Partial shading

Partial shading happens when part of a cell or module gets less sunlight because something blocks it. Common causes are trees, poles, chimneys, nearby buildings, cables, soiling, or the next row of modules. Cells and modules are wired in series, so the power lost is much larger than the shaded area alone would suggest. The shade pattern moves with the time of day and the season. A shaded cell can be pushed into reverse bias. It then uses up power and heats, which can create a hot spot or hot cell. Hot spots from long-lasting shade can lead to burn marks, bypass diode failure, glass breakage, arcing or fire. They can also speed up module aging. Frequent shading keeps bypass diodes running hot, which raises their failure rate. Hot spots from shading also add thermomechanical stress that fatigues cell interconnections. Hot cells caused by shading can also lead to discoloration, cell cracks, delamination and loss of electrical insulation. You can find shading by looking at the array, by comparing monitoring data from shaded and unshaded strings, or by running shading simulations. Infrared thermography can also help. Shading is best handled at the planning stage. Options include string and module layout, module-level power electronics, MPPT settings, and shade-tolerant modules such as half-cell or back-contact designs.