Measurement procedures for materials used in photovoltaic modules - Part 1-7: Encapsulants - Test procedure of optical durability

IEC 62788-1-7:2020 is designed as a more rigorous qualification test, using accelerated UV exposure at elevated temperature to determine whether polymeric encapsulants can suffer loss of optical transmittance. IEC 61215-2 already includes a UV preconditioning test (MQT 10), however, the parameters for that test only represent a limited level of exposure (~weeks of UV dose). This test procedure is intended for representative coupon specimens, applying stress at a greater intensity (designed relative to Phoenix, AZ), using a radiation spectrum that is more similar to the terrestrial solar spectrum, and using a duration of exposure that is more relevant to the PV application (i.e., equivalent to several years of outdoor exposure). This test quantifies the degradation rate of encapsulants so that the risk of the materials losing optical transmittance during operation in the terrestrial environments can be managed. The quantitative correlation between climate (or location of use), a specific application (utility installation, residential-installation, roof-mount, rack-mount, use of a tracker, the system electrical configuration and its operation), and the test can be established for each specific encapsulant material, but is beyond the scope of this document.

Procédures de mesure des matériaux utilisés dans les modules photovoltaïques - Part 1-7: Encapsulants - Procédure d’essai de la durabilité optique

l’IEC 62788-1-7:2020 est conçue comme un essai de qualification plus rigoureux, qui utilise une exposition aux UV accélérée à température élevée destinée à déterminer si les encapsulants polymérisés peuvent présenter une perte de facteur de transmission optique. L’IEC 61215-2 comprend déjà un essai de préconditionnement aux UV (MQT 10), toutefois, les paramètres de cet essai ne représentent qu’un niveau limité d’exposition (~semaines de dose d’UV). La présente procédure d'essai concerne les éprouvettes de coupon représentatives et applique des contraintes d’une plus grande intensité (conçues par rapport à Phoenix, AZ), en utilisant un spectre de rayonnement qui est plus proche du spectre lumineux solaire terrestre et une durée d'exposition plus pertinente pour l’application PV (c’est à dire équivalente à plusieurs années d'exposition en extérieur). Cet essai quantifie la vitesse de dégradation des encapsulants de manière à pouvoir gérer le risque de perte de facteur de transmission optique des matériaux au cours du fonctionnement dans l’environnement terrestre. La corrélation quantitative entre le climat (ou le lieu d'utilisation), une application spécifique (installation de service public, installation résidentielle, montage sur toit, montage sur châssis, utilisation d'un traqueur, configuration électrique du système et son fonctionnement) et l’essai peut être établie pour chaque matériau d’encapsulation spécifique mais n’entre pas dans le domaine d’application du présent document.

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Publication Date
20-Apr-2020
Current Stage
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Completion Date
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IEC 62788-1-7
Edition 1.0 2020-04
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Measurement procedures for materials used in photovoltaic modules –
Part 1-7: Encapsulants – Test procedure of optical durability
Procédures de mesure des matériaux utilisés dans les modules
photovoltaïques –
Partie 1-7: Encapsulants – Procédure d’essai de la durabilité optique
IEC 62788-1-7:2020-04(en-fr)
---------------------- Page: 1 ----------------------
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---------------------- Page: 2 ----------------------
IEC 62788-1-7
Edition 1.0 2020-04
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Measurement procedures for materials used in photovoltaic modules –
Part 1-7: Encapsulants – Test procedure of optical durability
Procédures de mesure des matériaux utilisés dans les modules
photovoltaïques –
Partie 1-7: Encapsulants – Procédure d’essai de la durabilité optique
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
INTERNATIONALE
ICS 27.160 ISBN 978-2-8322-8035-5

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

Attention! Veuillez vous assurer que vous avez obtenu cette publication via un distributeur agréé.

