Comparison of Degradation on Aluminum Reflectors for Solar Collectors due to Outdoor Exposure and Accelerated Aging

Transcription

1 energies Article Comparison Degradation on Aluminum Reflectors for Solar Collectors due to Outdoor Exposure Accelerated Aging Johannes Wette 1, *, Florian Sutter 1, Aránzazu Fernández-García 2, Stefan Ziegler 3 Reinhard Dasbach 4 1 DLR German Aerospace Center, Institute Solar Research, Plataforma Solar de Almería, Ctra. Senés Km. 4, Apartado 39, E04200 Tabernas, Almería, Spain; florian.sutter@dlr.de 2 Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas (CIEMAT), Plataforma Solar de Almería, Ctra. Senés Km. 4, P.O. Box 44, E04200 Tabernas, Almería, Spain; afernez@psa.es 3 Alanod GmbH & Co. KG, Egerstr. 12, Ennepetal, Germany; Stefan.Ziegler@alanod.de 4 Almeco GmbH, Claude Breda Straße 3, Bernburg, Germany; Reinhard.Dasbach@almecogroup.com * Correspondence: Johannes.Wette@dlr.de; Tel.: Academic Editor: Francisco Manzano Agugliaro Received: 28 September 2016; Accepted: 1 November 2016; Published: 5 November 2016 Abstract: Reflectors for concentrated solar rmal technologies need to withst 20 or even 30 years outdoor exposure without significant loss solar specular reflectance. In order to test durability innovative reflectors within a shorter period time, an accelerated aging methodology is required. The problem with accelerated testing is that poor correlation between laboratory field test results has been achieved in past. This is mainly because unrealistic degradation mechanisms are accelerated in wearing chambers. In order to define a realistic testing procedure, a high number accelerated aging tests have been performed on differently coated aluminum reflectors. The degradation mechanisms accelerated tests have been classified systematically compared to samples that have been exposed at nine different exposure sites outdoors. Besides stardized aging tests, innovative aging procedures have been developed in such way that agreement to degradation pattern observed outdoors is increased. Although degradation depends on location, five generic degradation mechanisms were detected. Stardized tests only reproduced one or two five mechanisms detected outdoors. Additionally, several degradation effects that were not observed outdoors appeared. The innovative accelerated aging tests artificially soiled samples were able to reproduce three five mechanisms observed outdoors, presenting a much more realistic overall degradation pattern. Keywords: concentrated solar power; accelerated aging; degradation; durability; aluminum solar reflector; corrosion 1. Introduction Industry is one main consumers energy worldwide, around 30% [1], with direct rmal energy (known as industrial process heat, IPH) representing a large share this energy dem in many sectors [2]. As temperature requirements IPH applications range from 60 C to 260 C [3], concentrating solar rmal systems are becoming essential for covering such rmal energy dem with renewable energies [4]. These small-sized concentrating solar rmal systems (including small-sized parabolic-trough collectors, PTC, compound parabolic concentrators, CPC, solar cookers) are also important for rmal energy supply in sustainable cities, isolated locations rural areas, covering applications such as for solar cooking [5,6], domestic hot water, space heating, pumping irrigation water, desalination water treatment [7]. In addition, concentrating solar Energies 2016, 9, 916; doi: /en

2 Energies 2016, 9, rmal technologies for power production (typically named Concentrated Solar Power, CSP) currently represent most promising solution to supply a significant share in renewable energy mix [8,9]. Consequently, concentrating solar rmal systems are contributing to mitigate impact energy production in global warming, to achieve a significant share renewable energy to build more sustainable cities. The aging degradation study components for concentrating solar rmal technologies are a crucial factor for ir deployment [10,11]. Meeting assuring lifetime goals for components is important for success this renewable energy technology for its competitiveness with or energy production technologies, especially with conventional ones. The reflectors are a key component that concentrate sunlight direct it to receiver. Lifetime goals for reflector range up to 20 or even 30 years as demed by industry [12]. Nowadays, widest used type reflector is a silvered glass mirror [13]. The silver layer is applied to a 4 mm glass substrate. The silver is protected on back side by a copper layer a system typically three protective paint coatings. Two main alternative types mirrors are discussed for use in CSP plants: silvered polymer foils aluminum mirrors. This paper focuses on aluminum mirrors, which consist an aluminum substrate that is anodized protected on front side by additional layers. For some types, reflection enhancing layers are deposited by physical vapor deposition (PVD) onto anodized layer. Aluminum reflectors present some advantages respect to silvered glass reflectors, such as a high formability, lightweight properties a cost reduction potential [14]. These properties make aluminum mirrors most suitable type for designing manufacturing small-sized concentrating solar collectors [4]. The drawbacks are lower initial reflectance values higher scattering compared to silvered glass mirrors. Also, corrosion resistance first aluminum mirror prototypes proved to be lower than for glass mirrors outdoors. In order to make a statement about lifetime a solar reflector material in a reasonable time (without waiting years to check durability under real operating conditions), several accelerated aging tests should be performed. Due to complexity material degradation, high number relevant environmental parameters that influence degradation process synergistic effects when more than one aging mechanism is present, correlations between accelerated aging tests outdoor exposure cannot be derived easily. Additionally, commonly applied accelerated aging tests originate from stards from automotive, photovoltaic or glass industry have not been specifically designed to test solar mirrors. Thus, testing time pass or fail decision cannot be adopted. This paper is focused on a comparison degradation mechanisms detected in real outdoor conditions with those reproduced in accelerated aging tests to determine realistic accelerated aging methods. 2. Materials Methods The most realistic way to assess durability a material at a certain site is to perform an outdoor exposure test. However, this methodology by itself is not affordable in most situations because it involves unreasonable testing time. The approach followed in this work is to microscopically compare appearing degradation its evolution in outdoor accelerated exposure to derive a statement about how realistic different accelerated aging tests are. The main source difficulty when real world accelerated aging tests are compared are synergistic effects that appear when more than one aging mechanisms are present. These effects are usually suffered in real outdoor exposure, where several wearing parameters are affecting at same time, but not always in accelerated aging tests, where only a certain set specific wearing conditions are applied, consequently, fewer degradation mechanisms may appear. Anor important challenge is to find testing conditions with high acceleration without producing unrealistic damages in material, both in stardized modified accelerated aging tests. To achieve se goals, several material samples were exposed outdoors in different locations also tested under a set accelerated aging conditions at Plataforma Solar de Almería (PSA).

