Monte Carlo Ray-tracing Methods Research Articles

Enhancing optical and thermohydraulic performance of linear Fresnel collectors with receiver tube enhancers: A numerical study

Linear Fresnel collector system is a type of solar thermal utilization installation. It is mainly used to focus solar radiation on the receiver tube to generate high temperature thermal energy which can be applied in many aspects. This study investigates five different types of receiver tube models using water as the flowing medium, which leads to improving heat transfer performance by means of being equipped with enhancers. The optical results and thermohydraulic performance of the Linear Fresnel collector system are evaluated through Monte Carlo Ray Tracing and Computational Fluid Dynamics (CFD) simulation methods. The obtained optical results reveal that the distribution of energy flux density concentrating on the receiver tube surpasses that of the parabolic trough collector system. In order to maintain an energy reception rate of 90.0% or higher, the installation height range of the receiver tube should be between 1468–1532 mm. When the sun-tracking error increases from 0.0° to 1.0°, the overall optical efficiency of the Linear Fresnel collector system decreases from 98.9% to 79.7%. The thermohydraulic performance shows that the enhancers can generate a swirling flow as its main feature, which effectively promotes the homogenization of the temperature field and destroys the boundary layer near the receiver tube wall, thus greatly improving the convective heat transfer in the receiver tube. However, the introduction of the enhancers also means that the pressure drop loss of the receiver tube increases. Furthermore, the impact of the Kenics mixer's enhancer on heat transfer and flow characteristics is further amplified as the rotation rate decreases, owing to its alternating and complex structure. Through a comparative analysis of the influence of different receiver tube models on Nusselt number (Nu), Frictional resistance coefficient (f) and comprehensive evaluation criterion (PEC) under four rotation rate conditions (y = 6.3, 8.1, 11.34, 18.9), the conclusions present that the receiver tube with an enhancer at a low rotation rate has a larger Nu than that at a high rotation rate, while the Frictional resistance coefficient changes little at different rotation rates. Therefore, the PEC is higher when the receiver tube is equipped with a low-rotation-rate enhancer. Moreover, the receiver tube equipped with a Kenics mixer at a rotation rate of 6.3 was found to be the overall optimum case, with the PEC value varying from 1.47 to 1.73.

Numerical study on a novel solar-thermal-reaction system for clean hydrogen production of methanol-steam reforming

Renewable energy-driven hydrogen production methods offer a promising solution for mitigating energy crises. The present study introduces a new solar-driven hydrogen production system that within methanol steam reforming (MSR) process. The numerical analysis is carried out using a cylindrical Fresnel linear concentrator coupled with a solar thermal reactor (STR). A comprehensive kinetic model of the MSR reaction was constructed and combined with analytical methods based on Monte Carlo ray tracing methods and finite volume methods to analyze the multi-physical properties involved in the reaction process. The results indicate that a novel solar-driven MSR-STR system can generate a sufficient driving force after adjusting the concentrator structure and relative position to match the energy demand of the MSR reaction. The accuracy of the model is compared with the experimental values, and the average error is 7.8 %. Tracking error of the component should be maintained below 1.5° to ensure the optical efficiency. The annual hydrogen production was analyzed and the levelized cost of hydrogen is $1.77/kg. Finally, a comparison is presented between the proposed MSR-STR system and parabolic trough concentrator based MSR hydrogen production system to evaluate the feasibility of the former in terms of the economics of implementation.

Heat transfer and thermal stress of a parabolic trough solar collector with secondary reflector using microencapsulated phase change material slurries

The traditional parabolic trough solar collectors (PTSC) often face the problems of high thermal stress and the deformation of the tubular receiver. In this paper, a secondary reflector (SR) is put into the traditional PTSC, and the microencapsulated phase change material (MEPCM) slurries with 7 vol%, 12 vol%, and 15 vol% are used as the operating fluids. By associating the Monte Carlo ray-tracing (MCRT), finite volume (FVM), and finite element (FEM) methods, the optical-thermal–mechanical coupling problem of the absorber tube is settled, the flow and heat transmission of MEPCM slurries in the collector tube are numerically investigated by applying the Eulerian-Eulerian multiphase flow model. The results show that the solar flux and wall temperature of the absorber tube with the SR are more uniform along the circumferential direction. The use of SR enhances the local heat transfer of the MEPCM slurries at the phase change section in the absorber tube, while when the phase changes finish, the local heat transfers of the slurries in the PTSC with SR have no difference with the traditional PTSC. Compared with the water, the MEPCM slurries obviously enhance the heat transfer of the absorber tube due to the phase change and the interactive motion of the microencapsulated particle, and the higher the slurry concentration, the larger the mean Nusselt number, the smaller the thermal stress. The addition of SR significantly decreases the thermal stress of the absorber tube, the maximal equivalent stress is reduced by 81.8%

