Highly accurate simulations and benchmarking of molecule-surface reactions

Heterogeneous catalyzed processes are highly significant to the chemical industry. Dissociative chemisorption (DC) of molecules on surfaces is always considered as a step with a high degree of rate control for heterogeneous catalysis. Our ability to comprehend the different mechanisms underlying DC on metal surfaces could benefit significantly from the availability of an accurate database for the barrier heights of elementary molecule-metal surface reactions. In this work, we used a recent implementation of specific reaction parameter (SRP) to density-functional theory (DFT) (SRP-DFT) to develop potential energy surfaces (PESs) for molecule-surface reactions. The PESs were used for molecular beam simulations and compared with molecular beam experiments. The barrier heights extracted from these PESs are now used for benchmarking.Theoretical Chemistr

A new approach for electric modeling of vertical axis wind turbine rotors in view to the construction of a global electric model for the wind energy conversion system / Nouvelle approche de modélisation électrique des rotors d’éoliennes à axe vertical en vue de la construction d’un modèle électrique global de la chaine de conversion éolienne

The combine effects of climate changes, the continue and considerable increase of energy demand in the world, the instability of fossil fuel prices as well as the energetic dependence of some countries who are not fuel producers leads to important development of renewable energies. More again, the energy bill in its dizzying rise causes the prices increase for raw materials, goods and even basic necessities. It is therefore imperative to go in for diversification of energy sources on the one hand and means to produce energy at a relatively low cost on the other hand. This thesis proposes a novel approach for aerodynamic modelling of Darrieus type vertical axes wind turbines (VAWTs). Indeed, it is a contribution to the build-up of a global electric model for vertical axis wind turbine conversion system with a view to improve monitoring, diagnosis and maintenance. The goal of this research work is double: firstly, to propose an innovative approach for aerodynamic modelling of VAWTs and secondly, based on this approach, to build a new equivalent electric model for the study, analysis and aerodynamic optimisation of Darrieus type VAWTs. More broadly, this work aims to contribute to the development of new tools, techniques and methods to improve the design, optimization, diagnosis, prognosis and monitoring of Darrieus VAWTs to better ensure their maintenance. The purpose is to provide the scientific community and the wind turbine industry with new tools that can ameliorate the reliability and thus increase performance and efficiency of Darrieus type VAWTs. An equivalent electric circuit model for three-blade DT-VAWT rotors was builded. The new model that we named the Tchakoua model is based on a recently developed approach for modelling DT-VAWT rotors using the equivalent electrical circuit analogy. The proposed model was built from the mechanical description given by the Paraschivoiu double-multiple streamtube model and was based on an analogy between mechanical and electrical circuits. Thus, the rotating blades and the blades’ mechanical coupling to the shaft are modelled using the mechanical-electrical analogy, and the wind flow is modelled as a source of electric current. Model simulations were conducted using MATLAB for a three-bladed rotor architecture, characterized by a NACA0012 profile, an average Reynolds number of 40,000 for the blade and a tip speed ratio of 5. The results obtained show strong agreement with findings from both aerodynamic and computational fluid dynamics (CFD) models in the literature. Les effets conjugués du réchauffement climatique, l’augmentation considérable et continue de la demande énergétique mondiale, la volatilité des prix des énergies fossiles, la dépendance énergétique de certains pays non producteurs de pétrole, font que les énergies renouvelables bénéficient actuellement d’un essor très important. De plus, la facture énergétique dans sa hausse vertigineuse entraîne les matières premières, les biens de consommation et même les produits de première nécessité. Il est donc impératif de réfléchir à la diversification des sources d’énergie d’une part et aux moyens de produire de l’énergie à un coup relativement bas d’autre part. Cette thèse propose un modèle électrique équivalent pour les rotors d’éoliennes à axes vertical. Elle constitue une contribution à la construction d’un modèle électrique global de la chaîne de conversion éolienne. L’objectif de cette recherche est d’une part de proposer une nouvelle approche de modélisation des éoliennes à axe vertical; et d’autre part d’élaborer, sur la base de cette approche, un nouveau modèle pour l’étude, l’analyse et l’optimisation aérodynamique des éoliennes à axe vertical de type Darrieus. De façon plus large, ce travail vise à contribuer au développement de nouveaux outils, techniques et méthodes devant permettre d’améliorer la conception, l’optimisation, le diagnostic, le pronostic et la surveillance des éoliennes à axe vertical de type Darrieus afin de mieux assurer leur maintenance. La finalité étant de mettre à la disposition de la communauté scientifique et de l’industrie de l’éolienne de nouveaux outils pouvant permettre d’accroître la performance aérodynamique ainsi que le rendement énergétique des éoliennes à axe vertical de type Darrieus. Un modèle électrique équivaut pour rotor d’éolienne de type Darrius tri-pales a été développé. Ce nouveau modèle que nous avons nommé le ‘Tchakoua model’ est basé sur une nouvelle approche de modélisation des rotors d’éoliennes à axe vertical utilisant des circuits électriques équivalents. Le modèle a été construit sur la base de description à double disque actuateur à multiple tubes de vent faite par Paraschivoiu et est basé sur l’analogie circuits mécaniques et circuits électriques appliquée à l’aérodynamique. Ainsi, les pales et le couplage mécanique à l’arbre lent sont modélisées en utilisant l’analogie mécanique électrique tandis que l’écoulement de vent est modélisé comme source de courant. Les simulations ont été effectuées avec le logiciel MATLAB pour un rotor tripal caractérisé par un profil NACA0012, un nombre de Reynold moyen de 40000 et une vitesse spécifique de 5. Une validation croisée de ces résultats avec les données issues des modèles aérodynamiques ou même des modèles numériques de dynamique des fluides montre de bonnes concordances

