Soiling Related Abrasion and the 

Development of a PV Abrasion Standard

David C. Miller, Asher Einhorn, Chaiwat Engtrakul, Clare L. Lanaghan, 

Leonardo Micheli, Helio R. Moutinho, Matthew T. Muller, Bobby To, 

Sarah Toth, and Lin J. Simpson

International PV Soiling Workshop

Golden Ballroom Salon D, Denver West Marriott hotel, Golden CO

08:40-09:00 Thursday, November 01, 2018

NREL/PR-5K00-72850
2

Motivation

•AR and/or AS coatings are applied to PV to directly 

increase e- generation and/or reduce effects of soiling.

•Coated modules are typically cleaned.

•The degradation and durability of popular coatings 

remains to be understood.

Coatings used in PV front surfaces. 

Einhorn et. al., J PV, in review.

Vendor cleaning building glazings (at NREL campus).

•Much of the damage to coatings results from cleaning 

(equipment and/or contamination as abrasive).

•PV leverages cleaning methods and equipment from the 

building glazing industry.

•Goal: develop an industry standard concerning abrasion.
3

IEC 62788-7-3 (PV Abrasion) Standard Is Under Development

Methods

•Falling sand test.

-Natural abrasion (wear from typical meteorological conditions). 

-Front surface coatings & backsheets.

Schematic of falling sand test. From 

Nishioka et. al., Proc. Asian PVSEC 2013. 

•On review, no existing standard from other industries was found readily suited for PV. 

-Example: frosted –glass- specimens. See: Miller et. al., NREL/TP-5J00-66334, 2016, 1-25.

⇒Accelerated abrasion standard for PV surfaces is presently being developed in IEC WG2.

Example (PB-8100) linear 

machine abrasion tester. 

www.byk.com

•Artificial machine abrasion.

-Cleaning of PV. 

-Includes slurry or dry dust abrasive.

-Linear translation or rotating brush.

Schematic of forced sand impingement 

test. From Klimm et. al., Proc. Euro. 

Weathering Symp. 2015.

•Forced sand impingement test.

-Covers severe storms (infrequent, but high velocity wind). 

-Front surface coatings & backsheets & vehicle integrated PV.
4

Key Details of the Linear Brush Test for Artificial Machine Abrasion

Some details can be leveraged from standards for other industries. 

Abrasive

•Consider contamination (size & composition) present on a PV module.

•AZ test dust: ISO 12103 A3 (“medium” particle size distribution).

Brush bristle

•Consider common/existing equipment.

•Material: Nylon 6,12 (other standards use/moving towards this material).

•Length: 3.8 cm (many water fed brushes use longer bristles). 

•Geometry: round, extruded bristles with no taper (standardize).

Brush geometry

•Standardize to legacy methods: area (3.5 x 8.5 cm), bristle count, tuft pattern.

Machine

•Standardize to legacy methods. 

-454 g contact force; 255 mm brush stroke length; 37 cpm test rate.

Next slides: explore aforementioned aspects in detail. 
5

What Size of “Abrasive” is Typically Found on PV Surfaces? 

cdf of particulate population of field sites 

(NC samples, 1y) verified at NREL. 

Nayshevsky et. al., 

Proc Intl Soiling Work., 2018. 

•15 μm ∅ (median) in PV literature.

•1.5 μm ∅ (50th percentile) in NREL 

field studies was < 15 μm PV literature.

-Results repeatable, Keyence & ImageJ.

-Loss of large particles during shipping?

-Cementation (e.g., Dubai & Kuwait)?

-Characterization methods & reporting 

is not standardized.

•PM2.5: from combustion, 

chemical processes.

•Airborne mass concentration 

of PM2.5 subject to evolution. 

•PM10: from mechanical 

origins. 

•Mass concentration field 

sites similar to expected 

airborne. 

Mass concentration of airborne PM. 

Wilson & Suh, J. Air & Waste Manage. Assoc., 1997.

