Soiling Effect Mitigation Obtained by Applying Transparent Thin-Films on Solar Panels: Comparison of Different Types of Coatings

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Article Soiling Effect Mitigation Obtained by Applying Transparent Thin-Films on Solar Panels: Comparison of Different Types of Coatings

Małgorzata Rudnicka and Ewa Klugmann-Radziemska *

Department of Energy Conversion and Storage, Faulty of Chemistry, Gdansk University of Technology, 80-233 Gda´nsk,Poland; [email protected] * Correspondence: [email protected]

Abstract: Dust accumulation on the front cover of solar panels is closely linked to location and orientation of photovoltaic (PV) installation. Its build-up depends on the module tilt angle, frequency of precipitation, humidity, wind strength and velocity, as well as grain size. Additionally, soil composition is determined by solar farm surroundings such as local factories, agricultural crops, and traffic. Over time, molecules of atmospheric dust agglomerate on top of each other and cause gradual reduction in generated energy. Manual cleaning techniques are required to restore working conditions of PV installation to their original conditions; however, they are time consuming and may lead to damage of the glass coverage. Therefore, implementing a different approach by utilizing self-cleaning and anti-dust coatings on front covers of module surfaces is thought of as a competitive manner of cleansing. Based on the varying properties of such thin-films, a division was made into Citation: Rudnicka, M.; hydrophobic, hydrophilic, and anti-dust coatings. In this article, the authors would like to present a Klugmann-Radziemska, E. Soiling Effect Mitigation Obtained by comprehensive review of those types of transparent films. Moreover, a few hydrophobic coatings Applying Transparent Thin-Films on available on the Polish market were analyzed by applying them on glass tiles and covering them Solar Panels: Comparison of Different with three types of dust. Types of Coatings. Materials 2021, 14, 964. https://doi.org/10.3390/ Keywords: photovoltaics; dust accumulation; hydrophobic; hydrophilic; anti-dust ma14040964

Academic Editors: Thomas Stergiopoulos and 1. Introduction Antonio Bartolomeo Solar energy is the steady growing green energy source with both large companies as well as individual people willing to invest in this market. As the need to reduce carbon Received: 28 December 2020 footprints is common in our everyday lives, one should not be surprised that photovoltaic Accepted: 15 February 2021 establishment tries to mitigate any drawbacks connected to solar farms. One such difficulty Published: 18 February 2021 is connected to pollutant deposition on photovoltaic (PV) module surfaces, which directly corresponds to the lowered energy yield possible to obtain from installation. Soil build-up Publisher’s Note: MDPI stays neutral progressing over time causes the reduction of the glass transparency of the front module with regard to jurisdictional claims in published maps and institutional affil- cover [1–3]. This, in turn, leads to decreases of solar irradiance reaching semiconductor iations. cells and decrements in current output [4–7]. Such effects could be partially mitigated if precipitation occurs and washes off a part of the accumulated pollution [8]. The rate of dust accumulation on top of the front glass coverage of PV modules is closely linked to the tilt angle, latitude, and environmental surroundings in the close proximity of solar power sources [9]. The more horizontal the tilt angle, the higher the Copyright: © 2021 by the authors. speed of soil settlement. Additionally, dust molecules tend to gather more at the bottom Licensee MDPI, Basel, Switzerland. of PV modules because of gravitational forces dragging them down [10]. Any occurrence This article is an open access article distributed under the terms and of heavy wind and storm or precipitation would likely cause a regain in efficiency [11,12]. conditions of the Creative Commons On the other hand, frequent sand storms in desert regions negatively impact the power Attribution (CC BY) license (https:// generation of solar energy, as they lead to complete coverage of glass surfaces, with sand creativecommons.org/licenses/by/ almost completely cutting off sun radiation reaching solar cells [13]. Similarly, a sparse 4.0/).

Materials 2021, 14, 964. https://doi.org/10.3390/ma14040964 https://www.mdpi.com/journal/materials Materials 2021, 14, 964 2 of 12

