Shading is a problem in PV modules since shading just one cell in the module can reduce the power output to zero.
- Shading one cell reduces the output of the whole string of cells or modules.
- Excess power from the unshaded cells is dissipated in the shaded cell.
- Bypass diodes isolate the shaded cell.
Shading of a Single Cell
The output of a cell declines when shaded by a tree branch, building or module dust. 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.
Shading of a Cell in a Module
An individual solar cell has an output of 0.5 V. Cells are connected in series in a module to increase the voltage. Since the cells are in series, the current has to be the same in each cell and shading one cell causes the current in the string of cells to fall to the level of the shaded cell. Typically, the module ISC is reduced to the lowest ISC of all the cells in a string. Shading just one cell in a module to half causes the output power of the whole module to fall to half. No matter how many cells there are in the string, completely shading one cell causes the output power of the module to fall to zero. The lost output power of all the unshaded cells is dissipated in the shaded cell. It is even worse at the system level, where multiple modules are in series to increase the system voltage to 600 or 1000 V and shading one cell would affect the entire module string.