Calculating and exporting DNI

[tafadzwa]

Hi

I would like to know how PVsyst calculates DNI values if give a weather data file that contains GHI but does not come with DNI. If it is indeed the case that it uses the Erbs model (DNI = (GHI - DHI)/cos(Z), where Z is the zenith angle), I would like to know how PVsyst handle cases where the zenith angle is 90 degrees (or relatively close to that).

Also, is there a way of exporting PVsyst's calculated DNI? It appears the only meteo data closest to DNIthat one can export is the Horizontal Beam Irradiation.

[André Mermoud]

As you pointed out, when not present in the data the DHI is evaluated using the Erbs model.

For the very high zenith angles (low sun height on the horizon), the DNI is indeed not well defined. If its value becomes too high from the GHI/DHI values, it is limited to the clear day model.

This is especially the case when importing POA values. If the time definition of the data doesn't correspond to the time used for the calculation of the solar geometry, we can have very high uncertainties in the morning or the evening. This is the reason of the "Time shift" correction in PVsyst.