Investigation and review of mirrors reflectance in parabolic trough solar collectors (PTSCs)

Jamali, Hamzeh Article Investigation and review of mirrors reflectance in parabolic trough solar collectors (PTSCs) Energy Reports Provided in Cooperation with: Elsevier Suggested Citation: Jamali, Hamzeh (2019) : Investigation and review of mirrors reflectance in parabolic trough solar collectors (PTSCs), Energy Reports, ISSN 2352-4847, Elsevier, Amsterdam, Vol. 5, pp. 145-158, https://doi.org/10.1016/j.egyr.2019.01.006 This Version is available at: https://hdl.handle.net/10419/243572 Standard-Nutzungsbedingungen: Die Dokumente auf EconStor dürfen zu eigenen wissenschaftlichen Zwecken und zum Privatgebrauch gespeichert und kopiert werden. Sie dürfen die Dokumente nicht für öffentliche oder kommerzielle Zwecke vervielfältigen, öffentlich ausstellen, öffentlich zugänglich machen, vertreiben oder anderweitig nutzen. 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If the documents have been made available under an Open Content Licence (especially Creative Commons Licences), you may exercise further usage rights as specified in the indicated licence. https://creativecommons.org/licenses/by-nc-nd/4.0/ Energy Reports 5 (2019) 145–158 Contents lists available at ScienceDirect Energy Reports journal homepage: www.elsevier.com/locate/egyr Review article Investigation and review of mirrors reflectance in parabolic trough solar collectors (PTSCs) Hamzeh Jamali Persian Gulf University, Faculty of Engineering, Bushehr, Iran article info Article history: Received 25 September 2018 Received in revised form 10 January 2019 Accepted 14 January 2019 Available online xxxx Keywords: Coating Mirror Reflectance Solar collector Substrate Thermal efficiency abstract As mirrors used in concentrating solar systems influence the thermal efficiency of the systems collectors to a large extent, the reflectance of mirrors plays a critical role in the thermal efficiency of parabolic trough solar collectors. The studies already done by scientists in terms of how to prepare mirrors of high reflectance and quality to be used in parabolic trough solar collectors are investigated in this review paper. In other words, the paper is aimed at investigating the reflectance of various mirrors already studied by researchers as an important parameter influencing the thermal efficiency of parabolic trough solar collectors.Thisinfluenceisnumericallyshownthroughtwoinstancesappliedinacasestudy.Manyfactors and preparation methods influencing solar mirrors reflectance and quality including mirror preparation technique, mirror material, and material configuration, which were studied by scientists, are investigated. In addition, the useful and important strategies, already studied, influencing the reflectors performance and quality are discussed within the paper sections and briefly through tables. Finally, although a nearly ideal reflectance of about 0.99 in both silver and aluminum mirrors – as the most common and efficient mirrors – has been achieved by scientists, it becomes evident that more work is still required to be done in the future in order to achieve mirrors of ideal reflectance and quality. ©2019 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Contents 1. Introduction....................................................................................................................................................................................................................... 146 1.1. Mirror preparation techniques............................................................................................................................................................................ 146 1.1.1. Physical vapor deposition (PVD).......................................................................................................................................................... 146 1.1.2. Sol–gel deposition ................................................................................................................................................................................ 147 1.1.3. Lamination ............................................................................................................................................................................................ 147 1.1.4. Thermal decomposition of organometallic compounds .................................................................................................................... 147 1.2. Glass substrates and superstrates ....................................................................................................................................................................... 147 1.2.1. Silver mirrors ........................................................................................................................................................................................ 147 1.2.2. Aluminum mirrors................................................................................................................................................................................ 148 1.3. Substrates and superstrates of other materials.................................................................................................................................................. 148 1.4. Silver-containing reflective and protective configurations............................................................................................................................... 148 1.4.1. Aluminum–silver alloy as reflector material ...................................................................................................................................... 148 1.4.2. Influence of lamination on reflectance................................................................................................................................................ 149 1.4.3. Incidence angle effect on reflectance .................................................................................................................................................. 149 1.4.4. Sol–gel method effect on silver reflectors production....................................................................................................................... 149 1.4.5. Silvered polymer reflectors.................................................................................................................................................................. 149 1.4.6. Reflectance-enhancing double layers effect ....................................................................................................................................... 150 1.4.7. Comparative discussion of some reflectors ........................................................................................................................................ 150 1.5. Aluminum-containing reflective and protective configurations ...................................................................................................................... 151 1.5.1. Electron gun usage................................................................................................................................................................................ 151 1.5.2. Integration of aluminum firstand second-surface mirrors.............................................................................................................. 152 1.5.3. Miro2 reflector...................................................................................................................................................................................... 153 1.5.4. Influence of lamination on reflectors .................................................................................................................................................. 