microcracks in solar modules · 2017-01-30 · cell and module manufacturers work to prevent...

MICROCRACKS IN

SOLAR MODULES

George McClellan

Senior Technical Sales Manager

REC Americas LLC

Origin, Detection and Prevention

2 © 2013 REC All rights reserved. Confidential

Agenda

1. What is a Microcrack?

2. How do Microcracks occur

– In the factory

– In shipment

– In field

4. How do we detect Microcracks?

– What is Electroluminescence and how does it work?

– MicroCrack Classification

5. How do we prevent Microcracks

– During Manufacturing

– During Transportation

– During Installation


What is a microcrack

The silicon used in the solar cells is very thin (180 to 200 microns) – the cells

are flexible – to a point

Solar panels expand and contract as a result of thermal cycling.

During the day the solar panels expand again because of a higher temperature.

Small imperfections in the silicon cell can lead to larger microcracks due to

thermal cycling

Through this thermal cycling process, ‘microcracks’ can originate in the solar

cells

Microcracks reduce current paths and can cause electrical conduction to drop

off

This has a negative impact on the performance and lifetime of the solar array

Performance degradation due to microcracks is rarely evident in the

commissioning – degradation shows up after system has been in operation for a

year or more

Microcracked modules will typically flash test within spec


What is a microcrack


What is a microcrack?


How do we detect microcracks?

A scanned module showing microcracks


How do Microcracks occur?

Microcracks can be caused by a number factors

– During Manufacturing

– During Storage/Shipment and Transportation

– During Installation

Cell and Module manufacturers work to prevent microcracks in cells and

modules during manufacturing and assembly

Wafers and cells

– Chipping in the wafer and cell can lead to microcracks

– Manufacturers perform incoming and outgoing inspection

– Vision and IR systems

– Electroluminescence

Module assembly

– Assembly can cause microcracks

– Stringing and tabbing

– Rework of strings

– Module manufacturers perform 100% pre and post assembly microcrack inspection





Wafers and cells





– Safe handling methods



Wafers and cells





– Safe handling and transportation methods



Mcrocracks can occur during storage, shipping and transportation

Be aware of pallet stacking limitations



Field generated microcracks are typically caused during

installation/handling on site, or excessive module loading

Modules twisted or torqued excessively by non-planar racking may be

subject to post commissioning microcracking

Modules that have been walked on during installion or maintenance

may show microcracks

Excessive or asymmetrical loads may cause microcracks



What is Electroluminescence and how does it work?



Electroluminescence (EL) is the form of luminescence in which electrons are

excited into the conduction band through the use of electrical current by

connecting cell in forward bias mode.

EL method requires the solar cells to be in the forward bias condition in order

for it to emit infrared radiations. The luminescence ranges from 950 nm to 1250

nm with the peak occurring at approximately 1150 nm.

Emission intensity is dependent on the density of defects in the silicon, with

fewer defects resulting in more emitted photons.

Low defect areas emit more photons and appear brighter in the CCD image

The more defects present, the darker the area will appear



Most module manufacturers will perform 100% EL pre and post lamination



Field Electroluminescence tools are deployed in Europe starting to gain traction

in the US



Not all microcracks are defects that would cause a module to be scrapped

– Some microcracks are self limiting

– Parallel conduction paths exist

Microcrack detections and classification criteria

– It is not enough just to detect Microcracks – we must be able to understand if this will be

a potential problem in the field

– If the sort criteria is too loose then problem modules could be released to the field

– If the sort criteria is too tight then we could be unnecessarily eliminating good modules,

causing yield to go down and module costs to go up.


How do we prevent Microcracks

During Manufacturing

– Design

– Half cut cells decrease the amount of area

– 4 and 5 bus bar cells increase the amount of redundant paths for current flow and collection

– Stronger materials ‒ glass/glass construction

‒ Thicker frames

– Processing

– Optimize soldering and QC/inspection program

– Focus on handling procedures pre-lamination and in rework

– Inline EL

– Pre/Post processing EL

During Transportation

– Handling

– Vertical packaging

– Safe palletization

During Installation

– Installation and design protocols to reduce the potential for bending/torqueing

– Compliant racking and clamping


Pallet Design reduces shipping damage

Protect bottom module damage from fork lift / pallet jack

Multiple ribs provides a stiffer pallet


Pallet Design reduces shipping damage

Vertical palletization decreases module flexing

Increases module damage potential for less than full pallet movement


Preventing Microcracks During Transportation

EL image of module as it left the

factory

EL image of the same module as

it arrived on site prior to install

How did this happen?


Preventing Microcracks During Transportation

Avoid stacking tools, materials or equipment on the pallet of modules

This can generate uneven loads that damage modules


Preventing Microcracks During Installation

Avoid walking, standing, sitting or

kneeling on the modules – even

on the rails and frames!

This can generate point loads

that damage modules



Microcrack damage does not show up immediately

Modules with microcracks usually commission within spec

Microcracks turn into cell level cracks and cause power loss and

hot spots after a few years in the field



Avoid walking, standing, sitting or kneeling on the modules – even




Microcrack damage does not show up immediately

Microcracks turn into cell level cracks and cause power loss and

hot spots after a few years in the field


So what did we cover?

Microcracks are small defects in the cell crystalline structure that can turn in to

larger cracks due thermal cycling.

– Microcracks can lead to array underperformance

Microcracks can occur during manufacturing, shipping/handling and during

installations

Microcracks are difficult to detects – electroluminescence (EL) is currently the

most popular detection method

Microcracks are a hidden risk to module reliability and array power production

– Materials, design and manufacturing are used to help reduce microcrack occurance

– Proper transportation and handling are required to reduce microcracks

– Array design and installation processes can also help prevent microcracks

THANK YOU

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is strictly prohibited. The information in this presentation may not be accurate, complete or up to date, and is provided without warranties or representations of any kind, either express or implied. REC, as well as its

directors, officers and employees, shall not be responsible for and disclaims any liability for any loss or damages, including without limitation, direct, indirect, incidental, consequential and special damages, alleged to

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microcracks in solar modules · 2017-01-30 · cell and module manufacturers work to prevent...

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