Derisking photovoltaic modules - Sequential and combined accelerated stress testing

IEC TR 63279:2020 reviews research into sequential and combined accelerated stress tests that have been devised to determine the potential for degradation modes in PV modules that occur in the field that single-factor and steady-state tests do not show. This document is intended to provide data and theory-based motivation and help visualize the next steps for improved accelerated stress tests that will derisk PV module materials and designs. Any incremental savings as a result of increased reliability and reduced risk translates into lower levelized cost of electricity for PV. Lower costs will result in faster adoption of PV and the associated benefits of renewable energy.

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Status
Published
Publication Date
20-Aug-2020
Current Stage
PPUB - Publication issued
Completion Date
21-Aug-2020
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IEC TR 63279
Edition 1.0 2020-08
TECHNICAL
REPORT
colour
inside
Derisking photovoltaic modules – Sequential and combined accelerated stress
testing
IEC TR 63279:2020-08(en)
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---------------------- Page: 2 ----------------------
IEC TR 63279
Edition 1.0 2020-08
TECHNICAL
REPORT
colour
inside
Derisking photovoltaic modules – Sequential and combined accelerated stress
testing
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS 27.160 ISBN 978-2-8322-8737-8

Warning! Make sure that you obtained this publication from an authorized distributor.

® Registered trademark of the International Electrotechnical Commission
---------------------- Page: 3 ----------------------
– 2 – IEC TR 63279:2020 © IEC:2020
CONTENTS

FOREWORD ........................................................................................................................... 5

1 Scope .............................................................................................................................. 7

2 Normative references ...................................................................................................... 7

3 Terms and definitions ...................................................................................................... 8

4 Framework for sequential and combined stress testing .................................................... 8

5 Sequential and cyclic sequential test methods ................................................................. 9

5.1 Extended damp heat and addition of ultraviolet light ............................................... 9

5.2 Sequential/combined testing with damp-heat, thermal cycling and ultraviolet

light ...................................................................................................................... 10

5.3 Consideration of interaction of UV radiation and damp heat .................................. 12

5.4 Test-to-failure—A sequential test protocol ............................................................. 13

5.5 Sequential test protocol optimized for differentiating backsheets ........................... 16

5.6 Mechanical stress testing in combination with damp-heat, humidity-freeze,

and thermal-cycling tests for examining cell cracking and its effects ..................... 20

6 Mechanism-specific multi-factor stress tests .................................................................. 22

6.1 General ................................................................................................................. 22

6.2 Testing for delamination ........................................................................................ 22

6.2.1 General ......................................................................................................... 22

6.2.2 Delamination – UV irradiation with high-temperature stress ........................... 22

6.2.3 Delamination – UV irradiation with thermal-cycling stress and humidity

freeze ............................................................................................................ 23

6.2.4 Delamination – UV irradiation with cyclic dynamic mechanical loading,

thermal cycling stress, and humidity freeze .................................................... 24

6.2.5 Delamination – Temperature, humidity, and electric field associated with

system voltage .............................................................................................. 25

6.3 Testing for potential-induced degradation ............................................................. 28

6.3.1 General ......................................................................................................... 28

6.3.2 Testing for potential-induced degradation with humidity, voltage, bias,

and light ........................................................................................................ 28

6.3.3 Factor of salt mist .......................................................................................... 29

6.4 Testing in damp heat with current injection and as a function of temperature ........ 30

6.5 Cell cracking and propagation in cyclic loading at various temperatures................ 31

7 Combined-accelerated stress testing ............................................................................. 33

7.1 Combined-accelerated stress testing for tropical environments ............................. 33

7.2 Combined-accelerated stress testing for multiple environments ............................ 36

8 Future directions............................................................................................................ 39

Annex A (informative) Overview of degradation modes and causal stress factors ................. 41

Annex B (informative) Failure modes plotted on a failure tree diagram for selected

clauses in this document ....................................................................................................... 43

Annex C (informative) Summary table of sequential and combined testing: Samples,

factors, combination, and stress-test results ......................................................................... 44

Bibliography .......................................................................................................................... 49

Figure 1 – Framework for sequential and combined stress testing, showing three axes

of comprehensiveness – testing samples, the number of stress factors of the natural

environment, and their sequence or combination of application. .............................................. 9

Figure 2 – Fraction power loss of modules though stress testing ........................................... 10

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IEC TR 63279:2020 © IEC:2020 – 3 –
Figure 3 – (a) Combined test sequence, and resulting (b) normalized power loss,

(c) short-circuit current (I ), and (d) fill factor (FF) [1] ........................................................ 11

