view factor for rear side

[ynesse]

Hello pvsyst community ! 

Can you please explain this said by pvsyst help in bifacial systems procedure  : 

how view factor depends on geometry of module choosen ( unlimited sheds or unlimited trackers) I don't understood the inicator said 

please any help will be appreciated

Quote

Reemission form factor :  represents the fraction of the irradiance from ground reaching the back side of your system. This is calculated as an integral for each point on the ground, and you have here the result of the average from the model. This only depends on the geometry. For tracking systems, it depends on the tracker's position. This is an indicator calculated from the model what is means

 

[Michele Oliosi]

The view factor is calculated with the following idea: given a point on the ground, what is the part of the top hemisphere that corresponds to the back side of the modules. Indeed some of the light reemitted  reflected by the ground will go towards the sky, other towards the structure, maybe some shading objects etc. With the view factor we are interested what is the proportion of reemitted light that actually goes to the back of the modules and not to some other direction.

Now, imagine that the size of the tables with modules varies. Depending on the size and spacing of the tables, for a given point of reemission on the ground, the fraction of light rays that reaches the back of the modules will vary. In short, the point on the ground may "see" more or less of the back of the modules.

Edited March 23, 2023 by Michele Oliosi
terminology

[laurahin]

Just a detail, but light in solar wavelengths is not reemitted by the ground.  At the ground temperature, emission will only be in IR wavelengths.  The light hitting the ground and going to the rear side of the module is scattered/reflected from the ground without absorption.

[Michele Oliosi]

@laurahin indeed my bad, i was using the term reemitted not really carefully..

[Vanesa Morales]

On 2/27/2023 at 5:25 AM, Michele Oliosi said:

The view factor is calculated with the following idea: given a point on the ground, what is the part of the top hemisphere that corresponds to the back side of the modules. Indeed some of the light reemitted  reflected by the ground will go towards the sky, other towards the structure, maybe some shading objects etc. With the view factor we are interested what is the proportion of reemitted light that actually goes to the back of the modules and not to some other direction.

Now, imagine that the size of the tables with modules varies. Depending on the size and spacing of the tables, for a given point of reemission on the ground, the fraction of light rays that reaches the back of the modules will vary. In short, the point on the ground may "see" more or less of the back of the modules.

Does this also depends on the IAM?

[Michele Oliosi]

On 4/18/2023 at 9:10 PM, Vanesa Morales said:

Does this also depends on the IAM?

Very good question. Indeed in PVsyst the view factor calculation incorporates the IAM factor, according to the Fresnel smooth glass model.