1,721,062 research outputs found
Simplified analysis of solar-weighted specular reflectance for mirrors with high specularity
No full textThe most relevant parameter to properly characterize solar mirrors is the solar-weighted near-specular reflectance. As this parameter cannot be directly measured with off-the-shelf instruments, a simplified procedure to be applied for highly specular solar mirrors is proposed in this paper. The approach, based on two criteria, was experimentally employed to check a wide variety of solar reflector materials. Only those mirrors with known high specularity passed the criteria, indicating that the proposed method is suitable. © 2016 Author(s)
Post-assembly in-situ check of parabolic trough modules by VISshed
No full textAn innovative instrument for the post-assembly in-situ check of parabolic trough modules is here reported. The instrument, named VISshed, is thought for the shed, where the parabolic trough modules are assembled just before their installation in the plant. The instrument is simply composed by one or more cameras installed on a motorized slide and a fake receiver. These components have to be precisely placed over the module; in the paper several indications for the accomplishment of this task are given. The sequence of images captured during the camera(s) translation from one rim of the module to the opposite one are processed according to the VIS method, here described in detail. The most important outputs are contour-map and mean value (for each facet) of the intercept factor. Furthermore, the image processing gives precise information to operators about how to improve the canting, as well as, at the end of optimization, the final shape quality of each facet (panel) installed in the module. This final panel check, after assembly, is a great opportunity for the customer to reject those panels which result too much deviating from the ideal parabola. © 2018 Author(s)
State of the art of performance evaluation methods for concentrating solar collectors
No full textFor the development and establishment of concentrating solar thermal collectors a reliable and comparable performance testing and evaluation is of great importance. To ensure a consistent performance testing in the area of low- temperature collectors a widely accepted and commonly used international testing standard (ISO 9806:2013) is already available. In contrast to this, the standard ISO 9806:2013 has not completely penetrated the testing sector of concentrating collectors yet. On that account a detailed literature review has been performed on published testing procedures and evaluation methodologies as well as existing testing standards. The review summarizes characteristics of the different steady-state, quasi-dynamic and fully dynamic testing methods and presents current advancements, assets and drawbacks as well as limitations of the evaluation procedures. Little research is published in the area of (quasi-) dynamic testing of large solar collectors and fields. As a complementary a survey has been conducted focusing on currently implemented evaluation procedures in this particular field. Among the ten participants of the survey were project partners of relevant industry and research institutions within the European project STAGE-STE (Work package 11 - Linear focusing STE technologies). The survey addressed general aspects of the systems under test, as well as required process conditions and detailed characteristics of the evaluation procedures. In congruence with the literature review, the survey shows a similar tendency: the quasi-dynamic testing method according ISO 9806:2013 presents the most common and advanced evaluation procedure mainly used in the context of tracking concentrating collectors for the performance assessment of parabolic trough collectors operating with thermal oil or pressurized water. These common solar systems can be evaluated with minor adaptions to the testing standard. Evaluation procedures focused on in-situ measurements in solar fields or collectors are scarce and complex as well as an evaluation of linear Fresnel collectors or other systems operating with non-common heat transfer media like molten salt and direct steam. As those are still presenting niche products and testing wise challenging systems under real test conditions a more sophisticated evaluation procedure such as the dynamic testing method is expected to be better suited. © 2016 Author(s)
Proposal of a new parameter for the comprehensive qualification of solar mirrors for CSP applications
No full textHemispherical and near-specular solar reflectance are today the main parameters used in solar mirror characterization. Unfortunately they can not properly describe the mirror effectiveness in redirecting the Sun radiation towards a receiver in CSP applications because the reflectance concept is developed by assuming the impinging radiation travelling as a plane wave, like that emitted by an infinitely far point light source. Besides that, the measurement of the near-specular reflectance is so much difficult that slowed down the progress of the SolarPACES reflectance guidelines, still stopped at the 2013 version. The present paper launches a new idea in the ongoing discussion on solar-mirror qualification to work around these difficulties: solar mirrors could be better characterised by the new parameter named Sun Conic Reflectance (SRC), which represents the amount of solar radiation reflected at a point of the mirror, and intercepted by the receiver with acceptance-angle 2φR. The standard measurement of SCR requires the setting of two conditions about the solar radiation: spectrum and divergence. The first is ASTM G173-03, for the second here was chosen that of a perfect clear-sky day, when the Sun-disk surface is uniformly radiating, and viewed from the Earth under 9.46 mrad, but any other could be chosen if it can experimentally implementable. For φ < 4.73 = 9.46/2 mrad, SCR and near-specular solar reflectance behave very differently, because the first represents the solar radiation intercepted by the receiver, while the second refers to the reflection of impinging plane-waves, with solar spectrum, towards directions deviating from the specular one of less than φ. When φ ≥ 4.73 mrad, the difference is much lower, and becomes null when φ is large enough. The second advantage of SCR is that it can be directly measured by means a simple experimental set-up which schema is shown and discussed in the paper. The formulae for computing SCR from near-specular reflectance are also reported. © 2016 Author(s)