Investigation into the effect of multiple line dipoles magnetic field through LS-3 parabolic trough solar system

This paper simulates the LS-3 parabolic trough solar collector with Monte Carlo Ray Tracing and computational fluid dynamics methods. The heat flux is non-uniform around the absorber tube. Therefore, multiple line dipoles magnetic field location can effect on solar collector performance. There is no article that has investigated the effect of this parameter in the parabolic trough solar collector. Also, this collector is simulated for the first time for June 21 and December 21, and the heat flux distribution for it is provided by the Monte Carlo Ray Tracing method. The multiple line dipoles magnetic field leads to a decrease in the maximum temperature of the absorber tube wall by 2.3 %. The magnetic field intensity also affects the solar collector's performance. If the magnetic flux density is equal to 100 G, 75 G and 50 G, then the value of Performance Evaluation Criteria is equal to 1.47, 1.43 and 1.39, respectively. Finally, the magnetic field location affects the Nusselt number but does not affect the Darcy friction factor. For example, if the multiple dipoles magnetic field are placed at angles of 0, 30, 60, and 90°, then the Nusselt number ratio is 1.50, 1.42, 1.44, and 1.65, respectively.

Phonon ray tracing calculations of ballistic temperature and heat flux profiles in nanostructures

Phonon ray tracing calculations have been used to quantify phonon boundary scattering in nanomaterials and to interpret thermal conductivity measurements. However, Landauer-based phonon ray tracing methods have not been able to access the temperature or heat flux profiles within nanomaterials, meaning that computationally intensive Boltzmann Transport Equation solvers are needed to gain insight into ballistic transport physics or model nanoscale temperature mapping experiments. Here, we derive and apply phonon Monte Carlo ray tracing methods to calculate the local temperature and local heat flux in semiconducting nanomaterials, with a focus on the ballistic transport regime. The derivation provides a straightforward interpretation of the local temperature in terms of a thermal conductance ratio, and the local heat flux in terms of the difference between forward- and reverse-oriented phonon trajectories crossing a control surface. After validating the method for several common transport regimes and geometries, we apply the method to optimize geometric parameters that lead to locally inverted temperature gradients in porous nanomeshes, and to evaluate the heat focusing capabilities of geometric ballistic phonon lenses. These applications illustrate how phonon ray tracing methods can be used to quantify ballistic thermal profiles and to design nanostructures that exhibit atypical thermal behaviors in the ballistic regime.

Calculating the view factor of randomly dispersed multi-sized particles using hybrid GRU-LSTM recurrent neural networks regression

This study presents three novel methods for the computational acceleration of simulations of high temperature radiative heat transfer in particulate media. The paper presents a novel algorithm for neighbor searching that can locate and sort multi-sized particles, based on their distance, with minimal computation. The traditional use of regression recurrent neural networks for time series data is modified to a regression function for distance-ordered data. LSTM and GRU cells are used in a deep recurrent neural network to predict radiation view factors of particle and face neighbors through the distance-ordered sequential data. The preprocessing of sequential data is addressed with two new transformation methods. Through the use of the transformation methods, both the particle and face neighbors are regarded as general objects, allowing one regression neural network to predict the view factor of any number of particles and faces sequentially, which represents a unique feature. With neural networks and preprocessing methods, it is possible to predict the view factor of different-sized objects (particles and faces), representing a significant development in the field. The models demonstrate high precision for objects located within the short-range radiation region. Objects located a long distance from the emitting particle have an over-prediction of view factor, but due to their negligible contribution to overall radiation heat transfer, a high coefficient of determination is obtained for particle and face neighbors, with several orders of magnitude improvement in computational speed compared to typical Monte Carlo ray tracing methods.