3 Energies 2016, 9, Equipment The following instruments are used to quantify solar reflector optical degradation in test campaign: A portable specular reflectometer model 15R-USB, manufactured by Devices Services. This instrument measures monochromatic specular reflectance with an incidence angle 15 in a wavelength range between nm, with a peak at 660 nm. The measurements were taken with an acceptance angle 12.5 mrad. The nomenclature used for this parameter is ρ s,ϕ (660 nm; 15 ; 12.5 mrad). Reflectance measurements are taken before, during after testing to evaluate optical quality different. A 3D light microscope model Axio CSM 700, manufactured by Zeiss. This microscope allows 5, 10, times magnification also measures roughness, defect features (size depth) surface priles. Microscopic pictures are taken during after completion certain tests to study appearance evolution defects. A scanning electron microscope (SEM) Gemini Ultra 55, manufactured by Zeiss, with an INCA FETx3 EDX system. This instrument is used to calculate thickness different reflector layers to detect existing elements. The laboratory wearing chambers employed to perform accelerated aging tests are listed below: Salt spray chambers. These chambers perform tests according to ISO-9227 stard [15]. The chamber model CSF-500 by Control Técnica is used for copper accelerated salt spray testing (CASS), while an Erichsen chamber (model 608) is used to carry out neutral salt spray (NSS) test. The SC 340 MH chamber by ATLAS is employed to perform wearing tests involving control temperature humidity, such as Damp Heat Humidity Freeze tests based on IEC stard [16]. The UV-Test chamber by ATLAS is utilized to perform UV + Condensation test, according to ISO stard [17]. The UVACUBE 400 chamber by Hönle is used for UV testing, with a filtered Fe-radiation source with an intensity 397 W/m 2 in wavelength range nm. The CKEST 300 chamber by Ineltec is utilized to perform corrosion testing under sulfuric environment (SO 2 cyclic Kesternich testing), according to ISO 6899 [18] DIN stards [19]. An inmersion chamber (Erichsen, model 530) was used to perform Machu test, according to Qualicoat specifications [20]. A soil pipe was manufactured installed at ageing lab based on DIN stard [21] to perform s trickling test. A sstorm chamber manufactured by ITS GmbH to perform tests based on stard MIL-STD 810G stard [22] Materials Nine different aluminum reflector from three manufacturers are tested. The material codes are chosen from A to I. The layer system different is shown in Figure 1 described in Table 1. The substrate all is a 0.5 mm thick aluminum sheet. In all cases aluminum surface is electrochemical polished anodized. The anodizing layer (Al 2 O 3 ) provides mechanical strength protects aluminum substrate from corrosion. The thickness anodizing layer varies from material to material. The structure all can be divided in two groups: with without a reflectance enhancing PVD layer system. The A, B, C, D

4 Energies 2016, 9, F have a PVD layer to increase reflectance (see Figure 1 left). It consists a high purity aluminum layer followed by two layers with different refraction indexes (SiO 2 TiO 2 ) to furr increase Energies 2016, 9, reflectance through positive interference. Additionally, all have a transparent coating on protective top for furr coating mechanical composition chemical manufacturing protection. processes. There are differences For in this protective C E coating top coating composition is polymer manufacturing based for processes. or For it is a sol-gel C Ecoating topbased coating on issilicon polymer dioxide based (SiO2). for or it is a sol-gel coating based on silicon dioxide (SiO 2 ). Figure Figure Layer Layer structure structure A, A, B, B, C, C, D E, E, G, G, H, H, I. I. Table Table 1 also also includes includes thickness thickness Al Al2O3 2 O 3 protective protective coating coating layers, layers, measured measured with with SEM, SEM, as as well well as as initial initial average average specular specular reflectance reflectance ρ ρs,φ s,ϕ (660 (660 nm; nm; 15 ; ; 12.5 mrad), mrad), measured measured on 49 on samples 49 samples per per material material before before aging aging testing. testing. As As can can be be observed, observed, reflectance reflectance varies varies from from (material (material I) I) to to (material (material A), A), whereas whereas group group with with PVD PVD coating coating present present a higher higher reflectance reflectance than than those without it. it. The sample size size is is cm 2 2 for forall all tests. This Thissize sizeis is small small enough enough to to measure measure samples samples with with spectrophotometer spectrophotometer large enough large enough to have ato sufficient have a sufficient measurement measurement surface not surface affected not by affected edge degradation by edge degradation effects. The effects. onlythe exceptions only exceptions are samples are samples tested intested UV + in Condensation UV + Condensation test, where test, where sample sample size issize 10 is cm 12 2 cm because 2 because holder holder in in corresponding corresponding chamber chamber is limited is limited to se to se dimensions. dimensions. Table Table Specular Specular reflectance reflectance structure structure different different tested. tested. Manufacturer Material Monochromatic Specular Reflectance Al2O3 2 O 3 Layer PVD Protective Coating ρs,φ ρ s,ϕ (660 nm; 15 ; ; 12.5 mrad) Thickness (µm) Coating Thickness Type Type A A ± Yes µm µm SiO SiO2 2 Sol Sol Gel Gel type type B B ± Yes 2.2 µm SiO SiO2 2 Sol Sol Gel Gel type type 2 2 C C ± Yes µm µm Polymer Polymer type type 1 1 D D ± Yes 2.5 µm SiO SiO2 2 Sol Sol Gel Gel type type E E ± No µm µm Polymer type type 2 2 F ± Yes 2.2 µm SiO F Yes 2.2 µm 2 Sol Gel type 4 SiO2 Sol Gel type G G ± No µm µm SiO SiO2 2 Sol Sol Gel Gel type type 5 5 H ± No 2.2 µm SiO H No 2.2 µm 2 Sol Gel type 6 SiO2 Sol Gel type 6 I ± No 2.5 µm SiO 2 Sol Gel type 7 I ± No 2.5 µm SiO2 Sol Gel type ExperimentalProcedure This This section section describes describes methodology followed followed to to find find those those accelerated aging aging tests tests that that reproduce degradation mechanisms suffered under natural aging conditions Outdoor Testing Outdoor Testing A broad outdoor exposure campaign started in spring 2012 latest samples were collected broad outdoor exposure campaign started in spring 2012 latest samples were in autumn A total 567 samples are being exposed at nine sites to different environments. collected in autumn A total 567 samples are being exposed at nine sites to different The sites cover a wide range outdoor conditions, from desert like to coastal inner city climates. environments. The sites cover a wide range outdoor conditions, from desert like to coastal The selected exposure sites corresponding climatic conditions are (see Figure 2 left): Tabernas, inner city climates. The selected exposure sites corresponding climatic conditions are (see Figure 2 left): Tabernas, Spain (semi-desertic); Almería, Spain (coastal, urban); Gran Canaria, Spain (coastal); Oujda, Morocco (desertic, urban); Missour, Morocco (desertic); Erfoud, Morocco (desertic); Zagora, Morocco (desertic); Tan Tan, Morocco (coastal, desertic) Abu Dhabi, UAE (coastal, desertic). Each site hosts 7 samples 9 material types (a total 63 samples per site). The samples are galvanically isolated exposure racks are facing south (see exposure rack Abu Dhabi in

