[Radiance-general] Annual illuminance simulation anomaly

[Andrew McNeil]

Hi David,

John mentioned that a half hour variance can create massive spikes in direct normal irradiance at sunset when calculated from horizontal irradiance data.   I think you have such a discrepancy at the root of your problem.

Irradiance data in weather files is typically the average over the previous hour.  So the irradiance for 10:00 is actually the average irradiance from 9:00 to 10:00.   If this is true of your weather data, then you should subtract  a half hour from the time in the weather data for your gensky command.  That way you are using the average sun position over the hour rather than the ending sun position.

Best,
Andy


On Mar 2, 2010, at 4:05 PM, David Smith wrote:

> Dear Radiance gurus,
> 
> First, thank you all very much for your help and suggestions. I also
> should have mentioned that I initially turned to Daysim, but it was
> having trouble with the geometry, and the only way to get the geometry
> to compile was to use meshes.
> 
> I guess I have two actual questions out of all of this:
> A. If I wanted to get a Radiance scene description that would [best]
> match the climate, what are the best data to use as inputs?
> B. Is gendaylit a prudent way forward instead of gensky CIE skies for
> this climate? What about other climates?
> 
> So, keeping those in mind, let's see if I can address some other
> people's questions:
> 1. I double checked everything and am quite certain I was picking the
> right columns (see test code below email).
> 2. As far as Greg's thought about basically having a large intensity
> to generate high enough horizontal illuminance values, shouldn't that
> be the case in the morning as well?
> 3. I had never used gendaylit before, but gave that a chance. I'm
> unsure how to catch errors on the output though, so I ended up
> ignoring the errors (script assumes illuminance is 0 and outputs that
> instead).
> 4. I also ran an empty Daysim scene using the same weather file (60
> minute step) with a sensor point at 0 0 0 in direction 0 0 1. I pulled
> in the .ill file to the mix to compare it to the rest as well.
> 5. The vector to the sun is parsed out of gensky output, I assume that
> it is very similar to the gendaylit vector, but I didn't check.
> 
> I ended up running the test loop over again with these additional bits
> of information as well as plotting the actual information within the
> weather file.
> 
> I've posted a chart of the June hourly averages for Abu Dhabi here:
> http://i47.tinypic.com/2r7a98m.png
> 
> Some thoughts:
> + I have a lot of confidence in the Daysim and gendaylit outputs that
> seem to follow each other and other lines' trends.
> + I haven't looked at the Radmap code to see how it handles the
> information, and I'm only part way through wrapping my head around
> some of the annual simulation readings that were recently introduced
> to the list.
> + I've read the ASHRAE papers on how the weather files were made. It's
> like making laws or sausages - best not to look behind the scenes if
> you want to appreciate them.
> 
> Any further insight is appreciated. Again, thank you.
> 
> Cheers,
> 
> --Dave
> 
> 
> 
> ============ Python code snippet =============
> 
> # Important: Python is white-space sensitive, your (or my) mail
> program may have fiddled with line breaks and the white-space
> 
> # Super simple unzipping of an epw file (comma separated values) into
> an indexed dictionary
> # Posted to Radiance general mailing list 20100302
> 
> import os
> 
> # define the files
> path = "/home/dave/rad/test_annual"
> epwfile = "abu.epw"
> 
> # define all the fields
> data_types = "year,month,day,hour,minute,uncertainty,dry_bulb,dew_point,relative_humidity,pressure,extraterrestrial_horizontal_radiation,extraterrestrial_direct_normal_radiation,horizontal_ir_radiation,global_horizontal_radiation,direct_normal_radiation,diffuse_horizontal_radiation,global_horizontal_illuminance,direct_normal_illuminance,diffuse_horizontal_illuminance,zenith_luminance,wind_direction,wind_speed,total_sky_cover,opaque_sky_cover,visibility,ceiling,weather_observation,weather_codes,precipitation,aerosol_optical_depth,snow_depth,days_since_last_snowfall"
> 
> # get data from the weather file
> os.chdir(path)
> handle = open(path+os.sep+epwfile,"r")
> raw = handle.read()
> handle.close()
> lines = raw.splitlines()
> 
> # figure out the average number of columns in each line, it's going to
> be really close to the lengths of the data lines
> counter = 0
> tally = 0
> for line in lines:
>    counter+=1
>    wxarray = line.split(',')
>    tally+=len(wxarray)
> linelength = int(round(1.0*tally/counter))
> 
> # try getting data out
> for line in lines:
>    if len(line.split(','))==linelength:
>        wxarray = dict(map(None,data_types.split(','),line.split(',')))
>        m = str(int(wxarray["month"]))
>        d = str(int(wxarray["day"]))
>        h = str(int(wxarray["hour"]))
>        direct = str(int(wxarray["direct_normal_radiation"]))
>        diffuse = str(int(wxarray["diffuse_horizontal_radiation"]))
> 
>        print m,d,h,direct,diffuse
> 
> _______________________________________________
> Radiance-general mailing list
> Radiance-general at radiance-online.org
> http://www.radiance-online.org/mailman/listinfo/radiance-general