Project design > Bifacial Systems > Bifacial systems procedure

Please also read the Bi-facial model principles. To define a bi-facial PV system:

1. - System definition

Currently, only Bi-facial models for unlimited sheds or unlimited trackers are available. This means that the edges of the system are not taken into account, and the contributing ground is from the first shed bottom, and the last shed + pitch. Threrefore the model is not applicable to experiments on a single (or double) array for example.

You should define a system compatible with these hypothesis:

-	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 3D shadings scene, as the shading effects would be accounted twice.

or in the 3D shading scene, you have to define "PV Tables as Sheds" or "Tracking PV planes". These objects define arrays. If you define several such objects, they should be 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 bi-facial model will establish a 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 Bi-facial 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 Bi-facial system.

BF_MainDialog

3. - Main bi-facial parameters

First, you choose the model according to the system you have defined.

Model Pages (unlimited sheds or trackers 2d)

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

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" value, i.e., according to your system definitions.

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 ones 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 meteo 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.

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 bi-facial 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.

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.

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 bi-facial 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.

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.