Monte Carlo Ray Tracing Research Articles

Non-line-of-sight multiple reflection underwater wireless optical communications channel model based on a capillary waves rough sea surface.

In a non-line-of sight reflective underwater wireless optical communications (UWOC) link, the transmitted beam relies on reflections from the sea surface to propagate to the underwater receiver. Most previous research on reflective channels has sufficiently considered single reflections from a smooth or rough surface, while ignoring the effect of multiple reflections. In fact, a rough sea surface may cause the reflected photons to hit the sea surface again, which is referred as a multiple reflection process. To make up for deficiencies in the existing literature, we first construct a capillary waves rough sea surface model, and then present a multiple reflection channel model with the help of the Monte Carlo ray tracing approach. The path loss and channel impulse response (CIR) were further evaluated based on the model for different communications scenarios. Numerical results suggest that multiple reflections increase the path loss by more than about 5dB, and reduce the CIR amplitude to less than one-third compared to a single reflection. The work done in this paper aims to provide theoretical support for UWOC system design.

Comparative optical performance investigation of Cross Compound Parabolic Concentrators designed with the elimination of multiple reflections principle

This paper evaluates low-profile solar concentrators with low-material consumption that improve the optical performance of Crossed-Compound-Parabolic-Concentrators (CCPCs) by using Eliminating-Multiple-Reflections (EMR). Three CCPC designs, LEMR, HEMR, and hybrid, were analysed using Monte-Carlo ray tracing, with optical efficiency and light flux uniformity as primary metrics. The LEMR and HEMR CCPCs achieved almost 88% average optical efficiency at the outlet aperture within the half acceptance angle. At the BCP, the LEMR CCPC attained an average efficiency of 85.7%, while the HEMR CCPC achieved an average efficiency of 85%. The hybrid CCPC achieved an average optical efficiency of 80% and 86% when the incidence angle was varied in the XY and YZ planes. The results showed that the non-uniformity of the light flux at the BCP remained low compared to the outlet aperture with changes in the incidence angle. A material consumption analysis revealed that the LEMR CCPC has the lowest material consumption as compared to the HMR and hybrid CCPCs and has a more uniform irradiance flux at the BCP. The LEMR CCPC has a relatively lower acceptance angle than the HEMR CCPC, making it suitable for a tracking system with a higher concentration ratio. The HEMR CCPC, on the other hand, can be used as a stationary concentrator due to its larger acceptance angle with a lower concentration ratio and can be applied in rooftops and building-integrated photovoltaics. Similarly, the hybrid CCPC has two different ranges of acceptance angles, making it advantageous for use in rooftops and building-integrated photovoltaics with seasonal tracking.

Ray-Trace Modeling to Characterize Efficiency of Unconventional Luminescent Solar Concentrator Geometries.

Luminescent solar concentrators (LSCs) are a promising technology to help integrate solar cells into the built environment, as they are colorful, semitransparent, and can collect diffuse light. While LSCs have traditionally been cuboidal, in recent years, a variety of unconventional geometries have arisen, for example, circular, curved, polygonal, wedged, and leaf-shaped designs. These new designs can help reduce optical losses, facilitate incorporation into the built environment, or unlock new applications. However, as fabrication of complex geometries can be time- and resource-intensive, the ability to simulate the expected LSC performance prior to production would be highly advantageous. While a variety of software exists to model LSCs, it either cannot be applied to unconventional geometries, is not open-source, or is not tractable for most users. Therefore, here we introduce a significant upgrade of the widely used Monte Carlo ray-trace software pvtrace to include: (i) the capability to characterize unconventional geometries and improved relevance to standard measurement configurations; (ii) increased computational efficiency; and (iii) a graphical user interface (GUI) for ease-of-use. We first test these new features against data from the literature as well as experimental results from in-house fabricated LSCs, with agreement within 1% obtained for the simulated versus measured external photon efficiency. We then demonstrate the broad applicability of pvtrace by simulating 20 different unconventional geometries, including a variety of different shapes and manufacturing techniques. We show that pvtrace can be used to predict the optical efficiency of 3D-printed devices. The more versatile and accessible computational workflow afforded by our new features, coupled with 3D-printed prototypes, will enable rapid screening of more intricate LSC architectures, while reducing experimental waste. Our goal is that this accelerates sustainability-driven design in the LSC field, leading to higher optical efficiency or increased utility.