153 1.5.5. Reflectance under various conditions ................................................................................................................................................. 154 E-mail address: [email protected]. https://doi.org/10.1016/j.egyr.2019.01.006 2352-4847/©2019 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). 146 H. Jamali / Energy Reports 5 (2019) 145–158 1.5.6. Dielectric usage in reflectors................................................................................................................................................................ 154 1.5.7. Sol–gel method in aluminum reflectors production.......................................................................................................................... 154 1.5.8. Reflectance-enhancing double layers effect ....................................................................................................................................... 154 2. Results and discussion...................................................................................................................................................................................................... 154 3. Conclusion ......................................................................................................................................................................................................................... 156 4. Prospective study.............................................................................................................................................................................................................. 157 Acknowledgment.............................................................................................................................................................................................................. 157 References ......................................................................................................................................................................................................................... 157 1. Introduction Solar energy as a sustainable and environmentally friendly source of clean energy has continuously been center of attention in the recent decades. For many years, scientists have been making lots of efforts in order to improve concentrating solar power systems efficiency. Thermal efficiency of parabolic trough solar collectors is one of the research issues which has always been focused on as a matter of great interest. The radiation heat transfer in parabolic trough solar collectors (PTSCs) plays an essential role in this regard, which is directly concentrated on solar mirrors used in the collectors. Since the sunlight density is really low regarding the temperature needed for the heat transferring fluid (HTF) in a collector absorber tube, mirrors as solar thermal reflectors are used in a collector to concentrate high solar flux on a small area of absorber tube in order to attain higher fluid temperature. The ideal reflectors should have a high reflectance, low maintenance and initial cost, and a long life. There are many factors which could affect the reflectance and the economic lifetime of a solar reflector. Therefore, reflectors manufacturing methods, as the ways to determine the favorable properties for the reflectors, should keep these properties substantially intact during the time which the reflectors may get degraded and stressed by storm, temperature cycling, water, humidity, vapor, ultraviolet (UV) radiation, dust, and physical abuse (Silver/Glass Mirrors,1985). Scientists have constantly been endeavoring to achieve a solar mirror with the ideal reflectance of 1. Although a solar mirror reflectance of 1 has not been practically achieved yet, scientists have already come up with various materials for the reflecting surfaces of mirrors through different methods each of which has its own advantages and disadvantages. Some of the mirrors prepared by them showed to have nearly ideal reflectance. However, corrosion as one of the most important problems causes degradation and consequently leads to a decrease in solar reflectance (Silver/Glass Mirrors,1985). Silver and aluminum as the most common materials with high reflectance used for solar reflectors, if not properly protected, can undergo corrosion due to exposure to atmosphere, leading to a significant decrease in their reflectance (Silver/Glass Mirrors, 1985). Therefore, it really becomes crucial to prepare mirrors of not only ideal reflectance, but also durable and resistant to corrosive agents. The review paper investigates the various materials and methods ever chosen and studied by researchers in order to prepare mirrors of better reflectance and quality. In Section 1, an introduction is given through discussing the paper purpose and then investigating the previous researches already done by scientists in this regard. Section 2discusses the influence of the reflectance of solar mirrors on the thermal efficiency of parabolic trough solar collectors through investigating two instances applied in a case study, accompanied with numerical results. In Section 3, the main conclusion is drawn from the two previous sections. And finally, Section 4predicts the prospective studies in this regard. 1.1. Mirror preparation techniques Although the article is not concentrated on mirrors preparation methods, it is worth reviewing the most common techniques already used by scientists in order to prepare mirrors with favorable properties needed for parabolic trough solar collectors. Below are mentioned the techniques: 1.1.1. Physical vapor deposition (PVD) Physical vapor deposition (PVD) process is a deposition process in which atoms or molecules of a material are vaporized from a solid or liquid source, transported in the form of a vapor through a vacuum or low-pressure gaseous environment, and then condense on a substrate (Mattox,2000). The process is also used to deposit films of elements, alloys, and compound materials as well as some polymeric materials (Mattox,2000). Typically, PVD process is used to deposit films with a thickness range of a few to thousands of angstroms, with deposition rates varying from 10 to 100 Å /s (Mattox,2000). There are various types of PVD process categorized as follows: Vacuum evaporation is one PVD process in which material from a thermal vaporization source reaches the substrate without collision with gas molecules in the space between the source and the substrate (Mattox,2000). Below are mentioned some of the most important advantages of vacuum evaporation process (Mattox, 2000): •High-purity films can be deposited from high-purity source materials. •High vaporization and deposition rates are possible. •Deposition process can be easily controlled. •The process is one of the least expensive processes of PVD ones. Sputter deposition is another PVD process which is the deposition of the particles vaporized from a surface by physical sputtering process as a non-thermal vaporization process where surface atoms are physically ejected by momentum transfer from an energetic bombarding particle accelerated by a gun (Mattox,2000). Below are mentioned some of the most important advantages of sputter deposition process (Mattox,2000): •It can be applied to elements, alloys, and compounds. •The sputtering source and target can take a defined shape in some configurations. •Reactive sputter deposition can be easily achieved. Arc vapor deposition is also one PVD process in which vapor is formed from the anode or cathode of a low-voltage high-current DC arc, in a low-pressure gaseous atmosphere (Mattox,2000). An arc can also be ignited in a vacuum, forming vacuum arc vaporization technique (Mattox,2000). Below are mentioned some of the most important advantages