® Registered trademark of the International Electrotechnical Commission
Marque déposée de la Commission Electrotechnique Internationale
---------------------- Page: 3 ----------------------
– 2 – IEC 62788-1-7:2020 © IEC 2020
CONTENTS

FOREWORD ........................................................................................................................... 3

INTRODUCTION ..................................................................................................................... 5

1 Scope .............................................................................................................................. 6

2 Normative references ...................................................................................................... 6

3 Terms and definitions ...................................................................................................... 7

4 Principle .......................................................................................................................... 7

5 Apparatus ........................................................................................................................ 7

5.1 Spectrophotometer for transmittance measurements ............................................... 7

5.2 Environmental chamber for weathering ................................................................... 7

6 Test specimens ............................................................................................................... 8

6.1 Specimen components and general considerations for all material types ................ 8

6.2 Test specimens for datasheet reporting .................................................................. 8

6.3 Use of alternate superstrate and substrate materials .............................................. 8

6.4 Witness specimens and experimental control .......................................................... 9

7 Measurement procedure .................................................................................................. 9

8 Artificial accelerated weathering ...................................................................................... 9

9 Calculation and expression of results .............................................................................. 9

10 Test procedure .............................................................................................................. 10

11 Pass/fail criteria ............................................................................................................. 10

12 Test report ..................................................................................................................... 11

Bibliography .......................................................................................................................... 13

---------------------- Page: 4 ----------------------
IEC 62788-1-7:2020 © IEC 2020 – 3 –
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
MEASUREMENT PROCEDURES FOR
MATERIALS USED IN PHOTOVOLTAIC MODULES –
Part 1-7: Encapsulants –
Test procedure of optical durability
FOREWORD

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

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

International Standard IEC 62788-1-7 has been prepared by IEC technical committee 82: Solar

photovoltaic energy systems.
The text of this International Standard is based on the following documents:
FDIS Report on voting
82/1669/FDIS 82/1704/RVD

Full information on the voting for the approval of this International Standard 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.

A list of all parts in the IEC 62788 series, published under the general title Measurement

procedures for materials used in photovoltaic modules, can be found on the IEC website.

---------------------- Page: 5 ----------------------
– 4 – IEC 62788-1-7:2020 © 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
• amended.
---------------------- Page: 6 ----------------------
IEC 62788-1-7:2020 © IEC 2020 – 5 –
INTRODUCTION

IEC 61215-2 (covering module design qualification and type approval) specifies a UV

-2 -2

preconditioning of 54 MJ⋅m (15 kWh⋅m ), which would be encountered after ~40 ideal sunny

days of exposure to the AM1.5G UV spectrum in IEC 60904-3. IEC 61730-2 presently specifies

4x the same UV exposure, i.e., 5 months UV dose. The International PV Quality Assurance

Task Force (PVQAT) leads global efforts to craft quality and reliability standards for solar

energy technologies. These standards will allow stakeholders to quickly assess a solar

photovoltaic (PV) module's performance and ability to withstand weather stresses, thereby

reducing risk and adding confidence for those developing products, designing incentive

programs, and determining private investments. As developed in conjunction with PVQAT, this

part of IEC 62788-1 is intended to supplement module qualification, which typically covers

reliability issues related to infant mortality, i.e., the first months of field use. This part of

IEC 62788-1 may also facilitate the pre-qualification of encapsulation materials using coupon

specimens, because long term weathering is not practical for larger module specimens. This

part of IEC 62788-1 also importantly uses high fidelity UV irradiation (relative to the terrestrial

solar spectrum), which is not practical to apply to module specimens (due to the lack of available

commercial equipment and the anticipated cost of operation).This part of IEC 62788-1 is not

presently specified for pre-qualification purposes in other standards, but may be used for that

purpose by module manufacturers.

The optical performance (transmittance) of polymeric frontsheets and backsheets is not covered

in this part of IEC 62788-1. These components are addressed in the IEC TS 62788-2.

