Hot spot heating happens in a string when there is one low current solar cell, from a number of several high short circuit current solar cells.
A single shaded cell in a string can reduce the current through the unshaded cells, leading to the unshaded cells producing higher voltages which can often reverse bias the shaded or bad cell.
What is Hot Spot Heating?
When the operating current of the whole series string approaches the short circuit current of the “shaded” cell, the total current produced becomes capped by the shaded cell current. The additional current produced by the unshaded good cells then forward biases the unshaded solar cells. When the series string is short circuited, the forward bias across all the unshaded cells reverse biases the shaded cell. Hot spot heating happens when a large number of series connected cells cause a large reverse bias across the shaded cell, leading to a huge dissipation of power in the shaded cells. Basically the entire extra generation capacity of the good cells is dissipated in the shaded cell. The huge power dissipation occurring in a small area leads to local overheating, or “hot spots”, that in turn causes defects, like cell or glass cracking, melting of soldering or solar cell degradation.
Main reasons for Hot Spots
There are many reasons for hot spots, either functional or operational.
- The functional reasons are grouped into two areas:
- Cell mismatch happens if cells of variable current production are connected in series.
- Cell damage can happen during the manufacturing process since the silicon cells can be subjected to different stresses during the processes of lamination, handling and transportation.
2.The operational causes for hot spots are linked to the solar array design and operation, and can include:
- Winter Shading
Panels will suffer from systematic shading of the bottom row of solar cells each morning and evening in winter season. Installation of panels in landscape orientation can enable the bypass diodes to operate and enable the generation of 10 percent electricity, even in winter conditions.
- Rooftop Conditions
Rooftops present different challenges as when cells are completely shaded (because of system design), this may not be enough for triggering the bypass diode, leading to increased temperature which causes degrading of the panel. Partial shading that is caused by trees or tall vegetation must be controlled.
Panels’ surfaces can be soiled because of dust, dirt or other contaminants during their lifetime. The Operation &Maintenance company must identify as well situations requiring cleaning. The frequency of cleaning visits will heavily rely on the climate conditions on location of installation.
Effect of Hot Spots on the Electrical Protection of the Panel
The consequences of hot spots can range from serious fires to increased aging of the materials. A hot spot occurring due to, or leading to, short circuit between the back and front of the cell will result in localized high intensity heating. This kind of hot spot can lead to melting of the back sheet and can cause fires.