Radiation on a tilted module seems to be inaccurate with imported climate data

[Reint]

Hi,

I tried to find a similar topic but I couldn't find one, so here's a new one.

I imported climate data from PVGIS for a location in the Netherlands between 2007 and 2016. Then, I compared the same project with both this imported climate data and the Meteonorm data between 1991 and 2010. When I compare the results, the annual sum of global irradiation is significantly lower for the Meteonorm climate data, however, the amount of generated energy is significantly higher. This seems very odd to me.

I suspect something is not going right with PVSol's calculation for the irradiance on a tilted surface with the imported climate data. When I compared the two energy balances I saw a large difference between the increases of radiation due to the orientation and inclination of the module surface in both balances. For the case with the imported climate data (2007-2016), the radiation increases with only 0,47% due to the orientation and inclination, while it increases with 8,88% for the case with the Meteonorm data.

I can't find out why this difference is so large, since it is exactly the same system. The changing ratio between direct and diffuse radiation over time might affect this percentage I guess, but not in this amount. What do you think?

PS. The first three images are from the system with imported climate data, the other three are with older Meteonorm climate data. 

PPS. The system has an inclination of 37 degrees and an orientation of 209 degrees.

[developer_mh]

Hi Reint,

that is very interesting! Could you provide the PVGIS data for Maastricht, please? A first guess would be that the timestamp might be wrong, so that the calculated elevation of the sun doesn't fit to the solar irradiance data.

Kind regards,

Martin

[Reint]

Hi Martin,

There you go!

Maastricht 2007-2016.txt Maastricht 2007-2016.csv

[developer_mh]

Hi Reint,

thanks for the data. I imported the data as is into PV*SOL, simulated and then had a look in the diagram editor at the series "irradiance onto horizontal plane" and "height of sun". These must be synchronous. So I searched for a clear sky day, like 24.06.:

As you can see, the two series are out of sync. This means the timestamp of the PVGIS has to be changed prior to the import in PV*SOL. Here, the solar irradiance seems to be araound one hour earlier than the height of sun, so we have to shift all PVGIS values one hour behind. In order to achieve that, I cut the last line of the PVGIS data and pasted it before the first line.

Maastricht p1			
50.5024,-5.3849,81,-1,-30			
			
Ta	Gh	FF	RH
0.32	0.0	3.03	96.82 //this was the last line before
0.14	0.0	3.1	96.37
-0.04	0.0	3.17	95.91
-0.22	0.0	3.25	95.45
-0.4	0.0	3.32	94.99
-0.58	0.0	3.39	94.53
-0.76	0.0	3.46	94.07
-0.93	0.0	3.53	93.61
-1.11	0.0	3.6	93.15
0.12	20.0	4.68	91.4
1.06	120.0	4.88	88.26
...

Imported and simulated, it looks like this:

So, now the irradiance seems to be slightly too late. Let us revert these changes and try with a 30 min shift. If you want to shift the data by 30min, you'll have to use excel to calculate the new values:

Imported and simulated the new data give this picture:

That is closer, but still there is a clear offset. So let's try to shift the values by 45min.

Here we go:

There is still as small offset, I would say, but it is ok now. Unfortunately I couldn't find any information on the timestamp on the PVGIS website. In PV*SOL we use the same time stamp as in Meteonorm (where we get most of our climate data from). That is, January 01, 14:00 refers to the time range between 13:01 and 14:00.

I attached the new climate data file, so you can import it yourself and see how the results look.

Hope that helps, kind regards,

Martin

 

Maastricht 2007-2016 45min.dat

[Reint]

Hi Martin,

Thank you very much for all the effort! The new energy balance with the imported data shifted by 45 min makes much more sense (check the attached file)! 

I have two additional questions:

- I'm not the biggest Excel expert. Did you have to calcalute all (8760*4=) 35040 cells "by hand" in Excel, or is there some automatic function to copy the same formula to every cell?

- How can I obtain the graphic with the height of the sun and the irradiance on a certain day in PVSol?

[Reint]

Hi Martin,

 

I the meantime, I have already solved my first question about preparing the Excel file by applying the formula to all cells.

The second question - regarding how to obtain a graphic with the height of the sun - remains still relevant to me. Thanks in advance!

[developer_mh]

Hey Reint,

on the results page there is the diagram editor. Select the data series you want to see and click 'ok':

Kind regards,

Martin