---------------------- Page: 7 ----------------------
– 6 – IEC 62788-1-7:2020 © IEC 2020
MEASUREMENT PROCEDURES FOR
MATERIALS USED IN PHOTOVOLTAIC MODULES –
Part 1-7: Encapsulants –
Test procedure of optical durability
1 Scope

IEC 61215-2 provides a set of qualification tests that indicate that a PV module design is likely

to be free of flaws that will result in early failure. However, IEC 61215-2 does not address the

long term wear-out of PV modules. This part of IEC 62788-1 is designed as a more rigorous

qualification test, using accelerated UV exposure at elevated temperature to determine whether

polymeric encapsulants can suffer loss of optical transmittance. IEC 61215-2 already includes

a UV preconditioning test (MQT 10), however, the parameters for that test only represent a

limited level of exposure (~weeks of UV dose). This test procedure is intended for

representative coupon specimens, applying stress at a greater intensity (designed relative to

Phoenix, AZ), using a radiation spectrum that is more similar to the terrestrial solar spectrum,

and using a duration of exposure that is more relevant to the PV application (i.e., equivalent to

several years of outdoor exposure). This test quantifies the degradation rate of encapsulants

so that the risk of the materials losing optical transmittance during operation in the terrestrial

environments can be managed. The quantitative correlation between climate (or location of use),

a specific application (utility-installation, residential-installation, roof-mount, rack-mount, use of

a tracker, the system electrical configuration and its operation), and the test can be established

for each specific encapsulant material, but is beyond the scope of this document.

The method herein is intended to qualify encapsulants for use in a PV module. This document

is intended to apply to encapsulants used in PV modules deployed under temperature

conditions of normal use, as defined in IEC TS 63126. The use of this method for encapsulants

in modules deployed under conditions of higher temperature is specified elsewhere, for example

IEC TS 63126. The method here is intended to be used to examine a particular encapsulant

and does not cover incompatibilities between the encapsulant and other packaging materials.

This document covers PV technology constructed using a transparent incident

surface/encapsulant/photovoltaic device construction, the relevance to other geometries where

the encapsulant layer is located behind the photovoltaic device layer, is outside the scope of

this document. In the case of bifacial cell technology, the module can accept light from its front

and back surfaces – the transmittance of a frontsheet (if used), encapsulant, and transparent

backsheet (if used) is relevant for both active surfaces. The optical durability of frontsheets and

backsheets, however, is addressed separately in the IEC TS 62788-2. Thin coatings that might

be added for antireflection or anti-soiling purposes are outside the scope of this document. The

method in this document can be used for other purposes (e.g., research and development);

many details of alternate uses of the method (e.g., alternate test durations or measurement

increments) are not described here.
2 Normative references

The following documents are referred to 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 61215-2, Terrestrial photovoltaic (PV) modules – Design qualification and type approval –

Part 2: Test procedures

IEC 61730-1, Photovoltaic (PV) module safety qualification – Part 1: Requirements for

construction
---------------------- Page: 8 ----------------------
IEC 62788-1-7:2020 © IEC 2020 – 7 –
IEC TS 61836, Solar photovoltaic energy systems – Terms, definitions and symbols

IEC 62788-1-4, Measurement procedures for materials used in photovoltaic modules. Part 1-4:

Encapsulants – Measurement of optical transmittance and calculation of the solar-weighted

photon transmittance, yellowness index, and UV cut-off wavelength

IEC TS 62788-7-2, Measurement procedures for materials used in photovoltaic modules –

Part 7-2: Environmental exposures – Accelerated weathering tests of polymeric materials

IEC TS 62915, Photovoltaic (PV) modules – Type approval, design and safety qualification –

Retesting

IEC TS 63126 , Guidelines for qualifying PV modules, components, and materials for operation

at high temperatures
ISO 291, Plastics – Standard atmospheres for conditioning and testing

ASTM G7, Standard practice for atmospheric environmental exposure testing of nonmetallic

materials
3 Terms and definitions

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

IEC 61730-1 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 Principle

The total spectral transmittance shall be quantified using a spectrophotometer equipped with

an integrating sphere (IEC 62788-1-4). Artificial weathering shall be performed at stable

specified irradiance, temperature, and relative humidity conditions using an environmental

chamber (IEC TS 62788-7-2). The changes in transmittance resulting from weathering shall be

quantified using subsequent spectrophotometer measurement(s). The results of this artificial

weathering test may be benchmarked against natural weathering, for example ASTM G7.

