Energie-forschungszentrum Niedersachsen (EFZN)

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Bockelmann M.,Clausthal University of Technology | Bockelmann M.,Energie-forschungszentrum Niedersachsen (EFZN) | Reining L.,Clausthal University of Technology | Reining L.,Energie-forschungszentrum Niedersachsen (EFZN) | And 4 more authors.
Electrochimica Acta | Year: 2017

Passivation of zinc electrodes caused by formation of oxy-products with low electrical conductivity is one of the most important phenomena which limits the current density during discharge and contributes to the poor cyclability of zinc-based batteries. The present review critically discusses the existing experimental findings related to passivation of zinc in alkaline electrolyte as well as the state of the mathematical modelling of this process. Despite numerous investigations over several decades, no general consensus about the underlying mechanisms and the strategies to prevent passivation could be achieved. Main reason for this uncertainty is that different processes can lead to passivation. On the one hand, precipitation of so-called type 1 films occurs above a critical zincate concentration while on the other hand the direct oxidation of the zinc surface takes place below a certain potential, leading to type 2 passive films. As long as formed films remain porous, passivation does not necessarily lead to complete breakdown of the electrode activity. It is therefore recommended to conduct further measurements employing a combination of techniques in which both the potential and the concentration of zincates at the electrode surface can be precisely controlled. It is also proposed to carry out systematic studies on the influence of temperature, electrolyte flow, and electrolyte composition – including additives – on the passivation time and the structure of the passive film. Finally, the purity and the structure of the zinc electrode may also have an influence on the passivation processes and should be considered during future studies. © 2017 Elsevier Ltd

Kaiser F.,Energie-forschungszentrum Niedersachsen (EFZN) | Busch W.,Institute For Geotechnik Und Markscheidewesen
WasserWirtschaft | Year: 2015

There are several new pumped hydro energy storage (PHES) schemes such as 'Gravity Power', 'Powertower' or the 'Lageenergie' storage concept. They rely on the principle of a hydraulic piston using the proven technology of conventional PHES plants. This category of so called 'piston pumped hydro energy storage' plants (PiPS) is in discussion for several years now. Yet, no commercial plants have been realized so far and only few articles address this topic on a scientific base. Therefore this paper tempts to describe the fundamental principles to make this technology comparable to the proven conventional PHES concept.

Hou Z.,Clausthal University of Technology | Hou Z.,Energie-forschungszentrum Niedersachsen (EFZN) | Hou Z.,University of Sichuan | Gou Y.,Energie-forschungszentrum Niedersachsen (EFZN) | And 5 more authors.
Environmental Earth Sciences | Year: 2012

The objective of this paper was to investigate the THM-coupled responses of the storage formation and caprock, induced by gas production, CO 2-EGR (enhanced gas recovery), and CO 2-storage. A generic 3D planer model (20,000 × 3,000 × 100 m, consisting of 1,200 m overburden, 100 m caprock, 200 m gas reservoir, and 1,500 m base rock) is adopted for the simulation process using the integrated code TOUGH2/EOS7C-FLAC3D and the multi-purpose simulator OpenGeoSys. Both simulators agree that the CO 2-EGR phase under a balanced injection rate (31,500 tons/year) will cause almost no change in the reservoir pressure. The gas recovery rate increases 1. 4 % in the 5-year CO 2-EGR phase, and a better EGR effect could be achieved by increasing the distance between injection and production wells (e. g., 5. 83 % for 5 km distance, instead of 1. 2 km in this study). Under the considered conditions there is no evidence of plastic deformation and both reservoir and caprock behave elastically at all operation stages. The stress path could be predicted analytically and the results show that the isotropic and extensional stress regime will switch to the compressional stress regime, when the pore pressure rises to a specific level. Both simulators agree regarding modification of the reservoir stress state. With further CO 2-injection tension failure in reservoir could occur, but shear failure will never happen under these conditions. Using TOUGH-FLAC, a scenario case is also analyzed with the assumption that the reservoir is naturally fractured. The specific analysis shows that the maximal storage pressure is 13. 6 MPa which is determined by the penetration criterion of the caprock. © 2012 The Author(s).