Mass concentration of Mesa field site (NC sample).

Mesa, 1y

>99th percentile cdf

<1 

μm resolution
6

“Abrasives” Used in the PV Community 

Cumulative distribution function for 

“abrasives” examined in the PV community.

(Supplier’s data).

Range of particle diameters for “abrasives” 

examined in the PV community.

(Supplier’s data).

•”Sands” have greater ∅ than “dust”.

•Sands have a more limited composition than dust.

(Sands mostly silica)

•Grain morphology may vary between the sands.

Chemical composition for DIN 

52348 sand. Busch GmbH.

MATERIAL

CONCENTRATION

{%}

SiO2 96.20

Al2O3 0.90

Fe2O3 0.02

TiO2 0.06

CaO + MgO 0.05

K2O + Na2O 0.55

Ignition loss 0.20

MATERIAL

CONCENTRATION

{%}

SiO2 99.3

Al2O3 0.5

Fe2O3 0.06

Ignition loss 0.2

Chemical composition for F36 sand. 

Quarzwerke GmbH.

SAMPLE

SIZE, 5th

PERCENTILE

{μm}

SIZE, 50th

PERCENTILE

{μm}

SIZE, 95th

PERCENTILE

{μm}

A1 dust 1.0 4.5 9.5

A2 dust 1.0 8.0 58.8

A3 dust 1.6 14.2 84.5

A4 dust 2.4 34.2 140.3

ASTM C778

graded 170 369 581

ASTM C778

20-30 607 732 960

DIN 52348 499 567 598

Qarzwerke F36 103 160 242

ISO 12103

(PTI) standard

Chemical composition for ISO 12103 test dust. PTI Inc, Safety Data Sheet.
7

Abrasive Composition: Additional Varieties May Ultimately Be Required

Comparison of dust from various locations.

Engelbrecht et. al., Inhalation Toxicology, 21, 2009.

•Local composition of soil (therefore PM10) varies 

between world locations.

MENA AZ

•”Compared with the Sahara, China, US, and world 

dusts, Middle East samples had lower proportions of 

SiO2 and higher proportions of CaO and MgO”, 

Engelbrecht et. al.

•Palygorskite [(Mg,Al) 2Si4O10 (OH)·4(H2O)] is a mineral 

commonly found in MENA locations (and SE USA), but 

not in AZ (ISO 12103 test dust).

• Palygorskite is affected considerably by water 

(including dew cycles), contributing to cementation.

•A future standardized “MENA test dust” may be appropriate.

•Location specific differences also exist within MENA.

•What location might be the benchmark for MENA?

•MENA dust products presently exist.

Chemical composition for 

ISO 12103 AZ test dust.

PTI Inc, Safety Data Sheet.
8

Bristle Materials Typically Used for PV and Their Characteristics

Nylon

•Hardest material. Slow wear rate ⇒ low cost of use.

•Easiest bristle material to clean ⇒ low cost of use.

•Nylon 6,6 swells more with water, may fatigue faster than Nylon 6,12.

Representative boar artwork.

http://www.nedgallagher.com/journal/archives/000841.html

Hog bristle

•Natural: obtained from along the spine of a boar's back. Premium price.

•Preferred in automotive industry (will not scratch clearcoat/paint).

•Not commonly used in MENA for cultural reasons.

Nylon 

6,6

Nylon 

6,12

Comparison of molecular structure of Nylons.

http://nxt-ubiquity.s3.amazonaws.com/wiley/plasticsengineering/april2016/UPLOADED_ASSETS/technicalpaper/T2.jpg

Other synthetics

•Includes: polyester, polystyrene, and polypropylene.

•Low cost resins.

•Softer materials ⇒ faster wear rate.
9

Representative Brushes for the PV Industry 

Images of representative brushes examined.

•Examine five brushes marketed to the PV industry 

(building glazing cleaning equipment vendors).

•Goal: verify characteristics to provide feedback to IEC 62788-7-3.

