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.

“The Durability of Polymeric Encapsulation 

Materials for Concentrating Photovoltaic Systems”

The 7th International 

Conference on Concentrating 

Photovoltaic Systems (CPV-7)

(Las Vegas)

2011/04/04 (Monday)

6:00-6:15 pm

NREL/PR-5200-50160

David C. Miller1*, Matt Muller1, Michael D. Kempe1, Kenji Araki2, 

Cheryl E. Kennedy1, and Sarah R. Kurtz1

1. National Renewable Energy Laboratory (NREL), 1617 Cole Blvd., Golden, CO, USA 80401

2. Daido Steel Co., Ltd. 2-30 Daido-cho, Minami, Nagoya 457-8545, Japan

* David.Miller@nrel.gov
Innovation for Our Energy Future

Motivation for the NREL Field Study

corrosion prevention, optical coupling : CPV systems typically use 

encapsulation to adhere optical component(s) or cover glass to the cell 

2

cross-sectional schematic of the components near the cell in CPV systems (not to scale)

encapsulation durability (25 year field deployment) is unknown: 

•identify field failure modes 

•gain insight related to failure mechanisms

•distinguish between material types

•identify materials for future study (HALT & qualification tests)
Innovation for Our Energy Future

Details of the Experiment (Specimens & Apparatus)

3

hydrocarbons

(representative types){

silicones

(representative grades)

{

test coupons are mounted in a modified CPV module product on a 2-axis tracker in Golden, CO

quartz/encapsulation/quartz

secondary optic (homogenizer)

passive cooling;

no cell

domed Fresnel lens
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Details of the Experiment (Measurands & Schedule)

4

“Continuous” measurements:

ambient conditions (irradiance, temperature, wind...)

fixture temperature (via thermocouple)

Periodic measurements:

transmittance (T[λ], hemispherical & direct) 

mass

appearance (photograph)

⇒from T[λ], calculate: yellowness index (D65 source,1964 10° observer), haze, λcut-on ...

fluorescence & RAMAN spectroscopy

Final measurements:

FTIR, NMR

TGA, DSC (polymer physics)

Test schedule:

0, 1, 2, 4, 6, 12, 18, 24, 30, 36 ... months
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The Field Conditions (Irradiance & Ambient Temperature)

Golden, CO: Steppe climate, elevation 1.79 km, 39.740 °N latitude

5

∫ = EdtW

T: Ambient shown.

Module’s back and 

inside monitored too.

raw field data for irradiance and temperature

W: Seasonal effects (winter cloudiness and latitude-related) observed 

during first 68 days 3.2 , int

,

= −

−

UV erwA

UV summerA

W

W 3.4 , int

,

= −

−

UV erwB

UV summerB

W

W

D.C. Miller, M.D. Kempe, C.E. Kennedy, S.R. Kurtz, Optical Engineering, 50 (1), 2010, 013003.

W: Consider seasonal, optical design, and material’s action spectrum 
Innovation for Our Energy Future

The Field Conditions (Specimen Temperature)

•Specimen temperature proportional to optical (IR) absorptance

(thermal management “system”: conduction to the frame.)

6

specimen and lens temperature, determined using optical thermography (inset) 

• ~40°C temperature rise observed. Tmax 70-80°C in summer.

Audience: does this compare to temperatures within your CPV module? 

IR thermography
Innovation for Our Energy Future

Results of Discovery Experiments

(The Homogenizer)

7

EVA: without homogenizer, rapid discoloration ⇒ combustion (smoke) 

D.C. Miller, S.R. Kurtz, Solar Energy Materials and Solar Cells, 2011.

optical images of EVA in (a) & (b), and PDMS in (c). 

inset shows: voided center, char, cracked coverglass, discoloration, delamination

silicone: without homogenizer ⇒ combustion 

•Likely motivated by local hot spots (101 to 103 · Cg)
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Results of Discovery Experiments

The Effect of Contamination)

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•Intentionally introduce soil, Al, PE, or bubbles into EVA or silicone

EVA: soil, Al, PE motivated localized discoloration ⇒ combustion

silicone: soil, Al ⇒ localized cracking. (no primer present)

• elapsed time: minutes – days/weeks

• bubbles: no failure @Cg=500, despite measured T[λ] reduction 

time sequence: optical images of test specimens
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•PVB was the first material to 

demonstrate thermal runaway 

mediated failure

•The radius of the affected region 

was seen to slowly grow during the 

cold winter months

9

Results of the Formal Experiment

(Hydrocarbon Specimens)

time sequence: optical images of test specimen 

optical images of test specimen at:

(a) 6 months and (b) 10 months
10 Innovation for Our Energy Future

Results of the Formal Experiment

(Hydrocarbon Specimens)

•Transmittance & YI not significantly 

affected, despite impending failure

•A characteristic with predictive 

capability is preferred!!!

time sequence: transmittance of the PVB specimen

optical fluorescence spectrum of PVB, for λI

= 280 nm

•Optical & Raman spectroscopy 

clearly indicate fluorescence

•These techniques may help 

understand the degradation 

mechanism (e.g., chromophores) 
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•Observations of silicone specimens include: (a) 

densification, (b) cracking, and (c) haze formation

11

Results of the Formal Experiment

(Silicone Specimens)

optical images of silicone specimens, including those obtained using (a) cross-polarization or (c) back-lighting

No mass loss with time for the (5) densified

specimens ⇒ likely occurred during molding

Crack advancement occurred during cold weather 

periods only ⇒ likely motivated by CTE misfit

Haze formation is attributed to one material’s 

unique formulation
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•Densification is not delamination

•Densification does scatter direct light

12

Results of the Formal Experiment

(Densified Silicone Specimens)

Problematic for CPV?

•Current limited condition (blue light) •Optical attenuation (less power)

⇒May not be significant in thin bond layers 
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Summary & Conclusions

Discovery experiments:

•Quickly confirmed the importance of an optical homogenizer

•Al, soil, polymeric contamination ⇒ T runaway & combustion of EVA

•Al, soil contamination ⇒ cracking of silicone

13

Formal experiment:

•17 of 25 specimens not discussed today! •3 of 25 specimens “failed”.

PVB: localized discoloration ⇒ thermal runaway ⇒ combustion

Fluorescence & Raman spectroscopy may provide prediction & diagnosis

Silicone: densification, cracking, haze-formation

Densification affects the direct transmittance

I am looking for your feedback about CPV module temperature 

conditions and observed failure modes

Field study of the durability of polymeric encapsulation materials for CPV
14 Innovation for Our Energy Future

Acknowledgements

•NREL: Dr. Keith Emery, Dr. Daryl Myers, Dr. John Pern, Matt Beach, 

Christa Loux, Tom Moricone, Marc Oddo, Bryan Price, Kent Terwilliger, 

Robert Tirawat

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

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

Energy Laboratory.

Paper: “The Durability of Polymeric Encapsulation Materials for 

Concentrating Photovoltaic Systems”, in preparation.