Shading

The amount of energy that a PV system produces has a direct relationship to the amount of sunlight that it receives. Shade therefore has a significant effect upon production. It is ideal for PV modules to be in a location without any shading during the entire day and throughout the year, unfortunately this is often not the case. As the sun rises or falls in the sky, its angle becomes lower and the shadows that are cast become longer, which means that shade is an inevitability in most locations. If a PV module can be kept shade free from 9:00 to 15:00, its production will not be severely affected, but shading during these crucial hours when the sun is at its strongest will cause a significant drop in production. How badly production will be affected will depend upon a few different factors:

• 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

As the sun changes position in the sky throughout the day and throughout the year, it is important to consider shade during the entire year when designing and installing a PV system. Shading issues become more pronounced at higher latitudes. For more information on the position of the sun through the year, see Tilt and orientation.

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  Shadow cast at 12:00 on December 21 in Bariloche, Argentina (41°S).

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  Shadow cast at 12:00 on June 21 in Bariloche, Argentina (41°S).

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 current from other modules or other cells within the module will simply bypass the poorly functioning columns. The typical PV module only has three different strings of PV cells, connected along the long axis of the module, with each one having its own bypass diode. 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 (Triangular symbol at top).

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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.

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  60-cell module with low intensity shading.

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  60-cell module with a line of high intensity shading.

Evaluating shade

It can be difficult to evaluate the impact of shading from mountains or trees in another season when evaluating a potential project. There are various tools available that can help perform an analysis of the impact of shade on production throughout the year. This information can be critical to design and If there are significant 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.

Module orientation and shading

If there is strong shading from an obstacle like a mountain to the East (shading in the morning) or West (shading in the afternoon), orienting the panel somewhat in the direction with the better solar resource should be considered in order 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.