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Predicting the Performance of Edge Seal Materials for PV

National Renewable Energy Laboratory – Photovoltaic Module Reliability Workshop NREL-PVMRW Michael Kempe Dhananjay Panchagade Arrelaine Dameron Matthew Reese March 1, 2012 NREL/PR-5200-54582

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Edge Seals - Introduction •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.

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Seal EncapsulantGlass

GlassH2O

w

3

Outline

• Ca film method for moisture ingress determination.

• Finite element modeling of moisture ingress.

• Investigation of failure modes. – Edge Pinch – UV Light – Heat and Humidity

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Test Sample Designed to Mimic Module Edge

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Seal EncapsulantGlass

GlassH2O

w

50 mmGlass (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

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Ca + 2 H20 → Ca(OH)2 + H2

Mirror-Like → Transparent

→

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Moisture Ingress Varies Greatly in Encapsulants

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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

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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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Moisture Ingress Rate Governed by Diffusion

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Moisture Ingress Rate Governed by Diffusion

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( )( )

( )( )CalciumC

XtDCCCC

XtDC P

nmP

nmP

nmP

nmP

nmP

nm −

∆∆

−++++∆∆

= −+−++

,21,1,,1,121

, 41

H2O

H2O

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Permeation Measured at Low RH

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Low RH Measurements Reduce Extrapolation Errors

0102030405060708090

100

0 10 20 30 40 50 60 70 80 90 100

Rela

tive

Hum

idity

(%

)

Temperature (⁰C)

500-550450-500400-450350-400300-350250-300200-250150-200100-15050-1000-50

Hours/Year

Bangkok Thailand RH and Temperature for outside of a Glass/Glass Rack Mounted Module

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Edge Seal Modeling • The use of fillers, pigments, and desiccants makes the

determination of modeling parameters much more difficult.

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%100%RHeSS kT

Ea

om

s

−

=

−

= kTEa

oeff

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 that immobilizes the water. OHR

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Getting the Modeling Parameters

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OHR2

Measured by weighing samples before humidity exposure, after humidity exposure, and after drying.

So EaS ,Measured by exposing to controlled humidity then drying in a TGA to determine moisture loss. Curvature of K vs %RH is determined by the ratio of S to RH2O

Do EaD , Estimate from other parameters and fit to Ca data. Specifically the difference between 45 and 85⁰C curves.

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0

0.02

0.04

0.06

0.08

0.1

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

Ingr

ess D

epth

(cm

)

Time (months)

Insulated Back, Glass/Polymer

Close Roof, Glass/Glass

Open Rack, glass/glass

Open Rack, Glass/Polymer

Ingress Estimated Using Finite Element Analysis

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Denver Colorado

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

tKX =

15

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0 2 4 6 8 10

Ingr

ess D

epth

(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

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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

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

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Edge Seal Failure Modes and Stresses

• Heat. • Humidity (85C/85% RH). • Adhesion to edge delete region. • UV Light. • Edge Pinch

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Laser Edge Delete Did Not Increase Ingress

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0.1

1

10 100 1000 10000

Dist

ance

(cm

)

Time (h)

Edge Deleted Sides

Non-Deleted Sides

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Edge-Seals May Have Edge Pinch

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Edge Pinch (lamination pressure cause the glass to bend around the perimeter)

Large tensile stress in polymer

Idea Edge Profile (no bend in glass at the module perimeter)

Very little stress in polymer

Glass Encapsulant/Edge Seal Glass

Glass Encapsulant/Edge Seal Glass

Schematic side views of module edge

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Edge Seal Test Specimen

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0.30 mm of edge pinch

0.2 mm thick polymer

Schematic side view of test sample Glass (3.18 mm) Ca film (100 nm) Edge Seal Glass (3.18 mm)

Photographic top view 0.5 mm thick polymer

100 mm

50 m

m

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Edge Seals With Pinch Resist 85°C and 85% RH

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No Exposure 674 h 85°C/85% RH 170 h 85°C/85% RH Only small signs of minor delamination on ends exposed to tensile stress.

Edge pinch is 0.31±0.01 mm for all exposures.

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UV Light Can Delaminate Edge Seals With Pinch

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No Exposure 0.32±0.01 mm pinch

621 h 60°C/60% RH/2.5 UV Suns

0.02±0.01 mm pinch

165 h 60°C/60% RH/ 2.5 UV Suns

0.02±0.01 mm pinch

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UV Light Can Delaminate Edge Seals With Pinch

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No Exposure 0.32±0.01 mm pinch

621 h 60°C/60% RH/2.5 UV Suns

0.02±0.01 mm pinch

165 h 60°C/60% RH/ 2.5 UV Suns

0.02±0.01 mm pinch Light exposure on non-Ca film backside.

Very significant delamination on ends exposed to tensile stress.

Polymer pulled away from edge

Delamination on front side

Intact Ca Film

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UV Light Alone is Much Less Damaging

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50 m

m

50 m

m

Unexposed 1962 h 60°C/60%RH/2.5 UV suns

PIB #2

PIB #1

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IEC TC82 WG2 Edge Seal Standards Development

• Under IEC TC82 WG2 a group has formed to work on developing standard test methods for testing PV packaging materials. – Encapsulants – Back Sheets and Front Sheets – Adhesives – Edge Seals/Pottants

• If you would like to help with the edge seal standards development, please contact me.

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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 will 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.

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Conclusions

• An edge seal width of 1 cm can be capable of keeping moisture out for 20 years in almost any climate.

• Delamination is the main concern for edge seal performance.

• Edge Seals should be assembled without edge pinch to ensure good adhesion.

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Acknowledgements

Sarah Kurtz John Wohlgemuth

David Miller Joshua Martin

This work was supported by the U.S.

Department of Energy under Contract No. DE-AC36-08-GO28308 with the National Renewable Energy Laboratory.