[Radiance-general] Annual illuminance simulation anomaly

[John Mardaljevic]

David,

As it happens, I use the EPW Abu Dhabi weather data to illustrate a  
point re: the Pearls Design System (sort of LEED specifically for Abu  
Dhabi). A visualisation of direct normal and diffuse horizontal  
illuminances doesn't show the effect you refer to -- scroll down to  
section 'Abu Dhabi climate':

http://www.iesd.dmu.ac.uk/~jm/doku.php?id=academic:daylight-compliance

[See page 'Daylight and Compliance' if the link gets mangled by your  
mailer.]

I had a closer look at the weather file data and it seems OK.

I have noticed an effect when processing weather files that can result  
in direct normal illuminances steadily (and unrealistically)  
increasing throughout the day.  However, that happens when direct  
normal has to be calculated from global horizontal and diffuse  
horizontal.  An offset in the time calibration of just half an hour  
can result in massive sunset values for direct normal.  Also, I have  
come across EPW files where the the direct normal in particular can be  
dodgy -- perhaps because it was derived from other quantities with  
flakey time calibration.

None of this however helps to explain what you noticed.  Also, I would  
have thought that your method would have resulted in a more-or-less  
exact match between weather file direct normal and predicted direct  
normal (ab=0).  I'm guessing you are aiming your sensor towards the  
sun using the direction vectors you get from gensky, yes?

I'm puzzled too.

-John

-----------------------------------------------
Dr. John Mardaljevic
Reader in Daylight Modelling
Institute of Energy and Sustainable Development
De Montfort University
The Gateway
Leicester
LE1 9BH, UK
+44 (0) 116 257 7972
+44 (0) 116 257 7981 (fax)

jm at dmu.ac.uk
http://www.iesd.dmu.ac.uk/~jm