Iso-shadings diagram

[Lunatic09]

I've tried to simulate vertically installed bifacial PV system(Tilt : 90 degree, Azimuth :0 degree).

I have a few questions about iso-shading diagram(attached image file)

1. Why there is each persentage(1%, 5%, 10%, 20% and 40%) of shading losses? and what does it mean? 

2. Is the shading loss mixed direct shading and diffuse shading? if it mixed, is there way to seperate the direct and diffuse shadings ?

[Michele Oliosi]

Hi,  the iso shading diagram is exactly what the name says, it displays the sky positions that produce the same shading factor as a continuous lines.
These iso-shading lines are akin to isolines on a map, which show you the points at a given altitude.

In general you can read the lines as follows: take the 40% line: as long as the sun is positioned at a sky position below that line, there will be at least 40% of shaded surfaces.

The iso-shading diagram just shows the direct shading factor. The diffuse one is computed separately.

On a separate note, I have never seen an iso shading diagram like yours. Please make sure that all the objects in the 3D scene are well defined and do not intersect each other. Also make sure that all PV surfaces have the correct orientation.

[Lunatic09]

Thank you for your response.

I simulated vertically installed bifacical pv system without any shading objects.

 

[Simulation Conditions]

PV module tilt : 90˚  / azimuth 90˚ (Faced west side) / shed distacne 100m (for eliminate interference between modules)

Even if there is no shading objects around PV modules, there is shading loss around 10~20˚ Azimuth.

Could you explain me why those losses occures?

[Michele Oliosi]

Ah yes, if you see the shading factor table

you will notice that the 90° height is always 1, because the sun shines on the side of the table, which is considered full shading. The iso-shading diagram was aimed at interrow shading, so it does not handle this case well. You can disregard the shading lines in the isoshading diagram.

[AmsteinWalthert]

Another question regarding the iso shading diagram and its interpretation. 

We simulated a bifacial system 65° tilted and -50° east oriented on a sloped hill with 17.9° according to the images below.

Questions for the iso shading diagramm:

• Why is there still partial shading at sun heigths > 15° even though the limit angle is set to 15°  in the 3D-scene?
• How can the plateau from azimuth angles -100 to 0 be explained? Why is it flat and not more round? Why does it go all the way till 0° azimuth and drops not already at -50°?

Is the answer somehow related to this second post here? 

Thank you very much

Best

S

 

Edited November 22, 2023 by AmsteinWalthert

[Michele Oliosi]

No the two questions are not really related I think.

Here you are dealing with 3 things:

• The iso-shading diagram comes from the shading factor table. The table has discrete values. This is why the lines are "squared".
• Moreover the angles at which the table is evaluated is 2, 10, 20, ... which means that for 15° PVsyst will interpolate shadings from 10° to 20°, i.e.,the iso-shading will show non zero shading all the way to 20° even though the limit angle is 15°. If you want to avoid using interpolation, you can still switch to the "slow mode". If you use that, in the simulation, the shading will be accurately calculated for each simulation step.
• You are showing the electrical shadings table, which behaves more extremely. Even though you shade a small area, this means a lot of shading. In your case the shading at 10° is therefore close to 100% (electrical shadings).

[AmsteinWalthert]

Thank you for the explanation and answer.

One more detailed question: is the shown Albedo value in the iso-shading diagram the yearly average weighted by energy? Or how can it be interpreted?

[AmsteinWalthert]

Thank you for the explanation and answer. I suggest, the described interpolation effect shows only in the graph and not in the calculation itself? I suggest there the 3D Scene is used as basis?

One more detailed question: is the shown Albedo value in the iso-shading diagram the yearly average weighted by energy? Or how can it be interpreted?

[Michele Oliosi]

It depends on whether you have chosen the “fast” mode or the “slow” mode. In the fast mode, the interpolation is used in the simulation. In the slow mode, the interpolation is shown in the iso-shading graph, but the simulation does the calculation properly with the 3D scene.

It is a shading factor for albedo, i.e., it is applied to the albedo irradiance contribution to the front-side of the modules. This is why it is not weighted per energy, but is just a geometric factor.