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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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2Copyright © DuPont 2017. All rights reserved.

Outline

• Introduction

• Field Examples

• Key Stress Conditions in the Field

• Accelerated Stress Conditions and their Relation to Field

• Conclusions


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


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


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


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


Key Stress Conditions in the Field

PID and corrosion


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


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


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


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


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


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


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


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


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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