Performance comparison of two monte carlo ray-tracing methods for calculating radiative heat transfer

Monte Carlo ray tracing (MCRT) methods are important for solving the radiative transfer equation. However, the particular ray-tracing procedure used has a significant impact on the calculational performance. In this paper, we analyze and compare the performances of MCRT pathlength (PL) and collision-based (CB) methods for different surface and medium parameters and different degrees of uniformity. The results show that in a gray medium and with a radiation heat balance system, the PL method is superior when the surface emissivity is less than 0.15 for surface elements, and when the mean optical thickness per element (MOTE) is small and the emissivity is large for space elements; otherwise, the CB method is superior. Also the overall performance of the PL method is better than that of the CB method. However, PL method is more sensitive to non-uniformity of medium parameters than the CB method, and that the PL method is more sensitive to uniformity of the scattering coefficient than to the absorption coefficient. In addition, for a gray body and uniform optical parameters, reducing the number of grids improves the performance of the MCRT. And the performance index of the PL method is the best when the cutoff level of energy beam tracking is 10–7 and 10–6 for surface and space elements respectively. These quantitative conclusions will help in selecting suitable MCRT methods for engineering calculations, based on the physical properties of materials and conditions.

Simulation and implementation of multiplexing modes on cold-neutron triple-axis spectrometer SIKA

SIKA, a high-flux cold-neutron triple-axis spectrometer at the open-pool Australian lightwater reactor (OPAL) at the Australian Nuclear Science and Technology Organisation (ANSTO), is equipped with a 13-blade analyser and position-sensitive detector. This multiplexing design endows SIKA high flexibility to run in both traditional triple-axis and multiplexing analyser modes. In this study, two different multiplexing modes on SIKA are simulated using Monte Carlo ray-tracing methods. The simulation results demonstrate SIKA’s capabilities to work in these operational modes, especially, the multi-Q const-Ef mode, where Q is the scattering wavevector and Ef is the final neutron energy. This capability was demonstrated by measuring the phonon dispersion of a Pb single-crystal sample with the multi-Q const-Ef mode on SIKA. Compared to the traditional and multi-analyser triple-axis spectrometers, multiplexing modes on SIKA combine the advantages of the high data-acquisition efficiency and flexibility to focus on local areas of interest in the momentum-energy transfer space, namely, the (Q, ħω) space.

Numerical simulation of a concentrating photovoltaic-thermal solar system combined with thermoelectric modules by coupling Finite Volume and Monte Carlo Ray-Tracing methods

During the last decades, the adoption of more strict safety and environmental regulations, as well as a rise in energy costs, sparked an increasing interest in the design of renewable energies systems, particularly solar systems, to supply both electrical power and heat. Because of their capability to simultaneously supply both electricity and heat, concentrating photovoltaic-thermal and thermoelectric hybrid systems have recently attracted scholarly attention. In this study, a detailed three-dimensional computational model of a novel concentrating photovoltaic-thermal solar system combined with thermoelectric modules in an integrated design with a triangular absorber and corresponding numerical simulations are presented. For this purpose, a three-dimensional integrated model combined the Finite Volume method with the Monte Carlo Ray-Tracing method is employed. After validating simulation results and providing some discussions on the effects of temperature and cooling on the photovoltaic modules performance, the impacts of varying two design parameters of the system, the aperture width of the reflector and the apex angle of the absorber, on the solar energy flux distribution and output power of photovoltaic modules are discussed. Finally, considering the conditions of the case study, a range of aperture widths for the reflector and apex angles for the triangular absorber are proposed.

Study on the coupled photon-thermal-stress integration method, characteristics with time and failure criterion in the solar molten salt cavity receiver