5 Energies 2016, 9, Spain (semi-desertic); Almería, Spain (coastal, urban); Gran Canaria, Spain (coastal); Oujda, Morocco (desertic, urban); Missour, Morocco (desertic); Erfoud, Morocco (desertic); Zagora, Morocco (desertic); Tan Tan, Morocco (coastal, desertic) Abu Dhabi, UAE (coastal, desertic). Each site hosts 7 samples Energies 2016, 9, material types (a total 63 samples per site). The samples are galvanically isolated Figure exposure 2 right), racks are facing tilt angle south is 45 (seeto exposure horizontal rack plane. Abu Dhabi During Figure exposure 2 right), samples tilt angle are not is cleaned, 45 to due horizontal to logistical plane. difficulties During exposure also because samples re are not is no cleaned, single stard due to logistical procedure difficulties in industry also because application re is no single to small stard samples procedure would be in difficult. industry The absence application a cleaning strategy to small provokes samples would that bevalidity difficult. The this absence study is limited a cleaning for strategy large scale provokes CSP plants, that where validity regular this cleaning study is limited applied for by large scale O&M CSP staff. plants, However, where regular results cleaning obtained is applied are by highly O&M appropriate staff. However, for those applications results obtained which are are highly located appropriate in isolated for /or those applications rural areas which (solar are cooking, located domestic in isolated hot /or water, space rural areas heating, (solar pumping cooking, irrigation domestic hot water, water, desalination space heating, pumping water treatment) irrigation water, where desalination aluminum mirrors water are treatment) most suitable where aluminum type solar mirrors collectors are are most not suitable frequently type cleaned. solar Every collectors 6 months, are not hosting frequently partners cleaned. send Every 9 samples 6 months, (one hosting each material partners type) sendto 9 samples PSA in (one Spain each for material detailed analysis. type) to The PSA collected in Spain samples for detailed are kept analysis. for reference The collected not samples placed are back kept outdoors. for reference The sites not in Spain placed backabu outdoors. Dhabi The were sites installed Spain spring Abu Dhabi were In summer installed in2013, spring five additional In summer sites were 2013, installed five additional in Morocco. sitesthe were so installed far available in Morocco. data involve The soa far minimum availableexposure data involve duration a minimum 24 months. exposurethis duration period 24 is months. relevant This enough period for isthis relevant type enough mirrors, for this which typepresent mirrors, degradation which present even after degradation a few months even after in some a fewwearing months inconditions, some wearing to conditions, cover goal tothis cover work, which goal is this to identify work, which degradation is to identifymechanisms degradation appearing mechanisms outdoors appearing to find outdoors proper to accelerated find proper aging tests accelerated to reproduce aging tests m. toafter reproduce completion m. After completion project, remaining project, samples remaining will stay samples on racks will stay for on possible racks furr for possible testing furr to make testing long term to make exposure longdata termavailable. exposure data available. Figure2. 2. Exposuresites sitesin innorth Africa Spain ; ; typical exposure rack Accelerated Aging Testing Accelerated Aging Testing The description performed accelerated aging tests can be seen in Table 2. In this table, testing The description performed accelerated aging tests can be seen in Table 2. In this table, testing conditions are specified, in particular testing time, temperature (T), relative humidity (r.h.), ph, air conditions are specified, in particular testing time, temperature (T), relative humidity (r.h.), ph, air velocity (v), particle diameter (d) additive concentrations. Furr information about testing velocity (v), particle diameter (d) additive concentrations. Furr information about testing conditions stard tests (NSS, CASS, Damp Heat, UV Condensation, Humidiy Freeze conditions stard tests (NSS, CASS, Damp Heat, UV + Condensation, Humidiy Freeze Kesternich) can be found in [23] or in corresponding stards. Kesternich) can be found in [23] or in corresponding stards. Test Table 2. Description performed accelerated aging tests. Stard Testing Time (h) Testing Conditions Materials Tested NSS a ISO 9227 [15] 3000 [NaCl] = 50 ± 5 g/l; T = 35 ± 2 C; r.h. = 100%; ph = CASS a ISO 9227 [15] 100 [NaCl] = 50 ± 5 g/l; [CuCl2] = 0.26 ± 0.02 g/l; T = 50 ± 2 C; r.h. = 100%; ph = Damp Heat a IEC [16] 2000 T = 85 ± 2 C; r.h. = 85% ± 5% UV + Condensation a ISO [17] h: UV (340 nm); T = 60 ± 3 C 4 h: T = 50 ± 3 C; r.h. = 100% Humidity Freeze a IEC [16] cycles: T from 40 to 65 C (10 min in maximum minimum T) 40 cycles: 20 h T = 65 C;

6 Energies 2016, 9, Table 2. Description performed accelerated aging tests. Test Stard Testing Time (h) Testing Conditions NSS a ISO 9227 [15] 3000 CASS a ISO 9227 [15] 100 [NaCl] = 50 ± 5 g/l; T = 35 ± 2 C; r.h. = 100%; ph = [NaCl] = 50 ± 5 g/l; [CuCl 2 ] = 0.26 ± 0.02 g/l; T = 50 ± 2 C; r.h. = 100%; ph = Materials Tested Damp Heat a IEC [16] 2000 T = 85 ± 2 C; r.h. = 85% ± 5% UV + Condensation a ISO [17] 2000 Humidity Freeze a IEC [16] 1500 Kesternich a ISO 6988 [18] DIN [19] h: UV (340 nm); T = 60 ± 3 C 4 h: T = 50 ± 3 C; r.h. = 100% 400 cycles: T from 40 to 65 C (10 min in maximum minimum T) 40 cycles: 20 h T = 65 C; r.h. = 85% ± 5% + 4 h T = 40 C 35 cycles: 8 h [SO 2 ] = 0.067%; T = 40 ± 3 C; r.h. = 100% + 16 h ambient T r.h. UV a W/m 2 ; ambient T r.h. Machu a Qualicoat [20] 48 T = 37 C; [NaCl] = 50 ± 1 g/l; [H 2 O 2 ] = 5 ± 1 ml/l; [CH 3 COOH] = 10 ± 1 ml/l Immersion a [NaCl] = 50 ± 5 g/l; Ambient T B, E, I NSS/UV + Cond b week NSS, 1 week UV + Condensation A, E, I NSS + S b Damp Heat + S b UV + Cond + S b Humidity Freeze + S b [NaCl] = 50 ± 5 g/l; T = 35 ± 2 C; r.h. = 100%; ph = S on samples T = 85 ± 2 C; r.h. = 85% ± 5% S on samples 4 h: UV (340 nm); T = 60 ± 3 C 4 h: T = 50 ± 3 C; r.h. = 100% S on samples 400 cycles: T from 40 to 65 C (10 min in maximum minimum T) 40 cycles: 20 h T = 65 C; r.h. = 85% ± 5% + 4 h T = 40 C S on samples B, C, E, F Soil pipe b DIN [21] g s, SiO 2 d = µm Sstorm chamber b MIL-STD 810G [22] 10 min, v = 12.5 m/s, [SiO 2 ] = 125 mg/m 3 a Conventional test; b Innovative (non-stardized) test. C, D, I B, C, D, I B, E, F Although UV, Machu immersion tests do not follow a stard, y have been previously applied. The UV test is performed under ambient temperature humidity for 2000 h. In Machu test, performed according to Qualicoat procedure, samples are immersed during 48 h in a salt solution containing acetic acid hydrogen peroxide at 37 C. The back samples is protected with a paint layer to prevent chemical attack substrate. The immersion tests were carried out by immersing samples into a 5% salt solution for 1000 h. A combination NSS UV + Condensation test has been performed to study possible acceleration in material degradation when effects chloride, condensation und UV radiation are combined. The samples are alternated between two chambers in one week cycles (one week NSS followed by one week UV + Condensation so on). This cycle is repeated until 1000 h testing time is reached. The second important modification traditional testing procedures this work is performance some conventional tests (NSS, Damp Heat, Humidity Freeze UV + Condensation) with artificially soiled samples. S from ground at test site in Tabernas was used to cover samples with a thin layer soil. These soiled samples were n tested in different accelerated tests. The procedure applying dust on sample surface consists diluting dust with water, introducing it with sample in a small basin waiting until water is evaporated.