of arc vapor deposition process (Mattox,2000): •Film-ions can be highly accelerated. •A high vaporization rate can be obtained in anodic arc. •There is low radiant heat load in cathodic arc. •The arc plasma activates reactive species and makes them more chemically reactive. •Alloy materials can be readily vaporized in cathodic arc. Last but not least, ion plating is one PVD process which utilizes concurrent or periodic bombardment of energetic particles – which are usually ions of an inert or reactive gas – from the depositing film material to modify and control the composition and properties H. Jamali / Energy Reports 5 (2019) 145–158 147 of the depositing film (Mattox,2000). Ion-plating can be done in a vacuum environment where the bombarding ions are produced from an ion gun, the process being called ion beam assisted deposition (IBAD) (Mattox,2000). Below are mentioned some of the most important advantages of ion plating process (Mattox,2000): •Surface coverage can be improved. •Film properties can be controlled. •Film properties are less dependent on the angle of incidence of the flux of depositing material in comparison with sputter deposition and vacuum evaporation techniques. 1.1.2. Sol–gel deposition The sol–gel process includes a number of stages such as gel formation, for synthesizing materials from solutions (Gaurav Bahuguna,2016). The most popular type of sol–gel process is based on the phase conversion of a sol obtained from organometallic or alkoxide precursors (Gaurav Bahuguna,2016). During the process, the above-mentioned sol gets polymerized at low temperature, and forms a wet gel (Gaurav Bahuguna,2016). Finally, a dry gel can be formed by eliminating solvent through exposure to standard heating (Gaurav Bahuguna,2016). The main advantages of sol–gel process over other conventional ones are low synthesistemperature, high purity,high homogeneity, betteryield, novel materials, and low cost (Gaurav Bahuguna,2016). 1.1.3. Lamination Lamination is the technique of manufacturing a material in multiple layers, so that the composite material improves in terms of strength, stability, sound insulation, appearance or other properties through applying various materials (Wolf,2010). There are two types of lamination which are extrusion-based lamination and adhesive-based film lamination, in order for making a composite construction (Wolf,2010). The performance of the both types largely depends on the types of polymeric, adhesive, and surface treatment ingredients (Wolf,2010). Adhesive-based laminates can be made by dry bonding, wet bonding, UV/EB curing, and hot melt adhesion processes (Wolf,2010). Extrusion-based laminates are made by extruding a thin tie-layer of a plastic material to bond two other materials such as a polymer film, paper or foil (Wolf,2010). Also, co-extrusion of materials can be done by extruding them at the same time and then pressing them together (Wolf,2010). For incompatible layers, a thermoplastic adhesive as a tie-layer is used to laminate them together (Wolf,2010). 1.1.4. Thermal decomposition of organometallic compounds Thermal decomposition, or thermolysis of organometallic compounds is a chemical decomposition process in which the compounds are decomposed by heat. Organometallic compounds are those in which the carbon atoms of organic groups are bound to metal atoms (Hassan,2002). As a result of the low electronegativity of metals, carbon–metal bonds are polarized so that carbon becomes the negative pole and metals tend to form positive ions (Hassan,2002). Therefore, the bonds to carbon break with the electron pair remaining on carbon (Hassan,2002). In reactions between organometallic compounds and compounds with electronegative substituent, attractive interaction between two carbon atoms is thus possible and reactions of this type lead to many useful and important carbon–carbon bond syntheses (Hassan,2002). Accordingly, organometallic compounds are increasingly preferred as the precursors for depositing materials on various substrates via thermal decomposition of the metal compound (Crabtree,2005). 1.2. Glass substrates and superstrates Glass as a classic material has been being used in solar mirrors since long time ago. In first-surface solar glass mirror, glass plays the role of a substrate which is coated by appropriate protective films and the reflective film is positioned at the mirror front side, while in second-surface solar glass mirror, glass plays the role of a superstrate which is coated by appropriate protective films and the reflective film is positioned at the mirror back side and protected by other layers. In order to prepare both first-surface and secondsurface glass mirrors, glass substrates and superstrates need to be coated by appropriate reflective and protective films through deposition processes. There are various types of glass used in many researches,includingsoda lime floatglass used inthe researches by Goodyear (1980), Coyle (1980), Dennis (1980), Correa (1998), Martinez (2000), and Almanza (2009), Corning Microsheet glass (Marion,1980), aluminosilicate (Corning 0317) used in the researches by Coyle (1980) and Pitts (1984), borosilicate (Corning 7809) used in the researches by Pitts (1984) and Czanderna (1986), CGW7809 (Coyle,1980), microscopic glass substrate (Nwosu,2017), and simple glass substrates used in the researches by Hass (1982), Pitts (1984), Silver/Glass Mirrors (1985), Susemihl (1987), Almanza (1988), Almanza (1992a), Almanza (1995), Fend (2003), Kennedy (2005), Hernandez (2007), and Ho (2013). Each glass type has its own advantages and disadvantages depending on technological applications and purposes researchers and manufacturers aim for. As the principal optical attributes of an ideal reflector are: (1) High optical performance, that is, ideal reflectance, specularity, and geometrical configuration (2) Low maintenance (3) Low initial cost (4) Long life, mirrors materials and manufacturing processes have always been undergoing modifications in order to achieve an ideal result. Below are mentioned two common types of glass mirrors: 1.2.1. Silver mirrors In some researches, glass thickness and composition were modified in order to obtain new types of glass with favorable characteristics (Silver/Glass Mirrors,1985). In terms of thickness, glass was produced as thin as possible to reduce double absorption during transmission of the incidental solar radiation over as wide a spectrum as possible, still keeping glass strength requirements fulfilled (Silver/Glass Mirrors,1985). The reflectance of Soda Lime glass, Ford glass, SCHOTT B270 glass, ASG LUSTRAGlass, and Corning Type 0317 were studied regarding glass thickness reduction (Silver/Glass Mirrors,1985). For most of the glass types studied whose thickness tended to be zero, the reflectance showed to be higher than 0.9 (Silver/Glass Mirrors,1985). In terms of composition, purity of silica was heightened and glass reflectance was consequently increased by lowering the amount of unfavorable chemicals present in glass like iron and some oxides (Silver/Glass Mirrors,1985). Then, the reflectance of Fused silica, 96% silica (Vycor), Borosilicate (Pyrex), Aluminosilicate, Soda-lime silica, and Lead-alkali was analyzed regarding their composition (Silver/Glass Mirrors,1985). Likewise, In one study, John et al. prepared silvered mirrors using a high-transmittance low-iron glass manufactured at Ford Motor Company’s Tusla glass plant, discussing the influence of the presence of iron on the performance of the solar energy reflectors (Goodyear,1980). It was demonstrated that low iron flat glass with relatively flat surface can be made by the float process and that this glass would make an excellent back-surface mirror with a solar reflectance of 0.896 (Goodyear,1980). In another study, R. T Coyle et