I am wondering if there is a way to take into account the amount of absorption for a layer of glazing system in Irradiation nodes simulation.
just assume we have 2 different glass panes with the same Tvis but with different SHGC.
as far as I understand, for radiance materials as glass the only parameter is Transmission:
Void glass id
3 rtn gtn btn
Even for dielectric material, there is no absorption parameter:
Void dielectric id
5 rtn gtn btn n hc
Should I define a Trans material to control the absorption of a glass layer?
Void trans id
7 red green blue spec rough trans tspec
Thank you very much in advance.
The trans material is not ideal, because it does not account for the directional incidence of light in terms of its reflection / absorption. The glass material, as far as I am aware, will use the simple Fresnel equations to calculate reflectance, and the rest of the light is absorbed or transmitted as per the definition in order to produce a normalized Tvis of whatever is set. You should be able to simply define a new glass material using the SHGC values (translated into transmissivity) for the irradiation nodes simulation.
You can even, roughly, make the SHGC of the total glazing system for a single glazing surface: SHGC_total = SHGC_1 * SHGC_2. Otherwise, some more advanced tools like Optics are required.
Thank you, Alstan.
The problem is whenever we are using such a selective material with different Tvis and Tsolar we have to be careful about the results of Irradiation. I think your solution is useful.
I actually wanted to compare the Irradiation values with what I've expected based on the optical properties in LBNL Window. and I find it confusing... :(
Do you have any information about the Sky condition in Radiance. dose it use the visible spectrum or the solar spectrum?
Well Radiance is explicitly putting out a 'fake' solar spectra based on a 179 lumens / watt efficacy for any visible light calculation with the CIE skies, etc. That's not very useful.
Gendaylit; however, can output the explicit solar irradiance:
Usage: gendaylit month day hour [...]
or: gendaylit -ang altitude azimuth [...]
followed by: -P epsilon delta [options]
or: [-W|-L|-G] direct_value diffuse_value [options]
or: -E global_irradiance [options]
-P epsilon delta (these are the Perez parameters)
-W direct-normal-irradiance diffuse-horizontal-irradiance (W/m^2)
-L direct-normal-illuminance diffuse-horizontal-illuminance (lux)
-G direct-horizontal-irradiance diffuse-horizontal-irradiance (W/m^2)
-E global-horizontal-irradiance (W/m^2)
Output specification with option:
-O [0|1|2] (0=output in W/m^2/sr visible, 1=output in W/m^2/sr solar, 2=output in candela/m^2), default is 0
gendaylit version 2.4 (2013/09/04)
This output isn't available from the DIVA GUI, but it can be done by hacking the source files and running them yourself in C:\DIVA\Temp\ProjectName\ by experts. Once you switch to the solar spectrum for your sources, all of your material properties should also be in terms of solar reflectance.