An investigative study on neutron emissions from titanium- deuterium system under thermal shock

The purpose of this study is to investigate the titanium-deuterium system under thermal shock, as a potential neutron source. The expected neutron emission is unique, i.e. it is monoenergetic with energy of 2.45 MeV, which is valuable for calibrating neutron detectors. In our study, titanium was loaded with deuterium gas at room temperature in an experimental system, and the system was subjected to rapid thermal cycling by repeated cooling with liquid nitrogen, followed by rapid warm up phases to create a non-equilibrium condition in titanium lattice. Neutron bursts were monitored using a [superscript 3]He detector, which responds to slow neutrons, a moderated [superscript 3]He detector, which responds to slow and fast neutrons, and a proton recoil detector, which responds to fast neutrons. The pressure and temperature of the system was monitored throughout the experiments. The result of this work shows that: 1) loading of titanium with deuterium gas should be done under high vacuum conditions (<1 X 10[superscript 6] torr) to remove environmental contaminants, which was found to inhibit the titanium-deuterium reaction, 2) cracks observed in titanium samples from lattice stress varied in size and location in titanium lattice and dependent on the level of deuterium loading. The presence of cracks in some locations indicates that the titanium-deuterium reaction is a local effect, 3) low level neutron burst were observed in less than 23% of all experiments and involved the detection of a single neutron burst, suggesting that neutron emission is a statistical process occurring at low probability. The neutron burst was observed from partially deuterated titanium samples. The level of neutrons detected is consistent with what has been reported in literature. 4). A large temperature increased from room temperature to 450 [degree sign]C during phase transition from [delta]-titanium to [delta]-titanium occurred, but no neutrons were observed. The temperature increased is likely associated with the exothermic reaction that occurs during hydride formation, which does not lead to neutron emission. 5) No evidence of tritium or nuclear transmutation was observed in our experimental system

