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Variability analysis of renewable power generation in complex terrain and the contribution of the spatio-temporal synergies for a resilient power supply. Methodology development and Ecuador case study
No full textSouth American countries highly rely on hydropower for their electricity supply and the deployment of remaining untapped hydropower potential is the cornerstone of national generation capacity expansion plans. However, given the dependency of hydropower on water availability, risks associated with weather and climate variability could jeopardize the security of electricity supply of these countries.
In the case of Ecuador, according to government plans, fossil fuel thermal power plants will continue playing an important role in meeting the demand during the dry periods. A more sustainable and resilient strategy would be to diversify the power mix focusing on exploiting the complementarities between hydropower and other variable renewable energies, such as wind and solar. However, the deployment of these technologies in the country is still at a very early stage and there are some challenges to be tackled.
Due to the varying nature of solar and wind resources, the optimal planning and deployment of solar and wind power that could potentially complement hydropower requires detailed knowledge of the spatial and temporal variability of the resources. Unfortunately, long-term, high-quality solar irradiance and wind speed measurements are generally scarce and sparsely distributed, challenging the characterization of solar and wind resources at a country level. The complex climatic characteristics and topography of Ecuador represent another challenge to better understand the spatio-temporal dynamics between renewable resources, as well as the potential synergies of solar and wind power generation to compensate hydropower during the dry periods.
The main research goal of this dissertation is to develop tools and data to support the optimal planning of a more sustainable and resilient power system in Ecuador by systematically investigating the spatio-temporal variabilities and synergies of renewable resources in the complex terrain of Ecuador. For this purpose, climate data, machine learning techniques, and power system modeling tools are used.
The lack of solar and wind resource data is addressed by processing satellite-derived solar irradiance data and by using numerical weather prediction models to simulate wind resources. The generated meteorological datasets for Ecuador have a temporal resolution of one hour and a spatial resolution of 3 x 3 km. They comprise 21 years (1998–2018) of solar irradiance data and 14 years (2005–2018) of bias-corrected wind speed and wind direction data at a turbine hub heigh of 80 m.
A novel methodology to characterize the spatio-temporal variability of gridded solar and wind resource datasets is proposed and demonstrated. Spatial functional data analysis (sFDA) is used to identify spatial subregions with similar intra-annual variability patterns of solar radiation and wind speed. Finally, the ability of geographically-dispersed photovoltaic (PV) and wind power systems to reduce the power output variability and provide reliable power generation is assessed by using power system performance models.
This dissertation provides the first comprehensive spatio-temporal characterization of solar radiation and wind speed in Ecuador. The identified subregions from the sFDA regionalization approach are the basis for the assessment of the complementarity between solar and wind to water resources of existing and planned hydropower plants. One of most important finding is that solar and wind resources have a strong spatio-temporal complementary behavior with water resources from both the Amazon and Pacific basins. This demonstrates that the seasonal variability of hydropower can be compensated by geographically-dispersed PV and wind power systems.
Another important finding is that the joint operation of PV and wind power systems from different subregions reduces the intrinsic variability of each resource. Wind power at a high-resource site (52% capacity factor) paired with PV from different subregions can provide the highest level of firm capacity (up to 5.5% of the combined capacity) for 87.5% of the time in a year. Furthermore, wind power from subregions with high resources stabilizes PV power output at diurnal timescales during the windy months (June–September), suggesting that both technologies could serve as baseload during this period, thus reducing the requirements for energy storage. The identified operational benefits of the spatio-temporal synergies among renewable power generation may provide economic incentives to increase the participation of PV and wind power in the Ecuadorian power mix.
These findings demonstrate that solar and wind power can play an important role in shaping a more sustainable and resilient power system in the country. The insights gained in this dissertation, as well as the provided data and tools, will support power sector planners and decision-makers in the development of strategies for the optimal expansion of solar and wind power technologies to complement hydropower and to reduce the dependencies on fossil fuel thermal power.
