14 Externe wissenschaftliche Einrichtungen
Permanent URI for this collectionhttps://elib.uni-stuttgart.de/handle/11682/15
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Item Open Access Spatio-temporal and polarisation dynamics of semiconductor microcavity lasers(2004) Hamm, Joachim; Hess, Ortwin (Prof. Dr.)Microcavity semiconductor lasers are known for their inherent tight coupling between active material and light-field. The dynamic interaction between the carrier and the photon subsystems is influenced equally strong by both, the dynamics of carriers within the quantum-well and the intra-cavity light-field dynamics. In this work, we develop theoretical models and investigate the nonlinear spatio-temporal behaviour of two prominent types of microcavity lasers, the vertical-cavity surface-emitting laser (VCSEL) and the vertical extended cavity surface-emitting laser (VECSEL). Today's aim to build faster and more powerful semiconductor laser devices goes hand in hand with a miniaturisation of the semiconductor laser structures down to the nanometerscale. Difficult even for simple bulk semiconductor devices, the even tighter coupling of the carrier and light-field sub-systems with respect to time- and length-scales disallow a separate dynamical treatment of the physical processes which take place within such novel microcavity semiconductor lasers. Due to their flexibility and their physical nature, time-domain simulations constitute an appropriate tool for targeting the entangled dynamics within the cavity, the structure and the active quantum-wells. We predict that along with the technological progress of microcavity semiconductor lasers and the availability of inexpensive computing power time-domain methods will gain more importance and constitute a valuable tool to analyse the optical and electronic properties of these devices.Item Open Access Enhancement of the REMix energy system model : global renewable energy potentials, optimized power plant siting and scenario validation(2014) Stetter, Daniel; Müller-Steinhagen, Hans (Prof. Dr.-Ing. habil. DEng/Auckland)As electricity generation based on volatile renewable resources is subject to fluctuations, data with high temporal and spatial resolution on their availability is indispensable for integrating large shares of renewable capacities into energy infrastructures. The scope of the present doctoral thesis is to enhance the existing energy modelling environment REMix in terms of (i.) extending the geographic coverage of the potential assessment tool REMix-EnDaT from a European to a global scale, (ii.) adding a new plant siting optimization module REMix-PlaSMo, capable of assessing siting effects of renewable power plants on the portfolio output and (iii.) adding a new alternating current power transmission model between 30 European countries and CSP electricity imports from power plants located in North Africa and the Middle East via high voltage direct current links into the module REMix-OptiMo. With respect to the global potential assessment tool, a thorough investigation is carried out creating an hourly global inventory of the theoretical potentials of the major renewable resources solar irradiance, wind speed and river discharge at a spatial resolution of 0.45°x0.45°. A detailed global land use analysis determines eligible sites for the installation of renewable power plants. Detailed power plant models for PV, CSP, wind and hydro power allow for the assessment of power output, cost per kWh and respective full load hours taking into account the theoretical potentials, technological as well as economic data. The so-obtined tool REMix-EnDaT can be used as follows: First, as an assessment tool for arbitrary geographic locations, countries or world regions, deriving either site-specific or aggregated installable capacities, cost as well as full load hour potentials. Second, as a tool providing input data such as installable capacities and hourly renewable electricity generation for further assessments using the modules REMix-PlasMo and OptiMo. The plant siting tool REMix-PlaSMo yields results as to where the volatile power technologies photovoltaics and wind are to be located within a country in order to gain distinct effects on their aggregated power output. Three different modes are implemented: (a.) Optimized plant siting in order to obtain the cheapest generation cost, (b.) a minimization of the photovoltaic and wind portfolio output variance and (c.) a minimization of the residual load variance. The third fundamental addition to the REMix model is the amendment of the