Module Accelerated Stress Testing and Prediction of Field Performance
SNEC 11th (2017) International Photovoltaic Power
Generation Conference & Exhibition
April 19, 2017
William Gambogi, Thomas Felder, Bao-Ling Yu, and T. John Trout, DuPont Photovoltaic Solutions, Wilmington, DE, USA
Hongjie Hu and Zhen Pan, DuPont (China) Research & Development and Management Co., Ltd., Shanghai, P.R.C
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Outline
• Introduction
• Field Examples
• Key Stress Conditions in the Field
• Accelerated Stress Conditions and their Relation to Field
• Conclusions
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Introduction
• IEC qualification test conditions were developed to identify early
life failures due to poor component selection, manufacturing
process and/or module design
• Extended IEC qualification testing (2x or 3x DH, TC, HF) does not
predict long term performance
• New accelerated test methodologies need to be developed to
accurately predict long term performance and adequately assess
changes to materials, module design and processing
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Defects of PV modules in the field are not uncommon, with most of these
defective modules using non field-proven materials. Defects are seen even
among systems in use less than five years. Field study results:
Global Concerns of PV Module Field Failures
22% of global modules have
shown visual defects1
Backsheet defects = 7.5%
PV module defects increased from 19% in
2013 to 48% in 20152
Backsheet defects = Particularly Serious
1 From a global field-module survey including more than 70 global installations, (1,9 MM+ modules at 450+ MW) in NA, EU and AP.
2TUV Rheinland Intersolar 2015, Roundtable Solarpraxis
No defect
78.0%
Cell
11.3%
Backsheet
7.5%
Encapsulant
2.7%
Other
0.5%
DuPont 2016 Field Study TUV Rheinland 2015 Study
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Field Examples of Backsheet Inner Layer Yellowing
Backsheet Yellowing on Sun-side
2 year old fielded module in a desert
region in USA- PVDF-based backsheet9 year old fielded module in Tibet- PET-based
backsheet-100% modules yellowed in 10kW field
Field examples of Backsheet Outer Layer Yellowing27 26.1 25.9 26.4 25.8 25.5 24.9 25.6 26.4 26 26.2 26.8 24.8 25.2 25.8 25.4 25.8 26.3 25.6 25.5 25.6 25.8 25.5 25.7 24.5 25.3 26.1 24.9 25.2 25.8 26.3 25.6 25.2 25.7 25.1 24.626.5 25.8 25.7 26.2 25.5 25.1 24.8 25.2 25.9 25.5 25.7 26.5 23.9 24 25.1 24.3 23.9 25 25.6 25.1 25.8 26.2 25.7 25.3 23.9 24.9 25.8 20.2 23 25.3 26.2 25.3 25 25.6 24.5 24.326 24.8 24.7 24.9 24.8 24.1 24.5 25.2 24.7 26.2 22.9 23.4 24.1 22.8 24.5 26 25.3 26.1 25.3 25.3 23.3 23.1 24 20.6 23 25.2 24.4 24.3 24.8 24.326.1 24.9 24.5 24.8 24.4 24.4 24 24.3 25.1 24.9 24.7 25.9 22.5 22.7 23.7 22.8 23 24.2 25.7 25.2 26 25.5 25.5 25.2 23.9 23.9 24.7 23.5 22.9 24.2 24.6 24.2 24.3 25.8 25 24.624.2 22.7 22.5 22.9 22.4 22.4 21.9 22.4 23.4 22.4 22.6 23.6 19.9 19.9 21.2 20 19.6 21.4 24.3 23.8 24.2 23.6 24 23.5 21.9 22 23.2 22 21.1 22.5 22.8 22.6 23.1 24.9 24.7 24.223.4 20.9 19.8 20.3 19.7 21.1 20.3 19.7 20.8 19.6 20.9 23.1 18.6 16.4 17.8 16.8 17.6 20.8 23.4 20.7 20.4 19.7 20.1 21.2 21.8 20.7 21 20.1 19.4 22 21.9 20.3 21.7 24.7 24.7 24.323.1 20.4 18.6 19.2 18.4 20.6 19.5 18.5 19.4 18.6 20.1 21.9 18.3 14.8 16 14.6 16.6 20.2 22.9 19.7 18.6 17.9 18.4 20.3 21.6 20.1 20 18.1 18.6 21.3 21.4 19.6 20.8 24.8 24.6 24.321.3 19.2 17.6 18.9 17.5 18.8 16.7 16.6 18 16.7 17.8 19.3 15.9 13.7 15 13.4 14.4 17.7 20.7 18.3 17.7 17.3 17.7 