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Solar Energy 96 (2013) 283–291

Effect of soiling on photovoltaic modules

Reinhart Appels a,⇑, Buvaneshwari Lefevre a, Bert Herteleer a, Hans Goverde a,b,Alexander Beerten a, Robin Paesen a, Klaas De Medts a, Johan Driesen a, Jef Poortmans b

a ESAT/ELECTA, KU Leuven, Kasteelpark Arenberg 10, B-3001 Heverlee, Belgiumb IMEC vzw, Kapeldreef 75, B-3001 Heverlee, Belgium

Received 1 December 2012; received in revised form 18 May 2013; accepted 2 July 2013

Communicated by: Associate Editor Bibek Bandyopadhyay

Abstract

In the past, the phenomenon of dust deposition on the glass cover of photovoltaic modules has been studied mainly in the MiddleEast, but little is known about the phenomenon in Central Europe. This paper focuses on the magnitude of the problem in Belgium(Koppen climate classification: Cfb). A variety of measurements were performed to determine the effect of dust settlement on the poweroutput of photovoltaic modules. The physical properties of the collected dust were examined using a scanning electron microscope(SEM). A potential solution for the phenomenon, namely the usage of special coatings on the cover glass, was investigated. The resultsshow that the problem of dust settlement on photovoltaic modules in Belgium is not as severe as in the Middle East. Nonetheless theproblem exists and results in a constant power loss between 3% and 4% for the optimal tilt angle in Belgium which is 35� and with periodsof regular rainfall. Please note that these results do not reflect a one year energy loss, further experiments are needed. Rain seems to havelittle cleaning effect on smaller dust particles (2–10 lm), but on bigger particles (pollen, approx. 60 lm) the cleaning effect is clearly vis-ible. The use of special coatings on the glass have a potential reduction in power loss caused by dust settlement. However, at thismoment, the extra cost associated with these coatings is not justified for photovoltaic modules in Belgium. Cleaning panels should onlybe done when soft tap water or demineralized water is available.� 2013 Elsevier Ltd. All rights reserved.

Keywords: Air pollution; Dust; Energy efficiency; Environmental factors

1. Introduction

Especially in residential use, little attention is given tothe efficiency of, and environmental effects on, installedPhotovoltaic Modules (Appels et al., 2012). About 7 dec-ades ago, Hottel and Woertz (1942) noticed a decrease inperformance of 4.7% after 2 months of exposing thermalcollectors with a tilt angle of 30�. Garg (1974) (India), Say-igh et al. (1985) (Kuwait) and Nahar and Gupta (1999)(India) observed an increase in the amount of depositeddust with decreased tilt angle. They concluded that solarpanels subjected to desert conditions should be cleaneddaily. Later, after 1990 (Mani and Pillai, 2010), interest

0038-092X/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.

http://dx.doi.org/10.1016/j.solener.2013.07.017

⇑ Corresponding author.E-mail address: Reinhart.Appels@esat.kuleuven.be (R. Appels).

in the effects of dust deposition on solar panels increased.Mohammad and Fahmy (1993) studied the effect of thephysical properties of dust (mainly particle size), and theinfluence of the amount of dust on the output of a solarpanel. In another paper (Mohammad and Fahmy, 1993)the same authors summarized the results of the study onthe influence of particle size on the reduction of transmit-tance. Their work showed that smaller particles have afar greater effect than larger particles on the transmittanceof glass. Goossens et al. (1993) studied the effect of windspeed on the deposition of dust in Israel and concluded thateven the slightest turbulence in the atmosphere has an effecton the movement of dust particles because of their extre-mely small inertia. Adel (2001) investigated dust depositionon 9 glass panels with different tilt angles in Egypt. Hemeasured the transmittance every day for 30 days, using