module REMix-OptiMo with a new power transmission model based on the DC load flow approximation. Moreover, electricity imports originating from concentrating solar power plants located in North Africa and the Middle East are now feasible. All of the new capabilities and extensions of REMix are employed in three case studies: In case study 1, using the module REMix-EnDaT, a global potential assessment is carried out for 10 OECD world regions, deriving installable capacities, cost and full load hours for PV, CSP, wind and hydro power. According to the latter, photovoltaics will represent the cheapest technology in 2050, an average of 1634 full load hours could lead to an electricity generation potential of some 5500 PWh. Although CSP also taps solar irradiance, restrictions in terms of suitable sites for erecting power plants are more severe. For that reason, the maximum potential amounts to some 1500 PWh. However, thermal energy storage can be used, which, according to this assessment, could lead to 5400 hours of full load operation. Onshore wind power could tap a potential of 717 PWh by 2050 with an average of 2200 full load hours while offshore, wind power plants could achieve a total power generation of 224 PWh with an average of 3000 full load hours. The electricity generation potential of hydro power exceeds 3 PWh, 4600 full load hours of operation are reached on average. In case study 2, using the module REMix-PlaSMo, an assessment for Morocco is carried out as to determine limits of volatile power generation in portfolios approaching full supply based on renewable power. The volatile generation technologies are strategically sited at specific locations to take advantage of available resources conditions. It could be shown that the cost optimal share of volatile power generation without considering storage or transmission grid extensions is one third. Moreover, the average power generation cost using a portfolio consisting of PV, CSP, wind and hydro power can be stabilized at about 10 €ct/kWh by the year 2050. In case study 3, using the module REMix-OptiMo, a validation of a TRANS-CSP scenario based upon high shares of renewable power generation is carried out. The optimization is conducted on an hourly basis using a least cost approach, thereby investigating if and how demand is met during each hour of the investigated year. It could be shown, that the assumed load can safely be met in all countries for each hour using the scenario's power plant portfolio. Furthermore, it was proven that dispatchable renewable power generation, in particular CSP imports to Europe, have a system stabilizing effect. Using the suggested concept, the utilization of the transfer capacities between countries would decrease until 2050.Item Open Access Numerical simulations of soot formation in turbulent flows(2008) Di Domenico, Massimiliano; Gerlinger, Peter (PD Dr.-Ing.)This work deals with the simulation of soot formation phenomena under gasturbine-like conditions. Main goal is the development of a reliable CFD simulation tool able to predict trends of soot formation under different operating conditions. A detailed, finite-rate chemistry combustion model is presented and validated. Since soot particles are the result of thousands of reactions involving hundreds of species, an efficient, new sectional approach for soot precursors and related reactions is chosen in this work. Effects of turbulent fluctuations in temperature and species concentration on the chemical reaction rate are included by employing an assumed Probability Density Function approach. Finally, the simulation of a semi-technical scale burner under gasturbine-like conditions will demonstrate the validity of the developed tools. Although discrepancies with experimental soot particle distributions are observed, numerical simulations are able to reproduce the pressure dependence of the peak soot volume fraction quite well.Item Open Access Storage technologies for the electricity transition : an analysis of actors, actor perspectives and transition pathways in Germany(2020) Frey, Ulrich J.; Wassermann, Sandra; Deissenroth-Uhrig, MarcThis article analyses actors in the storage niche during the German electricity transition. Thus, we develop a more differentiated understanding of actors and their storage activities. For that, we employ the analytical multi-level-perspective (MLP) framework to focus on interactions between old and new storage technologies. Using data from expert interviews, we investigate whether the storage pathway resembles any of the four ideal types of transition pathways