Há décadas a energia solar atrai pesquisadores de todo o planeta devido ao seu enorme potencial de aplicações como fonte de energia térmica. Com a crescente preocupação com o Meio Ambiente e a busca por fontes renováveis de energia, o interesse pelo aproveitamento da energia proveniente do Sol cresceu ainda mais. Apesar de desafios como a intermitência e a sazonalidade solares, e a dificuldade de transporte desta forma de energia, a busca por técnicas de armazenamento visa mitigar estes problemas. Uma alternativa termoquímica é a produção de combustíveis solares, ou seja, combustíveis obtidos por meio do uso da energia concentrada do Sol que possam ser estocados e transportados para onde e quando houver demanda de energia. Uma aplicação de destaque neste tema é a gaseificação de biomassa para a produção de gás de síntese (CO + H2). A gaseificação visa aumentar o potencial energético da biomassa sólida ou líquida ao realizar uma reação endotérmica que transfere a energia acumulada do Sol (por meio de concentradores) para as ligações químicas, com a recombinação dos átomos na forma de gás de síntese. Este gás, além do seu maior poder calorífico, é matéria-prima para muitos processos da indústria química. Desta forma, o objetivo desta reação é obter um combustível mais homogêneo e de fácil manuseio. A gaseificação é uma alternativa importante para o aproveitamento energético da biomassa, pois esta é normalmente bastante heterogênea. Sendo assim, o uso de gaseificadores viabiliza o uso de madeira, resíduos urbanos e agrícolas e outras matérias orgânicas como fontes de energia. Assim, este trabalho apresenta um estudo da utilização de concentradores solares para o processo de gaseificação de biomassa e a concepção de um reator químico solar para operação em escala laboratorial para a sequência dos estudos neste campo de pesquisa. Por meio de uma modelagem térmica, o trabalho destaca a viabilidade técnica da gaseificação solar de biomassa, comprovando um ganho energético e uma redução das emissões de gás carbônico em comparação com a queima direta da biomassa. Por fim, são apresentados os parâmetros de construção e operação de um gaseificador de pequena escala. / From decades, solar energy has instigated researchers worldwide due to its enormous potential. With raising concern for the environment and the search for renewable energy sources, the interest in using the Sun as a power supply grew even more. Despite challenges as the solar seasonality and intermittence, and difficulties transport its energy, innovative storage techniques aim to mitigate those issues. A thermochemical alternative is the production of solar fuels - fuels obtained from concentrated solar power (CSP) - which can be stocked and transported to the time and place there is demand. A forthcoming application in this field is biomass steam-gasification for syngas production (CO + H2). Gasification increases the thermal potential of liquid and solid biomass by means of an endothermic reaction that transfers solar energy (via radiation concentrators) to chemical bonds, re-combining the atoms as syngas (hydrogen and carbon monoxide). This gas mixture, besides its thermal potential, is a raw material for numerous processes in chemical industry. The purpose of this process is to obtain a fuel more homogenous and easy to manipulate. The gasification is an important alternative to biomass energy use, once it is generally heterogeneous. Thus, gasifiers enable the use of wood, algae, solid waste, agricultural byproducts, and other organic matter as power supplies. Therefore, this project presents the study of solar concentrators use in the steamgasification of biomass and the conception of a solar chemical reactor for laboratoryscale tests for the future steps of the research in this field. Models and bibliographic references points to the technical feasibility of solar steam-gasification of biomass, verifying a significant energy gain and carbon emission reduction comparing to direct burn of biomass. Ultimately, the dissertation presents the constructive and operational parameters of a small-scale gasifier.

