Monte Carlo Ray-tracing Model Research Articles

A solar air receiver with porous ceramic structures for process heat at above 1000 °C – Heat transfer analysis

Abstract Concentrated solar energy can be used as the source of heat at above 1000 °C for driving key energy-intensive industrial processes, such as cement manufacturing and metallurgical extraction, contributing to their decarbonization. The cornerstone technology is the solar receiver mounted on top of the solar tower, which absorbs the incident high-flux radiation and heats a heat transfer fluid. The proposed high-temperature solar receiver concept consists of a cavity containing a reticulated porous ceramic (RPC) structure for volumetric absorption of concentrated solar radiation entering through an open (windowless) aperture, which also serves for the access of ambient air used as the heat transfer fluid flowing across the RPC structure. A heat transfer analysis of the solar receiver is performed by means of two coupled models: a Monte Carlo (MC) ray-tracing model to solve the 3D radiative exchange and a computational fluid dynamics (CFD) model to solve the 2D convective and conductive heat transfer. Temperature distributions computed by the iteratively coupled models were compared with experimental data obtained by testing a lab-scale 5 kW receiver prototype with a silicon carbide RPC structure exposed to 3230 suns flux irradiation. The receiver model is applied to optimize its dimensions for maximum efficiency and to scale up for a 5 MW solar tower.

Optical characterization of a solar concentrating dish system

Solar dishes concentrate the quasi-collimated sun light towards a focal point. Typically, the optical characterization consists of experimental flux map measurements and Monte-Carlo ray-tracing (MCRT) simulations, which are fitted based on experimental data and serve for the prediction of the distribution of the irradiation in receivers or reactors. However, the MCRT simulations usually rely on idealized mirror geometries (i.e. parabolic mirrors), neglecting the actual dish geometry resulting from its construction method (such as petals, facets or segments). Thus, the parameter fitting might yield unrealistic parameters and local radiative flux peaks (“hot spots”), which are often experimentally detected behind the focal plane, cannot be predicted. Here, we characterized the 7m diameter solar dish at EPFL comprising 27 petals with a nominal focal length of 3.8 m and a rim angle of 50.3°. The measured peak concentration was 1781 suns and the received integrated solar power was 20.0 kW over an 18 cm diameter spot. We proposed an advanced geometry approach for the MCRT model considering the curvature of the dish, i.e. geometries mathematically described by a parabola with an exponent not equal, but close to 2. The advanced geometry approach was then applied for predicting experimentally measured flux distribution in focal and off-focus planes. The fitted reflectivities were 70% and 86% for the idealized and advanced geometry approach, respectively. The idealized geometry approach found an unrealistically low value whereas the advanced geometry approach’s value was within 1% of the manufacturer’s specification. The advanced geometry approach predicted a parabola exponent of 1.94, which emphasizes that the best geometry to describe the solar dish deviates from a perfect paraboloid. We conclude that MCRT models should consider non-perfect parabolic geometries for solar dishes in order to obtain more realistic values for the fitted parameters.

Robustness of Single- and Dual-Energy Deep-Learning-Based Scatter Correction Models on Simulated and Real Chest X-rays

Chest X-rays (CXRs) represent the first tool globally employed to detect cardiopulmonary pathologies. These acquisitions are highly affected by scattered photons due to the large field of view required. Scatter in CXRs introduces background in the images, which reduces their contrast. We developed three deep-learning-based models to estimate and correct scatter contribution to CXRs. We used a Monte Carlo (MC) ray-tracing model to simulate CXRs from human models obtained from CT scans using different configurations (depending on the availability of dual-energy acquisitions). The simulated CXRs contained the separated contribution of direct and scattered X-rays in the detector. These simulated datasets were then used as the reference for the supervised training of several NNs. Three NN models (single and dual energy) were trained with the MultiResUNet architecture. The performance of the NN models was evaluated on CXRs obtained, with an MC code, from chest CT scans of patients affected by COVID-19. The results show that the NN models were able to estimate and correct the scatter contribution to CXRs with an error of <5%, being robust to variations in the simulation setup and improving contrast in soft tissue. The single-energy model was tested on real CXRs, providing robust estimations of the scatter-corrected CXRs.

