2D Bifacial model conditions

Bifacial model and 3D scene

The bifacial model presently available in PVsyst is based on a simplified 2-dimensional representation, with similar assumptions as the "unlimited sheds" or “unlimited Trackers” PVsyst simplified calculation.

Now as an approximation, this calculation may be extended to a 3D scene, provided that this 3D scene is sufficiently well represented by the "Unlimited sheds or trackers" situation (see below for the detailed conditions).

For this, the program tries to identify the parameters to be used by the simplified 2D model, from the 3D scene characteristics. It has namely to define:

The plane tilt or tracking parameters (axis tilt),
The table width, which is usually well defined in the 3D,
The pitch between rows, from the most representative ones in the 3D scene (usually an approximation),
The number of sheds or trackers, as the first and last ones have different shading effects.

For the bifacial model, you will also have to define:

The height above the ground. This is not defined in the shadings 2D model, but it is crucial for the bifacial model. It is not reliable in the 3D scene as the ground is not always well defined (sloped). Therefore you should define it explicitly in the bifacial dialog,
The specific 2D parameters like albedo of the ground, etc.

Conditions with shading scene "Fixed tilted plane"

The pitch between the tables has to be regular enough. PVsyst internally computes the RMS deviation on the pitches between all neighboring tables in the scene. The resulting value must then lie below a threshold set by default at 0.4 m. The threshold can be modified in the advanced parameters, by changing the value "Pitch RMS limit for bifacial 2D models". The threshold is four times the value of the advanced parameter.
The table heights must be homogeneous across the whole scene. PVsyst will require tables to have a difference in heights of at most 1 cm or 1%. Note that the height can be found in the PV table edition window, or under "width" in the window "List and management of objects".
In PVsyst 7 and previous versions, it was required that there be one single orientation. In some cases when the table base followed the terrain, it was therefore recommended to group orientations by increasing the tolerance in the "Orientations management tool". This is not needed anymore in PVsyst 8 onwards.

Conditions with shading scene "Tracking, horizontal axis N-S"

The (horizontal) E-W pitch between the trackers has to be regular enough. PVsyst internally computes the RMS deviation on the pitches between all neighboring trackers in the scene. The resulting value must then lie below a threshold set by default at 0.4 m. The threshold can be modified in the advanced parameters, by changing the value "Pitch RMS limit for bifacial 2D models". The threshold is four times the value of the advanced parameter.
The tracker widths must be homogeneous across the whole scene. PVsyst will require tables to have a difference in widths of at most 1 cm or 1%. Note that the width can be found in the tracker edition window, or in the window "List and management of objects".
The average axis tilt in the orientation must lie below a threshold given by default of 2°. The threshold can be modified directly in the advanced parameters, by changing the value "Max. tilt axis for the bifacial 2D model".

Note on modifying the thresholds

The different limits set by default ensure that the model is applied only in situations where its results will be accurate. Although it is possible to circumvent some of these limits, this will also mean that the bifacial contribution to your PV production may suffer from some inaccuracies, due to working with an idealized model using only average values.