Daysim/Gendaylit

radiance-general
1

Dear all,
I have a question about Daysim and Gendaylit.

I`m simulating a simple room to link the lighting performance to
thermal ones and to do it i`m using Daysim and gendaylit.

I add into Daysim the Direct Normal Radiation and the Diffuse
Horizontal Radiation from epw file (London).

Then I lunched the simulation in Daysim only with an external sensor
point, first simulation with no bounces and then with ab= 4 and I got
Direct and Global Horizontal hourly illuminance values (Lux) and
Irradiance values (W/mq)

The parameters used to do it are:

-ab 4 -ad 1024 -as 256 -ar 512 -aa 0.1 -lr 2 -st 0.015 -sj 1 -lw
0.004 -dj 0.5 -ds 0.001 -dr 2 -dp 512

Check: there are little difference (1-2%) about Horizontal DIffuse
and Direct Irradiance and during sunrise and sunset this difference is
about 5% for Direect Irrradiance.

Then I created a sky by gendaylit (Radiance) for 3 hours (9:30, 12:
30, 15:30) and for three days (21 March, 21 June and 21 December),
adding the hourly irradiance values from epw, to check the difference
to illuminace provided by Gendaylit (Radiance) and Daysim by means of a
external sensor point.
Data values are:

Time Dir Norm Rad Diff Ho Rad
Mar 09:30 420.00 182.00
Mar 12:30 422.00 238.00
Mar 15:30 184.00 167.00
      
Jun 09:30 425.00 292.00
Jun 12:30 316.00 374.00
Jun 15:30 505.00 222.00
      
Dec 09:30 162.00 50.00
Dec 12:30 513.00 69.00
Dec 15:30 0.00 10.00

gendaylit $months $days $hours -a 51.5 -o 0 -m 0 -W $direct $diffuse

skies/perez_sky_$months.$days.$hours.rad

Dome.mat

skyfunc glow skyglow
0
0
4 1 1 1 0

skyglow source sky
0
0
4 0 0 1 180

oconv -w skies/perez_sky_$months.$days.$hours.rad dome.mat >
oct/perez_sky_$months.$days.$hours.oct

rtrace -h- -w- $param -I -opv oct/perez_sky_$months.$days.$hours.oct
< point.grd | rcalc -e '$1=$1; $2=$2; $3=$3; $4=($4*0.265+$5*0.670+$6*0.
065)*179' > result.illum.$months.$days.$hours.rs &

Parameters for rtrace are the same used to generate the sky by
Daysim.

I have found these difference (illuminance):

Time Dir Norm Rad Diff Ho Rad Horiz_Direct Ill (lux)
- Daysim Horiz_Direct Ill (lux) - Gendaylit
Mar 09:30 420.00 182.00
18000
18213
Mar 12:30 422.00 238.00
24302 24494
Mar 15:30 184.00 167.00
6319 6294
      
Jun 09:30 425.00 292.00
30854 31099
Jun 12:30 316.00 374.00
26763 26899
Jun 15:30 505.00 222.00
31907 31870
      
Dec 09:30 162.00
50.00
1133 1197
Dec 12:30 513.00 69.00
10444 10519
Dec 15:30 0.00
10.00
0 0

Time Dir Norm Rad Diff Ho Rad Horiz_Glob Ill (lux) -
Daysim Horiz_Glob Ill (lux) - Gendaylit
Mar 09:30 420.00 182.00
41477 30154
Mar 12:30 422.00 238.00
54439 39691
Mar 15:30 184.00 167.00
25891 16146
      
Jun 09:30 425.00 292.00
66633 49155
Jun 12:30 316.00 374.00
70027 48607
Jun 15:30 505.00 222.00
59991 46200
      
Dec 09:30 162.00
50.00
7539 4463
Dec 12:30 513.00 69.00
20718 15751
Dec 15:30 0.00
10.00
1144 597

As you can look at there are important difference in the Horiz
Diffuse Illuminace.

I don`t know if my way is right but I`d like to know something about
it and what sky is better to use to get the horly internal illuminace
values to link these ones to EnergyPlus or Trnsys.

Thanks a lot

Sorry for my personal english.

Roberto

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2

Ciao Roberto.

I'll start with the end of your e-mail:

I don`t know if my way is right but I`d like to know
something about it and what sky is better to use
to get the horly internal illuminace values to link
these ones to EnergyPlus or Trnsys.

So you really want to know internal illuminance values for a given
scene for the whole year? (I hope you know you can get the external
illuminance from the epw file.)

My summary of your problem:

Your idea is to use Daysim (or Radiance) with a climat data file to
calculate the internal illuminance values and plug these into E+ or
whatever. Your first step is to calculate external values in an empty
scene to "validate" your sky model. Doing this in Daysim and Radiance
(with gendaylit) you got two sets of results with significant
differences.

The parameters used to do it are:

-ab 4 -ad 1024 -as 256 -ar 512 -aa 0.1 -lr 2 -st 0.015 -sj 1 -lw
0.004 -dj 0.5 -ds 0.001 -dr 2 -dp 512

I hope you did have a -I option in there some where. You also only
need to increase your "-ab" to 1 to get the "diffuse" part.

Now the interesting bit:

If you look at your results or the global horizontal values you will
see that the Daysim values are roughly 25%-30% higher than those
calculated in Radiance. However, they are consistently higher (which
is good). I would expect that the difference is a result of the
approach the two applications have:

1) Daysim calculates a sky component and just maps the sky onto this
distribution to get a particular result.

2) Radiance used the sky directly in it's raytracing calculation.

Both sets of results are bound to be different from the actually
observed illuminance on the ground. You can extract the illuminance
values from the EPW file and compare them to the calculated results.
If you find that one of the sets is reasonably accurate go with that.

You can also use the measured values to 'calibrate' your calculations
and scale the internal results according to a reference calculation
outside. If your reference point value is different from the EPW
record you increase or decrease the calculated illuminance to
compensate for the overall difference.

You can do this either with the Daysim or Radiance results. I would
guess that a calculation for a whole year would be faster in Daysim.

Regards,
Thomas