Question-and-Answer Resource for the Building Energy Modeling Community
Get started with the Help page
Ask Your Question

Simulation time with complex external shading

asked 2015-08-07 11:46:42 -0500

updated 2015-11-07 17:36:13 -0500

I am modeling some complex external shading alternatives with OS/E+ and there are now thousands of shading surfaces in my OS SketchUp model (architecture!). Unfortunately the shadow calculations are now crippling the run time of my model.

Are there any simulation control tips, or other considerations, to help reduce simulation run time when there are very large numbers of shading surfaces present?

Some observations: under simulation settings > shadow calculation in OS, I noticed the default number for 'calculation frequency' is set to 7 days while the E+ Input-Output reference suggests a default of 20 days. Also the 'maximum figures in shadow overlap calculations' is set to 15,000 and my eplusout.err file gives the warning 'DeterminePolygonOverlap: Too many figures [>15000] detected in an overlap calculation.'

Any insight is appreciated, thanks!

image description

edit retag flag offensive close merge delete


Holy shading devices, Batman!

__AmirRoth__'s avatar __AmirRoth__  ( 2015-08-07 14:40:49 -0500 )edit

Hei @LyleK how did you model those vertical fins?

poppo92's avatar poppo92  ( 2017-01-28 05:36:53 -0500 )edit

@LyleK ......................

poppo92's avatar poppo92  ( 2017-01-28 05:40:25 -0500 )edit

2 Answers

Sort by ยป oldest newest most voted

answered 2015-08-07 12:09:22 -0500

@LyleK, we've done some work on the shading calculations in the current iteration and they have sped up significantly, although I am not sure whether the 15,000 surface limit for overlap calcs is still there. I believe these changes are already in the main EnergyPlus "develop" trunk (@Edwin?). If you are feeling adventurous, you could build your own EnergyPlus from source. Come on, what else do you have going on this weekend? :)

edit flag offensive delete link more

answered 2015-08-07 12:10:53 -0500

updated 2015-08-07 12:12:53 -0500

By default EnergyPlus mirrors shading surfaces at runtime. That may be important for vertical fins, but if your shading surfaces are horizontal then you can turn this off. It might also be nice to see an image of the model to see why there are so many surfaces.

  • Are you doing a lot of site shading of adjacent structures? If so make sure to only draw shading surfaces that face your structure or would directly affect the shading. If these adjacent structures have tiny little undulations in plan, you may consider drawing it as a flat plane instead.
  • Do you have tons of punched windows each with their own shade? If so you could consider window or zone multipliers. Although not sure how using a window multiplier would affect the sunlit fraction of the base surface, since it would only have one shade vs. one for every window. Would take some thought.
  • Back to not mirror shading surfaces, I'm not exactly sure if that is something in the IDF or through EP Launch. Would take some work to see how to set that for an OpenStudio run.
  • The calculation frequency is a good suggestion. That is available in OpenStudio in the simulation settings. We default to 30.

You might also look at your timestep in general too see if it can be relaxed, and also see what solar distribution you need for your model.

edit flag offensive delete link more


Unfortunately the shading elements are vertical, so mirroring the surfaces is necessary. I will try setting the shadow calculation frequency to 30 as suggested. I am using a timestep of 15 minutes currently. I am liberally using zone multipliers to begin with as this building is over 800k SF.

Can you expand on the solar distribution? It's currently set to 'FullExterior' which to my understanding would be the most basic calculation that takes into account exterior surfaces.

I also included a screenshot of the design in question above, this is one section of the exterior shading.

Lyle K's avatar Lyle K  ( 2015-08-07 12:42:15 -0500 )edit

I think FullExterior is good, vs. FullInteriorAndExterior, you don't want to use Minimal Shadowing. Thanks for the updated question with the screenshot. In a way your shading surfaces are acting like a screen with variable shading fraction based on the sun azimuth. One way to mimic this would be to create a large single shading surface with a scheduled seasonally and time of day adjusted transmittance schedule. Would be a cool measure to generate that from geometry :)

If this was only over the window I would recommend a WindowMaterial:Blind object with a vertical slat orientation.

David Goldwasser's avatar David Goldwasser  ( 2015-08-07 14:24:04 -0500 )edit

Your Answer

Please start posting anonymously - your entry will be published after you log in or create a new account.

Add Answer


Question Tools

1 follower


Asked: 2015-08-07 11:46:42 -0500

Seen: 1,214 times

Last updated: Aug 07 '15