18.6 18.9 18.6 19 17.2 17.3 19.7 19.8 18.5 19.7 23.6 24.3 24.223.3 20.9 19.2 19.7 19 20.9 18.4 18.4 19.4 18.4 19.2 21 17.3 13.9 15.4 13.8 15.2 19.2 21.9 19.2 18.2 18 17.9 20.3 20.6 19.5 19.3 17.6 18.1 20.9 20.7 19.1 20.2 24 24.5 24.621 20 20.3 17 18.1 19.9 18.1 18.3 19.1 17.3 19.5 21.1 18.5 15.9 17.1 15.5 16.6 20.3 18.4 16.7 17.6 14.5 15.1 18.3 22 19.7 18.3 17.9 18.8 20.1 20 18.6 17.4 25 24.9 24.918.7 18.3 18.5 16.1 15.5 17.4 15.9 16.4 17.8 16 16.1 18.4 17.3 14.6 17 15 14.5 17.4 16.6 15.4 17.1 14.2 13.1 15.2 20.3 18.4 16.8 17.2 17.4 17.2 19.1 17.8 19.4 23.5 24.1 24.320.4 19.2 18.8 16.5 19.7 18.4 17.4 18.5 19 21.4 18.7 15.6 17.4 16.7 20.6 18.5 16.3 17.7 14.2 18.1 22.2 19.6 17.8 18.6 20.8 20.4 18.8 20.6 24.7 2520.9 20.2 20.2 18.8 18.5 20.6 19.4 19.3 20 19.2 20.2 22.2 19.7 18 19.2 18.5 18.3 21.4 19.3 18.5 19.5 17.5 17.3 19.6 22.8 21.1 19.7 20.2 20.5 21.6 21.2 20 21.2 24.6 24.8 24.621.1 21.5 21.7 21.2 20.4 21.5 21.1 21.6 22.1 21.3 21.4 22.4 20.7 20.9 21.9 21.1 20.6 21.9 20.3 20.9 21.9 20.8 19.8 21.1 22.8 22.6 22.2 22.5 22.6 22.4 22.2 22 22.7 24.3 24.4 24.723.5 23.6 24.3 23.6 23.1 24.4 23.3 23.5 24.4 23.6 23.6 24.6 23.2 23.3 24.1 23.6 23.1 24.5 23.1 23.1 24.2 23.4 22.9 23.9 24.7 23.9 24.5 24.7 24.5 24.3 24.2 23.8 24 25.2 24.6 24.424.4 24.5 25.4 24.8 24.7 25.4 24 24.3 25.5 24.8 24.9 25.5 24.3 24.7 25.5 25.1 24.6 25.4 24 24.3 25.4 24.5 24.4 25.1 25 23.7 24.6 24.9 24.8 24.6 25.2 24.7 24.4 25.4 24.4 24.424.7 24.9 25.8 25.5 25.3 25.7 24.6 24.9 25.6 25.6 25.4 26 24.8 25.2 26.2 25.8 25.7 26.1 24.6 25.1 26 25.4 25.4 26 24.7 24 24.3 24.5 24.6 24 26 25.3 25.1 25.3 24.6 24.125 25.7 26.5 26.7 26.3 26.5 25.4 25.6 26.3 26.3 26.2 26.6 25.1 25.7 26.3 26.3 26.5 26.8 25.6 25.9 27 26.5 26.7 26.9 26 25.4 25.9 26 25.6 24.9 26.6 26.1 25.7 26.1 25.3 25.2A-Down1A-Down1A-Down1A-Down1A-Up6A-Up6A-Up6A-Up6 A-Up5A-Up5A-Up5A-Up5 A-Up4A-Up4A-Up4A-Up4 A-Up3A-Up3A-Up3A-Up3 A-Up2A-Up2A-Up2A-Up2 A-Up1A-Up1A-Up1A-Up1
A-Down6A-Down6A-Down6A-Down6 A-Down5A-Down5A-Down5A-Down5 A-Down4A-Down4A-Down4A-Down4 A-Down3A-Down3A-Down3A-Down3 A-Down2A-Down2A-Down2A-Down2MaxMaxMaxMax HighHighHighHigh27.027.027.027.024.224.224.224.221.421.421.421.418.718.718.718.715.915.915.915.913.113.113.113.1MinMinMinMin LowLowLowLow
b* valueb* valueb* valueb* valueYellowness measured on 12 rooftop modules deployed 15 years in Japan.
Significant yellowing even on the interior of array (b* of 13-20)
Highest yellowing is along edges with highest UV exposure (b* up to 27)
PET-based
BacksheetsDetail of module from the installation
shown on left
Source: Modules provided by AIST; DuPont analysis
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Field Examples of Backsheet Outer Layer Cracking
4 years old module from Spain- 2.3 MW field, 2 module types, large percentage affected-PA backsheet
5 year old module from Italy –large crack tripped inverter- PA backsheet
4 year old module from Canada- four 10 kW
installations, 57% of modules affected- PVDF
backsheet
9 year old module from Canada –large crack in PET-based backsheet
Remains of
backsheet
4 year old module from Canada with
backsheet in advanced state of
degradation- PVDF backsheet
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Key Stress Conditions in the Field
Direct UV/visible light exposure
• New encapsulants transmit more UV to the backsheet inner layer
• Higher UV stability for backsheet needed
High temperature and thermal cycling exposure
• Closed roof mounting
• Localized heating from shading and hot spots
Mechanical stress exposure
• Snow, wind, etc.
