Monte Carlo Ray-tracing Technique Research Articles

A New Solar Reactor Aperture Mechanism Coupled with Heat Exchanger

Concentrated solar energy finds applications for power generation and as a source of heat for solar thermochemical processes. However, solar energy reaching the earth’s surface is intermittent and fluctuates with weather conditions, position of the sun throughout the day and other seasonal changes. This causes a major drawback in receiver efficiency as semi-constant temperatures are required for efficient operation of solar thermochemical processes. This paper introduces a new variable aperture mechanism which is coupled with a heat exchanger to collect unused heat during peak times. The paper presents an optical and heat transfer analysis of the concept using Monte-Carlo ray tracing technique via TracePro and an in-house developed heat transfer code. The heat transfer analysis of the proposed concept shows the optimum aperture diameter with a compromise between reactor temperature and reradiation losses. It also predicts the losses incurred by the variable sized aperture mechanism when the incoming solar radiation changes.

Experimental and Numerical Heat Transfer Analysis of an Air-Based Cavity-Receiver for Solar Trough Concentrators

We report on the field testing of a 42 m-long full-scale solar receiver prototype installed on a 9 m-aperture solar trough concentrator. The solar receiver consists of a cylindrical cavity containing a tubular absorber with air as the heat transfer fluid (HTF). Experimental results are used to validate a heat transfer model based on Monte Carlo ray-tracing and finite-volume techniques. Performance predictions obtained with the validated model yield the following results for the receiver. At summer solstice solar noon, with HTF inlet temperature of 120 °C and HTF outlet temperature in the range 250–450 °C, the receiver efficiency ranges from 45% to 29% for a solar power input of 280 kW. One third of the solar radiation incident on the receiver is lost by spillage at the aperture and reflection inside the cavity. Other heat losses are due to natural convection (9.9–9.7% of solar power input) and re-radiation (6.1–17.6%) through the cavity aperture and by natural convection from the cavity insulation (5.6–9.1%). The energy penalty associated with the HTF pumping work represents 0.6–24.4% of the power generated.

Modelling and optimization of electric current spreading in III‐nitride LEDs

AbstractIn this work, we perform numerical simulations of a conventional GaN LED chip within the framework of the combined electrical/thermal/light approach. A GaN LED chip grown on a sapphire substrate is investigated. We calculate the dependencies of the forward voltage, light output power, wall‐plug efficiency, and uniformity of current spreading on the variations in the electrode geometry. Modelling is performed using a self‐developed software based on the Finite Volume method and the Monte Carlo ray tracing technique. The simulation results show that a well‐designed electrode pattern can provide more than 8% improvement in the wall‐plug efficiency. The implementation of contact optimization is required for designing highly efficient LED devices. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

COMBINED TWO-FLUX APPROXIMATION AND MONTE CARLO MODEL FOR IDENTIFICATION OF RADIATIVE PROPERTIES OF HIGHLY SCATTERING DISPERSED MATERIALS

An identification procedure is developed for obtaining spectral radiative properties of highly scattering dispersed materials such as porous ceramics. Traditional techniques based on measurements of the directional-hemispherical reflectance and transmittance are of limited use because of difficulties in fabricating sufficiently thin and mechanically stable samples to obtain reliable values of directional-hemispherical transmittance. However, one can use the directional-hemispherical reflectance measurements for optically thick samples to obtain the transport scattering albedo. A one-dimensional analytical solution employs the modified two-flux approximation for the identification of transport scattering albedo. An additional transmittance measurement is required to identify the transport extinction coefficient. Binormal narrow cone transmittance is measured for this purpose. Because the one-dimensional analytical solution is not applicable to model the binormal narrow cone transmittance, the Monte Carlo ray-tracing technique is used to identify the transport extinction coefficient. The identification procedure is applied to obtain near-infrared radiative properties of porous ceria ceramics used in solar thermochemical reactors. The identified transport scattering coefficient is shown to be in good agreement with theoretical estimates based on the Mie theory for polydisperse pores and grains. This verifies the applicability of a model based on independent scattering and Mie theory for theoretical predictions of radiative properties of two types of ceria ceramics with porosity of 0.08 and 0.72, and for extrapolating the properties of both ceramics in a limited near-infrared range to the range of significant absorption.

