Problems with Bifacial Modeling in PVsyst (Unconstrained Parameters)

[kjs55]

As of PVsyst 6.7.8: This applies to both fixed tilt and tracking (grid-connected projects). I set nb. sheds = 1. I would not think collector pitch and width matter in this case but perhaps they do b/c of built-in unlimited sheds modeling assumptions? When I vary pitch from 6.6 m to a large value like 100000 m, the energy of the system goes from roughly 4700 to 14000 kWh/year. For a given pitch (nb. sheds = 1), varying width changes energy. Also, you need to close and reopen the project in order for changes to nb. sheds, pitch, or width to take effect. I will update this post later w/ more information and examples when I have time. Edited December 16, 2019 by kjs55

[André Mermoud]

It is clearly mentioned in the Help that at the moment, the model available in PVsyst only applies to "unlimited sheds"-like systems, and with a significant number of sheds. The edge effects are not taken into account, neither in with, nor in the perpendicular direction (i.e. number of sheds). The pitch and collector width are basic parameters of the model.

This means that the model doesn't apply to a single module or table on a roof.

This feature will be developed in a future version.

[ckremin]

Dear André, dear PVsyst development team,

would it be possible to get an indication on the timing, when you are planning to release further updates regarding bifacial modelling? Are there any plans to revise the bifacial modelling methodology?

We are currently looking at a significant number of large scale PV opportunities, but are struggling to acurately simulate the bifacial yield gain with PVsyst. Measurements indicate that the numbers are very conservative, when it comes to model the bifacial energy gain.

Kind regards,

C. Kremin

[André Mermoud]

Our next improvement for Bi-facial will concern the vertical systems, which need to deeply review the Transposition model, especially for the circumsolar component.

However this doesn't affect the usual calculations (reasonable tilts < 40°). We don't have any indication that the model has to be revised.

Several studies with experimental data are on the way, and don't indicate that the bifacial gains are strongly under-evaluated in the unlimited sheds (or trackers) hypothesis.

The treatment of "single sheds" will not be available before several months.

[kjs55]

Nonetheless, I think it is important that users understand the impact of pitch for bifacial

## Mono-facial (v6.84)

Pitch = 10.22 m; Annual Energy = 2803 kWh/yr

Pitch = 100000 m; Annual Energy = 2820 kWh/yr

Impact: 0.6%

## Bifacial (v6.84)

Pitch = 6.6 m; Annual Energy = 4479 kWh/yr

Pitch = 100000 m; Annual Energy = 12694 kWh/yr

Impact: 283%

(NB: The mono-facial system above is not the same as the bifacial system apart from bifaciality; so, please do not compare 4479 to 2803 kWh/yr, etc.; "impact" above is the annual energy deviation b/w pitch #1 and pitch #2 for either mono-facial or bifacial; "impact" can be compared across mono- vs. bi-facial)