Demand for reserves and moral hazard in a Monetary Union

L’objectif de ce papier est d’examiner l’existence ou non d’un comportement d’aléa moral dans la demande de réserve internationale des pays membres d’une union monétaire, avec application à la CEMAC. Il se base sur des éléments de statistiques descriptives et se veut le début d’une analyse plus poussée à l’aide d’outils économétriques adéquats. De nos analyses, il ressort que les réserves internationales sont un instrument important qui permet de résorber les déséquilibres de payements, et de ce fait, leur demande dans une union monétaire entraîne un comportement d’aléa moral. Ce comportement se caractérisant par un laxisme dans la gestion économique et un tirage abusif sur le pooling. Ce comportement a été clairement mis en évidence dans le cas de la CEMAC, mais à des degrés divers pour les pays. Une solution à ce problème, qui ne devrait pas détruire l’union, serait de catégoriser les pays suivant leur risque de déficit afin d’exiger de chacun un coût de participation préalable à l’union.Demande de réserve. aléa moral, union mondetaire, CEMAC

SBH17: benchmark database of barrier heights for dissociative chemisorption on transition metal surfaces

Accurate barriers for rate controlling elementary reactions on metal surfaces are key to understanding, controlling, and predicting the rate of heterogeneously catalyzed processes. While barrier heights for gas phase reactions have been extensively benchmarked, dissociative chemisorption barriers for the reactions of molecules on metal surfaces have received much less attention. The first database called SBH10 and containing 10 entries was recently constructed based on the specific reaction parameter approach to density functional theory (SRP-DFT) and experimental results. We have now constructed a new and improved database (SBH17) containing 17 entries based on SRP-DFT and experiments. For this new SBH17 benchmark study, we have tested three algorithms (high, medium, and light) for calculating barrier heights for dissociative chemisorption on metals, which we have named for the amount of computational effort involved in their use. We test the performance of 14 density functionals at the GGA, GGA+vdW-DF, and meta-GGA rungs. Our results show that, in contrast with the previous SBH10 study where the BEEF-vdW-DF2 functional seemed to be most accurate, the workhorse functional PBE and the MS2 density functional are the most accurate of the GGA and meta-GGA functionals tested. Of the GGA+vdW functionals tested, the SRP32-vdW-DF1 functional is the most accurate. Additionally, we found that the medium algorithm is accurate enough for assessing the performance of the density functionals tested, while it avoids geometry optimizations of minimum barrier geometries for each density functional tested. The medium algorithm does require metal lattice constants and interlayer distances that are optimized separately for each functional. While these are avoided in the light algorithm, this algorithm is found not to give a reliable description of functional performance. The combination of relative ease of use and demonstrated reliability of the medium algorithm will likely pave the way for incorporation of the SBH17 database in larger databases used for testing new density functionals and electronic structure methods.Theoretical Chemistr

SBH17: benchmark database of barrier heights for dissociative chemisorption on transition metal surfaces

Accurate barriers for rate controlling elementary reactions on metal surfaces are key to understanding, controlling, and predicting the rate of heterogeneously catalyzed processes. While barrier heights for gas phase reactions have been extensively benchmarked, dissociative chemisorption barriers for the reactions of molecules on metal surfaces have received much less attention. The first database called SBH10 and containing 10 entries was recently constructed based on the specific reaction parameter approach to density functional theory (SRP-DFT) and experimental results. We have now constructed a new and improved database (SBH17) containing 17 entries based on SRP-DFT and experiments. For this new SBH17 benchmark study, we have tested three algorithms (high, medium, and light) for calculating barrier heights for dissociative chemisorption on metals, which we have named for the amount of computational effort involved in their use. We test the performance of 14 density functionals at the GGA, GGA+vdW-DF, and meta-GGA rungs. Our results show that, in contrast with the previous SBH10 study where the BEEF-vdW-DF2 functional seemed to be most accurate, the workhorse functional PBE and the MS2 density functional are the most accurate of the GGA and meta-GGA functionals tested. Of the GGA+vdW functionals tested, the SRP32-vdW-DF1 functional is the most accurate. Additionally, we found that the medium algorithm is accurate enough for assessing the performance of the density functionals tested, while it avoids geometry optimizations of minimum barrier geometries for each density functional tested. The medium algorithm does require metal lattice constants and interlayer distances that are optimized separately for each functional. While these are avoided in the light algorithm, this algorithm is found not to give a reliable description of functional performance. The combination of relative ease of use and demonstrated reliability of the medium algorithm will likely pave the way for incorporation of the SBH17 database in larger databases used for testing new density functionals and electronic structure methods.Theoretical Chemistr