7 Energies 2016, 9, With this procedure, a homogeneous adherent soiling layer is deposited on mirror surface. The contamination testing chambers was avoided by capturing dripping condensate in a vessel. Therefore execution tests according to corresponding stards was assured. For abrasion tests in soil pipe s storm chamber, conditions are not specified in a stard. The testing parameters were varied during course project, especially concerning particle size, s mass, air velocity test duration. tested samples are checked in detail for appearing degradation its evolution during accelerated aging testing. 3. Results Discussion Results obtained in experimental campaign, both in outdoor testing accelerated aging testing, are included in this section Outdoor Testing The five main degradation mechanisms that were detected during outdoor testing campaign are following: Corrosion aluminum PVD layer Micropitting in PVD layer Non-removable chemical deposition on surface Pitting corrosion S abrasion Table 3 summarizes degradation mechanisms suffered for each material at every exposure site. Table 3. Material type presenting each degradation mechanism under outdoor exposure testing, for every exposure site. Location Tabernas (Spain) Almeria (Spain) Abu Dhabi (UAE) Canarias (Spain) Oujda (Morocco) Missour (Morocco) Erfoud (Morocco) Zagora (Morocco) Tan Tan (Morocco) Exposure Time (Months) PVD Layer Corrosion Micropitting in PVD Layer Deposits on Surface Pitting Corrosion S Abrasion 6 - A, B, C B, D A, B, C D, F A, B, C F - 6 A, B, D A, B, C A, B, C, D, F A, B, C A, B, D, F, G, H, I - 24 A, B, C, D, F A, B, C A, B, D, F, G, H, I - 6 A, B, D, F A, B, C E - 18 A, B, D A, B, C A, B, G, H - 24 A, B, D, F A, B, C A, B, D, G, H, I - 6 A, B, D, F A, B, C A, B, C - 18 A, B, C, D A, B, C A, B, C, D, E, F, I - 24 A, B, C, D, F A, B, C A, B, C A, B, D, F A, B, C H - 24 B, D, F A, B, C A, B, C B, D, F A, B, C - 24 D, F A, B, C - 6 A, B, D, F A, B, C A, B, D, F A, B, C - 24 A, B, C, D, F A, B, C H 6 A, B, D, F A, B, C - 20 A, B, D, F A, B, C - 24 A, B, D, F A, B, C - 6 A, B, D, F A, B, C A, B, D, F, G, H, I - 20 A, B, C, D, F A, B, C A, B, D, F, G, H, I - 24 A, B, C, D, F A, B, C A, B, D, F, G, H, I -

8 Energies 2016, 9, Energies 2016, 9, The first mechanism consists a corrosion produced in pure Al layer, which is typical for PVD-coated aluminum reflectors ( A, B, C, D F). A detailed description this degradation mechanism can be found in [24]. Examples this degradation type can be seen in Figure 3. According to results presented in Table 3, in general se corrosion defects mainly appeared in A, B, D F ( also in samples C exposed in coastal sites after longer exposures, although with a lower intensity). For se four material types, PVD layer corrosion was detected in many sites already after 6 months. Typical spot sizes range between 100 µm a few millimeters. The number defects detected was higher for D F samples for aggressive climates (close to coast), such as Tan Tan, increased with exposure time. Also on sites with severe abrasion, extent this defect type is elevated. Here protective function top layers is decreased abrasive defects act Figure 3. Microscopic views degradation due to corrosion aluminum PVD layer. material as Energies starting 2016, points 9, 916 for corrosion. D after 6 months outdoor; material B after 12 months The second mechanism (micropitting) consists small perforations aluminum PVD layer. Under light microscope, se kinds defects are visible as small black spots a size below one micrometer. Micropitting is only observed on A, B C, as indicated in Table 3. As all se samples are from one manufacturer it is likely that a characteristic in production process causes this susceptibility. Typical spots in material C (see Figure 4 right) are smaller denser than spots detected in material A B (see Figure 4 left). Micropitting degradation appeared in all exposure sites after 6 months. SEM images actual holes in Al layer are presented in Figure 5. The damaged PVD layer system can be observed in central part image presented in Figure 5 left, without delaminated protective top coating. Figure 5 right shows a picture same area but with higher magnification. In this case, small holes can be easily observed in Al layer, corresponding to micropitting defects. The third degradation mechanism detected consists different kinds deposits on surface, which cannot be removed by usual cleaning process (demineralized water a st tissue when necessary). The settled environmental particles seem to chemically interact with Figure Figure Microscopic Microscopic views views degradation degradation due due to to corrosion corrosion aluminum aluminum PVD PVD layer. layer. material protective coating (see Figure 6). As can be seen in Table 3, this is most common defect material observed D after after 6 months months outdoor; outdoor; material material B after after months. months. in outdoor exposure study because it was detected in all all exposure sites. The second mechanism (micropitting) consists small perforations aluminum PVD layer. Under light microscope, se kinds defects are visible as small black spots a size below one micrometer. Micropitting is only observed on A, B C, as indicated in Table 3. As all se samples are from one manufacturer it is likely that a characteristic in production process causes this susceptibility. Typical spots in material C (see Figure 4 right) are smaller denser than spots detected in material A B (see Figure 4 left). Micropitting degradation appeared in all exposure sites after 6 months. SEM images actual holes in Al layer are presented in Figure 5. The damaged PVD layer system can be observed in central part image presented in Figure 5 left, without delaminated protective top coating. Figure 5 right shows a picture same area but with higher magnification. In this case, small holes can be easily observed in Al layer, corresponding to micropitting defects. The third degradation mechanism detected consists different kinds deposits on surface, which cannot be removed by usual cleaning process (demineralized water a st tissue when necessary). The settled environmental particles seem to chemically interact with protective coating (see Figure 6). As can be seen in Table 3, this is most common defect observed Figure Microscopic Microscopic images images degradation degradation due due to to micropitting micropitting corrosion corrosion in in material material A A C C in outdoor after 12 exposure months in study Almería. because it was detected in all all exposure sites. after 12 months in Almería. The second mechanism (micropitting) consists small perforations aluminum PVD layer. Under light microscope, se kinds defects are visible as small black spots a size below one micrometer. Micropitting is only observed on A, B C, as indicated in Table 3. As all se samples are from one manufacturer it is likely that a characteristic in production process causes

