 cells) scale at HEXIS AG revealed that operation in relevant conditions, with low gas flow rates, resulted in accelerated degradation of the Rh/CGO anode. During a 1451 hour period of galvanostatic operation, with redox cycles and overload treatments, a voltage degradation of 19.2% was observed. Redox cycling was noted to briefly recover performance of the stack before rapidly degrading back to the pre-redox cycling performance, though redox cycling does not affect this anode detrimentally. Instead, a more severe, underlying degradation mechanism, most likely caused by instability and agglomeration of Rh nanoparticles under operating conditions, is responsible for this observed degradation. Furthermore, exposure of the SOFC to fuel utilisations of >100% (overloading) had little effect on the Rh/CGO co-impregnated LSCT[sub](A-) anodes, giving a direct advantage over the standard HEXIS SOFC. Finally, elevated ohmic resistances caused by imperfect contacting with the Ni-based current collector materials highlighted that a new method of current collection must be developed for use with these anode materials.