Has the software taken into account the low-light performance on the rearside ？

[Chen]

Dear sir：

  Has the software taken into account the low-light performance on the rearside of bifacial module？ thanks

[Chen]

Because the  irradiance of the front side or the rear side of PV module will be different, in this case, there will be a difference in conversion efficiency between the front and back sides,with respect to 1000 W/m². 

 

[Michele Oliosi]

Yes, actually the irradiance of front side + backside*bifaciality factor are put together, which gives an effective irradiance evaluation. Only after that, the PV conversion model applies. Indeed, the light reaches the same cells, be it from the front or the back. The same low-light performance is therefore taken into account for front and back (up to the bifaciality factor).

[Chen]

I understand. What I mean is, if the irradiation intensity on the back of the module is relatively low while that on the front is relatively high, so does it imply that the low-light performance of the rear side is worse? 

[Luca Antognini]

Clarifying Definitions to Avoid Confusion

At PVsyst, the term "low-light performance" is usualy defined in the context of a PV module illuminated solely from the front side.

Let’s consider the following example:

• At a reference irradiance of 1000 W/m², the module has an efficiency of 22%

• At a lower irradiance of 200 W/m², the efficiency drops slightly to 21.34%

We define low-light performance as the relative difference in efficiency:

(21.34 - 22) / 22 × 100 = -3% (relative) 

This means the efficiency at the lower light level is 3% lower relative to that at full irradiance.

This is the standard definition used in PVsyst.

Important Note: A more negative number (e.g., -4%rel or -5%rel) does not necessarily imply worse performance. In fact, it often indicates that the module performs better at high irradiance (1000 W/m²), which is an observed trend in modern PV modules, largely due to reduced series resistance

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Bifacial Modules and Rear-Side Illumination

When it comes to bifacial modules, if only the rear side is illuminated (for the sake of the discussion), the module’s efficiency is proportional to the front-side efficiency, scaled by the bifaciality factor f.

This factor applies equally at both high and low light levels. Therefore, the relative low-light performance of the rear side is identical:

(f × 21.34 - f × 22) / (f × 22) = -3% (relative) 

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Link to the Bifaciality Factor Definition

To relate this to my colleague’s earlier explanation: the bifaciality factor is defined as the ratio of power output when the module is illuminated from the rear versus the front. This difference in power is primarily due to optical property differences between the two sides of the solar cell. Indeed:

• The cell materials are different on the front and rear sides

• Any differences in charge transport or extraction are negligible (These effects are much much smaller than the typical uncertainty in datasheet bifaciality (e.g., 80% ± 5%))

Thus, the bifaciality factor can effectively be understood as a scaling factor for the photogenerated current.

In practical terms, the total irradiance received by a bifacial module can be calculated as:

Irrad_Tot = front-side irradiance + (rear-side irradiance × bifaciality factor) 

The PV module performance modeling can then proceed without needing to differentiate which side the light is coming from—the model is agnostic to the direction of light incidence and so is the low-light performance.