Shading can occur due to various sources, such as trees, buildings, clouds, dust, or snow. Shading can be partial, where only some cells are covered, or complete, where the whole module is covered. Shading can also be static, where the source does not move, or dynamic, where the source moves with the sun or the wind. Shading can affect series-connected modules in different ways, depending on the extent, location, and duration of the shade.
When a cell is shaded, its current output drops significantly, while its voltage output remains relatively constant. This creates a mismatch between the shaded and unshaded cells in the same module or string. The unshaded cells try to force more current through the shaded cells, which causes them to heat up and dissipate power as heat. This reduces the overall current and power of the module or string, and can also damage the shaded cells over time.
To prevent this, most modules have bypass diodes connected in parallel with each cell or group of cells. These diodes allow the current to flow around the shaded cells, instead of through them. This reduces the power loss and heat generation due to shading, but also reduces the voltage output of the module or string. The bypass diodes only activate when the voltage difference between the shaded and unshaded cells exceeds a certain threshold, usually around 0.5 V.