NREL is a national laboratory of the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

Calcium Based Test Method for Evaluation 

of Photovoltaic Edge-Seal Materials

Michael Kempe, 

Arrelaine Dameron, 

Matthew Reese

2011 NREL PV Module 

Reliability Workshop

February 16, 2011

NREL/PR-5200-50839
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Experimental Objectives

Many PV technologies are sensitive to moisture. Even with 

impermeable front- and back-sheets, moisture can 

penetrate from the sides. Edge seals are incorporated 

around the perimeter to prevent this ingress.

Here we use a Ca-based method to evaluate the moisture 

ingress time for edge seal materials.

Then we use this data to model the performance when 

deployed outdoors.

2

Seal Encapsulant

Glass

Glass

H2O

w
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Test Sample Designed to Mimic Module Edge

3

Seal Encapsulant

Glass

Glass

H2O

w

50 mm

Glass (3.18 mm)

Polymer Film (~0.5 mm)

Ca (100 nm)

Glass (3.18 mm)

H2O

Ca + 2 H20 → Ca(OH)2 + H2

Module Edge

Test Sample
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Oxidation of Ca Indicates Moisture Ingress

4

Ca + 2 H20 → Ca(OH)2 + H2

Mirror-Like → Transparent

→
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Moisture Ingress Varies Greatly in Encapsulants

5

PDMS 

Ionomer

#1

Exposed to 

85°C and 

85% RH 

0 h 1.5 h 3 h 4.5 h

0 h 67 h 240 h 652 h

50 mm

50 mm
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Polyisobutylene Edge Seals Slow Ingress

6

PIB #1 

PIB #2

Exposed to 

85°C and 

85% RH 

0 h 163 h 652 h 1230 h

0 h 1490 h 2780 h 4664 h

50 mm

50 mm

Delaminations

Reactions
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0

0.5

1

1.5

2

2.5

0 5 10 15 20 25 30 35

Moisture Ingress Distance (cm)

Time (h)

EVA Exposed to 85C/85% RH

Moisture Ingress Rate Governed by Diffusion

7
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0.01

0.1

1

10

0.1 1 10 100 1000 10000 100000

Distance (cm)

850C/85% RH Exposure Time (h)

PDMS 

(0.80)

EVA 

(0.38)

PVB #1 

(0.25)

TPU #1 

(0.23)

Ionomer 

#1 (0.067)

Epoxy #1 

(0.058)

PIB #1 

(0.024)

PIB #2 

(0.018)

Material (K [cm/h1/2])

Typical Edge 

Seal Width

Ingress Rates Vary Greatly (850C)

8

= tKX
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0.01

0.1

1

10

0.1 1 10 100 1000 10000 100000

Distance (cm)

450C/85% RH Exposure Time (h)

PDMS #2 

(0.50)

EVA 

(0.16)

PVB #1 

(0.25)

TPU 

(0.11)

Ionomer 

(0.015)

Epoxy #1 

(0.012)

Epoxy #2 

(0.013)

PIB #1 

(0.0083)

PIB #2 

(0.004)

Material 

(K [cm/h1/2])

Typical Edge 

Seal Width

Ingress Rates Vary Greatly (450C)

9

= tKX
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EVA Water Permeation Parameters
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Moisture Ingress Rate Governed by Diffusion

11

( ) 



 



 



 





−=

Dt

x CtxC erf eq 2

1,

1-D Semi-Infinite Solid H2O
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Moisture Ingress Rate Governed by Diffusion

12

( ) 



 



 



 





−=

Dt

x CtxC erf eq 2

1,

1-D Semi-Infinite Solid

( ) ( ) ( ) ( ) ( )

∑ ∑

∞

=















 +

∞ −

=















 + −















 +

+ 



 



 +

+ = −

0

12

0

12

2

2

2 2

2

2 2

12 sin

12

12 1 sin

12

,, 116

n

l

tnD

m y

l

tmD

eq x

eq x y

e

l

Yn

n

e

l

Xm

C m

tYXCC π π

π π

π

2-D Ingress, Infinite rectangular bar

H2O

H2O
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Moisture Ingress Rate Governed by Diffusion

13

( ) 



 



 



 





−=

Dt

x CtxC erf eq 2

1,

1-D Semi-Infinite Solid

( ) ( ) ( ) ( ) ( )

∑ ∑

∞

=















 +

∞ −

=















 + −















 +

+ 



 



 +

+ = −

0

12

0

12

2

2

2 2

2

2 2

12 sin

12

12 1 sin

12

,, 116

n

l

tnD

m y

l

tmD

eq x

eq x y

e

l

Yn

n

e

l

Xm

C m

tYXCC π π

π π

π

2-D Ingress, Infinite rectangular bar

( ) ( ) ( ) C ( ) Calcium

X

tD CCCC

X

tD C P

nm

P

nm

P

nm

P

nm

P

nm

P

nm  −



 





∆

∆

−++++

∆

∆

=

+ − + −

+

2 ,1 1,1,,1 2 ,

1

, 41

2-D Finite Element Analysis with Ca-H20 Reaction

H2O

H2O
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D and S allow for Modeling of Performance

14

0

0.5

1

1.5

2

2.5

3

3.5

4

0 50 100 150 200 250 300

Distance (cm)

Time (h)

EVA #2

1-D Infinte 

Calculation

2-D Finite 

Calculation

2D FEA

0

1

2

3

4

5

6

7

8

9

0 20 40 60

Distance (cm)

Time (h)

GE RTV615 #1

1-D Infinte 

Calculation

2-D Finite 

Calculation

2D FEA 

Calculation

EVA Silicone
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Edge Seal Modeling

The use of fillers, pigments, and desiccants makes the 

determination of modeling parameters much more difficult.

