Differing POA Irradiance Depending on Weather Granularity

[JStief]

Hello - I recently ran a test on the same variant using 60-minute, 15-minute, and 1-minute TGY (typical GHI year) data from a satellite weather provider. The 15-minute Egrid was less than 60-minute (due to sub-hourly clipping), but (surprisingly) the 1-minute result was the highest out of the three. This was due to higher Global Incident in coll. plane (POA, plane of array). The top line GHI varied by one - due to rounding differences per the satellite provider explanation.

I'm hoping to better understand why the pvsyst simulation would have had this result. I reviewed the PhiAng and the hourly tracking angles are identical. See image below of the three different weather file runs' waterfalls. 

Thank you

1min, 15min, 60min pdf.pdf

Edited Friday at 10:07 PM by JStief
adding as attachment for better readability

[Michele Oliosi]

Dear JStief,

This is puzzling, especially because PVsyst will average data to 1-hour steps anyway, meaning that the modeling should be identical.

The first thing that springs to mind is that the 1-min, 5-min, and 60-min files are not simply related by averaging. In other words, if you average the 1-minute data, you won't obtain the 60-min data. Is there a chance to check that ?

[JStief]

Hello Michele - I had a similar thought so I compared the GHI total (from the weather input files). Is this what you were meaning? It could be possible this is all due to the rounding effect noted by the satellite weather data provider.

1-minute total: 1759.7 kW/m^2

60-minute total: 1758.8 kW/m^2

I also looked at it this way to see if there were any obvious patterns:

[Michele Oliosi]

Yes this is what I meant ! However, the differences seem to be small, I do not think they would account for the transposition difference. 

Another possibility that could exacerbate the effect of small GHI differences could be if DHI was not included in the file or not used at import ?

Is there a chance you could send the weather data files (MET) or the raw data over to support@pvsyst.com? This would make the analysis a bit simpler.