What are Bypass diodes in solar panels?

The bypass diodes were discussed briefly in the design of the solar panel, but their complete functionality was left for later. Bypass diodes are actually employed in solar panels to reduce the adverse effects of shading. Bypass diodes are high-power diodes (to withstand high current in the solar panel), that remain reverse biased if the solar panel is unshaded and no current flows through them. However, when a solar panel or a part of its shaded, the bypass diode gets forward biased and bypasses a string/s of cells in the solar panel or bypasses the whole panel. Usually, three bypass diodes are provided in the solar panel for a string of twenty (20) cells in a panel of 60 cells. The bypass diodes protect the solar panel from damage due to the creation of hot spots in shading conditions.

Current mismatch and hot spot creation

Current mismatch in series-connected solar cells occurs when the cell is shaded or somehow, is producing a lower current than other solar cells. When such a situation occurs, the shaded solar cell becomes reverse-biased, and essentially the current from all of the unshaded solar cells passed through it, and the shaded cell starts dissipating heat energy instead of creating it. This excessive heat dissipation concentrated in a small area leads to the formation of a “hot spot”, as shown in Fig. 1(a). The hot spot can be potentially fatal for a solar panel as it can crack the encapsulant glass layer in the solar panel. When this happens, the whole solar panel becomes useless and should be discarded. The bypass solves this problem and bypasses the entire string of cells where shading or mismatch is present, as depicted in Fig. 1(b). Another way of avoiding hot spot creation without using bypass diodes will be to drive a lower current equal to the current produced by the shaded cell. This scheme lowers the current in the entire string of panels (as they are connected in series), and therefore, yields very little power.

Fig. 1: (a) Current mismatch in string of solar cells, and hot spot creation. (b) Flow of current under normal and mismatch conditions.

Benefit of bypass diodes by different cases

In order to understand the functionality of the bypass diode further, we can create multiple scenarios from a string of 5 solar panels (with 60 cells and three bypass diodes) that is attached to an inverter. The inverter drives the current through the entire string. Under normal conditions, all cells contribute to power generation and we get 100% power output from the solar panels where each panel contributes 20% of total power. These scenarios are depicted in Fig. 2. For simplicity, if one cell from a string of cells is shaded, we will call that string to be shaded and if three strings from a single panel are shaded, then that panel, as a whole, will be considered to be shaded.

Fig. 2: Increase in power output from a string of solar panels by using bypass diodes explained with the help of three cases. Case-1: 1 string of cells in 1 panel is shaded. Case-2: 2 panels and 1 string are shaded. Case-3: All panels are shaded.

Case-1

In case-1, one cell (one string) from panel 5 is shaded by 50%. In this condition, the inverter can either drive the 50% current through the entire string or use one bypass diode in solar panel 5 which increases the power output.

Power without using bypass diodes = 50%

Power with using bypass diodes = 20%+20%+20%+20%+2/3*20% = 93.33%

In this case, the inverter designed to give maximum power will use bypass diodes and provide 93.33% power output.

CASE-2

In case-2, two panels and one string from panel 3 are shaded by 50%. In this condition, the inverter can either drive the 50% current through the entire string or use bypass diode which increases the power output.

Power without using bypass diodes = 50%

Power with using bypass diodes = 20%+20%+2/3*20% = 53.33%

In this case, the inverter designed to give maximum power will use bypass diodes and provide 53.33% power output.

CASE-3

In case-3, all panels are shaded by 50%. In this condition, the inverter can either drive the 50% current through the entire string or use bypass diode which will reduce the power output to zero.

Power without using bypass diodes = 50%

Power with using bypass diodes = 20%+20%+2/3*20% = 53.33%

In this case, the inverter designed to give maximum power will drive 50% current through the string of panels to achieve 50% power output, instead of using bypass diodes.

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