04 Fakultät Energie-, Verfahrens- und Biotechnik
Permanent URI for this collectionhttps://elib.uni-stuttgart.de/handle/11682/5
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Item Open Access Long-term capacity expansion planning with variable renewable energies : enhancement of the REMix energy system modelling framework(2017) Fichter, Tobias; Thess, André (Prof. Dr.)The large-scale integration of variable renewable energies (VRE) like Photovoltaics and wind power into the power system is crucial for the transition towards a sustainable electricity supply. However, due to the inherent characteristics of VRE, i.e. the site-specific, highly variable, and unreliable power generation, as well as their low variable generation costs, the large-scale deployment of VRE causes adequacy-, grid-related, and balancing-impacts for the residual system. These impacts and the related costs need to be considered for a concerted capacity expansion planning with VRE in order to identify cost-efficient and reliable transition pathways. Traditionally applied capacity expansion planning models have limitations to consider the value of energy at its time of the delivery of VRE and their impacts on the system due to the applied low system-operational detail. Hence, new planning methods are required to ensure a successful transition towards a sustainable electricity supply. This work enhances the REMix energy system modelling framework to allow for a concerted long-term capacity expansion planning with VRE. The outcome of this is the REMix-Capacity Expansion Model (REMix-CEM). The optimization model bridges the gap between traditional long-term capacity expansion planning and short-term power system operation models. This enables the model to consider the value and the impacts of a large-scale integration of VRE into the power system accurately within capacity expansion planning. This thesis describes the challenges of long-term capacity expansion planning with VRE and presents the developed model in detail. This includes a principle description of how REMix-CEM is typically applied by DLR for a science-based consultancy of planning authorities in developing and emerging countries. To demonstrate its capabilities, the flexible formulation of the model is used to investigate two important issues within a model-based long-term capacity expansion planning with VRE - the model foresight and the applied system-operational detail. Both issues can have a significant influence on results and computational effort of the model. These correlations are investigated within two case studies for a fictitious but representative power system of a developing country. Results of the first case study indicate that the type of model foresight (single-year myopic, multi-annual rolling horizon, or perfect foresight) has a strong influence when some of the input parameters change suddenly at one point of the planning time frame, while its influence is less pronounced when parameters changes rather continuously over the period of study. Only a large model foresight enables the model to anticipate future occurrences well in advance and to adopt its investment strategies accordingly. Furthermore, the analysis shows that the larger the model foresight the higher is the competitiveness of VRE and dispatchable RE, because their advantage to produce electricity at stable costs over the lifetime can be captured more precisely. However, it is also demonstrated that a larger model foresight means also a higher computational effort to solve the capacity expansion optimization problem. In addition, a large model foresight with perfect information over the planning time frame might not fully capture the decision frame-work of real-life decision makers. To keep computational effort manageable for long-term capacity expansion planning with VRE, investment decisions are typically based on a limited number of representative dispatch periods. These dispatch periods have the aim to represent the temporal variability of load and RE resources over the year as accurate as possible. Within the second case study it is shown that the average day method, which uses average values to assign values for RE resource availability to the utilized dispatch periods, is inappropriate for capacity expansion planning with VRE. The value of energy at its time of the delivery of VRE is modeled inaccurately and system flexibility requirements, caused by the integration of VRE, are underestimated systematically. The representative day method, which uses a sample of “real” historical days instead of average values, is significantly more suitable because extreme values are not averaged. This leads to a better approximation of VRE electricity generation, which allows a more accurate consideration of the value of energy at its time of the delivery of VRE and system flexibility requirements. System flexibility requirements can be captured within capacity expansion optimization especially by considering unit commitment constraints (UCCs) of thermal generators. However, this requires a large number of integer decision variables that