Sombreado

La cantidad de energía que produce un sistema FV tiene una relación directa con la cantidad de luz solar que recibe. Por tanto, la sombra tiene un efecto significativo sobre la producción. Es ideal para PV modules estar en un lugar sin sombra durante todo el día y durante todo el año, desafortunadamente este no suele ser el caso. A medida que el sol sale o cae en el cielo, su ángulo se vuelve más bajo y las sombras que se proyectan se vuelven más largas, lo que significa que la sombra es inevitable en la mayoría de los lugares. Si un módulo FV puede mantenerse libre de sombra de 9:00 a 15:00, su producción no se verá gravemente afectada, pero la sombra durante estas horas cruciales, cuando el sol está más fuerte, provocará una caída significativa en la producción. La gravedad de la producción dependerá de algunos factores diferentes:

• 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 azimuth.

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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 and this information can be critical when designing a system. There are various tools available that can help perform an analysis of the impact of shade on production throughout the year. The most economical and appropriate for off-grid use is 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.