amount of rain or high levels of humidity cause a formation of a cement-like layer, which dries into to a hard shell on the glass surface [9]. Dust properties, namely, the size, shape, mass, and composition, depend on the location of solar power plants. They can also be influenced by nearby industry, heavy traffic or agricultural crops [14]. The relations found between soiling tempo and particle size is that for grains with a smaller diameter, the loss of glass transmission is greater as such molecules can be stacked closely and block irradiance more efficiently [10]. Manufacturers handling worldwide sales outline in documents concerning conserva- tion of PV modules that it is mandatory to clean them manually [15–19]. It may help if the installation is small; however, this can hardly be an optimal solution for solar plants consist- ing of dozens or more devices. Not only is it time-consuming but wasting of water ought to be avoided if possible, given much more frequent and prolonged periods of drought. Addressing this issue, alternative cleaning methods are developed, namely, electrostatic, mechanical, and surface enhancement [20]. Electrostatic cleaning utilizes standing and traveling waves to expel dust from electrostatic fields. Standing waves move the dust downwards or upwards, and horizontal movement is achieved by traveling waves. Mechanical cleaning offers four methods—robotic, air-blowing, water-blowing, and ultra- sonic. Blowing air and water also result in reducing the temperature of PV surfaces. Lastly, surface enhancement revolves around introducing an additional layer on top of the front glass coverage. It utilizes nanotechnology in order to design hydrophobic and hydrophilic thin films. Hydrophobic coatings allow water drops to roll down and carry away dirt residue. Hydrophilic ones reduce the amount of dirt via photo-catalytic reactions. This article aims at familiarizing the reader with the overview of the current state of knowledge in the field of thin films developed specifically for usage in the photovoltaic industry. Additionally, experimental analysis of a few hydrophobic coatings available on the Polish market was performed.

2. Hydrophilic Coatings Hydrophilic coatings are characterized by their high surface energy. After spraying them with water they distribute it evenly over the whole surface and thus clean it. They are oftentimes based on silica particles [21]; however, the photo-catalytic effect may also be incorporated by utilizing titanium or wolfram [21,22]. Hee et al. analyzed the effect of dust settlement on glass slides without any coverage and discovered their transmission was reduced by 3% over the course of one month despite heavy rainfall occurring in Singapore [23]. The experiment was carried out for four months and after an initial steep linear decrease in glass transparency, this phenomena progressed more slowly for a day number over 32 (Figure1). They added another conclusion such that the rate of transmission reduction was faster at the bottom of the glass tile, meaning the debris after precipitation tended to gather there due to gravitational force. Later on, glass tiles were covered by a layer of TiO2 intended as a self-cleaning coating. Two different thickness levels were implemented—40 nm and 60 nm. Although a slower reduction of transmittance was observed for glass tile with 60 nm coating, it also had much lower initial transparency (Figure2). The mitigation of dust settlement was hardly compensated in this situation, since the TiO2 layer negatively impacted glass transmittance. Introducing composites of TiO2/SiO2 was therefore proposed. Varying molar rates were tested by de Jesus, and the conclusion was drawn that adding molecules of silicon improved the transmittance of glass [24]. However, as is visible from Figure3, the lesser the amount of titanium in the complex, the closer the resemblance of the transmittance function between the bare glass plate and the covered one. Another utilization of hydrophilic properties included coatings prepared by Jang [25]. They were made out of SiO2 nanoparticle layers and a silica binder ﬁlm between the nanoparticles and protected mirror surface. They showed complete water spreading, with a water contact angle (WCA) between 57.5◦ and 4.8◦, and no negative impact on glass transparency was observed. Afterwards, they were subjected to external conditions Materials 2021, 14, 964 3 of 12

Materials 2021, 14, 964 3 of 12

for 234 days. The mirror with the lowest WCA layer experienced the least reduction in Materials 2021, 14, 964 3 of 12 reﬂectance overall, but the difference between values for all mirrors was clearly visible after some exposure time, over at least 100 days (Figure4).

Figure 1. Graph of the average glass plate transmission as a function of the number of days (■) and transmission of the top (♦) and bottom (▼) part of the glass plate (based on: [23]).

Later on, glass tiles were covered by a layer of TiO2 intended as a self-cleaning coating. Two different thickness levels were implemented—40 nm and 60 nm. Although a slower reduction of transmittance was observed for glass tile with 60 nm coating, it also had much lower initial transparency (Figure 2). The mitigation of dust settlement was Figure 1. Graph of the averageFigure glass 1. Graphplate transmission of the average as glass a function plate transmission of the number as aof function days (■) of and the transmission number of days of the () and hardly compensated in this situation, since the TiO2 layer negatively impacted glass top (♦) and bottom (▼) parttransmission of the glass plate of the (based top ( on:) and [23]). bottom (H) part of the glass plate (based on: [23]). transmittance.

Figure 2. Transmittance spectrumspectrum forfor uncoateduncoated glassglass surface,surface, purepure TiO TiO22ﬁlms, films, and and TiO TiO22/SiO/SiO2 compositecomposite films ﬁlms after placing them inin aa furnacefurnace atat varyingvarying temperaturestemperatures [[23].23].

