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2016-08-22 07:03:40

and mechanical properties of a pv protective glass AbstractRecent developments in climate change have increased the frequency of dust storms in the Middle East. Dust storms significantly influence the performances of solar energy harvesting systems, particularly (photovoltaic) PV systems. The characteristics of the dust and the mud formed from this dust are examined using various analytical tools, including optical, scanning electron, and atomic force microscopies, X ray diffraction, energy spectroscopy, and Fourier transform infrared spectroscopy. The adhesion, cohesion and frictional forces present during the removal of dry mud from the glass surface are determined using a microtribometer. Alkali and alkaline earth metal compounds in the dust dissolve in water to form a chemically active solution at the glass surface. This solution modifies the texture of the glass surface, thereby increasing the microhardness and decreasing the transmittance of the incident optical radiation. The force required to remove the dry mud from the glass surface is high due to the cohesive forces that result from the dried mud solution at the interface between the mud and the glass. The ability altering the characteristics of the glass surface could address the dust/mud related limitations of protective surfaces and has implications for efficiency enhancements in solar energy systems. IntroductionGlass is widely used in solar energy harvesting applications to protect active devices from harsh environments, such as dust, heavy rain, wind, etc. Heavy dust storms are an environmental concern around the world, particularly in the Kingdom of Saudi Arabia. Changes in climate beyond normal expectations contribute considerably to the frequency of dust storms in the Middle East1, which has an adverse effect on the environment, urban life, and most industries. Air humidity and patchy rainfall cause dust accumulation to be severely problematic after dust storms. Dust accumulation occurs because of the ionic compounds in dust, which dissolve in water and modify the interfacial forces between the dust and surfaces. For dry dust particles that land on a solid surface, the interfacial forces are mainly governed by van der Waals forces. However, the interfacial forces increase in the presence of liquids, thereby resulting in mud formation on substrate surfaces. In this case, the cohesive forces due to the dried mud solution at the interface contribute to the increases in the interfacial forces. The dust particles are composed of compounds containing alkali (NaOH) and alkaline earth metals (CaCO3)1 that dissolve in condensed water vapor and increase the pH of the water. After mud forms from dust particles and water, some of the components of the dust, such as the alkali/alkaline earth compounds, dissolve into the water and form a chemically active solution. Because mud is composed of porous structures, the solution (water with dissolved ionic compounds) forms sediments at the interface between the substrate and the mud. The solution dries and forms a crystalline layer between the dried mud and the substrate. Because the mud formation and removal are interrelated, these processes are complex and require a thorough investigation of the after effects of mud deposition, including the chemical, optical, morphological, and mechanical (adhesion, friction, hardness, etc.) effects on the substrate surfaces. Consequently, investigation of the mud formed from dust on glass surfaces and its after effects on the surface characteristics is essential. A considerable number of research studies have been performed to examine the effects of dust on various solid surfaces. Dust accumulation on an insulator and the adhesion force between dust particles and the insulator surface were studied by Wang et al.2. They found that the charging of an insulator surface produces long range attractive forces on the dust particles; however, such charging had little influence on the adhesion force. The effects of sand and dust accumulation on photovoltaic modules were studied by Beattie et al.3. They demonstrated that the reduction in the active area of the modules was mainly due to the formation of clusters of particles on the surface, which reduced the available area for light capture to a much smaller area compared to particles resting directly on the glass surface. The adhesion of dust particles to common indoor surfaces in an air conditioned environment was examined by Tan et al.4. They found that dust and activated carbon adhesion were highly sensitive to surface roughness with an inverse relationship between adhesion force and roughness due to the reduction in contact area between the particle and a rougher material surface. The effect of drought on dust production in the Sudano Sahelian zone of the Sahara Desert was investigated by Middleton5. The data showed that dust storm activity in the west and east of the Sudano Sahelian belt had dramatically increased during