for interactions between niche and regime. Through our interviews, we identify five types of actor in the storage market: Big 4 (EnBW, RWE, E.ON, Vattenfall), project developers, innovative municipal utilities, small rural municipal utilities and independent green electricity providers. For each actor, we analyse four main aspects (1) previous orientation and motivation, (2) structural strategies, (3) institutional strategies, and (4) product-related strategies. Parallel to the classification of actors, we also classify available storage technologies according to their primary field of application. We conclude that interactions between regime and niche actors are cooperative, but weak, and no specific actor type currently dominates the niche activities. Hence, applications in the storage niche are not yet ready for a larger market. In sum, our results point to a future system that is characterized by reconfiguration, not substitution or transformation of current market actors.Item Open Access Three dimensional finite difference time domain simulations of photonic crystals(2004) Hermann, Christian; Hess, Ortwin (Prof. Dr.)In this work fundamental optical properties of various photonic crystal structures are analysed numerically within the framework of three dimensional finite-difference time-domain (FDTD) simulations. After a discussion of the underlying physical and mathematical principles from electrodynamics and solid state physics leading to the formation of a photonic bandgaps, two important example systems are discussed in detail. First, we study two-dimensionally patterned layer-by-layer systems. These system are promising with respect to applications in integrated optics, but suffer severely from out-of-plane radiation losses. With fully three dimensional simulations we analyse this loss mechanism for several vertical layer-setups, obtaining new and important results regarding the understanding of the interaction of in-plane structure and surrounding. E.g. we describe for the first time the existence of cladding modes in complicated layer-by-layer structures. Second, we analyse the strong space-, frequency- and polarisation dependance of spontaneous emission within the weak coupling limit in an three-dimensionally structured inverted opal crystallite of finite size. The inverted opal, exhibiting a complete bandgap, shows (beside the supression of emission for bandgap frequencies) strong enhancement for emitters placed on dielectric interfaces. These results are important for interpretating luminescence experiments of inhomogeneously infiltrated dye molecules. In the last part of the work we discuss the numerical method itself with special focus on the necessary adaptations for the treatment of photonic crystal structures.Item Open Access Microscopic spatio-temporal dynamics of semiconductor quantum well lasers and amplifiers(2007) Böhringer, Klaus; Hess, Ortwin (Prof. Dr.)This work discusses light-matter interaction and optical nonlinearities in semiconductor nanostructures and presents a detailed numerical analysis of the spatio-temporal dynamics in novel high-power diode lasers. We derive a microscopic, spatially resolved model that combines a density matrix approach to the carrier and gain dynamics in semiconductor quantum well gain media with the macroscopic Maxwell equations for the electromagnetic field dynamics. We present Maxwell semiconductor Bloch equations in full time-domain that cover many-body interactions, a diversity of time scales and gain saturation mechanisms, and comprise the fast-oscillating carrier wave and a sub-wavelength spatial resolution. Our work focuses on ultrafast carrier effects, a quantitative understanding of optical nonlinearities, the engineering of the mode structure in microcavities, and their impact on the laser emission characteristics. Optical dephasing and carrier relaxation due to the screened Coulomb interaction and scattering with phonons are explored in detail. This work aims to improve the quantitative understanding of lasing systems of technological or fundamental relevance by performing numerical experiments: Within the framework of the paraxial wave approximation, we study the excitation of multiple transverse modes, multi-mode dynamics and the occurrence of unstable optical filaments in broad area edge-emitting lasers. We analyse vertical cavity surface-emitting laser devices with a periodically structured defect as an example of a photonic band edge band gap laser. In particular, we explore the utilisation of photonic crystal structures: gain enhancement for band edge modes