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

How do I determine the sky conditions when using EnergyPlus/ Radiance in OpenStudio

asked 2014-11-20 16:02:59 -0500

updated 2014-11-20 21:07:29 -0500

Is there a way to determine sky conditions for a given date and time as interpreted by EnergyPlus or the Radiance package for OpenStudio?

Basically, I have narrowed down all the other factors regarding showing compliance for LEED IEQc8.1 and this is the only thing I cannot seem to get my head around.

For now I am taking the Radiance output > Space101map.ill file in the\1-Ruby-0\out\files\in\model\radiance\output\ts\Space_101\maps folder and ascertaining whether the space meets 2 single-point in time (3/21 9am & 3pm) metric of 10/500fc.

I know that the weather file is used to generate the sky conditions, but is there an ouput that tells me the exact condition for the hours chosen? If so, is there a way to override that condition to what is considered a "clear-sky."


edit retag flag offensive close merge delete

2 Answers

Sort by ยป oldest newest most voted

answered 2014-11-20 16:48:37 -0500

Hi @nfonner,

Good question. As you've stated, the sky conditions for EnergyPlus and Radiance simulations done in OpenStudio are dictated by the information in the weather file (.epw) you assign to your model. The older LEED rating systems used the more simplistic point-in-time compliance path, which assumes these ideal "clear" and "overcast" skies that rarely exist in Real Life[TM]. Hence the move toward true climate based daylight modeling (CBDM), a compliance path approved in LEED 4.0, even (wow!).

What you could do is look at the weather file (EPW) in a tool such as Climate Consultant to visualize the weather data, and hope that there are representative "clear" and "overcast" days near the equinox that you could use as exemplars for the LEED submittal. Basically, a really high direct normal irradiation value paired with a low diffuse horizontal value suggests a clear sky, and the inverse ratio suggests an overcast one. You may need to "look around" the equinox date(s) a bit to find truly exemplar days, which begs the question "will the LEED reviewer accept days other than 3-21 or 9/21". "Unfortunately", true CBDM is the only way to do an annual simulation in OpenStudio at the moment. You could write a measure that uses the daylight coefficient data from the OpenStudio-Radiance run and generates the simple clear/overcast skies with gensky to give you your data, and for that matter, we should add such an option directly to OpenStudio. In the meantime a little script in the interpreted language of your choice may be the fastest way right now (I am happy to help you with this, if you want to work offline a bit; we can post the results here).

P.S.: Details on what EnergyPlus does with the weather info can be found here. In the case of Radiance (by way of OpenStudio), what happens is the epw file is converted to the Daysim ".wea" format, which the Radiance program dctimestep uses to generate skies based on the Perez model.

edit flag offensive delete link more


Looking at the .ill file, the ratio of direct normal/ diffuse irradiation is 5.9 @ 9am & only 2.2 @ 3pm. The scale tops out at ~ 11.5 in Dec for the weather file. (Chicago O'hare epw.)

This is definitely on the clear side for March/ September, but I will most likely need to look at overriding the sky condition with a script. (I have been burned a few times with LEED + common sense.) I'll poke around to see if I can't at least get started with the daylight coefficient data and take in the 2 references.

Thanks again.

nfonner gravatar imagenfonner ( 2014-11-20 17:07:41 -0500 )edit

I have been asked same question to determine clear sky cover, overcast sky etc for my study.Please help me once you find the answer.thanks

hema gravatar imagehema ( 2014-11-21 04:00:05 -0500 )edit

answered 2014-11-21 10:33:36 -0500

updated 2014-11-21 10:37:25 -0500

One option that I passed over initially as a "gotta get this out now" solution was to generate a "custom day" (SizingPeriod:DesignDay object) in E+ for my 3/21 equinox using the ASHRAEClearSky Solar Model Indicator and a Sky Clearness of 1.0 (1.2 is actually the max.) I believe this would be a very simple task for measure writing in OS.

There are clear disadvantages when using E+ (vs Radiance) for daylighting studies as Rob as explained. But for a quick LEED compliance report...?

Is it fair to say this seems to be a case of simple tools for simple answers? (LEED not daylighting studies.)

I still think Rob has the right solution in integrating this into OS/Rad, btw.

edit flag offensive delete link more


Hey @nfonner, sorry, I didn't see this comment 'till now. If you want a quick answer you can also just use gensky and pipe that sky to dctimestep, giving you an illuminance result for the CIE ideal clear/overcast skies.

rpg777 gravatar imagerpg777 ( 2014-12-09 13:53:21 -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: 2014-11-20 16:02:59 -0500

Seen: 770 times

Last updated: Nov 21 '14