A high-performance solar simulator pursuing high flux and fine uniformity: Modelling, optimization, and experiment

The high flux solar simulator is a significant experimental tool in concentrated solar power applications. For a simulator, the uniformity of its light spot is as vital as high flux, since uneven light spots may result in the thermal-stress failure of the solar receiver. However, most constructed simulators have only pursued extremely high flux but ignored uniformity, thus degrading the simulator’s overall performance. Considering that improving uniformity comes at the expense of decreasing the flux, a high-performance solar simulator must meet a reasonable compromise between high flux and fine uniformity. In this paper, a simulator whose input power reached 30 kWe was designed and optimized using an in-house Monte Carlo Ray Tracing (MCRT) model, and corresponding processes were demonstrated to provide universal guidelines. To improve the uniformity while maintaining satisfying high flux, a non-coaxial reflector with optimized parameters was implemented. Based on the design and optimization results, a realistic solar simulator was manufactured and calibrated through experimental methods. The results demonstrate that the manufactured simulator achieved an about 10% reduction in non-uniformity while reaching a peak flux of 8.6 × 106 W·m−2, meeting the design expectation for a high-performance simulator.

Analysis of the absorptance and emittance in particle curtains by means of a discrete MCRT method

Apparent optical properties, absorptance and emittance, in particle curtains are analyzed by means of a discrete Monte Carlo Ray-Tracing model. The effect of solid volume fraction, particle diameter and curtain thickness on these properties is investigated for two types of particles with different absorptivity and emissivity: sand and CARBO HSP. Absorptance and emittance achieved by curtains made of these particles are greater than the values of absorptivity and emissivity of material. The increase in curtain absorptance/emittance compared to the particle absorptivity/emissivity is higher as the particle property is poorer, such as in sand. Both absorptance and emittance are maximum for opaque curtains, which entails high thicknesses. Assuming opaque curtains, the absorptance and emittance increase as the solid volume fraction decreases. The results obtained with the Monte Carlo Ray-Tracing model are used to estimate particle curtain radiative losses including emission and reflection losses. If material properties absorptivity and emissivity are used instead of the obtained curtain properties absorptance and emittance, sand curtain losses are up to 35 pp and only 9 pp in the case of CARBO HSP. Absorptivity is more important than emissivity, that is, it is preferable having high absorptivity than low emissivity to achieve low radiative losses. Emission losses are less important than reflected losses being very similar in both types of curtains. Sand curtains lose 33% of the incoming power while CARBO curtains lose only 7%. Radiative losses are significantly higher than advection losses in sand curtains. However, in CARBO curtains, advection losses are higher.

Development of the Ames Global Hyperspectral Synthetic Data Set: Surface Bidirectional Reflectance Distribution Function

AbstractThis study introduces the Ames Global Hyperspectral Synthetic Data set (AGHSD), in particular the surface bidirectional reflectance distribution function (BRDF) product, to support the NASA Surface Biology and Geology (SBG) mission development. The data set is generated based on the corresponding multispectral BRDF products from NASA's MODIS satellite sensor. Based on theories of radiative transfer in vegetation canopies, we derive a simple but robust relationship that indicates that the hyperspectral surface BRDF can be accurately approximated as a weighted sum of the soil surface reflectance, the leaf single albedo, and the canopy scattering coefficient, where the weights or coefficients are spectrally invariant and thus readily estimated from the multispectral MODIS products. We validate the algorithm with simulations by a Monte Carlo Ray Tracing model and find the results highly consistent with the theoretic derivation. Using reflectance spectra of soil and vegetation derived from existing spectral libraries, we apply the algorithm to generate the AGHSD BRDF product at 1 km and 8‐day resolutions for the year of 2019. The data set is biogeochemically and biogeophysically coherent and consistent, and serves the goal to support the SBG community in developing sciences and applications for the future global imaging spectroscopy mission.

Modeling and comparison of bulk and thin-film luminescent solar concentrators based on colloidal perovskite quantum dots.

Photovoltaic integrated luminescent solar concentrators (LSCs) can be embedded in modern buildings to serve as power-generation units. In this Letter, we demonstrate and develop a Monte Carlo ray-tracing model and a numerical description for the performance and loss evaluation of LSCs based on colloidal quantum dots. The performance differences between bulk and thin-film LSCs are systematically analyzed at different sizes and concentrations. It is found that large-area thin-film LSCs generally perform better, which is attributed to the suppression of scattering and the retention of quantum yield by this structure with twice the performance of bulk LSCs.

