Solar Reflectors Research Articles

Performance investigation of linear evacuated absorber of 2-stage solar Linear Fresnel Reflector module under non-uniform flux distribution

Performance and heat loss analyses have been carried out for a 2-stage parabolic receiver system of the solar Linear Fresnel Reflector module. Performance of the system with non-uniform distribution of the flux on the absorber and the influence of the wind on the receiver system are investigated. Solar flux distribution on the surface the absorber is applied by user-defined function to analyse the heat loss under the real-time scenario. The performance of the absorber tube when coated or uncoated with a selective material is studied for different flux conditions. This provides a perception to adopt appropriate coating material for different locations under varied DNI conditions. The investigation has been carried out to analyse the heat loss effects when the absorber is placed under evacuated and non-evacuated conditions. All the modes of heat transfer are considered in the study and the dominance of each mode of heat transfer is highlighted. Performance of the numerical data with the non-uniform flux distribution on the absorber with the measured velocity and emissivity values are validated with the experimental data. Deviation of ±10 % is observed between the experimental and the numerical values.

Phase Change Materials in Transparent Building Envelopes: A Strengths, Weakness, Opportunities and Threats (SWOT) Analysis

Building envelopes can play a crucial role in building improvement efficiency, and the adoption of Phase Change Materials (PCMs), coupled with transparent elements, may: (i) allow a better control of the heat flows from/to the outdoor environment, (ii) increase the exploitation of solar energy at a building scale and (iii) modulate light transmission in order to prevent glare effects. Starting from a literature review, focused on experimental works, this research identifies the main possible integrations of PCMs in transparent/translucent building envelope components (in glazing, in shutters and in multilayer façade system) in order to draw a global picture of the potential and limitations of these technologies. Transparent envelopes with PCMs have been classified from the simplest “zero” technology, which integrates the PCM in a double glass unit (DGU), to more complex solutions—with a different number of glass cavities (triple glazed unit TGU), different positions of the PCM layer (internal/external shutter), and in combination with other materials (TIM, aerogel, prismatic solar reflector, PCM curtain controlled by an electric pump). The results of the analysis have been summarised in a Strengths, Weakness, Opportunities and Threats (SWOT) analysis table to underline the strengths and weaknesses of transparent building envelope components with PCMs, and to indicate opportunities and threats for future research and building applications.

Space Charge Accumulation Characteristics in Fluorinated Insulating Material under DC High Stress

Electron beam irradiated fluorinated materials, which are used for an OSR (Optical Solar Reflector) of spacecraft have been investigated using a space charge measurement system under DC high stress. It was found that a large amount of space charge accumulated in the electron beam irradiated ethylene-tetrafluoroethylene (ETFE) and fluorinated ethylene-propylene copolymer (FEP) under DC high stress, while it was not observed in non-irradiated them. In this report, a dependence of relaxation time after the irradiation on the space charge accumulation characteristics was also studied.

Studies on Thin Film Multilayer Coatings Deposited using Sputtering Process

Multilayer coatings have been modelled and prepared experimentally for optical and mechanical properties for solar reflector applications on ceramic substrate. Aluminium thin films are ideally suited for reflector. This value of reflection is reduced reasonably by integrating multilayer thin films based on dielectric materials, behaving as bond and protective layer, aluminium thin films in combination with alumina coatings are also suitable for solar reflector coatings, aluminium thin films were coated on ceramic substrate using DC magnetron sputtering process in argon atmosphere. The effect of thickness of bond layer and protective layers on aluminium coatings of 400nm are characterized for optical and mechanical and properties has been presented in this paper. Multilayer coatings deposited with substrate temperature at RT with coating thickness of (100nm) Al2O3 as bond layer and protective layer as lower surface roughness (Ra<6nm) compared to the coating deposited with coating thickness of (300nm) Al2O3 with aluminium as reflective layer, as a result the coating had a hardness of 7 GPa, while that of pure aluminium deposited at RT with coating thickness of (100nm) Al2O3 as bond layer and protective layer had a hardness of 4.5 GPa. The simulation and experimental responses of multilayer of aluminium thin films deposited at in the wavelength range of 250-1500 nm were similar.