9 Energies 2016, 9, this susceptibility. Typical spots in material C (see Figure 4 right) are smaller denser than spots detected in material A B (see Figure 4 left). Micropitting degradation appeared in all exposure sites after 6 months. SEM images actual holes in Al layer are presented in Figure 5. The damaged PVD layer system can be observed in central part image presented in Figure 5 left, without delaminated protective top coating. Figure 5 right shows a picture same area but with higher magnification. In this case, small holes can be easily observed in Al layer, corresponding to micropitting Energies 2016, 9, defects Energies 2016, 9, Figure SEM Figure SEM image image micropitting micropitting corrosion corrosion in in Al Al layer layer same same spot spot at at two two magnifications, magnifications, 2 µm 1 µm. 2 µm 1 µm. The third degradation mechanism detected consists different kinds deposits on surface, which cannot be removed by usual cleaning process (demineralized water a st tissue when with protective coating necessary). The settled environmental particles seem to chemically interact (see Figure 6).SEM Asimage can be in Table 3, this is most common defect in outdoor Figure 5. seen micropitting corrosion in Al layer same spot observed at two magnifications, exposure 2 µmstudy because 1 µm.it was detected in all all exposure sites. Figure 6. Microscopic SEM image deposits on surface material A. The fourth mechanism, called pitting corrosion, is a punctual corrosion that penetrates into aluminum substrate (see Figure 7), presenting irregular shape sizes around µm. As can be noted in Table 3, this degradation mechanism almost exclusively happens on coastal sites (Almería, Canarias, Abu Dhabi Tan Tan). A remarkable behavior was detected in Tan Tan, where most suffer this degradation already after 6 months outdoor exposure (except C Figure Microscopic SEM SEM image image deposits deposits on on surface surface material material A. A. E). For afigure deeper Microscopic understing, pitting holeswere observed with 3D microscope, as seen in prile a cross section in Figure 7, right. As can be observed, depth a typical pitting The fourth mechanism, called pitting corrosion, is a punctual corrosion that penetrates into fourth mechanism, pitting is a punctual corrosion thatten penetrates intowith hole The is around µm. Incalled addition corrosion, areas surrounding pitting holes are covered aluminum substrate (see Figure 7), presenting irregular shape sizes around µm. As can aluminum substratereaching (see Figure 7), presenting irregular shape sizes around corrosion products considerable fractions samples. The formation pitting µm. holesas in be noted in Table 3, this degradation mechanism almost exclusively happens on coastal sites can be noted in Table 3,with this degradation mechanism almost happens on coastal sites aluminum is associated presence chloride ions [25], exclusively which explains strong sensitivity (Almería, Canarias, Abu Dhabi Tan Tan). A remarkable behavior was detected in Tan Tan, (Almería, Canarias, mechanism Abu Dhabi Tan Tan). sites. A remarkable behavior was detected in Tan Tan, where to this degradation at coastal where most suffer this degradation already after 6 months outdoor exposure (except C most suffer this degradation already after 6 months outdoor exposure (except C E). E). For a deeper understing, pitting holes were observed with 3D microscope, as seen For a deeper understing, pitting holes were observed with 3D microscope, as seen in in prile a cross section in Figure 7, right. As can be observed, depth a typical pitting prile a cross section in Figure 7, right. As can be observed, depth a typical pitting hole is hole is around µm. In addition areas surrounding pitting holes are ten covered with around µm. In addition areas surrounding pitting holes are ten covered with corrosion corrosion products reaching considerable fractions samples. The formation pitting holes in aluminum is associated with presence chloride ions [25], which explains strong sensitivity to this degradation mechanism at coastal sites.

10 aluminum substrate (see Figure 7), presenting irregular shape sizes around µm. As can be noted in Table 3, this degradation mechanism almost exclusively happens on coastal sites (Almería, Canarias, Abu Dhabi Tan Tan). A remarkable behavior was detected in Tan Tan, where most suffer this degradation already after 6 months outdoor exposure (except C E).2016, For 9,a 916 deeper understing, pitting holes were observed with 3D microscope, as Energies 10seen 16 in prile a cross section in Figure 7, right. As can be observed, depth a typical pitting hole is around µm. In addition areas surrounding pitting holes are ten covered with products fractions fractions samples. formation pitting holes in aluminum corrosionreaching productsconsiderable reaching considerable The samples. The formation pitting holes in is associated with presence chloride ions [25], which explains strong sensitivity to this aluminum is associated with presence chloride ions [25], which explains strong sensitivity degradation mechanism at coastal sites. sites. to this degradation mechanism at coastal Figure 7. Microscopic Microscopic image degradation due pitting corrosion material H depth depth Figure 7. image degradation due to to pitting corrosion in in material H prile typical spot. Energies 2016, 9, 916 prile typical spot. The fifth mechanism is an abrasive damage top coating caused by airborne dust The fifth mechanism is an abrasive few top coating caused airborne dust8 s particles. The size defectsdamage ranges from micrometers to around 0.5by mm. In Figure s particles. few micrometers around mm. Indefects Figurein 8 an overview can bethe seensize on leftdefects a ranges higher from magnification picture to shows 0.5 different an overview can be seen on left a higher magnification picture shows different defects in top coating on right. So far this mechanism has only been detected at desertic Moroccan top coating on right. So far this mechanism has only been detected at desertic Moroccan sites. sites. Figure Microscopic Microscopic view view s s abrasion abrasion produced produced in in material material A Afrom fromzagora, Zagora,10 10 Figure magnification magnification 100 magnification red square area in left picture. 100 magnification red square area in left picture. The site Zagora is especially aggressive concerning this mechanism as it appears after only 6 The site Zagora is especially aggressive concerning this mechanism as it appears after months spreads over entire surface all samples with a high density maximum defect only 6 months spreads over entire surface all samples with a high density maximum size. In Erfoud Missour defects only appear after longer exposition durations with less defect size. In Erfoud Missour defects only appear after longer exposition durations with dense distribution as well as a smaller size. less dense distribution as well as a smaller size. Additionally, reflectance was measured in all all sites, after testing period Additionally, reflectance was measured in all all sites, after testing period (see (see Figure 9.) Figure 9.) As can be seen in Figure 9, hemispherical reflectance losses are significantly lower than specular reflectance losses, because main process causing losses is scattering. Concerning reflectance decay for different, it can be noticed that in general A, B C show higher losess (mainly in hemispherical reflectance due to micropitting). And with respect to sites, most significant result is that Zagora ( also Erfoud, but in a lower importancy) suffered a considerably higher reflectance decrease. In this site, F, G I present highest reflectance

