Research ArticleReduction of Soiling on Photovoltaic Modules by a TrackerSystem with Downward-Facing Standby State

Yasuyuki Ota ,1 Akira Nagaoka,2 and Kensuke Nishioka2

1Organization for Promotion of Tenure Track, University of Miyazaki, Miyazaki 889-2192, Japan2Faculty of Engineering, University of Miyazaki, Miyazaki 889-2192, Japan

Correspondence should be addressed to Yasuyuki Ota; y-ota@cc.miyazaki-u.ac.jp

Received 17 May 2019; Revised 5 August 2019; Accepted 6 September 2019; Published 29 October 2019

Academic Editor: Giulia Grancini

Copyright © 2019 Yasuyuki Ota et al. This is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

The radiation received by solar cells within photovoltaic modules is lower than that arriving at the module surface. One of thecauses of this energy loss is soiling of the module surface. Therefore, the influence of dust adhesion on photovoltaic modulesmust be studied. In this study, we prepared two tracker systems: a new system and a typical system. During the night, theformer can switch to a downward-facing standby state, while the latter assumes an upward-facing standby state. The soiling onthe polymethylmethacrylate and glass set on the tracker systems with both standby states was evaluated for 20 months inMiyazaki, Japan. By adopting the tracker system with the downward-facing standby state, a direct transmittance that was more-than-5% higher than before was consistently obtained at 500 nm in both cases with polymethylmethacrylate and glass.

1. Introduction

As the density of solar energy is low, photovoltaic (PV)systems must be installed on a large scale to generate suffi-cient amounts of energy. The enormous potential can befully exploited if the world’s deserts are made available forharvesting the solar energy. For example, the Gobi Desert,which is located at high altitudes, receives large amountsof solar radiation (4.59 kWh/m2/day) [1–3]. While installingPV systems in deserts, we must consider the effect of thecollision and adhesion of sand on PV modules. When sandremains on the module surface, it can become firmly fixedby moisture such as dew, interrupting the transmittance oflight. Therefore, this type of surface adhesion on PV modulesmust be prevented.

The type of dust on the PV module is related to thesite conditions. PV module installed near the airport isexposed to dust from airplane exhaust, and PV modulesurface is subject to receiving chloride damage in a coastalarea [4]. In addition, various industries produced muchdust, and it has adhered on the PV module [5]. Forpreventing the accumulation of dust, the type of dust isan important factor.

The radiation received by the cells inside PV modulesis lower than that arriving at the module surface. Thecauses of this energy loss include soiling on the modulesurface and losses due to light reflection and absorptionby the materials covering the cells [5, 6]. Recently, theself-cleaning coating is used to prevent dust depositionon the PV module [7, 8]. We also developed an antisoilingcoating for PV module using WO3 and silica-based material[9, 10]. However, it was necessary to add cost for preparationof coating.

A concentrator photovoltaic (CPV) system uses an opti-cal system to collect and focus sunlight onto the solar cells.As the CPV system can only work with direct sunlight andcannot focus scattered light onto the solar cells, a signifi-cant portion of the light is lost due to scattering when thecollector surfaces are soiled. By contrast, conventional,nonconcentrating PV systems can work with both directand indirect sunlight, which implies that they are much lesssensitive to soiling than CPV systems [11].

Tracker systems are necessary for CPV systems, becausethey use optics to concentrate sunlight and the acceptanceangle of the optics is narrow. In the case of 500 sun concen-tration systems, the acceptance angle is less than 1° [12–14].

HindawiInternational Journal of PhotoenergyVolume 2019, Article ID 1296065, 8 pageshttps://doi.org/10.1155/2019/1296065

Therefore, accurate tracking of the sun is necessary for CPVsystems. In addition, tracker systems are applied to conven-tional nonconcentrating PV systems. In the morning andevening, the acceptance surface area of sunlight is very smallfor fixed PV systems. By adopting a tracker system, theoutput of the PV system in the morning and evening candrastically increase [15].

During the night or waiting times, PV modules ontrackers should preferably be horizontal to the groundbecause of the effect of wind resistance. Therefore, commontracker systems adopt the upward-facing standby state duringthe waiting time. However, sand and dust adhere and accu-mulate on the PV modules during this period. In this study,we developed a tracker system with a downward-facingstandby state. Common tracker systems have used linearactuator or slew drive for controlling the elevation angle.Since linear actuator has a narrow moving angle, the trackersystem with a downward-facing standby state requires theuse of a slew drive. It can switch to the downward-facingstandby state (upside-down state) during the night andsandstorms, preventing the accumulation of dust and sand.Also, the system can switch between the two states, upward-and downward-facing standby states, without additionalcost, because the upward- and downward-facing standbystates are controlled by operating program. Thus far, therehave been no reports discussing the effect of tracker systemswith a downward-facing standby state during the waitingtime. In this study, the soiling on the polymethylmethacry-late (PMMA) and glass set on the tracker systems withupward- and downward-facing standby states was evaluated.We also estimated the annual energy yield of CPV and PVsystems based on the degradation of transmittance ofPMMA and glass and irradiance database.