and the reduction of optical losses. The complex interplay between the intracavity optical field and quantum well gain dynamics is investigated for realistic optically pumped external cavity surface-emitting laser structures. We also consider the interaction of high-intensity femtosecond and picosecond pulses with semiconductor optical amplifiers and absorbers. We identify the microscopic origin of the fast nonlinearities, obtain nonlinear gain coefficients and recovery rates, and analyse the nonlinear pulse reshaping, i.e. changes and asymmetries in the amplified pulse shape and spectrum. Built upon efficient numerical algorithms and the increased availability of inexpensive high-performance computing resources, our microscopic time-domain approach is well suitable for the engineering and design optimisation of modern nanostructured high-power diode lasers.Item Open Access Entwicklung und Charakterisierung von anodenseitigen Diffusionsbarriereschichten für metallgetragene oxidkeramische Festelektrolyt-Brennstoffzellen(2009) Franco, Thomas; Müller-Steinhagen, Hans (Prof. Dr. Dr.-Ing. habil.)Die Langzeitbeständigkeit einer metallgetragenen SOFC (MSC, „Metall Supported Cell“) wird insbesondere durch thermisch aktivierte Diffusionsprozesse, wie z.B. der anodenseitigen Interdiffusion von Fe, Cr und Ni, limitiert. Während des MSC-Betriebs steht die Nickelphase der Ni/ZrO2-Anode bauartbedingt in di-rektem Kontakt mit der FeCr-Matrix des metallischen Trägersubstrates. Dadurch kann ein di-rekter Stofftransport von Ni aus der Anode in das FeCr-Substrat und vice versa von Fe bzw. Cr aus dem Substrat in die Ni-Matrix der Anode stattfinden. Die damit einhergehenden Gefü-geumwandlungen können zur beschleunigten Zelldegradation bis hin zum völligen Zellversagen führen. Dieser Prozess gilt derzeit als einer der hauptsächlichen Schädigungsmechanis-men bei der metallgetragenen SOFC, den es im Sinne eines angestrebten langzeitstabilen SOFC-Betriebs zu minimieren gilt. Die Integration einer oxidkeramischen Trennschicht, als Diffusionsbarriere (DBL) an der Grenzfläche „Substrat-Anode“, stellt für diesen Zweck nicht nur einen naheliegenden, sondern auch sehr aussichtsreichen Lösungsansatz dar. Im Rahmen der vorliegenden Arbeit wurden ausgehend von den hohen Anforderungen, die an eine zusätzliche Diffusionsbarriereschicht im SOFC-Betrieb gestellt werden, systemati-sche SOFC-relevante Untersuchungen zur Qualifizierung geeigneter Materialien durchge-führt. Betrachtet wurden dabei insbesondere unterschiedlich dotierte Perowskite auf Basis von LaCrO3, die mit Hilfe des „Atmosphärischen Plasmaspritzens (APS)“ auf dem metallischen Trägersubstrat appliziert werden können. Durch gezielte Schichtentwicklung mit einigen qua-lifizierten Materialien konnten im Ergebnis plasmagespritzte Barriereschichten entwickelt werden, deren Tauglichkeit im Langzeitbetrieb einer MSC erfolgreich demonstriert werden konnte.Item Open Access Steuerung für redundante Robotersysteme: Benutzer- und aufgabenorientierte Verwendung der Redundanz(2004) Schreiber, Günter; Hirzinger, Gerd (Prof. Dr. Ing.)In Zukunft ist ein großes Marktpotenzial in der maschinellen Bearbeitung von Dienstleistungsaufgaben ("Service Robotik"), wie z.B. zur Unterstützung behinderter oder älterer Menschen, in der Sicherheitstechnik, der Reinigungstechnik, der Raumfahrt, zu erwarten. Dort werden flexibel nutzbare und mit Sensoren ausgestattete Roboter gebraucht, da in diesen apriori unbekannten Umgebungen nicht alles voraus geplant und simuliert werden kann. Für diese Anwendungen werden Roboter benötigt, die leicht sind und dennoch Kraft ausüben können, die effizient und flexibel Kollisionen vermeiden können, die interaktiv auf Menschen reagieren können, während sie ihre Aufgaben verrichten, wie z.B. eine Last halten und dennoch auf Berührung ausweichen. Typischerweise können diese Aufgabenstellungen mit Hilfe von kinematisch redundanten Robotern erfüllt werden, da hier zusätzliche Freiheitsgrade vorhanden sind, mit denen auf variierende Aufgabenstellungen und veränderliche Umwelteinflüsse reagiert werden kann. Zur Nutzung von kinematisch redundanten Robotern wurden bereits viele Algorithmen und Methoden entwickelt. Ausgehend von den am Institut für Robotik und Mechatronik entwickelten Systemen einer neuen Robotergeneration ist es nun möglich, ein Rahmenkonzept zur Nutzung von redundanten Robotern zu schaffen, analog zu früheren Tagen, als Interpolatoren, Kinematik, inverse Kinematik etc. zu Industrierobotersteuerungen zusammengefasst wurden, die damit den Industrierobotern zum Durchbruch verhalfen. Daher wird in