In PV modules, diodes—semiconductors that allow current to flow only in one direction—provide a path for electrons to bypass a shaded section of a module. The more bypass diodes, the better the shade mitigation.
While some flexible thin-film modules incorporate bypass diodes on every cell, standard rigid crystalline PV modules typically provide only one bypass diode per 18 or more cells, since bypass diodes can only be wired in the module junction box. (Note: Even modules that come prewired with “quick connects” have small junction boxes that house these diodes.) For example, a 36-cell module will typically have only two bypass diodes. Each diode is wired across 18 cells, essentially isolating one-half of the module. In this case, one completely shaded cell would cause 50% of that module’s output to be lost. With a 48-cell module that has three diodes, one shaded cell would cause a loss of only 33% of the output.
If shading happens to more than one cell, the impact will depend on how much shading occurs and where it occurs. If several cells in each section of the PV module are shaded, then pathways for the electricity for that module are shut down and the module is disabled. (For more information, see “Bypass Diodes” in HP107.)