2. Methods

Figure 1 displays photographs of the tracker system thatcan switch to a downward-facing standby state by rotatingevery PV module during the night and sandstorms. Toallow the modules to rotate from an upward to downwardorientation, the modules were divided into two on bothsides across a pillar.

Acrylic and glass are primarily used as Fresnel lens mate-rials for the CPV modules [4, 16]. Glass is also used as thecover glass for nearly every flat-plate PVmodule. In this study,acrylic PMMA (SUMIPEX-E000©, Sumitomo Chemical Co.,Ltd., size: 3 × 3 cm2, thickness: 2mm) and photovoltaic whiteglass (Optiwhite™, PILKINGTON, size: 3 × 3 cm2, thickness:3mm) plates were used as substrates.

In this study, we prepared two tracker systems: a newsystem and a typical system. During the night, the formercan switch to a downward-facing standby state (downwardsystem), while the latter assumes an upward-facingstandby state (upward system). The typical tracker systemtracks the sun during the day and faces upward at night.In contrast, the downward system tracks the sun duringthe day but switches to a downward-facing orientationfrom sunset to sunrise.

The PMMA and glass substrates were set on the twotracker systems and were exposed to outdoor conditions atthe University of Miyazaki (Miyazaki, Japan, 31°49′N,131°24′E). The exposure period spanned from September30, 2014, to May 24, 2016, and data were collected everytwo weeks. The transmittance of the samples was measuredusing a spectrophotometer (JASCO, V-570) with andwithout an integrating sphere.

3. Results and Discussion

Figure 2 shows the transmittances of the PMMA and glasssubstrates on the downward and upward systems after 20months of exposure (May 24, 2016). To consider thetransmitted light for nonconcentrating PV systems, whichcan work with both direct and indirect light, the transmit-tance was measured with an integrating sphere, whichallows both direct and indirect (scattered) beams to bedetected, as shown in Figure 2(a). To consider the trans-mitted light for CPV systems, which can only work withdirect light, the transmittance was measured without anintegrating sphere, as shown in Figure 2(b). The detectable

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Figure 1: Photographs of the tracker system that can switch to adownward-facing standby state. During sandstorms and the night,this is performed by rotating all the PV modules.

2 International Journal of Photoenergy

angle of the spectrophotometer was ±0.5°, which is nearlyidentical to the acceptance half-angle of CPV systems witha high concentration ratio of 1000, which is our targetvalue [17].

In all cases, transmittance decreases after the full expo-sure period (20 months). The results indicate that the soilingduring the exposure causes the transmittance to decrease,and the decrease in transmittance for both direct and indirectlight is considerably lower than that for direct light. It isfound that a considerable amount of irradiated light was scat-tered by the accumulated dust on the substrates. These resultsindicate that CPV systems are much more sensitive to soilingthan nonconcentrating PV systems. The decrease in trans-mittance of the glass substrate is less than that of the PMMAsubstrate. The existence of hydroxyl (-OH) groups on thesurface is important for the prevention of dust adhesion.The presence of electrostatic charges on the surface of thesubstrates is one of the main factors affecting the adhesionof dust, and the charges can be suppressed by hydroxylgroups that adsorb water on the surface [18]. A very thinlayer of water is formed on the surface, which prevents thelocalization of electrostatic charges owing to the conductivityof water. The charges scattered by the water layer can easilybe discharged into the environment [19]. Moreover, the thinlayer of water makes the surface hydrophilic, causing theaccumulated dust to be easily washed away by rainfall. Theglass surface possesses several hydroxyl groups on its surface.Therefore, the transmittance of the glass substrate maintainsa higher transmittance than the PMMA substrate.

