Non-contact Electroluminescence Imaging of Outdoor Modules (ROI 14-95)
Fig. 1. Images are acquired using a Si CCD camera. The top row shows conventional EL using a forward bias with 2.5 A of current. The middle row shows contactless EL using an LED excitation light source. The images for each cell are collected as separate top and bottom halves and then stitched together. The bottom row shows PL imaging using uniform 810-nm laser light for excitation.
Photovoltaic (PV) module performance can be characterized by imaging using baseline and periodic images to track defects and degradation. Both thermal images, which can be acquired during sunny operating conditions, and photoluminescence (PL) images, which could be acquired at night, can be collected without electrical connection. Electroluminescence (EL) images, which are useful to detect many types of defects such as cracks, interconnect and solder faults, and resistances, have typically required electrical connection to drive current in the cells and modules.Description
Scientists at the National Renewable Energy Laboratory (NREL) have invented a contactless EL imaging technique which provides an EL image without the need for electrical connection to drive current through the module. Such EL imaging has the capability to be collected at night without disruption to daytime power generation.
Contactless EL imaging uses excitation light to induce a voltage across the cell and generate an EL image. Similarly, a voltage can be induced to generate a thermal image in a variation of illuminated lock-in thermography (ILIT). These contactless imaging techniques can be used to image modules while they are still mounted in the field. The modules can be imaged at night so as not to disrupt power production during the day. The contactless images can be used for a variety of characterization needs, as listed here:
• Field imaging to check for shipping and installation damage, such as cracked cells, for quality assurance;
• Imaging to monitor module performance over time to compare to baseline images, evaluate defects, and find safety issues such as hot spots and severely cracked cells before problems escalate;
• Supplement aerial, steady-state daytime thermal images to better determine causes of failures and/or degradation.
By performing imaging in the field, modules are not submitted to extra stresses and damage due to removing, transport, and remounting. Contactless imaging can be automated for nighttime data collection without any need to disconnect cables. The benefits of outdoor module imaging can lead to better reliability data of photovoltaic modules in the field. Such characterization can help the manufacturer, the customer, and the financer to understand failure and degradation issues, detect and prevent poor shipping or installation practices, and ensure confidence that the array will continue to function properly for its expected lifetime.
An example of the EL images that result from this characterization technique can be seen in the video below. NOTE: the cell characterized in this video does not contain any defects; for an example of EL imaging of a defective cell using this technique please view Fig. 1 above.
· Electroluminescence images can be gathered without disconnecting the module from the array
· Decreases time needed to test PV modules
· Requires less manual labor
· Increases safetyApplications and Industries
· Photovoltaic cells
· Photovoltaic systems
· Solar energy
· Solar power generationTechnology Status
|Technology ID||Development Stage||Availability||Published||Last Updated|
|NREL ROI 14-95||Development||Available||11/02/2015||11/02/2015|