Aujourd’hui, comprendre comment les plantes réagissent au stress hydrique est essentiel pour relever le défi de développer de nouveaux cultivars et de nouvelles stratégies d’irrigation, compatibles avec le maintien de la productivité des cultures sous les effets néfastes du réchauffement climatique. Dans ce contexte, l’étude des relations eau/plante présente un intérêt majeur pour la modélisation des réponses des plantes et des organes aux contraintes biotiques et abiotiques. Paradoxalement, il existe très peu de méthodes directes et non invasives pour quantifier et mesurer le niveau et le débit de l’eau dans les plantes.Dans le cadre de ce projet, nous rapportons le développement d’une méthodologie innovante basée sur la relaxation par résonance magnétique nucléaire à champ faible (RMN). Un dispositif RMN dédié pour effectuer des mesures RMN sur des plantes vivantes a été construit dans une chambre climatique qui permet un contrôle et une modification minutieuse des paramètres environnementaux pendant l’expérimentation sur des longues périodes au laboratoire. En parallèle, une imagerie RMN complémentaire à haut champ magnétique pour étudier, l’anatomie, la teneur en eau, le transport du phloéme et du xylème chez les plants de sorgho a été réalisé. La combinaison de ces approches nous permet de déterminer des biomarqueurs éco-physiologiques innovants et de concevoir de nouvelles expériences en laboratoire et même dans les champs.Un résultat particulièrement intéressant concerne l’étude de la distribution spatiale de l’eau dans les tiges (nœuds et entrenœuds) à partir de la relaxométrie RMN à faible champ et des images IRM 3D à haute résolution. La modification des paramètres de relaxation RMN au cours du cycle diurne dynamique sera présentée dans des conditions normales et en situation de stress abiotique. Une application directe permet d’extraire des biomarqueurs écophysiologiques qui permettent d’explorer et de modéliser les flux d’eau en période de stress hydrique et d’analyser leur impact sur le développement du sorgho.Notre but ultime est d’effectuer ces études RMN directement dans les champs. Ainsi, un appareil RMN portable fait maison, fonctionnant à 336 kHz est présenté. Le développement et l’optimisation de l’homogénéité d’un aimant résistif et de bobines Radio Fréquence ainsi que des séquences d'impulsions RMN, afin de respecter la polyvalence et les conditions thermiques pour maintenir la plante ,intacte sont décrits en détails. Enfin nous présenterons nos investigations menées avec ce dispositif en laboratoire, dans les serres et dans les champs de sorgho. / Today, understanding how plants respond to water stress is essential to meet the challenge of developing new cultivars and new irrigation strategies, consistent with the maintenance of crop productivity with the evidence of global warming. In this context, the study of plant /water relations is of central interest for modeling plant and organ responses to biotic and abiotic constraints. Paradoxically, there are very few direct and non-invasive methods to quantify and measure the level and the flow of water in plants.For this purpose, we report on the development of an innovative methodology based on low-field Nuclear Magnetic Resonance Relaxometry (NMR). A dedicated NMR device to perform NMR measurements on living plants has been built in a climatic chamber that allows a control and careful modification of environmental parameters during experimentation over reliable periods of time at the laboratory. In parallel, complementary NMR imaging at high magnetic field to study, the anatomy, water content, phloem and xylem transport in sorghum plants were performed. The combination of these approaches allows us to determine innovant eco-physiological biomarkers and to design new experiments in the laboratory and even in the fields.One particular interesting result concertns the investigation of the spatial distribution of water stems (node and inter node) from low field NMR Relaxometry and 3D high resolution MRI images. The modification of the NMR relaxation parameters during dynamic diurnal cycle will be presented in normal and abiotic stress conditions. A direct application permits to extract eco-physiological biomarkers which allows to explore and model water fluxes during water stress and to analyze their impact on the development of sorghum plant. Our ultimate goal is to perform these NMR studies directly in the fields. Thus, a home made portable NMR device, working at 336kHz(8mT) is presented. The development and optimization of the homogeneity of a resistive magnet and Radio Frequency coils, NMR pulse sequences in order to respect the versatility and thermal conditions to maintain the plant intact are described in details. Finally we will present our investigations conducted with this device in the laboratory, in the greenhouses and in sorghum fields.

