Silvered-glass Reflectors Research Articles

Performance Estimation of Copper-Free Solar Reflectors

Copper (Cu) has been an essential ingredient in the production of silvered-glass reflectors used in the concentrated solar power (CSP) systems. However, due to added material cost and the environmental burden induced by Cu mining, researchers have started to look for ways to develop copper-free coating systems for reflectors used in solar thermal applications. In this study, we employed accelerated aging tests to obtain the aging characteristics of reflectors with and without Cu and exposed reflectors outside for natural aging. Samples were subjected to Copper Accelerated Acetic Acid Salt Spray (CASS), Ultraviolet-Humidity (UVH) and UV tests, and the changes in the hemispherical and specular reflectance were measured and compared.

GPU-based Monte Carlo ray tracing simulation considering refraction for central receiver system

The heliostat in the central receiver system usually adopts silvered-glass reflectors, where a glass layer covers a specular reflection layer. Previous flux density distribution simulation methods ignored the refraction effects caused by the glass layer. This paper proposes a more accurate Monte Carlo ray-tracing simulation method considering refraction and total internal reflection (TIR) effects caused by the ray transmission in the glass layer. The proposed simulation method is fully designed and implemented on a graphics processing unit (GPU), which enables the algorithm performance to remain effective even when the simulation is more consistent with the real situation. Experiments and simulations reveal that refraction has non-negligible effects on the simulation results. Compared with the classical Monte Carlo ray-tracing simulation method that only considers the ray's reflection, refraction reduces the maximum radiative flux and total energy by up to 80% and 50%, respectively. Refraction also causes the flux spot reflected on the receiver panel to spread greatly. In several extreme cases, the ray is trapped in the glass due to TIR.

Influence of gaseous pollutants and their synergistic effects on the aging of reflector materials for concentrating solar thermal technologies

Concentrating solar thermal technologies have experienced an important boost in the last few years. Besides the production of electricity, they are particularly useful for the supply of industrial process heat. The industrial atmospheres affecting these solar plants typically contain gaseous pollutants that are likely to promote corrosion on the components of the solar facility, specifically solar reflectors, thereby compromising their optimal performance and the overall system efficiency. Seven accelerated aging tests were designed to study the effects of three air pollutants (H2S, SO2 and NO2) on the durability of two commercially available reflector types (silvered-glass and aluminum), both in single-gas tests and in multicomponent gas mixtures. Additionally, the same material types were exposed outdoors at five representative polluted sites, including industrial, urban and coastal environments. Reflectance and optical microscope monitoring corroborated which degree of corrosion was developed on a specific type of reflector in the different tests with gaseous pollutants, as well as the synergistic effects of gas combinations. For example, tests with sulfur were harmful for silvered-glass reflectors (up to a total of 16 corrosion spots), whereas aluminum was particularly affected by tests with NO2 (numerous micro spots of around 50 μm size). Moreover, comparisons of the corrosion patterns found in accelerated-aging and outdoor exposures revealed which laboratory test reproduced the different real polluted atmospheres in the most realistic way, which is the main goal of this work. For instance, the degradation found at Site 2 was reproduced by Test NO2+SO2, with an acceleration factor of 27.

Acceptance criteria for accelerated aging testing of silvered-glass mirrors for concentrated solar power technologies

Solar reflectors for Concentrating Solar Power (CSP) technologies are required to maintain their optical properties in demanding environments for more than 20 years of service-life. The durability of the commonly used silvered-glass reflectors is typically qualified by means of accelerated aging. Recently, the Spanish standardization committee UNE has published the first specific standard for this topic, which defines a set of accelerated aging tests for CSP reflectors. However, the standard does not contain pass/fail criteria. This paper proposes useful acceptance criteria for the accelerated tests defined by UNE, helping to interpret the obtained degradation results. The criteria have been determined by analyzing the collected accelerated aging data over the past 5 years in the OPAC laboratory, a joint research group of DLR and CIEMAT. Data from six different 4 mm silvered-glass manufacturers is presented, covering nearly the entire market of commercially available silvered-glass mirrors, and going way beyond the recommended testing times of the UNE standard. The data may be used to benchmark initial reflective properties (before aging) and the performance during accelerated durability testing. In addition, recommendations for improvements of the standard are given and an estimate of the acceleration factor of the Copper Accelerate Salt Spray (CASS) test with respect to a highly corrosive outdoor environment is presented.