11 Figure 8. Microscopic view s abrasion produced in material A from Zagora, 10 magnification Energies 2016, 9, magnification red square area in left picture. losses. The These site results Zagora are is useful especially for aggressive a better understing concerning this mechanism performance as it appears behaviour after only 6 different months spreads in over studied entire sites. surface However, all reflectance samples with measurements a high density are not able maximum to distinguish defect among size. In Erfoud different degradation Missour defects mechanisms only appear, consequently, after longer exposition y do notdurations show a clear patterns with less because dense distribution all possibleas effects well are as a mixed. smaller Therefore, size. reflectance loss was not selected as a representative parameter Additionally, to derive reflectance proper accelerated was measured aging tests in all that reproduce all degradation sites, after suffered testing period real outdoor (see Figure conditions. 9.) Figure Figure Hemispherical Hemispherical specular specular reflectance reflectance loss loss after after outdoor outdoor exposure exposure for for all all sites. sites. As can be seen in Figure 9, hemispherical reflectance losses are significantly lower than specular 3.2. Accelerated Testing reflectance losses, because main process causing losses is scattering. Concerning reflectance decay Table for 4 summarizes different, resultsit can be accelerated noticed that aging in tests general performed with A, B different C show. higher The losess columns (mainly show in hemispherical typical degradation reflectance mechanisms due to micropitting). that were already And discussed with respect in to outdoor sites, testing most (seesignificant Section 3.1) result is that rows Zagora represent ( also accelerated Erfoud, but aging a tests lower conducted. importancy) There suffered is ana additional considerably column, higher titled reflectance Side Effects, decrease. where In this degradation site, mechanisms F, G that do I present not happen during highest outdoor reflectance exposure losses. These are included. results are Images useful for typical a better appearances understing side performance effects are linked behaviour in table. different This way it can be directly in studied seen which sites. testhowever, has whichreflectance effect on a material. measurements The effects are not able twoto abrasion tests (soil pipe sstorm) are not included in this table because y are specifically designed as short-term mechanical tests thus only reproduce abrasive defects. From results conventional test program, following conclusions can be drawn: The PVD-layer corrosion can be provoked in NSS test in A, B, D F (see Figure 10a), but not as strong as detected in coastal outdoor sites. It only appears sporadically mainly on surface damages. The CASS test does reproduce this corrosion type (in A, B, C, D F) even for only 100 h testing, but always accompanied by strong pitting. Finally, this degradation mechanism was also detected in material B during Humidity Freeze test (where B, E F were included) also during immersion test (where B, E I were tested). Again in last two tests it only appeared to a very small extent. Micropitting only appears in UV + Condensation test in A, B C (see Figure 10b) in Damp Heat test (material C). The deposits on surface were not reproduced with conventional tests applied. Pitting corrosion was only reproduced in CASS test (see Figure 10c). In this case, pits found in all were bigger, more numerous with more deposits around spots than those detected in outdoor exposure tests (see Figure 10d).

12 Energies 2016, 9, Abrasion defects are only detected in tests including mechanical impacts particles, wher it is from falling particles in soil pipe (Figure 10p) or in air stream sstorm chamber (Figure 10o). Table 4. Degradation mechanisms different under accelerated aging testing. Test Exposure Time (Hours) PVD Layer Corrosion Micropitting in PVD Layer Deposits on Surface Pitting Corrosion Side Effects NSS a 2000 A, B, D, F A, B, C: cracks, delamination (Figure 10e,f) 1000 D, F A, B, D, F CASS a 100 A, B, C, D,F - - Damp Heat a C - - UV + Condensation a A, B, C - - Humidity Freeze a 1500 B A, B, C: cracks, delamination (Figure 10e,f) : strong pitting (Figure 10d) : unrealistic deposition (Figure 10g) C: layer changes (Figure 10h) A, B, D, E, F: cracks (Figure 10i) C: layer changes (Figure 10j) B, F: scratches, cracks (Figure 10k) Kesternich a UV a Machu a Immersion a 1000 B B: delamination, cracks (Figure 10e,f) NSS/UV + Cond b 1000 A A - - A, E: cracks (Figure 10j) A, E, I NSS + S b 1000 B B B, C, E, F - Damp Heat + S b 1000 D C C, D - UV + Cond/S b 1000 B, F A, B, C A, C, I - Humidity Freeze + S b 1000 B, F - B, D, E - a Conventional test; b Innovative (non-stardized) test. B, C: delamination, cracks, deterioration (Figure 10m) C: s cannot be removed (Figure 10n) A, B, D: cracks (Figure 10j) C: s cannot be removed (Figure 10n) B, D: cracks (Figure 10k) Tested B, E, F B, E, I B, C, E, F C, D, I A, B, C, D, F, I B, E, F Additionally, following side effects not detected outdoors were also found in conventional accelerated tests: A particular defect was detected in A, B C after NSS test, consisting cracks in top layers toger with ir delamination (see Figure 10e,f). This effect was also found in material B after immersion test. Some unrealistic deposits were detected in all after Damp Heat test, covering large parts sample surfaces (see Figure 10g). In UV + Condensation Damp Heat tests, elevated temperature is likely to cause degradation polymer top coating material C. Pictures se unrealistic changes in polymer layer are presented in Figure 10h (Damp Heat test) Figure 10i (UV + Condensation test). Additionally, anor side effect appeared in UV + Condensation test (in A, B, D, E F), consisting a significant amount cracks in top layers (see Figure 10j).