Erosion evolution and mass loss of polyimide in AO environment using 3D Monte Carlo method

Polyimide films coated on the surface of satellites are exposed to atomic oxygen (AO) environment during service. This will suffer variation of microscopic structure and morphology, leading to corrosion and mass loss in macroscopic. In this work, a 3D undercutting model based on Monte Carlo method (MC) and Ray Tracing method was constructed, with which the AO related erosion feature of polyimide film was evaluated. Then, we predicted the evolution of erosion cross-section of the coated material with defect of different sizes and shapes exposed in higher AO fluence. The macroscopic mass loss was derived by the eroded microscopic volume and material density, during which the actual roughness was considered in our proposed undercutting model. This work provides insights into the study of microscopic 3D erosion morphology and connects with macroscopic mass loss in experiment at higher fluences.

Numerical analysis of a modified combined volumetric filtered concentrating PV/T system using MCRT and FVM coupled method

To solve the problem that solar cell temperature limits high-quality thermal output from a traditional concentrating PV/T (CPV/T) system, this paper proposes a linear CPV/T system with a vacuum tube receiver incorporated with a combined volumetric filter. In which a solid absorption glass along with liquid filter is used as the combined volumetric filter for the silicon CPV module. However, the entire photovoltaic/photothermal conversion process in the combined filtered CPV/T system is complicated. To achieve more accurate predicted performance, a three-dimensional numerical model of this system is presented and the combined Monte Carlo Ray-Tracing (MCRT) method with the Finite Volume Method (FVM) is employed. The stated optical models are validated against the experimental results from an indoor test under one sun. And the coupled method of MCRT and FVM is also verified. The optimal filter combination is obtained by optical simulation using LightTools software based on the MCRT method firstly. Then, the solar energy flux distribution on the receiver achieved from LightTools is adopted as a boundary condition for CFD simulation. Based on these models and the MCRT-FVM coupled method, parametric study is conducted to quantify optical efficiency, temperature distribution, energy and exergy efficiency of the system. The results show that the optimal combined filters for silicon PV module are HB700+water and HB700+propylene glycol (PG). Because the optical absorption of water differs from that of PG, the PV maximum temperature is 308.36 K using HB700+water and it is 321.94 K using HB700+PG, whereas the outlet temperature of water and PG exceeds 352 K. The greater thermal and total exergy efficiencies can be achieved at higher inlet temperature and lower mass flow rate, while the electrical exergy efficiency shows slight change. To further reduce heat loss, an indium tin oxide (ITO) coating with low emissivity is applied on the receiver surface. The results reveal the receiver with 240° ITO coating achieves the highest total exergy efficiency of 14.92% and optical efficiency of 88.94%.

Light absorption by Volvocaceae colonies consisting of equidistant optically soft photosynthetic cells in a transparent spherical extracellular matrix

This study aims to develop a methodology to predict the absorption cross-section of colonial microalgae such as those of the Volvocaceae family including Eudorina, Pleodorina, and Volvox consisting of an ordered assembly of large and optically soft absorbing cells embedded within a non-absorbing spherical extracellular matrix (ECM). The absorption cross-section of spherical colonies, such as Eudorina, containing 16, 32, and 64 equidistant photosynthetic cells distributed on the surface of a sphere within a concentric spherical ECM was predicted by the superposition T-matrix method for ECM size parameters as large as 500 and by the Monte Carlo ray-tracing (MCRT) method for ECM size parameters as large as 900. The predicted absorption cross-sections given by the two methods were in excellent agreement despite the fact that the conditions for geometric optics were not rigorously satisfied. The absorption cross-section of the microalgae colonies considered was found to increase with increasing cell radius, absorption index or cell pigment concentration, and/or number of cells. Shading among cells decreased the mass absorption cross-section and was increasingly important for colonies with strongly absorbing cells, large cell radius, and/or large number of cells. These results demonstrate that accounting for shading effects is necessary to accurately predict the absorption cross-section of microalgae colonies. Furthermore, the study demonstrated that the MCRT method is an accurate and efficient method for modeling light absorption by an ensemble of many large, ordered, and optically soft particles. Finally, the impact of colony formation as well as pigment and biomass concentrations on the local (LRPA) and mean (MRPA) rates of photon absorption within a microalgae culture was also assessed. At low biomass concentrations, the LRPA and MRPA decreased in the presence of colonies. This effect was more pronounced at higher pigment concentrations.

