# Comparing Radiation behind Different Glass

Hi!

I am currently doing some study comparing 2 different glass materials;

1. Laminated glass (U = 3.90 W/sqmK, VLT = 0.65, SC = 0.61)
2. Double glazed unit (U = 1.57 W/sqmK, VLT = 0.57, SC = 0.37)

The radiation is evaluated behind the glass and I am getting similar results for both types of glass. I am hoping that someone can help me figure out if the result is correct or I have set it up wrongly.

Thank you in advance. Attachments are the screenshot of the material setting, results, and GH file used.

comparison.gh (99.0 KB)

1. Solar Energy Transmittance = 44%
2. Solar Energy Transmittance = 28%

Cheers,

@yewi

Since you don’t add a rad_mod_ argument to your aperture, it looks like by default HB will define a generic glass modifier for the Radiance simulation. So both constructions have the same glass modifier during the simulation:

void glass generic_exterior_window_vis_0.64
0
0
3 0.697576181538 0.697576181538 0.697576181538


You can fix this by adding your own glass modifiers, but since Radiance doesn’t model the thermal properties of your windows, the resulting interior radiation isn’t accounting for the impact of U-value and SHGC on the radiation balance. So I think you should redo this study in EnergyPlus to capture the surface heat balance.

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Saeran is correct that the Radiance properties of Honeybee objects are completely separate from the energy properties and so setting the EnergyPlus construction like that has no bearing on the Radiance simulation.

You can use Radiance to evaluate the amount of solar energy coming through the glass as long as you’re running simulations of solar irradiance (not visible light) and you set the properties of all the objects based on solar transmittance and reflectance (not visible transmittance and reflectance). Using EnergyPlus as Searan suggests is also a good suggestion.

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@chris @SaeranVasanthakumar Thanks a bunch guys. Will try the using the energy plus instead.

Cheers,
yw

you can use Radiance to evaluate the amount of solar energy coming through the glass as long as you’re running simulations of solar irradiance (not visible light) and you set the properties of all the objects based on solar transmittance and reflectance (not visible transmittance and reflectance).

This is good to know since I wasn’t sure how Radiance handled solar radiation parameters!

As an inexperienced Radiance user this also raises two other questions in my mind:

1. EP defines solar radiation as including the visible, UV and (some) infrared wavelengths (0.3 to 2.537 microns) (and so the solar material parameters are supposed to represent values averaged over the entire spectrum), do you have an idea of what Radiance considers solar radiation? I did my best to look through the Radiance docs, but can’t seem to find the appropriate section…

2. Assuming Radiance has a similiar definition of solar radiation as EP, it must perform its solar radiation simulation by expanding the spectrum it considers from just visible light to include the longer and shorter-wavelengths that compose solar radiation. However, since there’s only one input for radiation transmissivity (\tau) in the glass modifier, and none for reflectance (\rho) and absorption/emissivity (\alpha); only one other variable can be calculated from \tau, which I assume is \rho = 1 - \tau. So how does the Radiance simulation account for the fraction of solar radiation absorbed and emitted by the glass?

Feel free to point me towards the appropriate section of the Radiance docs instead of answering. As I said, I tried to look this up myself but didn’t have much luck.

Hi @SaeranVasanthakumar , the Radiance program that allows the incorporation of full spectrum solar radiation is gendaylit. The sky definitions are defined through the Perez Sky Model. I haven’t dug deep enough to know the exact wavelength ranges that are considered for visible and total solar radiation. This discussion and this blog-post by Ian Ashdown should help you get started though. As per the report referenced in his post, it appears that the spectrum considered is from .2 to 10 microns.

Regards,
Sarith

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Thanks @sarith, the gendaylit documentation that was extremely helpful. I wasn’t able to find a specific number, but the .25 - 3, and .2 - 10 were all mentioned, and seem to indicate it generally covers (probably depending on setup) a broader spectrum. I asked this question because I was trying to understand if Radiance was omitting the visible/infrared range, which would explain the omission of separate \rho, \tau, \alpha properties for those ranges in the HB glass modifier. Since Radiance covers a broader range of the solar spectrum, my earlier speculation seems correct: the glass modifier’s \tau, \rho measures represents an average over the entire spectrum.I believe this means custom glass scenarios where you have very high visible light with low solar heat gain (i.e. Solarban) can’t be modeled in HB right now, since the single glass \tau, \rho input would average over those extremes in the visible and infrared range.

When I have time I’ll look deeper into the documetation to see if the absorption question is answered.

This is correct but this EP capability is not currently exposed on any Honeybee components or within the LBT core libraries. In order to make use of EP’s ability to model the different parts of the solar spectrum, you have to use Glazing materials with the Spectral Optical Data Type. All of the Glazing materials that you can currently create with LBT Honeybee and that ship with Honeybee use SpectralAverage data instead of detailed Spectral data. At some point in the future, I would like to add support for Spectral data attached to glazing materials but we currently don’t have support for it.

The Radiance capabilities that are currently exposed are somewhat similar to EP. The only difference is that you have 3 channels exposed for RGB values instead of a single spectral average. However, a lot of the time, people tend to build models with completely greyscale materials (with RGB values all the same), which is essentially a spectral average method. As Sarith suggests, there are ways to get Radiance to model larger parts of the spectrum if you need it. But you’ll likely need to do some customization beyond what we’ve currently exposed in Honeybee.

Thanks @chris, it’s good to deepen my understanding of EP and Radiance.

However, I was actually asking a slightly different question:

My post goes through my reasoning why I think (but am not able to confirm) Radiance isn’t accounting for the absorption/emittance of solar radiation through panes of glass. I’m trying to understand the trade-off in simulating solar radiation behind a glass in EP versus Radiance. Specifically, whether the Radiance model of radiation, while much more accurate then EP in terms of reflectance/refraction/viewfactor/spectrum etc, might be slightly overcounting radiation behind glass by discounting absorption, since it doesn’t use a surface heat balance like EP to model radiant transfer through glass panes.

I will see if I can dig up the answer to my question in the docs when I have some time.

Sorry if my answer was tangential to what you were asking. You are right that modeling the Solar Transmittance of the glass with Radiance is not the same as modeling the total Solar Heat Gain though the glass, which should account for BOTH transmitted solar AND the solar energy that gets absorbed by the glass and then conducts to the interior.

So it just comes down to whatever metric @yewi needs to evaluate the glass. If transmitted solar is all that’s needed (eg. if you were studying the impact of shortwave solar on thermal comfort), then using Radiance is fine. But, if the total solar heat gain is needed for something like a Room load balance, then you’re right that EnergyPlus should be used instead of Radiance.

@chris