Jörg Petrasch

A Novel 50kW 11,000 suns High-Flux Solar Simulator Based on an Array of Xenon Arc Lamps

A novel high-flux solar simulator, capable of delivering over 50 kW of radiative power at peak radiative fluxes exceeding 11,000 suns, is operational at the Paul Scherner Institute. It comprises an array of ten Xe arcs, each close-coupled with ellipsoidal specular reflectors of common focus. Its optical design, main engineering features, and operating performance are described. The Monte Carlo ray-tracing technique is applied to optimize the geometrical configuration for maximum source-to-target transfer efficiency of radiative power. Calorimeter measurements indicated an average flux of 6800 kW/m2 over a 60-mm-diameter circular target, which corresponds to stagnation temperatures above 3300 K. This research facility simulates the radiation characteristics of highly concentrating solar systems and serves as an experimental platform for investigating the thermochemical processing of solar fuels and for testing advanced high-temperature materials.

Tomographic Characterization of a Semitransparent-Particle Packed Bed and Determination of its Thermal Radiative Properties

Keywords: computerised tomography ; extinction coefficients ; Monte Carlo methods ; particle size ; porosity ; two-phase flow Reference EPFL-ARTICLE-184803doi:10.1115/1.3109261 Record created on 2013-03-04, modified on 2017-07-28

Hydrogen Production via the Iron/Iron Oxide Looping Cycle

The iron/iron-oxide looping cycle has the potential to produce high purity hydrogen from coal or natural gas without the need for gas phase separation: Hydrogen is produced from steam oxidation of iron or Wustite yielding primarily Magnetite; Magnetite is then reduced back to iron/Wustite using syngas (CO+H2 ). A system model has been developed to identify favorable operation conditions and process configurations. Process configurations for three distinct temperature ranges, (i) 500–950 K, (ii) 950–1100 K, and (iii) 1100–1200 K have been developed. The energy content of high temperature syngas from conventional coal gasifiers is sufficient to drive the looping process throughout the temperature range considered. Temperatures around 1000 K are advantageous for both the hydrogen production step and the iron oxide reduction step. Simulations of a large number of subsequent cycles indicate that quasi-steady operation is reached after approximately 5 cycles. Comparison of simulations and experiments indicate that the process is currently limited by chemical kinetics at lower temperatures. Therefore, product recirculation should be used for a scaled-up process to increase reactant residence times while maintaining sufficient fluidization velocity.Copyright

Inverse identification of intensity distributions from multiple flux maps in concentrating solar applications

Radiative flux measurements at the focal plane of solar concentrators are typically performed using digital cameras in conjunction with Lambertian targets. To accurately predict flux distributions on arbitrary receiver geometries directional information about the radiation is required. Currently, the directional characteristics of solar concentrating systems are predicted via ray tracing simulations. No direct experimental technique to determine intensities of concentrating solar systems is available. In the current paper, multiple parallel flux measurements at varying distances from the focal plane together with a linear inverse method and Tikhonov regularization are used to identify the directional and spatial intensity distribution at the solution plane. The directional binning feature of an in-house Monte Carlo ray tracing program is used to provide a reference solution. The method has been successfully applied to two-dimensional concentrators, namely parabolic troughs and elliptical troughs using forward Monte Carlo ray tracing simulations that provide the flux maps as well as consistent, associated intensity distribution for validation. In the two-dimensional case, intensity distributions obtained from the inverse method approach the Monte Carlo forward solution. In contrast, the method has not been successful for three dimensional and circular symmetric concentrator geometries.

Tomography based numerical simulation of the demagnetizing field in soft magnetic composites

The magneto-static behaviour of soft magnetic composites (SMCs) is investigated using tomography based direct numerical simulation. The microgeometry crucially affects the magnetic properties of the composite since a geometry dependent demagnetizing field is established inside the composite, which lowers the magnetic permeability. We determine the magnetic field information inside the SMC using direct numerical simulation of the magnetic field based on high resolution micro-computed tomography data of the SMCs microstructure as well as artificially generated data made of statistically homogeneous systems of identical fully penetrable spheres and prolate spheroids. Quasi-static electromagnetic behaviour and linear material response are assumed. The 3D magnetostatic Maxwell equations are solved using Whitney finite elements. Simulations show that clustering and percolation behaviour determine the demagnetizing factor of SMCs rather than the particle shape. The demagnetizing factor correlates with the slope of a 2-point probability function at its origin, which is related to the specific surface area of the SMC. Comparison with experimental results indicates that the relatively low permeability of SMCs cannot be explained by demagnetizing effects alone and suggests that the permeability of SMC particles has to be orders of magnitude smaller than the bulk permeability of the particle material.

