Heißwasserextrahierbarer Kohlenstoff und Bodenatmung als Parameter zur Abschätzung der potentiellen Kohlenstofffreisetzung aus organischen Böden

Durch Ihre hohen Gehalte an Kohlenstoff (C) und organischer Bodensubstanz (OBS) und besitzen Moorböden eine herausragende Rolle im globalen Kohlenstoffkreislauf. Bei unsachgemäßer Nutzung setzen diese organischen Böden besonders hohe Mengen an C, z.B in Form von CO2 frei. Der labile und aktive Anteil der OBS, der potentiell besonders leicht freigesetzt werden kann, lässt sich allgemein mit dem Parameter heißwasserextrahierbarer Kohlenstoff (Chwe) abschätzen, da diese Fraktion große Mengen leicht umsetzbarer Bestandteile wie etwa hohe Anteile an mikrobieller Biomasse, Einfachzucker oder Ligninmonomere enthält. Bis jetzt ist aber unklar, wie gut sich dieser Parameter zur Ableitung der potentiellen C-Freisetzung aus Moorböden eignet. Für verschiedene Mineralböden konnten bereits enge Korrelationen zwischen dem Chwe und der jeweiligen Bodentamung aufgezeigt werden. Studien zur Beziehung der CO2-Freisetzung und dem Parameter Chwe speziell für organische Böden fehlen bisher. Ziel der vorliegenden Untersuchung war es deshalb, diese möglichen Korrelationen für organische Böden zu untersuchen. Dazu wurde der Chwe an über 50 unterschiedlichen Moorbodensubstraten ermittelt. Hier wurde eine Extraktionsmethode angewandt, welche speziell an die hohen Anteile an OBS angepasst wurde. Daneben wurde die jeweilige Bodenatmung mittels Inkubationsversuchen im Labor gemessen und mit dem Gehalt an Chwe verglichen. Die bisherigen Ergebnisse zeigen mittlere bis hohe Korrelationen zwischen der Bodenatmung und dem Chwe, so dass davon auszugehen ist, dass der Chwe zur Abschätzung einer potentiellen C-Freisetzung auch für organische Böden herangezogen werden kann, um damit die Empfindlichkeit gegenüber Kohlenstoffverlusten beschreiben zu können. Die gewonnenen Daten sollten allerdings durch zusätzliche Untersuchungen, vor allem an bisher nicht genügend berücksichtigten Moorbodensubstraten, weiter überprüft werden

Heterogeneous integration of contact-printed semiconductor nanowires for high-performance devices on large areas

In this work, we have developed a contact-printing system to efficiently transfer the bottom-up and top-down semiconductor nanowires (NWs), preserving their as-grown features with a good control over their electronic properties. In the close-loop configuration, the printing system is controlled with parameters such as contact pressure and sliding speed/stroke. Combined with the dry pre-treatment of the receiver substrate, the system prints electronic layers with high NW density (7 NWs/μm for bottom-up ZnO and 3 NWs/μm for top-down Si NWs), NW transfer yield and reproducibility. We observed compactly packed (~115 nm average diameters of NWs, with NW-to-NW spacing ~165 nm) and well-aligned NWs (90% with respect to the printing direction). We have theoretically and experimentally analysed the role of contact force on NW print dynamics to investigate the heterogeneous integration of ZnO and Si NWs over pre-selected areas. Moreover, the contact-printing system was used to fabricate ZnO and Si NW-based ultraviolet (UV) photodetectors (PDs) with Wheatstone bridge (WB) configuration on rigid and flexible substrates. The UV PDs based on the printed ensemble of NWs demonstrate high efficiency, a high photocurrent to dark current ratio (>104) and reduced thermal variations as a result of inherent self-compensation of WB arrangement. Due to statistically lesser dimensional variations in the ensemble of NWs, the UV PDs made from them have exhibited uniform response

Modification of Fine-Grained Powder to Facilitate a Moving Reaction Bed for Thermochemical Energy Storage

Dispatchable power generation is one important advantage of concentrated solar power (CSP) plants, if high-performance thermal energy storages are included. Thermochemical energy storages can make a decisive contribution because of their low material cost, high energy densities and heat transformation possibility. For an appropriate temperature range (~500°C) the reversible gas-solid reaction of calcium oxide with water vapor to calcium hydroxide is investigated: CaO(s) + H2O(g) ⇌ Ca(OH)2 + ΔHR In order to release a constant power level over a long period of time (night cycle), it is necessary to separate the power from the capacity of the storage unit. For this purpose, the gravity-induced flow of the reaction material through small reactor geometry is highly promising. However, due to its cohesive characteristic, the material tends to agglomerate during cycling and shows poor flow behavior leading to plugging within the reactor. Approaches of the bulk industries have demonstrated improvements of flowability of organic materials by adding nanoparticles to bulk material in order to decrease the Van-der-Waals forces. However, this enhancement is not established for high temperatures, reacting conditions and inorganic materials. In this contribution, the approach to optimize the flow behavior of thermochemical storage material and the prevention of agglomeration by addition of nanomaterial is presented. Furthermore, miscellaneous advantages of the gravity-induced moving bed reactor are given

Untersuchung und Optimierung von Fließ- und Durchströmungsverhalten von Schüttungen zur thermochemischen Wärmespeicherung