describes the unit commitment status. This leads to high computational complexity. Hence, UCCs are typically neglected during capacity expansion optimization. Within the second case study it is however demonstrated that neglecting UCCs within capacity expansion planning with VRE leads to an overestimation of the competitiveness of VRE and an underestimation of the need for flexible generation and storage technologies. This work shows that by a linear relaxation for UCCs system flexibility restrictions can be captured accurately during long-term capacity expansion optimization with comparably low additional computational effort.Item Open Access An investigation of hydrogen generation via steam reforming of liquid fuels(2017) Martin, Stefan; Thess, André (Prof. Dr. rer. nat. habil.)Im Rahmen der vorliegenden Arbeit wurde die Dampfreformierung von Biodiesel, Diesel und Bioethanol experimentell und theoretisch untersucht. Flüssige Brennstoffe zeichnen sich durch eine hohe volumetrische und gravimetrische Energiedichte und eine bereits vorhandene Verteilungsinfrastruktur aus. Die dezentrale Wasserstofferzeugung aus Flüssigbrennstoffen durch Reformierung kann mit dazu beitragen, die Marktdurchdringung von Brennstoffzellenfahrzeugen zu beschleunigen. Weitere Anwendungsmöglichkeiten bestehen im industriellen Sektor, etwa für metallurgische Prozesse oder für die Herstellung von Flachglas. Im Rahmen der experimentellen Arbeiten wurde ein vertieftes Verständnis der Katalysatordeaktivierung erzielt. Insbesondere wurden geeignete Betriebsbedingungen ermittelt, um die initiale Kohlenstoffbildung auf der Katalysatoroberfläche zu verhindern. Geringe Temperaturen und hohe Brennstoffmassenströme begünstigen eine Katalysatordeaktivierung durch Verkokung. Die Kohlenstoffbildungsneigung nimmt in der Reihenfolge Bioethanol < Biodiesel < Diesel zu. Durch entsprechende Wahl der Katalysatoreintrittstemperatur und des Brennstoffmassenstromes wurde für die jeweiligen Flüssigbrennstoffe ein stabiler Versuchsbetrieb (100 Stunden) nahe am chemischen Gleichgewicht nachgewiesen. Im Falle von fossilem Diesel hat sich gezeigt, dass die Langzeitstabilität der Dampfreformierung durch eine vorhergehende Entschwefelung des Kraftstoffs weiter verbessert werden kann. Die experimentellen Arbeiten werden ergänzt durch eine Simulationsstudie. Ziel der Untersuchung ist die verfahrenstechnische Optimierung einer Wasserstofferzeugungseinheit aus Biodiesel (50 Nm3/h H2) bestehend aus den Komponenten Dampfreformierung, Wassergasshift-Stufe, Druckwechseladsorption und Gas-Flüssig-Brenner. Neben einem positiven Druckeinfluss zeigen die Ergebnisse ein Optimum des molaren Dampf:Kohlenstoff-Verhältnisses bei 2,78. Aufbauend auf einem verfahrenstechnisch optimierten System wird ein wärmeintegrierter Wasserstoffgenerator entwickelt mit einem thermischen Systemwirkungsgrad von 75,6 % (bezogen auf den unteren Heizwert).Item Open Access Development and investigation of oxygen evolution reaction catalysts for proton exchange membrane electrolyzers(2018) Wang, Li; Friedrich, K. Andreas (Prof. Dr. rer. nat.)Hydrogen as an energy carrier is expected to play a vital role in the future renewable dominated energy system. One of the promising technologies to produce high purity H2 is the proton exchange membrane (PEM) electrolyzer. However, its large-scale commercialization is hindered by the high investment cost, even though the technology is becoming mature. One main cost contributor is the anode catalyst, which requires Ir-based materials with a high loading to overcome the high overpotential of oxygen evolution reaction (OER). Aiming to address the challenge to reduce the amount of Ir required in the anodes, several OER catalysts were developed in this work by adopting different strategies. On the one hand, Ir nanoparticles were supported on electro-conductive ceramic materials, including Magnéli phase Ti4O7 and SnO2:Sb aerogel, to increase the Ir utilization. Both Ir/Ti4O7 and Ir/SnO2:Sb aerogel catalysts show significantly improvement in Ir mass activity. In particular the latter one achieved the same performance compared to the unsupported counterpart while only containing ca. 30 wt.% Ir on the electrode. On the other hand, to increase the specific OER activity for each active site, one unsupported Ir-rich catalyst was derived from IrRuOx by electrochemical Ru leaching. After stabilization of Ru leaching, it demonstrates a 13-fold OER activity greater than state-of-the-art, rutile phase IrRuO2. The developed catalysts were physically characterized by X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS) and transmission electron microscopy (TEM). Subsequently, the materials were electrochemically evaluated by rotating disc electrode (RDE) technique, consisting of cyclic voltammetry (CV), linear scanning voltammetry (LSV) and chronopotentiometry. Besides, CO-stripping and Cu underpotential deposition (Cu-UPD) were employed to investigate their electrochemical surface area (ECSA). Advanced operando techniques were used to investigate the stabilization and catalysis mechanisms of the given