Degradation of concentrating solar thermal reflectors in acid rain atmospheres

Given the importance that concentrating solar power technologies have had in recent years, the interaction between humid atmospheres in which SO2 is the main pollutant and the metal reflecting layers of the solar concentrators is a major concern that should be addressed. Previous durability studies have urged the importance of designing an accelerated aging test protocol for simulating aggressive industrial atmospheres that solar reflector materials are bound to encounter. Therefore, three types of reflectors were tested in an acid-rain (or Kesternich) chamber at various temperatures and gas concentrations based on the DIN 50018 and ISO 6988 standards. The results showed the significant effect of high SO2 concentrations rather than high temperatures on silvered-glass reflectors, although synergy should not be disregarded. Strong reductions in specular reflectance were found for one type of silvered reflector compared to another type, highlighting the significance of material processing and its effects on their final performance. Aluminum reflectors were not damaged as much as silvered-glass reflectors, but under microscopic inspection, a wide range of noticeable corrosion defects could be found in all the materials. Comparisons between the most representative Kesternich test and samples from an outdoor industrial site permitted realistic lifetime correlations for commercial silvered-glass reflectors.

Assessment of the erosion risk of sandstorms on solar energy technology at two sites in Morocco

This multi-disciplinary research paper should help solar power plant developers to perform an advanced site assessment in arid locations where the annual irradiance levels are high, but significant quantities of airborne sand and dust increase the risk of optical energy losses due to extinction, soiling, erosion damage (also known as abrasion), etc. Due to these effects sandstorms have a direct consequence on the operation and maintenance (O&M) costs. The work presented in the following characterizes airborne sand and dust material and later focuses on the resulting erosion effects. Some important meteorological and geological parameters for sandstorm occurrence and the resulting erosive damage on glass materials by impacting windblown material are extracted from literature. The respective parameters have been measured at two locations in Morocco (Zagora and Missour). After evaluation of wind and humidity data and a comprehensive soil analysis, the erosion risk was estimated to be higher in Zagora. The specular reflectance loss of exposed silvered-glass reflectors of 5.9% in Zagora and 0.8% in Missour after 25 months of exposure verified this estimation. Additionally, a specular reflectance analysis on a mirror sample that has been exposed for nine months in Kuwait is shown. On that sample specular reflectance losses of more than 40% were measured. A checklist with seven items is given in the conclusion to help solar plant developers to evaluate the risk of component aging due to sand storm erosion.

Effects of reduced sulphur atmospheres on reflector materials for concentrating solar thermal applications

The degradation of reflector materials for concentrating solar thermal applications is analysed. Corrosion of their metallic reflective layer is considered a major problem in facilities which are located near industrial sites, where reduced sulphur gases may be present. Accelerated ageing tests were performed to study the influence of H2S on the corrosion of two types of silvered glass reflectors and one aluminium reflector. Different degradation patterns were found for silvered glass reflectors, whereas aluminium reflectors did not corrode in the presence of the sulphurous gas. Therefore, industrial pollution caused by this type of gas may decrease the solar collectors’ performance.

