Optical Characterization of PV Glass Coupons and PV Modules Related to Soiling Losses, Greg P. Smestad, Ph.D., Sol Ideas Technology Development December 6th, 2017, 11:35 AM - 12:00 PM Session 5: Characterization (Chair: Xiaohong Gu, NIST) Atlas/NIST Workshop on PV Materials Durability December 5-6, 2017, Gaithersburg, Maryland National Institute of Standards and Technology, Gaithersburg, Maryland https://www.nist.gov/el/mssd/agenda

1. Greg P. Smestad, Ph.D.
Sol Ideas Technology Development
Leonardo Micheli, Ph.D.,
Na>onal Renewable Energy Laboratory
(NREL)
Thomas A. Germer, Ph.D.,
Na>onal Ins>tute of Standards and
Technology (NIST)
Eduardo F. Fernández, Ph.D.
University of Jáen, Spain 12/11/17
Op#cal Characteriza#on of PV Glass Coupons
and PV Modules Related to Soiling Losses
Session 5: Characterization (Chair: Xiaohong Gu, NIST)
December 6th, 2017, 11:35 AM - 12:00 PM

2. • Soiling Overview
• Ångström formula
• Experimental Procedure
• Transmission Results
• Coupon vs. PV Module
• Angle of Incidence effects
– Model vs. Experiments
• Conclusions
Outline
Ambient Particulates
(PM)
Deposited PM Reduces
Transmittance to PV
Graphic: M.H. Bergin et al, Duke University, Durham, NC

3. Ambient Particulates
(PM)
Deposited PM Reduces
Transmittance to PV
Graphic: M.H. Bergin et al, Duke University, Durham, NC
"Large Reduc>ons in
Solar Energy
Produc>on Due to
Dust and Par>culate
Air Pollu>on", Mike H.
Bergin, Chinmay
Ghoroi, Deepa Dixit,
James J. Schauer, and
Drew T. Shindell,
Environ. Sci. Technol.
LeX., 2017.

4. Soiling: Defini#on
Accumula>on of dust, dirt and par>cles on
the surface of PV modules.
Drop in power output: can be > 50%.
Drop in energy yield: 0 to 6% in the U.S.
4

5. https://earth.nullschool.net/#current/particulates/surface/level/
overlay=pm2.5/
5
Real time visualization of PM2.5

6. Defini#on of Par#culate MaBer
ø < 10
um ø < 2.5
um
Particulate matter (PM): concentration (µg/m3) of solid
particles and liquid droplets suspended in 1 m3 of air.
PM10 Sources:
Crushing or grinding
operations
Dust stirred up by
vehicles on roads
PM2.5 Sources:
Motor vehicles,
power plants,
residential wood
burning, forest
fires, agricultural
burning, industrial
processes.
Source: https://www.airnow.gov/index.cfm?action=aqibasics.particle6

7. PV EL and Soiling
associated with a more constant voltage potential between the
external glass surface and the active cell circuit of the
module—but lower net current magnitude [17].
a
b
Fig. 4. Subtractive electroluminescence images taken at 4.1-A
forward-bias current on module type A with salt; (a), 73.3% power
remaining; (b) without salt, 84.7% power remaining. Degraded
areas appear dark.
Peter Hacke, et al, Effects of
Photovoltaic Module Soiling on
Glass Surface Resistance and
Potential-Induced Degradation,
Conference Paper, NREL/
CP-5J00-64492 December 2015
7

8. Diagram Courtesy of Al Hicks, NREL8
The Optical Path of the Light (clean vs. soiled)

9. Particle size is smaller
than one- tenth the
wavelength of light
DIFFUSE
(a) RAYLEIGH SCATTERING
Particle size is
approximately
one-fourth the
wavelength
of light
DIFFUSE
Particle size is
larger than the
wavelength
of light
(b) MIE SCATTERING
An Introduction to Solar Radiation,
Muhammad Iqbal, Academic Press,
New York, 1983, Chapter 6.
Atmospheric Scattering
9
can be applied to
particles on the glass