catalysts. Near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) was applied on rutile phase RuO2 and IrRuO2. The results unveil that the abundant unstable Ru(OH)x formation on the electrode surface leads to the fast degradation of RuO2 while Ir prevents its formation in the case of IrRuO2 thereby stabilizes the electrode. Additionally, the amorphous IrOx and rutile phase IrO2 were comparatively studied by both NAP-XPS and soft X-ray adsorption near edge structure (XANES). A correlation between the concentration of anion OI- species and the OER activity on both Ir@IrOx and IrO2 electrodes was observed, suggesting the OER catalysis mechanism on Ir oxides is likely to involve anion rather than cation red-ox chemistry regardless of the oxide structure (amorphous vs. rutile). At the end, the above developed Ir-rich catalyst that shows promising performance from RDE measurements was tested in a PEM electrolyzer. It demonstrated an apparent higher OER activity compared to IrRuO2 and no cell potential increase was observed for ca. 400 h of electrolysis operation. To correlate the fundamental understanding and those acquirements from engineering perspective, a simulation model based on Butler-Volmer equation was established to extract the key kinetic parameters for the anode catalyst from the electrolyzer cell test. Ir-rich catalyst and IrRuO2 were implemented with the model, their exchange current density (i_oa) and charge transfer coefficient (α_an) were obtained, then the corresponding Tafel slopes were calculated, with which the OER catalysis mechanisms are elucidated accordingly.Item Open Access Entwicklung und Integration neuartiger Komponenten für Polymerelektrolytmembran- (PEM) Elektrolyseure(2018) Lettenmeier, Philipp; Friedrich, K. Andreas (Prof. Dr.)Item Open Access Hydrogen production by partial catalytic dehydrogenation of kerosene(2016) Pearson, Karolina; Thess, André (Prof. Dr. rer. nat.)Die Verknappung fossiler Energieträger erhöht den Bedarf an effizienten Technologien in der Luftfahrt. Für die Stromerzeugung an Bord eines Flugzeugs, während des Bodenbetriebs, können anstelle eines konventionellen Hilfstriebwerks alternativ Brennstoffzellensysteme eingesetzt werden. Der dafür notwendige Wasserstoff kann durch partielle katalytische Dehydrierung des an Bord verfügbaren Kerosins bereitgestellt werden. In dieser Arbeit werden zwei alternative Prozesskonzepte für die Wasserstofferzeugung aus Kerosin entwickelt und auf Ihren elektrischen Wirkungsgrad energetisch bewertet. Für diesen Zweck wird die partielle katalytische Dehydrierung von Kerosin detailliert experimentell untersucht und die Ergebnisse in die Modellierung der Prozesskonzepte eingebunden.Item Open Access Techno-ökonomische Prozessbewertung der Herstellung synthetischen Flugturbinentreibstoffes aus CO2 und H2(2016) König, Daniel H.; Thess, Andre (Prof. Dr. rer. nat.)Technologieverbesserung, Optimierung der Betriebsabläufe und effiziente Infrastrukturgestaltung tragen zur Reduzierung der Umweltwirkung des stetig wachsenden Flugverkehrs bei. Um jedoch die Ziele zur Emissionsminderung und Dekarbonisierung des Flugverkehrs zu erreichen sind alternative Treibstoffe notwendig. Die Konvertierung von H2 und CO2 in flüssigen Flugturbinentreibstoff mit dem Power-to-Liquid-Verfahren (PTL) stellt dabei einen möglichen Herstellungspfad dar, welcher in dieser Arbeit unter technischen und ökonomischen Gesichtspunkten analysiert und bewertet wird.Item Open Access Techno-ökonomische Optimierung eines Hochtemperatur-Latentwärmespeichers(2018) Hübner, Stefan; Thess, André (Prof. Dr. rer. nat.)Diese Arbeit beschreibt die technische und ökonomische Optimierung eines Hochtemperatur-Latentwärmespeichers, der z.B. in solarthermischen Kraftwerken mit Direktverdampfung zur Speicherung der Verdampfungsenthalpie des Wassers eingesetzt wird. Die Wärmeübertragung erfolgt über axial berippte Rohre, deren Form, Material und Anbindung in dieser Arbeit theoretisch optimiert wird, das Optimierungsergebnis wird anschließend mit sehr zufriedenstellenden Ergebnissen experimentell überprüft.Item Open Access Experimental investigation of Ca(OH)2 as thermochemical energy storage at process relevant boundary conditions(2017) Schmidt, Matthias; Thess, André (Prof. Dr. rer. nat. habil.)Die Reaktion von Calciumhydroxid zu Calciumoxid und Wasserdampf eignet sich aufgrund der bereits demonstrierten Reversibilität besonders gut zur Speicherung thermischer Energie. Zudem ist das Material kostengünstig, auf der ganzen Welt in großen Mengen verfügbar und bietet potentiell eine hohe erreichbare Speicherdichte. Da der Reaktionspartner Wasserdampf jedoch ca. 40 % der Energie des Speicherprozesses enthält, ist es für einen effizienten Speicherbetrieb zwingend notwendig den energetischen Aufwand für die Bereitstellung in den Anwendungsprozess zu integrieren. Dies führt wiederum dazu, dass das Reaktionssystem in einem sehr weiten Druck- und Temperaturbereich betrieben werden muss. Da bisher kaum experimentelle Erkenntnisse zum Betrieb unter solchen realen Prozessbedingungen vorhanden sind, werden diese Betriebsweisen im Rahmen der vorliegenden