284 R. Appels et al. / Solar Energy 96 (2013) 283–291

a pyranometer, investigating the behavior of the decreasein transmittance with respect to time. Hegazy noticed thatthe speed of decrease in transmittance decreases with timeand eventually reaches a saturation point. Hassan et al.(2005) came to a similar conclusion that the decrease intransmittance reaches a point of saturation after 30 daysof exposure. The power output of the PV panels reduced33.5% to 65.8% for an exposure of respectively 1 and6 months. Elminir et al. (2006) (Egypt) used 100 glass pan-els with different tilt angles and measured a dust depositionof 15.84 g/m2 (0�) and 4.48 g/m2 (90�). Kimber et al. (2006)measured a linear increase in the losses at a rate of0.0011 kWh/kWp/day between rain showers. A linearincrease in the losses was also modeled for Madrid (Spain)by Ransome and Sutterlueti (2012) who determined thatthe soiling behavior dominates the cost in regions with longperiods without rainfall. El-Nashar (2009) investigated theseasonal effect of dust deposition in Abu Dhabi, where dur-ing the summer months frequent sandstorms occur. Heconcluded that the dust deposition on glass tubes, used ina solar desalination plant, caused a monthly drop in trans-mittance of 10–18%, advising a weekly cleaning to get themaximum annual water production. Rahoma and Hassan(2010) came to similar results after a nine-year measure-ment period of aerosol optical depth measurements in Hel-wan (Egypt), although they only measured the dust in theair, not the dust deposition, which is the focus of thispaper. Tests (Vivar et al., 2010) in Madrid (Spain) andCanberra (Australia) concluded that CPV systems aremore sensitive to soiling than flat panels, reaching opticallosses of up to 26% for CPV systems after 4 months ofexposure. Pavan et al. (2011) determined in Italy that theinfluence of soiling was higher for a 1 MW PV power planton sandy soil, with 6.9% annual losses, compared to anannual loss of 1.1% for compact soil. Stridh (2012) simu-lated that cleaning of PV power plants has an economicalvalue for the south of Europe (Murcia, Spain), but proba-bly not in the North (Helsinki, Finland), especially consid-ering snowfall.Andrews and Pearce (2012) developed amethodology for predicting losses based on readily avail-able meteorological data, especially for snowfall. Ibrahim(2011) discovered a reduction by 2.78% per day of Isc anda decrease by 0.863% per day of Voc due to accumulationof dust. Although a cleanliness monitoring system has beenintroduced by Garcıa et al. (2012) and the influence of theloss distribution has been investigated in Lee et al. (2012),many questions still remain, and we are especially inter-ested in the influence of the dust settlement on photovoltaicmodules in Belgium.

Although Qasem et al. (2011a); Qasem et al. (2011b) didsimilar research in Kuweit (Koppen climate classification(Jordan et al., 2013): BWh) with the type of dust of thatarea, Zorrilla-Casanova et al. (2012) included the effect ofrainfall in Spain (classified as BSh under the Koppen cli-mate classification), the research described in this papergoes a step further by comparing a set-up shielded by rainwith an unshielded set-up using a spectrometer and a

scanning electron microscope. An other interesting resultfrom Zorrilla-Casanova et al. (2012) is the seemingly sym-metric losses caused by dust throughout a day, indicatingthat the losses have an angle dependence. Although in1980 Shelby et al. (1980) reviewed the weathering resistanceof different types of glass, the effect of special new types ofglass, such as glass with a self-cleaning or an anti-reflectioncoating (based on nanostructured glass packaging (Sakhujaet al., 2012)), and their potential are unknown.

2. Methodology

This part of the paper will detail the methodologyemployed to investigate the dust deposition on tilted glasssamples. First, the spectrometer set-up will be presented.Next, the experimental verification of the relationshipbetween the decrease in transmittance and the decrease inoutput power will be highlighted. Followed by some sam-ples were examined using a scanning electron microscope(SEM), to identify some components in the dust. Nextthe effect of some prototype coatings are investigated andfinally, the set-up to determine the effect of dry residue isintroduced.

