The Solar Design Book

Shadows

Shadows can kill the output of a module or system, more than you would expect. Depending on the design of the module, shadowing just one cell can drop the output to almost zero. If just half of one cell is shadowed, the output of the whole module can drop to almost half.

Why is this? Sunlight provides the electrons in the cells that allows current to flow through them. No sunlight, no electrons. No electrons, no current. Since the cells are wired in series, this blocks the current from flowing through all the cells. In the same way, one cell shadowed in a module that is connected in series with other modules will kill the output of all the modules. Bypass diodes can allow current to flow around the module with the shadowed cell, but the output still suffers.

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If the voltage builds up enough across a cell, the cell will break down, forcing current through. This can be very hard on the cell, causing it to overheat and can ruin the cell, in turn ruining the module. This condition can happen if modules are connected in series. To prevent these hot spots, bypass diodes are provided in the junction boxes that shunt current around the shadowed cell or cells. This prevents the hot spot, but does not change the fact that the module with the shadowed cell is not producing the normal power.

Do not allow shadows on modules. Clean off bird droppings, leaves, or other debris from the modules. If the array does not get sun until later in the day, or is shadowed in the afternoon, the output will suffer. The SolarSizer program, supplied with the Solar Design book, will calculate how much the power is reduced by shadowing in the morning and afternoon.

Amorphous silicon modules are not as effected as crystalline modules. That is because the cells are long and skinny and hard to completely shadow. Also, amorphous cells will allow some current to pass even when shadowed.