MICROCRACKS IN SOLAR MODULES Origin, Detection and Prevention George McClellan Senior Technical Sales Manager REC Americas LLC
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 2
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 3
What is a microcrack 4
What is a microcrack? 5
What is a microcrack? Microcracked module that flash tested within spec Why are microcracks bad? A. Kottantharayil et al. Lessons Learned from the All India Survey of Photovoltaic Modules. NREL PV Reliability Workshop. Golden, CO: 2016. 6
How do we detect microcracks? A scanned module showing microcracks 7
How do Microcracks occur? Microcracks can be caused by a number factors 8 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
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 9
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 10
How do Microcracks occur? Mcrocracks can occur during storage, shipping and transportation Be aware of pallet stacking limitations 11
How do Microcracks occur? Mcrocracks can occur during storage, shipping and transportation 12
How do Microcracks occur? Mcrocracks can occur during storage, shipping and transportation 13
How do Microcracks occur? Mcrocracks can occur during storage, shipping and transportation 14
How do Microcracks occur? Mcrocracks can occur during storage, shipping and transportation 15
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 16
How do we detect microcracks? What is Electroluminescence and how does it work? 17
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 18
How do we detect microcracks? Most module manufacturers will perform 100% EL pre and post lamination 19
How do we detect microcracks? Field Electroluminescence tools are deployed in Europe starting to gain traction in the US 20
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. 21
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 22
Pallet Design reduces shipping damage Protect bottom module damage from fork lift / pallet jack Multiple ribs provides a stiffer pallet 23 2013 REC All rights reserved. Confidential
Pallet Design reduces shipping damage Vertical palletization decreases module flexing Increases module damage potential for less than full pallet movement 24 2013 REC All rights reserved. Confidential
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? 25
Preventing Microcracks During Transportation Avoid stacking tools, materials or equipment on the pallet of modules This can generate uneven loads that damage modules 26
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 27
Preventing Microcracks During Installation 28 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
Preventing Microcracks During Installation 29 Avoid walking, standing, sitting or kneeling on the modules even on the rails and frames!
Preventing Microcracks During Installation 30 Avoid walking, standing, sitting or kneeling on the modules even on the rails and frames!
Preventing Microcracks During Installation 31 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 32
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