10. β → 0.0 to 0.5 or even higher
• An index representing the
amount of aerosols (particles)
present
• Wavelength (λ) exponent
• Generally 0.5 to 2.5 (Ångström suggested 1.3)
ma is the optical path length
Ångström turbidity formula
𝛼=4 for small non-absorbing particles
𝛼=1 for small absorbing particles
𝛼=0 for large particles
10

11. Spectral impact of soiling: Experiments
11

12. Experimental procedure
- Seven 4 cm x 4 cm x 3 mm-thick low iron glass coupons shipped to
each location.
- Coupon 1 to 6 installed outdoors at zero tilt angle for eight weeks.
- Coupon 0 kept in a dust free container and used to calibrate each
spectrophotometer.
- Weekly transmission measurements for coupons 1, 2, 3.
- Daily weather and particulate matter (PM) concentration recorded.
- A dry cleaning is performed by using a microfiber cleaning cloth.
• Coupon 1 cleaned every week, Coupon 2 every four weeks.
12

13. Aaer 8 weeks (San José, CA)
13

14. Ångström equation returns in both cases high R2 (≥90%), and low RMSE (<0.3%).
Fidng San José and Chennai Data
(aaer 8 weeks)

15. Ångström equation returns in both cases high R2 (≥90%), and low RMSE (<0.3%).
…but fitting at low wavelengths can still be improved.
Fidng San José and Chennai Data
(aaer 8 weeks)
15

16. First results
Absolute direct and hemispherical transmittance of coupon 3 in Golden,
CO. Wavelengths between 500 and 1100 nm have been averaged.

17. Hemispherical transmittance (8 weeks)
Hemispherical transmittance in the visible and NIR range of coupon 5 for all the sites
(above) and for the low soiling sites (below), referenced to the transmittance of
coupon 0. The spectra were measured using a PerkinElmer Lambda 1050 UV/Vis
spectrophotometer with a 150 mm integrating sphere at NREL and processed using a
local regression technique to remove noise.

18. Par>cle Area and Hemispherical TransmiXance
• Average particle area was determined by high resolution optical
microscopy.
• Percentage of the surface covered by particles was estimated.
• A linear correlation, with R2 higher than 0.99, is found by comparing the
percentage area covered by particles to the hemispherical transmission.
• The broadband hemispherical transmission could be directly obtained from
the coverage area, independently of location, dust type and composition.
• See the subsequent plot (slide).
• Broadband hemispherical transmittance (300-2500 nm)
was measured.
18

19. Fractional Loss versus Particle Coverage
5% 10% 15% 20% 25% 30% 35% 40%
0%
5%
10%
15%
20%
25%
FractionalParticleCoverage
Fraction Loss
slope = 0.80±0.02
R2
= 0.99
19

20. The Dust & Rain in Spain falls mainly…
Predication: 6-months of outdoor exposure at Jáen, Spain.
Estimated weekly evolution of Soiling Ratio index estimated for three PV materials.
20
From the transmission of glass coupons times the solar spectrum times the PV spectral response or QE. There was a
Saharan dust storm at week 6 and then rain.

21. 28 Chapter 2
Fig. 2.10 Geometry for calculation of the absorptivity from a polished (left side) and
textured (right side) light absorber. In each case, a summation of multiple reflections must
be made in order to calculate the absorptivity. The angle q is measured from the surface
normal.
These equations can be used to predict the reflectivity, otherwise known as
the reflection coefficient, of a solar cell absorber material. The index of n value
for Si, for example, is approximately 4 at a wavelength of 600 nm (see Fig. 2.3).
Library of Congress Cataloging-in-Publication Data
Smestad, Greg P.
Optoelectronics of solar cells / by Greg P. Smestad.
p. cm.-- (SPIE Press monograph ; PM115)
Includes bibliographical references and index.
ISBN 0-8194-4440-5 (softcover)
1. Solar cells. 2. Optics. I. Title. II. Series.
or soiled
Op>cs of a PV Module (right) does not
match that of the coupons (lea)
or clean
glass
air

22. iθ
rθ
rφ
i 180φ = °
rdΩ
β
Angular Measurements and Model
scattering azimuth
or
“out-of-plane
scattering angle”
scattering angle
incident angle
differential
solid angle
incident azimuth
sample rotation
Thomas A. Germer, Sensor Science Division, NIST