IntroducingNabemoto proposed composites aphoto-catalytic of TiO2/SiO2 was anti-soiling therefore andproposed. hydrophilic Varying layer molar that rates was wereobtained tested by by modifying de Jesus, WOand3 thewith conclusion partially hydrolyzedwas drawn that tetra-ethyl adding orthosilicate molecules of [ 26silicon]. Its effect was observed on a surface of polymethylmethacrylate (PMMA), since such material improved the transmittance of glass [24]. However, as is visible from Figure 3, the lesser theis used amount as Fresnel of titanium lenses in for the concentrator complex, the photovoltaics. closer the resemblance The difference of wasthe transmittance significant, as Figure 2. Transmittance spectrum for uncoated glass surface, pure TiO2 films, and TiO2/SiO2 composite films after placing functionthe reduction between in the the measured bare glass current plate and output the incovered the concentrator one. photovoltaic system was them in a furnace at varying temperatures [23]. 9.6% without additional coating and 3.3% with coating. The same approach was analyzed by another group from Japan. However, Sueto Introducing composites of TiO2/SiO2 was therefore proposed. Varying molar rates decided to further expand the number of added layers [27], and so PMMA was first were tested by de Jesus, and the conclusion was drawn that adding molecules of silicon spin-coated with an acrylic urethane capping layer, then with a nanograded intermediate improved the transmittance of glass [24]. However, as is visible from Figure 3, the lesser layer, and finally with enhanced WO as a photocatalytic layer. It helped to shield PMMA the amount of titanium in the complex,3 the closer the resemblance of the transmittance from photocatalytic properties and to reduce reflectivity. The measured mass of dust function between the bare glass plate and the covered one.

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Materials 2021, 14, 964 agglomerated on the surface without coating exceeded 0.01 g, whereas for PMMA4 of 12 with the hydrophilic layer, it oscillated around 0.005 g.

Figure 3. Transmittance spectrum for uncoated glass surface, pure TiO2 films, and TiO2/SiO2 composite films after placing them in furnaces at varying temperatures [24].

Another utilization of hydrophilic properties included coatings prepared by Jang [25]. They were made out of SiO2 nanoparticle layers and a silica binder film between the nanoparticles and protected mirror surface. They showed complete water spreading, with a water contact angle (WCA) between 57.5° and 4.8°, and no negative impact on glass transparency was observed. Afterwards, they were subjected to external conditions for 234 days. The mirror with the lowest WCA layer experienced the least reduction in FigureFigure 3. 3. TransmittanceTransmittance spectrum spectrum for uncoated for uncoated glass surface, glass pure surface, TiO2 films, pure and TiO TiOﬁlms,2/SiO2and TiO /SiO reflectance overall, but the difference between values for all mirrors was2 clearly visible2 2 compositecomposite films ﬁlms after after placing placing them them in fu inrnaces furnaces at varying at varying temperatures temperatures [24]. [24]. after some exposure time, over at least 100 days (Figure 4). Another utilization of hydrophilic properties included coatings prepared by Jang [25]. They were made out of SiO2 nanoparticle layers and a silica binder film between the nanoparticles and protected mirror surface. They showed complete water spreading, with a water contact angle (WCA) between 57.5° and 4.8°, and no negative impact on glass transparency was observed. Afterwards, they were subjected to external conditions for 234 days. The mirror with the lowest WCA layer experienced the least reduction in reflectance overall, but the difference between values for all mirrors was clearly visible after some exposure time, over at least 100 days (Figure 4).

FigureFigure 4. Solar 4. specularSolar specular reﬂectance reflectance as aas function a function of of time time for for mirrors mirrors with hydrophilic hydrophilic coatings coatings with with varying varying water water contact contact anglesangles (4.8◦, 55(4.8°,◦, 56 55°,◦, 57.556°, ◦57.5°))[25 ].[25].

TestingNabemoto performed proposed on thea photo-catalytic PV modules wasanti-soiling doneby and Canete hydrophilic [28]. A layer compound that was made obtained by modifying WO3 with partially hydrolyzed tetra-ethyl orthosilicate [26]. Its of hybrid polymer and metal-oxide nanoparticles was ﬂow-coated on the surface. Inter- estingly, initial power output was 1–1.5% higher after depositing this coating, suggesting better anti-reﬂectivity. Current–voltage characteristics were measured through one year,

and modules were constantly kept outside; thus, they were subjected to real-time at- Figure 4. Solar specular reflectancemospheric as a function conditions. of time for Current mirrors valueswith hydrophilic are presented coatings with in Figure varying5 water as a contact function of time, angles (4.8°, 55°, 56°, 57.5°) [25]. with additional insight into rainfall statistics. In the dry months of summer, the greatest differenceNabemoto between proposed modules a photo-catalytic with and without anti-soiling coating and hydrophilic was observed, layer corresponding that was to approximatelyobtained by modifying 5%. WO3 with partially hydrolyzed tetra-ethyl orthosilicate [26]. Its