Second-Law Considerations in Monte Carlo Ray-Trace and Discrete Green’s Function Analysis of Coupled Radiation and Conduction Heat Transfer

Abstract A generic Monte Carlo ray-trace (MCRT) engine for computing radiation distribution factors (RDFs), working in tandem with an efficient finite volume formulation based on discrete Green’s functions (DGFs), has been used to solve a massive (2636 nodes) coupled radiation/conduction problem. Distribution factors computed using the MCRT method are known to produce unconditionally stable solutions to pure radiation problems even though the RDFs themselves do not conform exactly to the reciprocity principle. However, when these same RFDs are introduced into time-dependent finite volume conduction formulations based on DGFs, the resulting model is found to be fundamentally unstable due to inherent violations of the second law of thermodynamics. A novel approach to addressing this instability is presented and demonstrated for the case of a telescope typical of those employed to monitor the planetary energy budget from low Earth orbit.

Numerical investigation of an enhanced PTC absorber tube using cylindrical inserts

AbstractThe main purpose of this study is to investigate numerically the thermal performance of a parabolic trough solar collector's absorber tube that contains a novel kind of inserts with the objective to improve the heat transfer between the heat transfer fluid and the absorber tube. In the first part of this paper, the diameter and the length of the cylindrical inserts are investigated based on finite volume method and Monte Carlo ray tracing method for Reynolds number ranges from 2.36 × 104 to 7.09 × 104. In the second part, the eccentricity of the cylindrical inserts is investigated under the same operating conditions. The Therminol®VP1 is the HTF that is used in this investigation's intermediate fluid. The numerical simulation indicates that the perturbators enhance the thermal behavior of the receiver and reduces the absorber tube's temperature difference.

A secondary reflector geometry optimization of a Fresnel type solar concentrator

The present paper aims to develop a new shape of secondary reflector for Fresnel linear concentrator; it represents the continuation of a series of optimization carried out on a solar prototype with Fresnel type concentration, the new developed form represents a Double Parabolic Concentrator (DPC). This study was carried out through an optimization scheme in order to determine the parameters that influence on the prototype's performances. For this purpose, a numerical model is proposed. It aims at determining the different performance of the solar prototype. In this context, a parametric study and an optimization were carried out in order to determine the influence of certain parameters on the prototype's performances, namely: 1) the secondary reflector: Double Parabolic Concentrator (DPC), and 2) the different elements that characterize it such as the focal length, the aperture, the spacing, and the tube position. The modeling of the various parameters describing the solar field was carried out by the Monte Carlo ray-tracing (MCRT) method. The results are analyzed and presented in the form of tables and curves. In fact, they allowed the determination of the optimal configuration of the solar receiver. According to the optimization results, we can notice that the optimal DPC is not a double parabola. Moreover, the new form of DPC has a power gain of 10 to 13% compared to the existing form and also gives an optical efficiency of the receiver which reaches 100 %, that means that all the rays which cross the glass are absorbed by the tube. As a result, this configuration gives better theoretical and experimental performance. On the basis of this optimal configuration, several thermal power stations based on LFR technology have been produced and marketed.

A numerical and experimental comparison of heat transfer in a quasi two-dimensional packed bed

Heat transfer plays a major role in many industrial processes taking place in packed beds. An accurate and reliable simulation of the heat exchange between particles is therefore crucial for a reliable operation and to optimize the processes in the bed. The discrete ordinates method (DOM) provides an established numerical technique to model radiative heat transfer in granular media that offers the possibility to consider the directional dependence of the radiation propagation. In this work, DOM is compared with Monte Carlo ray tracing, which provides an alternative method for heat transfer simulations. Geometrically simple configurations are used to investigate the influence of the angular discretization on the accuracy of the results and the computation time in both methods. The obtained insights are then transferred to a more complex configuration of a quasi two-dimensional test rig consisting of metal rods for which also experimental results are available. Our results show that both DOM and Monte Carlo ray tracing allow for an accurate simulation of heat transfer in packed beds. Monte Carlo ray tracing requires thereby computation times that are surprisingly competitive (although still somewhat slower) compared to DOM and allows for an easier computation of highly accurate reference solutions. In our preliminary comparison to the experimental test rig, Monte Carlo ray tracing also provides the advantage that it can more easily model highly specular materials such as stainless steel. Both methods are comparable for diffuse materials such as magnesium oxide.