the drought years; by a factor of 6 in Mauritania and up to a factor of 5 in Sudan. The transport of Asian dust around the globe was reported by Uno et al.6. The findings revealed that Asian dust could influence the global radiation budget by stimulating cirrus cloud formation and marine ecosystems by supplying nutrients to the open ocean. The eolian dust deposition in the western United States due to human activity was studied by Neff et al.7. They indicated that the larger dust flux, which persisted into the early twenty first century, resulted in more than fivefold increase in inputs of K, Mg, Ca, N and P to the alpine ecosystems, with implications for surface water alkalinity, aquatic productivity and terrestrial nutrient cycling. A study of the impact of airborne dust deposition on the performance of solar photovoltaic (PV) modules was performed by Jiang et al.8. They demonstrated that dust pollution had a significant impact on PV module output and that the reduction of efficiency had a linear relationship with the dust deposition density; however, no differences due to the cell type were noted. The effectiveness of self cleaning and antireflective packaging glass for solar modules was examined by Verma et al.9. They indicated that non lithographic nanostructuring of the packaging glass surface resulted in both reduced reflection at the air/glass interface and self cleaning characteristics. The effects of dust on the transparent covers of solar collectors were studied by Elminir et al.10. The findings revealed that the reduction in glass normal transmittance depended strongly on the dust deposition density in conjunction with the plate tilt angle and the orientation of the surface with respect to the dominant wind direction. The electrodynamic screen performance for dust removal from solar panels and solar hydrogen generators was investigated by Mazumder et al.11. When the electrodes were activated by phased voltage, the dust particles on the surface of the film became electrostatically charged and were removed by the traveling wave generated by the applied electric field. The influence of the dust deposition on the performance of parabolic trough solar collectors was studied by Niknia et al.12. They developed a new correlation for the thermal performance of a parabolic collector due to various dust thicknesses in comparison to a clean collector. The outdoor performance and durability testing of anti reflecting and self cleaning glass for photovoltaic applications was presented by Sakhuja et al.13. The nanostructured glass samples resulted in enhanced self cleaning and improved PV performance over a long exposure period. The effect of dust accumulation on the performance of evacuated tube collectors was examined by El Nashar14. He reported the performance decline of evacuated tube collectors due to dust accumulation over periods extending from one month to an entire year. The energy yield loss caused by dust deposition on photovoltaic panels was investigated by Sayyah et al.15. They provided a database for predicting anticipated soiling losses at different locations around the world, which could be used to assess effective cleaning methods for restoring a system's energy yield. Suppression of dust adhesion on a photovoltaic concentrator module using an anti soiling photocatalytic coating was studied by Sueto et al.16. Their findings revealed that the presence of electrostatic charges on the surfaces of the samples was a main factor in the adhesion of sand, which could be suppressed by the anti soiling photocatalytic layer. The importance of cleaning concentrated photovoltaic arrays in a desert environment was presented by Khonkar et al.17. They indicated that in a desert environment, some extreme weather events occur that require cleaning of all types of PV modules for improved system performance. The effect of soiling on photovoltaic modules was investigated by Appels et al.18. They demonstrated that special coatings on glass had potential for reducing the power loss caused by dust settlement; however, the extra cost associated with these coatings was not justified for photovoltaic applications. Although many research studies have been performed on dust accumulation on PV and solar thermal surfaces15,16,17,18, the chemo mechanical effects of dust accumulation on glass surfaces in humid environments requires further investigation. Therefore, in the present study, the dust characteristics and effects of dust accumulation and mud formation (reflecting the influence of the humid air environments) on plain glass surfaces are investigated. The chemical effects of mud on glass surfaces are assessed using various analytical tools, including optical, electron scanning and atomic force microscopies, X ray diffraction, energy dispersive spectroscopy, and Fourier transform infrared spectroscopy. The adhesion force between mud and glass surfaces is obtained from microtribometer data, and the optical transmittance of the glass after the mud removal is measured. It should be noted