From: Bruno Bueno <[email protected]>
Date: November 29, 2013 2:10:29 AM PST
Thanks Greg and German for your comments. As I mentioned in my email, I'm considering only a simple double-pane window, without any concentrating system.
Finally, the problem was not the 3pm but the shading effect of the reveal, which we included in the 3pm but not in EnergyPlus. The thickness of the wall was 0.35 m, the glass was located at 0.05 m from inside and there were three windows on a south-facing facade. With this configuration, the solar transmission decreased up to 30% due to shading effects from the reveal for high incidence angles.
We've compared the solar transmission from the 3pm with classic Radiance, and they match well.
Cheers,
Bruno
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Message: 1
Date: Sat, 23 Nov 2013 13:45:52 -0800
From: Greg Ward<[email protected]>
Hi Bruno,
The underprediction is expected for fenestration systems with any concentrated through component, as the Klems matrix representation will spread out any transmission to the resolution of its patch sizes, which is around 15 degrees or so (square). In other words, the direct component will be spread out by the resolution of the Klems BTDF. I don't know whether or not EnergyPlus is using the Klems BTDF, and if it does, it might use it differently so there isn't this spreading of irradiation, but that's a trade-off between different types of errors. Radiance spreads the light based on BTDF angular accuracy, but the integrated value (and therefore the subsequent interreflections) will carry the correct amount of energy. A different energy sampling approach might dump the energy into a smaller area with higher concentration, just not get it in the right place. It's difficult to say which approach is better.
Cheers,
-Greg
From: Bruno Bueno<[email protected]>
Date: November 20, 2013 7:16:41 AM PST
Hello,
I'm using the three-phase method to calculate indoor illuminance, and it works fine. However, when I use it to calculate total solar transmission (irradiance) in a room, I get an underprediction of around 30% (compared to other methods such as EnergyPlus). Can anybody tell me what I'm doing wrong?
- I have an office with three south-oriented windows. Each window has only double-pane glazing.
- I use the -O 1 option in gendaylit to get radiance integrated over the solar range.
- I use the solar properties of my fenestration system instead of the visible ones to calculate BSDF data.
- I calculate solar transmission by creating two grids of sensors 1mm away from the windows (1000 sensors for 4 m2 window area), one facing to the windows (front) and another one in the opposite direction (back). The total solar transmission in [W] is the irradiance of the "front" sensors minus the irradiance of the "back" sensors multiplied by the window area.
The three-phase method matrices are calculated as:
Sky vector
gendaylit %i %i %1.1f -a %f -o %f -m %f -W %f %f -O 1 -w -l %i |genskyvec -m 4 -c 1 1 1 > daylight/tempSol.skv (%* are replaced by actual numbers)
View matrix
rcontrib -f klems_int.cal -bn Nkbins -fo -o daylight/irrSurfFront_%s.vmx -b kbinS -m window_south -I+ -ab 12 -ad 50000 -lw 2e-5 daylight/vmx.oct < daylight/intSurfFrontSensor.pts
Daylight matrix
genklemsamp -vd 0 -1 0 daylight/SouthWindow.rad | rcontrib -c 1000 -e MF:4 -f reinhart.cal -b rbin -bn Nrbins -m sky_glow daylight/dmx.oct > daylight/south.dmx
Then, for the "front" grid, I'd just multiply the matrices as:
rlam '!dctimestep daylight/irrWinFront_window_south.vmx daylight/tmxs%i.xml daylight/south.dmx daylight/tempSol.skv' | rcalc -e '$1=($1+$4+$7+$10+$13)*0.333+($2+$5+$8+$11+$14)*0.333+($3+$6+$9+$12+$15)*0.333'
Similarly for the "back" grid.
Thank you in advance for your help!
Bruno
--
Dr.-Ing. Bruno Bueno
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Message: 2
Date: Sat, 23 Nov 2013 20:03:53 -0300
From: Germ?n Molina Larrain<[email protected]>
I think I can add something else that S. Vidanovic told me:
"Well, main limitation is that integration is not really done patch by
patch (at least not for solar part)... If you for example take a look at
equation 280 you will notice that integration is averaged over sky and
ground elements. In solar calculations, I do not care from which part of
the sky radiation is coming, I simply do averaging over entire sky and just
apply that into equations. I have no idea how much error that will
introduce into calculations. One important note is that I do care what is
current sun position and therefore I do know what is incoming patch at
given time, and will know really precisely to calculate absorbed solar
radiation on interior sides."
meaning that only the beam radation is passed thorugh the BSDF, and the
rest is averaged.
Also, EnergyPlus should add the absorbed and reemited radiation...? are you
considering that?
I am not sure about what EnergyPlus does about the Basis issue that Greg
mentioned...
German