Influence on imaging performance and evaluation of Wolter-I type mandrel fabrication errors.

The electroforming replication process has been widely used in the fabrication of nested x-ray telescopes. The imaging performance of the mirrors is determined largely by the shape accuracy of the mandrels. To predict the imaging performance of mirrors replicated from mandrels with different parameter and fabrication errors, a special Monte Carlo ray tracing model is established and verified by experiments. Then, based on ray tracing numerical calculation, the influence of each major fabrication error is discussed. Furthermore, according to the results obtained by the simulation of slope error, a method for evaluating the relationship between the mandrel full-band errors and imaging quality is proposed and then verified by experiments. The results show that the power spectral density (PSD) reference given by the method can well reflect the quality of the mandrels, and guide the fabrication process.

A Superimposed QD-Based Optical Antenna for VLC: White LED Source.

Visible light communication (VLC) is a versatile enabling technology for following high-speed wireless communication because of its broad unlicensed spectrum. In this perspective, white light-emitting diodes (LED) provide both illumination and data transmission simultaneously. To accomplish a VLC system, receiver antennas play a crucial role in receiving light signals and guiding them toward a photodetector to be converted into electrical signals. This paper demonstrates an optical receiver antenna based on luminescent solar concentrator (LSC) technology to exceed the conservation of etendue and reach a high signal-to-noise ratio. This optical antenna is compatible with all colors of LEDs and achieves an optical efficiency of 3.75%, which is considerably higher than the similar reported antenna. This antenna is fast due to the small attached photodetector—small enough that it can be adapted for electronic devices—which does not need any tracking system. Moreover, numerical simulation is performed using a Monte Carlo ray-tracing model, and results are extracted in the spectral domain. Finally, the fate of each photon and the chromaticity diagram of the collected photons’ spectra are specified.

Multiplexed spatially-focused localization of light in adipose biological tissues

Last decades the effects of localization and focusing of light in turbid randomly inhomogeneous tissue-like scattering medium have been attracting a particular attention. Weak localization of light in disordered and weakly ordered biological tissue, polarization memory effect, correlations in transmission matrices, focusing light by wavefronts shaping have been widely exploited. Here, we represent an experimentally observed and theoretically confirmed new type of spatial localization of light within biological tissues. General description of the observed phenomenon based on Monte Carlo ray tracing model is provided. We find that innate body arrangements of individual adipocytes can act as a cascade of quasi-ordered microscale lenses confining propagation of light within adipose tissues similar to lens lightguides. The observed spatially-resolved longitudinal multi-focusing of light within disordered adipose biological tissues can naturally lead greater spatial control and enhance light-tissue interactions.

Super-suppression of long phonon mean-free-paths in nano-engineered Si due to heat current anticorrelations

The ability to minimize the thermal conductivity of dielectrics with minimal structural intervention that could affect electrical properties is an important capability for engineering thermoelectric efficiency in low-cost materials such as Si. We recently reported the discovery of special arrangements for nanoscale pores in Si that produce a particularly large reduction in thermal conductivity accompanied by strongly anticorrelated heat current fluctuations [1] – a phenomenon that is missed by the diffuse adiabatic boundary conditions conventionally used in Boltzmann transport models. This manuscript presents the results of molecular dynamics simulations and a Monte Carlo ray tracing model that teases apart this phenomenon to reveal that special pore layouts elastically backscatter long-wavelength heat-carrying phonons. This means that heat carriage by a phonon before scattering is undone by the scattered phonon, resulting in an effective mean-free-path that is significantly shorter than the geometric line-of-sight due to the pores. This effect is particularly noticeable for the long-wavelength, long mean-free-path phonons whose transport is impeded drastically more than is expected purely from the usual considerations of scattering defined by the distance between defects. This “super-suppression” of the mean-free-path below the characteristic length scale of the nanostructuring offers a route for minimizing thermal conductivity with minimal structural impact, while the stronger impact on long wavelengths offers possibilities for the design of band-pass phonon filtering. Moreover, the ray tracing model developed in this paper shows that different forms of correlated scattering imprint a unique signature in the heat current autocorrelation function that could be used as a diagnostic in other nanostructured systems.