15

%100

RH% eSS kT

Ea

om

s 



 





−

=





 



 −

=

kT

Ea

eff o

D

eDD

Mobile phase water absorption is split between the 

polymer matrix and the mineral components.

Assume linearity with relative humidity.

Mobile phase water diffusivity is an effective 

diffusivity. This accounts for a rapid equilibration 

between adsorbed and dissolved water.

A non-reversible reaction with water. R

2OH
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Getting the Modeling Parameters

16

R

2OH

Measured by weighing samples before humidity 

exposure, after humidity exposure, and after drying.

EaSo S ,

In progress: Measured by exposing to controlled 

humidity then drying in a TGA to determine moisture 

loss. Currently assuming EaS of 16 KJ/mol.

D EaDo ,

Estimate from other parameters and fit to Ca data.

0.01

0.1

1

10

10 100 1000 10000

Distance (cm)

Exposure Time (h)

Typical Edge 

Seal Width 850C/85%RH

450C/85%RH
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Ingress Estimated Using Finite Element Analysis

17

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.1

0 1 2 3 4 5 6 7 8 9 10 11 12

Ingress Depth (cm)

Time (month)

Insulated Back, Glass/Polymer

Close Roof, Glass/Glass

Open Rack, glass/glass

Open Rack, Glass/Polymer

Denver Colorado

Used TMY3 Data and Temperature estimates similar to King et al, and Kurtz et al.

= tKX
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0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

0 5 10 15 20 25 30

Ingress Depth (cm)

Time (years)

Insulated Back, Glass/Polymer

Close Roof, Glass/Glass

Open Rack, glass/glass

Open Rack, Glass/Polymer

Square Root Relation Works to Longer Times

18

Denver Colorado

Used TMY3 Data and Temperature estimates similar to King et al, and Kurtz et al.

= tKX
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Preliminary Results for Different Climates

19

A sensitivity analysis gave about ±15% on K and Width, and ±30% on 20 yr equivalent time.

9.22

56

7.77

16

0.0327

0.00076 0.32 316 0.7

0.00078 0.33 330 0.8

0.00087 0.36 408 0.9

0.00092 0.38 454 1.0

0.00081 0.34 353 0.8

0.00082 0.34 364 0.8

0.00089 0.37 432 1.0

0.00093 0.39 471 1.1

0.00099 0.41 525 1.2

0.00101 0.42 551 1.3

0.00114 0.48 705 1.6

0.00121 0.51 795 1.8

0.00119 0.50 767 1.8

0.00122 0.51 805 1.9

0.00138 0.58 1,029 2.4

0.00146 0.61 1,161 2.7

0.00168 0.70 1,520 3.5

0.00171 0.72 1,580 3.7

0.00187 0.78 1,889 4.4

0.00195 0.82 2,062 4.8

0.00198 0.83 2,115 4.9

0.00201 0.84 2,192 5.1

0.00220 0.92 2,625 6.1

0.00230 0.96 2,867 6.6

20 yr equivalent 

at 850C/85% RH

20 yr equivalent 

at 450C/85% RH

 (cm/h (cm) (h) (years) 1/2)

Open Rack, glass/glass

Close Roof, Glass/Glass

Open Rack, Glass/Polymer

Insulated Back, Glass/Polymer

Do (cm2

/s)=

EaD (kJ/mol)=

So (g/cm3)=

EaS (kJ/mol)=

Reactive Ca absorption (g/cm3)=

Open Rack, Glass/Polymer

Insulated Back, Glass/Polymer

Open Rack, glass/glass

Close Roof, Glass/Glass

Open Rack, Glass/Polymer

Insulated Back, Glass/Polymer

Open Rack, glass/glass

Close Roof, Glass/Glass

Open Rack, Glass/Polymer

Insulated Back, Glass/Polymer

Open Rack, glass/glass

Close Roof, Glass/Glass

MUNICH

PHOENIX SKY HARBOR 

INTL AP

MIAMI INTL AP

DENVER/CENTENNIAL 

[GOLDEN - NREL]

Open Rack, glass/glass

Close Roof, Glass/Glass

Open Rack, Glass/Polymer

Insulated Back, Glass/Polymer

Open Rack, glass/glass

Close Roof, Glass/Glass

RIYADH

BANGKOK

Open Rack, Glass/Polymer

Insulated Back, Glass/Polymer

K 

20 y required 

width
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What edge seal parameters are important?

1. Adhesion is the most important parameter.

a) Must be maintained after environmental exposure.

b) Residual stress in glass may affect adhesion.

c) Material may expand as it absorbs water.

d) Good surface preparation is necessary.

2. Breakthrough time is the next most important.

a) The 12 mm edge delete perimeter should be wide enough to 

keep moisture out.

3. Module mounting configuration is not important.

a) Hotter installations tend to dry out the module partially 

countering the effects of increased diffusivity.

4. The steady state transmission is less important. 

a) The amount of permeate is very low.

b) Ideally one will not reach steady state.

20
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Conclusions:

1. An edge seal width of 1 cm can be capable of 

keeping moisture out for 20 years in almost any 

climate.

2. The mounting configuration is not a significant factor 

for determining the diffusion based lifetime of an 

edge seal.

3. The climate a module is deployed in very significantly 

impacts edge seal performance.

4. Exposure to between 500 h and 3000 h of 85C and 

85% RH will equate to about 20 years of moisture 

ingress through an edge seal.

21