Reexamination of Photovoltaic Hot Spotting to Show Inadequacy of the Bypass Diode

Hot spotting is a reliability problem in photovoltaic (PV) panels where a mismatched cell heats up significantly and degrades panel performance. High temperatures due to hot spotting can damage cell encapsulant and lead to second breakdown; both cause permanent damage to the PV panel. Although bypass diodes are used for protection and qualification tests are used to reduce cell mismatch, these strategies are shown to be insufficient for hot spot prevention. This paper reexamines the hot spot problem in PV strings through simulation and load-line analysis. Results show that cells in typical panel string lengths are susceptible to hot spotting because of reverse bias behavior. A number of existing and emerging solutions aimed at hot spot prevention are discussed and evaluated. Commercially available active bypass switches are an improvement over passive diodes but do not prevent hot spotting. Cells with low breakdown voltages limit power dissipation but are not fully vetted as a long-term solution. A combination of hot spot detection and open-circuit protection is a complete solution to hot spotting.