Simulation of solar-induced chlorophyll fluorescence by modeling radiative coupling between vegetation and atmosphere with WPS

Recent advances in instruments and retrieval methods enable measurements of solar-induced chlorophyll fluorescence (SIF) across a wide range of scales. Radiative transfer (RT) models for simulating scattering and (re-)absorption of SIF provide a powerful tool to study the upscaling of SIF signal from leaf level to terrestrial ecosystems. Based on the Monte Carlo ray-tracing (MCRT) model, WPS (Weighted Photon Spread), we made major extensions with new functionalities and systematic evaluation of the new modules. By modeling the radiative coupling between atmosphere and land surface with the same MCRT method, the non-fluorescent and SIF radiance received by sensors can be simulated at levels from top-of canopy to top-of-atmosphere (TOA) in a coherent manner. New extension to represent the three-dimensional (3-D) canopies with geometrical primitives composed of turbid medium makes the hyperspectral simulation (especially SIF) for a sensor with medium spatial resolution at kilometer-scale feasible and practical. Evaluations through ROMC (Radiation transfer model intercomparison Online Model Checker) show that the accuracy of the new module of 3-D structure representation in WPS is within 1% of the reference solution. The spectra of TOA radiance and SIF and their components simulated at nadir by WPS agree closely with those simulated by the coupled SCOPE and MODTRAN models with the coefficient of determination (R2) higher than 0.99 and the average absolute relative error (AARE) lower than 6.39%; for angular distributions of TOA radiance and SIF at 685 nm and 740 nm, R2 is higher than 0.81 and AARE is lower than 6.94%. Comparisons of the spectra of TOA radiance and SIF and their components simulated at nadir by WPS and the DART model give R2 higher than 0.99 and AARE lower than 3.5%; R2 is higher than 0.92 and AARE is lower than 5.92% for the TOA angular simulations. The WPS model was also evaluated by hyperspectral measurements through unmanned aerial vehicle at different altitudes, which shows that WPS can reproduce the spectral features of a rapeseed crop. WPS can be used as a versatile tool to assess the impacts of various factors on the SIF signal and to evaluate the SIF retrieval methods under different conditions.

Optical properties and scattering distribution of thermographic phosphors

The optical properties and scattering distribution of thermographic phosphors have been demonstrated using a combined experimental and numerical method. ZnO:Zn and BaMgAl10O17:Eu2+ (BAM) are two types of widely used phosphors due to their stable physical properties and high-temperature sensitivities. To study their inter-phosphor light transfer, the angular scattering distribution and light propagation of these two phosphor suspensions was measured using a spectro-goniometric system. A collision-based Monte Carlo ray-tracing model was developed to extract their optical properties, including extinction coefficient, scattering albedo, asymmetry factors, and scattering fraction. With the void fraction around 0.98, the extinction coefficient of ZnO:Zn was determined to be 4.719, while that of F grade BAM and N grade BAM were 11.584 and 9.777, respectively. In addition, BAM had a higher scattering fraction (α = 0.99) than ZnO:Zn (α = 0.88). Due to the higher values of extinction coefficient and scattering fraction, BAM demonstrated more significant scattering than ZnO:Zn. Light transmission through phosphor suspensions was predicted along the direction of the light path. For ZnO:Zn, 70% of flux was scattered when the distance increases to 0.5, while for BAM, the distance was 0.15. Furthermore, with the same mass loading, smaller particle sizes can promote scattering and reduce the amount of light transmitted through phosphor suspensions.

