GTI instead of GHI

[Florent D.]

Hello !

I have GTI data from site (TMY format). I would like to know if it is possible to use them directly in my PVsyst simulation ? 

It means that no transposition from GHI would be allowed. 

If possible, how ? 

Thanks,

Best Regards,

Florent D.

[Linda Thoren]

Dear Florent D.

You can import your measured GTI data as a “Custom file” in the PVsyst database. For the Meteo data, tick the box marked “Measured global on plane” and indicate the column with the GTI data, then in the Variables tab you specify the Azimuth and the tilt.

Regards

[MicheleANE]

Dear Linda,

While it's true you can import GTI data this way, I believe PVsyst will once calculate GHI from this imported GTI and then calculate GTI internally. This 2 GTIs will probably differ by some value. I think Florent was asking if it's possible to keep the exact values as imported.

Of course correct me if I am wrong, best regards.

[Florent D.]

Dear Michele and Linda,

Thanks for your feedbacks. 

Yes, I would like to keep the exact values as imported. 

If PVyst recalculate the GHI then GTI from the measured GTI, there is no benefit. 

Are we sure that PVsyst would recalculate ?

[Michele Oliosi]

Yes, PVsyst does at the moment recalculate the GTI from a GHI and DHI values. GHI and DHI may be generated from an input measured GTI.
This is because as things stand, a MET file can be used with any orientation, i.e., it should be useable with an orientation other than that of the measured device. Moreover, the GTI itself does not have all the needed irradiance information, we need to decompose into diffuse components and direct component in order to make all calculations e.g., IAM or shadings.

However, recomputed GTI and original GTI will in general differ by at most 1%.

[rMotta]

I see that the .MET file always compute GlobHor and DiffHor from GlPMeas.
So I tried to change manually the GlPMeas, GlobHor and DiffHor to my field measurements.

Even using Hay in the Project Settings, my simulation output shows:

• GlobHor = modified GlobHor;
• DiffHor = modified DiffHor;
• GlobInc < modified GlPMeas (4,9%).

So what is the rule? Is there a range that GlobHor can change from GlPMeas and disable the GTI measurements? 

Is there any Advanced parameters to change and allow  the use of measured GTI?

[Michele Oliosi]

In principle, you cannot input all of GlobHor DiffHor and GlPMeas when importing data. Which ones did you select in your MEF ?

In order to use the GTI you need import only the GlPMeas.

Where did you change the values manually ? I do not clearly see how to do that in PVsyst.

[rMotta]

In My MEF only selected the GTI and Tamb.

So I create "reference MET" and then change manually (encode62) the GlPMeas, GlobHor and DiffHor to my field measurements.

Lets call the last one as "edited MET".

When I use "edited MET" to simulate the system, the hourly output shows:

• GlobHor = edited GlobHor;
• DiffHor = edited DiffHor;
• GlobInc < edited GlPMeas (4,9%).

The simulation don't use my edited GlPMeas.

[Michele Oliosi]

I see. This is normal then, because the edited GHI and DHI were chosen in order to give back GlobInc close to GlPMeas when transposed. If you change them, you will not get back the same value.

When comparing transposed measured GHI and DHI to GlPMeas, you are accounting for:

1. fundamental uncertainty in the measurement (calibration, noise, ...)
2. uncertainty in the measurement system installation (orientation, position...)
3. Hay model uncertainty

Because of this I cannot tell you what is the single source of the 4.9% discrepancy.

One thing that is on our roadmap is use one of either measured GHI or DHI in addition to GlPMeas in order to have a better equivalence at the GHI and DHI level. I agree that it would be better with such an option.