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The Bifacial tool is available in the "System" part. It is only available (visible) when you choose a bifacial module.
Bifaciality Factor
Nowadays more and more Sicrystalline modules are able to use light from the rear side to produce electricity.
In PVsyst, such "Bifacial modules" will be characterized by their "Bifaciality Factor", i.e., the ratio of the nominal efficiency at the rear side, with respect to the nominal efficiency of the front side. Remember that the nominal efficiency is simply the nominal Power (under STC) expressed in [kWp], divided by the area of the PV module [m²].
PVsyst considers that the behavior of the rear production is similar to the behavior of the front, i.e. obeys to the Onediode model with the same parameters. See here for details.
Irradiance on the ground
In most cases (except vertical plane systems), the useable irradiance at the rear side is mainly the reemission of some part of the incident light on the ground.
Therefore we should first evaluate the incident light on the ground. However, this irradiance is not uniform and we have to evaluate it for each point of the ground.
 The beam component reaches the ground only between the PV modules. Therefore at a given time, a given ground point will receive (or not) the beam component. This depends of course on the sun's position, so that the beam irradiance distribution has to be computed at each time step of the simulation.
 The diffuse component may be evaluated  for a given ground point  by integrating the diffuse rays received from all directions of the space which are not "hidden" by collectors. For this calculation, we have to use the same hypothesis as for other diffuse models, i.e., that the diffuse is isotropic. We can then evaluate  for each ground point  the received diffuse by an integral, analogous to the integral for the transposition model, over all parts of the sky "seen" by this point. Therefore, the diffuse acceptance is a distribution function over the ground points, independent of the sun's position. It only depends on the geometry of the system, and may be computed only once for a fixed tilt system and a given ground point. For tracking systems, the geometry changes with every tracker movement, and the diffuse light distribution has to be recalculated at every simulation step.
 Shed transparent fraction: The shed may not be entirely opaque to the sun light. There may be spacings between the cells and between the modules, that are not obstructed by components or mounting structures. This will lead to additional light reaching the ground. We will not involve a complex model for this usually small contribution. We will just assume an additional contribution to the light received by each ground point that will be proportional to the Global horizontal irradiance and the specified transparency factor.
NB: In the "unlimited sheds" model, the rows are supposed to be continuous. As a first approximation, this transparency factor may also be used to take spacings between tables into account, if they are not too wide.
Irradiance on the rear side  View Factor
Now we have to evaluate the irradiance on the rear side of the PV modules. This will be characterized by the amount of irradiance reemitted by the ground, which we name "Albedo".
The light remitted from a given ground point is the received irradiance, multiplied by the albedo factor of the ground. You can find some examples of usual Albedo factors here. However this parameter is very important in the bifacial situation, and should be estimated with care for each particular system. It may change when the ground is wet, with snow, or with time (ageing of the surface), and even eventually with the sun's height. PVsyst doesn't take such changes into consideration in the present time, except that you can define seasonal values for the albedo.
NB:  Please don not confuse the albedo of the project with this albedo for bifacial. The albedo of the project characterizes the ground in front of the installation, far from the system. It is used in the Transposition Model and affects the GlobInc value. The albedo for bifacial is a property of the ground just below your PV installation. 
A very important hypothesis of PVsyst is that the light reemitted by a point of the ground has an isotropic distribution. This means that the light is reemitted with the same intensity whatever the direction of the space (halfsphere above this point). There is no peculiar reflection. This is a lambertian distribution, i.e., each ray is multiplied by the cosine of the incidence angle.
We name "View Factor" (or "Form factor") the fraction of this light effectively reaching the PV module. This is again the result of an integral over all PV modules directions "seen" by this point. The light reemitted to the sky is obviously lost. The View Factor is also a property of each ground point, only depending on the geometry. The distribution of view factors for any point is calculated only once.
In fact we have to evaluate 2 kinds of "View factors": one concerning the rear side of the collectors, and another one representing the irradiance reaching the front side. This last contribution will be added to the usual incident irradiance. This View factor integrals involve an IAM correction for each ray, which is of particular importance for the front side.
Besides these contributions, we also have to take the diffuse part directly seen by the rear side (again result of an integral), as well as the beam eventually falling on it (in the morning and evening in summer).
All the irradiance calculations for the rear side take into account the IAM losses and are always calculated using the simple Fresnel model for glass without antireflective coating.
Finally, we can have some mechanical structures behind the module (including the junction box). Therefore we also have to define a shading factor for the rear side.
NB: all these calculations are done in terms of irradiance. The involved energies should be re normalized by the concerned areas. i.e.:
  the total energy on the ground is the irradiance multiplied by the total ground area concerned by the installation. 
  the total energy on the rear side is the irradiance multiplied by the collector's area. 
Therefore, the energy ratio computed by the view factor integral should be multiplied by the area ratio:
Irradiance (Rear) = Irradiance (Ground) * View factor * Ground Area / Collectors area.
This renormalization was not done correctly in the versions 6.60 ... 6.63 of PVsyst, and should not be used for bifacial evaluations.
The version 6.64 is correct, but does not take some marginal contributions into account, like the Beam on the rear side or the reflexions of the near ground on the front side.
Reflected Irradiance on the front side
From the diagram above, we see that there is a contribution of the albedo reflexion, reaching also the front side of the collector.
We can evaluate this contribution in the same way as for the rear side, by defining a View factor for the front side. This contribution is very low for low tilts, and is emitted by ground regions below the shed, which are weakly illuminated. But it may become significant when the tilt increases (especially for the vertical bifacial case).
This contribution is evaluated during the simulation of bifacial systems, and is appearing on the loss diagram as "Ground reflexion on front side" (variable named ReflFrt).
NB: This contribution is present with any PV system, not only bifacial. However, in the usual simulations it is neglected, as we don't have the modeling framework (definition of ground albedo, ground points, shading of other tables, etc) for this evaluation. To our knowledge, no other software is taking this contribution explicitly into account.
Tracking systems
For tracking systems, we can use the same model and hypothesis. The precalculations of the ground points integrals (diffuse, view factors) should be performed for different tracker's positions. For oneaxis trackers, we perform this calculation for 7 Phi orientations of the trackers. Then, the simulation will interpolate between these values (using cubic splines) at each hour, as function of the exact position of the trackers.
Conversion into electrical power
The irradiance on the rear side will give rise to an increase of the global PV module output power. During the simulation, PVsyst simply adds the rear irradiance (weighted by the bifaciality factor) to the front incident irradiance before computing the onediode model.
Now, the irradiance is not uniform on the rear side of the modules. Remember the cell with the lower current determines the current in the whole string. Therefore, we also have to take a Mismatch loss factor into account. Currently, there is no model for the estimation of this mismatch loss factor, it is set to 10% by default and can be changed by the user. This mismatch loss is only applied to the rear side part.
Summary of the Hypothesis
The main hypothesis for the Bifacial model in PVsyst are:
  The diffuse irradiance is isotropic. 
  The reemission of each ground point is isotropic, with a specified albedo factor. 
  The additional irradiance on the rear side is added to the front irradiance for the application of the onediode model. 
Application models
PVsyst will propose different models for the calculation of bifacial systems:
  Simple "unlimited sheds" with a 2dimensional calculation. This provides also a set of pedagogic tools for a deeper understanding of the different contributions mentioned above. 
  Simple "unlimited trackers" (horizontal axis) with a 2dimensional calculation involving a full precalculation for several positions of the trackers. 
  Model based on your 3dimensional shading scene. This will be developed in a next version. 
  General treatment of tracking systems with the 3D scene. This will be developed in an ultimate version... 