Radiation propagation in a hierarchical solar volumetric absorber: Results of single-photon avalanche diode measurements and Monte Carlo ray tracing analysis

Volumetric solar receivers are essential components in high-temperature concentrated solar power plants. Their optical design is crucial for achieving efficient photon-thermal energy conversion; however, their three-dimensional geometry complicates a reliable and accurate optical characterization. This work proposes a new methodology for the 3D optical-performance analysis of the volumetric absorber having an open hierarchical structure. Firstly, the solar absorber is optically characterized using a test bench that mainly comprises a solar simulator and a customized instrument equipped with single-photon avalanche diodes, which holds the absorber and measures light flux on its external surface. Then experimental measurements are compared with a Monte Carlo ray-tracing numerical model. The results are consequently employed to understand light propagation in the absorber. This procedure is successfully applied to characterize a complex three-dimensional self-similar structure manufactured by Selective Laser Melting. The proposed experimental technique is a promising candidate for becoming a robust in operando method to characterize the radiation propagation within the complex porous structures employed for volumetric receivers.

Monte Carlo simulation of a LSC based on stacked layers of fiber arrays with core-coating different absorbing properties.

In this work, a Monte Carlo ray-tracing model for the simulation and optimization of a fiber Luminescent Solar Concentrator (LSC) based on stacked layers of fiber arrays is developed and validated. The fiber LSC efficiency improvements are compared against a conventional planar LSC. We developed a new model to analyze the performance of different configurations of bulk-doped fibers and fibers constituted by a doped coating and a passive core. These configurations are analyzed also varying fiber packing geometry diameters, and length. Due to the exceptionally low absorption coefficient of the silica fibers (αwg ≈ 10-4 cm-1), concentration factors of up to 1.9 are predicted when dimensions are scaled over 1 m2, which improve more than twice the maximum concentration factor ever reported. These results serve as a preliminary theoretical study for the future development of a new LSC design based on flexible silica micro-fibers coated with Si-QDs doped poly(lauryl methacrylate) (PLMA) layers.

A linked-scale coupled model of mass erosion and redistribution in plasma-exposed micro-foam surfaces

Surface evolution due to exposure to harsh environments is of importance in many scientific and technological applications. In plasma-exposed materials, the surface receives charged particles from the plasma, leading to a series of processes that drive the system far from equilibrium and may lead to the severe deterioration of the surface properties. Although surface morphological changes are driven by atomic collisions taking place over picoseconds and nanometers, these processes result in mass loss and redistribution of matter over much larger length and time scales. This necessitates a multi-scale approach capable of capturing the range of processes linking primary atomic collision events with engineering-level surface geometry changes. In this paper, we develop a computational model to simulate the morphological evolution and effective erosion rate of micro-architected tungsten foams during low-energy plasma ion bombardment. The model acts on several length scales, with the energy and angular dependence of the sputtering yield of flat tungsten surfaces determined using the SRIM code based on the binary collision approximation. This information is introduced into a low-fluence, short-term Monte Carlo raytracing model, and further into a high-fluence, long-term particle transport model. In the latter, material particles representing billions of atoms are described in a digitized 3-D representation of the foam structure from X-ray tomography data, and are sputtered off and redeposited using the atomistic information. We show that the redeposition of sputtered atoms leads to partial self-healing in the bottom layers with a sharp reduction in the sputtering coefficient of low density foams, roughly 25% of the solid W value. This is in qualitative agreement with recent experiments on low porosity W structures. At high fluence, the foam structure degrades considerably as there are fewer ligaments available to recapture sputtered atoms and, consequently, the sputtering rate increases again.

Analysis of optical properties in particle curtains

Optical properties in particle curtains are investigated by means of a Monte Carlo Ray-Tracing (MCRT) model. The MCRT model calculates absorptance, transmittance and reflectance of curtains formed by discrete particles under different conditions. The effect of particle diameter, curtain thickness and solids volume fraction is studied for particles made of two materials with different absorptivity: sand (αp = 0.34) and CARBO (αp = 0.87). The absorptance of curtains with sand particles and CARBO particles can increase up to 0.75 and 0.97, respectively, in opaque curtains with solids volume fractions close to zero, which requires very thick curtains. If the opacity of the curtain decreases due to a smaller thickness, a greater particle diameter or a smaller solids volume fraction, the curtain absorptance decreases. Besides the MCRT model, an analytical model is developed to predict the optical properties of particle curtains in a simpler way than the MCRT method. The MCRT is used to validate the analytical model. The maximum difference in the curtain absorptance calculated by the MCRT model and the absorptance calculated by the analytical model is 0.048 in the case of sand and 0.02 in the case of CARBO particles. These small differences make the analytical model a very useful tool to integrate the calculation of optical properties in complex models of particle systems.

