Bifacial gain for east west pv system

[Agnieszka K]

I would like to know the best way to simulate an east-west PV system that takes into account the gain of the bifacial modules. Could you please tell me what is the typical value of bifacial gain for this kind of layout?

[Michele Oliosi]

Do you mean vertical EW or EW domes ?

 

[Agnieszka K]

I mean east-west array (domes) on the ground

[Michele Oliosi]

It is not currently possible to simulate with PVsyst as this has two orientations (we handle only one for now, we will update this but not before 2024 I think).
However the bifacial gain is usually negligible (there is no light arriving on the backside), unless the domes are high enough over the ground.

[Agnieszka K]

The height of the lower edge of the module above ground level is 0.8 m. Is this value high enough to have impact on the bifacial gain?

[Michele Oliosi]

in this case you should space the EW rows sufficiently to get try to get some ground reflection

[Agnieszka K]

But how am I supposed to run this simulation in case I can't use bifacial model for EW domes?

[Michele Oliosi]

As mentioned above, it is not directly possible at the moment, sorry.

You could estimate the bifacial gain using a single orientation, replacing each EW dome by a single horizontal PV table with the same width as the original EW dome basis. However this is an extremely crude approximation.

[TClement]

Hi everyone,

Would it be more accurate to do 2 simulations (1 for each azimuth) with a 3D scene, representing one azimuth PV Field, for exemple the East PV Modules, and replace the West PV Modules of the domes by a roof ? Like this, no reflection from the ground nearby, but only the one between the arrays ?

I've done this test, and have a significant increase of the theoretical PV Production (1599 kWh/kWp/y with monofacial East exposed VS 1771 kWh/kWp/Y with bifacial East exposed, same for West), but I don't know if my results are correct and usable...

 

[Michele Oliosi]

No, sorry, the results are not really useable like this. The backside irradiation model (bifacial model in PVsyst) is a 2D approximation that currently only works on single orientation rows. It does not take into account shading objects from the shading scene, as the roof you are adding instead of the other rows.

What is the height of the modules ? Are they at ground level ? If so, you can expect zero bifacial gain. If they are on a structure that lifts the rows above ground (like on a carport) then there may be some bifacial gain.

If the rows are well above ground, you can consider approximating your system by a tilt 0 flat disposition. In terms of bifacial gain, you should get an ok result.

[TClement]

Hi Michele,

Thank you for your answer ! In fact modules have a 10° tilt angle and the lowest point is at 90cm from the ground.

Is the approximation of tilt at 0° would be ok ?

[Michele Oliosi]

The approximation will probably be slightly optimistic for the bifacial gain.
There is a way to not change your 3D scene tilts, it would be to go to the 3D scene > Tools > Orientation management, increase the tolerance (more than 20°) and click on Identify. In this way you can trick PVsyst into considering only the single average orientation with tilt 0°. The transposition gains for the front side will however be a bit off.