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Durability of Poly (Methyl Methacrylate) Lenses 

Used in Concentrating Photovoltaics

3rd International Workshop on 

Concentrating Photovoltaic Power 

Plants 

Bremerhaven, Germany

2010/10/21 (Thurs)

17:00 – 17:20 (Germany)

9:00-9:20 (US)

*Presenter:

David.Miller@nrel.gov

NREL/PR-5200-49901

David Miller*, Lynn Gedvilas, Bobby To, Cheryl Kennedy, and Sarah Kurtz
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Purpose- and Details- of Screen Test at NREL

•Literature ⇒ studies initiated ≥20 years ago

•Goal here: characterize the durability of a broad range of 

contemporary specimens subject to indoor HAST

•Test instrument: ATLAS Ci4000 Weather-ometer

(Xenon-arc lamp @ 2.5x UV suns. Chamber @ 60°C/60%RH); *1 exception

2 5/8” 

1 3/4 3x ” 

Test specimens (4.4 x 6.7 cm2)

DESCRIPTION SPECIMEN TYPES

stock (unpatterned) 11

linear focus lens 1

spot focus lens 8

veteran (fielded) lens 3

•Veteran specimens on tracker in desert

site, seldom cleaned (8, 22, 27 yrs)
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Details of NREL Screen Test

•Measurands: 

Periodic

optical appearance (photograph) 

optical transmittance (hemispherical)

mass

contact angle (sessile drop, H2O on 1st surface)

“End of life”

haze (from direct transmittance)

prism facet geometry (lenses: section then SEM)

surface morphology (SEM or AFM)

indentation (Vicker’s hardness, toughness)

rheometry (E’, E’’, Tg)

XPS or ESCA (surface chemistry, before & after cleaning)

•Test schedule:

0, 1, 2, 4, 6, 12, 18, 24, 30, 36 months

≥8 acceleration factor (irradiance and 24 hour operation)

pull 1 of replicates every 12 months
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Transmittance is Reduced by Aging

Measured transmittance at 0, 6 months for best and worst sheet stock specimens 4

• “Optical Durability” = transmittance as f[t] 

•Lambda 900 (Perkin-Elmer) spectrophotometer (w/ integrating-sphere)

T↑ rounded fcut-on

spectral bandwidth

“typical” aging

#11 (most-affected): 

no UV absorber, other additives

#8 (increased T):

loss of additive/ 

chromophore production #1 (least-affected): 
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Effect of H2O on Transmittance

•Verified in direct- & hemispherical-T measurements

•H2O solubility (≤2.5 wt.% ) primarily affects unharvested IR λ’s

•UV & vis (PV) unaffected; weathering results are not H2O absorption!

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•Specimens maintained for >1 month in dry box (≤0.1 ppm H2O) or DI

•Traditional λ’s to estimate H2O dissolved in polymers: 1.9, 2.7 μm

Kapur, et. al., Proc. IEEE PVSC, 001210-001214 (2009).

Swonke and Auer, Proc. SPIE, 7412, 74120A, (2009).

Measured transmittance for wet & dry stock specimens (unaged) 

w=7067, 5244, 4630, 3708, 3384 cm-1
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Transmittance is Reduced by Soiling

•Contamination absorbs, scatters, and back-reflects light

•Effect most significant as λ↓ (Mie scattering: 0.6/n<π∅/λ<5)

Measured transmittance, as-received and after cleaning for 22, 8 year old Fresnel lens specimens

optical

durability

T↓

6-29%

φPV↓ 15, 2, 1% 

φUV↓ 29, 6, 2%

*Remember also:

•Direct light (CPV) more severely affected than hemispherical (FP-PV)

•Optical durability & soiling affect color balance, e.g. top-cell limited I
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Yellowness Index (YI) Distinguishes 

Formulations, Aging Methods

•YI (ASTM E313 & E308 [D65 source, 10° observer]) quickly quantifies degradation

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YI determined from raw hemispherical transmittance measurements (proportional to visual appearance). *Outdoor YI plotted for time only. 

•Range of results, depending on material formulation. Not all the same!

•Damage rate indoors/outdoors = 220x for same (unstabilized) material

•Suggests synergy between UV, temperature, and/or humidity
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Volatile Species (Mass Loss) Resulting From Aging

•Non-linear asymptotic trend (as great as 1.27%)

•Process of outward diffusion? (Degradation products and/or additives).

•Literature: 4.4% mass loss for chain scission (70 hours @ 300 nm)

•If photolysis, then volatile content proportional to cumulative damage

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mass loss determined , starting after 1 month indoor aging.

