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 <k...@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 > >
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