Há décadas a energia solar atrai pesquisadores de todo o planeta devido ao seu enorme potencial de aplicações como fonte de energia térmica. Com a crescente preocupação com o Meio Ambiente e a busca por fontes renováveis de energia, o interesse pelo aproveitamento da energia proveniente do Sol cresceu ainda mais. Apesar de desafios como a intermitência e a sazonalidade solares, e a dificuldade de transporte desta forma de energia, a busca por técnicas de armazenamento visa mitigar estes problemas. Uma alternativa termoquímica é a produção de combustíveis solares, ou seja, combustíveis obtidos por meio do uso da energia concentrada do Sol que possam ser estocados e transportados para onde e quando houver demanda de energia. Uma aplicação de destaque neste tema é a gaseificação de biomassa para a produção de gás de síntese (CO + H2). A gaseificação visa aumentar o potencial energético da biomassa sólida ou líquida ao realizar uma reação endotérmica que transfere a energia acumulada do Sol (por meio de concentradores) para as ligações químicas, com a recombinação dos átomos na forma de gás de síntese. Este gás, além do seu maior poder calorífico, é matéria-prima para muitos processos da indústria química. Desta forma, o objetivo desta reação é obter um combustível mais homogêneo e de fácil manuseio. A gaseificação é uma alternativa importante para o aproveitamento energético da biomassa, pois esta é normalmente bastante heterogênea. Sendo assim, o uso de gaseificadores viabiliza o uso de madeira, resíduos urbanos e agrícolas e outras matérias orgânicas como fontes de energia. Assim, este trabalho apresenta um estudo da utilização de concentradores solares para o processo de gaseificação de biomassa e a concepção de um reator químico solar para operação em escala laboratorial para a sequência dos estudos neste campo de pesquisa. Por meio de uma modelagem térmica, o trabalho destaca a viabilidade técnica da gaseificação solar de biomassa, comprovando um ganho energético e uma redução das emissões de gás carbônico em comparação com a queima direta da biomassa. Por fim, são apresentados os parâmetros de construção e operação de um gaseificador de pequena escala. / From decades, solar energy has instigated researchers worldwide due to its enormous potential. With raising concern for the environment and the search for renewable energy sources, the interest in using the Sun as a power supply grew even more. Despite challenges as the solar seasonality and intermittence, and difficulties transport its energy, innovative storage techniques aim to mitigate those issues. A thermochemical alternative is the production of solar fuels - fuels obtained from concentrated solar power (CSP) - which can be stocked and transported to the time and place there is demand. A forthcoming application in this field is biomass steam-gasification for syngas production (CO + H2). Gasification increases the thermal potential of liquid and solid biomass by means of an endothermic reaction that transfers solar energy (via radiation concentrators) to chemical bonds, re-combining the atoms as syngas (hydrogen and carbon monoxide). This gas mixture, besides its thermal potential, is a raw material for numerous processes in chemical industry. The purpose of this process is to obtain a fuel more homogenous and easy to manipulate. The gasification is an important alternative to biomass energy use, once it is generally heterogeneous. Thus, gasifiers enable the use of wood, algae, solid waste, agricultural byproducts, and other organic matter as power supplies. Therefore, this project presents the study of solar concentrators use in the steamgasification of biomass and the conception of a solar chemical reactor for laboratoryscale tests for the future steps of the research in this field. Models and bibliographic references points to the technical feasibility of solar steam-gasification of biomass, verifying a significant energy gain and carbon emission reduction comparing to direct burn of biomass. Ultimately, the dissertation presents the constructive and operational parameters of a small-scale gasifier.

This thesis presents a method of tracking and correcting for the swaying of a central receiver tower in concentrated solar production plants. The method uses a camera with image processing algorithms to detect movement of the center of the tower. A prototype was constructed utilizing a CMOS camera connected to a microcontroller to control the movements of three surrounding heliostats. The prototype uses blob-tracking algorithms to detect and correct for movements of a colored model target. The model was able to detect movements in the tower with average error of 0.32 degrees, and was able to correctly orient the surrounding heliostats to within 1.2 and 2.6 degrees of accuracy while testing indoors and outdoors, respectively.