Wagner S.,Energie-forschungszentrum Niedersachsen (EFZN) | Oberland A.,Energie-forschungszentrum Niedersachsen (EFZN) | Turek T.,Energie-forschungszentrum Niedersachsen (EFZN)
Energy Technology | Year: 2016

A new method for evaluation of battery cells containing porous thin-layer electrodes is developed. For this purpose, a typical concept of chemical reaction engineering to classify reactors by dimensionless characteristic values is used. Therefore, the system of equations describing the porous electrodes in a battery cell is transferred to dimensionless notation and corresponding dimensionless properties are developed. The required kinetic data are measured with the help of electrochemical test cells. A complete set of parameters for the description of the lithium-ion battery is determined and the characteristic properties for the chosen example are calculated and analyzed. Finally, the performance of the anode and cathode using lithium manganese oxide and surface-modified graphite as active materials is compared with the aid of the newly developed dimensionless parameters. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Baumann A.L.,Fraunhofer Institute for Telecommunications, Heinrich-Hertz-Institut | Guenther K.-M.,Energie-forschungszentrum Niedersachsen (EFZN) | Saring P.,University of Gottingen | Gimpel T.,Fraunhofer Institute for Telecommunications, Heinrich-Hertz-Institut | And 4 more authors.
Energy Procedia | Year: 2012

Samples of crystalline silicon for use as solar cell material are structured and hyperdoped with sulfur by irradiation with femtosecond laser pulses under a sulfur hexafluoride atmosphere. The sulfur creates energy levels in the silicon band gap, allowing light absorption in the infrared wavelength regime, which offers the potential of a significant efficiency increase. This Black Silicon is a potential candidate for impurity or intermediate band photovoltaics. In this paper we determine the laser processed sulfur energy levels by deep-level transient spectroscopy (DLTS). We present how the number of laser pulses per sample spot influence the sulfur energy levels and hence the DLTS spectra. Further we show that changing the laser pulse by splitting it with a Michelson interferometer setup results in altered absorption which is most likely due to altered sulfur energy levels. This contribution focuses on the possibility of controlling the sulfur in Black Silicon through manipulating the laser pulse shape. As a first step samples of microstructured silicon are fabricated with doubled laser pulses at two different laser pulse distances and the absorption spectra by integrating sphere measurements are compared.

Kontermann S.,Fraunhofer Institute for Telecommunications, Heinrich-Hertz-Institut | Gimpel T.,Fraunhofer Institute for Telecommunications, Heinrich-Hertz-Institut | Baumann A.L.,Fraunhofer Institute for Telecommunications, Heinrich-Hertz-Institut | Guenther K.-M.,Energie-forschungszentrum Niedersachsen (EFZN) | And 2 more authors.
Energy Procedia | Year: 2012

We present a femtosecond laser pulse process that induces a texture-like surface structure on silicon wafers and optionally incorporates sulfur into the silicon lattice for emitter formation depending on the processing atmosphere. Such laser processed Black Silicon provides an easily adjustable surface roughness for good light trapping in silicon solar cells. The structure is independent of the silicon crystal orientation and is easily applied on one wafer side only. A sulfur emitter can be formed within the laser structuring process, and allows electric current extraction from a solar cell structure manufactured from this material. Then the advantage is that no further emitter formation step like diffusion is necessary compared to other Black Silicon solar cell approaches, where the Black silicon is created wet chemically. By incorporating sulfur in the silicon crystal lattice, we can show that this Black Silicon absorbs in the infrared wavelength regime. This characteristic can potentially be used to better exploit the energy in the sun spectrum. We manufacture a laser processed Black Silicon solar cell prototype without any emitter diffusion step and achieve the highest efficiency of 4.5 % reported for this cell type.

Bockelmann M.,Clausthal University of Technology | Kunz U.,Energie-forschungszentrum Niedersachsen (EFZN) | Turek T.,Energie-forschungszentrum Niedersachsen (EFZN)
Electrochemistry Communications | Year: 2016

The development of a powerful, cyclically stable and electrically rechargeable zinc-oxygen battery with a three-electrode configuration is reported. A copper foam was used as stable substrate for zinc deposition in flowing potassium hydroxide electrolyte, while oxygen reduction and evolution were accomplished by a commercial silver electrode and a nickel foam, respectively. The cell could be charged and discharged with up to 600 mA cm- 2, delivered a peak power density of 270 mW cm- 2, and performed for more than 600 cycles, although short circuits by dendrite formation could not yet be completely avoided. At a current density of 50 mA cm- 2 and a temperature of 30 °C, a promising energy efficiency of 54% was achieved. © 2016 Elsevier B.V. All rights reserved.

