radiation on vertical surface

I have performed different simulations to compare the radiation striking vertical PV modules either east or west orientation.
The radiation on the west orientation is much lower than the est orientation this difference is about 40% using the PEREZ model and 34% using the isotropic model.
I performed the same simulations with PVsyst and the difference is only 1% (est greater than west).
Can you please tell me to justify such results?
Thank you
Regards
Marco

Hi Marco,

I assume you used the same weather file and array tilt and azimuth values for your comparison between SAM and PVsyst.

Could you reply with a copy of the .sam file and weather file you used for this analysis?

Best regards,
Paul.

Hi Marco,

Thank you for emailing your files.

I have a qualitative explanation for the results you are seeing. The following heat map shows the hourly DNI data from your weather file. I added a green horizontal line to emphasize Noon. You can see that more solar energy is available over the year in the morning than in the afternoon, so I would expect an east-facing array to collect more energy over the year than a west facing one.



I have to look into the rear-side calculations for the bifacial model with tilt = 90 and azmiuth = 90 or 270. The model was originally developed based on data from arrays with tilt angles between 15 and 45 degrees and facing the equator, so I am not sure it was tested for vertical east- and west-facing arrays.

In any case, if you compare the monthly electrical output of the two arrays, the difference is much smaller than the difference in monthly plane-of-array front-side and rear-side irradiance values. I will follow up when I've had time to investigate further.

Best regards,
Paul.

Dear Paul, thank you very much for your kind and useful answer. In your answer, you referred to "The following heat map shows the hourly DNI data from your weather file."
I wasn't able to find this map, can you please check if this map is available?
I'm looking forward to receiving your following up on this topic when available.
Best regards
Marco
 

Hi Marco,

Sorry about that! I forgot to attach and insert the picture -- it should be there now.

Best regards,
Paul.