Durability of solar reflector materials for secondary concentrators used in CSP systems

Secondary concentrators are used in solar concentrating systems to redirect solar beams reflected by the primary concentrators to the focal point or line. These components allow to increase the concentrated solar flux density and hence to lower thermal radiation losses. Solar reflectors for secondary concentrators are permanently exposed to environmental conditions, high radiation fluxes and elevated temperatures that potentially cause stress and degradation throughout the time. Therefore, analyzing solar reflectors of secondary concentrators by simulating these conditions is crucial. No previous research works about the durability of solar reflector materials for secondary concentrators have been reported. The present work is focused on studying the degradation of the reflector materials by simulating accelerated aging, caused by several ambient parameters and the effect of concentrated radiation. Both cooled and uncooled systems for secondary concentrators are included in this study. According to results obtained, aluminum reflectors and thin silvered-glass reflectors glued to an aluminum structure showed minimum reflectance losses and structural degradation under the operation conditions of cooled 3D secondary concentrators (tower systems). Following critical aspects to avoid reflector degradation were identified: to select a suitable adhesive material to glue the thin silvered-glass reflector to the support aluminum structure, to properly protect reflectors edges, to design a suitable cooling system and to avoid the combination of high radiation fluxes with mechanical stress. In addition, laminated silvered-glass reflectors have shown to be suitable for uncooled 2D secondary concentrators (Fresnel collectors). Furthermore, a comparison with naturally aged secondary concentrators using silvered-glass reflectors glued to an aluminum structure revealed that the simulated degradation under accelerated conditions performed in this work did reproduce the most frequent degradation patterns suffered in real operating conditions.

An Economic and Performance Evaluation of Parabolic Trough Component Improvements

Improved parabolic trough concentrating collectors will result from better design, improved fabrication techniques, and the development and use of improved materials. This paper presents and implements an easily applied methodology that quantifies the performance potential of selected parabolic through component improvements from a systems viewpoint and uses performance data to determine the economic value of each improvement. Specifically considered are evacuated receivers, silvered glass reflectors, improved receiver selective coatings, higher optical accuracy concentrators, and higher transmittance receiver glazings. Upper-bound costs for each improvement are provided as are estimates of the increased solar energy system rates of return made possible by these improvements. The performance and economic potentials of some of these improvements are substantial, especially at higher collector operating temperatures.

Our Book Shelf

POSSESSORS of what Mr. Webb calls “common telescopes,” will be pleased to have another edition of this most useful adjunct to their instruments, with corrections and additions up to the present time. Now that silvered glass reflectors are so cheap, and apertures little below six inches not uncommon in the hands of amateur astronomers, the author's definition of a common telescope is probably too limited, but these limits are extended as we proceed with the book and find mention of objects barely visible with nine inches. The advice on the use of telescopes, and the mode of observation is sound and good, and too much stress cannot be laid on the necessity of a good solid stand; a good telescope will be absolutely useless with an unsteady mounting. The description of the various phenomena to be viewed in the members of the Solar system may lead possessors of small telescopes to expect too much, the separation of Saturn's rings, the markingson Jupiter's satellites, to wit, although mention is made of the apertures required to view the features mentioned but this may also make the boak useful for work with larger instruments. We must take objection to the great contrast of light and shade, as is often the case in other works, in the cuts of Venus and Jupiter's moons, the dark markings on Venus being infinitely too black, they in reality being only just visible, with first-rate instruments, to a practised astronomer. Drawings of this kind only represent position and shape, but it must be remembered that an amateur expects to see through the telescope exactly what he sees in a drawing. One-third of the book is taken up with a selection of double stars and nebula, as in the former editions, with measures of position and distance up to later dates. Altogether the book will be found most useful to every incipient astronomer, but perhaps there may be too strong a tendency to star-gazing induced by it, and we should have been more gratified to have seen directions to readers having telescopes of certain sizes how to make their observations of real use and not a mere pastime. For instance, double image micrometers can be used on less apertures than 6-in. without clockwork; and some instructions in the use of them, and in reducing their observations so as to show the motions of binaries, would be of great service in teaching amateurs to do useful work; a hint, also, on drawing the ever-changing belts of Jupiter, any extraordinary spots on the sun, the larger nebuke, and last, not least, the star clusters. As soon as amateurs have seen the planets and a few double stars, they should begin to make themselves useful, otherwise they soon get tired of the mere star-gazing and the telescope becomes to them a thing of the past. Celestial Objects for Common Telescopes. By the Rev. T. W. Webb Third edition, revised and enlarged. (London: Longmans, Green, and Co, 1873.)