Helsch G.,Clausthal University of Technology | Mos A.,Clausthal University of Technology | Deubener J.,Clausthal University of Technology | Deubener J.,Energie-forschungszentrum Niedersachsen (EFZN) | Holand M.,Interstate University of Applied Sciences and Technology
Solar Energy Materials and Solar Cells | Year: 2010

Quarter-wave anti reflectance (AR) coatings were applied on silica glass by a solgel dip coating process to minimize reflective losses of the receiver cover materials for solar tower power plants. To investigate the thermal resistance of the coatings prepared from silica sols of 20 nm, mean particle sizes, solar transmittance, thickness, and density were determined as a function of isothermal heat treatments using UVvisNIR spectroscopy, profilometry, and X-ray reflectometry, respectively. The initial solar transmittance τ of the AR coated silica glass, cured at 773 K, was 0.973 for a film thickness d=111 nm. Heat treatments in the range from 1273 to 1423 K for 15240 min reduce both film thickness and solar transmittance of the crystal-free films. The isothermal shrinkage rate of the film thickness was described for a relative density >0.634 using MackenzieShuttleworth (MS) model. Viscosity data calculated from the sintering rate constant of the MS model were compared with literature data of silica glasses of known water content prepared by sintering and fusion. This analysis indicates that the sintering kinetics of the nanoporous film were controlled by viscous flow of amorphous silica particles of a water content of ≈1100 ppmw associated with an activation energy of 321 kJ mol-1. © 2010 Elsevier B.V. All rights reserved.

Helsch G.,Clausthal University of Technology | Deubener J.,Clausthal University of Technology | Deubener J.,Energie-forschungszentrum Niedersachsen (EFZN)
Solar Energy | Year: 2012

Antireflective coatings for solar cover glasses and photocatalytic coatings for self-cleaning glasses are two types of nano-functionalized thin films, which are extensively investigated at present. Broadband antireflective coatings typically receive their function by the low refractive index of nano-porous silica (<1.4), whereas the most established photocatalytic coatings consist of high refractive index materials, such as titania (>2.0). Compatibility of these two functional materials was investigated using sol-gel dipcoating-technology. Therefore silica sol, used to prepare nano-porous antireflective coatings, was mixed with different amounts of titanium n-butoxide to achieve titania concentrations in the range from 1 to 50wt%. The fired coatings on silica glass showed a decrease in solar transmittance and an increase in photocatalytic activity with increasing titania concentration. With respect to applications in solar energy conversion systems compatible SiO 2-TiO 2 materials of low titania fractions (10wt%) were established, which combine minor loss in transmittance and exceptional high photocatalytic activity. In comparison to conventional nano-porous silica, the decrease in the solar transmittance was only 0.7%, whereas the degradation rate of stearic acid was 30times faster. © 2011 Elsevier Ltd.

Beck H.-P.,Energie-forschungszentrum Niedersachsen (EFZN) | Franz O.T.,Energie-forschungszentrum Niedersachsen (EFZN)
Underground Storage of CO2 and Energy - Proceedings of the Sino-German Conference, and the Sino-German Workshop "Eor and New Drilling Technology | Year: 2010

Germany plans a swift increase of renewable energy to supply its population with electrical power. The change of the power source from nuclear and thermal power plants to renewable sources will have a huge impact on the German power grid. Historically, nuclear and thermal power plants were installed close to the power consumer in order to avoid loss during shipment of the generated electrical power. The units installed are as big as possible, in order to be efficient. A change in the existing power grid is necessary, because renewable energy sources are rarely concentrated in the same location as the existing power plants. At the moment the only feasible way to increase renewable energy is an increase of wind farming offshore. With the move offshore the consumer and producer regions drift further apart. There is a high demand for an increase of the capacity of the existing power grid. However, this increase, the building of new lines, has a lot of opponents. Thus, the increase cannot keep up with the demand. A higher utilization of the power grid is necessary to solve the problem and to keep power costs affordable. Electric power from wind farms and other renewable sources fluctuates. Storage capacity for the German network is necessary to solve this fluctuation problem and to guarantee high utilization of the grid. There are different options in discussion. One proven technology is pumped-storage hydroelectricity. However, this technology has two downfalls: 1) Steep mountains are needed with a high difference of altitudes between the two storage reservoirs. 2) The storage reservoirs consume big areas and alter landscapes. The first downfall limits the areas available and the second leads to environmental concern of local inhabitants. Both problems can be solved if abandoned mines could be utilized for pumped-storage hydroelectricity. A transdisciplinary group of researchers of the EFZN performed a study to investigate all abandoned mines in Germany and to find a way to utilize these mines as energy storage which contributes to load balancing of the German Power Grid.

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