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

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@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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