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Using Radiance to generate solar surface gain schedules

asked 2018-02-22 07:58:52 -0500

kostas gravatar image

Hi all, From my understanding I need values for absorbed radiation in both opaque ( using the SurfaceProperty:SolarIncidentInside object) and transparent elements (using the ComplexFenestrationProperty:SolarAbsorbedLayers object) .Although it is straightforward to calculate solar gains on the inside face of walls, ceiling and floor I am a bit confused about how to calculate absorbed radiation on each layer of the glazing system since we model windows as single surface in Radiance.

Regards, Kostas

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answered 2018-03-05 15:10:32 -0500

Hi Kostas, You can generate layer absorption coefficients by incident angle with window. In Window preferences turn on "create debug output" in the options tab. Then in optics tab turn on "use matrix method for specular glazing".

Then run the calc for the IGU in Window.

in the directory: C:\Users\Public\LBNL\WINDOW7.6\BSDFs you'll find {systemname}_0_AngularData.csv That csv file will have absorption coefficients for each layer. Below I've pasted the column names in the angular data file for your reference. The columns with absorption are named g1-[fb]Abs[1-9] where [fb] indicates front or back incident direction (you almost certainly want front) and [1-9] indicates the surface.

You can use those coefficients to calculate total absorption by integrating the energy in all incident angles.


columns in the Angular Data csv file: n Theta Phi g1-SHGC g1-Tfsol g1-Tbsol g1-Rfsol g1-Rbsol g1-fAbs1 g1-bAbs1 g1-fAbs2 g1-bAbs2 g1-Tfvis g1-Tbvis g1-Rfvis g1-Rbvis g1-Tdw-K g1-Tdw-ISO g1-Tuv

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I should also add that it's possible to create a radiance matrix file with these layer absorption coefficients. Then you can create a daylight matrix, and a sky vector in radiance and do the integration really quickly. If you have a glazing system with three layers then you would create a 3x145 matrix window absorption coefficient matrix. Window Layer Energy Absorption = (Wabs coefficient matrix) * (Daylight Matrix) * (sky vector)

Andyrew gravatar image Andyrew  ( 2018-03-05 15:19:32 -0500 )edit

This war really helpful, many thanks for the response.

Regards, Kostas

kostas gravatar image kostas  ( 2018-03-07 10:46:29 -0500 )edit

answered 2018-02-22 10:06:09 -0500

Looks like you're wanting to use Radiance and EnergyPlus together, perhaps with OpenStudio? If so, the built-in "Radiance Daylighting Measure" creates a simple glazing material (Radiance 'glass' or 'trans' primitive) based on the VLT of the window construction found in the OpenStudio model. While Radiance does look at the full spectrum, you'll find many of the inputs for materials are concerned mainly with the visible.

While it's true that windows are typically modeled as a single polygon with Radiance, for the utmost accuracy you should use two if you're doing an actual IGU. But of course, you need the right inputs, then. What I typically do is get the SHGC, U-value, and VLT for the whole unit (IGU) and plug those into a simple glazing material for EnergyPlus, and a single glass material applied to a single polygon in the Radiance model (and this is how the Radiance measure works). If you do not have those values, i.e., you are modeling some experimental system or doing an optimization on the UGU's components, you will need to use LBL's Window and Optics programs to mock up your components and get these values that way. Or take a wild guess. =)

Unfortunately, there is no way to use a custom glass material in the Radiance measure workflow. If you are trying to use this measure, we can talk further about how you might enable that.

P.S. There is a bit more info on the Radiance glass modeling details in this post.

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First, many thanks for the reply. Yes, I attempted to calculate the solar gains in Radiance, and then feed Energy Plus through a Schedule:file object containing the results from Radiance. My problem is that I don't understand how to calculate absorbed solar radiation in the layers of the glazing system.

Regards, Kostas

kostas gravatar image kostas  ( 2018-02-22 14:29:30 -0500 )edit

Again, you'll need to use Window or Optics to sort that out. Even if you do that though, you will need to translate the Radiance results into something useful for EnergyPlus. I think the best, recent, thread on this topic is over on the radiance-general list. Here's a link to the archive: Calculating scheduled solar gains for EnergyPlus' surfaces

rpg777 gravatar image rpg777  ( 2018-02-22 14:58:19 -0500 )edit

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Asked: 2018-02-22 07:58:52 -0500

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Last updated: Mar 05 '18