Figure 4 – Power degradation of modules in 85 °C and 85 % relative humidity as a

function of extent of preconditioning under Xe light [9] .......................................................... 13

Figure 5 – (a) Overview of the test-to-failure sequences, and (b) results showing

module power normalized to their post-light-soak values for seven module types.................. 14

Figure 6 – Examples of field-relevant degradation modes seen in modules tested in

the test-to-failure protocol ..................................................................................................... 15

Figure 7 – Module accelerated sequential tests (MAST) ........................................................ 17

Figure 8 – Degradation modes from MAST and fielded modules ........................................... 19

Figure 9 – (a) Front-side mini-module exposure in a xenon weathering chamber with

water spray; (b) fielded module with six years of service in North America with 30 %

power loss [21] ..................................................................................................................... 20

Figure 10 – (a) Test-stage description; (b) relative change in standard test condition

(STC) module parameters as a function of stage and maximum power determined at

STC [23] ............................................................................................................................... 21

Figure 11 – (a) Stress testing at 65 °C combined with UV radiation dose of 180 W/m

in the range of 300–400 nm, 900 h; (b) 75 °C without UV radiation, 1 000 h [28] .................. 23

Figure 12 – Delamination in sequential test ........................................................................... 25

Figure 13 – Delamination associated with system voltage ..................................................... 27

Figure 14 – Degradation of three modules with and without UV-A light irradiance in

chamber at 60 °C, 85 % RH, and 1 000 V (positive or negative polarity depending on

the sample) ........................................................................................................................... 29

Figure 15 – Sheet resistance measured on glass surfaces with various soil types, as a

function of relative humidity (RH %), at 60 °C [41] ................................................................ 30

Figure 16 – Cyclic unidirectional 4-point bending with loading alternating between 0 N

and 500 N at different temperatures as shown, with duration of 4 s at each of the high-

and low-pressure dwells, 10 000 to 30 000 cycles with pressure (“Press”) from the

front-glass side or backsheet side [49] .................................................................................. 32

Figure 17 – Example of 24 h PV module combined accelerated stress-testing protocol

modified from ASTM D7869 .................................................................................................. 34

Figure 18 – Shrinkage of polymer C backsheet leading to delamination and cracking .......... 35

Figure 19 – Multiple-environment C-AST sequence ............................................................... 37

Figure 20 – Failure of two mini-modules with a polymer B outer-layer backsheet type

undergoing different multiple-environment C-AST sequences ............................................... 38

Table 1 – Extended damp heat and ultraviolet light ............................................................... 10

Table 2 – Sequential/combined testing with damp-heat thermal cycling and ultraviolet

radiation ............................................................................................................................... 12

Table 3 – Ultraviolet light and damp-heat interaction ............................................................. 13

Table 4 – Test-to-failure – Sequential test protocol ............................................................... 16

Table 5 – Module accelerated stress test 1 (MAST #1) .......................................................... 18

Table 6 – Module accelerated stress test 2 (MAST #2) ......................................................... 18

Table 7 – Module accelerated stress test 3 (MAST #3) ......................................................... 18

Table 8 – SML-TC-HF sequential test ................................................................................... 21

Table 9 – UV irradiation under high-temperature conditions .................................................. 23

Table 10 – UV irradiation with TC stress ............................................................................... 24

Table 11 – UV irradiation with DML-TC-HF sequential test .................................................... 25

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– 4 – IEC TR 63279:2020 © IEC:2020

Table 12 – DH – Negative system bias stress sequential test ................................................ 28

Table 13 – UV irradiation – negative system bias stress combined test ................................. 29

Table 14 – Bending load test at various temperatures ........................................................... 33

Table 15 – Partial list of observed degradation modes, attributed mechanisms, and

stress factors seen in the first application of the combined accelerated stress-testing

protocol based on ASTM D7869 ............................................................................................ 35

Table 16 – Combined-accelerated stress test (Tropical 24 h ASTM D7869-based

sequence) ............................................................................................................................. 36

Table 17 – Multiple-environment combined-accelerated stress test ....................................... 38

Table A.1 – Degradation modes and potential stress factors that can lead to their

manifestation ........................................................................................................................ 42

Table C.1 – Table summarizing sequential and combined stress testing ............................... 44

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IEC TR 63279:2020 © IEC:2020 – 5 –
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
DERISKING PHOTOVOLTAIC MODULES – SEQUENTIAL
AND COMBINED ACCELERATED STRESS TESTING
FOREWORD

1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising

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8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is

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9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of patent

rights. IEC shall not be held responsible for identifying any or all such patent rights.

