[Radiance-general] Annual illuminance simulation anomaly

[Greg Ward]

Hi David,

In the case of "cnuhw.png," it looks like you are specifying a  
horizontal irradiance due to direct solar that is pretty high all the  
way to sunset.  Since the direct horizontal is proportional to the  
sine of the solar altitude, and this goes to zero as the sun nears the  
horizon, the corresponding radiance of the sun has to increase  
dramatically to compensate.

A more realistic scenario would have the direct horizontal component  
dropping to zero as the sun reaches the horizon.  My guess is that  
your weather files aren't calibrated properly to the hour, or their  
direct measurements actually include a significant diffuse component.

Does anyone else have experience with this?

Cheers,
-Greg

> From: David Smith <dbs176 at gmail.com>
> Date: February 26, 2010 3:47:52 PM PST
>
> Dear list,
>
> I was beginning an attempt to look at annual illuminance using the
> information in a weather file (Energy Plus). As an initial test, I
> used gensky while providing explicitly the direct normal radiation and
> diffuse horizontal radiation from the weather file.
> There was no actual geometry in the scene, just the sun, sky, skyglow,
> etc. I then tested the illuminance with rtrace -I for a sensor
> directed in the +Z direction as well as looking directly at the sun
> where ever it was at that time.
>
> What I found was that at the end of the day, the sensor point directly
> facing the sun spikes dramatically. I've tried it with two different
> weather files with similar results. Also, other sensor points pointed
> in the general direction of the sunset spike correspondingly.
>
> Here are links to two images, each showing the hourly averages for the
> month of June:
>
> Abu Dhabi: http://i49.tinypic.com/cnuhw.png
> New York: http://i47.tinypic.com/20gz991.png
>
> Does anybody have any insight to what may be causing this?
>
> Cheers,
>
> --Dave
>