that the work presented is original and has not been reported elsewhere. ExperimentalPV protective glass samples with dimensions of 30mm30mm2mm (widthlengththickness) were used as workpieces. The chemical composition of the glass was 76.5% SiO2, 9.9% CaO, 1.2 MgO and 12.4% Na2O. The dust was collected from PV modules in the area of Dhahran in Saudi Arabia after a dust storm in 2014. Characterization of the dust was performed using SEM, EDS, and XRD. A JEOL 6460 scanning electron microscope was used for the SEM and EDS examinations, and a Bruker D8 Advanced diffractometer with a Cu K radiation source was used for XRD analysis. The typical settings of the XRD instrument were as follows: 40kV and 30mA for the x ray source and a scanning angle (2) range of 20 80. Roughness measurements and surface profile characterization were performed using a 5100 AFM/SPM Microscope by Agilent in contact mode. The probe tip was made of silicon nitride (r=20 60nm) with a manufacturer specified force constant, k, of 0.12N/m. A Micro Photonics digital microhardness tester (MP 100TC) was used for the surface microhardness measurements. The standard test method for the Vickers indentation hardness of advanced ceramics (ASTM C1327 99) was adopted. The measurements were repeated five times at each location to ensure the consistency of the results. A linear microscratch tester (MCTX S/N: 01 04300) was used to determine the friction coefficient of the glass surfaces. The contact load was set at 0.03N, and the end load was set at 2.5N. The scanning speed was 5mm/min, and the loading rate was 0.01N/s. The total length for the scratch tests was 0.5mm. The optical transmittance was measured using a UV spectrometer (Jenway 67 Series spectrophotometer), and Fourier transform infrared spectroscopy (Bruker VERTEX70) was performed to collect the infrared absorption spectrum of the glass. To investigate the effects of dust and mud on the Ray Ban RB3362 Sunglasses Arista Frame Crystal Brown Gradient Le
Ray Ban RB3362 Sunglasses Arista Frame Crystal Brown Gradient Le surface characteristics of the glass, actual dust accumulation and mud formation were simulated in a laboratory. In actual environments, the mud formed from accumulated dust particles due to the condensation of water vapor onto the particles. The accumulated dust thickness was measured over the period of two weeks during a dust storm in Saudi Arabia in 2014. This accumulation was on the order of 300m. To simulate dust accumulation in the laboratory, 300 m layers composed of dust particles collected from the local environment were formed on the cleaned glass surfaces. Desalinated water, which was equal to the amount of water vapor that condensed on the same volume of the dust in the open environment, was dispensed gradually onto the dust layer. The initial condensation tests were performed in ambient humid air to estimate the amount of condensate that accumulated over time. Moreover, the dispensed water was left on the surface of the dust layer without mechanical mixing to resemble water condensation from humid air. Therefore, the simulated formation of mud on the glass surfaces was similar to the deposition that occurred naturally in the open environment. Next, the glasses were kept in ambient air at room temperature for three days to dry. Scratch tests were performed to measure the tangential force required to remove the mud from the glass surfaces. The tangential force provided information regarding the adhesion, cohesion and frictional work during the dry mud removal. To examine the after effects of the mud on the glass surfaces, the dry mud was removed from the workpiece surfaces using a desalinated water jet that was 2mm in diameter with a velocity of 2m/s. The cleaning process was applied for 15minutes to each glass surface. Finally, the morphology, optical transmittance, molecular characteristics, and microhardness of the mud removed glass surfaces were analyzed using the analytical tools. The microhardness, friction and adhesion tests were repeated 12 times to secure the confidence levels for the experimental uncertainty assessments. Based on the distribution of the experimental data, the confidence level of 95% was resulted; in which case, the mean () of the data distribution was within1.75 of the standard deviation of the distribution of a single measurement from that distribution. The experimental uncertainty analysis revealed that the uncertainty less than 2% was resulted for the microhardness measurements while the uncertainty of about 3% was obtained for the friction and adhesion tests. Results and DiscussionCharacterization of the environmental dust is presented, and the influence of the mud formed from the dust particles in the humid environment on the chemical and optical characteristics of the glass is examined. The frictional, adhesion, and cohesion forces required to remove the dry mud from the glass surface are also measured. Figure 1 shows SEM micrographs of dust particles with various sizes, and Fig. 2 shows the size distribution