This dissertation contributes to the ongoing discussion on renewable energy complementarities in the region and the proposed methodology can be transferred to other countries to support optimal capacity expansion planning. Furthermore, the methods and data developed in this dissertation provide the groundwork for further research into energy system modeling
Modeling and utilizing a vanadium redox flow battery for easier grid and market integration of wind power
No full textUltrathin Multiple Quantum Wells Solar Cell Based on Silicon/Germanium Nanostructures
No full textIn this work, we report a proof-of-concept for an ultrathin multiple quantum wells (MQW) solar cell based on silicon (barrier)/germanium (QW) heterostructures integrated as a subwavelength photonic resonator below 50 nm. The multiplication of the QWs number in a periodic configuration of 6 times 2.5 nm-thick QWs allows the enhancement of photocurrent while maintaining high voltage and fill factor. Compared to a single QW (SQW) nanoabsorber of 20 nm, the implementation of MQW architecture with similar total thickness results in a significant enhancement of the photoconversion efficiency from 3 % to 5%, yielding a relative improvement of about 65%. Based on optoelectronic modeling of the optical field distribution and the electronic structure at the QW/barrier heterojunctions, the output characteristics of the subwavelength MQW solar cell device are analyzed
Modellierung und Verwendung einer Vanadium Redox Flow Batterie für bessere Netz- und Strommarktintegration der Windenergie
No full textPower grid and market integration of wind energy is a challenge due to the fluctuating and intermittent power output resulting from the variable nature of wind resource. Energy storage is a promising alternative for effective grid integration of renewable energy. One storage technology which is under the spotlight in the recent years is the vanadium redox flow battery (VRFB) which could have certain advantages when utilized at large-scale grid connected applications. In this study, a megawatt scale VRFB was modeled based on experimental data with a kilowatt scale real life unit. The dependence of the overall system efficiency on the state of charge and power was determined. By using the model, optimal number of modules for certain power levels during charging and discharging operations were estimated for megawatt scale operations. In order to evaluate the power grid integration of wind power at a single wind farm level, a second simulation model which combines the megawatt scale VRFB model and a medium sized (10 MW) wind farm was developed and the battery was utilized to compensate for the deviations resulting from the forecast errors in an electricity market bidding structure. Using an existing electricity market model based on deviation penalties and penalty multipliers, economics of the system were evaluated by determining the payback periods for a dedicated VRFB installation at this medium sized, single wind farm level
A Smart Charging Assistant for Electric Vehicles Considering Battery Degradation, Power Grid and User Constraints
Get PDFDer Anstieg intermittierender Stromerzeugung aus erneuerbaren Energiequellen erschwert zunehmend einen effizienten und zuverlässigen Betrieb der Versorgungsnetze. Gleichzeitig steigt die Zahl der Elektrofahrzeuge, die zum Aufladen erhebliche Mengen an elektrischer Energie benötigen, rapide an. Energie- und Mobilitätssektor sind somit unweigerlich miteinander verbunden, was zur Folge hat, dass zuverlässige Elektromobilität von einer robusten Stromversorgung abhängt. Darüber hinaus empfinden Fahrzeugnutzer ihre individuelle Mobilität als eingeschränkt, da Elektrofahrzeuge im Vergleich zu Fahrzeugen mit Verbrennungsmotor derzeit eine geringere Reichweite aufweisen und mehr Zeit zum Aufladen benötigen.
In der vorliegenden Arbeit wird daher ein neuartiges Konzept sowie eine Softwareanwendung (Ladeassistent) vorgestellt, die den Nutzer beim Laden seines Elektrofahrzeuges unterstützt und dabei die Interessen aller beteiligten Akteure berücksichtigt. Dafür werden zunächst Gestaltungsmerkmale möglicher Softwarearchitekturen verglichen, um eine geeignete Struktur von Modulen und deren Verknüpfung zu definieren. Anschließend werden anhand realer Daten sowohl Energieverbrauchs- als auch Batteriemodelle entwickelt, verbessert und validiert, welche die Fahr- und Ladeeigenschaften von Elektrofahrzeugen abbilden. Die wichtigsten Beiträge dieser Arbeit resultieren aus der Entwicklung und Validierung der folgenden drei Kernkomponenten des Ladeassistenten.
