Bi-facial modules

Nowadays more and more Si-crystalline modules are able to use 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 considers that the behavior of the rear PV production is similar to the behavior of the front, i.e. obeys to 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 will increase the total useable irradiance, and therefore affect the efficiency as function of irradiance. This makes sense, as the efficiency behavior as function of the irradiance seems to be mainly determined by the series resistance, i.e. related to the ohmic loss (quadratic as function of the power) in this resistance.

Now the bifaciality factor is always less than 1 (manufacturers usually claim that it attains 0.8 to 0.9, it may be significantly lower for some technologies). If it would be greater then one, this would mean that the rear side should be used as front side ! The optical properties of the rear side are worse than the front side, the glass properties may be lower (no AR coating, higher IAM loss), and they may be affected by the eventual shading of the junction box.

In the model, the Rear side IAM is suppposed to correspond to a "normal glass".