of the dust particles. The dust collected on the glass surfaces consists of large and fine particles and is a heterogeneous mixture of particles with various morphologies. In this case, no standard set of sizes is dominant in the dust particles mixture. However, in general, small dust particles are aggregated in clusters of 1 3m (Fig. 1). The large particles are on the order of 20m in diameter, and some small particles are attached to the surfaces of the large particles because of the charges. Although the small particles that are attached to the large particles vary in size, in general, these particles are sub micron in size (Fig. 1). The slight bright appearance of the small particles in the SEM micrographs is associated with electron charging during the imaging. This charging indicates that these particles possess charges that allow them to attach to the large particles. As indicated in an earlier study19, the smaller particles (average particle diameter 2.5m) reside in the atmosphere for prolonged times and interact with solar radiation for longer durations than the large particles. Therefore, prolonged exposure to the atmosphere in regions closer to the sea causes the attachment of ionic compounds. Moreover, although the particles have irregular shapes, the average dust particle size based on the particle distribution is on the order of 1.2m (Fig. 2). Figure 1SEM micrographs of the dust particles: (a) dust particles of various sizes, (b) aggregated small particles (highlighted by the circle), (c) small particles attached to large particles, and (d) flake like particle. By analyzing the SEM images of the dust particles, two principle quantities were identified: the aspect ratio and the shape factor. This result is in agreement with the previous study20. The aspect ratio is, where A is the cross sectional area, and Lproj is the longest projection length of the dust20. The shape factor is, where P is the perimeter of the dust particle20. The particle diameter and area can be generated from these measurements. The diameter of a circle with equivalent area is considered for circular dusts, and for non circular dusts, an ellipse model is used by assuming the longest projection as the major axis and preserving the cross sectional area of the particle20. The aspect ratio is related to the particle roundness and approximately represents the ratio of the major axis to the minor axis of the ellipsoid best fit to the particle. In addition, the shape factor is the inverse of the particle circularity, which is associated with the complexity of the particle. In this case, a shape factor of unity corresponds to a perfect circle. The aspect ratio and the shape factor are found to change with the particle size. Although no linear relation holds between the aspect ratio or the shape factor and the particle size, the particle aspect radio decreases with increasing particle size, while the shape factor increases with increasing particle size. Nevertheless, the shape factor approaches unity for the smaller particles, while for the large particles, the median shape factor almost reaches 3. Table 1 presents the EDS data for the dust particles. The presence of oxygen, iron, sulfur, chlorine, calcium, silicon, sodium, magnesium, and potassium are evident, and their concentrations vary at different locations in the dust. This result indicates that the dust is composed of non uniformly distributed elements and compounds. The quadrangular particle, which appears to be deformed from a cubic particle, is rich in sodium and chlorine; however, the aggregated particles are rich in calcium and oxygen. Flake like particles are also observed; these particles are rich in calcium and silicon (Fig. 1). Figure 3 shows an X ray diffractogram of the dust particles. Potassium, sodium, calcium, sulfur, chlorine, and iron peaks are clearly visible. The iron peak is coincident with the aluminum and silicon peaks. The presence of sodium and potassium peaks is associated with sea salt because the region where the dust was collected is close to the Arabian Gulf. The concentration of chlorine changes for different dust particles, and the EDS data do not satisfy the molar ratio for NaCl (Table 1); therefore, the dust particles do not contain salt crystals; instead, NaCl is dissolved in the compound form. The sulfur may form a monomer layer during the aging process in the atmosphere. However, the sulfur can be correlated with the calcium in the dust, such as the anhydrite or gypsum component (CaSO4). The iron is most likely related to clay aggregated hematite (Fe2O3). Figure 4 shows SEM micrographs of the top surface and a cross section of the mud formed on the glass surface. The mud is formed using the collected dust and the desalinated water application that mimics the condensation of water from humid air. During the formation of the mud, no mechanical mixing is used, and the mud was left to dry at room temperature for two days prior to analysis. The mud surfa.

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