[TRNSYS-users] Zenith angle/Problem with meteonorm indian weatherfile

[David BRADLEY]

Dear Users,
   There have been some questions recently about result differences 
between Type15 and Type109. When we updated Trnsys from version 16 to 
version 17 we identified a number of algorithm differences between Types 
109 and 15. The decision was made to recommend Type15 and to discontinue 
Type109.

  As regards the specific question about zenith angle, the algorithms 
used to compute zenith angle involve various SIN() and COS() functions 
that "blow up" with divisions by zero at sunup and sundown. Further, the 
sun is not a point in the sky but takes some time to fully rise above or 
set below the horizon. As timesteps get smaller and smaller, this 
becomes a more difficult problem to solve. Each model makes some 
algorithm assumptions about what to do with solar radiation at sunup and 
sundown. There isn't any consistently "correct" answer for all weather 
files. Further, the solar radiation data in the weather files (whether 
TMY, Meteonorm, EnergyPlus, IWEC, or anything else) is all modeled, not 
measured; the creators of those files have made algorithm decisions 
about how to clean up the data as well.

Regards,
  David


On 10/23/2010 12:10, thiemann.stefan at vdi.de wrote:
> Hello,
>
> I would like to pick up a subject already metioned on 7.July 2010
>   concercing the incidence angle.
> Comparing Type 109 with Type 15 for a TMY2-file I noticed some
>   differences in the zenith anlge especially near sunrise and sunset.
>   When the sun is down Type 109 is 99° (??) while Type 15 ist nearly
>   (approx. 2-3 degrees lower) than 90°. But why?
> Is there any Trnsys convention on the definition of the zenith angle
>   during the night?
>
> I have another problem concerning indian weatherfiles. Calculating the
>   solar radiation for vertical facades on the east and west I got very
>   different and strange results. Does anybody know about a similiar
>   problem?
>
> Thank you, Stefan
>
>
>
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-- 
***************************
David BRADLEY
Principal
Thermal Energy Systems Specialists, LLC
22 North Carroll Street - suite 370
Madison, WI  53703 USA

P:+1.608.274.2577
F:+1.608.278.1475
d.bradley at tess-inc.com

http://www.tess-inc.com
http://www.trnsys.com