Ce travail de thèse porte sur l’étude de la réduction de CO 2 et H 2 O en CO et H 2 au moyen de cycles thermochimiques. Ces cycles utilisent des oxydes métalliques pour réaliser ces réductions en deux étapes, permettant de diminuer la température nécessaire. Dans une première étape endothermique, l’oxyde métallique est réduit à haute température (>1200°C) grâce à un apport d’énergie solaire concentrée. Dans une seconde étape exothermique réalisée à plus basse température (<1200°C), cette espèce réduite est ré-oxydée en présence d’eau ou de CO 2 , produisant H 2 ou CO et régénérant l’oxyde métallique pour un autre cycle. Le mélange de H 2 et CO (syngas), ainsi produit uniquement grâce à de l’énergie solaire peut ensuite être transformé en carburant liquide conventionnel par un procédé catalytique de type Fischer-Tropsch. Cette étude s’intéresse particulièrement aux cycles à base d’oxydes volatiles, ZnO/Zn et SnO 2 /SnO, dont le produit de la première étape de réduction est sous forme gazeuse à la température de réaction, puis se condense sous forme de nanoparticules. Tout d’abord, des moyens et méthodes ont été développés pour l’étude de la cinétique des réactions de réduction à hautes températures, en particulier une méthode inverse utilisant la mesure en ligne de l’oxygène produit dans un réacteur solaire, et un dispositif de thermogravimétrie solaire. Par ailleurs, différents moyens de diminuer la température des réactions de réduction ont été étudiés, à savoir la diminution de la pression et l’emploi d’un agent réducteur carboné. L’impact de la diminution de la pression sur la cinétique de réduction a été quantifié pour SnO 2 et ZnO.Une étude de l’évolution physico-chimique de poudres de SnO durant la deuxième étape d’oxydation du cycle a ensuite été réalisée, montrant l’importance de la réaction de dismutation de SnO en Sn et SnO 2 sur la réactivité des poudres dans la gamme de température étudiée. / This PhD thesis focuses on the study of the CO2 and H2O reduction into CO and H2 using thermochemical cycles. These cycles use metal redox pairs for stepwise reduction at lower temperature. The first step consists of the endothermic high temperature reduction of the metal oxide (>1200°C) using concentrated solar energy. The second step, operated at a lower temperature (<1200°C), uses the reduced specie to reduce CO2 or H2O, yielding CO or H2 and regenerating the metal oxide. The CO and H2 mixture (syngas), produced using solar energy, can then be converted into liquid fuel using a conventional Fischer-Tropsch catalytic process. The study considers more specifically the volatile oxide cycles, ZnO/Zn and SnO2/SnO, for which the reduced specie is obtained in gaseous phase at the reaction temperature, and is then condensed as nanoparticles. First, means and methods for studying the kinetics of reduction reactions at high temperatures were developed, namely an inverse method based on the online analysis of O2 production in a solar reactor and a solar-driven thermogravimeter. In addition, the study of reduced pressure operation and the use of a carbonaceous reducer were considered as efficient means to decrease the operating temperature and to promote a fast reaction. The impact of reduced pressure was quantified for SnO2 and ZnO reduction. A study of the evolution of the morphology and chemistry of the SnO powder during the second oxidation step was then conducted, emphasizing the importance of SnO disproportionation on the powder reactivity.