In this paper, the studies on the characteristics with time and failure criterion in the solar molten salt cavity receiver are carried out based on the coupled photon-thermal-stress integration method. Especially, the truly non-uniformed solar heat flux in the cavity receiver with different time are simulated using the optical simulation approach coupling the Monte Carlo ray tracing and Gebhart methods proposed by our team. Besides, the complex coupled heat transfer process of conduction, convection and radiative in the receiver are also comprehensively considered. Based on the integrally model, the solar flux, temperature and thermal stress on the absorber surfaces with different time are obtained. Then, the influences of different tube materials, flow layouts of molten salt on the coupling performances of photon-thermal-stress conversion process are discussed. Finally, the stress failure criterion in the molten salt cavity receiver is proposed based on the fracture mechanics theory. The results show that the temperature and thermal stress are influenced significantly by the non-uniformed solar flux, the distribution of temperature and thermal stress are also extremely non-uniformed. The solar flux and thermal stress change remarkably with different time, the peak heat flux and stress are maximum at 12:00 noon, and the peak thermal stress could increase by 11% and 10% compared with 11:00 and 13:00. It is also found that the peak thermal stress would be reduced effectively when the receiver tube with better thermal conductivity property and the molten salt flow layout which is from low-flux area to high-flux area are adopted. For example, compared with 800H receiver tube, the peak thermal stress of 316H tube could be reduced by 37%. Meanwhile, the peak thermal stress of flow layout which is from low to high flux area would be 20% lower than that with the opposite layout. The fracture toughness of receiver tube, which is influenced by the molten salt corrosion, is one of the key parameters to assess stress failure. Based on the stress failure criterion of critical solar flux proposed by this paper, the critical heat flux of stress failure would exhibit a decreasing trend when the fracture toughness degradation is serious in the molten salt receiver.

Direct three-dimensional design of compact and ultra-efficient freeform lenses for extended light sources

Achieving accurate control of the spatial energy distribution of extended sources is an ultimate goal of illumination design and has considerable significance in practical applications. To achieve this goal, we, for the first time, present a direct method to design compact and ultra-efficient freeform lenses for extended sources in three-dimensional geometries. Instead of using the traditional Monte Carlo ray-tracing and feedback methods to address the challenges caused by the extended sources, we develop a novel design method to numerically calculate both surfaces of the freeform lens directly. The proposed method is very efficient and robust in designing freeform lenses for Lambertian and non-Lambertian extended light sources. The final freeform lenses are very compact and highly efficient and can significantly reduce glare and light trespass.

Integrated numerical study on the coupled photon-thermal conversion process in the central solar molten salt cavity receiver

In a solar power tower system, the concentrating and collecting subsystem, including the heliostat field and the receiver, contains some of the most costly and technically challenging components. Many types of solar receivers make up the solar power tower system. Among them, the cavity molten salt receiver has been widely applied due to its lower heat loss. The accurate simulation of the coupled heat transfer process in the cavity receiver and the prediction of receiver performance are of great importance for the receiver design and safe operation. The solar flux distribution on cavity interior surfaces is extremely non-uniform, which has great effect on the receiver thermal performance. Taking the non-uniform solar flux distribution into consideration, the photon-thermal conversion process is simulated integrally in this paper. A hybrid simulation approach, which couples Monte Carlo ray tracing and Gebhart methods, is applied to investigate the complete solar radiation transfer process in the solar power tower system with a cavity receiver and obtain the non-uniform solar flux distribution. On this basis, the coupled photon-thermal conversion process in the molten salt cavity receiver is simulated. The effects of the fluid flow layout on the receiver performance under the non-uniform solar flux distribution are particularly revealed. Also, the variation of the receiver performance with time is analyzed in detail. The results show that the solar flux distribution on the interior surfaces of the cavity receiver is extremely non-uniform, under which the molten salt flow layout has great effects on the receiver performance. In the case of flow layout that the cold molten salt flows into the receiver in the high-flux area, the temperatures of both the absorber wall and the molten salt increase rapidly along the flow path. The temperature of the whole absorber wall is relatively high, which leads to greater heat loss and less amount of hot molten salt, and even seriously, “heat transfer deterioration” near the outlet of the receiver. When the cold molten salt flows into the receiver in the low-flux area, the temperature of both the absorber wall and the molten salt increase slowly along the flow path. The temperatures of the whole absorber wall is relatively low, which results in less heat loss and larger amount of hot molten salt. Although the shape of the solar flux distribution on the interior surfaces varied greatly with time, the evolutions of the molten salt (absorber wall) temperature at different time display similar variation tendency along the flow path. Moreover, the reflective loss varies with time greatly, while the heat loss varies with time slightly.