al. studied a new sheet glass (CGW-7809) manufactured by the Solar Energy Research Institute (SERI) at Corning Glass Works (CGW) (Coyle,1980). By minimizing the total iron content, adjusting the conditions to oxidize all of the ferrous iron (Fe+2) to ferric iron (Fe+3), and also adjusting the levels of alumina and soda, the resulting CGW-7809 composition finally represented a 148 H. Jamali / Energy Reports 5 (2019) 145–158 logical compromise among solar transmission (∼0.91), thermal expansion, life, and cost, compared with soda-lime float glass and CGW-0317 aluminosilicate fusion glass (Coyle,1980). Glass substrates have always been being used for solar reflectors by researchers within various reflector configurations. Silver, vapor deposited on float glass, had specular reflectance ranging from 0.943 to 0.978 for various silicon resins, as protective coatings, tested by Dennis (1980). In a study, the mechanical and optical properties of silver/glass mirrors were reported in a study by Silver/Glass Mirrors (1985). The mirrors had high optical reflectance of 0.97 above (compared with aluminum which had reflectance of 0.91–0.92) within the wavelength band of 300 <λ< 700 nm (Silver/Glass Mirrors,1985). They also showed very good specularity, durability and ability to resist distortion from forces, and tolerance to impurities and mirror compositional variations (Silver/Glass Mirrors,1985). In another study, regarding the fact that silvered mirrors with a solar reflectance of approximately 0.97 can degrade when exposed to terrestrial environment, Pitts et al. developed silvered mirrors by the thermal decomposition of organometallic resinate solutions, obtaining an adhesion between the silver and the glass substrates (Corning 0317 and Corning 7809) ten times as strong as silvered mirrors formed by evaporated silver, and improving the corrosion resistance of silver films (Coyle, 1980). As long as cost is one of the most important factors in mirrors design and configuration, C. Kennedy et al. reported a low-cost reflector material as an advanced solar reflective mirror (ASRM), developed by NREL (Kennedy,2005). The mirror had a silvered glass substrate protected by an alumina coating several microns thick which was deposited by ion-beam-assisted deposition (IBAD) (Kennedy,2005). They also tested some samples which maintained a hemispherical reflectance of 0.95 after more than 3 years accelerated and outdoor exposure, having the potential for a manufacturing cost lower than $10.76 /m2at the time (Kennedy, 2005). In addition, as a new mirror configuration, C. Nwosu et al. developed new reflectors by applying nanoscale thin films of Ag and Cu (Nwosu,2017). Ag thin film of 100 nm was deposited by thermal evaporation onto a thin microscopic glass substrate while the Cu thin film (50 nm) was deposited on the Ag surface via sputtering (Nwosu,2017). The deposited thin films were finally backed with 54 µm Pb based paint (Nwosu,2017). As a result, the configuration of Pb-paint (54 µm)/Cu (100 nm)/Ag (50 nm)/glass showed an ideal reflectance of 0.96–0.99 (Nwosu,2017). 1.2.2. Aluminum mirrors In one research, first-surface aluminum reflectors with float glass (soda lime glass) as substrate were studied by W. Dennis et al. using various silicone resins in order to be used as protective coatings (Dennis,1980). Aluminum, vapor deposited on float glass had specular reflectance of 0.883, being protected by the best silicone resin tested (Dennis,1980). However, vapor deposited silver on float glass with a higher reflectance than aluminum had much poorer adhesion to glass than aluminum and was more difficult to protect (Dennis,1980). In another research, R. Almanza et al. studied first-surface mirrors obtained by using Si2O3and SiO films as the front-surface over aluminum films thermally evaporated over soda lime glass substrates (Almanza,1992b). An aluminum film was deposited on a clean 3-mm glass substrate (Almanza,1992b). Then two types of first-surface mirrors were formed, one with SiO, and the other one with Si2O3as the front-surface, each one with a thickness of 3000 Å (Almanza,1992b). Finally, the reflectance valuesachieved were0.82and 0.86,respectively (Almanza,1992b). In order to test the samples durability, after one year of exposure to the environment, no degradation was detected in them (as an advantage), but after two years a slight amount of corrosion appeared (as a disadvantage) (Almanza,1992b). At the same direction, a research into production of aluminum firstand second-surface solar mirrors by sputtering processes using linear magnetrons on soda lime glass substrate areas was done by Correa (1998). The goals of this technique were film purity, large areas coating capability, and good uniformity on deposition thickness (Correa,1998). In these mirrors, there was a better adherence of aluminum to the glass compared to silver mirrors, a fewer corrosion problems, a higher reflectance compared with second-surface mirrors from the visible to far IR regions, and a UV reflectance higher than in silver mirrors (Correa,1998). The aluminum was protected automatically by a silicon dioxide (SiO2) front film (Correa,1998). However, in preliminary stages, SiO2layer appeared yellowish affecting the mirror reflectance(Correa,1998).To eliminate suchcolor (as thereis some absorbance on the visible spectrum), small amounts of O2was added to the sputtering gas (argon), so a reactive sputtering would achieve the purity (Correa,1998). Therefore, a higher reflectance of ∼0.89 was obtained (Correa,1998). In a nearly different way, Iván Martínez et al. made aluminum first-surface solar mirrors over soda-lime glasses with concave geometry (Martinez,2000). The glasses were thermally sagged in a furnace up to 600 ◦C for about 2 h (Martinez,2000). The reflecting film was made with thermal evaporation of aluminum while the protective film Al2O3 was made through sputtering technology with a planar magnetron (Martinez,2000). According to the tests, the mean life of each glass mirror was expected to be ten years or higher, with a reflectance of 0.85 (Martinez,2000). 1.3. Substrates and superstrates of other materials Metal substrates are other options selected and used by many researchers including stainless steel substrate used in the researches by Czanderna (1986), Ashley (1988), Reed (1988), Jorgensen (1993), Schissel (1994), and Brogren (2004), aluminum substrate used in the researches by Susemihl (1987), Schissel (1987), Jorgensen (1991), Jorgensen (1993), Morales (1999), Fend (2000), Fend (2003), and Good (2016), silver–aluminum substrate (Adams,1979), and molybdenum substrate (Hass,1982). In addition, many different types of polymer substrates have already been investigated by many researches, including polyester (EC534, cast polymer, extruded polymer films) used in the researches by Griffin (1980), Susemihl (1987), Jorgensen (1991), Kennedy (1997), and Good (2016), plastic used in the researches by Almanza (1988), Almanza (1992a), Jorgensen (1994), and Almanza (1995), and PET(polyethylene terephthalate) used in the researches by Jorgensen (1993), Jorgensen (1994), and Kennedy (1994). Both metal and polymer substrates were tested in the above-mentioned researches and mostly proved to be good substitutes for glass substrates in terms of cost reduction and durability in outdoor conditions. Metal substrates, in some researches, were painted and covered by special materials – as mentioned in the following sections of the paper in detail – in order to increase the substrates resistance to corrosion. Also, various polymer substrates were prepared and applied in the above researchers – as mentioned in the following sections of the paper in detail – in order to reach one with best durability while maintaining the reflectance of the reflective material. 