23. Describing the scaBer from a
delocalized scaBerer
The Bidirectional Scattering Distribution Function
(BSDF) is the fraction of power scattered per unit
projected solid angle.
r
r
0
i r
lim
cos
P
f
P θ→Ω Ω
= [sr-1]
Scattered
power [W]
Incident power [W]
Solid angle [sr]
Polar scattering
angle
Thomas A. Germer, Sensor Science Division, NIST23

24. IEST-STD-CC 1246E Distribution
The cumulative distribution is
the number of particles between
1 µm and D. Shown here are
the distribution density, dN(D)/
dD, and the distribution density
weighted by area and volume.
Notice how much the peak
appears to change. It is hard to
eyeball a peak size in a
distribution from an image.
1 10 100 1000
0.000
0.002
0.004
0.006
0.008
0.010
0.012
0.014
0.016
NormalizedDistributionDensity
D / µm
N(D)
N(D) D2
N(D) D3
Thomas A. Germer, NIST 24

25. -90° -60° -30° 0° 30° 60° 90°
10- 5
10
- 4
10- 3
10
- 2
10- 1
10
0
101
10
2
103
10
4
105
C7
C3D
Control
Model
BTDF/sr-1
θt
The Transmission Data (San José)
Thomas Germer, NIST25

26. Reflectance for collimated incident light
Ωr
With a lot of experimental geometries, the incident light is close
to collimated. Then, the reflectance is
r
r
i i r r i i r r r
r r r r r
r
i i r r
( , ; ) ( , ; , ) cos d
d d sin cos ( , ; , )
f
f
ρ θ φ θ φ θ φ
θ φ θ θ φ φ
θ
θ θ
Ω
Ω
=Ω Ω
=
∫
∫
Program INTEGRATED.MIST calculates the hemispherical
reflectance for any BRDF model. Thomas A. Germer, NIST

27. -90° -60° -30° 0° 30° 60° 90°
10- 4
10
- 3
10
- 2
10- 1
10
0
10
1
102
10
3
10
4
105
2.5°
70°
60°C7
Model
BRDF/sr-1
θs
45°
The Reflection data
Thomas A. Germer, NIST

28. 0° 30° 60° 90°
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
ρ(θi
,2π)
θi
specular reflectance of glass
IEST-STD-CC 1246E
Cleanliness 1000
Predicted Reflectivity vs incidence angle
Thomas A. Germer, NIST28

29. 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
0.026
0.027
0.028
0.029
0.030
0.031
0.032
ρ(θi
,2π)
λ / µm
Calculated Hemispherical Reflectance of Soot Particles
Using IEST-STD-CC 1246E Distribution Cleanliness 1000
Thomas Germer, NIST29

30. Conclusions (Optics of PV soiling)
• Estimating soiling losses using the transmission from glass
coupons may not easily translate to knowledge about
power losses from PV modules.
• The affect on direct transmission is greater than
hemispherical.
• There is a linear correlation between the area covered by
particles and the broadband hemispherical transmittance.
• Soiling produces a higher attenuation at shorter
wavelengths (Ångström turbidity formula) compared to
longer wavelengths.
• The impact of soiling is likely higher on PV materials with
larger bandgap (a-Si, CdTe).
• Soiling Losses are certainly a function of input angle.

31. • EPSRC SUPERGEN SuperSolar Hub’s “International and
industrial engagement fund” for the project “Global investigation
on the spectral effects of soiling losses”
• University of Exeter, UK; National Renewable Energy Laboratory
(NREL), USA; University of Jáen, Spain; Sol Ideas Technology
Development, San José (CA), USA; Tezpur University, Tezpur,
India; Robert Gordon University, Aberdeen, UK; South Valley
University, Qena, Egypt; British University in Egypt, El Sherouk
City, Egypt; BITS Pilani, Dubai Campus, Dubai, UAE; Academy of
Scientific and Innovative Research, Chennai, India; CSIR-Central
Electronics Engineering Research Institute, Chennai, India; Indian
Institute of Technology Madras, Chennai, India.
Acknowledgments
Greg P. Smestad, Sol Ideas Technology Development
smestad@solideas.com — www.solideas.com