A novel integrated simulation approach couples MCRT and Gebhart methods to simulate solar radiation transfer in a solar power tower system with a cavity receiver

An integrated simulation approach, which couples Monte Carlo ray tracing (MCRT) and Gebhart methods, is proposed to simulate solar radiation transfer in a solar power tower system with a cavity receiver. The MCRT method is used to simulate the solar radiation transfer process from the heliostat field to interior surfaces of the cavity receiver, and the Gebhart method is used to simulate the multiple reflections process of solar radiation within the cavity. This integrated simulation method not only reveals the cavity effect on receiver performance but also provides real-time simulation results. Based on this method, the reflection loss of the cavity receiver and solar flux distributions are discussed in detail. The results indicate that the cavity effect can significantly reduce the reflection loss and homogenize the concentrated solar energy distributed on interior surfaces to some extent. Moreover, the surface absorptivity has less effect on the reflection loss when cavity effect is considered. The cavity effect on homogenizing solar flux distributions is greater with lower surface absorptivity. In addition, although the concentrated solar energy is distributed on the cavity aperture with similar shapes at different times, the shape of the solar flux distribution on interior surfaces varies greatly with time.

Mixed-level optical simulations of light-emitting diodes based on a combination of rigorous electromagnetic solvers and Monte Carlo ray-tracing methods

Over the last two decades, extensive research has been done to improve light-emitting diodes (LEDs) designs. Increasingly complex designs have necessitated the use of computational simulations which have provided numerous insights for improving LED performance. Depending upon the focus of the design and the scale of the problem, simulations are carried out using rigorous electromagnetic (EM) wave optics-based techniques, such as finite-difference time-domain and rigorous coupled wave analysis, or through ray optics-based techniques such as Monte Carlo ray-tracing (RT). The former are typically used for modeling nanostructures on the LED die, and the latter for modeling encapsulating structures, die placement, back-reflection, and phosphor downconversion. This paper presents the use of a mixed-level simulation approach that unifies the use of EM wave-level and ray-level tools. This approach uses rigorous EM wave-based tools to characterize the nanostructured die and generates both a bidirectional scattering distribution function and a far-field angular intensity distribution. These characteristics are then incorporated into the RT simulator to obtain the overall performance. Such a mixed-level approach allows for comprehensive modeling of the optical characteristic of LEDs, including polarization effects, and can potentially lead to a more accurate performance than that from individual modeling tools alone.

Study on optical and thermal performance of a linear Fresnel solar reflector using molten salt as HTF with MCRT and FVM methods

Abstract A novel linear Fresnel reflector which employs the evacuated tube, CPC secondary reflector, and uses molten salt as the heat transfer fluid (HTF) was designed and studied in this paper. A 3D optical model was developed to simulate the radiation transmission within the system with Monte Carlo Ray Tracing (MCRT) method. Based on the model, firstly, the optical performance of the systems using cylindrical and parabolic mirrors was compared. Then the local solar flux distribution on the absorber surface and the optical efficiency were computed. Then the effects of the slope error, time and location, etc. were investigated. Finally, the thermal performance was investigated by coupling the MCRT with the Finite Volume Method (FVM). The optical simulation results indicate that the system with optimized cylindrical mirrors can achieve nearly the same performance as the one with parabolic mirrors. The solar flux distribution on the absorber exhibits a non-uniform characteristic which can be improved by using mirrors with proper slope error. The instantaneous optical efficiency of 65.0% at normal incidence and the annual mean optical efficiency which ranges between 55.2% and 34.8% from the equator to N50° can be achieved. The numerical results indicate that the temperature profiles on the absorber follow the non-uniform solar flux. The collector efficiencies are all above 46.0% under the studied conditions. Both the thermal efficiency and the collector efficiency increase with decreasing salt temperature and with increasing radiation. These results suggest that the introduced system is a feasible choice for using molten salt as the HTF in Fresnel system.