Arbeit untersucht. Dazu wurde ein Reaktionsbett für eine repräsentative Masse (2.4 kg) an Speichermaterial und einer generischen Geometrie entwickelt. Mit diesem experimentellen Aufbau im Labormaßstab wurde durch systematische Parametervariation erstmalig die thermische Leistungsfähigkeit des Reaktionssystems im gesamten prozessrelevanten Betriebsbereich untersucht. Als ein wesentliches Ergebnis der experimentellen Untersuchung konnten im gesamten Druckbereich technisch relevante thermische Be- und Entladeleistungen demonstriert werden. Jedoch zeigte sich auch, dass bei geringen Wasserdampfdrücken (1.4 - 50 kPa), die langsame Reaktionsgeschwindigkeit des Rohmaterials die erzielbaren Be- und Entladetemperaturen, teilweise einschränkt. Auf Basis der experimentellen Erkenntnisse wurde der Einsatz des Speichers in einem bestehenden thermischen Solarkraftwerk konzeptionell untersucht. Dabei zeigte sich, dass insbesondere dann eine hohe Speichereffizienz erzielt werden kann, wenn die für die Speicherentladung notwendige Wasserdampfbereitstellung thermisch in den stromerzeugenden Kraftwerksprozess integriert wird. Insgesamt eröffnen die in dieser Arbeit demonstrierten Betriebsweisen den effizienten Einsatz des Speichersystems für unterschiedlichste Anwendungen.Item Open Access Combined electricity and water production based on solar energy(2015) Moser, Massimo; Thess, André (Prof. Dr.)Several studies carried out at DLR such as [AQUA-CSP 2007], [MED-CSD 2010] and [MENAWATER 2011] have shown that the current water supply of several countries of the Middle East and North Africa (MENA) relies to a large extent on fossil groundwater extractions. Such extractions are characterized by continuously increasing energetic and economic efforts, which causes depletion of precious water resources and negative impact on the environment. The gap between water resources and water demand is likely to be sharpened by global changes such as population and economic growth and climate change. However, a series of technical approaches exist in order to mitigate water scarcity. Amongst others, water supply can be increased by means of unconventional solutions such as seawater desalination. Thereby, large part of the greenhouse gases emissions related to this process can be avoided by the introduction of renewable energy technologies such as concentrating solar power (CSP), photovoltaic (PV) and wind power. The main objective of this dissertation is the development of a flexible model for the integrated techno-economic assessment of seawater desalination plants using renewable energy. A number of simulation models have already been implemented for the design and the simulation of renewable plants or desalination units. However, so far no established tool exists for the simulation of such integrated systems. The simulation tool INSEL has been selected for the analysis. This commercially available tool combines a modular structure with simple handling and low computational effort. The core of the present doctoral thesis consists in the extension of the currently available INSEL library with new models for a number of desalination technologies, i.e. multi-effect distillation (MED) and reverse osmosis (RO) as well as CSP components such as solar field, thermal energy storage and power block. One of the focuses of this work has been the techno-economic evaluation and comparison of MED and RO plants. In the last few years RO has gained a dominant position in the global desalination market. This success is due to low capital cost, significant improvements in membranes (salt rejection rate, life time) and reduction of specific energy consumption. The optimal recovery ratio of RO plants, i.e. the ratio between produced drinking water and feed water, results from a compromise between minimization of investment cost, energy consumption and risk of membrane fouling. Feed water pre-treatment represents a challenging issue for RO plants. MED is characterized by relatively high investment cost, which also depends to a large extent on metal price, higher water consumption than RO but less demanding feed water pre-treatment. Heat cost for MED is a function of power supply technology, fossil fuel price and heating steam pressure. The optimal number of stages in a MED plant results from a trade-off between minimization of thermal energy requirements and maximization of plant efficiency. The INSEL library has been further extended by a number of CSP components, which include parabolic trough, linear Fresnel and central receiver. The solar field models base on a steady-state thermal energy balance between incoming radiation, geometrical and optical losses, heat gains of the heat transfer fluid (HTF) and heat losses to the environment. The model takes into account layout and losses of the HTF system. In addition, transient effects are considered by means of a simplified approach. This is an important improvement which is not considered by the majority of the existing system analysis simulation tools. A two-tank molten salt storage has been selected as reference thermal energy storage, while the power