2.1. Transmittance of natural dust deposition

The influence of the settlement of dust on the outputpower of solar panels was determined in an indirect way.First we collected dust using a set-up containing 4 identicalnormal glass samples exposed to the elements at differenttilt angles (0�, 30�, 60� and 90�). The set-up was placedon the rooftop of the Department of Electrical Engineering(ELECTA/ESAT at N 50.9�, E 4.7� (Woyte et al., 2003)) atKU Leuven. Two such set-ups were used where one wasshielded from the rain. The transmittance of the glass sam-ples was evaluated every 2–3 weeks, consistently measuredright after a rain shower, using a spectrometer (AvantesAVASPEC-2048-2-USB2). The transmittance measure-ments were performed with the set-up shown in 1. Themeasurements were performed on the spectrometer set-up, where a 180� measuring probe was used together witha 500 W halogen lamp light source. The set-up wasdesigned in such a way that it measures the spectrum ofthe light passing through a predefined surface, where theglass samples can be mounted.

The transmittance in function of the wavelength (Tk) fora glass sample (Tk,G), a glass sample contaminated by soil-ing (Tk,G&S) and the soiling (Tk,S) has been determined as:

T k ¼Z

Ik

I0kdk ð1Þ

T k;G&S ¼ T k;S � T k;G ð2Þ

T k;S ¼T k;G&S

T k;Gð3Þ

With I0k the measured irradance in function of the wave-length for the setup without a medium, Ik,G the irradance

Fig. 1. Set-up for transmittance measurements. Fig. 2. Set-up used to measure the I–V-curve using artificial dustdistributed between two plexiglass plates.

R. Appels et al. / Solar Energy 96 (2013) 283–291 285

in function of the wavelength for the setup with the cleanedglass sample and Ik,G&S the irradance in function of thewavelength for the setup with the, by soiling contaminated,glass sample. It goes without saying that, even though thelamp is connected to a voltage stabelizer, it takes some timefor the illumination to saturate.1

2.2. Transmittance of artificial dust deposition

Secondly the relation between the dust deposition andoutput power was determined. For the two different typesof solar panels (a Sanyo HIP-210 NKHE1 and an Euroso-lare PL160) were artificially contaminated with differenttypes of dust (white sand (250 lm), clay (68 lm) andcement (10 lm)). As shown in 2, the dust was weighedaccurately and applied on a reference glass sheet with asieve before placing the glass on the solar panel. For differ-ent amounts of deposited dust the I–V curve was registeredand the decrease of the maximum power point was calcu-lated. For this experiment natural sunlight was used. Themeasurements were done as fast as possible on a clearday, removing the glass sheet before and after every mea-surement for calibration. In the same way small glass sam-ples (identical to the ones used to collect natural dust) wereartificially contaminated with the same dust that was usedon the solar panels. For different amounts of depositeddust the transmittance was measured using the set-up of1. These results together with the I–V curves lead to a rela-tion between the decrease in transmittance and outputpower. The power decrease was determined by:

1 This can take a few hours for a new lamp and quickly droppes to abouthalf an hour for a frequent used lamp.

P MPP ;S½%� ¼P MPP ;G&S

P MPP ;G� 100% ð4Þ

With PMPP,S the maximum power available due to soiling,PMPP,G&S the maximum power measured on the contami-nated glass, and PMPP,G the maximum power measuredfor a cleaned sample (0 [g/m2]).

2.3. Physical properties of the collected dust

Some glass samples where examined using a scanningelectron microscope to identify the source of the collecteddust.

2.4. The effect of special coatings

The effect of special coatings to enhance the reflectionand self-cleaning properties of the front glass of solar pan-els was investigated. A set-up containing coated glass sam-ples with a tilt angle of 35� was used to collect natural dustand the transmittance was evaluated after three weeks ofexposure.