The effect of incidence angle on the reflectance of solar mirrors

Solar reflectors for Concentrating Solar Technologies require a high reflectance in the terrestrial solar spectrum (280–4000nm). Besides the wavelength, the reflectance of solar mirror materials is also dependent on the incidence angle of the incoming sunlight. The commonly used measurement equipment measures the reflectance at fixed near-normal incidence angles, typically between 8° and 15°. In this work, the annual incidence angle frequency distribution has been calculated for a LS3/Eurotrough-type parabolic-trough collector located at different sites, and for the heliostat field of the solar tower system CESA-1 located at the Plataforma Solar de Almería in Tabernas, southern Spain. It was found that the most frequent incidence angles registered in the solar field are quite higher than the ones at which reflectance is measured with state of the art instruments, obtaining mean incidence angles in the range of 28–35° depending on the type and location of the solar field.A methodology to predict the off-normal reflectance of silvered-glass mirrors based on near-normal reflectance and transmittance measurements of the uncoated glass is presented. The complex refractive index of 2, 4 and 5mm thick solar glass and the deposited silver was determined and used to model the solar weighted reflectance of silvered-glass mirrors at different incidence angles. The model was compared to experimental measurements. For this purpose, the Spectral Specular Reflectometer (S2R) has been improved and updated with a polarizer crystal to measure reflectance at perpendicular (s-pol) and parallel (p-pol) polarizations up to incidence angles of θ = 70°.Eight solar mirror materials (three silvered-glass mirrors of different glass thicknesses, two anti-soiling coated glass mirrors, two enhanced aluminum reflectors and a silvered polymer film) have been measured over a broad range of incidence angles and the results have been weighted with the annual incidence angle frequency distribution. The obtained incidence angle-weighted reflectance is a suited parameter to compare the efficiency of solar mirror materials taking into account their use in a specific collector type and location.

Heat loss investigation of 125kWth solar LFR pilot plant with parabolic secondary evacuated receiver for performance improvement

Numerical investigation is carried out on the secondary reflector system of the pilot solar linear Fresnel reflector module of 154 m2 in Vallipuram, Tamil Nadu. Initially, optical investigation on the design of the Linear Fresnel Collector module and the secondary reflector system is carried out. The optimized flux thus obtained is applied as a boundary condition to the thermal loss analysis. Investigation of the total heat losses (convective, radiative and conductive) with three different boundary conditions namely, the constant flux, variable flux and non-uniform flux distribution is carried out. The heat loss study has been carried out when the absorber is under non-evacuated and evacuated state. The study has been carried out with the DNI ranging between 250 W/m2 and 1000 W/m2. Uncoated and selectively coated absorbers with the emissivity of 0.01–1 are analyzed. Comprehensive analysis on the influence of wind flow on the receiver system is carried out. The variation of wind velocity from the ground level is incorporated in the forced convection. Investigation on the effect of the wind speed (0 m/s–10 m/s) and the wind direction (0°–90°) on this second stage reflector system is analyzed. The deviation between the different flux boundary conditions is comprehensively analyzed under different DNI and wind conditions. The variable flux distribution shows less deviation of about 15% from the non-uniform flux distribution for the DNI of 1000 W/m2 and is found to be less than 5% for 500 W/m2 under evacuated conditions. To avoid the computational complications in applying non-uniform flux boundary conditions to the absorber for DNI greater than 500 W/m2, variable boundary flux condition can be applied as an equivalent flux condition by augmenting 15% non-uniform error percentage to the final heat loss value.