The performance of solid oxide fuel cells (SOFCs) at the cell and system levels is studied using computer simulation. At the cell level, a new model combining the cell micro and macro models is developed. Using this model, the microstructural variables of porous composite electrodes can be linked to the cell performance. In this approach, the electrochemical performance of porous composite electrodes is predicted using a micro-model. In the micro-model, the random-packing sphere method is used to estimate the microstructural properties of porous composite electrodes from the independent microstructural variables. These variables are the electrode porosity, thickness, particle size ratio, and size and volume fraction of electron-conducting particles. Then, the complex interdependency among the multi-component mass transport, electron and ion transports, and the electrochemical and chemical reactions in the microstructure of electrodes is taken into account to predict the electrochemical performance of electrodes. The temperature distribution in the solid structure of the cell and the temperature and species partial pressure distributions in the bulk fuel and air streams are predicted using the cell macro-model. In the macro-model, the energy transport is considered for the cell solid structure and the mass and energy transports are considered for the fuel and air streams. To demonstrate the application of the cell level model developed, entitled the combined micro- and micro-model, several anode-supported co-flow planar cells with a range of microstructures of porous composite electrodes are simulated. The mean total polarization resistance, the mean total power density, and the temperature distribution in the cells are predicted. The results of this study reveal that there is an optimum value for most of the microstructural variables of the electrodes at which the mean total polarization resistance of the cell is minimized. There is also an optimum value for most of the microstructural variables of the electrodes at which the mean total power density of the cell is maximized. The microstructure of porous composite electrodes also plays a significant role in the mean temperature, the temperature difference between the hottest and coldest spots, and the maximum temperature gradient in the solid structure of the cell. Overall, using the combined micro- and micro-model, an appropriate microstructure for porous composite electrodes to enhance the cell performance can be designed. At the system level, the full load operation of two SOFC systems is studied. To model these systems, the basic cell model is used for SOFCs at the cell level, the repeated-cell stack model is used for SOFCs at the stack level, and the thermodynamic model is used for the balance of plant components of the system. In addition to these models, a carbon deposition model based on the thermodynamic equilibrium assumption is employed. For the system level model, the first SOFC system considered is a combined heat and power (CHP) system that operates with biogas fuel. The performance of this system at three different configurations is evaluated. These configurations are different in the fuel processing method to prevent carbon deposition on the anode catalyst. The fuel processing methods considered in these configurations are the anode gas recirculation (AGR), steam reforming (SR), and partial oxidation reformer (POX) methods. The application of this system is studied for operation in a wastewater treatment plant (WWTP) and in single-family detached dwellings. The evaluation of this system for operation in a WWTP indicates that if the entire biogas produced in the WWTP is used in the system with AGR or SR fuel processors, the electric power and heat required to operate the plant can be completely supplied and the extra electric power generated can be sold to the electrical grid. The evaluation of this system for operation in single-family detached dwellings indicates that, depending on the size, location, and building type and design, this system with all configurations studied is suitable to provide the domestic hot water and electric power demands. The second SOFC system is a novel portable electric power generation system that operates with liquid ammonia fuel. Size, simplicity, and high electrical efficiency are the main advantages of this environmentally friendly system. Using a sensitivity analysis, the effects of the cell voltage at several fuel utilization ratios on the number of cells required for the SOFC stack, system efficiency and voltage, and excess air required for thermal management of the SOFC stack are studied.

The performance of solid oxide fuel cells (SOFCs) at the cell and system levels is studied using computer simulation. At the cell level, a new model combining the cell micro and macro models is developed. Using this model, the microstructural variables of porous composite electrodes can be linked to the cell performance. In this approach, the electrochemical performance of porous composite electrodes is predicted using a micro-model. In the micro-model, the random-packing sphere method is used to estimate the microstructural properties of porous composite electrodes from the independent microstructural variables. These variables are the electrode porosity, thickness, particle size ratio, and size and volume fraction of electron-conducting particles. Then, the complex interdependency among the multi-component mass transport, electron and ion transports, and the electrochemical and chemical reactions in the microstructure of electrodes is taken into account to predict the electrochemical performance of electrodes. The temperature distribution in the solid structure of the cell and the temperature and species partial pressure distributions in the bulk fuel and air streams are predicted using the cell macro-model. In the macro-model, the energy transport is considered for the cell solid structure and the mass and energy transports are considered for the fuel and air streams. To demonstrate the application of the cell level model developed, entitled the combined micro- and micro-model, several anode-supported co-flow planar cells with a range of microstructures of porous composite electrodes are simulated. The mean total polarization resistance, the mean total power density, and the temperature distribution in the cells are predicted. The results of this study reveal that there is an optimum value for most of the microstructural variables of the electrodes at which the mean total polarization resistance of the cell is minimized. There is also an optimum value for most of the microstructural variables of the electrodes at which the mean total power density of the cell is maximized. The microstructure of porous composite electrodes also plays a significant role in the mean temperature, the temperature difference between the hottest and coldest spots, and the maximum temperature gradient in the solid structure of the cell. Overall, using the combined micro- and micro-model, an appropriate microstructure for porous composite electrodes to enhance the cell performance can be designed. At the system level, the full load operation of two SOFC systems is studied. To model these systems, the basic cell model is used for SOFCs at the cell level, the repeated-cell stack model is used for SOFCs at the stack level, and the thermodynamic model is used for the balance of plant components of the system. In addition to these models, a carbon deposition model based on the thermodynamic equilibrium assumption is employed. For the system level model, the first SOFC system considered is a combined heat and power (CHP) system that operates with biogas fuel. The performance of this system at three different configurations is evaluated. These configurations are different in the fuel processing method to prevent carbon deposition on the anode catalyst. The fuel processing methods considered in these configurations are the anode gas recirculation (AGR), steam reforming (SR), and partial oxidation reformer (POX) methods. The application of this system is studied for operation in a wastewater treatment plant (WWTP) and in single-family detached dwellings. The evaluation of this system for operation in a WWTP indicates that if the entire biogas produced in the WWTP is used in the system with AGR or SR fuel processors, the electric power and heat required to operate the plant can be completely supplied and the extra electric power generated can be sold to the electrical grid. The evaluation of this system for operation in single-family detached dwellings indicates that, depending on the size, location, and building type and design, this system with all configurations studied is suitable to provide the domestic hot water and electric power demands. The second SOFC system is a novel portable electric power generation system that operates with liquid ammonia fuel. Size, simplicity, and high electrical efficiency are the main advantages of this environmentally friendly system. Using a sensitivity analysis, the effects of the cell voltage at several fuel utilization ratios on the number of cells required for the SOFC stack, system efficiency and voltage, and excess air required for thermal management of the SOFC stack are studied.