5 Apparatus
5.1 Spectrophotometer for transmittance measurements

A double beam or single beam spectrophotometer equipped with an integrating sphere and

conforming to the requirements of IEC 62788-1-4 shall be used.
5.2 Environmental chamber for weathering

An artificial weathering apparatus shall be used, as specified in IEC TS 62788-7-2. The

weathering apparatus shall meet the requirements of the artificial accelerated weathering

method specified, for example IEC TS 62788-7-2, method A3.
___________
Under preparation. Stage at the time of publication: IEC/DTS 63126:2019.
---------------------- Page: 9 ----------------------
– 8 – IEC 62788-1-7:2020 © IEC 2020
6 Test specimens
6.1 Specimen components and general considerations for all material types

Specimens shall be constructed according to the geometry, methodology, and number of

replicates for weathering test specimens as specified in IEC 62788-1-4.

Silica glass shall be used for the encapsulant specimens to standardize the results for the

purpose of datasheet reporting of the durability. Silica glass may be used to achieve the worst-

case weathering results, i.e., the most accelerated, because it is fully UV transmitting. The silica

shall fulfill the requirements of IEC 62788-1-4.

To limit the propagation of localized weathering damage through the specimens, the indicated

specimens should be marked (with a serial number or other identifier) on the side that is not

facing the incident radiation. Because of the aggressive nature of the combined stress factors

applied during artificial weathering, it is recommended to mark by physically scribing the

substrate.

All specimens shall be laminated or processed in a manner similar to that used to fabricate PV

modules.
6.2 Test specimens for datasheet reporting

Encapsulant specimens shall consist of glass/encapsulant/glass coupons as specified in

IEC 62788-1-4.

Some module geometries are constructed using separate front encapsulant (that is UV

transparent) and back encapsulant (that is not UV transparent). The durability of each

encapsulant material shall be examined separately using separate coupons containing each

specific material.
6.3 Use of alternate superstrate and substrate materials

For the purpose of research and development, other materials including photovoltaic glass and

UV attenuating encapsulant may be used in coupon specimens. The test may reveal material

specific degradation, particularly when product glass and/or encapsulants are used.

Photovoltaic glass may be used to achieve weathering that is more representative of that

encountered in a PV module. Photovoltaic glass, however, shall not be used for the purpose of

datasheet reporting. If a superstrate glass other than silica is used, its transmittance should be

similar to the manufactured superstrate. In the case of module representative specimens, the

glass used in weathering shall have a solar weighted transmittance that is equal to or greater

than that of the glass used in PV modules between the UV cut-off wavelength and 400 nm. The

representative glass used in weathering shall also have an initial UV cut-off wavelength within

±2,5 nm that of the glass in used in PV modules, for example the same glass from the same

manufacturer. If the manufacturer’s tolerances for UV cut-off wavelength and solar weighted

transmittance are available for fabricated lots of photovoltaic glass used for test specimens,

then those should be used in place of the aforementioned limits.

In the case of modules using a polymeric frontsheet or superstrate that is not composed of

glass, that superstrate material may be used in the encapsulant coupons for weathering in the

place of glass. It is recommended to verify that the solar weighted transmittance of the

superstrate is equal to or greater than that of the superstrate used in PV modules between the

UV cut-off wavelength and 400 nm. The superstrate used in weathering shall also have an initial

UV cut-off wavelength within ±2,5 nm the superstrate used in PV modules, for example the

same superstrate material from the same manufacturer. If the manufacturer’s tolerances for UV

cut-off wavelength and solar weighted transmittance are available for fabricated lots of

superstrate material used for specimens, then those should be used in place of the

aforementioned limits.
---------------------- Page: 10 ----------------------
IEC 62788-1-7:2020 © IEC 2020 – 9 –

