Solar cells are the main component of photovoltaic modules and any defects inside these cells – such as cracks, fissures, or faulty contacts – will affect their performance and lead to losses in the panels’ power and generation. Electroluminescence (EL) tests are increasingly utilized in PV plant quality control inspections, together with thermography tests, to gauge the condition of photovoltaic modules.
The high-resolution images acquired through EL testing provide useful information on cell condition, such as busbar welding quality and the presence of cell abnormalities resulting from manufacturing problems or cell breakage. Visualizing a wide range of these defects makes it easier to assess the condition of the solar panels.
However, although it is perfectly integrated into solar module quality control activities, this technology is not easy to apply. In particular, the need to polarize the modules to inject a current that produces cell luminescence, and the low signal intensity and high ambient noise, require working at night when the solar panels are not generating electricity.
In this context, the GdS Optronlab Group and Enertis Applus+ developed a device, using near-infrared sensitive cameras and special acquisition procedures, which permits daytime electroluminescence measurements in solar radiation environments, with no need for panel disassembly. Developed within the framework of an R&D project by the Center for the Development of Industrial Technology (CDTI), this device is a breakthrough in the field that may greatly increase test efficiency in the long term.
The prototype incorporates software with a wide range of image capture and processing procedures which make it possible to obtain images under different conditions (nighttime and daytime) by automated processes. The resulting images offer good signal-to-noise quality, as daytime EL measurements are obtained from the module while eliminating the light (or noise) generated by nearby objects when receiving natural light. High signal-to-noise quality is particularly important when an electroluminescence image of sufficient quality is required to analyze the quality status of a module. In addition, the system is controlled by a micro-PC and can be run on any Android device such as tablets or cell phones.
The prototype was tested in different contexts: both in real operating conditions in several PV plants under different values of irradiance and cloud-cover, as well as in the controlled laboratory conditions of a pilot plant.
These tests have proven that the prototype affords efficient and competitive daytime electroluminescence testing, optimizing image acquisition time and reducing staff requirements. Specifically, the system enables simultaneous polarization of entire strings of modules, thereby reducing the number of connections and disconnections, which increases the module inspection rate while decreasing accidental error probability. Thus, plant tests yielded am inspection rate of up to 80 panels per hour in daytime electroluminescence with high quality images, which was comparable to those obtained in a laboratory. These results also pave the way for substantial improvements in acquiring nighttime electroluminescence images.
In addition, the prototype presents significant operational advantages, such as reduced ancillary elements and wireless connection capability which facilitates mobility in the difficult settings where PV plants are usually built. This opens the door to vast electroluminescence tests in real daytime operating conditions, as, for example, in the case of post-shipment analyses of PV modules, and it also significantly improves nighttime technical-operational conditions.
Published in PV Magazine