The main task of IEC technical committees is to prepare International Standards. However, a

technical committee may propose the publication of a technical report when it has collected

data of a different kind from that which is normally published as an International Standard, for

example "state of the art".

IEC TR 63279, which is a Technical Report, has been prepared by IEC technical committee 82:

Solar photovoltaic energy systems.
The text of this Technical Report is based on the following documents:
Enquiry draft Report on voting
82/1657/DTR 82/1692B/RVDTR

Full information on the voting for the approval of this technical report can be found in the report

on voting indicated in the above table.

This document has been drafted in accordance with the ISO/IEC Directives, Part 2.

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– 6 – IEC TR 63279:2020 © IEC:2020

The committee has decided that the contents of this document will remain unchanged until the

stability date indicated on the IEC website under "http://webstore.iec.ch" in the data related to

the specific document. At this date, the document will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
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IMPORTANT – The 'colour inside' logo on the cover page of this publication indicates

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of its contents. Users should therefore print this document using a colour printer.

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IEC TR 63279:2020 © IEC:2020 – 7 –
DERISKING PHOTOVOLTAIC MODULES – SEQUENTIAL
AND COMBINED ACCELERATED STRESS TESTING
1 Scope

This document reviews research into sequential and combined accelerated stress tests that

have been devised to determine the potential for degradation modes in PV modules that occur

in the field that single-factor and steady-state tests do not show. This document is intended to

provide data and theory-based motivation and help visualize the next steps for improved

accelerated stress tests that will derisk PV module materials and designs. Any incremental

savings as a result of increased reliability and reduced risk translates into lower levelized cost

of electricity for PV. Lower costs will result in faster adoption of PV and the associated benefits

of renewable energy.
2 Normative references

The following documents are referred in the text in such a way that some or all of their content

constitutes requirements of this document. For dated references, only the edition cited applies.

For undated references, the latest edition of the referenced document (including any

amendments) applies.

IEC 60721-2-1, Classification of environmental conditions – Part 2-1: Environmental conditions

appearing in nature – Temperature and humidity

IEC 61215-1:2016, Terrestrial photovoltaic (PV) modules – Design qualification and type

approval – Part 1: Test requirements

IEC 61215-2:2016, Terrestrial photovoltaic (PV) modules – Design qualification and type

approval – Part 2: Test procedures

IEC 61730-2:2016, Photovoltaic (PV) module safety qualification – Part 2: Requirements for

testing
IEC TS 61836, Solar photovoltaic energy systems – Terms, definitions and symbols

IEC TS 62782:2016, Photovoltaic (PV) modules – Cyclic (dynamic) mechanical load testing

IEC 62788 (all parts), Measurement procedures for materials used in photovoltaic modules

IEC TS 62804-1, Photovoltaic (PV) modules – Test methods for the detection of potential-

induced degradation – Part 1: Crystalline silicon

IEC TS 62804-1-1, Photovoltaic (PV) modules – Test methods for the detection of potential-

induced degradation – Part 1-1: Crystalline silicon – Delamination

ASTM D7869-17 Standard Practice for Xenon Arc Exposure Test with Enhanced Light and

Water Exposure for Transportation Coatings
---------------------- Page: 9 ----------------------
– 8 – IEC TR 63279:2020 © IEC:2020
3 Terms and definitions

For the purposes of this document, the terms and definitions given in IEC TS 61836 apply.

ISO and IEC maintain terminological databases for use in standardization at the following

addresses:
• IEC Electropedia: available at http://www.electropedia.org/
• ISO Online browsing platform: available at http://www.iso.org/obp
4 Framework for sequential and combined stress testing

A number of researchers, companies and testing laboratories have explored aspects of

sequential and combined stress testing to fill outstanding needs. Such needs include testing

beyond IEC 61215-2, which for the most part does not purport to examine for end-of-life wear-

out and failure mechanisms. In other cases, stresses are sequenced and combined to elicit

failure modes that have been seen in the field that existing IEC tests may not evaluate.

A framework for organization is proposed that implements stress factors of the natural

environment, sequences and combinations of applying them, and sample types that may be

employed for evaluation. To illustrate this, Figure 1 is introduced, which gives a three-

dimensional plot with the axes of sample, factor, and combination, that together indicate the

comprehensiveness of test methods to represent the effects of the natural environment on the

sample in accelerated testing.

First the sample comprehensiveness axis of Figure 1 is discussed. As a new material is

explored, the material itself is studied to achieve a basic understanding of its intrinsic

degradation mechanisms and durability. Thus, material and coupon tests as they are performed

now according to IEC 62788 series material tests will be valuable. However, failures often occur

at the interfaces between materials, and the performance of one component of the module often

depends on the behaviour of another component or material in the assembly. Therefore, to

represent the material interactions, boundary conditions in actual use, and stresses

experienced, it is necessary to examine mini-modules, and most comprehensively, full-size

modules with all their components.