Hemispherical Reflectance Results of the SolarPACES Reflectance Round Robin
No full textMirrors are the first link in the energy-conversion chain from Sun to electricity-delivery in the grid. Shape and solar reflectance are the key-parameters of mirrors, respectively affecting how solar radiation is concentrated around the focus, and how much of the impinging solar power is reflected. In SolarPACES Task III, an expert group is drafting the solar reflectance guidelines; in order to speed up the discussion the SRRR round robin was launched at the beginning of 2013. Identical kits, each one consisting of ten specimens collected from eight cooperating producers, were distributed and measured at six research institutes, acting as evaluators. The kit includes both traditional (glass based) and innovative (first-surface) solar mirrors. The paper only reports on the simplest task among those of SRRR: the solar hemispherical reflectance measurement. Near-specular solar reflectance was also measured and compared but the results are still under investigation and are not part of this paper. The measurements were accomplished according to the guidelines. The differences among the achieved results are within the typical accuracy of spectrophotometers, demonstrating the reliability of the reflectance guidelines. The statistic of the deviations from the true value is analysed separately for each evaluator, and allows us to infer information abut the gauging-status of the adopted reference mirror, as well as the measurement reproducibility. © 2015 The Authors. Published by Elsevier Ltd
Approximated method for modelling hemispherical reflectance and evaluating near-specular reflectance of CSP mirrors
No full textCommercial first-surface solar-mirrors, which are alternatives to conventional glass mirrors, reflect solar radiation in a broader manner because of light scattering. However, in concentrated solar power all the radiation reflected in the solid angle of receiver-viewing is useful. This makes a mirror-evaluation methodology which takes into account not only specular, but also near-specular reflectance, mandatory. Two laminated mirrors and one traditional were analysed with a new method based on near-normal incidence reflectance measurements of the hemispherical spectrum, and near-specular at 405.5, 543.5 and 632.8. nm, for several acceptance angles. For a given acceptance angle, the wavelength-behaviour of the ratio near-specular/hemispherical is found well modelled by the well-known Total Integrated Scattering relationship. The angular-behaviour of the hemispherical reflectance is predicted by a simplified optical model of the mirror. As final result, the behaviour of solar reflectance versus the incidence-angle over the investigated acceptance-angle range was obtained. © 2013 Elsevier Ltd
The effect of incidence angle on the reflectance of solar mirrors
No full textSolar reflectors for Concentrating Solar Technologies require a high reflectance in the terrestrial solar spectrum (280–4000 nm). 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 5 mm 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. © 2017 Elsevier B.V
Solar mirror qualification setup, the key instrument in a new strategy for evaluating off-normal near-specular solar-reflectance
No full textSolar reflectance is one of the most important parameters characterizing the mirrors used to redirect the solar radiation toward a receiver in concentrating solar power plants. In Solar Power and Chemical Energy Systems (SolarPACES) Task III, an international group of experts is drafting the reflectance guidelines and debating the hard task of measuring off-normal near-specular solar-reflectance. Because of the lack of suitable commercial instruments, several research institutes are studying and setting up special equipment. Since 2012, the Italian National Agency for New Technologies, Energy and Sustainable Economic Development (ENEA) proposed an articulated strategy based on modeling the hemispherical spectral reflectance with the equivalent model algorithm and the ratio of near-specular to hemispherical reflectance with the total integral scattering equation; in the latter, the roughness is replaced by the heuristic parameter σφ , nonphysically dependent on the acceptance angle. The solar mirror qualification experimental setup was conceived for measuring σφ . Recently, SolarPACES approved a new project to support the working group for improving the proposed instruments. The paper summarizes the ENEA strategy and presents the latest version of the instrument, SMQ2, discussing its several important new features: self-calibration, setup simplification, straightforward applicability to high specular mirrors, measurement-time cutoff, and acceptance-angle range extended up to 50 mrad
Upgrading of ENEA Solar Mirror Qualification Set-up
No full textAbstractRecently a new method for characterization of mirrors for concentrating solar power has been proposed: starting from the hemi- spherical reflectance spectrum and the data obtained with a dedicated experimental set-up, the solar near-specular reflectance at off-normal incidence and in the range 0-20 mrad of (half) acceptance-angle is predicted. The limits of the reflectometers commonly used in specularity study, and the advantages of the new experimental set-up are discussed. The most relevant modifications, among the latest ones carried out in view of the forthcoming set-up commercialization, are described: direct measurement of diffuse re- flectance beyond 20 mrad; adoption of cheaper, more compact, and less coherent lasers; new procedure of data processing to get more reliable coherent-free assessment
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