The detailed results of the bifacial model are available on the Loss diagram.
Here is an example, with a description of all output variables involved:
In the Report editor, you can also export the values into Excel (menu "Export > Loss diagram values), which gives some explanations about how to calculate some values:
Detailed variables description:
GlobGnd | Global incident on the ground. |
| This is the total irradiance reaching the ground area below the installation (beam and diffuse). |
| This irradiance [W/m2] is referred to the reflecting reference area (in 2D models: proportional to the pitch). |
ReflLss | Ground reflection loss (albedo) |
| This is the loss of irradiance due to the reflection (1 - Albedo), referred to the ground reference area. |
BkVFlss | View factor for rear side |
| Each point of the ground is supposed to receive an irradiance (luminous power) specific to its position on the ground, and the sun's position. |
| After reflection, this point will re-emit the same power in all directions (isotropic hypothesis, i.e., Lambertian distribution), according to the albedo factor. |
| Now a part of this power will be intercepted by the rear side of the collectors, the rest is lost (sky). The view factor is the fraction of this interception for each given point. |
| The total power received by the rear side is the sum of all ground points contributions. This power will result in an irradiance [W/m²], which is now normalised to the m² of collector area. |
| Therefore when expressed as irradiance (not Power), this view factor should be multiplied by the Ground area and divided by the Collector area (i.e., 1/GCR). |
DifSBak | Sky diffuse on the rear side |
| This is the contribution of the diffuse irradiance, coming directly from the sky. |
BeamBak | Beam effective on the rear side |
| This is the contribution of the eventual beam coming directly from the sun: |
| - with sheds, possible in the morning and evening when the sun passes behind the East or West. |
| - with vertical East-West rows, when the sun is opposite to the front side, |
| - with tracking systems, never. |
BackShd | Shading loss on rear side |
| Due to the mechanical structures (corresponding to the specified "Structure shading factor"). |
GlobBak | Global irradiance on rear side |
| Is the sum of all these contributions. |
| This value is mentioned in [kWh/m²], and as a percentage of the GlobEff value. |
EArrNom | Array Nominal energy at STC efficiency |
| The reference energy of the array is defined as the sum of: |
| - the GlobEff value, multiplied by the PV modules area and the STC efficiency, |
| - the GlobBak value, multiplied by the PV modules area, the STC efficiency and the bifaciality factor. |
There are 2 other contributions linked to the bifacial model:
MismBak | Mismatch for back irradiance |
| This is the result of the Mismatch parameter specified by the user (see Bifacial Procedure). |
| At this stage of the Array losses, on the results diagram this is referred as percntage of the full system energy. |
| As an example, if you have specified a loss of 10% (of the bi-facial contribution), and you have a bi-facial contribution of 15% of GlobEff, this will be 10% of GlobBakEn / (GlobBakEn + GlobEff). |
| NB: Here, the GlobBakEn is GlobBak * Bifaciality factor. |
ReflFrt | Ground reflexion on front side |
| This represent a contribution of the irradiance reflected by the close ground (between rows), and reaching the front side of the collectors (weighted by the IAM loss, very significant here). |
| This contribution is indeed present in any PV system (bifacial or not). However nobody takes it into account, as its calculation involves the full Bi-facial model, with an accurate definition of the ground, its albedo and the geometry, and the full calculation of the ground points view factors. |
| This contribution is very low with sheds and trackers systems, but becomes crucial with vertical East-West bifacial systems. |