Bifacial systems procedure

Please also read the Bifacial model principles. To define a bifacial PV system:

1. System definition

Currently, only 2D bifacial models, akin to unlimited sheds or unlimited trackers, are available. This means that the edges of the system are not taken into account, and the contributing ground area is from the bottom of the first row, to the last row + pitch. Threrefore the model is well adapted to experiments on a single row for example (in this case, some workarounds exist).

You should define a system compatible with these hypotheses:

either, in the "Orientation" dialog, you should choose "Unlimited sheds" or "Horizontal axis Unlimited trackers". These involve a generic analytic calculation of the mutual shadings. In this case, you should not define a corresponding 3D model,
or, in the 3D shading scene, you have to define sufficiently regular row arrangements of fixed tilt tables or single axis trackers. These objects define arrays of trackers or fixed tilt tables. If you define several such objects within a single orientation, they should be close to identical (same orientation, pitch or table/trackers widths). In this case, you have to define "Fixed orientation" or "Tracking horizontal axis" in the "Orientation" part, and the bifacial model will establish a corresponding dummy "Unlimited Sheds or Trackers" corresponding to these parameters.

2. PV module choice

In the "System" part, you have to choose a PV module specified as Bifacial to define a bifaciality factor.

When choosing such a PV module, the button Bifacial system will appear just above the PV module definition. This opens the following dialog, with the general parameters related to a Bifacial system.

3. Main bifacial parameters

First, you choose the model according to the orientation type you have defined (only the relevant model is enabled, the other one is greyed out).

Model Pages (unlimited sheds or trackers 2D models)

This opens the corresponding page, where the basic system parameters (sheds or trackers width, pitch, height above ground, etc) are pre-defined according to the concerned subsystem.

See 2-dimensional unlimited sheds or 2-dimensional unlimited trackers.

In this page, you can play with these parameters to analyze the effect of different configurations. However, when exiting this page, the parameters will be reset to their default values based on your system definitions, unless certain options are checked in the "Parameters Mode" group within the General Simulation Parameters tab. These options specify whether user modifications to 'Pitch' and 'Number of Sheds' are retained for the simulation or reset upon leaving the window.


Height above ground	the only parameter without correspondence in the system's definitions is the height above ground that you have to define here. This is the height of the bottom of the sheds or of the axis in the tracker case.

General Simulation Parameters page

Among the parameters on the general parameters page, several are fixed by other calculations:


Beam ground factor	is the fraction of the Beam on horizontal plane reaching the ground between the sheds. This evolves according to the sun position of course, and depends on the weather data. This is not really a parameter, but an indicator.
Diffuse ground factor	is the fraction of the Diffuse on horizontal plane, as seen by each point of the ground (integral over all directions). This depends on the system geometry, the average of all contribution ground points is given here. In tracking systems, this depends on the tracker's position.
Reemission form factor	represents the fraction of the irradiance from ground reaching the back side of your system. This is calculated as an integral for each point on the ground, and you have here the result of the average from the model. This only depends on the geometry. For tracking systems, it depends on the tracker's position. This is an indicator calculated from the model.
Module Bifaciality factor	is the ratio of the rear side yield under STC, with respect to the front side STC performance. This is a specification of the PV module.

Other parameters are instead to be fixed by the user:


Module transmission factor	The module transmission factor describes how much light can pass through a row of modules, and reach the ground beneath (i.e., contributing to the ground irradiance). It should include spacings between cells (if transparent) and spacing between modules (if transparent). You can also use this parameter as an approximation if you have some spacing between tables, provided that this is not too large. This parameter is usually null except for spacing between tables.
Ground albedo	is the albedo property of the ground below your system, contributing to the bifacial reflection. This has obviously nothing to do with the albedo defined in the Project, which characterizes the (far) terrain in front of your installation. This parameter may be defined in monthly values to take the eventual snow into account.
Structure shading factor	is the shading factor of any obstacle between the ground and your sensitive rear side. As a first approximation, this may be the ratio of the mechanics area to the sensitive area. If close to the rear side, this should take the electrical mismatch into account: as the current in a string is the current in the worst cell, if this mechanics covers x% of one cell, the shading effect will be x% for the concerned string. This loss may be reduced by a factor of 2 if the mechanics is covering half a cell (i.e., the shade is distributed on 2 cells). NB: If you have a tracker with a torque tube: - If you have only one module in the width of the tracker, the torque tube will necessarily throw shades on the module. In this case , the shading factor will be the diameter of the tube, divided by the cell's size, and half this value if the tube is shading 2 cells. - With trackers of 2 modules, where the tube is between the modules (usual case), the shading factor will be null.
Mismatch loss factor	in a bifacial system, all parts of the module will be irradiated differently. Remember that the current in a string is limited by the current of the weakest sub-module. The uniformity of the rear irradiance will increase with the height above the ground. If you have several strings in your system you should put all the modules of a given string on a same row. This way, only the non-uniformity across one module width will be significant (provided that there are no non-uniformities in the length of the row). Sorry, PVsyst does not have any model nor well-established value to propose here for now. The default of 10% is just a rough estimation and is not based on a calculation.