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

[David Bradley <bradley@tess-inc.com>]

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@icax.co.uk

www.icax.co.uk

 

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