Antwort: Creating sky files for low solar altitudes

Dear Wendelin,

Thank you so much for your answer. I found the skartveit.c in the Daysim source files. I will take a look at it tonight.

Anyway, I want to ask one more thing; I create a WEA weather data file based on illuminance values instead of irradiance. Then I use ds_shortterm to create a WEA file with 1 minute intervals. As you see in the following, around sunset on 31st of July, the illuminance values suddenly (in only minute) dropped from 107 to 0, however it should take much longer in real.

7 31 20.158 0 122

7 31 20.175 0 117

7 31 20.192 0 112

7 31 20.208 0 107

7 31 20.225 0 0

7 31 20.242 0 0

7 31 20.258 0 0

Is there any way to estimate the length of this period (from 100 lux to 1 lux)? It is probably different with different sky conditions, but if we think about clear sky, is there any studies that mention this period or not?



On Monday, April 27, 2015 2:15 PM, "[email protected]" <[email protected]> wrote:
Dear Majid,

please be aware of the -i option in gendaylit.

-i time_interval[min]
If gendaylit is used with weather files, the specified instantaneous points of time may be incorrect.
This error occurs due to the fact that measurement results are frequently defined for time intervals,
not for specific points of time. Although gendaylit is working correctly, this may lead to wrong out-
puts especially at low sun altitudes. The -i option allows to specify the time interval of the mea-
surements in minutes, causing the solar position to be corrected for low sun altitudes. A warning mes-
sage is returned if a correction has been performed.

The method you are looking for is the Skartveit-Olseth method. Unfortunately, I do not have any program code for it.
Have you already looked in the Daysim code? I think it is implemented there.

Best regards

Dr. Wendelin Sprenger
Division Thermal Systems and Buildings
Fraunhofer Institute for Solar Energy Systems ISE
Heidenhofstr. 2, 79110 Freiburg, Germany
Phone: +49 (0) 7 61/ 45 88-57 45
[email protected]

-----Majid Miri <[email protected]> schrieb: -----An: Radiance General Discussion <[email protected]>
Von: Majid Miri <[email protected]>
Datum: 27.04.2015 12:30
Betreff: [Radiance-general] Creating sky files for low solar altitudes

Hi all,
I want to simulate skies during a period very close to sunrise and sunset when the global horizontal skylight is between 100 and 1 lux).I tried to use Daysim and Meteonorm 6.0 to create a weather data file with 1 minute intervals, but both of them are based on watt and their minimum value is 1 watt. When converting from irradiance to illuminance, it gives me around 140 lux as minimum. (I use “gendaylit.exe 3 21 12.0 -a 59.7 -o -18.0 -m -15.0 -W 0.0 1.0 -O 2“ to create a sky with only 1 watt horizontal diffuse irradiance)
However I need to estimate the times that the skylight is less than 100 lux.I use Gensky to produce automatically the sky file for the times that the sun altitude is close to horizon (below and above horizon), but it gives me much higher values than I expected. I checked the IES handbook and it says that to estimate diffuse skylight (the Sky ratio method is not accurate for low solar altitude, and I guess that Gensky use the same method).Therefore, I really appreciate if anyone can help me to produce a series of sky with 1 minute intervals for the time the sun is very close to horizon (below or above horizon)
Radiance-general mailing list
[email protected]