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effect was observed on a surface of polymethylmethacrylate (PMMA), since such material is used as Fresnel lenses for concentrator photovoltaics. The difference was significant, as the reduction in the measured current output in the concentrator photovoltaic system was 9.6% without additional coating and 3.3% with coating. The same approach was analyzed by another group from Japan. However, Sueto decided to further expand the number of added layers [27], and so PMMA was first spin- coated with an acrylic urethane capping layer, then with a nanograded intermediate layer, and finally with enhanced WO3 as a photocatalytic layer. It helped to shield PMMA from photocatalytic properties and to reduce reflectivity. The measured mass of dust agglomerated on the surface without coating exceeded 0.01 g, whereas for PMMA with the hydrophilic layer, it oscillated around 0.005 g. Testing performed on the PV modules was done by Canete [28]. A compound made of hybrid polymer and metal-oxide nanoparticles was flow-coated on the surface. Interestingly, initial power output was 1–1.5% higher after depositing this coating, suggesting better anti-reflectivity. Current–voltage characteristics were measured through one year, and modules were constantly kept outside; thus, they were subjected to real-time atmospheric conditions. Current values are presented in Figure 5 as a function of time, with additional insight into rainfall statistics. In the dry months of summer, the Materials 2021, 14, 964 greatest difference between modules with and without coating5 of 12was observed, corresponding to approximately 5%.

FigureFigure 5. 5.Short-circuit Short-circuit current current values values over the over course the of 12course months of with 12 months daily rainfall with data daily [28 ].rainfall data [28]. 3. Hydrophobic Coatings 3. HydrophobicHydrophobic coatings Coatings exhibit a high water contact angle, over 90◦, as well as low surfaceHydrophobic energy. As a result, coatings they encourageexhibit a dewhigh and water rain tocontact form into angle, spheres, over causing 90°, as well as low suchsurface droplets energy. to slide As off a of result, the surface they and encourage carry off any de pollutantw and rain residue to [form29–31 ].into spheres, causing The approach utilizing SiO2 matrix with two sized particles (8 nm, 60 nm) was proposedsuch droplets by Bahattab to slide [32]. off After of applying the surface it onto and a glass carry substrate, off any it waspollutant stated that residue trans- [29–31]. parencyThe increased approach by a fewutilizing percentile SiO points.2 matrix Annual with exposure two tosized theoutside particles conditions (8 nm, 60 nm) was inproposed Saudi Arabia by causedBahattab a 13% [32]. transmittance After applying reduction it foronto glass a tilesglass with substrate, hydrophobic it was stated that coatingstransparency and 19% increased for the surface by without a few any percentile film. The blowing points. of compressedAnnual exposure air proved to the outside toconditions be enough toin almost Saudi completely Arabia cleancaused the coated a 13% surface, transmittance since the transparency reduction loss for was glass tiles with 2%. However, it was not as successful for the other glass, as transparency loss reached 8%. Highlyhydrophobic transparent coatings silica nanoparticles and 19% synthesized for the withsurface the addition without of tetraethyl any film. orthosil- The blowing of icatecompressed and ethanol air were proved the aims to ofbe study enough carried to outalmost by Polizos completely [33]. They clean were the deposited coated surface, since onthe top transparency of the glass surface loss and was left 2%. under However, external conditions it was not for threeas successful months. Afterwards, for the other glass, as ittransparency was stated that loss the transmittance reached 8%. of theHighly uncoated transparent tile was around silica 20–25% nanoparticles lower. synthesized with Transparent hydrophobic coatings based on silica sol (SS) and SiO2 nanoparticles were proposedthe addition by Quan of tetraethyl [34]. Their hydrophobic orthosilicate character and et couldhanol be were adjusted the by aims changing of study the x carried out by numberPolizos of [33]. (SS-SiO They2)x. Increasingwere deposited the number on x alsotop resultedof the inglass heightened surface transmittance, and left under external and it was explained by the formation of nanovoids limiting reflection: x = 1 or x = 3 thin layers exhibit similar transparency as bare glass. For x > 5, transmittance rises a few percentile points and the sliding angle plummets. Bare glass and glass enhanced with thin layers were inspected after artificially blowing dust over each surface. From Figure6, the conclusion can be drawn that the magnitude of the hydrophobic effect does not greatly impact the reduction in transmittance after polluting surfaces. Bare glass faces more than 1.5% transparency reduction and for glass with coatings, this number is less than 0.6%. Low surface energy and the course structure of such films is enough to hinder particle adhesion. The SiO2 nanoparticle layer and silica binder film between nanoparticles analyzed by Jang could also be altered to exhibit hydrophobic properties [25]. Additional thermal vapor deposition of fluorosilane over this thin film performed at a temperature of 120 ◦C set the water contact angle value between 111◦ and 165◦. Afterwards, the testing surfaces were placed outside for 234 days, where mirror with the highest WCA layer outperformed other coatings (Figure7). Materials 2021, 14, 964 6 of 12