Numerical study on the start-up strategy of cylindrical tubular receiver filled with nickel foam

A cylindrical tubular receiver filled with nickel foam (CTRNF) appears to be a promising receiver concept due to its capability to provide temperatures over 900 °C at a high pressure. Nevertheless, CTRNF is challenged by the unsteadiness and high intensity of radiant flux may cause inaccurate start-up time predictions, providing unrealistic and unstable outlet temperatures. In this article, these matters are addressed by a numerical model coupling the Monte Carlo ray tracing (MCRT) method and discrete coordinate (DO) radiative transfer model. The model considers the cold, semi-cold, semi-hot, and hot start-up strategies on the CTRNF using N2 and Ar as the working medium. Three preheating methods and their influence on the thermal characteristic of the CTRNF are proposed. Temperature and equivalent stress distribution of internal heat absorber is simulated and analyzed. The results indicate that the duration for four start-up strategies takes 1711 s, 2189 s, 1212 s and 1032 s as the outlet temperature reaches to 1200 K, respectively, when Ar is used as the working medium (10 g/s) and radiative power of 10.5 kW incidents on the solar receiver. Compared with the 68.44 kg Ar consumption of cold start-up strategy, the Ar working medium consumption decrease by 24.44 kg, 51.68 kg, and 63.16 kg for semi-cold start-up, semi-hot start-up, and hot start-up strategy. However, the maximum equivalent stress of the CTRNF under the hot start-up strategy is 100 MPa higher than under the cold start-up strategy, which requires accurate monitoring of the cavity temperature in solar receiver.

Optimizing triangle mesh lenses for non-uniform illumination with an extended source.

Precise control of irradiance distribution is a complicated problem for freeform lens design, especially when the target is non-uniform. Realistic sources are often simplified as zero-etendue ones in cases designed for content-rich irradiance fields while the surfaces are usually assumed smooth everywhere. These practices can limit the performance of the designs. We developed an efficient proxy for Monte Carlo (MC) ray tracing under extended sources, with the linear property of our triangle mesh (TM) freeform surface. Our designs show finer irradiance control compared to their counterparts from the LightTools design feature. One of the lenses is fabricated and evaluated in an experiment, and performed as expected.

Effect of Mask Geometry Variation on Plasma Etching Profiles

It is becoming quite evident that, when it comes to the further scaling of advanced node transistors, increasing the flash memory storage capacity, and enabling the on-chip integration of multiple functionalities, "there's plenty of room at the top". The fabrication of vertical, three-dimensional features as enablers of these advanced technologies in semiconductor devices is commonly achieved using plasma etching. Of the available plasma chemistries, SF6/O2 is one of the most frequently applied. Therefore, having a predictive model for this process is indispensable in the design cycle of semiconductor devices. In this work, we implement a physical SF6/O2 plasma etching model which is based on Langmuir adsorption and is calibrated and validated to published equipment parameters. The model is implemented in a broadly applicable in-house process simulator ViennaPS, which includes Monte Carlo ray tracing and a level set-based surface description. We then use the model to study the impact of the mask geometry on the feature profile, when etching through circular and rectangular mask openings. The resulting dimensions of a cylindrical hole or trench can vary greatly due to variations in mask properties, such as its etch rate, taper angle, faceting, and thickness. The peak depth for both the etched cylindrical hole and trench occurs when the mask is tapered at about 0.5°, and this peak shifts towards higher angles in the case of high passivation effects during the etch. The minimum bowing occurs at the peak depth, and it increases with an increasing taper angle. For thin-mask faceting, it is observed that the maximum depth increases with an increasing taper angle, without a significant variation between thin masks. Bowing is observed to be at a maximum when the mask taper angle is between 15° and 20°. Finally, the mask etch rate variation, describing the etching of different mask materials, shows that, when a significant portion of the mask is etched away, there is a notable increase in vertical etching and a decrease in bowing. Ultimately, the implemented model and framework are useful for providing a guideline for mask design rules.