dieser Arbeit ein Konzept zur Nutzung und Bedienung kinematisch redundanter Roboter entwickelt. Aufgrund der Tatsache, dass sehr viele verschiedene Verwendungszwecke der kinematischen Redundanz möglich sind, die einander möglicherweise sogar widersprechen, ist die Wahl solcher Verwendungszwecke, Methoden und Algorithmen sehr aufgabenspezifisch. Im ersten Teil der Arbeit werden Algorithmen und Methoden zur Verwendung der kinematischen Redundanz einander gegenübergestellt, aus diesen wird ein weittragender Algorithmus ausgewählt und in der Tiefe untersucht. Dieser wird in vielen verschiedenen Anwendungsfeldern eingesetzt, wie in der Singularitätsbehandlung, verschiedenen Verwendungszwecken der kinematischen Redundanz und der interaktiven intuitiven Verwendung von redundanten Manipulatoren. Im zweiten Teil wird eine Systemarchitektur und eine Benutzerschnittstelle konzipiert, mit der ein Nicht in die Lage versetzt wird, einen kinematisch redundanten Roboter zu bedienen. Eines der Hauptergebnisse aus der Gegenüberstellung der verschiedenen Algorithmen zur Behandlung kinematischer Redundanzen ist, dass es nicht "den Besten" gibt. Daher sollte die Einbettung mehrerer verschiedener Algorithmen in die Systemarchitektur eines ServiceRoboters vorgesehen werden. Ein weiteres Ergebnis ist, dass durch die Wahl von Methoden der "Constraint Optimization" anstelle der weitverbreiteten Moore-Penrose Pseudo Inverse Anforderungen realer Systeme, wie z.B. endliche Gelenkgeschwindigkeiten, leicht eingebettet werden können. Auch die Koordination von hochgradig redundanten mobilen Manipulatoren Nebenbedingungen kann mit solchen konvexen Optimierungsproblemen in Echtzeit behandelt werden. Die Behandlung von Singularitäten nutzt eben diese Ungleichungs-Nebenbedingungen. Die Singularitätsbehandlung wird in dieser Arbeit an nicht-redundanten Industrierobotern untersucht, der Formalismus wird danach auf redundante Manipulatoren ausgedehnt. In der Erweiterung des Formalismus auf redundante Manipulatoren werden verschiedene Verwendungszwecke untersucht, wie z.B. Geschicklichkeitssteigerung, Sollkonfiguration, Kollisionsvermeidung, interaktive Verwendung der Redundanz etc. Weiterhin werden Punkt-zu-Punkt-Bewegungen untersucht, da durch die Verwendung numerisch iterativer Algorithmen die Konvergenz in Echtzeit nicht garantiert werden kann. Diese Betriebsart wird exemplarisch beim Einfangen eines geworfenen Balles erprobt. Interaktive intuitive Beherrschung der kinematischen Redundanz ist ein wichtiges Thema, um einem Bediener das Vormachen von Aufgaben mit einem Roboter zu ermöglichen. Während bei den nicht das Vormachen der Aufgabe im Bezug auf das Werkzeug ausreicht, muss bei redundanten Manipulatoren zusätzlich die kinematische Redundanz betrachtet werden. Das interaktive Konzept wird dann auf die Aufgabenausführung ausgedehnt, was am Beispiel des inversen Pendels exemplarisch untersucht wird. Eine Systemarchitektur wird konzipiert, in der es möglich ist, viele verschiedene Algorithmen und Methoden zu verwalten, diese mit Daten zu versorgen und um weitere Komponenten flexibel zu erweitern. Zusätzlich wird ein Bedienkonzept für redundante Manipulatoren entwickelt, und die notwendigen Bedienelemente werden skizziert. Die entwickelten Methoden und Algorithmen werden an verschiedenen Szenarien, die auch reale Roboter mit bis zu 10 Freiheitsgraden umfassen, erprobt.Item Open Access Numerical modeling of ignition processes in single- and multiphase flows(2014) Boyde, Jan Michael; Aigner, Manfred (Prof. Dr.-Ing. )In this work a combustion model is developed and presented which is applicable to a wide range of conditions through modifications to the Turbulent Flame Speed Closure model. The model predictions show that for all examined test cases including single- and multiphase conditions, a satisfying agreement with available experimental data is achieved. This underlines the usefulness of numerical tools for the investigation of ignition processes in the context of aircraft engines.Item Open Access Numerical simulations and experimental investigations on quasi-static and cyclic mixed mode delamination of multidirectional CFRP laminates(2011) Naghipour, Parya; Voggenreiter, Heinz (Prof. Dr.