VBPV system calculation process of beam and diffuse component for rear-side

[Luis Zimmermann]

Hello together, 

is the calculation process of the beam and diffuse component on the rear side of a VBPV system (bifacial model) the same as for the front-side?

If not, please explain me the simplification and differences. 

[Linda Thoren]

Hello Luis Zimmermann,

In PVsyst, the beam and diffuse components and the rear side of a vertical bifacial PV system are calculated similarly as on the front side, by computing the integral of the visible sky. For the rear side irradiance, this is done using a 2D intersection method, same as the "unlimited shed" field type. If the layout is done in the 3D scene, the visible sky is instead in 3D. 

With bifacial systems, the view factor model also estimates the contribution of ground reflection to the front and rear side (not considered in mono facial simulations). 

[Luis Zimmermann]

Can you please explain a bit more detailed the difference between the calculation of beam and diffuse components for the front and for the rear side?
And what about the AlbInc (reflected irradiance from the far ground), how is this component calculated for the front and for the rear side?

[Linda Thoren]

This is further explained in the following help page:
https://www.pvsyst.com/help/project-design/bifacial-systems/index.html#irradiance-on-the-rear-side-view-factor

[Luis Zimmermann]

What is the difference in the calculation model (do both use the similar view factor model? ) between the calculation of ReflFrt / ReflBak (close ground reflection) and AlbInc (far ground reflection)? 
And is the far ground reflection included in the rear side irradiance GlobBak?

[Robin Vincent]

There are two main differences between ReflFrt and AlbInc.

AlbInc uses the albedo value defined in the project setting, while ReflFrt uses the albedo defined in the bifacial model.

The far albedo is only seen by the 1st row, while ReflFrt is computed for all tables. 

 

The computation itself is also slightly different (integral of visible sky portion vs integral of the reflection on several ground points) but this should have a extremely small impact on the final results.