A Modular Ceramic Cavity-Receiver for High-Temperature High-Concentration Solar Applications

A high-temperature pressurized air-based receiver is considered as a module for power generation via solar-driven gas turbines. A set of silicon carbide cavity-receivers attached to a compound parabolic concentrator (CPC) are tested on a solar tower at stagnation conditions for 35 kW solar radiative power input under mean solar concentration ratios of 2000 suns and nominal temperatures up to 1600 K. A heat transfer model coupling radiation, conduction, and convection is formulated by Monte Carlo ray-tracing, finite volume, and finite element techniques, and validated in terms of experimentally measured temperatures. The model is applied to elucidate the effect of material properties, geometry, and reflective coatings on the cavity’s thermal and structural performances.

A solar dish concentrator based on ellipsoidal polyester membrane facets

The design of a solar parabolic dish concentrator is proposed based on an array of polyester mirror membrane facets that are clamped along their edges by elliptical rims and focused by applying a slight vacuum underneath the membranes, creating an ellipsoidal shape. The axes ratio of the elliptical rims varies with the position on the dish to approach the paraboloidal shape. The elastic mirror membrane deformation under uniform pressure load is simulated by finite element structural analysis and the resulting radiative flux distribution at the focal plane of the dish is determined by the Monte Carlo ray-tracing technique. Optimization of the membrane deflection is accomplished for maximum solar flux concentration at the focal plane. Two dish geometries are examined: (i) a 1.5-m radius 3-m focal length small dish, comprising 19 facets of 0.275-m radius with four different curvatures, yielding a peak solar concentration ratio of 5515 suns and a mean solar concentration ratio of 1435 suns with an intercept factor of 90% over a 3-cm radius disk target and (ii) a 10.9-m radius 11-m focal length large dish, comprising 121 facets of 0.9-m radius with 15 different curvatures, yielding a peak solar concentration ratio of 23,546 suns and mean solar concentration ratio of 8199 suns with an intercept factor of 90% over a 10.4-cm radius disk target. The performance of the second geometry is compared to that of the more conventional design of a multi-facet dish concentrator consisting of identical circular facets and shown to reach - on the same target area - a 12% higher mean solar concentration ratio as well as a 6.6% higher intercept factor. The simulated membrane shape is experimentally verified with photogrammetrical measurements carried out on a prototype facet of the small dish.

高准直太阳模拟器的设计与仿真

This paper gives the design specifications of high-collimating solar simulator,introduces basic principle and structure,summarizes the calculation method of the solar simulator's optical system size,and gives the general formula.This paper also optimizes the optical system,and presents the optimization principle of energy transfer system,points out the difference between the complex lighting system and other imaging systems.The design of the solar simulator's optical and mechanical structure is described.Finally,the simulation design on solar simulator using the Monte Carlo ray-tracing technique is conducted,and the simulation results are given quantitatively by diagrammatic form.

A Solar Trough Concentrator for Pill-Box Flux Distribution Over a CPV Panel

An integral methodology is formulated to analytically derive the exact profile of a solar trough concentrator that delivers a uniform radiative flux distribution over a flat rectangular target area at the focal plane. The Monte Carlo ray-tracing technique is applied to verify the analytical solution and investigate the effect of sun shape and mirror surface imperfections on the radiation uniformity and spillage. This design is pertinent to concentrating photovoltaics at moderate mean solar flux concentration ratios of up to 50 suns.

Optical Design of a Novel Two-Stage Solar Trough Concentrator Based on Pneumatic Polymeric Structures

An innovative concept for fabricating solar trough concentrators based on pneumatic polymer mirrors supported on precast concrete frames is presented. Optical aberration is corrected by means of a secondary specular reflector in tandem with a primary cylindrical concentrator. The optimal design is formulated for maximum solar flux concentration. The Monte Carlo ray-tracing technique is applied to determine the effect of reflective surface errors and structural beam deformations on the performance of the combined primary and secondary concentrating system. The numerical results are validated with field measurements on a 49.4 m length, 7.9 m width sun-tracking prototype system. Theoretical maximum solar concentration ratio is 151 suns; the measured one with a flat secondary reflector was 55 suns.

Study on the Simulation Model for the Optimization of Optical Structures of Edge-lit Backlight for LCD Applications

The optical performances of 15-inch edge-lit backlight were simulated by using a Monte Carlo ray-tracing technique. The backlight model was built by combining a wedge-type light guide plate, a diffuser sheet, a tubular fluorescent lamp with a lamp reflector, and two crossed prism sheets. Angular distributions of the luminance on each optical component obtained from simulation were consistent with those obtained from experiments on a real 15-inch backlight. The constructed backlight model was used to evaluate the optical performances of a micro-pyramid film. It was found that the on-axis luminance gain on the pyramid film is higher than that on one prism film but much lower than that on the two crossed prism films. These results suggest that a reliable simulation model can be used to develop new hybrid films and to optimize the optical structure of edge-lit backlight in order to reduce the developmental period.

