PV Degradation Rate

[BigInch]

The degradation rate of PV current production is yidely reported with values between 0.5% and 1%/year.

From example calculations I have seen this rate appears to be used as a constant, but I don't understand how it could possibly be a constant. Certainly, with a degradation rate of 0.5%, the average power output in any given year must be = Wo * (1-.005)^(year-1). Are the examples I have seen in error?

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[GregLocock]

Do you know what causes the degradation? I can see two obvious continuous mechanisms - degradation of the encapsulation (yellowing) due to UV, and cosmic rays etc degrading the semiconductor. There'd also be discrete things like cracked cells and failed solder joints.

I have seen numbers like 80% output after 20 years,



Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[BigInch]

Right Greg. Most major mfgrs do guarantee their panels to 80% for 25 years. They just say degradation is a result of exposure to solar radiation and resulting tarnishing. I suspect it may have a molecular component, but I don't know those details. But, treating 0.005 as a constant will give a net production of 0.8750 vs 0.8347 of the original after 25 years. Not much I suppose in terms of the variability of sunshine over the years, but in terms of being theoretically correct, and if you're dreaming in multi-megawatts, I like to be on the side I can justify, so trying to find out if anybody knows the theory behind it.

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[GregLocock]

If you are doing this commercially then check out this book

Green, MA. Silicon solar cells: advanced principles and practice. Sydney, Bridge Printery, 1995.

I think it will have some discussion of degradation. You could write to Martin at UNSW, he is a very helpful guy.






Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[BigInch]

Found it. Apparently its not truely linear, but still fits a linear correlation for all practical purposes. Actually it is more rapid during initiatial exposures. Its caused by UV radiation effects on the p-n junctions.

http://www.nrel.gov/docs/fy02osti/31455.pdf

CONCLUSIONS

Analysis of data from a long-term solar weathering
study of commercial crystalline Si modules has revealed a
slow Isc degradation, which began after the rapid initial lightinduced
degradation caused by oxygen contamination in
boron-doped Si solar cells. The rate of the slow degradation
may be intrinsic to individual solar cell manufacturing
processes. Because of a linear correlation between the
slow degradation and the UV exposure dose, and because
the same rates were observed under UVA-340 fluorescent
illumination, the degradation is probably caused by UV absorption
at or close to the top Si surface. Quantum efficiency
measurements indicate that the bulk of the Isc losses
(both rapid and slow) occurred in the response region above
700 nm. Considering previous literature reports of Isc losses
with our results, it is very unlikely that the slow degradation
was caused by encapsulation browning. The degradation
also does not seem to be thermally activated.

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"What gets us into trouble is not what we don't know, its what we know for sure" - Mark Twain