13 Energies 2016, 9, samples are very hydrophobic, soiled surfaces tend to retain humidity thus stay wet for a Finally, some scratches were detected in A F after Humidity Freeze test (see longer duration during exposure. Second, as natural s includes many different impurities, Figure 10l). aggressive compounds like chlorides can be introduced which act as corrosion activators. In The same first side innovative effects test observed performed, in a combination conventional tests NSS were also detected UV + Condensation in tests with test was chosen s. For to benefit example, fromcracks advantages in UV + Condensation+S both tests. This test was only Humidity applied tofreeze a small + set S tests. (A, In addition, E I). As innovative can be seen tests inpresented Table 4, material some side A suffered effects caused from both by PVD- s layer application, corrosion micropitting that is, NSS corrosion + S test in this showed test. In noticeable addition, deterioration it is worth to points remark( that typical B cracks C, see detected Figure in 10m) UV + Condensation s could not test be (see removed Figure from 10j) also appeared sample surface in this combined material C test, both in in Damp A E. Heat +S test in UV + Condensation + S test (see Figure 10n). Figure 10. PVD corrosion in material D after NSS test (compare to outdoor results in Figure 3); Micropitting in material A after UV UV + + Condensation test test (compare to to outdoor results results in Figure in Figure 4); (c) 4); Pitting (c) Pitting corrosion corrosion in material in material G after G after CASS CASS test (compare test (compare to outdoor to outdoor results results in Figure in 7); Figure (d) Strong 7); (d) Pitting Strong corrosion Pitting corrosion after after CASS test CASS (detected test (detected in all ); in all ); (e) Cracks (e) Cracks delamination delamination material in Amaterial after A NSS after test; (f) NSS Cracks test; (f) Cracks delamination delamination material in C after material NSS C after test; (g) NSS Deposits test; (g) on sample Deposits surface on sample aftersurface Damp after Heat Damp test (detected Heat test in(detected all ); in all (h) ); Layer changes (h) Layer in material changes in C after material Damp C after Heat Damp test; (i) Heat Layer test; changes (i) Layer inchanges materialin C material after C UV after + Condensation UV + Condensation test; (j) Cracks test; in (j) material Cracks in F after material UV F after + Condensation UV + Condensation test; (k) Scratches test; in(k) material Scratches A after in material Humidity A after freeze test; Humidity (l) Surface freeze deposits test; (l) Surface on surface deposits on material surface A after material UV + A Condensation after UV + test Condensation with s (compare test with s to outdoor (compare results to outdoor in Figure results 6); (m) in Figure Deterioration 6); (m) Deterioration in material Bin after material NSS B after + S NSS test; (n) + S Non-removable test; (n) Non-removable s depositss in material deposits C after in material Damp C after Heat + S Damp tests; Heat (o) Abrasion + S tests; defects (o) after Abrasion sstorm defects chamber after sstorm test; (p) Abrasion chamber test; defects (p) after Abrasion soil pipe defects test. after soil pipe test. 4. Conclusions In outdoor results, indications were found that soiling has an influence on degradation behavior. The outdoor In general, exposure samples with nine stronger different soiling aluminum also showed mirror more types or tested defects. at nine Therefore, exposure a series sites showed tests with that artificially type soiled amount samples degradation has been carried depends out. on The soiled samples location. have However, been tested five generic degradation mechanisms have been detected: aluminum corrosion pure PVD-Al layer,

14 Energies 2016, 9, in conventional tests. The first important result is that Tabernas dust indeed seems to enhance corrosion mechanisms. In particular, following events were detected: In UV + Condensation + S test (material B F) as well as in Damp Heat + S test (material D), PVD-layer corrosion was detected on soiled samples, although this was not case for clean samples. This degradation mechanism was also found in Humidity Freeze + S test ( B F) NSS + S test (material B), but in this case results are not as significant because it was also noticed in same tests without s. It is important to highlight that corrosion that appeared in two tests was stronger than those detected in same test without s. Micropitting corrosion was only detected in conventional tests when Damp Heat or UV + Condensation conditions were applied. However, when s is added on sample surfaces, this degradation pattern was detected not only in se two tests, but also in NSS test (for material B). The effect incompletely removable deposits on samples, observed after outdoor tests, is reproduced by all innovative accelerated aging tests with s (see Figure 10l). Finally, pitting corrosion was not noticed on innovative tests performed with s. There are two main factors explaining higher susceptibility to degradation soiled samples. First, soiling changes wetting phenomena on sample surface. While clean samples are very hydrophobic, soiled surfaces tend to retain humidity thus stay wet for a longer duration during exposure. Second, as natural s includes many different impurities, aggressive compounds like chlorides can be introduced which act as corrosion activators. The same side effects observed in conventional tests were also detected in tests with s. For example, cracks in UV + Condensation+S Humidity Freeze + S tests. In addition, innovative tests presented some side effects caused by s application, that is, NSS + S test showed noticeable deterioration points ( B C, see Figure 10m) s could not be removed from sample surface material C both in Damp Heat +S test in UV + Condensation + S test (see Figure 10n). 4. Conclusions The outdoor exposure nine different aluminum mirror types tested at nine exposure sites showed that type amount degradation depends on location. However, five generic degradation mechanisms have been detected: aluminum corrosion pure PVD-Al layer, micropitting pure PVD-Al layer, pitting corrosion Al substrate, mechanical top coating degradation, non-removable surface deposits due to chemical interaction between local dust particles protective coatings reflector samples. Degradation is more severe on sites close to coast on sites with high wind velocities airborne s particles. Materials with reflectance enhancing PVD layers show improved reflectance but also a higher susceptibiltiy to degradation. Some mechanisms only appear on se one mechanism even originates from a distinctive feature production process one manufacturer. The same sample types have been tested in a large accelerated aging testing campaign including stardized innovative tests. Stardized tests only reproduced a maximum one or two five mechanisms detected outdoors. Some short term tests do not provoke any degradation at all. In addition, several degradation effects that were not observed outdoors appeared. A realistic degradation pattern was thus not obtained for any conventional tests. The difference in material behavior concerning mechanisms is similar to one found during outdoor exposure. The innovative accelerated aging tests artificially soiled samples with Tabernas dust were able to reproduce three five mechanisms observed outdoors. None stard tests reproduce deposit on surface degradation, though it is systematically reproduced by innovative tests