A cavity-receiver containing a tubular absorber for high-temperature thermochemical processing using concentrated solar energy

A solar chemical reactor consisting of a cylindrical cavity-receiver containing a tubular ceramic absorber is considered for performing thermochemical processes using concentrated solar radiation as the energy source of high-temperature process heat. The model chemical reaction selected is the thermal dissociation of ZnO into its elements, which proceeds endothermically at above 1800 K and is part of a 2-step H 2O-splitting thermochemical cycle for H 2 production. A lab-scale 5 kW reactor prototype is fabricated and subjected to high-flux solar irradiation in the range 448–2125 kW/m 2. A heat transfer reactor model is formulated that encompasses the governing mass and energy conservation equations coupling radiation/convection/conduction heat transfer to the chemical kinetics, and their solution by Monte Carlo ray-tracing and finite difference techniques. Validation was accomplished by comparing numerically computed and experimentally measured temperatures and reaction rates in the 1780–1975 K range. The reactor model is further applied to simulate a continuous thermochemical process, identify major sources of irreversibility, and predict solar-to-chemical energy conversion efficiencies.

Spectropolarised ray-tracing simulations in densely packed particulate medium

Light scattering from particulate medium is simulated using the Monte Carlo ray-tracing technique. The medium is modelled as a randomly packed medium of ellipsoidal grains with stochastically rough surfaces, with an optional thin coating. Optical properties are modelled using a wavelength-dependent complex refractive index and taking Fresnelian reflections and refractions from the interfaces. The size and shape of the grains are assumed to be large and smooth enough for geometric optics to apply reasonably well. The ray-tracing technique uses parallel, weighted rays for computing simultaneously over a wide wavelength spectrum and a small roughness range, and scaling to obtain a large range of sizes and absorbities simultaneously. Polarisation is fully accounted for. The multiobservation technique is effectively used at each scattering point. The scattering from thinner sample layers is also received as a subresult. Simulations are run for a set of model samples to study the effects and sensitivities regarding the values of certain parameters. It has been found that the size and composition of the grains affect the scattering in a unique and invertible way. The shape of the grain causes similar significant effects that must certainly be taken into account if any accuracy is required, although inverting for the shape is difficult without further constraints. The packing density has a small but observable effect. Polarisation can be used to study the composition of low-albedo objects.

Band-approximated radiative heat transfer analysis of a solar chemical reactor for the thermal dissociation of zinc oxide

A solar chemical reactor for the thermal dissociation of ZnO is modeled by means of a detailed heat transfer analysis that couples radiative transport to the reaction kinetics. An extended band-approximated radiosity method enables the analysis of directional and wavelength depended radiation exchange. Boundary conditions included the incident concentrated solar radiation, determined by the Monte Carlo ray-tracing technique, and the hemispherical and band-approximated optical properties derived for the quartz window. Validation was accomplished by comparing the numerically modeled and experimentally measured window temperatures, reaction rates, and energy conversion efficiencies. The experimentally measured solar-to-chemical energy conversion efficiency increased with temperature, peaked at 14% for a reactor temperature of 1900 K and ZnO dissociation rate of 12 g/min, and decreased as the reactor approached its stagnation temperature. The conditions for which this efficiency can be augmented are discussed.

Tomography-based Monte Carlo determination of radiative properties of reticulate porous ceramics

A 3D digital representation of a reticulate porous ceramic (RPC) sample, generated by computer tomography (CT), is employed to determine its porosity, surface-to-volume ratio, and the minimum size of a representative elementary volume (REV) for continuum domain. Subsequently, the Monte Carlo (MC) ray-tracing technique is applied to calculate the extinction coefficient and scattering phase functions based on the probabilistic distribution functions of the extinction path-length and of the directional cosine of incident radiation. The methodology and governing equations are presented for diffusely and specularly reflecting surfaces. The isotropic assumption is justified by demonstrating that the extinction coefficient is directionally independent.

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 50kW 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 6800kW∕m2 over a 60-mm-diameter circular target, which corresponds to stagnation temperatures above 3300K. 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.

