Shading is a problem faced in the operation of all PV modules since the shading of just one single solar cell in the module can reduce the total power output.
Effects of Shade on PV Output
PV systems generate electricity based on the amount of incident sunlight on the module surface, so when a shadow is cast on the panel, the output power decreases. Yet, the decrease in power could be really worse than expected.
Initially it is expected that power output of the panel will be decreased proportionally to the area of the panel that is shaded. Yet, shading only one cell out of 36 cells in a small solar module can lead to reduction in power output by over 75%.
Shading of a Single Cell
The output of a cell decreases drastically when shaded by any blockage like a tree branch, a building or dust accumulating on the module. The output decreases proportionally to the percentage of shading. For completely opaque objects like leaves, the decrease in current output of the cell is proportional to the percentage of the cell that is blocked.
Shading of a Module
The solar cells in a PV module are all connected in series. Shading a single cell leads to the current in the string of cells to fall down to the level of the shaded cell.
Water flow Analogy
To really understand how it leads to such severe losses, it can be helpful to use the analogy of flow of water through pipes. The flow rate of water through pipes is constant, similar to the flow of current through a solar cell string, which is constant for a specific irradiance level.
The shading a solar cell is similar to the effect of the introduction of a clog in a water pipe. The clog in the pipe causes restriction to the flow of water through the whole pipe. The current through the entire string is similarly reduced when the solar cell is shaded. This is noteworthy as every cell in the cell string has to operate at the current level produced by the shaded cell. This leads to the unshaded cells operating at a level below the maximum power level. Thus, just a small amount of shading can lead to a dramatic effect on the output power of the solar panel.
Similar principles can relate to the PV modules connected together. The current flowing through the entire string of PV modules can be reduced heavily even if just one single module is shaded, leading to significant potential loss of the power output.
Fortunately, there are various different approaches which can be applied in the design of PV system so as to reduce losses from shading. These include using different bypass diodes, stringing arrangements and MLPEs (module level power electronics).