Metasurface Optical Solar Reflectors Using AZO Transparent Conducting Oxides for Radiative Cooling of Spacecraft

Optical solar reflectors are devices that combine high reflection for visible wavelengths with a strong emissivity in the infrared. Compared to the conventional rigid quartz tiles used on spacecraft since the 1960s, thin-film solutions can offer a significant advantage in weight, assembly, and launch costs. Here, we present a metasurface-based approach using an Al-doped ZnO (AZO) transparent conducting oxide as infrared plasmonic material. The AZO is patterned into a metasurface to achieve broad plasmonic resonances with an enhanced absorption of electromagnetic radiation in the thermal infrared. In the visible range, the transparent conducting oxide provides low losses for solar radiation, while intrinsic absorption losses in the ultraviolet range are effectively suppressed using a multilayer reflecting coating. The addition of high-emissivity layers to the stack eventually results in comparable emissivity values to the thin plasmonic device, thus defining a window of opportunity for plasmonic absorption...

Detection of Micro-Cracks on Highly Specular Reflectors: Dimensioning a Vision Machine Based on Optical Properties

This paper describes the complete dimensioning of a method and apparatus dedicated to the automatic detection of defects on optical mirror-like components called optical solar reflector (OSR), which are used to ensure the thermal control of satellites. First, the requirements of the method are defined according to the properties of the OSR (multilayer and semitransparent) and the defects to be detected. The defect dimensions are then measured and exhibit very dispersed micrometric values. Second, the optical characterization of the OSR and its sublayers is conducted, enabling the dimensioning of the machine with regard to the localization of the defect (surface, interface, or volume). Third, a semi-industrial prototype is developed and evaluated through two kinds of experiments. The first experiment illustrates the production capacity of the prototype and the second the measurement capability. Results showed that the prototype exceeded the initial requirements and could be used to replace the currently used control with confidence.

Aiming strategy optimization for uniform flux distribution in the receiver of a linear Fresnel solar reflector using a multi-objective genetic algorithm

Non-uniform solar flux may lead to negative effects in the receiver of the linear Fresnel reflector (LFR), including the failure of the receiver and the fluctuating operation. For reducing these effects, an aiming strategy optimization approach is presented by combining a multi-objective Genetic Algorithm (GA) and Monte Carlo ray tracing to homogenize the flux distribution in current work. Both the flux non-uniformity index and the optical loss (ηloss) are used as the objective functions. Based on the approach, first, the flux distributions in the Multi-Tube Cavity Receiver (MTCR) and the Single-Tube Receiver with a Secondary Collector (STRSC) are optimized at a typical condition. Optimal results indicate that the GA optimization strategy (S2) can reach a compromise between the flux non-uniformity and the optical loss in both MTCR and STRSC systems. Furthermore, the optimal strategy obtained at a specific transversal incidence angle can be applied in a relatively large range around it. Moreover, parameter study indicates that the aiming line number (naim) has little impact on the efficiencies of the two systems. naim has almost no effect on the flux non-uniformity in the MTCR, but the effect is visible in the STRSC. Finally, the application of S2 under a real-time condition indicates that fluxes in the two receivers can be homogenized efficaciously in the whole time range with a small drop of 0.2–3.8 percentage points in efficiency compared with those of traditional one-line aiming strategy (S1). It is also found that the flux non-uniformity indexes of the MTCR are greatly reduced from 0.77–1.09 to 0.02–0.06 when S1 is replaced by S2, and those of the STRSC are steeply reduced from 0.59–0.70 to 0.29–0.37. It is concluded that the present approach is effective and suitable for homogenizing the fluxes in the receivers of LFRs.