Am Institut für Technische Thermodynamik des DLR in Stuttgart werden neuartige Wärmespeicherkonzepte auf Basis von reversiblen Gas-Feststoff-Reaktionen untersucht und entwickelt. Ein geeignetes Reaktionssystem für den Hochtemperaturbereich stellt dabei die Reaktion von Calciumoxid (CaO) mit Wasserdampf (H2O) zu Calciumhydroxid (Ca(OH)2) dar. Anwendungsbedingt ist es notwendig, das feinkörnige Reaktionsmaterial zu optimieren, um trotz des kohäsiven Charakters der Schüttung eine Förderung durch schmale Reaktorgeometrien zu ermöglichen. Aufgrund der reaktionsbedingten Volumenänderung der Partikeln ist ein Pelletieren nicht möglich. Daher wird im Rahmen dieses Beitrags die Minimierung der Van-der-Waals Kräfte durch das Einbringen von Nanopartikeln untersucht. In der Schüttguttechnik ist das Prinzip der Erhöhung der Oberflächenrauhigkeit von Partikeln durch Einbringung von Nanopartikeln bekannt [1]. Für die thermochemische Wärmespeicherung stellen sich jedoch besondere Herausforderungen aufgrund der hohen Temperaturen (~500°C), der anorganische Materialien sowie der Volumen- und Oberflächenänderung des Materials während der chemischen Reaktion. Es wird ein Verfahren vorgestellt, welches unter den genannten Bedingungen eine Bewegung des Reaktionsbetts durch Verbesserung der Fließfähigkeit und Vermeidung der Agglomeration ermöglicht. Darüber hinaus werden die Ergebnisse im Kontext der Entwicklung zukünftiger kostengünstiger Wärmespeicher diskutiert

Improving Powder Bed Properties for Thermochemical Storage by Adding Nanoparticles

Thermochemical storage offers interesting potential to store thermal energy, especially in the field of industrial waste heat utilization or for concentrated solar power (CSP) plants. However, at present the development of thermochemical storage technology is in its initial stage with investigations mainly on material aspects or small lab-scale systems. With regard to its thermodynamics and kinetics, it has been shown that the CaO/Ca(OH) 2 reaction system is suitable for thermochemical heat storage at a temperature range of 400–600 C. However, the behaviour in a small lab-scale system was mainly dominated by heat and mass transfer limitations originating from the small particle size and changes in the bulk properties. It is shown that by the addition of small amounts of additives like nano- particles of SiO 2 (Aerosil ), the bulk properties can be stabilized and consequently the cycling stability ensured. In addition, channelling effects can be minimized resulting in a more homogeneous flow through the reaction bed improving the overall reaction behaviour of the thermochemical storage

SiC power module loss reduction by PWM gate drive patterns and impedance-optimized gate drive voltages

This paper presents a novel procedure to determine the internal gate-source voltage inside a multi-chip power module using the example of a SiC half bridge module. Based on the lumped elements of the gate circuit calculated by a quasi-static electromagnetic simulation, each field-effect transistor is represented by a single, voltage dependent capacitor. The procedure is validated by clamped inductive switching measurements of a SiC power module. Moreover, it is applied to determine the maximum permissible gate-source voltage range in compliance with the manufacturer's voltage rating for a given driver-module combination. In this context a significant extension of the gate drive voltage range and thus an increase of efficiency using impedance specific PWM patterns is demonstrated

Modified Materials for Thermochemical Energy Storage

Thermal energy storage plays an important role for utilizing Concentrated Solar Power (CSP) plants as dispatchable renewable electricity source. Thereby, thermochemical energy storages can make a decisive contribution due to their high energy density in combination with low material cost. A suitable reaction system in appropriate temperature range (~500°C) is the reversible gas-solid reaction of calcium oxide with water vapor to calcium hydroxide: CaO(s) + H2O(g) ⇌ Ca(OH)2(s) + ΔHR For application in CSP plants two main aspects are focused. On the one hand the separation of power and capacity is reasonable, if a constant power over a long period (e.g. night cycle) is required. In this case, the material which is stored in reservoirs, defining the storage capacity, has to be moved through a reactor providing the desired power level. To permit the flow of the cohesive material agglomeration effects have to be prevented and flow behavior needs to be improved. A systematical analysis of the improvement of powder bed properties by addition of nanomaterial is presented. It will be shown that agglomeration can be already prevented by using only small amounts of nanoparticles and the flow behavior can be clearly improved. On the other hand the working temperatures of current CSP plants range from ~250°C to ~550°C, whereas the minimum temperature of the CaO/Ca(OH)2 reaction system is ~400°C. In an approach to decrease the equilibrium temperature of hydration-/dehydration reaction several modifications on the material, as for example doping with other metal oxides and addition of catalysts were investigated to probe the possibility and range of an adjustment. A survey was made to proof if the characteristic features of the system CaO / Ca(OH)2 are also assignable to other alkaline earth metal oxides / hydroxides

Non-equilibrium thermochemical heat storage in porous media: Part 1 – Conceptual model

Abstract Thermochemical energy storage can play an important role in the establishment of a reliable renewable energy supply and can increase the efficiency of industrial processes. The application of directly permeated reactive beds leads to strongly coupled mass and heat transport processes that also determine reaction kinetics. To advance this technology beyond the laboratory stage requires a thorough theoretical understanding of the multiphysics phenomena and their quantification on a scale relevant to engineering analyses. Here, the theoretical derivation of a macroscopic model for multicomponent compressible gas flow through a porous solid is presented along with its finite element implementation where solid–gas reactions occur and both phases have individual temperature fields. The model is embedded in the Theory of Porous Media and the derivation is based on the evaluation of the Clausius–Duhem inequality. Special emphasis is placed on the interphase coupling via mass, momentum and energy interaction terms and their effects are partially illustrated using numerical examples. Novel features of the implementation of the described model are verified via comparisons to analytical solutions. The specification, validation and application of the full model to a calcium hydroxide/calcium oxide based thermochemical storage system are the subject of part 2 of this study