1.4. Silver-containing reflective and protective configurations 1.4.1. Aluminum–silver alloy as reflector material Silver as a commonly used reflector material has a reflectance of 0.97 (Adams,1979). However its reflectance, if not properly protected, could considerably decrease because of tarnishing in the atmosphere (Adams,1979). On the other hand, the reflectance of aluminum, as another commonly used reflector material, hardly changes in atmospheric conditions (Adams,1979). However, its reflectance is not as high as silver (Adams,1979). Therefore, in H. Jamali / Energy Reports 5 (2019) 145–158 149 order to obtain desirable characteristics for solar reflectors made of either silver or aluminum reflectors, and reach a reasonable trade-off, films of aluminum–silver alloys (prepared once by Harris and Siegel in 1948) were used as reflective layers by Adams et al., overcoming silver corrosion problem, although not having as high areflectanceas silver (Adams,1979). The reflectance of these alloys were in the range of 0.6 to 0.8 (Adams,1979). All the films referred to in the study were produced using an argon pressure of 60 mTorr and target potentials of 2000 V on the aluminum and 1000 V on the silver (Adams,1979). 1.4.2. Influence of lamination on reflectance R. Marion investigated glass/adhesive/backing laminate referred to as a reflector laminate for silvered mirrors (Marion, 1980). Since the glass used in linear parabolic trough concentrators must be formed to curvature, a very thin sheet glass was used in order to elastically deform the glass without causing failure (Marion, 1980). However, thin glass is extremely fragile and would be difficult to handle in sheets reasonably sized for mirror use (Marion, 1980). The simplest way in order to overcome this problem was to laminate the glass to a backing material to form a flat laminate (Marion,1980). In the study, the design, fabrication, and testing of reflector laminates using 0.25 or 0.51 mm Corning Microsheet glass was discussed (Marion,1980). The solar average specular reflectance for the reflector laminate with either 0.25 or 0.51 mm thick glass was 0.93 to 0.95 (Marion,1980). As a consequence, it was found that lamination concept not only facilitates handling of thin glass and allows it to be bent to a small radius of curvature, but also increases the glass resistance to fracture and abrasion, by the compression it brings about throughout the glass (Marion,1980). 1.4.3. Incidence angle effect on reflectance Since the reflectance weighted by solar irradiance spectrum strongly affects solar concentration ratio, some researches were done into this subject as one of great interest. In a research, absolute reflectometers such as the one by NIST was developed to measure bi-directional reflectance in the range 200–2500 nm (Proctor,1996). In a couple of studies, measurements of specular reflectance were reported for several reflective materials including back-silvered glass, metallized polymer films, and polished aluminum(Pettit,1977;Susemihl,1987).Since in noneof the previous researches is the measured spectral data over the solar spectrum at various incidence angles generally available for defined acceptance angles suited for solar applications, P. Good et al. in a recent work within Monte Carlo ray-tracing simulations employed a spectroscopic goniometry system that enables the spectral and directional measurement of reflectance, transmittance, and scattering with high accuracy over a wide range of wavelengths and incidence angles relevant for solar concentrating applications (Good,2016). They showed that the solar-weighted specular reflectance at near normal incidence and an acceptance half-angle of 17.5 mrad was 0.941 for back-silvered glass, and 0.908–0.926 for silvered polymer films, and 0.939–0.954 for silvered aluminum sheets (Good,2016). The angular scattering, quantified in terms of the standard deviation of a Gaussian distribution, was found to be negligible for backsilvered glass (<0.07 mrad), and noticeable for silvered polymer films (0.27–1.12 mrad) and silvered aluminum sheets (0.12–1.66 mrad). In addition, the spectral transmittance of semi-transparent materials suitable for protective covers was measured, yielding solar-weighted normal transmittance values of 0.913 and 0.946 for 100 µm thin films of ETFE (ethylene-tetrafluoroethylene) and FEP (fluorinated ethylene propylene), respectively (Good,2016). Consequently, it was found that the optical efficiency and the solar concentration ratio are strongly dependent on the reflectance and transmittance of materials, while the effect of non-specularity is secondary and affects the solar flux distribution to some extent (Good,2016). In another study, H. Liang et al. proposed a novel optical simulation method which combined the advantages of two models, Monte Carlo Method (MCM) and Finite Volume Method (FVM), in order to optimize optical efficiency of parabolic trough solar collectors with a cavity receiver (Hongbo Liang,2017). They investigated the CSP optical efficiency regarding the parameters such as rays number, rays relative error, inclination angle of side absorber of cavity, and absorber tube depth (Hongbo Liang,2017). The optical efficiency showed to be insensitive to the inclination angle between 0◦and 15◦(Hongbo Liang,2017). The highest optical efficiency of the whole receiver they obtained was 85.30% at the highest absorber tube depth (Hongbo Liang,2017). The results also showed that inclination angle of side absorber of cavity was not a sensitive factor for optical efficiency of the cavity receiver, but for heat transfer and concentration ratio within the absorber tube (Hongbo Liang,2017). 1.4.4. Sol–gel method effect on silver reflectors production C. Ashley et al. investigated the planarizing effect of sol–gel films on several stainless steel substrates for solar silvered mirrors (Ashley,1988). By first planarizing the metal substrate using sol–gel derived films, the ultra-smooth surface required for high specular reflectance was obtained (Ashley,1988). Through the study, a new configuration of Sol–gel overcoat/Primary protective coat/Silver layer/Sol–gel planarizing layer/steel substrate was evaluated, which gave a highly improved reflectance of ∼0.93 (Ashley,1988). S. Reed et al. also studied and designed frontsurface silver mirrors coated with sol–gel derived films as protective coatings for silver (Reed,1988). Regarding the fact that silver can be degraded by any subsequent processing (heating, coating, outdoor exposure, or H2SO4exposure), sol–gel films proved to be dramatically reducing the thermal degradation of silver (Reed, 1988). In addition, a primary protective layer of SiO2was sputterdeposited before sol–gel film application onto a 2000 Å thick 403 stainless steel in order to minimize damage to silver from sol– gel deposition and subsequent heating (Reed,1988). Finally, the results also showed that silver degradation seemed to originate at the edges suggesting that less degradation would be observed in mirrors with sealed edges (Reed,1988). In another study, A. Morales et al. also investigated sol–gel front-surface silvered mirrors(Morales,1999). Sol–gel thin andthick silica protectedsilvered mirrors presented reflectance values of 0.96 and 0.93 and outdoor durabilities of over 4 and 5 years measured in a QUV weathering chamber, respectively (Morales,1999). In consequence, all the above researches showed how an ultra-smooth reflecting surface could be obtained using sol–gel method which yielded a good reflectance, also suggesting protective layers as a solution to the problem of unfavorable effects on silver layer caused by the method (if any). 