Structural, elastic, electronic and vibrational properties of BaRh2P2 and SrIr2As2 superconductors: A DFT study

The structural, elastic, electronic, Vickers-Hardness, vibrational, Optical and thermo dynamical properties of potentially technologically important superconductors BaRh2P2 and SrIr2As2 are calculated using density functional theory (DFT) with CASTEP code for the first time. The structural and other physical properties of BaRh2P2 and SrIr2As2 are compared with the results where available and show well accord. The phonon dispersion curve is calculated and the dynamical stability of this compound is investigated. Both the compounds show mechanically stable under the Born stability conditions. For the above condition BaRh2P2 behaves ductile nature and SrIr2As2 indicates the brittle nature. The Mulliken dislocation bonding population and charge density maps show stronger bond between As-Ir compared with Ir-Ir bond. The overall superior conversation reflects that the chemical bonding in BaRh2P2 and SrIr2As2 superconductors can be denominated as an extremely anisotropic connection between ionic, covalent and metallic interactions. At the Fermi level valence band and conduction bands overlapped, so the compounds are metal. From the Fermi surface and charge density map it is observed that both ionic and covalent bond exist. Vickers hardness reveals the both superconductors relatively soft with compare to Diamond. The optical and acoustic modes are observed clearly. We calculate the Helmholtz free energy (F), internal energy (E), entropy (S), and specific heat capacity (Cv) from the phonon density of states. Various optical parameters are also calculated. The reflectance spectrum shows that this compound has the potential to be used as an efficient solar reflector. Debye temperature of BaRh2P2 and SrIr2As2 superconductors are 273.91 K and 341.03 K calculated by using present elastic constants data. The superconducting parameter indicates the phonon-mediated medium coupled BCS superconductors. The acquired results in present investigation could provide a great spur for future studies.

Efficiency improvement of flat plate solar collector using reflector

Abstract Solar collectors are the main components of a solar heating system. The collectors collect the sun’s energy, transform this radiation into heat, and then transfer this heat into a fluid, water or air, which has many household or industrial applications. This paper introduces a new technology to improve the performance of the solar thermal collectors. The solar reflector used here with the solar collector to increase the reflectivity of the collector. Thus, the reflector concentrates both direct and diffuse radiation of the sun toward the collector. To maximize the intensity of incident radiation, the reflector was allowed to change its angle with daytime. The radiations coming from the sun’s energy were converted into heat, and then this heat was transferred to the collector fluid, water. A prototype of a solar water heating system was constructed and obtained the improvement of the collector efficiency around 10% by using the reflector. Thus, the present solar water heating systems having the best thermal performance compared to the available systems.

A Study on the Possibilities of Obtaining Intermetallic Coating of Al-Cu and Cu-Zn Systems Deposited on Metal Carrier

The research aimed at investigating the possibility of obtaining thin intermetallic films, which can be used as thermal control coatings for spacecraft. The coating films build the passive part of the overall thermal control system of a spacecraft, thus they must provide strong adhesion with the carrier material and have high functional characteristics, optical ones in particular. The study concerned issues of synthesis of stable intermetallic phase of Al4Cu9, Al2Cu, Cu5Zn8 on aluminum and copper carriers by magnetron layer-wise sputtering of reagents. They have looked into patterns of emergence of intermetallic coating applying modes of "rapid" and "slow" sputtering of reagents, applying various thicknesses of sputtered layers of reagents, temperature of the carrier, heat treatment of sprayed coatings. Incomplete and complete modes of synthesis of intermetallic coverage have been discovered. Obtained cross-sectional images of coatings, outcomes of microanalyzer scanning of reagent distribution through the thickness of sputtered coatings, microhardness values, optical absorption and emission ratios, unit of electric resistances, adhesion to the carrier. Obtained prototypes of intermetallic thermostatic coatings could be used in "solar reflectors" and "solar absorbers" classes. The results of measurements of optical and strength characteristics revealed that the intermetallic films can be used not only as thermostatic coatings for space technology, but also in general mechanical engineering, due to high-end mechanical properties.

