GlobHor and GlobInc for horizontal trackers

[MarcoMalagnino]

Dear all,

I have a question about the calculation of the irradiance on module plane for single axis horizontal trackers.

Comparing the hourly values of the GlobHor and GlobInc I have noted that at the noon the GlobInc is slightly higher (in my case between 0,5% and 1,8%, avg 1%) than the respective GlobHor at the same hour.

Actually I expected that at noon, because the tracker is in the horizontal position, the GlobHor and GlobInc have the same value. But it is not the case.

Why there is this difference between them?

Thanks in advance

[André Mermoud]

Are you sure that your noon corresponds exactly to the "solar noon" (i.e. sun's Azimuth = 0)?

This cannot be the case along a full year.

Therefore you always have a slight tilt of your trackers, which may explain this difference.

[MarcoMalagnino]

Dear Mermoud,

thanks for your quickly replay, but I am not sure that this is the only cause of that difference.

I have simulated another location, this time in USA (CA).

I have imported a TMY3 file from NREL (but using the Meteonorm format because the data comes from NSRDB Data viewer) with the hourly values of GlobHor, GlobDiff, Tamb and Wind. I am always using an horizontal tracking system and in this case the hourly values of the calculated GlobTilt is really higher than the respective hourly values of the entered GlobHor: e.g. At end of April the max GlobHor is about 745 W/m² and the respective GlobInc is 1235 W/m².

Obviously the irradiance level plays an important role for such systems, above all for the clipping losses of the inverter.

I will send you in a separate EMail the meteo file and the PVsyst Project.

Could you please advise me which is and where is the mistake I make in generating the GlobInc values?

Thanks in advance.

Marco

[MarcoMalagnino]

Dear Mermoud,

I have reviewed my simulation and I have noticed that, even if I have entered the time-shift correction (-29 minutes for this location), PVsyst has not changed it. After several attempts I was able to get the desired time-shift modification and now it seems that the irradiance levels on module plane are more plausible (now the max GlobInc is 1100 W/m²).

Thanks for your time and disposability.

Best regards

Marco