Solatube BSDF Simulation


#1

Hello,

I am trying to use Honeybee[+] to run a daylight simulation using the Solatube BSDF files and wondering if anyone has experience working with these? My results seemed much lower than I expected so I tried using Honeybee[+], Honeybee, and DIVA for comparison. I am wondering if I have an incorrect setting with Honeybee because the DIVA results seem much more in line with my expectations and all three models are using the same geometry, materials, etc.

Below is information on the BSDF files from solatube with some instructions:
http://www.solatube.com/support/technical-resources-list?f[]=field_tech_category:146

Here are my results for an illuminance calculation from DIVA (left), Honeybee+ (center), and Honeybee (right). These were attempting to replicate a report from solatube indicating that 25 devices would produce a workplane illuminance of 112 fc, so even DIVA is significantly lower than what they are claiming.

Below is a dropbox link to download my files for reference as well as a sample report from Solatube. Any help would be appreciated!

Thanks,
Steve


#2

Unless they switched things up under-the-hood, DIVA is still running Daysim binaries for annual simulations. And Daysim does not support BSDFs. Are you running a point in time simulation? If so, under what sky conditions?
Can you share the Radiance folders that are written out by DIVA, HB and HB[+]? I might be able to get back to you by the weekend.

Regards,
Sarith

Any past experiences with Solar Tube BSDFS …@PeterZatko or @MohammadHamza ?


#3

Yes I know DIVA is still using Daysim so my plan was to use HB[+] for annual simulations. For these point in time simulations I was using the Perez sky for Falls City, NE (January 20th at 13:00, 20% cloud cover). I also tested the CIE clear sky with sun at multiple times with similar results.

I also ran some images and it was interesting that the distribution inside the space was quite different between DIVA and HB/HB[+]. In DIVA the skylights near walls would produce a fairly uniform/diffused effect whereas the HB/HB[+] images showed more of an optical distribution. I didn’t save these but will try to recreate them to share.

Below are the radiance folders for each of these models:

Thanks,
Steve


#4

In your files, the settings for point-in-time simulations for illuminance simulations are different for DIVA and Honeybee (both versions):
DIVA:
!gendaylit -ang 29.57 7.16 -O 0 -W 851 91 -g 0.2
Honeybee(s):
!gendaylit 1 20 13.0 -a 40.08 -o 95.6 -m 90.0 -W 851.0 91.0 -O 0 | xform -rz 0.0
I am not sure if altitude/azimuth of 29.57 and 7.16 corresponds to the sun position for 20th Jan in Falls City. This might explain the difference in point-in-time illuminance values for DIVA and Honeybee.

For the image-based simulations, I would suggest increasing -ad by a factor of 2, reducing -aa to 0.05 and also double -ar (all through rad Parameters).

Some other comments:
I also noticed that your absolute units are very high for the model. Is this in inches or is your room 700+ meters? Usually, the practice is to follow SI units in Honeybee.


Some of the allocation of rendering parameters in Radiance are based on absolute dimensions, so this might play into the results (not completely sure about this one).

For Honeybee[+] you should be seeing some convergence in results if you set your values of -ad to around 20000 or so (this is specific to your model).

Your Radiance model and the orientation of the BSDFs are okay and the same across all three cases. I did some tests to ensure if this was the case:
2018-11-25%2020_20_38-Window 2018-11-25%2020_21_15-Window

Regards,
Sarith

Update: I thought the values for radiation were different for gendaylit because I was thrown off by the decimals. Anyway, I will give this one more go tomorrow.


#5

Sarith,

Thank you so much for looking into this. I checked the sun position and it looks like that is correct for the date/location but could explain some slight differences due to rounding. I was modeling in inches which may have thrown things off, especially considering the application note below from Solatube:

All Solatube Daylighting System Radiance BSDF file product/aperture dimensions are reported in decimal meters.

I updated the model to meters and ran the calculations again using the parameters you suggested. They seem more reasonable now, but still fairly different between DIVA and HB.

As I mentioned before it seems like DIVA is handing the BSDF file quite differently which may explain the differences in calculations. See test images below using the CIE clear sky (6/21 @ 12:00, Falls City NE). Any thoughts on why the distribution would be so different HB & DIVA? I have the same geometry and materials connected directly through grasshopper so I can’t think of any reasons why.

DIVA%206%2021%201200%20CIE%20ClearSkyWithSun

honeybee%206%2021%201200%20CIE%20clear%20sky%20with%20sun

Thanks,
Steve


#6

Hi Steve, can you share the HDR files rendered through DIVA and Honeybee (the ones you posted above)? I would like to snoop around inside the metadata to see what is going on. Both images are rendered through rpict, however, there might be differences relating to settings and versions.

Regards,
Sarith


#7

Sarith,

Absolutely. I just zipped both directories with RAD files in case you wanted to look at anything else. I don’t think that you can see how DIVA is calling rpict in their files, but I have reached out to them as well to try figure out why this is happening.


Thanks,
Steve


#8

So, there are a few things at play here.

The DIVA version of Radiance is 5.0, which nearly three years old. The NREL version used by Honeybee is 5.2, which was released in June. See the last lines in each screenshot.
Honeybee[+]


DIVA

There have been several updates to code since 2015 which affect how BSDFs are handled. However, it is hard to say if that is affecting the results in this specific scenario.

There is also an issue relating to how ambient calculations are being handled in DIVA and Honeybee[+]. If you see the screenshot for DIVA, you’d notice that there is a -af flag in the rpict command. They are likely running an overture calculation before generating the final image. The overture calculations have been implemented for image based simulations in Honeybee-legacy but do not appear to be ported over (yet) to Honeybee[+] (cc: @mostapha) . In very simplistic terms, overture calculations give better bang-for-buck for the same rendering settings. You can see more about them here. My suggestion for getting an “accurate” image-based result would be to use the latest version of Radiance with Honeybee-legacy. I think the splotches you see in the HB[+] image would lesser in Honeybee legacy. If they still don’t improve try reducing -aa to 0.01.

By the way, the skies for Honeybee and DIVA appear to be the same irrespective of the difference in the way they are being invoked by gendaylit. In the following images, the camera is at the global origin (0,0,0) and looking upwards.
DIVA:


Honeybee:

Arithmetic difference: DIVA - Honeybee.

Considering how bright the sun and circumsolar region are in previous images, the difference shown in the above image is miniscule.

Regards,
Sarith

(I found a somewhat unrelated issue when I was looking at the sky files. I raised a question for that on github.)