Dans le contexte énergétique mondial de raréfaction des ressources fossiles, et de réduction des émissions de gaz à effet de serre, l’agence internationale de l’énergie prévoit que la filière solaire thermodynamique fournisse en 2050 plus de 10% de l’électricité mondiale. De gros efforts de recherche sont nécessaires pour atteindre cet objectif. Les axes de développement actuels des technologies solaires à concentration portent, entre autres, sur les systèmes optiques composés de multiples miroirs (champs d’héliostats, concentrateurs linéaires de Fresnel, Beam-Down), sur les récepteurs solaires volumétriques (récepteurs à air, lits fluidisés) et sur les réacteurs (chimie à haute température, photobioréacteurs, dessalement par voie thermodynamique). Le transfert d’énergie par rayonnement est un des phénomènes prépondérants dans les systèmes optiques concentrateurs et dans les récepteurs solaires volumétriques. Les laboratoires Rapsodee et Laplace ont développé en quelques années d’étroite collaboration un savoir faire méthodologique sur la modélisation des transferts radiatifs et le calcul de sensibilité par la Méthode de Monte- Carlo et ils ont accumulé une expérience pratique, issue de la synthèse d’image, en programmation scientifique en interaction avec des chercheurs en informatique. Nous montrons dans ce manuscrit dans quelle mesure l’association de ces compétences théoriques et pratiques permet de répondre à certains besoins de la communauté du solaire à concentration et nous donnons des éléments de réponses ou des pistes à explorer en vue de surmonter les difficultés restantes que nous avons rencontrées. / The decrease of the fossil energy resources and the reduction of the emissions of greenhouse effect gas are major environmental issues. In this global situation, the International Energy Agency expects that solar power will provide more than 10% of the world electricity in 2050. Significant research efforts are needed to achieve this goal. Radiative transfer is one of the main physical phenomena in solar optical concentrators and in volumetric solar receivers. In few years of closely work, the laboratories Rapsodee and Laplace developed a methodological know-how in using Monte-Carlo methods for the modeling of radiative transfer and the sensitivity computations. They have also accumulated some experience in scientific programming and algorithms optimisation. We show in this dissertation how the combination of these physicists theoretical and practical skills can meet certain needs of the community of solar concentration. We give some answers or clues to be explored to get through the remaining difficulties we encountered.

Dans le contexte énergétique qui se profile, la production d’électricité par voie solaire thermodynamique s’avère une solution prometteuse, que ce soit pour des considérations économiques, d’échelle de production ou environnementales. Une voie d’amélioration du rendement des centrales solaires à tour consiste à utiliser des cycles thermodynamiques à haut rendement type cycles combinés. Cela nécessite de pouvoir fournir un fluide de travail pressurisé à très haute température (10bar et 1000°C minimum). Ce manuscrit présente les travaux menés afin de développer et de viabiliser un concept d’absorbeur solaire surfacique modulaire en céramique (carbure de silicium) capable de répondre à ces exigences. Le choix du carbure de silicium s’est imposé pour sa résistance aux hautes températures et aux problèmes d’oxydation. Cependant, l’utilisation d’une céramique comme matériau implique un risque de casse des modules. Les céramiques sont en effet fragiles lorsqu’elles sont soumises à des contraintes de traction. C’est la connaissance et la maitrise de ce risque qui fait l’objet de cette étude. L’approche adoptée combine le développement d’outils numériques et d’études expérimentales réalisées sur le site de la centrale solaire Thémis (Targassonne, 66, France). La méthodologie desimulation développée permet de prédire le comportement thermique et le comportement mécanique de l’absorbeur. Ceci permet de réduire les risques encourus par l’absorbeur et d’en connaitre les performances. Cette méthodologie a été éprouvée à l’aide des résultats expérimentaux. / For the future, using thermodynamical solar power plant seems to be a good solution to ensure electrical production. Solar tower plants are able to produce electricity in significant amount, are environmentally friendly and economically competitive. One way to increase the yield of these plants is using high efficiency thermodynamical cycles, like combined cycle. That requires to providing a working fluid at high temperature and high pressure (10bar and 1000°C at least). This PHD thesis presents the works performed to develop and enhance a concept of modular plate solar ceramic absorber that can ensure the required air production. We chose the silicon carbide as material due to its resistance to high temperatures and oxidation problems. The drawback is ceramic modules are weak to traction stresses. The study focuses on the knowledge and the control of this phenomenon. This work combines the developments of numerical tools and experimental studies performed at Thémis power plant (Targassonne, 66, FRANCE). The numerical method permits simulations to predict the thermal behavior and the mechanical behavior of a solar module absorber. It allows the reduction of the mechanical stresses undergone by solar receiver and the prediction of its performances. This methodology was tested using experimental results.