• Installation and mount-related stress
• Localized stress from cell edges, busbar ribbons, tabbing wires
Reflected UV/visible light exposure
• UV exposure of the backsheet outer layer from ground (albedo)
• Roof, water, ground cover depending on installation
Mechanical abrasion
• Sandstorms
• Installation
Internal voltage stress
• Potential-induced degradation (PID) and corrosion
• Heat generated from resistive losses
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Accelerated Conditions and their Relation to Field
• Accelerated testing protocols attempt to recreate stress
conditions in the field at higher stress levels including
– higher temperature
– higher humidity
– higher irradiation intensity
– larger temperature range
– faster temperature variation
in order to identify field-related failures and eliminate the cause
• Simultaneous and sequential exposure of these accelerated
stress conditions allows better prediction of synergistic effects
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Module Accelerated Sequential Testing (MAST)
Repeated sequential stress mimics field degradation not detected by single tests and current industry standards.
Sequential exposures are applied to same module.
1000
hours1000
hours
Damp Heat
Thermal
Cycling
Thermal
Cycling
Thermal
Cycling
UVA
200X 200X 200X
1000 hours
1000 hours
1000 hours
UVAUVAUVA
4000 Hours
in a UVA ChamberAmounts to 24 years worth of UV stress
600 Thermal Stress
Cycles
1000 Hours
in a Humidity ChamberAmounts to > 25+ years
worth of stress Mimics thermal stresses
seen in the field
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Sequential Test Results Compared to Field Results
Polyamide backsheet -
cracking
Polyamide backsheet -
cracked, 5 years in Spain
PVDF backsheet -
cracking
PVDF backsheet –
Cracked, 5 years in Canada
MAST Sequential
Accelerated Test
Field Examples
PET backsheet- yellowing
PET backsheet- Yellowing,
15 years in Japan
DH 85C/85%RH
UVA 1.2W/sqm (340nnm), 70C BPT
TC 85oC <=> -40oC per IEC 61215
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MAST Alternative Testing Sequences
UVA & Water SimulationCombines UV and rainfall simulation, common weathering test conditions using commercially
available weatherometer.
Damp Heat & Thermal CyclingCombines two important stress factors in a
shorter test not requiring full size, expensive UV equipment.
DH/TC exposure to assess backsheet cracking in full size modules. ASTM G155 or SAE J1960 protocols in a weatherometer for backsheet coupon or minimodule
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Results of Alternative Sequential Test
Cracks with Polyamide based backsheet
Sequential
Accelerated
Test
Field Examples
Polyamide backsheet - cracked, 5 years in Spain
Cracks with single-sided PVDF backsheet
Cracked and degraded 1s PVDF
backsheet <5 years in the field
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Weathering Test Results
Yellowing in PET-based Backsheets Loss of Mechanical Properties in PET-based Backsheets
15 year rooftop- yellowed PET backsheet 9 year ground mount- cracked PET backsheet
Sequential
Accelerated
Test
Field Examples
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Comparison of Sequential Accelerated Test Results
UVPET /
HPET
UVPET /
PET
Test Sequence Measurement Format Unit(Hydrolitically-
stabilized)(Standard)
Yellowing
Full size
module,
Minimodule or
backsheet
samples
b* 0.8 OK 0.6 0.2 0.6 3.1 3.7 4.1
Mechanical Loss-
Cracking
Full size
module,
Minimodule or
backsheet
samples
observe or %
elongation
loss
OKMicro
CrackingOK Cracking 45 Cracking 50-100 100
2DH1000 2X TC200 Mechanical Loss-
CrackingMini module observe OK OK OK Cracking Cracking OK OK
YellowingMinimodule or
Backsheetb* 0.1 0.2 0 4.7 0.8 6.4
Mechanical Loss-
Elongation
Minimodule or
Backsheet
% Elongation
Loss0 0 0 50 10-100 80-100
Years in Field Years 34 4 4 26 7 9 7 6 19
Yellowing Modules OK OK 3-10 3-20
Cracking Modules OK OK Cracking Cracking Cracking Cracking Cracking
3
Sequential Test Summary
Field
Outdoor Performance
UVA1000-DH1000
2X(TC200-UVA1000)
UVX-water spray-3000
hours
1
Tedlar®PVF
TPTPVDF FEVE PVDF FEVE Nylon
Tedlar®
PVF TPE
Good correlation of sequential test results and field defects
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Conclusions
• Fielded module defects are commonly seen, recent surveys show
increasing levels of defects often due to untested newer
materials
• Backsheet defects include frontside yellowing and airside
yellowing and cracking- yellowing is an indication of polymer
degradation
• Sequential tests have been developed that match field exposure
levels and reveal defects better than single stress exposures
• Three sequential test conditions have been identified for module
testing, examples provided showing good predictability of field
defects
• Shorter test time with better prediction are the goals of current
accelerated test activities
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