-Ing)The structural applications of Carbon Fibre Reinforced Plastic (CFRP) composites are gradually expanding in aerospace industry as a result of their outstanding mechanical properties such as high stiffness to weight ratio and fatigue resistance. With the increasing application, the need for understanding their mechanical behaviour and failure mechanisms also rises. Interfacial cracking between layers or delamination is one of the most common failure types in laminated fibre-reinforced composites due to their relatively weak inter-laminar strengths. Typically, delamination failures initiate and propagate under mixed mode effect of normal and shear stresses. Therefore, mixed mode delamination failure in fibrous composites has been one of the major issues being studied extensively in recent years. In this scope, the development of predictive, reliable and robust numerical and experimental analysis tool for quasi-static or cyclic mixed mode delamination of CFRPs is the major focus of the thesis. Quasi-static and cyclic mixed mode delamination failure in unidirectional and multidirectional CFRP laminates are analyzed using fracture experiments, finite element (FE) simulations, analytical calculations, and Scanning Electron Microscopy (SEM). Quasi-static delamination tests under mixed mode bending (MMB) represented by a superposition of normal and shear loadings are conducted to obtain the load-displacement response and investigate the effect of fiber orientation and stacking sequence on the progressive mixed mode delamination failure. The experiments designate that varying fibre orientation and stacking sequences have a considerable effect on load-displacement response and mixed mode fracture toughness of multidirectional laminates. The other important outcome of the experiments is that delamination resistance in multidirectional laminates is also considerably higher than in their unidirectional counterpart. The numerical model of the laminate is described as an assembly of individual layers and interface elements. Each individual ply is assumed as an orthotropic homogenized continuum under plane stress, permitting the modelling of damage initiation in each ply under the combination of longitudinal, transverse, and shear stress states. The interface elements, the constitutive behaviour of which are implemented as a user element routine in ABAQUS, are represented via the cohesive zone concept with bilinear and exponential softening laws. The sensitivity of the interface element has also been tested with respect to input parameters, such as interface element length and initial stiffness, using numerical examples. The numerical results revealed that in order to achieve a closer response to experimentally obtained results there must be some limitations on input values, which in turn influence the computational cost of the simulation. The numerical model is able to successfully capture the experimentally observed effects of fibre angle orientations and variable stacking sequences on the global load-displacement response and mixed mode inter-laminar fracture toughness of the various laminates. A reliable numerical simulation requires a correct evaluation of quasistatic fracture toughness especially in between plies with different orientations. Therefore, the total mixed mode and decomposed fracture energies for different multidirectional laminates were estimated by an analytical approach based on the combination of classical laminated plate theory and linear fracture mechanics. The analytical approach produces quite accurate predictions of the fracture toughness values obtained experimentally. It can further be used as a widely applicable calculation tool of mixed mode delamination toughness. The analytical solution is further enhanced by adding the effect of thermal residual stresses. The fracture toughness values calculated with and without residual thermal stress terms indicate that for the chosen quasi-symmetric multidirectional laminates, the influence of thermal stresses can be neglected. Cyclic mixed mode delamination in multidirectional composite laminates subjected to high cycle fatigue loading is investigated by numerical simulations and cyclic MMB experiments. Similar to the quasi-static case, the numerical model includes lamina and interface elements. The description of the cyclic delamination crack growth rate is based on the cyclic degradation of bilinear interface elements, with subsequent unloading/ reloading cycles. In other words, the interfacial fatigue damage evolution law, added to the previously implemented user element routine, is a cohesive law that links fracture and damage mechanics to establish the evolution of the damage variable in terms of the cyclic crack growth rate. The constitutive cyclic damage model is calibrated by means of mixed mode fatigue experiments and reproduces the experimental results successfully and with minor error.
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