conditions for three months. Afterwards, it was stated that the transmittance of the uncoated tile was around 20–25% lower. Transparent hydrophobic coatings based on silica sol (SS) and SiO2 nanoparticles were proposed by Quan [34]. Their hydrophobic character could be adjusted by changing Materials 2021, 14, 964 the x number of (SS-SiO2)x. Increasing the number x also resulted in heightened6 of 12 transmittance, and it was explained by the formation of nanovoids limiting reflection: x = 1 or x = 3 thin layers exhibit similar transparency as bare glass. For x > 5, transmittance rises a few percentile points and the sliding angle plummets. Bare glass and glass enhancedIn another with thin studies layers of were de Jesus, inspected SiO2 sol-gel after artificially treated with blowing hexamethyldisilazane dust over each surface. (referred toFrom as SM)Figure resulted 6, the conclusion in a hydrophobic can be drawn thin that ﬁlm the [35 magnitude]. In comparison of the hydrophobic with their effect previous TiOdoes2/SiO not greatly2 composite, impact it the had reduction much higher in transmittance transparency, after which polluting is even surfaces. superior Bare to glass the bare glassfaces more (Figure than8). 1.5% A soiling transparency test was reduction performed, and but for itglass was with concluded coatings, that this all number glass plates is withless than additional 0.6%. Low layers surface performed energy similarly and the withcourse regards structure to dustof such adhesion films is and enough were to easily cleanedhinder particle with water. adhesion.

Materials 2021, 14, 964 7 of 12

2 x ■ Figure 6. TransmittanceTransmittance loss loss for for bare bare glass glass surface surface (Δ ()∆ and) and glass glass with with (SS-SiO (SS-SiO) coating2)x coating ( ) [34]. ()[ 34].

The SiO2 nanoparticle layer and silica binder film between nanoparticles analyzed by Jang could also be altered to exhibit hydrophobic properties [25]. Additional thermal vapor deposition of fluorosilane over this thin film performed at a temperature of 120 °C set the water contact angle value between 111° and 165°. Afterwards, the testing surfaces were placed outside for 234 days, where mirror with the highest WCA layer outperformed other coatings (Figure 7).

FigureFigure 7. Solar 7. Solar specular specular reﬂectance reflectance as as a a function function of of timetime forfor mirrorsmirrors with hydrophobic hydrophobic coatings coatings with with varying varying water water contact contact anglesangles (164.9 (164.9°,◦, 153.3 153.3°,◦, 130 130°,◦, 111.2 111.2°)◦)[ 25[25].].

In another studies of de Jesus, SiO2 sol-gel treated with hexamethyldisilazane (referred to as SM) resulted in a hydrophobic thin film [35]. In comparison with their previous TiO2/SiO2 composite, it had much higher transparency, which is even superior to the bare glass (Figure 8). A soiling test was performed, but it was concluded that all glass plates with additional layers performed similarly with regards to dust adhesion and were easily cleaned with water.

Figure 8. Transmittance spectrum for uncoated glass plate and glass with the layer of hydrophobic film (SM), hydrophilic Si86Ti14 (ST1), hydrophilic Si40Ti60 (ST2), and pure TiO2 (T) [35].

4. A Study of Hydrophobic Coatings Available on the Polish Market An additional experiment concerning hydrophobic coatings provided by certified Polish companies has been carried out on nine identical glass plates sized 0.13 m × 0.18 m. Each plate was cleaned with isopropyl alcohol, and then 2 mm of the liquid substance was sprayed directly on the surface and evenly distributed with a clean cloth. One glass was kept clean and without any coating for the comparative analysis. Table 1 contains names of applied products with corresponding glass plate numbers. Price conversion from PLN to USD was calculated via the pl.investing.com website on January 24th.

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Figure 7. Solar specular reflectance as a function of time for mirrors with hydrophobic coatings with varying water contact angles (164.9°, 153.3°, 130°, 111.2°) [25].

In another studies of de Jesus, SiO2 sol-gel treated with hexamethyldisilazane (referred to as SM) resulted in a hydrophobic thin film [35]. In comparison with their previous TiO2/SiO2 composite, it had much higher transparency, which is even superior Materials 2021, 14, 964 to the bare glass (Figure 8). A soiling test was performed, but it was concluded that7 of all 12 glass plates with additional layers performed similarly with regards to dust adhesion and were easily cleaned with water.