1.4.5. Silvered polymer reflectors As an appropriate option for constructing durable, low-cost, light-weight solar concentrating collectors, silvered polymer reflectors have been being studied by many researchers since long time ago. In one study, silvered polymer reflectors were prepared by A. Czanderna et al. through the multilayer mirror configuration of polymer/silver/backing adhesive/substrate by thermal evaporation and dc sputtering onto 3-M ECP-300XP polymer, where Inconel or aluminum as backing materials, 3-M Doublestick-SPR as adhesive, and 316 stainless steel and Corning 7809 glass as substrates were applied (Czanderna,1986). Among all silvered polymers samples tested, 3-M ECP-300XP had maintained the best specularity of over 0.9 at 7 mrad for 65 weeks of outdoor weathering, because of topographical roughness of the polymer sheet before metallization (Czanderna,1986). In another study, silvered polymer reflectors were produced in a different way; I. Susemihl 150 H. Jamali / Energy Reports 5 (2019) 145–158 et al. investigated silvered transparent cast polymer sheets and silvered extruded polymer films (Susemihl,1987). Applied with an adhesive onto aluminum or glass substrates, or suspended as a membrane, silvered polymers showed a reflectance of over 0.9 (Susemihl,1987). The quality of the mirrors did not change significantlyover the wavelengthranging from400 to 1000nm and incidence angles between 20 and 60 degrees (Susemihl,1987). In a nearly similar work, P. Schissel et al. studied thin, flexible silvered polymers mounted on aluminum substrate with a significant cost reduction and an acceptable performance (Schissel,1987). The mirrors maintained a reflectance greater than 0.9 for one year outdoor exposure (Schissel,1987). They also studied polymeric stabilizers and stabilizers that do not absorb UV light to improve mirror durability, regarding the fact that conventional stabilizers – needed to maintain the reflectance of silver – may limit mirror durability (Schissel,1987). In a creative research, in terms of tunneling problem as a common problem in silvered polymer mirrors, G. Jorgensen et al. developed ways to improve tunneling resistance of silvered-polymer reflector materials (Jorgensen, 1991). A set of measures were taken including either thermal treatment of ECP-305/Substrate construction (after lamination) at ◦C for 40 h or application of Tedlar tape (a polyvinyl fluoride from DuPont) to protect all edges, thermal treatment of reflector elements, avoiding bare aluminum substrate material to prevent aluminum hydroxide formation which can initiate tunneling, and using alternative deposition methods like sputtering rather than evaporation of silver onto the polymer film (Jorgensen,1991). In nearly same direction, as a state-of-the-art research, P. Schissel et al. studied ECP-305 as silvered-PMMA film commercially available from the 3M Company and developed in collaboration with the National Renewable Energy Laboratory (NREL), in order to make progress in overcoming the three primary mechanisms causing ECP-305 to lose reflectance: (1) Photon-induced silver corrosion, (2) surface soiling, and (3) a form of delamination called tunneling (Schissel, 1994). For the problem of silver corrosion, UV absorbers like opaque layers, or a dielectric paint layer were added to polymer glazing to block the light and finally avert or slow corrosion (Schissel,1994). In terms of soiling problem, non-contact cleaning could regularly return the reflectance to more than 0.9 over a twoyear period (Schissel,1994). Finally, for the delamination or tunneling problem, annealing the reflector materials and protecting the mirror edges dramatically reduced tunneling in test samples (Schissel,1994). The test results also showed that edge-taping and thermal heat treatment before curving would provide protection against the mechanical stresses introduced into the reflector film during the bending process (Schissel,1994). In another research, however, regarding the two drawbacks of ECP-305 – the price of the material and the loss of reflectance during service – which kept it suspended from being employed on a wide-scale basis in solar industry, C. Kennedy et al., with the cooperation of NREL, introduced some advanced reflector materials including a commercial silvered-polymer solar reflector designated ECP-305+by 3M Company, all-polymeric reflector material prepared by Dow Chemical Company of Midland-Michigan, and silvered Teflon, all of which maintained a specular reflectance above 0.9 for a lifetime of ten years under outdoor conditions (Kennedy,1994). Among all, the exciting design which used ECP-305+laminated to a 4-mil-thick polyethylene terephthalate (PET) substrate, was to be promising for wide-scale usage in solar industry, especially for offering increased durability in terms of corrosion degradation and delamination resistance (Kennedy,1994). Additionally, C. Kennedy et al. studied a low-cost durable reflector of a silvered polymer mirror protected by an optically transparent alumina coating applied by ion-beam-assisted physical vapor deposition (IBAD) technique, in order to achieve a specular reflectance above 0.9 (Kennedy, 1997). 1.4.6. Reflectance-enhancing double layers effect As the application of metal reflecting coatings may be followed by some problems — mainly, poor adherence between the metal reflector and the substrate, and poor durability of the metal reflector, the usage of some reflectance-enhancing double layers as a solution to the above-mentioned problems can maintain a satisfactory adherence between the metal reflector and the substrate, protect the reflecting material and consequently increase its durability, and most importantly increase the reflectance by using them with alternately low and high indices of refraction (Hass, 1982). Regarding the issue, G. Hass et al. studied the UV, visible, and IR reflectance behaviors of the most frequently used mirror metal coatings Al, Ag, Au, Cu, Rh, and Pt at various angles of incidence, and also protective layers SiO, SiO2, SiOx, and Al2O3, in order to be used in front-surface mirrors (Hass,1982). They also presented new deposition techniques such as electron gun evaporation and electron beam evaporation for the deposition of high-meltingpoint metals and oxides (Hass,1982). The coatings were deposited by evaporation in high vacuum (Hass,1982). With the reflectanceenhancing double layer of Al2O3+CeO2, the reflectance of Ag mirrors increased from 0.983 to 0.993 (Hass,1982). Al2O3was used as an adherent layer, by electron beam evaporation onto Ag mirrors (Hass,1982). Evaporated nichrome was also a good adherent layer to be used between the substrate and the Ag film (Hass,1982). In another study, a reflectance-enhancing pair of aluminum oxide and tantalum oxide with controlled thickness was used by M. Viswanthan et al. to improve the silver mirrors reflectance in the visible region (Viswanathan,1988). They developed front and rearsurface mirrors with a reflectance greater than 0.95 in the visible and IR regions, and an excellent mechanical and environmental durability obtained by the thickness control of appropriate binder film and protective film materials (Viswanathan,1988). The mirrors were prepared using a binder layer and protective overlayer materials like aluminum oxides, tantalum oxide, yttrium oxide, silicon dioxide, and magnesium fluoride (Viswanathan,1988). A thin layer of chromium and a composite layer of chromium and silver as the binder coating for the front-surface mirrors and tantalum oxide for the rear-surface mirrors were used (Viswanathan,1988). The films were deposited by a vacuum evaporation technique (Viswanathan,1988). 