Wrong GHI reproduced with gendaylit

radiance-general
1

Hallo everybody,

I have a question about gendaylit. I used gendaylit it the following way:

# gendaylit 6 20 8:0 -a 52.067779555140284 -o -9.348720190266699 -m -15 -E 502.5495
# erbs_s0, erbs_kt, irr_dir_h, irr_diff: 719.095 0.699 379.049 123.501
# WARNING: the -E option is only recommended for a rough estimation!
# Local solar time: 7.60
# Solar altitude and azimuth: 32.9 -86.6

void light solar
0
0
3 5.562e+06 5.562e+06 5.562e+06

solar source sun
0
0
4 0.837790 -0.050318 0.543668 0.533000

void brightfunc skyfunc
2 skybright perezlum.cal
0
10 3.431e+01 1.719e+01 -0.968935 -0.326815 13.068499 -3.423653 0.301840 0.837790 -0.050318 0.543668 

skyfunc glow bright 
0
0
4 1 1 1 0

bright source lightq 
0
0
4 0 0 1 180

and expected that if I measure the irradiance in z-direction (with rtrace) that I measure around 502 W/m^2 but instead of this I measure around 300 W/m^2. Is there a mistake in my usage of gendaylit?

Greetings Philip

2

Hi Philip,

there are 2 issues:

  1. If you use -E, your input is Global (solar) horizontal irradiance,
    if you measure in sun direction you will get a higher value (in your
    case around 890W/m2 if you have a 20% ground reflectance)

  2. If you are interested in solar irradiance/radiance values then you
    need to use the -O 1 option, the default is using the visible range in
    the output, applying a luminance efficacy model.

so your command should be

gendaylit 6 20 8:0 -O 1 -a 52.067779555140284 -o -9.348720190266699 -m
-15 -E 502.5495

Then, if you calculate the horizontal irradiance you will get 500W/m2 as
expected :wink:

cheers

Jan

3

Thanks @Jan_Wienold for the fast reply you helped me a lot.

Greetings Philip