Bi-facial modules

Nowadays more and more Si-crystalline modules can utilize light from the rear side for producing electricity.

For defining a bifacial module in PVsyst, please use the concerned checkbox in the PV module definition dialog, page "Size and technology".

Bifaciality factor (\(\phi\))

In PVsyst, bifacial modules are characterized by their Bifaciality Factor (ϕ), which is defined by the IEC 61724-1 standard as the ratio of the nominal efficiency of the rear side to that of the front side. Remember that the nominal efficiency is simply the nominal Power (under STC) expressed in [kWp], divided by the area of the PV module [m²].

PVsyst assumes that rear-side PV production behaves similarly to the front side—that is, it follows the one-diode model with the same parameters. Therefore in the simulation, the irradiance on the rear side will simply be added to the front irradiance, weighted by the bifaciality factor when applying the one-diode model. It is also used when computing the bifacial performance ratio.

This increases the total usable irradiance and thus affects the efficiency as a function of irradiance. This makes sense because efficiency behavior as a function of irradiance appears to be determined primarily by series resistance—that is, by ohmic loss (quadratic as a function of power) in this resistance.

The bifaciality factor is always less than 1 (manufacturers typically claim it reaches 0.8 to 0.9, though it may be significantly lower for some technologies). If it exceeded 1, it would imply that the rear side should be used as the front side. However, the optical properties of the rear side are inferior to those of the front side: the glass properties may be lower (no anti-reflection coating, higher incidence angle modifier loss), and they may be affected by potential shading from the junction box.

In the model, the rear-side incidence angle modifier is assumed to correspond to "normal glass".