Sizing of partially closed surface-breaking microcracks with ...
MICROCRACKS IN SOLAR MODULES · 2017-01-30 · Cell and Module manufacturers work to prevent...
Transcript of 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
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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
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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
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What is a microcrack
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What is a microcrack?
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How do we detect microcracks?
A scanned module showing microcracks
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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
– Vision and IR systems
– Electroluminescence
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How do Microcracks occur?
Wafers and cells
– Chipping in the wafer and cell can lead to microcracks
– Manufacturers perform incoming and outgoing inspection
– Vision and IR systems
– Electroluminescence
– Safe handling methods
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How do Microcracks occur?
Wafers and cells
– Chipping in the wafer and cell can lead to microcracks
– Manufacturers perform incoming and outgoing inspection
– Vision and IR systems
– Electroluminescence
– Safe handling and transportation methods
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How do Microcracks occur?
Mcrocracks can occur during storage, shipping and transportation
Be aware of pallet stacking limitations
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How do Microcracks occur?
Mcrocracks can occur during storage, shipping and transportation
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How do Microcracks occur?
Mcrocracks can occur during storage, shipping and transportation
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How do Microcracks occur?
Mcrocracks can occur during storage, shipping and transportation
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How do Microcracks occur?
Mcrocracks can occur during storage, shipping and transportation
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How do Microcracks occur?
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
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How do we detect microcracks?
What is Electroluminescence and how does it work?
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How do we detect microcracks?
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
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How do we detect microcracks?
Most module manufacturers will perform 100% EL pre and post lamination
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How do we detect microcracks?
Field Electroluminescence tools are deployed in Europe starting to gain traction
in the US
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How do we detect microcracks?
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.
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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
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Pallet Design reduces shipping damage
Protect bottom module damage from fork lift / pallet jack
Multiple ribs provides a stiffer pallet
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Pallet Design reduces shipping damage
Vertical palletization decreases module flexing
Increases module damage potential for less than full pallet movement
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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?
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Preventing Microcracks During Transportation
Avoid stacking tools, materials or equipment on the pallet of modules
This can generate uneven loads that damage modules
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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
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Preventing Microcracks During Installation
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
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Preventing Microcracks During Installation
Avoid walking, standing, sitting or kneeling on the modules – even
on the rails and frames!
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Preventing Microcracks During Installation
Avoid walking, standing, sitting or kneeling on the modules – even
on the rails and frames!
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Preventing Microcracks During Installation
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
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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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