Because the optical characteristics of a glass or a polymeric superstrate can be affected by

weathering (e.g., solarization of glass [1] or discoloration of polymeric materials), it is

suggested to include reference specimens of the glass or superstrate material for weathering,

if the stability of the superstrate material is unknown. For superstrate or substrate materials

that are known to solarize, it is suggested to artificially solarize those materials before

constructing test specimens.
6.4 Witness specimens and experimental control

Measurement of witness specimens, as described in IEC 62788-1-4, shall be used to verify the

measurements within each measurement session. Control measurements shall be performed

at the start and end of each measurement session. Control measurements may also be

performed intermediately, during the measurement session. The control measurements should

be compared to quantify and correct for drift of the spectrophotometer through the weathering.

7 Measurement procedure

Transmittance measurements shall be performed as specified in IEC 62788-1-4, including the

equilibration of the spectrophotometer lamp(s), baselining of the instrument, and use of witness

specimen(s). Specimens with an impermeable superstrate (e.g., glass) shall be conditioned

prior to optical measurements as specified in ISO 291 class 2, for example (23 ± 2) °C, (50 ±

10) % RH for at least 15 min to allow thermal equilibration. Specimens with a permeable

superstrate (e.g., polymeric frontsheet) shall be conditioned prior to optical measurements as

specified in ISO 291 class 2, for example (23 ± 2) °C, (50 ± 10) % RH for at least 24 h to

facilitate moisture equilibration. Because the transmittance at short wavelengths may be used

to diagnose the effects of weathering, it is recommended to measure the transmittance from

200 nm to 2 500 nm.
8 Artificial accelerated weathering

For encapsulants used in PV modules deployed under temperature conditions of normal use,

artificial accelerated weathering shall be performed in conformance with IEC TS 62788-7-2,

method A3. For encapsulants in modules deployed under conditions of higher temperature,

weathering shall be performed according to IEC TS 63126.

NOTE The details of the test conditions in IEC TS 62788-7-2, including: irradiance; chamber air temperature; black

panel temperature; and chamber relative humidity, are specified for reference here as IEC 62788-1-7 is developed.

Details including the intent of the test conditions are also provided here to facilitate the development of IEC 62788-

1-7. All of these details, however, will be removed in the final version of IEC 62788-1-7 because having them solely

located in IEC TS 62788-7-2 will prevent confusion as both documents are revised in the future.

-2 -1

IEC TS 62788-7-2, method A3: irradiance of 0,8 W⋅m ⋅nm , controlled at 340 nm; 65 °C

chamber air temperature with a 90 °C black panel temperature; and chamber relative humidity

of 20 %.

Specimens shall be conditioned prior to weathering as specified in IEC TS 62788-7-2.

9 Calculation and expression of results

Results and their corresponding uncertainty shall be calculated from the transmittance

measurements, including the solar-weighted transmittance, representative solar-weighted

transmittance (solar-weighted transmittance of photon irradiance transmitted throughout the

range of the spectrum utilized by a representative PV device, as defined in IEC 62788-1-4),

yellowness index, and UV cut-off wavelength, as specified in IEC 62788-1-4. The initial values

___________
Numbers in square brackets refer to the Bibliography.
---------------------- Page: 11 ----------------------
– 10 – IEC 62788-1-7:2020 © IEC 2020

for the characteristics and the change in their performance following UV weathering shall be

reported.
10 Test procedure
The following test sequence shall be applied:
a) Specimen fabrication and preparation (as in IEC 62788-1-4).
b) Visual inspection (IEC 61215-2, MQT 01).

c) Specimen conditioning for measurement (as in IEC 62788-1-4 or ISO 291 class 2).

d) Initial transmittance measurement of the specimens (as in IEC
...

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