Effect of Morphology on Spectral Radiative Properties of Three-Dimensionally Ordered Macroporous Ceria Packed Bed

In this paper, radiative characterization of a packed bed of novel three-dimensionally ordered macroporous (3DOM) ceria particles is performed in the spectral range relevant to solar thermochemical processes, 0.35–2.2 μm. Normal–hemispherical transmittance and reflectance of three samples of various thicknesses are measured. Monte Carlo ray-tracing (MCRT) and discrete ordinate methods are employed to identify transport scattering albedo and transport extinction coefficient in the spectral range corresponding to weak absorption in the semi-transparency band of ceria. 3DOM ceria particles are characterized by weaker scattering in comparison to sintered ceria ceramics, and increased transparency in the near-infrared spectral range 0.7–2 μm. The ordered pore-morphology of the 3DOM ceria after thermochemical redox cycling between temperatures 1373 K and 1073 K is altered due to sintering of walls of the 3DOM structure. The absorption coefficient of the packed bed is found to be practically independent of morphology. Radiative characterization of 3DOM ceria ceramics before and after thermochemical cycling suggests that preserving the 3DOM structure can lead to scattering characteristics that permit longer attenuation path lengths of incident concentrated solar radiation in the material, as well as be favorable for confinement of the near-infrared radiation during thermochemical cycling leading to favorable thermochemical conditions for fuel production.

Robust Image Denoising Using a Virtual Flash Image for Monte Carlo Ray Tracing

AbstractWe propose an efficient and robust image‐space denoising method for noisy images generated by Monte Carlo ray tracing methods. Our method is based on two new concepts: virtual flash images and homogeneous pixels. Inspired by recent developments in flash photography, virtual flash images emulate photographs taken with a flash, to capture various features of rendered images without taking additional samples. Using a virtual flash image as an edge‐stopping function, our method can preserve image features that were not captured well only by existing edge‐stopping functions such as normals and depth values. While denoising each pixel, we consider only homogeneous pixels—pixels that are statistically equivalent to each other. This makes it possible to define a stochastic error bound of our method, and this bound goes to zero as the number of ray samples goes to infinity, irrespective of denoising parameters. To highlight the benefits of our method, we apply our method to two Monte Carlo ray tracing methods, photon mapping and path tracing, with various input scenes. We demonstrate that using virtual flash images and homogeneous pixels with a standard denoising method outperforms state‐of‐the‐art image‐space denoising methods.

An improved wavelength selection scheme for Monte Carlo solvers applied to hypersonic plasmas

A new databasing scheme is developed for Monte Carlo Ray Tracing methods applied to hypersonic planetary entry. In this scheme, the complex relationships for the emission wavelength selection of atomic and molecular species in nonequilibrium flows are simplified by developing random number relationships for individual transitions, as opposed to using relationships for the spectral emission coefficient of a given species. These new techniques speed up wavelength selection by about 2 orders of magnitude, and offer flexibility for use in weighted or part-spectrum Monte Carlo solvers.

Comparison of Monte Carlo ray-tracing and photon-tracing methods for calculation of the impulse response on indoor wireless optical channels

We present a comparison between the modified Monte Carlo algorithm (MMCA) and a recently proposed ray-tracing algorithm named as photon-tracing algorithm. Both methods are compared exhaustively according to error analysis and computational costs. We show that the new photon-tracing method offers a solution with a slightly greater error but requiring from considerable less computing time. Moreover, from a practical point of view, the solutions obtained with both algorithms are approximately equivalent, demonstrating the goodness of the new photon-tracing method.

An Air-Based Cavity-Receiver for Solar Trough Concentrators

Abstract A cylindrical cavity-receiver containing a tubular absorber that uses air as the heat transfer fluid is proposed for a novel solar trough concentrator design. A numerical heat transfer model is developed to determine the receiver’s absorption efficiency and pumping power requirement. The 2D steady-state energy conservation equation coupling radiation, convection, and conduction heat transfer is formulated and solved numerically by finite volume techniques. The Monte Carlo ray-tracing and radiosity methods are applied to establish the solar radiation distribution and radiative exchange within the receiver. Simulations were conducted for a 50 m-long and 9.5 m-wide collector section with 120°C air inlet temperature, and air mass flows in the range 0.1–1.2 kg/s. Outlet air temperatures ranged from 260°C to 601°C, and corresponding absorption efficiencies varied between 60% and 18%. Main heat losses integrated over the receiver length were due to reflection and spillage at the receiver’s windowed aperture, amounting to 13% and 9% of the solar power input, respectively. The pressure drop along the 50 m module was in the range 0.23–11.84 mbars, resulting in isentropic pumping power requirements of 6.45×10−4−0.395% of the solar power input.