Micro-Tomographic Investigation of Ice and Clathrate Formation and Decomposition under Thermodynamic Monitoring

Clathrate hydrates are inclusion compounds in which guest molecules are trapped in a host lattice formed by water molecules. They are considered an interesting option for future energy supply and storage technologies. In the current paper, time lapse 3D micro computed tomographic (µCT) imaging with ice and tetrahydrofuran (THF) clathrate hydrate particles is carried out in conjunction with an accurate temperature control and pressure monitoring. µCT imaging reveals similar behavior of the ice and the THF clathrate hydrate at low temperatures while at higher temperatures (3 K below the melting point), significant differences can be observed. Strong indications for micropores are found in the ice as well as the THF clathrate hydrate. They are stable in the ice while unstable in the clathrate hydrate at temperatures slightly below the melting point. Significant transformations in surface and bulk structure can be observed within the full temperature range investigated in both the ice and the THF clathrate hydrate. Additionally, our results point towards an uptake of molecular nitrogen in the THF clathrate hydrate at ambient pressures and temperatures from 230 K to 271 K.

DISCRETE VS CONTINUUM LEVEL SIMULATION OF RADIATIVE TRANSFER IN SEMITRANSPARENT TWO-PHASE MEDIA

abstract The mathematical formulation of the continuum approach to radiative transfer model-ing in two-phase semi-transparent media is numerically validated by comparingradiative fluxes computed by (i) direct, discrete-scale and (ii) continuum-scaleapproaches. The analysis is based on geometrical optics. The discrete-scale approachuses the Monte Carlo ray-tracing applied directly to real 3D geometry measured bycomputed tomography. The continuum-scale approach is based on a set of continuum-scale radiative transfer equations and associated radiative properties, and employs theMonte Carlo ray-tracing for computations of radiative fluxes and for computations ofthe radiative properties. The model two-phase media are reticulate porous ceramicsand a particle packed bed, each composed of semitransparent solid and fluid phases.The results obtained by the two approaches are in good agreement within the limits ofstatistical uncertainty. The continuum-scale approach leads to a reduction in computa-tional time by approximately one order of magnitude, and is therefore suited to treatradiative transfer problems in two-phase media in a wide range of engineeringapplications.Published by Elsevier Ltd.

CONTINUUM RADIATIVE HEAT TRANSFER MODELING IN MEDIA CONSISTING OF OPTICALLY DISTINCT COMPONENTS IN THE LIMIT OF GEOMETRICAL OPTICS

abstract Continuum-scale equations of radiative transfer and corresponding boundary condi-tions are derived for a general case of a multi-component medium consisting ofarbitrary-type, non-isothermal and non-uniform components in the limit of geometricaloptics. The link between the discrete and continuum scales is established by volumeaveraging of the discrete-scale equations of radiative transfer by applying the spatialaveraging theorem. Precise definitions of the continuum-scale radiative properties areformulated while accounting for the radiative interactions between the components attheir interfaces. Possible applications and simplifications of the presented generalequations are discussed.Published by Elsevier Ltd. 1. IntroductionRadiative transfer in media consisting of opticallydistinct components is encountered in multiple fields ofscience and engineering including chemical processing,combustion, nuclear and civil engineering, atmosphericsciences, and solar technology. 1 An important subset aremedia consisting of components in the range of geome-trical optics [1]. They find applications as reacting packedbeds, porous heat exchangers, radiant absorbers andburners, and insulating materials. Porous structures usedin high-temperature solar thermal and thermo–chemicalprocesses to generate power and produce chemicals are ofspecial interest. Radiative transfer in such materials isoften predicted by using continuum models employingappropriate continuum-scale radiative properties [2,3].