Bud Martin
Solar Wizard
- Joined
- Aug 27, 2020
- Messages
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Interesting reading from Sunpower. I was looking to learn about shading effect and bypass diode, I did not know about heating issue due to shading, I member one of the member shows the pictures of burnt up cell that melt the panel.
Reverse Bias and Bypass Diodes
When cells are put into reverse bias, instead of converting photons to electricity, they convert electricity to heat. This happens any time the current generated by the rest of the cells in the string of modules exceeds the current that a cell can support. Common causes of reverse bias are shading from leaves, chimneys, or the buildup of soiling along the bottom of a module.
If too much heat is dissipated from a reverse bias cell, particularly if that heat is concentrated to a small defect region on the solar cell, the heating can cause failure of the other components within the module, such as the backsheet, glass, or encapsulant. Conventional Modules rely on bypass diodes to deactivate the portion(s) of the module where one or more cells are shaded, typically resulting in a 33% power drop. While this has a negative effect on energy output, it mitigates the heating issue. One tradeoff of this design is that the reliability of the module is tied to the bypass diode.
Diode life depends on several factors such as diode quality, junction box handling, and module installation; but, all diodes will eventually fail with use. This is because an activated diode runs at an elevated temperature, reducing the remaining life of the diode [6], [7]. Depending on how a diode fails, it can permanently remove a substring from the module or allow a shaded cell to run unmitigated in reverse bias, leading to module failure.
SunPower’s cells run in reverse bias at much lower temperatures, so bypass diodes are not required to ensure long term reliability (Table 1). However, when enough cells are in reverse bias, the cumulative power loss from the shaded cells can exceed the power produced from the cells in forward bias, so SunPower includes diode protection to enhance energy yield.
Reverse Bias and Bypass Diodes
When cells are put into reverse bias, instead of converting photons to electricity, they convert electricity to heat. This happens any time the current generated by the rest of the cells in the string of modules exceeds the current that a cell can support. Common causes of reverse bias are shading from leaves, chimneys, or the buildup of soiling along the bottom of a module.
If too much heat is dissipated from a reverse bias cell, particularly if that heat is concentrated to a small defect region on the solar cell, the heating can cause failure of the other components within the module, such as the backsheet, glass, or encapsulant. Conventional Modules rely on bypass diodes to deactivate the portion(s) of the module where one or more cells are shaded, typically resulting in a 33% power drop. While this has a negative effect on energy output, it mitigates the heating issue. One tradeoff of this design is that the reliability of the module is tied to the bypass diode.
Diode life depends on several factors such as diode quality, junction box handling, and module installation; but, all diodes will eventually fail with use. This is because an activated diode runs at an elevated temperature, reducing the remaining life of the diode [6], [7]. Depending on how a diode fails, it can permanently remove a substring from the module or allow a shaded cell to run unmitigated in reverse bias, leading to module failure.
SunPower’s cells run in reverse bias at much lower temperatures, so bypass diodes are not required to ensure long term reliability (Table 1). However, when enough cells are in reverse bias, the cumulative power loss from the shaded cells can exceed the power produced from the cells in forward bias, so SunPower includes diode protection to enhance energy yield.
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