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The Sandia model for the modelling of PV Modules has been developed at the Sandia National Laboratories since 1992.

It is fully described in the report SAND2004-3535 (2004) directly available on the web.  

For a given PV module, the Sandia Model defines about 40 parameters, established on the basis of outdoor continuous measurements  during some days, which should be significant of various weather conditions.

The Sandia Laboratories propose a database of measured data comprising about 500 items, but corresponding to about 100 really measured modules, as for most of modules the actual measurements and a renormalization to the specified STC values are presented.  The Sandia model cannot be applied to other modules than those which have been fully measured using this procedure.

The Sandia model doesn't describe a full I/V characteristics, but only 5 points on the I/V curve:   (0, Isc),  (Vmp/2, Ix),  (Vmp, Imp),  (Vmm, Ixx), and  (Voc, 0)    (where Vmm = (Voc + Vmpp)/2).

Therefore for using it in the simulation, Pvsyst has to complete the I/V curve.  For a given operating condition (G,T) we first calculate the 5 points from the Sandia model equations, and then we establish the one-diode model passing by these 5 points.  

The Sandia model defines a spectral correction, as a function of the Air mass only (no dependence of the Kt). Therefore we understand that this correction should be applied to the beam component only. The spectral balance (gain for crystalline, loss for amorphous) is reported on the loss diagram.

Comparisons with the one-diode model

When opening a module in the database of PVsyst, if an equivalent module is available in the Sandia database, this is referenced in the "Remarks" (and in the choice lists), and you have the opportunity choosing the model to be used.

-  On the "Additional Parameter" page, the parameters of the model are visualized, along with eventual differences with the PVsyst model. Here you can also choose a Sandia module to be associated to the module.

- The button  "Sandia-Diode compar" opens a window with a close comparison of the results of both models.

If the STC values are matching, you can ask for adjusting the One-diode parameters to match the Sandia model at best. You cans get the discrepancies between both models, either at 25°C, or for all temperature conditions.

For crystalline modules, it is usually possible to find a very good agreement between both models.Keeping in mind that during simulation, the Sandia model will give a little spectral correction (gain)  which is not accounted here.

For amorphous modules there are very significant differences, which we don't understand in detail.