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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.

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.

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.

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).

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.

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