FigureFigure 8. 8. TransmittanceTransmittance spectrum spectrum for for uncoated uncoated glass glass pl plateate and and glass glass with with the the layer layer of of hydrophobic hydrophobic filmﬁlm (SM), (SM), hydrophilic hydrophilic Si Si8686TiTi1414 (ST1),(ST1), hydrophilic hydrophilic Si Si4040TiTi6060 (ST2),(ST2), and and pure pure TiO TiO2 (T)2 (T) [35]. [35 ].

4.4. A A Study Study of of Hydrophobic Hydrophobic Coatings Coatings Available Available on on the the Polish Polish Market Market AnAn additional additional experiment experiment concerning concerning hy hydrophobicdrophobic coatings coatings provided provided by certified certiﬁed Polish companies has been carried out on nine identical glass plates sized 0.13 m × 0.18 m. Polish companies has been carried out on nine identical glass plates sized 0.13 m × 0.18 m. Each plate was cleaned with isopropyl alcohol, and then 2 mm of the liquid substance was Each plate was cleaned with isopropyl alcohol, and then 2 mm of the liquid substance was sprayed directly on the surface and evenly distributed with a clean cloth. One glass was sprayed directly on the surface and evenly distributed with a clean cloth. One glass was kept clean and without any coating for the comparative analysis. Table1 contains names of kept clean and without any coating for the comparative analysis. Table 1 contains names applied products with corresponding glass plate numbers. Price conversion from PLN to of applied products with corresponding glass plate numbers. Price conversion from PLN USD was calculated via the pl.investing.com website on January 24th. to USD was calculated via the pl.investing.com website on January 24th. Table 1. Glass plate numbering and corresponding reference plate without coating (1) as well as plates with hydrophobic coatings (2–9), with information about the manufacturer.

No. Product Description of the Coating Manufacturer 1 No coating - - Suitable for: glass 2 Nano Window Expected durability: 1 year Hadwao Nanotechnologia Price of 100 mL: 7.37 USD Suitable for: glass 3 Nano Window Plus Expected durability: 3 years Hadwao Nanotechnologia Price of 100 mL: 11.39 USD Suitable for: glass coverage in PV modules and solar collectors 4 Nano Solar Hadwao Nanotechnologia Expected durability: 1 years Price of 100 mL: 9.25 USD Suitable for: glass coverage in PV modules and solar collectors 5 Nanocape Solar Nanosolution Expected durability: not speciﬁed Price of 100 mL: 9.38 USD Suitable for: glass coverage in PV modules and solar collectors; ceramic roof tiles 6 Eco hydrophobic impregnate H2O Nanotechnology Expected durability: 1–3 years Price of 100 mL: 27.88 USD Suitable for: glass coverage in PV modules and Nanostone Home Panel PV solar collectors 7 P&K J. Marciniak P. Bzukała protection Expected durability: 2 years Price of 100 mL: 31.90 USD Materials 2021, 14, 964 8 of 12

Table 1. Glass plate numbering and corresponding reference plate without coating (1) as well as plates with hydrophobic coatings (2–9), with information about the manufacturer.

No. Product Description of the Coating Manufacturer 1 No coating - - Suitable for: glass Hadwao 2 Nano Window Expected durability: 1 year Nanotechnologia Price of 100 mL: 7.37 USD Suitable for: glass Hadwao 3 Nano Window Plus Expected durability: 3 years Nanotechnologia Price of 100 mL: 11.39 USD Suitable for: glass coverage in PV modules and solar collectors Hadwao 4 Nano Solar Expected durability: 1 years Nanotechnologia Price of 100 mL: 9.25 USD Suitable for: glass coverage in PV modules and solar collectors 5 Nanocape Solar Nanosolution Expected durability: not specified Price of 100 mL: 9.38 USD Suitable for: glass coverage in PV modules and solar collectors; ceramic Eco hydrophobic H2O 6 roof tiles impregnate Nanotechnology Expected durability: 1–3 years Price of 100 mL: 27.88 USD Suitable for: glass coverage in PV Materials 2021, 14, 964 8 of 12 Nanostone Home modules and solar collectors P&K J. Marciniak P. 7 Panel PV protection Expected durability: 2 years Bzukała

Table 1. Cont. Price of 100 mL: 31.90 USD Suitable for: glass Nanostone Home P&K J. Marciniak P. No. Product8 Description of theExpected Coating durability: 3 years Manufacturer Windows protectionSuitable for: glass Bzukała Nanostone Home Windows 8 Expected durability:Price 3 years of 100 mL: 29.97P&K J. USD Marciniak P. Bzukała protection Price of 100 mL: 29.97Suitable USD for: glass; ceramic Nanostone HomeSuitable for: glass; ceramic P&K J. Marciniak P. Nanostone Home Shower 9 9 Expected durability:Expected 2 years durability:P&K 2 J.years Marciniak P. Bzukała protection Shower protection Bzukała Price of 100 mL: 26.54Price USD of 100 mL: 26.54 USD