block consists of a detailed thermodynamic model of a conventional Rankine cycle. The INSEL CSP models have been developed with information from the DLR groups of the Institute of Solar Research and of the Department of Thermal Process Technology. A potential application of the implemented INSEL models is shown in a final case study, which assesses the feasibility of combined power and water production plants. The analysis has been carried out for Marsa Alam, a remote touristic location in the South-East of Egypt. Under these assumptions RO provides slightly lower water production cost than MED; however, due to its robustness and simplicity of operation, MED could still be a competitive option wherever feed water pre-treatment is particularly challenging and RO plants would be prone to frequent fouling problems (e.g. Arabian Gulf). In addition, one of the main findings is that moving in due time towards an electricity supply system based on a mix of renewable and conventional technologies is not only convenient in the case of high fossil fuel price, but it also includes strategic advantages such as the reduction of the dependence on scarce resources and the stability of the supply cost.Item Open Access The value of concentrating solar power for a sustainable electricity supply in Europe, Middle East and North Africa(2018) Hess, Denis; Thess, André (Prof. Dr.)Dispatchable solar power from concentrating solar thermal power plants (CSP) combined with thermal energy storage and co-firing option can provide energy according to demand. A transfer of such electricity from solar thermal power plants in desert regions to distant consumer centres may therefore complement regional or domestic energies. The research question of the value of this transfer was already analysed in qualitative studies that found out a high potential of this idea. However, a detailed energy system modelling showing the value of concentrating solar power plants from Middle East and North Africa (MENA) for Europe (EU) was not yet done. This thesis closes the scientific knowledge gap applying an energy system model with a least-cost approach and detailed scenario analysis for the year 2050. The thesis describes the effects of including and excluding a transfer of CSP from MENA to EU. The transfer-system consists of a power plant and a high voltage direct current transmission (HVDC) and is therefore called CSP-HVDC (concentrating solar power - high voltage direct current) power plant. The techno-economic assumptions for this composed technology are strictly chosen to avoid its overestimation. The assessment of CSP-HVDC considers energy system evaluation criteria. These multi-criteria reveal the impact of CSP-HVDC on energy infrastructure, operational behaviour, cost and emission of the energy system. To evaluate national grid expansion, a new grid methodology is introduced as composed of transmission and distribution grid. This new model reduces complexity of the grid and analyses grid expansion considering different power plant park portfolios. As a result CSP-HVDC-application proves to require a future national grid expansion to a far lesser extent than a system with high shares of wind and photovoltaic energy. This is substantiated by a validation with a high-resolution grid model. Integrating CSP-HVDC into the national grid brings about reduction of grid stress due to lower capacity peaks in the transmission lines of the grid. Cost sensitivity analyses indicate the cost uncertainty of the energy system. The examination of system cost uncertainty shows that an appropriate share of dispatchable energy including CSP-HVDC generates a minimal system cost uncertainty. Analysis of cost relations of CSP-HVDC to all other used technologies (e.g. nuclear and carbon capture and storage technologies) point out how high the probability of an integration of CSP-HVDC is. Inside the EUMENA region (Europe, Middle East and North Africa) various regions are subdivided. Due to different characteristics of regional energy systems, it depends on the model region in EU how probable it is that CSP-HVDC is integrated. For some EU regions up to 66% integration probability can be achieved. The result of multi-criteria evaluation shows that power plant capacity, electrical storage expansion, and electrical curtailment cause a lower impact when CSP-HVDC is used to supplement the energy portfolio. However, the strict model conditions lead to a higher needed transmission infrastructure altogether, when CSP-HVDC is applied in high shares because of its long transfer distance. Lower CSP-HVDC shares can reduce the total transmission infrastructure of single regions. Another result demonstrates that system cost does not play the essential role in quantifying the value of CSP-HVDC because system cost differences may be small. The right share of CSP-HVDC offers a higher degree of freedom, more options and compromises for the design of a low carbon energy system under the included evaluation criteria mentioned above. Thus, a technologically diverse energy system with low carbon emission rather benefits from CSP-HVDC.