The output of a cell declines when shaded by a tree branch, building, module dust or any other factor. The output declines proportionally to the amount of shading. For completely opaque objects such as a leaf, the decline in current output of the cell is proportional to the amount of the cell that is obscured. Hot-spot heating occurs when there is one low current solar cell (because of shading) in a string of at least several high short-circuit current solar cells, as shown in the figure below:-
Hot-spot heating occurs when a large number of series connected cells cause a large reverse bias across the shaded cell, leading to large dissipation of power in the poor cell. Essentially the entire generating capacity of all the good cells is dissipated in the poor cell. The enormous power dissipation occurring in a small area results in local overheating, or “hot-spot”, which in turn leads to destructive effects, such as cell or glass cracking, melting of solder or degradation of the solar cell.
The causes are as below: –
- Shading/soiling: overhead objects (ex. trees, poles, etc.), vegetation overgrowth, surface fouling, foreign objects on surface
- Mechanical damage: broken glass, broken/bent frame, collisions of modules with each other or other objects, improper fixturing
- Internal module failures: cell material defects (ex. shunts, high series resistance, etc.), cell cracks, local de-lamination, poor solder joints
The effects of shading and soiling can be mitigated during the system design phase. A detailed study can be performed to determine the effects of trees, poles, or other objects that may shade the modules throughout the day and throughout the year. Soiling can be mitigated with periodic maintenance of the system.
In conclusion, it can be said that hot-spot can have severe effect on the plant life and its performance. But, it can be avoided through a robust O&M practices.
You can read further about the solar cells connection and hot-spot here.