LightingLighting intensity intensity was measuredwas measured with the with aid of the an luxmeteraid of an with luxmeter catalog numberwith catalog number AB-8809AAB-8809A manufactured manufactured by Abatronic by Abatronic company, company, that was placed that behind was placed the glass behind plate in the glass plate accordance with Figure9. in accordance with Figure 9.

Figure 9. Illuminance measurement layout carried out by an Abatronic luxmeter. Figure 9. Illuminance measurement layout carried out by an Abatronic luxmeter. Firstly, illuminance values for clean glass plates were gathered. Further consecutive tests involved obtaining the lighting intensity for different pollutant types and layers of dust. Three various soils, later referred to as 1, 2, and 3, with four dust densities—4.3, 8.5, 12.8, and 21.4 g/m2—were deposited on each glass plate to calculate normalized illuminance for clean glass plates (1) and normalized illuminance after soil adhesion (2). Every soil type applied in artiﬁcial dust experiments has been listed in Table2. Soil 1 was taken directly from roads in the vicinity of Chemistry building, on Gdansk University of Technology (GUT) campus. The size of its grains differs, as some are much smaller with a diameter of 0.2 mm, whereas others reach around 0.45 mm. Soils 2 and 3 were gathered from sand beach areas in the close proximity of a walking pier and tram loop, both located in Gda´nskBrzezno district. They are much more uniform concerning grain size; for soil 2, the average diameter falls in the range of 0.23–0.36 mm and for soil 3, it is 0.22–0.33 mm.

Table 2. Numbering of soil samples collected from photovoltaic modules, laboratory hall, and three various locations in Gdansk.

Soil Number Description Coordinates Taken from roads nearby the Chemistry C 1 54◦37” N 18◦62” E GUT building Taken from a sandy beach area near the 2 54◦41” N 18◦64” E walking pier in Gdansk Taken from a sandy beach area near the tram 3 54◦41” N 18◦62” E loop in Gdansk Materials 2021, 14, 964 9 of 12

Firstly, illuminance values for clean glass plates were gathered. Further consecutive tests involved obtaining the lighting intensity for different pollutant types and layers of dust. Three various soils, later referred to as 1, 2, and 3, with four dust densities—4.3, 8.5, 12.8, and 21.4 g/m2—were deposited on each glass plate to calculate normalized illuminance for clean glass plates (1) and normalized illuminance after soil adhesion (2). Every soil type applied in artificial dust experiments has been listed in Table 2. Soil 1 was taken directly from roads in the vicinity of Chemistry building, on Gdansk University of Technology (GUT) campus. The size of its grains differs, as some are much smaller with a diameter of 0.2 mm, whereas others reach around 0.45 mm. Soils 2 and 3 were gathered from sand beach areas in the close proximity of a walking pier and tram loop, both located in Gdańsk Brzezno district. They are much more uniform concerning grain size; for soil 2, the average diameter falls in the range of 0.23–0.36 mm and for soil 3, it is 0.22–0.33 mm.

Table 2. Numbering of soil samples collected from photovoltaic modules, laboratory hall, and three various locations in Gdansk.

Soil Description Coordinates Number 54°37″ N 18°62″ 1 Taken from roads nearby the Chemistry C GUT building E Taken from a sandy beach area near the walking pier in 54°41″ N 18°64″ 2 Gdansk E Taken from a sandy beach area near the tram loop in 54°41″ N 18°62″ 3 Gdansk E Materials 2021, 14, 964 9 of 12 5. Results and Discussion Figure 10 presents the normalized illuminance for clean glass plates with respect to 5.light Results intensity and Discussion in the laboratory hall that oscillated around 330 lux at the beginning of the experiment.Figure 10 presentsIt should the be normalized noted that illuminance all of the for glass clean plates glass platescaused with an respect approximate to 10% light light intensity in the laboratory hall that oscillated around 330 lux at the beginning of the experiment.intensity decrease, It should beregardless noted that allof ofwhether the glass they plates were caused covered an approximate with 10%coating, as this is lightattributed intensity to decrease, transmittance regardless reduction of whether caused they were by covered the glass with itself. coating, Glass as this covered is with Eco attributedhydrophobic to transmittance impregnate reduction exhibited caused slightly by the lower glass itself. illuminance Glass covered loss with than Eco other slides with hydrophobiccoatings; however, impregnate differences exhibited slightly between lower the illuminance light intensity loss than calculated other slides between with each plate coatings; however, differences between the light intensity calculated between each plate werewere minimal minimal and and did notdid exceed not exceed 1% with 1% regards with to regards glass tile to 6. glass Therefore, tile 6. it mayTherefore, be it may be statedstated that that hydrophobic hydrophobic surface surface enhancement enhancement leads to a leads slight decreaseto a slight of transparency decrease of transparency andand could could be be applied applied to photovoltaic to photovoltaic modules. modules.

