[Radiance-general] unit of irradiance using epw as input

[Greg Ward]

Hi David,

This looks correct to me.  There is probably an easier way to get the result based directly on the output of gendaylit, but I'm not familiar enough with this program to say which parameters in the file correspond to global irradiance.

Cheers!
-Greg

> From: David Davidson <this.is.david.davidson at gmail.com>
> Subject: Re: [Radiance-general] unit of irradiance using epw as input
> Date: May 4, 2015 7:43:07 AM PDT
> 
> Dear Greg,
> 
> Thank you very much!
> 
> Hope you and the list can shed light on the following question about Spectral Fraction value in Fluent solar load model based on your explanation: 
> 
> In Fluent (CFD simulation software) menu for its solar load model, the spectral fraction is defined as the fraction of incident solar radiation in the visible part of the solar radiation spectrum, which is calculated as V (the visible incident solar radiation) divided by V+IR (the total incident solar radiation , visible plus infrared).
> 
> I'd like to ask if the following calculation using Radiance is correct (using Singapore's epw weather file, Mar-20, 3pm as an example):
> 
> Step 1: get the global horizontal irradiance (full spectrum, in wh/m2)
> gendaylit  3 20 15 -a 1.37 -o -103.98 -m -120 -W 447.0 378.0  -O 1   >  ./gendaylit.sky
> 
> oconv ./gendaylit.sky ./gendaylit_sky.rad > ./gendaylit_sky.oct
> 
> echo 0 0 0 0 0 1 | rtrace -w -h -I+ -ab 1 ./gendaylit_sky.oct | rcalc -e "$1=$1*0.265+$2*0.670+$3*0.065" > ./gendaylit_global_horizontal_irradiance_in_Wh-m^2.txt
> 
> The content of the gendaylit_sky.rad file, which describes the sky geometry, is:
> skyfunc glow sky_glow
> 0
> 0
> 4 1 1 1 0
> 
> sky_glow source sky
> 0
> 0
> 4 0 0 1 180
> 
> The output is: 771.593 (wh/m2), which is close to 799 wh/m2 (global_horizontal_radiation) reported in the epw file.
> 
> 
> Step 2: get the global horizontal irradiance (integrated over the visible spectrum, in wh/m2) 
> 
> The three commands are basically the same except that the [-O] option of gendaylit is set as [-O 0]:
> gendaylit  3 20 15 -a 1.37 -o -103.98 -m -120 -W 447.0 378.0  -O 0   >  ./gendaylit_visible.sky
> 
> oconv ./gendaylit_visible.sky ./gendaylit_sky.rad > ./gendaylit_sky_visible.oct
> 
> echo 0 0 0 0 0 1 | rtrace -w -h -I+ -ab 1 ./gendaylit_sky_visible.oct | rcalc -e "$1=$1*0.265+$2*0.670+$3*0.065" > ./gendaylit_global_horizontal_irradiance_visible_in_Wh-m^2.txt
> 
> The output is: 465.338 (wh/m2), which corresponds to global horizontal illuminance of 465.388*179(lm/w) = 83304 (lux), which is close to 89300lux (global_horizontal_illuminance) as reported in epw file. 
> 
> So, the Spectral Fraction for Mar-20, 3pm is 465.338 / 771.593 = 0.603
> 
> Is it correct?
> 
> 
> On Mon, Apr 27, 2015 at 1:56 AM, Greg Ward <gregoryjward at gmail.com> wrote:
> Hi David,
> 
> The EPW weather file should contain total solar irradiance values in watts/sq.meter.  Radiance also reports values in watts/sq.meter, but only over the visible spectrum.  Therefore, we normally include an "efficacy" calculation that converts the EPW's radiometric values into these semi-photometric values.  (It's a bit confusing, but suffice it to say that Radiance's units have a relation to photometric units that employs a predefined efficacy/conversion value of 179 lumens/watt.)
> 
> If you want Radiance to actually give you total solar irradiance, then you should employ the -O1 option of gendaylit and gendaymtx.  This should forgo the usual correction factors.
> 
> Cheers,
> -Greg
> 
>> From: David Davidson <this.is.david.davidson at gmail.com>
>> Subject: [Radiance-general] unit of irradiance using epw as input
>> Date: April 26, 2015 12:22:55 AM PDT
>> 
>> Hi, may I ask if the unit of irradiance is Wh/m2 if I use the "epw weather file + gendaylit/gencumulativesky + rtrace" method to get the irradiance of a given point for a particular hour in a year or annual total irradiance? 
>> 
>> The three radiation variables in epw file are using the unit of Wh/m2, so I assume the output of rtrace simulation using epw file as input should have the same unit. Is it correct?
>> 
>> Thanks!
>