On-site EL is starting a quality revolution
EL inspection is commonly used in module production lines, most buyers request 100% module EL inspection before delivery. Unfortunately the same quality request is not applied to the PV plants.
The above photo shows an EL image of a PV array, serious cell breakage is found in the middle of the modules. This kind of breakage frequently happens during installation or maintenance, but only few really check it on the PV array. EPCs request a very high quality standard for modules, but many of them do not apply the same request level to their installations. We have seen many installers and panel cleaning workers walking on the modules, and some owners even invite guests to take photos standing on the modules.
No quality awareness, that is the problem. But it is going to be changed as on-site EL inspection is becoming a standard quality control for PV plants.
EL is not only used to check the cracks in the PV panels, but also a very good tool to find many other defects, for example, PID. PID can be detected by monitoring system, however it is recognized only after serious power decay, and the power generation loss had happened for months. With the on-site EL inspection, PID can be found in a very early stage. The photo above is an example we took in the field. The dark cells are PID infected, the power output of the cells was still ok at high irradiance, but the performance under weak light was becoming worse. On the monitoring system, the power generation was not greatly reduced, so it was not recognized as PID yet. With low current EL inspection, we can easily find the PID cells at early stage.
Low irradiance performance is another characteristic that EL can tell. The dark cells in the above image have lower power output at low irradiance. Because they have lower shunt resistance (Rsh), which results in lower performance under weak light. After replacing the modules, the power generation was greatly improved.
These defects were not treated seriously in PV plants, so the module and cell manufacturers did not have to sort these defects out. Even if they wanted to, they did not have suitable device to do it. But the situation is changing, the largest EL equipment supplier ASIC (more than 80% market share) released new models for module and cell production lines in the latest SNEC trade fair. The new models perform EL inspection at high and low current level to find out low Rsh cells in the modules, and the cycle time is still the same as tradition ones. It is important because it makes the in-line weak light performance sorting feasible. It won a great attention in the fair, many first tier manufactures are starting to use it in the production. Once the defect can be sorted out, the power at low irradiance can be greatly improved, and the annual yield will also be greatly enhanced. It is a revolution of the PV plant quality, just like EL inspection was first introduced into module production.
Therefore, new EL standard and on-site EL will surely become a standard control for PV plants, make sure you are ready for it !
About the author:
Dr. Jay Lin, Chief Consultant of PV Guider Consultancy
Task Force Leader in SEMI International Standards
Technical Committee of CNS National Standards
12 years experience (since 2004) in PV technology research and module production.
As an independent consultant, he has audited more than 50 module factories for insurance companies, investors and EPCs.