[Radiance-general] 3pm to calculate indoor solar transmission

[Greg Ward]

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 <bruno.bueno at ise.fraunhofer.de>
> 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
> Solar Facades
> Division Thermal Systems and Buildings
> Fraunhofer Institute for Solar Energy Systems ISE
> Heidenhofstr. 2, 79110 Freiburg, Germany
> Phone: +49(0)761 4588 5377
> bruno.bueno at ise.fraunhofer.de
> www.ise.fraunhofer.de
> 
> 
> 
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