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Derating factors, accuracy and precision


Hi Sean,
This question follows a previous one on comparing the output in a 'irradiation rich' and 'rather hot' location (South Western US desert) versus an irradiation 'not as rich' but 'rather cold' location (South Western prairies in Canada).
I noted that when calculating the energy output of a system, irradiation and its derating (by a number of factors) is accounted. However, I can't see that temperature is taken into consideration for the calculus (it is not seen as any of the derating factors).
Now, it is clear that when sizing the array as far as number of modules we require temperature (as we have seen in the exercises in the course), but yet again, I don't see temperature when sizing the actual energy output. (Or is it accounted in the efficiency?)
I believe that software as PVwatts considers temperature as it is based in a climate database and not just irradiation, but what if I am not relying in such software or similar for the calculus?


From my experience with the NABCEP Exams, they will give you different derating factors and you use what they give you. If they leave out the temperature correction, I would assume that it is buried in another derating factor.

The way it was before software was used to calculate production was not as precise. It may have been more accurate than precise, since where one factor is too low, the other factor would be high to make up for it.

If you look at shading and performance with a Solmetric Suneye or a Solar Pathfinder, it does a good job, but is also not giving us data that is exact. There are many foggy days that would benefit to some degree from open sky outside of the sunpath. You do not really see anyone taking that into consideration. It would have to do with the regional and seasonal quality of diffuse light.

Software is more precise and accurate, so we use software in real life situations. Modeling with derating calculations will help us understand how everything works.
Another thing you do not see with most software is how bypass diodes go into the equation. Weather is variable, most people agree that the climate is changing. The best we can do is take an educated guess and use as much historical data as we can.

Have you ever seen the Photon tests?


Here we find all kinds of variability. They are supposed to be testing the PV at STC and when they take it to a field in Germany and see what the production is over the years, there are some modules that are producing 10% less energy per year than other modules watt for watt installed. Many of the manufacturers do not like these tests for obvious reasons and a few really like the tests.

When large projects are done, often independent labs are hired to secretly procure average modules and test them. I got to see this lab:


and this lab:


Both labs were very impressive, with machines that shake, bake and freeze PV modules over and over again.

Sean White

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