Figure 3 shows the changes in transmittance of thePMMA and glass substrates on the downward and upward

systems during the exposure at a wavelength of 500 nm.Figure 3(a) shows the transmittance measured with an inte-grating sphere and Figure 3(b) shows that without anintegrating sphere. In all cases, the initial transmittance isidentical for the downward and upward systems. As thedays progress, the differences in transmittance of thesubstrates on the downward and upward systems (transmit-tance of substrate on downward system minus that onupward system: Tdown‐up) become clear in the first sixmonths of exposure and, subsequently, become saturated.The decrease in transmittance for direct light (Figure 3(b)) issignificantly higher than those for both direct and indirectlight (Figure 3(a)). As shown in Figure 3(b), the initial trans-mittance of the PMMA substrate measured without an inte-grating sphere is 91.8% (September 30, 2014) and those onthe downward and upward systems after the full exposureperiod (May 24, 2016) are 82.2 and 77.4%, respectively, indi-cating that the transmittance of the PMMA substrate on thedownward system is 4.8% higher than that on the upward sys-tem after the full exposure period (Tdown‐up: 4.8%). The initialtransmittance of the glass substrate measured without anintegrating sphere is 90.6% (September 30, 2014), and thoseon the downward and upward systems after the full exposureperiod are 86.8 and 81.2%, respectively, indicating that thetransmittance of the glass substrate on the downward systemis 5.6% higher than that on the upward system after the fullexposure period (Tdown‐up: 5.6%). The averaged Tdown‐up fromMarch 31, 2015 (after 6months of exposure), toMay 24, 2016(after 20 months of exposure), for the PMMA and glasssubstrates are 5.2 and 6.2%, respectively. By adopting thedownward tracker system, we can consistently achieve a

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3International Journal of Photoenergy

transmittance that is higher than that of the traditional systemby more than 5%.

Figures 4 and 5, respectively, display photographs andoptical microscope images of the (a) PMMA and (b) glass

substrates on the upward and downward systems after thefull exposure period (May 24, 2016). It is evident that thesample surfaces of the PMMA and glass attached to theupward system are soiled in comparison to those attached

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4 International Journal of Photoenergy

to the downward system. The soiling on the surface of the PVsystems can be prevented by using the downward tracker sys-tem. In all cases, millimeter-sized dust is not observed on thesurface of the substrates. The tracker systems tracked the sunand were oriented almost vertically in the morning and eve-ning. It is considered that millimeter-sized dust falls from thesurface of the substrates by gravity during the verticalorientation of the tracker systems.

To estimate the performances of the PVs, we estimatedthe output photocurrent of the PVs using the obtainedtransmittance data. To estimate the photocurrent ofnonconcentrating Si PVs, the solar spectrum of AM 1.5G(1000W/m2), transmittance of the PMMA and glass sub-strates measured with an integrating sphere, and quantumefficiency (spectral response) of the Si solar cell [20] weremultiplied. This estimation assumes Si PV modules withglass and PMMA covers. Nearly every Si PVmodule was cov-ered with glass. In contrast, to estimate the photocurrent ofCPVs, the photocurrent from each subcell of the InGaP/In-GaAs/Ge triple-junction solar cell was calculated by takingthe solar spectrum of AM 1.5D (900W/m2), transmittanceof the PMMA and glass substrates measured without an inte-grating sphere, and spectral response of the triple-junctionsolar cell [21] and multiplying them. This estimationassumes CPV modules with a PMMA Fresnel lens andsilicone on glass (SOG) Fresnel lens.

The photocurrent of the PVs (Jp) can be determined byEquation (1) [22, 23]:

Jp =ðEQE λð Þ × T λð Þ × F λð Þdλ, ð1Þ

where λ is the wavelength, EQEðλÞ is the quantum efficiencyof the solar cells, TðλÞ is the transmittance, and FðλÞ is thephoton flux of the solar spectra.

Figure 6 displays the changes in the calculatednormalized photocurrent of PVs using the PMMA andglass surfaces on the downward and upward systemsduring the exposure for nonconcentrating Si PVs andCPVs using the InGaP/InGaAs/Ge triple-junction solarcell. In this figure, the photocurrent was normalized bythe initial value on September 30, 2014.

The change in photocurrent during exposure exhibits atrend identical to the transmittance shown in Figure 3. Thedecrease in photocurrent of the Si PV is much lower than thatof the CPV because the Si PV can work with both direct andindirect light. As shown in Figure 6(b), after the full exposureperiod (May 24, 2016), the normalized photocurrents of theCPV with the PMMA Fresnel lens on the downward andupward systems are 0.90 and 0.85, respectively, indicatingthat the normalized photocurrent on the downward systemis 5% higher than that on the upward system. For the sameperiod, the normalized photocurrents of the CPV with SOGFresnel lens on the downward and upward systems are 0.96and 0.90, respectively, indicating that the normalized photo-current on the downward system is 6% higher than that onthe upward system. It is found that by adopting the down-

ward tracker system, we can achieve a more-than-5% higherphotocurrent after the full exposure period.