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.

A Detailed Numerical Study of a Nanofluid-Based Photovoltaic/THERMAL Hybrid System under Non-Uniform Solar Flux Distribution

The concentrated photovoltaic/thermal system (CPVT) adopting spectral beam splitting is a promising field of solar energy research. However, the thermo-electric properties of fluid-based CPVT collectors, which depend strongly on the non-uniform concentrated energy flux, remain unclear. This study aims to fill the gap and explore the thermo-electric properties of fluid-based CPVT collectors under non-uniform energy flux based on the finite volume method (FVM) with the Monte Carlo Ray-Trace (MCRT) method. The actual solar flux distribution on the receiver surface is obtained using Tracepro software. Then, the realistic non-uniform energy flux was employed in ANSYS Workbench 2022R1 software as a boundary condition to increase the accuracy of the CFD modeling of the system. The model is validated by comparing the results of the reference data. Moreover, the impact of uniform and non-uniform energy flux on the PV cell temperature is analyzed. In addition, the effects of mass flow rate on the electrical and thermal performance of the system are investigated. The results show that the PVT hybrid system has high conversion efficiency, with a total efficiency of more than 50%. Notably, the extreme non-uniformity of the solar-concentrated energy flux can result in local overheating of the PV cell, which may lead to irreversible damage.

Internally shielded receivers for parabolic trough solar concentrators operating with supercritical carbon dioxide: Analytical assessment

An urgent need to enable the operation of parabolic trough concentrators at higher temperatures for higher efficiency of the coupled power cycles is manifested in the state-of-art literature. Supercritical carbon dioxide is a promising fluid for the next generation of these concentrators. However, the inflated radiative thermal losses at elevated temperatures are the main challenge, and the relatively large absorber diameters are questionable for supercritical pressures. This study proposes a novel 1-D analytical model, coupled with Monte Carlo ray tracing, for evaluating internally shielded receivers based on the absorber tube’s diameter and eccentricity, the heat shield’s opening angle, as well as the operating conditions. The results suggest that increasing the tube diameter reduces the performance dependency on tube eccentricity, but also reduces the optical performance. Shielding the receiver is favorable for opening angles up to 160°. The impact of operating pressure is marginal, compared to flow rate and temperature. A shielded receiver with an opening angle of 160°, eccentricity of −10 mm, and tube diameter of 50 mm maximizes the exergy efficiency up to 45.9%, whereas a receiver with corresponding values of 80°, −15 mm, and 80 mm, respectively, maximizes the energy efficiency up to 77.2%. Shielded receivers also boost the efficiency by up to 10.6%, compared to conventional 70 mm receivers. These performance enhancements are expected to elevate the efficiency of the integrated systems with minimal economic and environmental impacts due to the simplicity of the proposed solution.

Performance Simulation of Solar Trough Concentrators: Optical and Thermal Comparisons

The solar trough concentrator is used to increase the solar radiation intensity on absorbers for water heating, desalination, or power generation purposes. In this study, optical performances of four solar trough concentrators, viz. the parabolic trough concentrator (PTC), the compound parabolic concentrator (CPC), the surface uniform concentrator (SUC), and the trapezoid trough concentrator (TTC), are simulated using the Monte Carlo Ray Tracing method. Mathematical models for the solar trough concentrators are first established. The solar radiation distributions on their receivers are then simulated. The solar water heating performances using the solar trough concentrators are finally compared. The results show that, as a high-concentration ratio concentrator, the PTC can achieve the highest heat flux, but suffers from the worst uniformity on the absorber, which is only 0.32%. The CPC can generate the highest heat flux among the rest three low-concentration ratio solar trough concentrators. Compared with the PTC and the CPC, the TTC has better uniformity, but its light-receiving ratio is only 70%. The SUC is beneficial for its highest uniformity of 87.38%. Thermal analysis results show that the water temperatures inside the solar trough concentrators are directly proportional to their wall temperature, with the highest temperature rise in the PTC and the smallest temperature rise in the TTC. The solar trough concentrators’ thermal deformations are positively correlated to their wall temperatures. The radial deformation of the SUC is much larger than those of other solar trough concentrators. The smallest equivalent stress is found in the SUC, which is beneficial to the long-term operation of the solar water heating system.

