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Module temperature correction


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I'm trying to figure out how to reconstitute the PVsyst Egrid Energy output from the following PVsyst variables outputs:

GInc, Tarray, Pnom, and PR

my logic is the following:

The Pnom corrected for module temperature is Ptc=Pnom*(1+gamma(Tarray-25)), gamma being the temperature coefficient of the modules

The Egrid_tc should be equal to Ptc*(Ginc/1000)*PR

This "logic" considers that the "PVsyst solution" is all incorporated in the simulated module temperature "Tarray".

The following extract table shows PVsyst output data and the two formulas listed above for one typical day.




I would like to understand why the first column and last column are not equal (Egrid calculated by PVsyst, and Egrid_tc calculated in excel using the above formulas).

I made sure to use the temperature coefficient gamma given in PVsyst .PAN file (0.0041). Is there another correction to be done?

Many thanks,

PS: Reference for the formulas used can be found here: http://cleanpath.com/sites/default/files/pdfs/news/SolarPro_Managing_Risks_June_2011.pdf

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The Performance Ratio PR, as it is used in PVsyst, is defined in the international standard IEC 61724.

It does not include a temperature correction for the reference yield, as you are showing in your formulas.

Therefore, the correct expression should read:

E_Grid = Pnom * GlobInc * 1kW/m2 * PR

You did not mention the Pnom for your example, but from the table you provide I calculated it to be 1.080 MW and the PVsyst values obey the above formula (up to the rounding errors).

See also the following FAQ : http://forum.pvsyst.com/viewtopic.php?f=30&t=697, or the PVsyst online help 'Project Design -> Results -> Normalized Performance Index'

Best regards,

Bruno Wittmer

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Further information.

First of all, on your table you don't mention the differences (in percentage), so that the visual analysis is very difficult.

Now in PVsyst the PR is computed from the whole simulation process.

This involves other losses which also vary along a day.

These are mainly the IAM loss, eventual shading effect, module efficiency according to the temperature, wiring resistance loss.

Other effects of second order may also occur.

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