15 Energies 2016, 9, including soiling. Unrealistic side degradation mechanisms like top coating cracking have not been prevented in se cases. However, a more realistic overall degradation pattern was observed compared to stardized testing clean surfaces. The investigation this approach is ongoing to identify best suited parameters (regarding soil application, test conditions repeatability). Although re was no single test that produced a perfectly realistic degradation pattern, results this study enable researchers to identify more realistic tests to combine m into a testing sequence. With this approach more mechanisms can be reproduced at once. It can also help avoid using tests test durations with unrealistic side effects. The research results presented in this paper are a useful tool for aluminum-reflector manufacturers to design perform ir accelerated aging testing strategy in order to improve manufacturing process product quality. In addition, significant information about real degradation mechanisms to be resolved at different representative sites is presented, giving manufacturers not only approach to be followed to advance with ir products, but also possible specific sites to be avoided when recommending m. The comparisons made in course this investigation also built a base for testing procedure for aluminum reflectors that was published via SolarPaces program [26]. This testing procedure is a powerful tool for research laboratories to serve as independent institutions in comparison validation prototype or marketed aluminum-reflectors. Acknowledgments: Financial support from German ministry for environment, nature conservation nuclear safety (BMU) within contract is gratefully acknowledged. The authors want to thank ITC Gran Canaria (especially Juan Antonio Suarez Ortega), MASDAR (especially Ali Al Masabai), IRESEN Mohammed First University in Oujda for hosting outdoor exposure racks sending samples to PSA. Also, Lucia Martínez from Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas (CIEMAT), Tomás Jesús Reche Navarro from EULEN, Florian Wiesinger, Alexer Oschepkov Peter Heller from DLR for ir valuable contributions. Author Contributions: Johannes Wette, Florian Sutter Aránzazu Fernández-García were responsible for execution exposure testing campaign as well as analysis. The whole campaign was planned in collaboration with Stefan Ziegler Reinhard Dasbach as material experts, y also contributed to interpretation results. Johannes Wette wrote article with much collaboration from Florian Sutter Aránzazu Fernández-García. Conflicts Interest: The authors declare no conflict interest. Nomenclature Units ρ s,ϕ (660 nm; 15 ; 12.5 mrad) monochromatic specular reflectance at wavelength 660 nm, incidence angle 15 acceptance angle 12.5 mrad (%) T temperature ( C) r.h. relative humidity (%) v wind velocity (m/s) d particle diameter (µm) Acronyms CASS CPC CSP NSS PSA PTC PVD SEM cupro-acetic acid salt spray compound parabolic concentrators Concentrated solar power neutral salt spray Plataforma Solar de Almería parabolic-trough collectors Physical vapor deposition scanning electron microscope References 1. International Energy Agency. Key World Energy Statistics; International Energy Agency: Paris, France, Do, T.-T.-H.; Schnitzer, H.; Le, T.-H. A decision support framework considering sustainability for selection rmal food processes. J. Clean. Prod. 2014, 78, [CrossRef]

16 Energies 2016, 9, Kalogirou, S. The potential solar industrial process heat applications. Appl. Energy 2003, 76, [CrossRef] 4. Fernández-García, A.; Rojas, E.; Pérez, M.; Silva, R.; Hernández-Escobedo, Q.; Manzano-Agugliaro, F. A parabolic-trough collector for cleaner industrial process heat. J. Clean. Prod. 2015, 89, [CrossRef] 5. Harmin, A.; Merzouk, M.; Boukar, M.; Amar, M. Design experimental testing an innovative building-integrated box type solar cooker. Sol. Energy 2013, 98, [CrossRef] 6. Narasimha Rao, A.V.; Subramanyam, S. Solar cookers Part I: Cooking vessel on lugs. Sol. Energy 2003, 75, [CrossRef] 7. Fernández-García, A.; Zarza, E.; Valenzuela, L.; Pérez, M. Parabolic-trough solar collectors ir applications. Renew. Sustain. Energy Rev. 2010, 14, [CrossRef] 8. European Solar Thermal Electricity Association (ESTELA). Solar Thermal Electricity, Strategic Research Agenda ; ESTELA: Brussels, Belgium, International Energy Agency (IEA). Technology Roadmap: Solar Thermal Electricity; IEA: Paris, France, García-Segura, A.; Fernández-García, A.; Ariza, M.J.; Valenzuela, L.; Sutter, F. Durability studies solar reflectors: A review. Renew. Sustain. Energy Rev. 2016, 62, [CrossRef] 11. Wiesinger, F.; Sutter, F.; Fernández-García, A.; Reinhold, J.; Pitz-Paal, R. S erosion on solar reflectors: Accelerated simulation comparison with field data. Sol. Energy Mater. Sol. Cells 2016, 145, [CrossRef] 12. Kennedy, C.E.; Terwilliger, K.; Milbourne, M. Development Testing Solar Reflectors; Technical Report No. NREL/CP ; National Renewable Energy Laboratory (NREL): Golden, CO, USA, Kennedy, C.E.; Terwilliger, K. Optical durability cidate solar reflectors. J. Sol. Energy Eng. 2005, 127, [CrossRef] 14. Sutter, F.; Ziegler, S.; Schmücker, M.; Heller, P.; Pitz-Paal, R. Modelling optical durability enhanced aluminum solar reflectors. Sol. Energy Mater. Sol. Cells 2012, 107, [CrossRef] 15. International Organization for Stardization (ISO). ISO 9227: : Corrosion Tests in Artificial Atmospheres Salt Spray Tests; ISO: Geneva, Switzerl, International Electrotechnical Commission (IEC). IEC 62108:2007: Concentrator Photovoltaic (CPV) Modules Assemblies Design Qualification Type Approval; IEC: Geneva, Switzerl, International Organization for Stardization. ISO 11507: : Paints Varnishes Exposure Coatings to Artificial Wearing Exposure to Fluorescent UV Lamps Water; ISO: Geneva, Switzerl, International Organization for Stardization (ISO). ISO 6988:2012, Metallic Or Non-Organic Coatings Sulfur Dioxide Test with General Condensation Moisture; ISO: Geneva, Switzerl, Deutsches Institut für Normung. DIN 50018: , Testing in a Saturated Atmosphere in Presence Sulfur Dioxide; Deutsches Institut für Normung: Berlin, Germany, QUALICOAT. Specifications for a Quality Label for Liquid Powder Organic Coatings on Aluminum for Architectural Applications, 13th ed.; QUALICOAT: Zurich, Switzerl, Deutsches Institut für Normung. DIN 52348: Testing Glass Plastics Abrasion Test S Trickling Method; Deutsches Institut für Normung: Berlin, Germany, Department Defense. MIL-STD-810G: Environmental Engineering Considerations Laboratory Tests; Department Defense: Philadelphia, PA, USA, Sutter, F.; Fernez-García, A.; Wette, J.; Heller, P. Comparison Evaluation Accelerated Aging Tests for Reflectors. In Proceedings SolarPACES 2013 International Conference, Las Vegas, NV, USA, September Sutter, F.; Wette, J.; Lopez-Martin, R. Corrosion Aluminum Solar Reflectors. In Proceedings International Conference on Concentrating Solar Power Chemical Energy Systems, SolarPACES 2012, Marrakesh, Morocco, September Szlarska-Smialowska, Z. Pitting corrosion aluminum. Corros. Sci. 1999, 41, [CrossRef] 26. Sutter, F.; Wette, J.; Fernández-García, A.; Ziegler, S.; Dasbach, R. Accelerated aging testing aluminum reflectors for concentrated solar power. In SolarPaces Guideline; SolarPACES: Almería, Spain, by authors; licensee MDPI, Basel, Switzerl. This article is an open access article distributed under terms conditions Creative Commons Attribution (CC-BY) license (