2.5. Cleaning aspects

Finally the effect of dry residue of different types ofwater were measured in the same way as explained earlier,where glass samples are sunk in a wooden frame, as shownin Fig. 3, leaving a gap of respectively 1 mm, 3 mm and5 mm between the top of the glass and the wood. Theset-up was placed horizontally, to allow vaporization ofthe different amounts of water with their constituent parti-cles and the transmission of light through the glass is mea-sured. Different types of water were poured onto the glass

Fig. 3. Sketch of the set-up to allow evaporation of the different watersolutions. Set-up placed horizontally.

286 R. Appels et al. / Solar Energy 96 (2013) 283–291

and given enough time to evaporate. When evaporated, theglass samples were cleaned on the bottom side and thetransmittance was measured.

Before the start of every experiment, all samples wherecleaned and measured. These measurements were used asa reference.

3. Results and discussion

Five experiments were performed. First, the collectionof natural dust on glass samples and the influence of thecleaning effect of rain is discussed, second, the effect of arti-ficial dust with different particle size on the cover glass isinvestigated, followed by an examination of the physicalproperties of the collected dust using a scanning electronmicroscope, next the effect of some special prototype coat-ings are examined and lastly, the dry residue of differenttypes of water are measured.

3.1. Transmittance of natural dust deposition

The full line of 4 shows the decrease of the transmittancefor the glass samples that were subject to rainfall. It shows

Fig. 4. Transmittance over time (measured from 01/03 to 28/04 2011). Pleasebehavior will most likely be (irregular-) saw-toot like.

that after 5 weeks of exposure, the decrease in transmit-tance saturates. This means that for the optimal tilt anglein Flanders Belgium (approx. 35�) the transmittancedecrease saturates between 3% and 4%. Saturation, how-ever, was not yet observed during the short measuring per-iod for a tilt angle of 0�. The dotted line of 4 displays thecase where the glass samples were shielded from the rain.The effect of rainfall on the transmittance is very clearwhen comparing the samples with and without roof in 4.The deposition of dust continues when the glass samplescannot be cleaned by the rain, resulting in a continuingdecrease of transmittance. The conspicuous decrease oftransmittance between 35 and 58 days of exposure to dustin the dotted line of 4 is caused by the high concentrationof airborne pollen (approx. 60 lm in size) in that period,corresponding with the beginning of Spring in Belgium. 4also indicates that dust deposition is a greater problemfor lower tilt angles.

3.2. Transmittance of artificial dust deposition

The values for the observed decrease in output power ofthe two solar panels and the decrease in transmittance ofthe glass samples with increasing (artificial) dust depositionare comparable. 1 shows the results for the measurementswhere white sand (250 lm), clay (68 lm) and cement(10 lm) were used. 5 visualizes the transmittance of 1. Sim-ilar results were found by Jiang et al. (2011), where another measurement technique - making dust airborne in atest chamber - was used. 1 shows that transmittance andpower decrease are approximately the same. The small dif-ference (<2.5%) is most likely caused by measurementerrors: first, the used method to distribute the dust is unli-kely to give a perfect equal distribution of dust and sec-ondly, the measurements were performed with naturalsunlight which has the tendency to fluctuate, even on a

note that the lines only connect the measuring points. The real transient

Table 1Transmittance and power decrease for sand, clay and cement. Abbreviations: Transmittance (T), Power (P) and solar panels Sanyo (S) and Eurosolare (E).

(g/m2) White sand (250 lm) Clay (68 lm) Cement (10 lm)

T (%) Pmax (%) T (%) Pmax (%) T (%) Pmax (%)

S E S E S E

0 100 100 100 100 100 100 100 100 10010 98.95 98.22 99.92 90.03 91.84 90.30 81.59 79.08 80.9420 95.98 95.16 94.54 79.97 80.62 79.64 59.39 59.35 60.2240 90.82 90.23 90.90 60.44 61.14 62.88 46.52 48.76 47.5660 84.97 85.26 86.24 51.58 51.23 52.79 33.34 35.84 34.32

Fig. 5. Transmittance for artificial contamination by white sand, clay and cement.