Experimental study of power generation utilizing human excreta

This study presents the energetic performance of the biomass to produce power for micro scale domestic usage. Human excreta are chosen as the subject of the study to investigate their potential to produce biogas under ambient conditions. Furthermore, the research examines the approaches by which biogas production can be enhanced and purified, leading to a high-power generation system. The experimental work focuses on the design and fabrication of a biogas digester with a reverse solar reflector, water scrubbing tower, and a dryer. Anaerobic digestion has been considered as the decomposition method using solar energy which is a heat providing source. Specifically, two types of experiments have been performed, namely, feces to water weight proportion and continuous feeding experiments, each involving a set of six samples. The effect of parameters such as pH, ambient temperature, and biogas upgradation reveals that volume of biogas and power generation can be best obtained when an 8:2 feces to water weight sample is employed and when the feeding is applied every fifth day. In addition, this study discusses the environmental prospects of the biogas technology, which is achieved by using the water purification method to improve the methane percentage to 85% and remove undesired gases. The motivation behind this work is to understand the potential of human excreta for the development of domestic power generating systems. The results obtained reveal that 0.35m3/kg of biogas is produced with 8:2 weight proportion sample, which generates 26.8kWh power from 35kg of waste. On the other hand, continuous feeding on the fifth day produces 0.7m3/kg biogas and generates 60kWh power.

Transient multi-day simulations of thermal storage and heat extraction for a finned solar thermal storage device

The effect of combining metal fins (Al) for heat spreading and recovery with phase change material (a mixture of NaNO3 and KNO3) on the performance of a thermal storage device is investigated. High-resolution transient simulations are performed covering two days of solar energy influx and heat extraction. The solver uses the enthalpy method to track melting, a strongly coupled implicit scheme to calculate conjugate heat transfer, and a dynamically refined mesh to ease grid creation and maximize computational efficiency. A potential application is thermal storage for solar cooking, although other applications can also be envisaged. The energy inputs of the simulations are based average solar radiation during a 48h solar cycle in New Delhi, India in June with a 1.5m2 solar reflector. Four different fin designs for an insulated latent heat thermal storage device (TSD) to be used with a solar cooker are tested. The four designs are compared based on their ability to spread heat evenly and rapidly into the phase change material (PCM) and the ease with which heat can be withdrawn from the device for cooking. The tests demonstrate the potential for using long term, multiday numerical simulations in the evaluation of TSD designs.

Assembly and Testing of a Thermal Control Component Developed in Brazil

The optical solar reflector is basically a mirror of second surface with low absorptivity/emissivity ratio and negligible degradation in the space environment, which makes it an excellent coating for thermal control of satellites. It works as a radiator and is used in particular parts of the external surfaces of the satellites in order to reject the undesirable heat to the deep space. In the Brazilian Space Programs, the radiators of the satellites are generally painted with special white paints in order to reject heat instead of the use of optical solar reflector. The problem of white-ink radiators is the high degradation of the thermo-optical properties that happen over the useful lives. Thus, a process of manufacturing and assembly of optical solar reflector was developed in Brazil. To validate this process in terms of mechanical and thermal properties, 3 types of optical solar reflector radiators were manufactured, and their absorptivity and emissivity properties at the temperature of 23 °C were measured. Optical solar reflector coupons were mounted on aluminum plates to perform vibration, thermal vacuum and thermal-shock tests. A study was also done to optimize the thickness of the glue to fix the structure of the satellite on the optical solar reflector. It showed an excellent environmental stability and maintained its thermo-optical characteristics after the tests.

Optimization of solar linear Fresnel reflector system with secondary concentrator for uniform flux distribution over absorber tube

Solar linear Fresnel reflectors (LFR) are attractive because of low cost and optimal utilization of land area. They provide high concentration ratio when used in conjunction with secondary concentrators. A pilot plant of LFR system with 154m2 of primary mirror field installed at Vallipuram, India has been considered for the present analysis. This type of collectors give rise to high variation of solar radiation flux in the circumferential direction of absorber for the cases with fixed aim line of primary mirrors. In the present work, the concept of variable aim lines for primary mirrors is introduced to defocus and spread the radiation flux in a more uniform manner over the absorber. In particular, both the tilt angles and the radii of curvature of the individual Fresnel mirrors are modified to obtain a better flux uniformity. In addition, a novel segmented parabolic secondary concentrator profile has also been developed, which in conjunction with the variable aim line concept leads to a high optical efficiency of 76.4% and a coefficient of flux variation value of 0.13. On the other hand, 74.9% and 70.9% of optical efficiencies along with 0.17 and 0.33 values of coefficient of variation are obtained with more commonly employed compound parabolic concentrator and trapezoidal concentrator profiles respectively.

