[RE-wrenches] Future low Vmp

[Corey Shalanski]

Hi Nathan (and Kent),

Back around 2020, I reached out to Sarah Kurtz about degradation
dependencies, and she offered the following:
"The degradation depends on the technology and, indeed, on the specifics of
the product... The historical observation that silicon module degradation was
dominated by short-circuit current is changing - more recent data suggest
that the newer cells are more likely to lose fill factor and/or voltage."

We base our calculations on the Vmp value at STC and on the ASHRAE 2% dry
bulb temperature averaged over the 'summer' months (June/July/August). In
order to estimate the cell temperature, we add the following according to
the array type:
  - Canopy / ground / pole +25 °C
  - Roof (rack/tilt-up) +30 °C
  - Roof (flush/low-slope) +35 °C
To predict future Vmp values, we apply an assumed 0.25% annual degradation
rate. Given the overall uncertainty with the dependencies, I believe this
assumption can be considered both reasonable and perhaps slightly
conservative.

Corey Shalanski
Cherry Street Energy
Atlanta, GA


On Thu, Apr 10, 2025 at 6:46 PM Kent <kent at coveoregon.com> wrote:

>
> Nathan,
>
> In the NREL report Photovoltaic Degradation Rates — An Analytical Review
> by Dirk C. Jordan and Sarah R. Kurtz
> <https://www.nrel.gov/docs/fy12osti/51664.pdf> the conclusion is that the
> 0.5% or so annual degradation of PV module output is mostly due to changes
> in PV module current. Other studies have come to the same conclusion. So,
> long term changes in voltage might not be worth worrying about.
> Kent Osterberg
> Blue Mountain Solar
>
>
> On 4/10/2025 4:31 PM, Nathan J. Stumpff via RE-wrenches wrote:
>
> Hi wrenches,
>
> I used to do these kind of calcs all the time but am wicked rusty. What
> assumptions do you use for future (degraded) hot temp Vmp of a PV string to
> check if you will still be able to operate inside a given MPPT range?
> Specifically interested in assumed yearly degradation to Vmp, assumed
> actual operating cell temp, and if it is appropriate to use STC value as
> the starting point and if not what would be better (NOCT?)
>
> Thanks,
> -Nathan
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org/attachments/20250415/b82c0024/attachment.htm>