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Improved Accelerated Test Methods for

Solar Panel Service Life Prediction

Chiou-Fu Wang, Ph.D.

DuPont Photovoltaic Fluoromaterials (PVFM)

SECI Workshop

June 25, 2015

Copyright © DuPont 2015. All rights reserved

Environmental Stresses in the Field

• Photovoltaic modules are exposed to a wide range of stress conditions

• Stresses operate on the module simultaneously and sequentially;

synergistic effects are observed


• 41% inspected modules exhibited some visual defect

• Findings consistent with BP and SunPower data

DuPont Collaborated with Downstream Stakeholders to Assess Solar

Systems, Modules, and Materials in the Service Environment

Subcomponent degradation mode pareto

based on visual inspection

Encapsulant, 4%

Backsheet, 9%

Cell, 24%

Glass, 2%

Mismatch, 1%

Others, 1% Defect Not

Detected, 59%

• Inspected >60 global installations (>200 MW & 1.5 million modules) in NA, EU, & AP ranging from 0-30 years installed

• Data includes c-Si modules

from > 45 module manufacturers

Accepted for presentation at IEEE PVSC (New Orleans, 2015, A. Bradley et al)


EVA

EVA

Glass

Cells

Backsheet

Inner layer

Middle Layer

Outer Layer

Encapsulant Side

Air side – this side is exposed to the climate

MOST CRITICAL

Backsheet is Critical to Protect a Solar Panel

• Durable and reliable backsheet materials can enable longer

lifetime, maximize power output and reduce risk


Risks Associated with Using Unproven Backsheet Materials

• Backsheet degradation increases potential safety risks such as loss of electrical protection • Yellowing is a one sign of polymer degradation and an early indicator of performance issues • Cracking typically is associated with yellowing


Outer Layer Backsheet Cracking and Yellowing

Polyester-based backsheet

• Years in service: 4 years

• Location: Spain

• 5,000 modules affected


Yellowing of PET and Tedlar® PVF-Based Backsheets (Field Data)

Field test: Measurement of backsheet color change after modules have been exposed for years in the field

Results: Small color change for Tedlar®-based backsheets

Significantly larger changes in b* for PET backsheets

Source: 227 modules with Tedlar® and 4 modules with PET-based backsheet from various locations and manufacturers 7

Tedlar® shows exceptional color stability;

PET color change indicates polymer damage & degradation.


Aged Modules with PET-Based Backsheet Have Visible

Degradation and Significantly Lower Power Output

1. Standard PET 2. Standard PET 3. Tedlar® PVF film

Defects

Years in field 10 years 12 years 11 years

Cell type Mono Poly Mono

Wet leakage Pass Fail Pass

b* (Yellowing) 9.0 14.2 2.7

Name plate rating 143 W 125 W 100 W

IV measurement 77 W 105 W 91 W

% power loss 46% 16% 9%

Annualized power loss

4.6% 1.3% 0.8%

Crack

Yellowing

8


Shortcomings of Current IEC Qualification Tests

• Current IEC qualification standards were designed to identify early

failures due to module design; does not predict long-term durability

• IEC qualification standards do not adequately address durability of

materials to UV exposure and weathering and synergistic effects of

multiple stress

• UV and weathering tests are not applied to modules due to the

equipment challenges


UV Dosage(kWh/m2) Desert Tropic Temperate

Annual UV (kWh/m2) 91.7 78.5 56.9

25 year UV exposure to the back of module

275 235 171

Current Standard 15 - 100

Passing IEC certification test does not imply

25 year durability for backsheet materials.

Assumption: 12% UV reflectivity of the ground

**UVA: 70 ◦ C, BPT 340nm, 1.2W/m²-nm @340nm, (65 W/m², 250-400nm), continuous, no condensation

***IEC 15 kWh = 70 days, so implied annual value is 78.2 kWh,

(1)M. Koehl, “Indoor and Outdoor Weathering of PV Modules”, Proc of SPIE Vol 7048 704806-4, (2008)

(2)M. Kempe, Ultraviolet light test and evaluation methods for encapsulants of photovoltaic modules, Sol.Energy Mater.Sol.Cells, 94 (2010) 246–253.,

(3) “Weathering of Plastics: Testing to Mirror Real Life Performance”, George Wypych, p17 (1999),

(4)R. Chadysiene and A. Girgzdys, “Ultraviolet Radiation Albedo of Natural Surfaces”, J. Env, Eng. Land. Mgmnt,, 16(2): 83-88 (2008)

UV dosage under different climate conditions

UV Ageing in Standard is Far Below than the Environment


PET-Based Backsheets Yellow, Lose % Elongation in UVA Exposures

UVA 65W/m2 (250-400nm), 70oC BPT, 3000 hours is equivalent to 18 year desert exposure

Yellowing Losing % Elongation

0

1

2

3

4

5

6

7

TPT TPE HPET1 HPET2

b*

UVA b* at 3000 hours

0 hours

3000 hours

0

50

100

150

200

250

300

TPT TPE HPET1 HPET2

% E

lon

gati

on

UVA % Elongation at 3000 hours

0 hours

3000 hours

Yellowing and loss of mechanical properties

are signs of polymer degradation


Recommended Sequential Tests

Test: Backsheet or Modules

Test: Full-Size Laminates or Module

Thermal Cycling

Test: Backsheets: weathering (Xe), 102m light, 18m light + water spray Test: Module: resistive loading, weathering exposure (above)

Observe: Backsheet Cracking

Observe: Backsheet Yellowing, Mechanical Loss Module: Power Loss

nX

Observe: Backsheet Yellowing, Cracking

) ) Damp Heat

UVA

Xenon Ageing

Damp Heat nX UVA ) ) Water Spray

Thermal Cycling


Tedlar® film-based backsheets endures various climates for decades

Gansu 27 years

Power degradation: ~10%

Hainan

23 years Power degradation: 6.1%

Zhejiang

23 years

Power degradation: 13.1%

1

2

3

Beijing

Shanghai

13

Only solar panels with reliable materials maximize power output

and 25 or more year system lifetimes


Conclusions

• Field inspections and fielded module analysis are key to fundamentally

understand PV component performance and durability

• There are risks associated with using unproven backsheet materials

• Passing IEC certification test does not imply 25 year durability for

backsheet materials. New testing method shall be created to better

simulate the environmental challenges, and new standard shall be

established to ensure the durability of module materials.

• DuPont proposes better single stress conditions including UV durability

and sequential testing using damp heat (DH), thermal cycling (TC) and

UV exposure (UVA) to better predict long term outdoor performance

Copyright © 2015 DuPont. All rights reserved. The DuPont Oval

Logo, DuPont™, The miracles of science™, Materials Matter™, and

all products denoted with ® or ™ are registered trademarks or

trademarks of E.I. du Pont de Nemours and Company or its affiliates.

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