gendaylit vs CBDM

[Chingiz Asadzade]

Dear Dr. Southall, 

I hope you are doing well. 

I have tried to change the gendaylit description using text edit node and irradiance values from .epw file. 

As an example instead of description of: "gendaylit -ang 54.985 33.381 -P 6.300000190734863 0.15000000596046448 -O 1 -g 0.0" I write the description of  "gendaylit -ang 54.985 33.381 -W 696 228 -O 1 -g 0.0" which i get from Radiance command prompt for 3rd of August at 13:00 in Stuttgart. 

The simulation works pretty easily, thank you for your help.

However, when I run the same time and same .epw file with CBDM exposure or hourly irradiance functions, the results vary both for visible and full spectrum from the results I got from "gendaylit". Sometimes the difference is small and sometimes the results are completely different, especially when the values for direct normal irradiation are low. What could be a possible reason for that?

As I understand, CBDM function creates Perez sky using "gendaymtx" command, dividies the sky to patches and performs "rcontrib". And the basic node uses "gendaylit" and "rtrace" functions.

Would you sat that it is more accurate to use CBDM function to simulate the irradiance for partially shaded modules?

[VI-Suite]

gendaylit output also requires a sky description to get light from the sky, as below, otherwise you just get light from the sun. This might account for the discrepancy when the direct sun contribution is low. Did you add this?

skyfunc glow sky_glow
0
4 1 1 1 0

sky_glow source sky
0
4 0 0 1 180

You broadly understand correctly.

Either method should work but CBDM subdivides the sky into patches, and the coarser the sub-division the greater the error in light/shadow boundaries. You can increase the CBDM resolution to counter this a bit. 

[Chingiz Asadzade]

Hi again Ryan.

I was not adding the sky description. I added and cross checked the results again and they are pretty the same with CBDM methods. Thank you very much for this correction.

When i increase the resolution to 2 for mor precise sky subdivison, the results within CDBM decreases twice. I mean if the result was 0.4 kWh/m2 it becomes  aprroximately 0.2 kWh/m2. But the same time and date for gendaylit gives the results as in 145 patches (resolution=1). Could it be because gendaylit also discretize the sky to 145 patches or this is not the case?

Thanks in advance.

Best,

Chingiz Asadzade
/

[VI-Suite]

That shouldn't happen. Gendaylit doesn't discretise. I can't replicate this on my linux box; results increase slightly going from 1 to 2 resolution. What platform are you on?

[Chingiz Asadzade]

I use Windows. The results of gendaylit and CBDM are close when the resolution is 1. 

If I simulate particular hour (for example 3rd of August start and end time 13:00) for .epw file from EnergyPlus for Stuttgart, just for a default plane as a light sensor in the center of Blender layout the results for "hourly irradiance" option are 0.81 kWh/m2 for resolution 1, and 0.20 kWh/m2 for resolution 2.

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