We estimated the annual energy yield of PV and CPVsystems based on the degradation of photocurrent of PMMAand glass, as shown in Figure 6. The outputs of PV and CPVsystems were calculated on meteorological test data for PVsystems (METPV-11) [24] and configuration of PV systems(number of junction and configuration of series and strings),and the annual energy yield was integrated them. Thedetailed analysis procedure of the annual energy yield isdescribed in References [25, 26]. As the estimation results,the annual energy yield of the CPV system on the downwardsystem was 4.01% for the PMMA Fresnel lens and 4.36% forSOG Fresnel lens higher than that on the upward system. Bycontrast, the annual energy yield of the PV system on thedownward system was 1.40% for the PMMA cover and1.88% for glass cover higher than that on the upward systembecause the PV system is much less sensitive to soiling thanthe CPV system.

The relationship between the rainfall during the standbystate at night and the effect of the downward system wasassessed as follows. The accumulated rainfall during thestandby state at night (from sunset to sunrise) (RFnight) foreach measurement interval (accumulated for two weeksbefore each measurement) and the differences in transmit-tance at 500nm between the substrate samples on the down-ward and upward tracker systems (Tdown‐up) for PMMA andglass were derived from the measured data. The relationshipbetween the two is presented in Figure 7. Here, the transmit-tance was measured without an integrating sphere. The opencircles represent the data after the first six months of expo-sure (from September 30, 2014, to March 31, 2015), whilethe filled circles represent the data from March 31, 2015(after six months of exposure), to May 24, 2016 (after 20months of exposure). As mentioned regarding Figure 3,Tdown‐up becomes distinct after the first six months of expo-sure and then becomes saturated. To assess the relationshipbetween RFnight and Tdown‐up, the data after the first sixmonths of exposure (filled circles) are discussed. Statistically,the filled circles exhibit a weak negative correlation to RFnightand Tdown‐up. The correlation coefficients for the PMMA andglass substrates are −0.27 and −0.42, respectively. The corre-lation coefficient from −0.25 to −0.40 is defined as a weaknegative correlation. When there is considerable rainfallduring the standby state, the substrates on the upwardtracker system are washed and Tdown‐up becomes small.

The environment of the test site used in this study wasrelatively clean as it was in a rural area and experienced signif-icant rainfall. While installing PV systems in dusty or dryareas, by adopting the downward system, the total exposuretime of the systems shortens and Tdown‐up becomes largebecause of the small amount of rainfall. Eventually, the effectof the downward system becomes clearer.

4. Conclusions

A tracker system that can switch to a downward-facingstandby state during the night was developed and can

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prevent the accumulation of dust and sand. The soiling onthe PMMA and glass set on the tracker systems with upwardand downward-facing standby states was evaluated for 20months in Miyazaki, Japan.

The initial transmittance of the PMMA substrate mea-sured without an integrating sphere was 91.8% (September30, 2014), and those on the downward and upward systems

after 20 months of exposure (May 24, 2016) were 82.2 and77.4%, respectively. The initial transmittance of the glasssubstrate measured without an integrating sphere was90.6% (September 30, 2014), and those on the downwardand upward systems after the full exposure period were86.8 and 81.2%, respectively. The averaged difference intransmittance of the substrates on the downward and upward

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systems (Tdown‐up) from March 31, 2015 (after six months ofexposure), to May 24, 2016 (after 20 months of exposure), forthe PMMA and glass substrates was 5.2 and 6.2%, respec-tively. By adopting the downward tracker system, we canconsistently achieve a more-than-5% higher transmittance.Moreover, we estimated the output photocurrent of the PVsusing the obtained transmittance data. The photocurrent ofthe CPV with the PMMA Fresnel lens on the downwardsystem was 5% higher than that on the upward system afterthe full exposure period. The photocurrent of the CPV withsilicone on glass Fresnel lens on the downward system was6% higher than that on the upward system after the full expo-sure period. It is found that by adopting the downwardtracker system, we can achieve a more-than-5% higherphotocurrent after the full exposure period.

The environment of the test site used in this study was rel-atively clean as it was in a rural area and not in an industrialarea. The effect of the tracker system with the downward-facing standby state would be even stronger for PV systemsinstalled at dusty sites, and the gain in the PV output wouldconcomitantly be larger.

Data Availability

The transmittance data used to support the findings of thisstudy are included within the article.

Disclosure

Some of the results and findings were presented in the27th International Photovoltaic Science and EngineeringConference, Shiga, Japan, November 2017; however,detailed data and discussions were presented in this work.

Conflicts of Interest

The authors declare no conflict of interest.

Acknowledgments

This work was supported in part by a grant for the ScientificResearch on Priority Areas from the University of Miyazaki.We would like to thankMr. Shota Kurogi of the University ofMiyazaki and Mr. Jun Hirota of THK Co., Ltd. for valuablediscussions.

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