Shading

The amount of energy that a PV system produces has a direct relationship to the amount of sunlight that it receives. Shade therefore will result in reduced production, but even a little bit of shade can cause severe production losses for a PV module depending on:

• The intensity of the shade (is it really weak from something in the distance or from something only a few meters away?)
• How much of the module is shaded
• What part of the module is shaded

Bypass diodes and shading

The cells of a PV module are connected in series. If one part of a part of a series circuit has high resistance or is not performing properly, it becomes a constraint for all of the other parts of the circuit. PV modules have incorporated bypass diodes in their junction boxes which automatically bypass strings of cells that are not performing properly. The typical PV module only has three different strings of PV cells, connected along the long axis of the module, protected by bypass diodes. This has important consequences for how a PV module performs when there is shade as bypass diodes respond variably depending upon the conditions. Shading along the short-axis (rows) of the module will have a greater impact than shading along the long-axis (columns) of the module

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  60-cell module. Two columns of PV cells are connected in series, which are then connected to a bypass diode.

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  60-cell module. Strong shading across an entire row of cells will trigger all of the bypass diodes. 100% of module production will be lost.

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  60-cell module. Strong shading down an entire column of cells will only trigger one bypass diode. 33% of module production will be lost.

Partial shading and shade intensity

Strong shade and shading that covers an entire PV cell is likely to trigger bypass diodes and will result in a significant loss in PV module production. Weak shading or partial shading on a PV cell may cause overall performance of the module to drop without being strong enough to trigger a bypass diode. A common example may shade resulting from a tree branch is close to the module and provides a thick dark line of shade, it will cause significant problems, but a distant branch causing a faint line of shade will not likely cause much production loss.

Evaluating shade

If there is a shading issue in one direction, it often makes sense to orient the module slightly in the other direction to maximize production. Shading frequently happens in the morning or in the afternoon as the sun’s angle is lower in the sky. In some places it is unavoidable, but if the modules are free from shading between 9am and 3pm, production will not be very dramatically affected.

Seeing as solar modules depend on the sun to function, shade is the enemy. The effect that shading parts of a solar module has upon a solar module is not proportionate to the amount of the module that is shaded because all of the solar cells are connected together in series. Even shading one cell completely can drop production of a module by more than 25%. The degree to which shading affects the output of a module depends upon a variety of factors: the percentage of the surface area of the cell(s) that is shaded, the intensity of the shade, and how many bypass diodes (devices that help to mitigate the effects of shade) the module has. In areas where there are trees or mountains that are likely to shade a solar module and affect production there is the option of using shade analysis tools to better understand the effects. The most practical and cost effective tool is called the Solar Pathfinder.