1,721,053 research outputs found
Conceptual Design of Operation Strategies for Hybrid Electric Aircraft
Full text linkAmbitious targets to reduce emissions caused by aviation in the light of an expected ongoing rise of the air transport demand in the future drive the research of propulsion systems with lower CO2 emissions. Regional hybrid electric aircraft (HEA) powered by conventional gas turbines and battery powered electric motors are investigated to test hybrid propulsion operation strategies. Especially the role of the battery within environmentally friendly concepts with significantly reduced carbon footprint is analyzed. Thus, a new simulation approach for HEA is introduced. The main findings underline the importance of choosing the right power-to-energy-ratio of a battery according to the flight mission. The gravimetric energy and power density of the electric storages determine the technologically feasibility of hybrid concepts. Cost competitive HEA configurations are found, but do not promise the targeted CO2 emission savings, when the well-to-wheel system is regarded with its actual costs. Sensitivity studies are used to determine external levers that favor the profitability of HEA
Aspekte der Transformation von Fernwärmesystemen: Verteilung, Quellen und Senken
Full text linkDas Pariser Abkommen von 2016 verpflichtet 196 Vertragsparteien, die globale Erwärmung auf deutlich unter 2 °C zu begrenzen und bis Mitte des 21. Jahrhunderts Treibhausgasneutralität zu erreichen. Deutschland verabschiedete 2019 das Bundesklimaschutzgesetz mit dem Ziel der Treibhausgasneutralität bis 2045.
2022 betrug der Anteil Erneuerbarer Energien am deutschen Bruttoendenergieverbrauch 20 %, im Stromsektor 46 % und in der Wärmebereitstellung nur 17 %, obwohl dort 70 % der Primärenergie genutzt werden. Fernwärmesysteme könnten durch ihre Speicher- und Verteilfunktion erneuerbare Energien integrieren, jedoch liegt ihr Anteil dort nur bei 18 %. Die Umstellung dieser Systeme auf Klimaneutralität ist durch das Kohleverstromungsbeendigungsgesetz dringender geworden. Die fossile Wärmeerzeugungsstruktur und hohen Netztemperaturen müssen den Anforderungen Erneuerbarer Energien angepasst werden.
Diese Arbeit untersucht die Transformation von Fernwärmesystemen, mit besonderem Fokus auf Wärmequellen und die Wärmeverteilung und nutzt dabei veröffentlichte Daten zu Fernwärmesystemen, Gebäuden, industriellen Betrieben sowie statistischen Daten zur Bevölkerung und Wärmeversorgung. Sie gibt eine Übersicht über Fernwärmesysteme in Deutschland und bewertet das Potenzial solarthermischer Wärme, industrieller Abwärme und Abwasserwärme. Außerdem untersucht sie die thermo-hydraulischen Auswirkungen der Absenkung der Netztemperaturen bei gleichzeitiger Integration Erneuerbarer Energien.
Ergebnisse zeigen, dass Fernwärmesysteme nach Netzlängen klassifiziert werden können und ein erhebliches Potenzial zur Integration Erneuerbarer Energien besteht. Die Absenkung der Netztemperaturen kann ohne größere Einschränkungen erfolgen. Engpässe lassen sich durch dezentrale Wärmequellen reduzieren oder durch zusätzliche Druckerhöhung beseitigen
ESTRAM: mathematical model documentation
No full text<p>This documentation outlines the nomenclature and equations for energy system optimization models developed within the ESTRAM framework by a research group of the Leibniz University Hannover and the Institute for Solar Energy Research Hamelin (ISFH).</p>
Degradation of proton exchange membrane (PEM) water electrolysis cells: Looking beyond the cell voltage increase
Full text linkThe degradation of proton exchange membrane water electrolysis cells is usually measured in a temporal increase of the cell voltage. Although this is sufficient to evaluate the stability of a system, it is less suitable for targeted material development. Thus, an overpotential-specific and temporally resolved electrochemical characterization protocol is proposed. In this the ohmic overpotential is determined with high frequency resistance measurements. These are also used in combination with polarization curves to distinguish between the kinetic and mass transport overpotentials and to determine kinetic key parameters, according to the Butler-Volmer and transition state theory. Complementary electrochemical impedance spectroscopy measurements further unravel the individual resistances. On this basis, the following statements can already be issued. The major share of the measured cell voltage increase, i.e. degradation, is of apparent nature as it is recovered once lower potentials are applied. It is suggested that this is due to changes in the oxidation states of the iridium-based catalyst. Real degradation occurs in the ohmic and mass transport overpotential mainly at higher current densities and longer operating times. The increasing kinetic overpotential with increasing operating time is primarily potential-driven. Interestingly, both the Tafel slope and the apparent exchange current density slightly increase over time. © 2019 The Author(s). Published by ECS
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe 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
Transformation der Energiesysteme : Ein kompakter Überblick zur Energieforschung an der Leibniz Universität Hannover
Full text link[no abstract available
ESTRAM: data documentation
No full text<p>This documentation offers an overview of data employed in energy system optimization models built with the ESTRAM framework by a research group of the Leibniz University Hannover and the Institute for Solar Energy Research Hamelin (ISFH). It is important to note that specific models may utilize distinct data as indicated in their respective studies.</p>
- …