Next the factors comprehensiveness axis is discussed. This is the number of stress factors of

the natural environment applied in testing of the sample. Moving from single stress factor tests

to multi-factor tests increases confidence of capturing the factors relevant to both known and

unknown degradation modes. Using one factor alone may be useful to evaluate an acceleration

factor or an activation energy associated with that stress for a specific degradation mode or

mechanism that is already understood to depend principally on that stress factor independently

of others.

Finally, the combination comprehensiveness axis is discussed. It represents the manner of

integration of the stress factors on the sample. We seek to sequence and combine the stress

factors in a manner that represents how they appear together in nature to increase the

probability of accelerating only the real degradation modes in the module as they would

manifest in nature. As stress factors are considered, individually or in combination, it is

necessary to understand whether stress levels applied are maintained within the levels of the

natural environment, or if they are exceeded. If exceeded, acceleration of the test may be

increased, but there is significantly increased potential of incurring degradation modes that are

artifacts—modes not necessarily representative of those that would be seen in the natural

environment.

Tables are given in this document for various experimental results in the framework of Figure 1

condensed into two dimensions. These serve to explain how the sequential and combined

accelerated stress tests, with consideration of sample type, factors, and their combination, have

served to produce particular failures or degradation modes. In these condensed two-

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IEC TR 63279:2020 © IEC:2020 – 9 –

dimensional plots, the various column-listed stress factors may be an individual stress factor

such as mechanical load, or an existing IEC 61215 stress test, such as damp heat or thermal

cycling, which in itself contains factors of temperature cycling and current through the cell

circuit. Annexes in which the failure modes are collected for reference are as follows: Annex A :

Overview of degradation modes and causal stress factors, Annex B: Failure modes plotted on

a failure tree diagram for selected clauses in this document, and Annex C : Summary table of

sequential and combined testing: samples, factors, combination, and stress test results of the

samples studied. The templates in these Annexes may be useful for classifying other failure or

degradation modes as they become understood in the future.
Points shown are possibilities for testing within this space.

Figure 1 – Framework for sequential and combined stress testing, showing three axes

of comprehensiveness: testing samples, the number of stress factors of the natural

environment, and their sequence or combination of application
5 Sequential and cyclic sequential test methods
5.1 Extended damp heat and addition of ultraviolet light

Extended damp-heat (DH) testing has frequently been used to attempt to differentiate durability

of PV modules. An example of this is shown in Figure 2a). Five modules undergo five iterations

of 1 000 h duration DH tests at 85 °C and 85 % relative humidity (RH). Four module types

exhibit great degradation after 2 000 h that is due to fill factor (FF) loss from metallization to

silicon contact-resistance increase. The degradation comes at test conditions with temperature

in combination with humidity significantly exceeding those found for modules in PV field

installations, and the degradation mechanisms observed with extended DH tests have

frequently been inconsistent with those seen in fielded PV modules [1] . Reviews of

agglomerated field-degradation data for crystalline silicon cell modules have shown degradation

primarily by short-circuit current (I ) loss followed by FF loss and the least degradation

exhibited by open-circuit voltage (V ) [2].

Excessive humidity may lead to unrealistically high levels of acetic acid formation, leading in

turn to unrealistically high FF losses through grid finger to silicon contact corrosion and other

mechanisms. Therefore, excessively long DH stress tests that produce very high acetic acid

__________
Numbers in square brackets refer to the Bibliography.
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– 10 – IEC TR 63279:2020 © IEC:2020

levels are believed to have limited use in evaluating the durability of conventional crystalline

silicon PV modules installed in the field.

If, after 2 000 h of DH testing, modules were transferred for ultraviolet (UV) exposure in a DH

environment with an 85 °C target module temperature, then the power losses were more modest

as shown in Figure 2b) and reported to be primarily associated with I degradation [1], which

is representative of what is observed in the field. Including UV radiation is necessary to

represent this stress factor of the natural environment. A summary of the sample type used,

stress factors, and their sequence and combination, along with resulting degradation modes

seen for the modules in the study, is given in Table 1.
a) b)

a) Module M1 with thermoplastic and modules M2–M5 with ethylene vinyl acetate encapsulant through 1 000 h of

85 °C and 85 % relative humidity damp heat cycles;
b) Modules through 2 000 h of the damp heat exposure in (a) fol
...

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