Characterizations

-Physical dimensions

-Chemical composition

-Glass & phase transition temperature(s)

(1) (2) (3) (4) (5)

Not considered: rotary disc style brush (manufacturer: Kärcher). Not Examined:

-Proprietary brushes.

-Microfiber/fabric “bristles”.

-Surfactants/liquids/solvents.

(detergents & anti-spotting agents are not allowed in CA water reclamation laws) 
10

INDEX

BRISTLE

COLOR

BRISTLE MATERIAL:

COMPOSITION

BRISTLE MATERIAL:

SAME AS IN 

VENDOR 

CATALOG?

BRISTLE 

DIAMETER

{mm}

BRISTLE 

LENGTH

(OUTER)

{mm}

BRISTLE 

LENGTH

(INNER)

{mm}

1 yellow or gray polyester yes 0.34 75 50

2 clear Nylon 6,12 yes 0.30 51 41

3 brown hog bristle yes 0.27 71 N/A

4 red polyester no 0.25 43 N/A

5 blue Nylon & polyester no 0.50 31 N/A

6 black horsehair yes NREL field study

Lessons Learned From the Representative Brushes

Key physical characteristics (bristle-diameter & -length) were verified.

The base material was verified against known reference materials

•Materials used do not always match catalog information. 

(PE and horse hair are cheap).

•Bristle diameter > 0.23 mm. kbend ∝ ∅4

•Bristle length(s) > 38 mm. kbend ∝ L-3

Example: the FTIR peaks ~3300 and ~2900 cm-1 

may be used to verify the structure of Nylon.

INDEX

Tg1=Tα1

{°C}

Tc1

{°C}

Tm1

{°C}

1 44 180 216

2 157 189 216

3 N/A N/A N/A

4 N/A 202 218

5 N/A 200 224

6 153 N/A N/A

Measured phase transition temperatures for the same brushes.

•Bristle material’s phase transition temperature may limit 

the local temperature on the specimen (dry brush test). 

•Nylon and PE have similar Tmelt, Tcrystallization

•Natural fibers may have Tg, will decompose at higher T. (No melt).
11

A Comparison of the Bristle Materials in Linear Abrasion

Photo of the brushes used in the comparative study. 

•Compare popular bristle materials

(Nylon 6,12; hog bristle; polyester) in dry linear abrasion test. 

•Correlate optical performance (transmittance), 

surface energy (contact angle), and surface roughness.

What is the quantitative significance of the bristle material in cleaning abrasion?

Experiments

•Brush test performed using A4 (coarse) AZ 

test dust on reference (“J”) glass.

•Up to 20k cycles (clean 1x daily for 25y, 

20kc also used in other industries’ standards).

BRISTLE MATERIAL:

COMPOSITION

Tg1=Tα1

{°C}

Tc1

{°C}

Tm1

{°C}

BRISTLE 

DIAMETER

{mm}

BRISTLE 

LENGTH

(OUTER)

{mm}

Nylon 6,12 125 188 215 0.30 38.1

hog bristle 176 N/A N/A 0.11 38.1

polyester 106 196 224 0.61 38.1

Characteristics of the brushes used in the comparative study. 

Brushes

•38.1 mm long bristles (some deceleration, 

longest length possible in abrasion tester).

•Identical brush blocks → similar packing 

pattern & density.
12

73

75

77

79

81

83

85

87

89

91

0 5,000 10,000 15,000 20,000

τd, Direct Transmittance {%}

n, Number dry brush cycles {dimensionless}

 

-15 

-2.5 

%Td. 

Bristle Materials Readily Distinguished For Dry Brush Test (1)

Effect of linear abrasion on direct transmittance results for J (reference glass).

•Results here quantify the 

feedback from the industry (hog 

bristle least damaging; Nylon most 

damaging). 

•Results for glass can be used as 

reference for artificial abrasion 

study and standard development.

•Next step: dry brush testing of 

coated specimens.