1.4.7. Comparative discussion of some reflectors G. Jorgensen reported a study into low-cost (likely to be less than $11 /m2at the time), durable solar reflectors, done by National Renewable Energy Laboratory (NREL) and 3M Company (Jorgensen,1993). In the early 1980s, a number of candidate metallized reflector materials were evaluated by many researchers, including acrylic,1silicone, fluoropolymers, polyacrylonitrile (PAN), polycarbonate, and polyester films, among which acrylic was determined to be the most promising polymer candidate from a cost-perperformance perspective, considering both silver and aluminum as reflective layers (Jorgensen,1993). A chronological cost-perperformance ratio of silvered polymer reflector materials was reportedtobe Scotchcal 5400>FEK-244>ECP-300x>ECP-300>ECP300A>ECP-305 by Jorgensen (1993). G. Jorgensen also evaluated both organic and inorganic candidate coat materials (Jorgensen, 1993). Organic layers of organosilicone, polyurethane (PUR), and acrylic, and inorganic coatings of Si3N4, diamond like carbon (DLC), SiOx, Al2O3, and other oxides were tested along with organic/ inorganic composite coatings (Jorgensen,1993). In their study, G. Jorgensen reported the substrate materials such as sheet stainless steel or aluminum or a polymer film such as polyethylene terephthalate (PET) used for front-surface reflectors (Jorgensen, 1Polymethylmethacrylate. H. Jamali / Energy Reports 5 (2019) 145–158 151 1993). In addition, fire protective and scratch-resistant top coatings (SRC), a UV-cured acrylic, a low emissivity coating, a thermalcured organosilicone, and a UV-cured organosilicone provided and tested by NREL were reported (Jorgensen,1993). Moreover, silvered polymers reflectors as candidate advanced material reflectors were studied by the NREL (Jorgensen,1993). In terms of durability, ECP-305 reflector degraded a little after more than 2 years of outdoor exposure, while ECP-300A reflector degraded a lot after 1 year of outdoor exposure. Additionally, silvered PMMA (Polymethylmethacrylate) reflectors with back protective copper layers of various thicknesses (0 Å, 100 Å,300 Å,and 600 Å), regarding the mirror configuration of PMMA/Ag/Cu/adhesive/substrate (Al or stainless steel), were examined, where the reflectance for 0 Å thickness significantly degraded after 300 h of exposure, while the reflectance for other thicknesses was maintained greater than 0.9 after 2000 h of exposure (Jorgensen,1993). A new silver mirror configuration, Top protective film (2 mil TeflonTM2)/Metal interlayer (Au, Cu, Cr, Ti)/Reflective layer (Ag)/Back protective layer (Inconel, Cu, Cr, Nichrome), was also investigated aiming to minimize corrosion problems, top protective coats being deposited via a low-voltage, ion-plating process (Jorgensen,1993). Among all of the samples tested for the above-mentioned configuration, the samples with copper back protection and either no interlayer or a titanium interlayer exhibited the best optical durability after 1 month exposure in Weather-Ometer (WOM) (Jorgensen,1993). In another comprehensive study, a research and development program by NREL into advanced reflector materials was reported by Jorgensen (1994). As one of the ways to reduce the cost of solar mirrors is to metallize an appropriate and inexpensive substrate material such as a polyethylene terephthalate (PET) film and then overcoat the reflector with an abrasion-resistant, durable, protective top layers, such as organosilicones, polyurethanes, or acrylics as organic layers, and Si3N4, diamond-like carbon, SiOx, Al2O3, and other oxides as inorganic layers, NREL has done some durability tests on some reflectors with this construction (Jorgensen, 1994). Following sputter deposition of SiOxover silver, and the configuration PET/Ag/SiyOx, the silvered mirrors optical durability and performance showed to be dependent on the stoichiometry and thickness of SiyOx(Jorgensen,1994). Polyurethane (PUR) was also evaluated on both glass and PET substrates, with and without copper protective layers behind the silver; where the silvered mirrors reflectance degraded during accelerated exposure testing (Jorgensen,1994). Directly deposited reflector materials, following the configuration of Top coat/Reflective layer/Levelizing layer/Substrate, were also investigated, the advantage of such materialsbeing that theyeliminate the needfor adhesivesandlamination during the manufacturing process (Jorgensen,1994). Samples severely degraded in accelerated exposure testing at NREL showing problems of interlayer adhesion and top coats showed to be turning yellow during weathering, as limitations of this method for solar applications (Jorgensen,1994). Additionally, a silvered TeflonTM reflector material was developed, whose advantage was that TeflonTM is inherently weatherable and nonhygroscopic, while the reflectance was generally low (0.8 at 650 nm and 8–12 mrad) (Jorgensen,1994). In the same work, a polymer multilayer reflective material (PML) was used by the Pacific Northwest Laboratory (PNL), under a subcontract with NREL, following the configuration of Si3N4and-or PML/Ag/PML/Substrate, where Si3N4is protective hard coat, and the PML encapsulates the silver reflective layer to prevent corrosion (Jorgensen,1994). The PML was deposited by a vacuum flash-evaporation technique having the potential for extremely high production-line speeds and consequent low production cost (Jorgensen,1994). A variety of ion-assisted sputterdeposited hard coats were also introduced to be applied to silvered 2Polytetrafluoroethylene. PET film, and intended to result in an improved optical durability (Jorgensen,1994). In another part of the study, all-polymeric reflector materials were produced by Dow Chemical Company, having the advantages that degradation of optical performance caused by corrosion of metallic reflecting layers is no more a concern, and such materials can be directly thermoformed into usable structures, thereby reducing costs associated with support elements (Jorgensen,1994). NREL characterized an approximate reflectance of 0.9 for some samples of the all-polymeric reflector materials, 60 mils thick and comprising 5000 alternating coextruded layers having a tailored gradation in layer thickness (Jorgensen,1994). As the final part of work, adhesively bonded reflector materials, with a matrix of TefzelTM/Silvered PET laminated samples bonded to tension frames, were developed, the advantages being that the material construction were low-cost, and the problems with poor specular reflectance associated with metalizing fluoropolymer films could be avoided (Jorgensen,1994); however, the main concern was the durability of the adhesive layer during weathering (Jorgensen,1994). In a comparative and comprehensive research, T. Fend et al., with the cooperation of NREL, introduced some lowcost reflectors with long durability and reflectance values up to 0.96, including Flabeg, Miro2, Naugatuck, SolarBrite95, Glaverbel, and Erie-Electroverre, among which Flabeg demonstrated the best optical durability (Fend,2003). Flabeg mirrors were produced by applying a silver reflective layer and an adhesion-promoting layer (usually copper) onto relatively thick (>1 mm) glass using wet chemistry processes followed by protective backside paint (Fend, 2003). SolarBrite95 mirror construction developed by Alco and evolved from EverBrite95 was a UV-stabilized polyester film having a metallic back-protective layer (Fend,2003); the film was laminated using a thermoset adhesive to a chemically pretreated aluminum substrate (Fend,2003). A painted coating was applied to the backside of the metal substrate for added durability (Fend, 2003). Consequently, the reflectance achieved was ∼0.92 and the cost ranged between 17 and 52 e /m2at the time (Fend,2003). The Naugatuck thin glass mirror showed a reflectance loss of 0.95 to 0.92 after 5.5 years exposure in Texas, 0.95 to 0.89 after 6.5 years exposure in Sacramento, and similar reflectance after 6 years in the WOM (Fend,2003); however, a negligible loss in reflectance occurred (after cleaning, and having the correct choice of adhesive to bond the thin glass to the substrate) in Phoenix and Florida after 7 and 5 years exposure, respectively (Fend,2003). As shown in Table 1, various silver mirror materials along with configurations have been studied by a lot of researchers in order to obtain silver mirrors with better reflectance and durability, and low cost. In some of the researches, there was a trade-off between the three factors of reflectance, durability, and cost, while in some other researches, one or a pair of factors were prioritized through the application of specific mirror materials and configurations regarding the demands made in solar industry. 