A generalized characterization of radiative properties of porous media using engineered features and artificial neural networks

At the core of many engineering applications is the evaluation of the radiative responses of materials and devices, particularly at high temperatures. Researchers have focused their attention for decades on the investigation of radiation heat transport (RHT) in porous media. Monte Carlo ray tracing (MCRT) is a reliable computational alternative to the otherwise expensive or infeasible experimental measurements for RHT in heterogeneous media. Despite the accuracy, the computational cost of MCRT simulations can be burdensome given the convergence requirements, the complexity of materials (e.g., large number of particles in a porous medium), and space of input physical specifications (e.g., angle and/or location of incoming rays, wavelength, etc.). This study is an attempt to replace MCRT simulations with supervised learning algorithms. Ground truth labeling data is generated using MCRT for random overlapping and non-overlapping circular packed porous media and solving the classical radiative transfer equation (RTE). A significantly low-cost physical and geometrical-based artificial neural network (ANN) model is developed to forecast the radiative characteristics of arbitrary porous configurations, using engineered geometric features. Using the ANN model, a sensitivity analysis is conducted to assess the relative importance of the designed features. This insight helps in identifying the most crucial features to improve the learning for more complex geometries. The precision of predictions is calculated based on different hypotheses made over the training data, i.e., whole-size, wall-wise, and pointwise classes. It is shown that when overlapping circles are used as training, the designed model can predict the radiative properties of out-sample data for the first two classes with the accuracy of R2>0.944 and R2>0.787, while more directional engineering features are required to achieve higher accuracies for the former class. Results show that the current model is highly generalizable and applicable to a variety of porous configurations.

Identification of the spectral complex refractive index of pure silica micrometric fibers versus temperature

A new methodology was developed to identify the spectral complex refractive index of semi-transparent solid phases within heterogeneous media up to high temperatures. In this methodology, numerical samples are first generated from statistical information about the microstructure of a given material obtained by X-ray micro-tomography. The radiative properties of these numerical samples (reflectance, transmittance, and emittance) are then simulated with a Geometric Monte Carlo Ray Tracing code while the radiative properties of the real material are measured by Fourier Transform Infrared spectroscopy. The spectral complex refractive index is then extracted thanks to an optimization algorithm, by minimizing the discrepancies between the experimental and numerical radiative properties. This methodology was applied to a fibrous sample made of pure silica. The complex index was extracted at 295 K and 1010 K. In both cases, it enabled a good prediction of the radiative properties of the material and proved the relevance of performing calculations with characteristics identified at the local scale instead of the bulk ones.

Computational Modeling of a Small-Scale, Solar Concentrating Device Based on a Fresnel-Lens Collector and a Flat Plate Receiver with Cylindrical Channels

The energy efficiency of a small-scale solar concentrating thermal device is investigated, based on Monte-Carlo Ray-Tracing (MCRT) and Computational Fluid Dynamics (CFD) modeling. The device consists of a Fresnel lens collector—engraved on a 1 m rectangular plate—and a 10 cm sized plate receiver, with drilled cylindrical channels with a diameter of 10 mm. Inlet velocities and heat transfer fluid (HTF) temperatures lie within the range of 0.25–1 m/s and 100–200 °C, respectively. The configurations examined involve the utilization of a selective coating on the absorbing surface of the receiver, increasing the channel diameter to 15 mm and the receiver size to 20 cm, and insertion of a glass envelope in front of the absorbing surface. Energy efficiency increases with increasing fluid velocity up to 80%, a level beyond which no further improvement is observed. The coating contributes to a reduction in heat losses; it brings substantial benefits for the lower velocities examined. The increase in channels diameter also contributes to an increase in the energy efficiency, while the increase in receiver dimensions leads to the opposite effect. The glass cover does not improve the performance of the collector, due to substantial optical losses.