Als Erstes wird das individuelle Mobilitätsverhalten der Nutzer modelliert und anhand von aufgezeichneten und halbsynthetischen Fahrdaten von Elektrofahrzeugen ausgewertet. Insbesondere wird ein neuartiger, zweistufiger Clustering-Algorithmus entwickelt, um häufig besuchte Orte der Nutzer zu ermitteln. Anschließend werden Ensembles von Random-Forest-Modellen verwendet, um die nächsten Aufenthaltsorte und die dort typischen Parkzeiten vorherzusagen.
Als Zweites wird gemischt-ganzzahlige stochastische Optimierung angewandt, um Ladestopps in einem zukünftigen Zeithorizont möglichst komfortabel und kostengünstig zu planen. Dabei wird ein graphenbasierter Algorithmus eingesetzt, um den Energiebedarf und die Eintrittswahrscheinlichkeit von Mobilitätsszenarien eines Elektrofahrzeugnutzers zu quantifizieren. Zur Validierung werden zwei alternative Ladestrategien definiert und mit dem vorgeschlagenen System verglichen.
Als Drittes wird ein nichtlineares Optimierungsschema entwickelt, um vorhandene Zeit- und Energieflexibilität in Ladevorgängen von Elektrofahrzeugen zu nutzen. Die Integration eines detaillierten Batteriemodells ermöglicht eine genaue Quantifizierung der Kosteneinsparungen aufgrund einer geringeren Batteriealterung und dynamischer Stromtarife. Anhand von Daten aus realen Ladevorgängen von Elektrofahrzeugen können Einflüsse auf die Rentabilität von Vehicle-to-Grid-Anwendungen herausgearbeitet werden. Aus der Umsetzung des vorgestellten Ansatzes in einer realistischen Umgebung geht ein Architekturentwurf und ein Kommunikationskonzept für optimierungsbasierte intelligente Ladesysteme hervor. Dabei werden weitere Herausforderungen im Zusammenhang mit standardisierter Ladekommunikation, Eingriffen der Energieversorger und Nutzerakzeptanz aufgedeckt
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Halbleiter-Isolator-Halbleiter-Heteroübergang mit Atomlagenabscheidung als Schottky-Tunnel-Solarzelle auf nanostrukturiertem Silizium
Get PDFDie vorliegende Arbeit befasste sich grundlegend mit der Herstellung und Untersuchung der Wirkungsweise einer Solarzelle der 3. Generation. Dabei wird eine nasschemisch geätzte Silizium-Nanostruktur zur Steigerung der Absorption genutzt. Statt eines klassischen p-n-Übergangs dient ein mit Atomlagenabscheidung (engl. ‚Atomic Layer Deposition‘, ALD) hergestellter Halbleiter-Isolator-Halbleiter-Heteroübergang (engl. ‚Semiconductor-Insulator-Semiconductor‘, SIS) zur Ladungstrennung. Dieser setzt sich aus Silizium, einer dünnen (< 3 nm) dielektrischen Isolatorschicht und einem transparenten leitfähigen Oxid (TCO) aus aluminiumdotierten Zinkoxid (AZO) zusammen.
Zunächst wird die Wirkungsweise des SIS-Überganges auf einer polierten Siliziumoberfläche durch Modifikationen des ALD-Prozesses, die Verwendung verschiedener Dotierungen des Siliziums und unterschiedlicher Dielektrika (Aluminiumoxid (Al2O3), Hafniumoxid (HfO2), Titanoxid (TiO2)) als Material der Isolatorschicht untersucht. Hier wird aufgezeigt, dass der SIS-Übergang maßgeblich durch die Grenzflächenbildung, fixierte Ladungen und die Qualität der Isolatorschicht beeinflusst wird. Anschließend befasst sich die Arbeit mit der Wirkung einer nanostrukturierten Siliziumoberfläche unter Anpassung der Morphologie und Strukturtiefe auf den SIS-Übergang. Übersteigt dabei die Strukturtiefe 2 µm, führen Rekombinationen aufgrund einer inhomogenen Beschichtung zu starken Leistungsverlusten der Solarzelle. Durch die strukturierte Oberfläche konnte neben einer erhöhten Absorption (> 90%) die Ladungsträgersammlung verbessert werden. Dies führt zu einer Steigerung des Wirkungsgrades um den Faktor 2,4 im Vergleich zu einer polierten Oberfläche
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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