R. Appels et al. / Solar Energy 96 (2013) 283–291 287

clear day and it is not easy to perfectly reproduce the samedegree of dust deposition on the glass samples and the PVmodules. The bend in 5 for white sand and clay is mostlikely caused by clustering of dust particles, where extraadded dust particles pile up on each other, adding moremass per square meter, without an associated decrease intransmittance, as confirmed by Beattie et al. (2012).

3.3. Physical properties of the collected dust

A scanning electron microscope (SEM) was used toexamine the dust that was collected on the glass samples.6 and 7 shows that, a glass sample that was shielded fromthe rain, was littered with pollen (approx. 60 lm in size).A bigger magnification reveals that between the pollen ablanket of particles ranging from 2 to 10 lm can be found.8 and 9 shows the image of the surface of a glass sample thatwas not shielded from the rain and reveals that the sameblanket of particles that can be categorized as ‘inhalablecoarse particles’ (Case et al., 2008) or particulate matter(PM) between PM2.5 and PM10, is present on the surfaceof the glass, but that the pollen have been completelywashed away by the rain. These glass samples were takeninside right after a rain shower which means that the

blanket of fine dust particles is not removed by rainfall. Thisexperiment reveals why the transmittance never reaches100%, not even after rainfall. 9 also reveals local spots onthe glass surface where bigger dust particles accumulate.A possible explanation is that resin particles, probably frompine, particles carried by the wind end up on the glass sur-face and trap dust particles. Another possibility is thatwater droplets evaporate and leave behind dust deposits.

3.4. The effect of special coatings

Prototype coated glass samples with three different typesof coatings were examined: an anti-reflection coating (AR),a self cleaning coating (SC) and a multilayer coating (ML)consisting of both AR and SC. These glass samples were setup in a similar way as in 3.1. The glass samples wereexposed to atmospheric conditions during 3 weeks (fromthe 5th of April 2011 to the 28th of April 2011). 2 showsthe results for the transmittance measurements. Thedecrease in transmittance is least when ML coating is used.The measurements show that the coatings improve the self-cleaning properties of the glass samples. The saturation ofthe transmittance was, however, not observed due to thelimited time of exposure. The economical potential of this

Fig. 6. SEM image of the surface of a glass sample that was shielded fromthe rain (30� tilt angle).

Fig. 7. SEM image of the surface of a glass sample that was shielded fromthe rain (30� tilt angle). Identification pollen: (a) pine, (b) maple and (c)oak.

Fig. 8. SEM image of the surface of a glass sample that was not shieldedfrom the rain (30� tilt angle).

Fig. 9. SEM image of the surface of a glass sample that was not shieldedfrom the rain (30� tilt angle).

Table 2Results transmittance measurements for the coated glass samples (3 weeksexposure).

Type of coating Transmittance decrease (%)

Regular glass 2.63Anti-reflection (AR) 1.75Self cleaning (SC) 1.30Multilayer (ML) 0.85

Fig. 10. Transmittance decrease due to dry residue after evaporation.

288 R. Appels et al. / Solar Energy 96 (2013) 283–291

solution depends heavily on the extra cost that comes withthe use of the coatings. This however is unknown duringthe writing of this paper as these coatings are not yet avail-able on the market.

Fig. 11. Transmittance of glass samples with dry residue of hard tap water.

2 French degrees.

R. Appels et al. / Solar Energy 96 (2013) 283–291 289

3.5. Cleaning aspects

Note that the distribution of dust (or other particles) onthe PV module will not be homogeneous. There are manyreasons for that, like for example (a) the turbulence onthe edge of the PV module is bigger than in the center,which has a big impact as explained in the introduction,(b) at the top of the module the cleaning effect of rain willbe less than at the bottom, due to the effect of rain strikingcombined with rain run-off on the module, but on the otherhand, (c) the bottom of the module could see more dirtdeposit than the top, because it is exposed to more dirt, dis-solved in the rainwater run-off, and (d) depending on thetilt angle of the PV module with a mounting frame, theremay be some stagnant water at the bottom of the module,increasing the effect of local soiling.