FigureFigure 10. 10.Illuminance Illuminance measurement measurement layout carried layout out carried by an Abatronicout by an AB-8809A Abatronic luxmeter. AB-8809A luxmeter. Results from further analysis carried out for three pollutant types and four different surface densities of dust are presented in Figure 11. Normalization was applied with regards to the clean glass plate 1, without hydrophobic coating, which corresponds to value 1. Such an approach enabled coating type analysis of transmittance losses for pollutant-covered glass. For every amount of soil there was a slight drop in light intensity passing through slides, and lower amount of soil did seem to have a positive effect in regaining some of the transparency. Smaller values of dust densities provided some level of difference in illuminance between various pollutant types; however, it never exceeded 1%. Materials 2021, 14, 964 10 of 12

Results from further analysis carried out for three pollutant types and four different surface densities of dust are presented in Figure 11. Normalization was applied with regards to the clean glass plate 1, without hydrophobic coating, which corresponds to value 1. Such an approach enabled coating type analysis of transmittance losses for pollutant-covered glass. For every amount of soil there was a slight drop in light intensity passing through slides, and lower amount of soil did seem to have a positive effect in Materials 2021, 14, 964 regaining some of the transparency. Smaller values of dust densities provided some level10 of 12 of difference in illuminance between various pollutant types; however, it never exceeded 1%.

FigureFigure 11. 11.Normalized Normalized illuminance illuminance for for glassglass platesplates polluted with with soil soil 1 1(■ (), ),soil soil 2 ( 2♦ (),), and and soil soil 3 ( 3▼ (H) at) at different different surface surface densities:densities: (a) ( 4.3a) 4.3 g/m g/m2,(2, b(b)) 8.5 8.5 g/m g/m2,,( (cc)) 12.8 12.8 g/m g/m2, 2(d,()d 17.1) 17.1 g/m g/m2. 2.

6.6. Conclusions Conclusions DecreasingDecreasing the the efficiency efficiency of of photovoltaic photovoltaic modules modules caused caused by by the the deposition deposition of pollu-of tantspollutants on their on surface their surface is a significant is a significant problem, problem, causing causing the reduction the reduction of generated of generated energy, andenergy, thus theand decreasethus the ofdecrease economic of economic viability. vi Anability. alternative An alternative approach approach to manual to cleaningmanual is modificationcleaning is modification of the cover materialof the cover and materi implementingal and implementing self-cleaning self-cleaning films on glass. films It would on beglass. useful It forwould application be useful in for PV application installations in onPV bothinstallations a large scaleon both and a alarge small scale scale. and a smallA scale. general conclusion can be drawn from the presented literature review that glass transparencyA general is not conclusion impacted, can irrespective be drawn offrom if such the presented a coating isliterature hydrophilic review or hydrophobic.that glass This,transparency however, is observednot impacted, for research irrespective carried ofout if atsuch an especiallya coating smallis hydrophilic scale, as itor was performedhydrophobic. under This, laboratory however, conditions. is observed Some for research negative carried impact out on at the an transparency especially small value wasscale, noted as it when was performed taking into under consideration laboratory coatings conditions. provided Some negative by local companies.impact on the It is recommendedtransparency value to ask was manufacturers noted when taking for providing into consideration details of coatings their product—namely, provided by local if theycompanies. performed It glassis recommended transmittance to analysisask manufacturers before and afterfor providing thin film application. details of their If such evaluationproduct—namely, is available, if they prosumers performed are glass able tran to choosesmittance in fullanalysis awareness before theand product after thin with the best possible properties.

Author Contributions: Conceptualization, E.K.-R.; methodology, E.K.-R.; software, M.R.; validation, E.K.-R.; formal analysis, E.K.-R.; investigation, M.R.; resources, E.K.-R.; data curation, M.R.; writing- original draft preparation, M.R.; writing-review and editing, E.K.-R.; visualization, M.R.; supervision, E.K.-R.; project administration, E.K.-R.; funding acquisition, E.K.-R. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Materials 2021, 14, 964 11 of 12

Data Availability Statement: The data presented in this study are available on request from the corresponding author. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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