Abouelezz and Waters, Studies on the Photodegradation of Poly(Methyl Methacrylate), NBSIR 78-1463, (1978), 1-55.
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CHARACTERISTIC UNIT 0 1 2 4 6

PV photon flux γ ⋅m-2⋅s

-1 723±3 724±3 721±4 725±3 719±6

UV energy flux W⋅m-2 14±8 17±8 17±7 17±8 16±9

IR energy flux W⋅m-2 7±0 7±0 7±0 7±0 7±0

YI unitless -1.2±0.1 0.2±1.4 0.6±1.4 1.2±1.7 3.3±2.9

λcut-on nm 363±36 369±28 370±28 364±33 379±24

∆m, MASS LOSS % N/A 0.000±0.000 0.408±0.048 0.646±0.059 0.705±0.074

Θ, CONTACT ANGLE degrees N/A 66±1 60±1 58±2 61±4

t, TIME {months}

The Expected Correlation Between YI, and UV/ PV Flux 

• Table summarizes the 9 standard transmitting stock specimens

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Contact Angle is Reduced by Aging

•Θ relates to accumulation and retention of particulate matter

•Θ: 66 → 58 → 43° (for unaged → aged → veteran)

•More easily cleaned, but more rapid to accumulate soil

•Partially restored by cleaning at 6 months ... surface accumulation?

eventually...
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Mechanical Damage from Aging: Cracks and Haze

Cracks:

•Radial cracks in domed spot-focus lens (85°/85%RH)

•Cracks could motivate failure on impact; could grow via fatigue

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Optical photograph , where arrows 

indicate cracks (3/3 specimens)

Optical photograph , where contrast 

indicates surface damage 

(each, 3/3 specimens)

Haze:

•As-received veteran specimens demonstrate haze

•Surface erosion or microcracks detrimental to direct solar flux
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Surface Roughness Identifies the Causes of Haze

•90x90 & 20x20 μm2 topography atomic force microscopy (AFM) scans

•Largest features (width/depth): 3850/340, 1540/88, 2350/72 nm

•Veteran specimens: 

(a) abrasion/erosion, (b) scratches from cleaning,

(c) embedded material (Si) in set-B

•Erosion much less significant for mounting ≥2m (less airborne PM).

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Roughness of unaged stock specimen relative to as-received (but cleaned) veteran specimens (27, 8, 8 years service)

Thornton, “The Effect of Sandstorms on PV Arrays and Components”, Proc. Solar, 81-85, (1992).
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A Summary of Photolysis From the Literature

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(after Aboulezz and Waters, “Studies on the Photodegradation of Poly(Methyl Methacrylate)”, 1978)

•Dominant mechanism suggested in literature & suspected here

•Random main chain scission by UV (photolysis) ⇒ Mw therefore Tg reduced

•Tg reduced ↓~5°C after 18 years outdoors 

L.G. Rainhart & W. P Schimmel, SAND 74-0241, 1974.

• Mw↓ likewise affects mechanical durability: KIC↓⇒σf↓ ... ∂a/∂N↑
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Fourier Transform Infrared (FTIR) Spectroscopy 

*May identify changes in the molecular structure, but not vulnerability 

•Magnitude major peaks reduced, consistent with chain scission

•All major peaks related to ester (C=O), (C-O), or methylene (-CH2-)

•Technique not recommended for regular diagnosis of PMMA

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Surface spectra of unaged and aged specimens (set#11) obtained using FTIR-ATR relative to reference spectrum for PMA monomer
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The Progress Report ... (Summary)

•Transmittance:

•UV cut-on f , spectral bandwidth compromised by aging

•Notable IR absorption @ 1415, 1907, 2697 nm from H2O

•Soiling⇒trends as in optical durability. (Blue) current limited?

•YI: This figure of merit may imply a synergy between UV, T, and/or RH

•Mass loss (on order of x%) may result from photolysis 

•Θ decreases with time. Formation of water soluble surface layer?

•Erosion, scratching of surface quantified for veteran specimens

•FTIR: changes after 6 months consistent with chain scission... will verify

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*Future: Work with a SOG lens? Your participation solicited for round 2!

“Theme”: from change in characteristics examined, photolysis suspected from the 

literature ... will verify in future with “end of life” (12 month) measurements
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Acknowledgements

•NREL: Mike Kempe, Daryl Myers, John Pern, Matt Beach, Christa 

Loux, Marc Oddo, Bryan Price, Kent Terwilliger , Robert Tirawat

•Others: Ralf Leutz, Hans Philipp Annen (Concentrator Optics Inc.), 

John Wiedner, Michael Longyear, Scott Steele 

(Arizona Public Service, S.T.A.R. facility)

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

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

Renewable Energy Laboratory.

See also:

This study: D.C. Miller, L.M. Gedvilas, B. To , C.E. Kennedy, and S.R. Kurtz, “Durability of Poly(Methyl Methacrylate) Lenses 

Used in Concentrating Photovoltaics”, Proc. SPIE, 2010, 7773-02.

Literature review: D.C. Miller and S.R. Kurtz, “Durability of Fresnel Lenses: A Review Specific to the Concentrating 

Photovoltaic Application”, submitted to Solar Energy Materials and Solar Cells
Innovation for Our Energy Future

Questions

xxx

•Have you specifically studied the affects of cleaning (abrasion)?

•What about acid rain (the chemsitry of outdoor rain)

•What can be learned/said about the 

(chemistry/additiives/mW) most durable formulations

•Is there a change in the refractive index?

•XXX

•XXX

•XXX

•XXX

•XXX

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Thermal Decomposition

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•Occurs readily for T>200°C

•Synergistic effect w/ irradiation (UV)⇒occurs at T<200°C

•Many classic studies of Ea vs. heating rate, atmopshere

•O2 suppresses decomposition

(after Aboulezz and Waters, “Studies on the Photodegradation of Poly(Methyl Methacrylate)”, 1978)

•Unzipping of main chain in methyl methacrylate (monomer)

•Autocatalytic process (zip length on order of 1000)

•Significant weight loss (vs. minimal for chain scission)