For this experiment, different types of water were cho-sen and poured on glass samples, sunk for 1 mm, 3 mmand 5 mm, as shown in 3 and allowed to be vaporizedbefore measuring. 10 shows the results of the transmittancemeasured for every case. The types of water employedwere: (a) hard tap water, (b) soft tap water, (c) rainwatercaptured at the start of a rainshower, (d) demineralizedwater, namely water where all ions are removed, (e) osmo-sis water made by using the reverse osmosis process, (f)some ammonia dissolved in demineralized water, and (g)some detergent dissolved in demineralized water.Osmosiswater, ammonia and detergent are often used by firms spe-cialized in cleaning solar panels. Leuven, a city close to thecenter of Belgium, is known for its hard tap water (34� f to

42� f2). The soft tap water used came from Beringen, in theNorth-East of Belgium (10� f to 15� f). Looking at 10,demineralized water gives the best result, closely followedby osmosis water. The negative slope of the ‘osmosiswater’ line cannot readily be explained. However, the mea-surement error of our system is larger than the differencebetween the demineralized water and the osmosis water.The vaporizing process was executed in our lab, which isnot a controlled dust-free environment. The uncertaintyof the measurements was not accurately determined. Thestriking curve of rainwater is most likely caused by the cap-turing of airborne dust particles when falling from the sky.Although the previous experiments indicate that rain has acleaning effect on solar panels, at first rain will deposit air-borne particles, making the panels dirtier. With continuingrain, the precipitated airborne particles are removed by thecleaner rain droplets. Of course, in reality, there will neverbe a time where 5 mm of water stays stationary on a PVpanel and evaporates, but over the years, the repeated dustdeposition and evaporation of water results in a similareffect and hence long term measurements are needed tobetter understand this process. Manual and automaticPV cleaning services are offered by emerging companies.Whether or not these services are cost effective, special careshould be taken when using, for example, an automaticsprinkler system to clean panels for two reasons. First,when sprinkling water with a high amount of dissolved

290 R. Appels et al. / Solar Energy 96 (2013) 283–291

particles on non-hydrophobic glass,it can leave a smallamount of water, leaving a dry residue which can buildup over time. Secondly, the small amount of water stuckto the glass can capture airborne dust particles - whichwould otherwise have been removed from the air by therain - more easily. 11 gives more detailed information onthe transmittance effect for hard tap water after evapora-tion and shows that the transmittance decrease dependson the wavelength of the incident light.

4. Conclusion

The results presented show that the effect of dust depo-sition or soiling in Belgium and by extension for the Kop-pen climate classification: Cfb is responsible for a constantpower loss between 3% and 4%, which saturates afterapproximately 5 weeks of exposure. Please note that theseresults do not reflect a one year energy loss, further exper-iments are needed. The results indicate that annual clean-ing of photovoltaic modules will have virtually no effecton the performance of the system. This, however, doesnot mean that regular cleaning of photovoltaic modulesis unnecessary. The results only take into account theeffects of dust deposition. Photovoltaic modules are sub-jected to other means of contamination like e.g. bird drop-pings, fallen leafs, chemicals, growth of moss, etc. whichwere not studied here. Rainfall seems to have a limitedeffect on the saturation level of the power losses causedby small dust particles (2–10 lm), although larger dust par-ticles (pollen), on the other hand, are washed away afterevery shower. The use of special coatings proves to be apotential solution. However, it is going to be a challengeto make these solutions cost-effective. When cleaning solarpanels with water, we suggest to use water with a low levelof hardness (<15� f). When using a detergent or dilutedammonia, the surface must be rinsed afterwards (althoughthis is probably not cost effective nor efficient), and if pos-sible, to dry the glass, as it otherwise becomes an idealdeposition surface for dust particles, worsening instead ofimproving the transmittance of the glass. Future workcould look at the cost-effectiveness of the cleaning methodsemployed. For example, comparing all automatical andmanual washing methonds.

Acknowledgement

The authors thank Imec for their financial support.

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