32. The International PV Quality Assurance Task Force
(PVQAT, "PV cat”) TG-12. Website: http://www.pvqat.org
Soiled Residential PV System of David Bernal
in Los Angeles (Photo by Greg Smestad)
Greg P. Smestad, Sol Ideas
Technology Development
smestad@solideas.com —
www.solideas.com

33. Background, Supporting Information
Leonardo Micheli, Eduardo F. Fernández, Greg P. Smestad, et
al, "A unique global investigation on the spectral effects of
soiling losses of PV glass substrates: preliminary results".
• Paper & Poster available
• Presented at ieee-pvsc.org/PVSC44/
• Download the materials for free here:
http://www.solideas.com/projects/pvquality/index.html

34. SEM (soiling site: San José, CA)
SEM image of particles from soiling in San José, CA.
Photo: Greg Smestad/NREL, Helio.Moutinho@nrel.gov

35. 6.2 Attenuation of Direct Solar Radiation 109
/ *Οηλ
Figure 6.2.2 Attenuation of direct radia-
tion through a homogeneous atmosphere.
J χ
ηλ
or Beer's law. According to Bouguer's law, the attenuation of light through a
medium is proportional to the distance traversed in the medium and to the
local flux of radiation. This simple law applies in most instances (with the
exception of absorption by gases), whether the energy is absorbed or scat-
tered. Moreover, this law applies to monochromatic radiation only. In terms
of this law,
4A = A)nAexp(-/cAm), (6.2.1)
where
kx is the monochromatic extinction or attenuation coefficient,
m is the optical path length, and
kxm is the monochromatic extinction optical thickness3
(dimensionless).
Bouguer's law - the attenuation
of light through a medium is
proportional to the distance
traversed in the medium.
kλ is the monochromatic
extinction or attenuation
coefficient
m is the optical path length
kλm is the monochromatic
extinction optical thickness
particularly useful when one or more of the atte
not obey the simple Bouguer's law. The monoch
ue to direct radiation can be written as
^ = r L
= ΪΊΚΑ,
transmittance due to a single process i and τλ is due t
ined. The transmittance due to a single process is g
τίλ = exp(-feamI).
w accurately represents attenuation processes of scAn Introduction to Solar Radiation, Muhammad Iqbal, Academic Press, New York, 1983, Chapter 6.
3
5

36. Figure 6,6,1 Variation of the horizontal visibility with a and ß.
I.OP
* 0.8
Id
U
i 0.6
Z
<
O 0.4
o
£T
UJ
<
0 . 2 h
0.0
L^^^^^iSm « 0.05
Γ x
•"
•
•
J ^.-~
-■»
«^* ^«-~·-"""~
l· " ^-^*
^-~—-·"""
ψΛ
--*·*^
1 ^>^ „--' / ?s^ / ' ^.
L x' ' ^ϊ^Γ X / X ^^<^/ / ' ^ ^ ' '/ ' s ^*^ *'
/ ' ~ <ί^^ ^
r/ / ^>^^ / ^^/ / ^^ ' ^*—
Ϊ / /? /^'^Γ / / ' '^^
/ / ' ' %έ^1 / / / Jr / / /^y a = 0.7
ψ /<>' —.-..31 ^* s
st '{ySS *
I'-*' 1 1 1 t 1 1 1 _L
^***^
~Ί
0.4 2 . 00.8 1.2 1.8
WAVELENGTH ( p )
Figure 6,6,2 Aerosol spectral transmittance as a function of a and ßm.
An Introduction to Solar Radiation, Muhammad Iqbal, Academic Press, New York, 1983, Chapter 6.

37. First results: Coupon 5 (2 months of outdoor exposure)
Broadband hemispherical transmittance (300-2500 nm), average particle area,
and percentage of the surface covered by particles, measured at the end of the
data collection. Unsoiled PV glass transmittance is 90.4%
• A linear correlation, with R2 higher than 0.99, is found by comparing the
percentage area covered by particles to the hemispherical transmission
• The broadband hemispherical transmission could be directly obtained from
the covered area, independently of dust type and composition.

38. Quantum Efficiency
(spectral response of PV technologies)