[TRNSYS-users] Sky temperature: which one to use?

[David Bradley]

Francois,
  This wasn't as straightforward to figure out as I would have thought.

  First, there are two different sky temperature correlations that are 
available in TRNSYS. One such correlation comes from

M. Martin, P. Berdahl, Characteristics of Infrared Sky Radiation in the 
United States, Lawrence
Berkeley Laboratory, University of California - Berkeley, Solar Energy 
Vol. 33, No. 3/4, pp. 321-
336, 1984.

and is implemented in Type69. The other is from ASHRAE (I believe) and 
is implemented in Type575 (in the TESS Utility Library). As a side note, 
there is at least one other sky temperature model out there that is 
being used in EnergyPlus. The correlation that is built into Type15 is 
the correlation from Type575.

There are actually two modes of Type69 available: one in which the 
opaque cloud cover is read as an input and one in which the opaque cloud 
cover is estimated from the ratio of diffuse to total radiation. The 
mode used in Type15 is the one in which the opaque cloud cover is read 
from the data file. I would recommend plotting out the opaque sky cover 
output from Type15 and looking at it to make sure that it looks 
reasonable and hasn't been set arbitrarily. In the comparison you 
performed, you selected the mode of Type69 in which the opaque cloud 
cover is estimated, which does indeed cause some differences in the sky 
temperature. A more appropriate comparison would have been between 
Type15 and the mode of Type69 in which the cloud cover is read from 
Type15. Make sure that in doing this, you select the "opaque cloud 
cover" output and not the "total cloud cover" output.

My recommendation would be that if the opaque cloud cover data from your 
meteonorm data file looks reasonable, use the effective sky temperature 
from type15. If it does not, then use Type69's ability to estimate the 
cloud cover based on solar radiation.
Kind regards,
 David


Francois Badinier wrote:
>
> Dear TRNSYS community,
>
>  
>
> It seems that there are some differences between the " Effective sky 
> temperature" available in the weather component (e.g. Type 15-6) and 
> the "fictive sky temperature" calculated with the Type69b. The 
> differences are quite important and I would like to know which one I 
> should "trust" to calculate some IR-emissions towards the sky.
>
>  
>
> I  attached a simple testing project in which I plotted the following 
> graphs:
>
>  
>
> 1.       The _"effective sky temperature"_ , straight from the weather 
> component
>
> 2.       The  _ambient (dry-bulb) temperature_
>
> 3.       The _average_ between "effective sky temperature" and 
> "ambient temperature"
>
> 4.       The _"fictive sky temperature"_ calculated by the Type69b 
> (using "*total diffuse* *radiation*" as an input)
>
>  
>
> What comes out of this graph is that the "average temperature" is 
> quite close to the "fictive sky temperature" calculated by Type69b.
>
> So where do the differences come from and which temperature should I use?
>
>  
>
> Thank you for your help.
>
>  
>
> François Badinier
>
> Development Engineer
>
> ICAX Ltd
>
> 1 Hatfield House
>
> Baltic Street West
>
> London EC1Y OST
>
> francois.badinier at icax.co.uk <mailto:alan.kiff at icax.co.uk>
>
> www.icax.co.uk <http://www.icax.co.uk>
>
>  
>

-- 
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Thermal Energy System Specialists (TESS), LLC 
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