1.5. Aluminum-containing reflective and protective configurations 1.5.1. Electron gun usage R. Almanza et al. obtained some advances in first-surface mirrors using SiO, SiO2, and Si2O3films as front-surface over an aluminum film thermally evaporated by an electron gun over a glass or plastic substrate 3 mm thick (Almanza,1988). The SiO2protected mirrors were obtained by evaporating pirex glass with an electron gun in a small evaporator (Almanza,1988). In order to obtain a higher reflectance in the mirrors, the SiO was evaporated in an atmosphere of oxygen at pressure 10−4Torr to produce a coating of Si2O3(Almanza,1988). Finally, the highest measured specular reflectance was 0.86 (Almanza,1988). In another study, R. Almanza et al. presented more advances in aluminum firstsurface mirrors regarding their previous studies (Ref. Ho,2013) 152 H. Jamali / Energy Reports 5 (2019) 145–158 Table 1 Brief data collection on configuration and reflectance of various silver-focused mirrors. Mirror material or configuration Reflectance Comment Aluminum–silver alloy mirror 0.6 to 0.8 This mirror type was produced in order to overcome silver corrosion problem, although not having as high a reflectance as silver (Adams,1979). Low-iron-glass mirror 0.896 Iron minimization improved the mirror reflectance (Goodyear,1980). Glass/adhesive/backing laminate 0.93 to 0.95 Corning Microsheet glass was used in this mirror (Marion,1980). Silicon resin-protected mirror 0.943 to 0.978 Silver was vapor deposited on float glass, then protected by silicon resin (Dennis,1980). Silver/glass mirror ≥0.97 The reflectance was measured within the wavelength band of 300 <λ<700 nm (Silver/Glass Mirrors,1985). Silver/corning glass 0317 or 7809 0.97 The mirror was produced by thermal decomposition of organometallic solutions (Guenther,1990). ASRM silver/glass mirror 0.95 The mirror was protected by alumina coating (Kennedy,2005). Pb-paint/Cu/Ag/glass 0.96 to 0.99 Nanoscale thin films of Ag and Cu were applied on a microscopic glass substrate (Nwosu, 2017). Silver/glass 0.941 The reflectance was measured at near normal incidence and an acceptance half-angle of 17.5 mrad (Good,2016). Silver/polymer 0.908 to 0.926 The reflectance was measured at near normal incidence and an acceptance half-angle of 17.5 mrad (Good,2016). Silver/aluminum 0.939 to 0.954 The reflectance was measured at near normal incidence and an acceptance half-angle of 17.5 mrad (Good,2016). Sol–gel overcoat/primary protective coat/silver/sol–gel planarizing layer/steel substrate 0.93 An ultrasmooth surface was obtained using sol–gel derived films (Ashley,1988). Sol–gel silvered mirror 0.93 and 0.96 The reflectance was measured after 5 and 4 years of QUV weathering chamber (Morales, 1999). PMMA/Ag/Cu/adhesive/Al or stainless steel substrate >0.9 The reflectance was measured after 2000 h of exposure, for different thicknesses of Copper layer (Jorgensen,1993). Silvered polymer (3-M ECP-300XP) Mirror >0.9 The mirror maintained reflectance for 65 weeks of outdoor weathering (Czanderna, 1986). Silvered polymer mirror >0.9 Silvered cast polymer sheet and silvered extruded polymer films were used (Susemihl, 1987). Silvered polymer mirror >0.9 The reflectance was measured after one year of outdoor exposure (Schissel,1987). Silvered polymer mirror >0.9 The mirror maintained reflectance for ten years under outdoor conditions (Kennedy, 1994). Silvered polymer mirror >0.9 The mirror maintained reflectance by overcoming three corrosion inducing mechanisms (Schissel,1994). Silvered polymer mirror >0.9 Protective alumina coating was used by IBAD technique (Kennedy,1997). Silvered mirror/reflectance-enhancing double layers 0.993 The reflectance increased from 0.983 to 0.993 by using reflectance-enhancing double layers (Hass,1982). Silvered mirror/reflectance-enhancing double layers >0.95 The films were deposited by vacuum evaporation technique (Viswanathan,1988). SolarBrite95 0.92 The mirror evolved from EverBrite95 (Fend,2003). Naugatuck 0.95 The reflectance decreased from 0.95 to 0.92 after 5.5 years of exposure in Texas (Fend, 2003) . (Almanza,1992a); first it should be noted that although silver mirrors have higher reflectance than aluminum mirrors, to obtain a good silver mirror five layers are needed compared with aluminum mirrors which need two layers (aluminum film and a protective transparent film) (Almanza,1992a); In addition, less corrosion is observed in aluminum mirrors than in silver mirrors, because the adherence to the glass in aluminum mirrors is better and the sulfide tarnishing does not occur (Almanza,1992a). As a new approach in their study, two electron guns were used in order to obtain higher quality in aluminum first-surface mirrors; one gun for aluminum evaporation permitting to eliminate or to minimize the pinholes, and the other one to allow the evaporation of SiO without any mirror contamination (Almanza,1992a). The second advantage was a better adherence achieved between the aluminum film and the Si2O3obtained by oxidization of SiO with some oxygen inside the evaporation chamber (10−4Torr), due to the use of two e-guns that permit not to open the chamber (Almanza, 1992a). Through the same way and in another creative work, a research and development of aluminum first-surface solar mirrors was done by Almanza (1995). They used two protective films for the aluminum layer: Si2O3and SiO2(Almanza,1995). In the firstsurface mirrors, the substrates were chosen to be glass, metals, or plastic (Almanza,1995). Then, two electron guns were used to manufacture aluminum first-surface solar mirrors (Almanza, 1995). With the two electron guns, there was no need to open the chamber during aluminum evaporation with tungsten filaments, and consequently, better adherence between the aluminum film and Si2O3or SiO2was obtained (Almanza,1995). The optimum thickness was 1000 Å or higher for the aluminum layer, about 2500 Å for Si2O3, and 3200 Å for the SiO2(Almanza,1995). The mirrors were tested in the environmental chamber for accelerated weathering (Almanza,1995); in consequence, SiO2and Si2O3proved to be corrosion resistant and protective, especially regarding the preparation process of SiyOxlayer beside the thickness of the layer (Almanza,1995). The mirrors possessed high specular reflectance (0.89) and high environmental stability (Almanza,1995). 1.5.2. Integration of aluminum firstand second-surface mirrors R. Almanza developed new solar aluminum mirrors by integration of aluminum firstand second-surface mirrors, composite aluminum first-surface mirrors with double layer, and combination of integrated and composite mirrors (Almanza,1999). As a consequence, the reflectance of the mirrors were 0.85–0.86 before and after aging tests for first-surface films and 0.73 for second-surface films (Almanza,1999). In the same direction, the optical parameters of double first-surface aluminum mirrors i.e. Glass/Al/SiO2/Al/ SiO2produced using magnetron sputtering method were studied by C. Hernandez et al., in order to develop the mirrors of longer life (Hernandez,2007). The manufactured mirrors showed a reflectance of 0.81–0.82 (Hernandez,2007). Through a new work,