Modeled GPI Does Not Match Imported Measured GPI

[kjs55]

PVsyst v6.84: I am importing measured irradiance data from a tilted global POA pyranometer (GlobInc aka GPI). I would expect Mod GlobInc to exactly align with the Mes GlobInc that I am importing; however, this is not the case.

The first measured GlobInc value is 446.7 W/m^2. Upon importing, I look at the modeled GlobInc value at the same timestamp. GlobInc (Hay model) for the same tilt and azimuth is 416.0 W/m^2. So, there is a 7% deviation b/w Mes and Mod GlobInc at this timestamp (at times the deviations are larger than 7%).

The deviations are notably larger at the edges of the day (morning/afternoon) compared to around solar noon.

I have set PVsyst to use the Hay transposition model per the following post:

https://forum.pvsyst.com/viewtopic.php?t=40/

Another note: For both Hay and Perez, it reports albedo = 0.2 for GlobInc; however, there is no option to define albedo when importing custom measured irradiance. Should albedo be a user input to the custom data import tool?

Edited December 16, 2019 by kjs55

[dtarin]

That article is quite old. Perez is the default transposition model. Using measured diffuse irradiance will result in higher transposition accuracy for GlobInc.

[kjs55]

Thanks @solarguru but the deviations I reported are larger (>10%) using Perez vs. Hay

I am interested to hear PVsyst's opinion on this

There are no issues reported for the imported measurement data however the PVsyst POA does not match the imported values

[jfolguera]

Hi,

Do we have any feedback on this? I have the same problem when importing GPI from my pyranometers. They don't match what PVsyst "calculates" after and the data on the report.

Thanks for the help.

[dtarin]

PVsyst POA is dependent on your weather file, location, and system design. There are a lot of questions one could ask, such as how well does your PVsyst model match actual site conditions, is there complex terrain for this site in question? If you are comparing measured to modeled and the results are way off, then perhaps you have an inaccurate weather file. Does GHI/DHI in your model match measured? If it doesnt, you shouldnt be expecting GlobInc to match measured. If your weather file is TMY, that is a P50 weather file.

[André Mermoud]

When importing POA data, the aim is to find GlobHor and DiffHor values wich will restitute the exact GlobInc corresponding to your input data (with the same transposition model).

Now this calculation (transposition) involves the solar geometry.

Therefore the time of your data should match the time at which PVsyst will calculate the solar geometry. You have to specify the TimeShift for getting this match.

Please use the tool "Databases > Meteo Tables and Graphs > Check Data quality".

And carefully read the help "Meteo Database > Notes on Meteo > Meteonote9_Time shift in imported meteo files".

As far as you don't correctly define this match at the import time, your data will be incorrect, and you will have discrepancies either in the morning or in the evening, depending on the sign of the time shift.