Experimental Determination of the Extinction Coefficient for a Packed-Bed Particulate Medium

The extinction coefficient of a powder mixture of ZnO and beech charcoal is determined at ambient temperature using two experimental set-ups: (a) radiation in the range 500–1000 nm is captured by a 200 μm-diameter fiber optic and sent to a spectrometer equipped with a Si/PbSe detector; (b) radiation in the range 350–1100 nm is captured by a 5 mm-diameter Si-photodiode. Both set-ups measure the attenuation of intensity from a 3273 K blackbody source. The experimental results are implemented in the numerical solution of the equation of radiative transfer, using the Monte-Carlo ray-tracing technique. The extinction coefficient is determined to be 10103 ± 615 m−1 at 1000 nm using set up (a), and 7850 ± 337 m−1 for the range 350–1100 nm using the more accurate set-up (b).

Steam-Gasification of Coal in a Fluidized-Bed/Packed-Bed Reactor Exposed to Concentrated Thermal RadiationModeling and Experimental Validation

The steam-gasification of coal in a fluidized-bed or a packed-bed chemical reactor is considered using an external source of concentrated thermal radiation for high-temperature process heat. The energy equation that couples heat transfer with the chemical kinetics is solved by means of a numerical model that incorporates the Monte Carlo ray-tracing technique for nonisothermal, nongray, absorbing, emitting, and scattering media. The reaction kinetics are described by Langmuir−Hinshelwood type rate laws. Validation is accomplished by comparing the numerically computed temperature profiles, product gas composition, and reaction extent with the experimentally measured values using a tubular quartz reactor directly exposed to high-flux irradiation. For the packed bed, the temperature increases monotonically because the internal radiative exchange approaches a conduction-like heat transfer within the bed. For the fluidized bed, the temperature increases rapidly in the first one-quarter of the bed and then reaches a constant value because of the strong fluidization in the upper bed region derived from the 5-fold volumetric growth due to gas formation and thermal expansion. Above 1450 K, the product composition consisted mainly of an equimolar mixture of H2 and CO, a syngas quality that is notably superior than that typically obtained in autothermal gasification reactors (with internal combustion of coal for process heat), besides the additional benefit of the upgraded calorific value.

Annular Compound Parabolic Concentrator

The annular compound parabolic concentrator (CPC) is a body of revolution consisting of two axisymmetric surfaces produced by rotating a classical two-dimensional CPC around an axis parallel to the CPCs axis. Its ability to further concentrate incoming radiation when used in tandem with a primary solar parabolic concentrator is analyzed by the Monte Carlo ray-tracing technique. Potential applications are found in capturing the annular portion of primary concentrated solar radiation and augmenting its power flux intensity.

ISOCAM Long Wave Detector Glitches: Data and Ray-Tracing Simulations

The affected pixel number distribution of the ISOCAM Long-Wave (LW) detector in cosmic ray-induced glitchesis calculated. The methods employed are Monte Carlo ray-tracing techniques and the taxi metric, which allows direct calculation of the affected pixels based on the knowledge of entry and exit points of the ray. The simulation results are compared with long-term experimental data. Based on the simulations combined with the CREME96 cosmic ray model for solar quiet period, it is estimated that the detector is on average traversed by ∼0.3 cosmic ray protons per second. For the experimental data, a corresponding minimum bound of ∼0.14 cosmic ray protons per second is obtained. Deviations in the simulated and measured pixel number distributions are discussed.

Raytran: a Monte Carlo ray-tracing model to compute light scattering in three-dimensional heterogeneous media

A model of radiation transfer in three-dimensional (3D) heterogeneous media is designed and evaluated. This model implements state-of-the-art Monte Carlo ray-tracing techniques and is dedicated to the study of light propagation in terrestrial environments. It is designed as a virtual laboratory, where scenes of arbitrary complexity can be described explicitly and where the relevant radiative processes can be represented in great detail, at spatial scales relevant to simulate actual measurements. The approach capitalizes on the existing understanding of the elementary radiative processes and recognizes that the major difficulty in accurately describing the radiation field after its interaction with a typical terrestrial scene results from the complexity of the structure and the diversity of the properties of the elements of the scene. The output of the model can be customized to address various scientific investigations, including the determination of absorption profiles or of light-scattering distributions. The performance of the model is evaluated through detailed comparisons with laboratory measurements of an artificial target as well as with other established reflectance models for plant canopies.