view factor for rear side

[115522]

Hello pvlib_python community !

I have a question about can I calculate view factor for rear side in bifacial system using pvlib
In fact , I 've consult pvlib.bifacial.infinite_sheds but I didn't find function that really can calculate view factor for rear side all functions are about sky view factor
any solution please ! many thanks in advance

[cwhanse]

View factor from the rear side to which surface, the ground or the sky? Or are you looking for something else?

[115522]

view factor from ground to rear side

[cwhanse]

The function pvlib.bifacial.infinite_sheds._vf_row_ground returns the view factor from a point on the module to the ground, and _vf_row_ground_integ integrates those view factors over the shaded and unshaded portions of the module. To specify front or rear, use the surface_tilt; for example, surface_tilt=30 will return view factors for the surface facing toward the sky at 30 tilt, surface_tilt=150 will return view factors for the side facing the ground.

These view factors are the fraction of radiation leaving the ground that arrives at the module. I assume that's what you want. Maybe our way to describing them isn't as clear as it could be. In pvlib documentation, we say "from the module to the ground" is as if one is viewing the ground from the module. I think heat transfer literature would describe this differently, as the fraction of radiation leaving the ground that is received on the module.

[115522]

so when I want to work with _vf_row_ground(x, surface_tilt, gcr) x is the hight of module from ground ?? can explain please

[cwhanse]

x is a fraction, of the distance from the bottom edge to the top of the module, along the slanted height. The _infinite_sheds model is simplified by not having a distance between the module and the ground, and thus modules can't "see" underneath adjacent rows.

[115522]

many thanks for your availibility , support , and fast replying @cwhanse
what I have understood that there is no distance between module and ground in infinite_sheds model ? another point in test x is set as an array x = np.array([0., 0.5, 1.0]) my question here how can I specifiy x value another question if I am using a tracking system surface_tilt will take many values how I will know which one is for rear side and which one for front side please help !availibility

[cwhanse]

In _vf_row_ground, x can be a single value or an array. surface_tilt can be an array. But this is important x and surface_tilt cannot both be arrays. We haven't vectorized these functions in that manner (yet).

To get a view factor for a length of the module, rather than at a single point, you want to use _vf_row_ground_integ, instead of _vf_row_ground. Use f_x to identify the fraction of the module (from the bottom edge). _vf_row_ground_integ can accept both f_x and surface_tilt as arrays as long as they are the same length.

You will have to keep track of the front and rear surfaces in your code. Usually, trackers rotate to face the front surface east in the morning. The surface_tilt for the rear side will be 180 - surface_tilt for the front sice.