Demande de réserves et aléa moral dans une Union Monétaire

L’objectif de ce papier est d’examiner l’existence ou non d’un comportement d’aléa moral dans la demande de réserve internationale des pays membres d’une union monétaire, avec application à la CEMAC. Il se base sur des éléments de statistiques descriptives et se veut le début d’une analyse plus poussée à l’aide d’outils économétriques adéquats. De nos analyses, il ressort que les réserves internationales sont un instrument important qui permet de résorber les déséquilibres de payements, et de ce fait, leur demande dans une union monétaire entraîne un comportement d’aléa moral. Ce comportement se caractérisant par un laxisme dans la gestion économique et un tirage abusif sur le pooling. Ce comportement a été clairement mis en évidence dans le cas de la CEMAC, mais à des degrés divers pour les pays. Une solution à ce problème, qui ne devrait pas détruire l’union, serait de catégoriser les pays suivant leur risque de déficit afin d’exiger de chacun un coût de participation préalable à l’union

Demande de réserves et aléa moral dans une Union Monétaire

L’objectif de ce papier est d’examiner l’existence ou non d’un comportement d’aléa moral dans la demande de réserve internationale des pays membres d’une union monétaire, avec application à la CEMAC. Il se base sur des éléments de statistiques descriptives et se veut le début d’une analyse plus poussée à l’aide d’outils économétriques adéquats. De nos analyses, il ressort que les réserves internationales sont un instrument important qui permet de résorber les déséquilibres de payements, et de ce fait, leur demande dans une union monétaire entraîne un comportement d’aléa moral. Ce comportement se caractérisant par un laxisme dans la gestion économique et un tirage abusif sur le pooling. Ce comportement a été clairement mis en évidence dans le cas de la CEMAC, mais à des degrés divers pour les pays. Une solution à ce problème, qui ne devrait pas détruire l’union, serait de catégoriser les pays suivant leur risque de déficit afin d’exiger de chacun un coût de participation préalable à l’union

Modelling wind turbine degradation and maintenance

degradation, inspection and maintenance processes. The model was developed based on the Petri net method that effectively captures the stochastic nature of the dynamic processes through the use of appropriate statistical distributions. The versatility of the method allows the details of the degradation and maintenance operations to be incorporated in the model. In particular, there are dependent deterioration processes between wind turbine subsystems, complex maintenance rules and the incorporation of condition monitoring systems for early failure indication to enable replacement prior to failure. The purposes of the model are to predict the future condition of wind turbine components and to investigate the effect of a specified maintenance strategy. The model outputs are statistics indicating the performance of the wind turbine components; these include the probability of being in different condition states, the expected number of maintenance actions and the average number and duration of system downtime under any maintenance strategy

A Critical Review on the Structural Health Monitoring Methods of the Composite Wind Turbine Blades

With increasing turbine size, monitoring of blades becomes increasingly im-portant, in order to prevent catastrophic damages and unnecessary mainte-nance, minimize the downtime and labor cost and improving the safety is-sues and reliability. The present work provides a review and classification of various structural health monitoring (SHM) methods as strain measurement utilizing optical fiber sensors and Fiber Bragg Gratings (FBG’s), active/ pas-sive acoustic emission method, vibration‒based method, thermal imaging method and ultrasonic methods, based on the recent investigations and prom-ising novel techniques. Since accuracy, comprehensiveness and cost-effectiveness are the fundamental parameters in selecting the SHM method, a systematically summarized investigation encompassing methods capabilities/ limitations and sensors types, is needed. Furthermore, the damages which are included in the present work are fiber breakage, matrix cracking, delamina-tion, fiber debonding, crack opening at leading/ trailing edge and ice accre-tion. Taking into account the types of the sensors relevant to different SHM methods, the advantages/ capabilities and disadvantages/ limitations of repre-sented methods are nominated and analyzed