•Hog bristle minimally affects 

glass. (∆τd ~ 2.5%)

•Nylon maximum affect on glass. 

(∆τd ~ 15%).

•Polyester gave significant damage.

•Only large ∅ bristle available from 

our vendor.
13

Bristle Materials Readily Distinguished For Dry Brush Test (2)

Effect of linear abrasion on surface energy (contact angle) for J (reference glass).

•Contact angle greatly reduced 

from baseline value with dry 

brushing for all bristle materials.

•Contact angle remained reduced 

(hydrophilic) through experiment 

for hog bristle and polyester.

•Contact angle evolved in complex 

manner with n for Nylon. 

•Additional characterization 

(surface roughness, SEM, XPS) 

should clarify if abrasion, 

contamination, or diffusion 

contribute to the surface energy.
14

nylon

polyester

hog bristle

∅avg

300μm

∅avg

640μm

∅avg

220μm

500 μm

Optical microscope image comparing the bristle types 

after 20k cycles of dry brush test.

Optical microscope image comparing the bristle types 

before 20k cycles of dry brush test.

•Morphology after 20,000 cycles evidences: 

bristle erosion (hog bristle); 

bristle rounding (nylon, temperature or electrostatic discharge); 

bristle deformation (polyester).

Brush Bristles Are Readily Distinguished For Dry Brush Test

20k 

cycles
15

Field Coupon Study (Background and Progress)

Test sites:

•Contamination and abrasion prone locations. 

•Mesa, Arizona; Sacramento, California; Mumbai, India; 

Kuwait City, Kuwait; Dubai, United Arab Emirates.

Cleaning methods:

•No clean (NC); dry brush (DB); low-pressure water spray (WS); wet sponge and squeegee (WSS). 

•Clean 1x/month.

•Examine 1 set of duplicates each year for 5 years.

Status:

•y1 specimens recently returned from all sites. Some y2 specimens have returned.

•Quantifying soiling performance (PAC) to down-select sites & cleaning methods of interest.

•Future: F/A, RE: abrasion, cleaning, and soiling. 

Samples:

•7.5 cm x 7.5 cm coupons.

•Includes AR, AS (-phobic & -philic), reference glass. 

•Black backpane (similar temperature to PV). 

Original specimen set deployed in Sacramento.
16

Abrasion Observed in the Field Study

Einhorn et. al., JPV, in press.

•The efficacy of cleaning was readily distinguished after 1 year.

•DB cleaning method greatly damaged some coatings after 1 year 

(12 cleanings) in Dubai. 

-Example: coating D extensive damage. B relatively intact.

-More frequent cleaning may help.

•WSS cleaning method much less damaging.

•Quantification & comparison between sites presently underway for DB.

AFM to asses damage regions for Dubai 

B specimen (left) and D specimen (right) for DB cleaning. 

3 4

3

~75 nm

1 2 4

1 2

~100 nm

B: relatively 

intact

D: relatively 

damaged
17

“The Abrasion of Backsheets & VIPV May Also Be Addressed”

Roy Choudhury et. al., IEEE PVSC, 2018.

•The erosion of backsheets was recently 

identified in veteran PV modules.

•Caveat: polymers may erode from 

photodegradation (loss of volatiles) in 

addition to physical abrasion.

•More evidence & understanding is 

sought within IEC 62788-2 

(backsheet standard group). 

•IEC WG2: consider backsheets within the scope of IEC 62788-7-3. 

•Accelerated test sequence for PV modules may ultimately include abrasion, 

in a test sequence (e.g., UV → abrasion → TC200 → HF10)

To participate in the backsheet abrasion survey, contact:

Jürgen JUNG <juergen.jung@agfa.com>

•IEC WG7: consider vehicle integrated PV within IEC 62788-7-3.
18

Summary

•An IEC abrasion test standard is being developed for the PV industry.

•Suite of: falling sand, forced sand impingement, and machine abrasion tests.

•Originally just for coatings, but may eventually address backsheets & VIPV.

