Bypass Diode (PV Module, Hot-Spot Protection)
A diode connected in parallel across a group of typically 18-24 series-connected solar cells within a PV module, with its forward direction opposite to the normal current direction of the cells, so that it does not conduct under normal operating conditions. When one or more cells in that group are shaded or become defective, they can no longer pass the current from the other, illuminated cells in the string; without a bypass diode, that cell would then be reverse-biased and forced to dissipate the full power of the remaining cells as heat, which can lead to local overheating (a 'hot spot') with the risk of permanent cell damage or even fire. In that case, the bypass diode automatically switches in parallel with the shaded cell group to carry the current, limiting the power loss to that single cell group and keeping the temperature of the shaded cells within safe limits. IEC 61730 (safety qualification) and IEC 61215 (design qualification) set requirements for the thermal testing and reliability of these diodes, since a defective bypass diode can itself also be a significant cause of fire in PV installations.
Bij een thermografische inspectie van een zonnedak valt een duidelijk warmere plek op één paneel op; nader onderzoek wijst uit dat de bypassdiode van die celgroep is doorgebrand en niet meer schakelt, waardoor de beschaduwde cellen structureel oververhit raken.
Ervan uitgaan dat bypassdiodes onderhoudsvrij en onbeperkt betrouwbaar zijn — bij herhaalde thermische cycli kunnen goedkope diodes voortijdig falen, wat bij een thermografische inspectie zichtbaar moet worden gecontroleerd.