Thermal performance evaluation of various nanofluids with non-uniform heating for parabolic trough collectors

Nanoparticles when used even in relatively low concentrations, can significantly alter the thermal properties of a base working fluid, thereby substantially enhancing the thermal performance of a power generation system. In the present study, numerical simulations were performed for a solar collector to test the effectiveness of six non-metallic nanoparticles, namely aluminum oxide (Al2O3), cerium oxide (CeO2), copper oxide (CuO), ferric oxide (Fe2O3), titanium dioxide (TiO2) and Silicon dioxide (SiO2). These nanoparticles were dispersed individually in three different base working fluids; therminol VP-1 (at 400 K), water (at 400 K) and molten salt (at 600 K) to form different nanofluids. Each of these nanofluids were then examined with three different volume fractions (2%, 4% and 6%) in addition to the pure base working fluid case for a range of Reynolds number (Re=104-105). For the simulation the Monte Carlo Ray Tracing (MCRT) model was used to represent the non-uniform heat flux around the absorber tube of the Parabolic Trough Collector (PTC). The results show that the enhancement of the thermal and the hydraulic performances depended upon the combination of the nanoparticles and the base working fluid. Silicon dioxide, SiO2, however, was found to be the most efficient nanoparticle regardless of the choice of the base working fluid for all the tested volume fractions. For example, for typical operating conditions for SiO2 with a volume fraction (VF) of 6%, the average Nusselt number of the water-based mixture was enhanced by 32.4%, with a thermal efficiency improvement of 5.11% and performance evaluation criterion (PEC) of 1.313. On the other hand, for a molten salt-based SiO2 mixture, the average Nusselt number was improved by 21.36% and thermal efficiency by 9.92% with a PEC of 1.155. Finally, the corresponding improvements with therminol VP-1-base fluid were 15.6%, 9.18% and 1.21 for the average Nusselt number, thermal efficiency and PEC respectively.

A 28 kWe multi-source high-flux solar simulator: Design, characterization, and modeling

Singapore’s first multi-source high-flux solar simulator (HFSS) that comprises an array of seven 4-kWe xenon short-arc lamps close-coupled with ellipsoidal reflectors is designed, constructed, and characterized. An indirect mapping system that consists of a water-cooled Lambertian target, a CMOS camera, and a heat-flux gauge is adopted herein to experimentally characterize the performance of the developed HFSS. A calibration constant of 192.8 ± 9.6 kW m−2 (GS µs−1)–1 is determined and used to transform the raw-flux maps with grayscale values to the absolute-flux maps. Utilizing a series of flexible and precise adjusters, the variability of ± 1 mm in the x- and y-directions of the peak flux points of the radiation modules is measured. A peak flux of 7740 kW m−2 is measured for the sum of all 7 radiation modules. Average radiative fluxes of 4191 kW m−2 and 2933 kW m−2 can be observed from circular target areas of Ø40 mm and Ø60 mm, respectively, associated with promising electricity-to-target transfer efficiencies of 18.8% and 29.6%. A Monte Carlo ray-tracing (MCRT) model is calibrated by the experimental data. Based on the minimum root-mean-standard deviation between the measured and simulated results of the radiative power on the target, the optimal parameters of the MCRT model include the arc radius of 3.4 mm and the reflector surface error of 1.1 mrad.

Pairing Directional Solar Inputs From Ray Tracing to Solar Receiver/Reactor Heat Transfer Models on Unstructured Meshes: Development and Case Studies

Abstract A novel method for pairing surface irradiation and volumetric absorption from Monte Carlo ray tracing to computational heat transfer models is presented. The method is well-suited to directionally and spatially complex concentrated radiative inputs (e.g., solar receivers and reactors). The method employs a generalized algorithm for directly mapping absorbed rays from a Monte Carlo ray tracing model to boundary or volumetric source terms in the computational mesh. The algorithm is compatible with unstructured, two and three-dimensional meshes with varying element shapes. Four case studies were performed on a directly irradiated, windowed solar thermochemical reactor model to validate the method. The method was shown to conserve energy and preserve spatial variation when mapping rays from a Monte Carlo ray tracing model to a computational heat transfer model in ansys fluent.