•ISO 12103 AZ test dust may be used for machine abrasion tests. 

•Nylon (most damaging) and hog bristle (least damaging) were readily 

distinguished in dry machine abrasion tests (transmittance, surface energy). 

•Popular Nylon 6,12 recommended as bristle material for machine abrasion. 

•Abrasion depends on tribology and bristle materials (Tmelt) characteristics.

•Please contact me if you are interested in the abrasion standard.
19

Acknowledgements

Thanks to field site collaborators: Bader Alabdulrazzaq, Ayman Al-Qattan, Pedro Banda, Ben 

Bourne, Telia Curtis, Zoe Defreitas, Jim J. John, Greg Kimball, Anil Kottantharayil, Marco 

Stefancich, Pr. Govindasamy Tamizhmani, Juzer Vasi, and Sonali Warade

There has been fantastic participation in PVQAT TG12 webinars & TG12-3. 

Thank you to the many participants for your ongoing support!!! 

•If interested in the TG12-3 activities or IEC 62788-7-3 standard, please contact 

David.Miller@nrel.gov

NREL STM campus, Dennis Schroeder

Your questions and feedback are much 

appreciated! Please help me to cover the 

important details & perspectives.

This work was supported by the U.S. Department of Energy under 

Contract No. DE-AC36-08-GO28308 with the National Renewable 

Energy Laboratory. This material is based upon work supported by 

the U.S. Department of Energy's Office of Energy Efficiency and 

Renewable Energy (EERE) under Solar Energy Technologies Office 

(SETO) Agreement Number 30311. The views expressed in the 

article do not necessarily represent the views of the DOE or the 

U.S. Government. The U.S. Government retains and the publisher, 

by accepting the article for publication, acknowledges that the U.S. 

Government retains a nonexclusive, paid-up, irrevocable, 

worldwide license to publish or reproduce the published form of 

this work, or allow others to do so, for U.S. Government purposes. 
20

Standards Discussion Session (1)

•What are the highest impact soiling problems that presently need to be 

addressed?

•What scientific information is needed to help address soiling?

•Are existing standards sufficient? What new standards are needed?
21

Standards Discussion Session (2)

•What do you believe is the future of:

(a) soiling monitoring

(b) soiling assessment

(c) prediction

(d) mitigation? 

What issues need to be resolved? 

What innovations are needed?

•Are the PVQAT TG12 groups sufficient to address the most pressing 

issues?
22

Task Group 12-1 (sensors and the monitoring of soiling)

 Contributed to IEC 61724-1 (quantifying effect of soiling on PV systems). 

 Contact: YuePeng DENG <Yuepeng.Deng@FIRSTSOLAR.COM>.

The Subgroups of PVQAT TG12 (Soiling)

Task Group 12-4 (modeling/analysis of effects of soiling on PV systems)

 Example: analysis of power production data from PV installations

 Reference: Deceglie et. al., Proc. IEEE J PV, 2018. also: 

http://www.nrel.gov/docs/fy16osti/65763.pdf

 Contact: Leo MICHELI <Leonardo.Micheli@nrel.gov>

Task Group 12-3 (antireflective and/or anti-soiling coatings)

 Recent focus on abrasion. 

 Web study group, including abrasion and characterization methods. 

 Reference: Miller et. al., NREL/TP-5J00-66334, 2016, 1-25. 

http://www.nrel.gov/docs/fy16osti/66334.pdf

 Goal: international artificial soiling test standard, e.g., used for R&D purposes.

 Contact: David MILLER <David.Miller@nrel.gov>

Task Group 12-2 (solutions for cleaning)

 Much recent interest. Goal: module cleaning standard (manual & robotic methods).

 Contact: Lin SIMPSON <Lin.Simpson@nrel.gov>

Task Group 12 Webinars (all general topics)

 Monthly webinars on soiling topics. 2nd Tuesday of each month. 

 Contact: Greg SMESTAD <smestad@solideas.com>