RE: [EnergyPlus_Support] Tubular Daylighting devices - Beam visible transmittance

[Kin Wai Tsang <ekwtsang@xxxxxxxxxxxx>]

Dear Peter,

Thank you for your reply.  And I have uploaded a file for your reference.  I have try to change the aspect ratio, the occurrence of Beam transmittance > 0 increase.  So I guess it has a impact on it.  It is welcome if you can find anything wrong for me.

Regards,
Ernest

"Ellis, Peter" <peter_ellis@xxxxxxxx> wrote:                              
 Hi Ernest,
  
 I'm not sure why your tubular daylighting device (TDD) isn't getting beam visible transmittance for greater incidence angles.  The beam visible transmittance accounts for the transmitted light from the direct beam of the sun.  Are you sure the TDD dome is not mounted on a sloped roof?  Is it possible there are other surfaces shading the dome at certain times of the day?  I think the documentation in the Engineering Ref is complete; there are no other assumptions that might be causing this behavior.  If you are still having problems, please upload your file to the Files area of the Yahoo groups web site.
  
 Thanks,
 P
 
 ________________________________
 
 From: EnergyPlus_Support@xxxxxxxxxxxxxxx on behalf of Ernest K W Tsang
 Sent: Thu 5/22/2008 12:50 AM
 To: EnergyPlus_Support@xxxxxxxxxxxxxxx
 Subject: [EnergyPlus_Support] Tubular Daylighting devices - Beam visible transmittance
 
 Dear all,
 
 I am currently modeling a lightpipe (tubular daylighting device) with 
 E+. However, I find a problem. My modeled lightpipe is a vertical one 
 with an aspect ratio of 11.2 and the pipe reflectance is 0.98. It is 
 noted that, the beam visible transmittance larger than 0 if and only if 
 solar altitude larger than 55deg (i.e. incidence angle should be smaller 
 than 35deg).
 
 As I know that the built-in model is based on Swift and Smith (1995) 
 model, and I has done a cross checking. It is noted that the 
 transmittance will approach zero only if it has a angle of incidence 
 more than 85deg. And even the angle of incident is 80deg, the 
 transmittance should still about 20%.
 
 So I would like to ask does the beam visible transmittance is account 
 for the transmittance of light from direct sun? Also does there is any 
 assumption other than those mentioned in Engineering Document used for 
 lightpipe calculation. I have check across the source code, and I 
 cannot find anything wrong in the code.
 
 Regards,
 Ernest Tsang
 
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