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

[David Davidson]

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.skyoconv ./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!
>
>
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>
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