Simulation of Bifacial Solar Generation with Ground Placed Shading Structures

[chuang]

I am using PVsyst 7.4 for bifacial solar module simulation, but I have noticed that the electricity generation does not change when obstacles are placed on the ground around the modules during the simulation. Is there a mistake in my settings?

 

[Michele Oliosi]

There is no mistake in your settings.
At the moment, the bifacial backside irradiance model is a 2D view factor model, that approximates the situation of the 3D scene. Shading objects are not represented, neither are differences in height of the tables and so on.
One partially related point is that the “Height above ground” in the backside irradiance model definition is not retrieved from the 3D scene either. You should therefore enter it manually according to your system specifications.

[chuang]

The question is, currently, is it not possible to study how obstacles on the ground affect reflection and simulate the electricity generation of bifacial modules?

[Michele Oliosi]

Indeed, at the moment this is not possible in PVsyst. We only model the impact of mutual shadings for the backside irradiance modeling.

[chuang]

Understood. Thank you very much for your response.

[chuang]

Dear  Oliosi

Sorry, I have one more question. When simulating "Height above ground" in PVsyst as shown in the figure below, theoretically, for bifacial modules, the higher the modules, the more direct sunlight should enter below the modules, and the global value should increase. Why does it tend to level off in the simulation when the height is above two meters?

 

 

[Michele Oliosi]

There is not more direct sunlight on the backside of the modules by increasing the height. Only reflected sunlight on the ground will increase.
The maximum irradiance on the ground is found when the tables are high enough so that their own shading of the ground has little impact. This happens relatively fast.
Intuitively, if you imagine tables very high up, you can imagine how the shading they cast on the ground is irrelevant in most hours. From that point on, it does not matter at what height they are.

[chuang]

Dear Oliosi

Sorry for asking a silly question.

The issue should be related to the values of "global on ground," as shown in the figure below. As you mentioned in your previous response, when the height of the solar modules increases, the reflected sunlight on the ground increases. This means that the value of "global on ground" should increase, right? Then, why, in the figure below, does the value of "global on ground" decrease when the height increases?

(From my current understanding, increasing the height does not increase the direct sunlight passing through the gaps between the modules, but it should increase the direct sunlight from the sides of the modules. Like second dialog shown below)

 

[Michele Oliosi]

Hmm I agree that this plots seems wrong. There may be a bug at that level. Sorry, we need to correct it.
By the way these plots are there just for information, they don't have an impact on the simulation.

[Bruno Wittmer]

This is a numerical imprecision in the calculation. This plot should indeed be flat, since the global light reaching the ground does not depend on the mounting height. The calculation is based on the 2D-section of the rows, and therefore does not consider border effects as suggested by chuang. It is only the light coming through the gaps in the rows that will be accounted for.

The numerical imprecision leads to a variation of around 2% in this example. This is quite extreme and probably due to a rather small pitch (high ground coverage ratio GCR). More common GCR values in the range of 0.4 - 0.6 lead to an imprecision of less than 1% in this calculation. This extreme example will translate to an error of 2% in the total bifacial irradiance contribution. For a bifacial irradiance gain of around 10% , this would lead to an imprecision of 0.2% in the total PV generation of the system, which is a rather small uncertainty for a bifacial simulation. Most likely the bifacial irradiance gain is much smaller than 10% in this high GCR example, making the error that arises from this numerical uncertainty negligible.