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Effect of a small solar cell tolerance to the total PV panel power

We can see in the article about bypass diodes that just one shaded solar cell may kill the entire panel's power production. When the shaded solar cells have a power loss of 25% or more, the total module current is equal to the current of the weakest cell when no bypass diodes are used.

But I asked myself: what is the effect of a small solar cell tolerance to the total PV module power? So I have simulated what happens if one solar cell has a little bit less return than the rest. The simulations are based on Sunpower monocrystalline silicon solar cells. The conclusion of my investigation is that it doesn't matter if one solar cell has a loss of say 5%.

This circuit is used for the SPICE simulations in Multisim:

SPICE simulation of PV panel mismatch loss due to solar cell tolerance
SPICE simulation of PV panel mismatch loss due to solar cell tolerance

The circuit has only 5 solar cells, but the simulations are also done with 20 solar cells.

PV panel MPP mismatch loss IV curve

Simulated is the situation, in which one solar cell has 5% loss. The solar current of the poorly cell is 4.75A and that of the rest is 5A. While the current difference is 5%, we see that the difference in power at the MPP is just 1.3%: thus the solar cell efficiency will be averaged out.

PV panel MPP mismatch loss IV curve
PV panel MPP mismatch loss IV curve

PV panel loss versus single solar cell loss

With a larger number of solar cells there will be better averaged out. To make this clear, I have simulated two PV panels, one with five solar cells in series and one with 20.

PV panel loss versus single solar cell loss with 5 solar cells
PV panel loss versus single solar cell loss with 5 solar cells

The graph is quadratic: small solar cell variations are averaged out. Large solar cell variations are not averaged out but will predominate; this is the case with shadow. In the graph we see that a bypass diode over the poorly solar cell will limit the total loss to 40%.

PV panel loss versus single solar cell loss with 5 solar cells
PV panel loss versus single solar cell loss with 5 solar cells

PV panel loss versus single solar cell loss with 20 solar cells
PV panel loss versus single solar cell loss with 20 solar cells

If one solar cell in a 20 cell PV module has a return of -5%, the total PV module power is just 0.4% less.

Conclusion

  • Small variations of solar cell returns are averaged out.
  • Small variations of solar cell returns are better averaged out than large variations.
  • If the solar cell variations are small, it makes no sense to match solar cells in a solar panel.

 

Do you have any comments? Please let me know.
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