Equipment and methods for measuring reflectance of concentrating solar reflector materials

The proper optical characterization of solar reflector materials is a challenging task. Although several commercial instruments exist to measure reflectance, they have been developed for other applications and often do not meet all the specific requirements demanded by the solar thermal industry. In particular, the characterization of solar reflectors involve the complete solar spectral wavelength range, an incidence angle range from near normal to 70° and most importantly a very narrow acceptance angle range from near specular to 20 mrad. The accurate measurement of reflectance as a function of all the previously mentioned parameters has not been commercially implemented. This paper reviews the different alternatives to measure reflector materials, describes reflectance models used to approximate the missing information and presents current research work on prototype reflectometers to fill the gap.

Comparative study of Mo2Ga2C with superconducting MAX phase Mo2GaC: First-principles calculations

The structural, electronic, optical and thermodynamic properties of Mo2Ga2C are investigated using density functional theory (DFT) within the generalized gradient approximation (GGA). The optimized crystal structure is obtained and the lattice parameters are compared with available experimental data. The electronic density of states (DOS) is calculated and analyzed. The metallic behavior for the compound is confirmed and the value of DOS at Fermi level is 4.2 states per unit cell per eV. Technologically important optical parameters (e.g., dielectric function, refractive index, absorption coefficient, photo conductivity, reflectivity, and loss function) are calculated for the first time. The study of dielectric constant (ɛ1) indicates the Drude-like behavior. The absorption and conductivity spectra suggest that the compound is metallic. The reflectance spectrum shows that this compound has the potential to be used as a solar reflector. The thermodynamic properties such as the temperature and pressure dependent bulk modulus, Debye temperature, specific heats, and thermal expansion coefficient of Mo2Ga2C MAX phase are derived from the quasi-harmonic Debye model with phononic effect also for the first time. Analysis of Tc expression using available parameter values (DOS, Debye temperature, atomic mass, etc.) suggests that the compound is less likely to be superconductor.

Materials for Radiative Cooling: A Review

Buildings use more than one-third of the total energy consumed in counties within the Mediterranean climate zones like in Turkey. During the summer months, the absorption of solar energy by the buildings increases the required cooling load significantly. One unconventional solution to this problem is the radiative cooling of the building surfaces and façades. If a surface benefits from daytime radiative cooling, its equilibrium temperature will be lower. This can be achieved only if 88% of solar radiation within the visible and near-infrared spectrum is reflected. For this purpose, paints and coatings may have an obvious advantage because of their low cost, simplicity and ease of application;however, strong solar reflectors may significantly mutate the color. On the other hand, the use of spectrally selective surfaces isof great value in practical applications such as collection of solar energy and spacecraft temperature control. In addition, the development of novel nano-patterning techniques and new designer materials for various applications are likely to surface. These developments may help to develop a wider data base for spectral and directional radiative properties of different materials. This review is to provide an overview of mainstream and such as cermets, paints and coatings, and metal oxides that can be used for radiative cooling of buildings. Also, a summary is provided for the theories, fabrication methods, usual design and possible future research opportunities to optimize these materials for spectral selectivity for specific applications.

Performance Assessment of Solar Reflector Supported Steam Generator for Cooking of Meals under Different Climatic Conditions

Performance Assessment of Solar Reflector Supported Steam Generator for Cooking of Meals under Different Climatic Conditions