# unit of irradiance using epw as input

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

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!

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@gmail.com>
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!

Dear Greg,

Thank you very much!

Hope you and the list can shed light on the following question about *Spectral

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.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" >

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*

./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" >

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@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@gmail.com>

*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

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!

_______________________________________________

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@gmail.com>
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

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

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@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@gmail.com>