Bifacial system: 2-dimensional unlimited sheds

For the basic principles of the bifacial simulation, please click here.

Important Hypothesis

This simplified 2D model is only applicable to large regular row-based systems (sheds). It neglects the edge effects at the extremities of the rows.

Therefore, it is not well adapted to systems with few modules, or systems with geometrically complex layouts.

This basic model treats - for the modeling of the backside irradiance - an orientation as a generic "Unlimited sheds" configuration, i.e., with identical pitch between rows, and without considering the row extremities. Note that it also does not differentiate between PV-"sensitive" areas and non-"sensitive" frames. Therefore you can use it:

either directly with a simplified geometry defined with the "Unlimited sheds" option in the "Orientation" window,
or with an orientation with a 3D representation, provided that the PV array is sufficiently large and regular to be approximated by the "Unlimited sheds" hypothesis.

The application of the model is as follows:

PVsyst will create an approximate "Unlimited sheds" geometry for each bifacial fixed tilt orientation, just for the calculation of backside and reflected irradiances during the simulation.
This geometric representation will try to match the 3D layout as closely as possible (the part corresponding to the same orientation). When irregularities in the 3D drawing exist (e.g. for the pitch), averages are used.
Prior to the simulation, the parameters of this "unlimited sheds" representation will by default be adjusted according to the 3D drawing.
The height above the ground, however, is not extracted from the 3D scene. It must be specified manually in the bifacial model window.
To compensate some limitations of the translation of the 3D drawing to a simplified geometry, the user can override the "number of rows" and "pitch" parameters. These will only impact the backside and reflected irradiances, not the front side calculations.
Since the remaining geometric parameters (tilt, azimuth, table width, etc.) defined in the bifacial model window are overridden by the 3D scene, they can be considered exploratory parameters that can be used within the bifacial model window to observe how the bifacial model reacts to differing conditions.

2-dimensional calculation

The "unlimited sheds" hypothesis allows a simple analytic calculation of irradiances in a row-based system, namely the mutual shading from row to row, considering only transverse directions.

This also simplifies the bifacial calculations: we can consider just the ground points distribution along the azimuth line, and we can analyze their behavior in terms of the row-to-row configuration.

Therefore, this window is also a powerful exploratory tool for the understanding of the bifacial system.

For this calculation, we have to specify the row-based arrangement geometric parameters (plane tilt/azimuth, pitch, table width, height above the ground), as well as the ground albedo.

The window shows:

The basic parameters of the rows arrangement,
An evaluation of the ground irradiance for some specified dates (clear day),
An evaluation of the daily irradiance values for each month (clear day conditions),
A drawing - with animation - of the sheds and ground irradiance behaviors (beam, diffuse acceptance, ground re-emission, etc),
Some distributions of parameters, see below.

Beam acceptance on ground: this highly varies according to the season. Here for November at latitude 43° North (instead of June just above):

Beam on Ground, and reflected : only a part of the illuminated ground re-emits to the collectors.

Diffuse acceptance: depends on the position of the ground point below the sheds.