The solar cells (60 or 72 full cells) in general, are (usually) connected in series in a solar module. As we know, the cells connected in series should have the same operating current flowing through it for fully realizing the module’s power output. However solar cells when in operation may be shaded due to leaves, clouds, bird droppings, etc. This would lead to mismatch in generation characteristics (i.e. current and voltage output) from a solar cell. When connected in series, the mismatch in voltage output does not impact the power output as such different voltages are just added up. Say in a string of 12 solar cells, 11 have voltage output of 0.6V whereas 1 cell has output of 0.55V, the total output voltage of the string is as follow: 11 x 0.6 + 1 x 0.55 = 7.15V. It is the mismatch in current output of the solar cell which is fatal for the string (and for the solar module). Say in a string of 12 solar cells, 11 cells have current output at maximum power of 7.5A whereas 1 cell (say cell X) has output of 7A. As the string operates at a single current value, the current of the cell with the lowest value (7A)presides over the string. In such case, the excess current (0.5A) from the remaining cells (11 in this case) is forced onto the cell X. This causes the operating temperature of cell X to shoot up which may lead to hotspot (A condition where the temperature of part of/entire cell rises up so much that it may rupture)on the cellin the long run (Figure 2).

Figure 2: Hotspot in a solar cell (Source: Google Images)

An effect similar to it can be observed in the solar module connected in series. Due to (say) shading, one of the solar module in an array may not able to generate the desired current (and hence the power output). Such module forces the entire array of modules to operate at a lower current. The excess power produced by the remaining modules is dumped into the partially/fully shaded module. This dumping leads to an increase in temperature of the shaded (and already stressed) solar module (Figure 3). If such shading is not mitigated, it may adversely impact the module causing hotspot and reduced power generation capacity of solar module. A single shaded module can typically bring down the power output of a plant from around 10% to as high as 40%. Additionally with formation of hotspot, the life of the solar module may be reduced to around 10 years (depending on the external conditions).

Figure 3: Hotspot in solar module due to current mismatch (Source: Google images)

In order to avoid such issues, the solar module are fitted with bypass diodes. These diodes are connected in opposite polarity in parallel to the string (usually 2) in the module (Figure 4). In normal operation, these diodes are inactive and do not participate in the generation. However if any of the string (or cell/cells in the string) is/are shaded, the entire string operates at lower current than the other strings which forces the shaded string to act as a load rather than generator (i.e. it operates in reverse bias mode). This reverse biasing (or reverse polarity) causes the bypass diodes to be in active state. They perform the following main functions:

Enable smooth operation of the module by bypassing the current from the shaded string
Protecting the shaded string from power dumping and hence from hotspot

Figure 4: Typical configuration of solar module along with bypass diode (Source: Waaree Energies)