[TRNSYS-users] Impact of Longwave radiation exchange model

[Sok, Eloise]

Dear all,

I am trying to see to what extend the longwave radiation exchange model in Trnsys has an impact on the thermal calculations.

For this purpose, I have modeled a rectangular sunspace room (2.5mx5mx2.7m) with one south-oriented fully glazed façade attached to a second rectangular room (5mx5mx2.7m) with no window.  I have launched 2 sets of simulations in LW radiation exchange standard (starnode approach) and detailed modes (Gebhart factors), both in detailed mode regarding beam and radiation distribution. I have looked at the different balance and surface outputs, as well as the sunspace air temperature and heating and cooling loads. Finally, I have obtained some results that I could not interpret easily:


1.       Different definition of Qcomo
In TRNSYS' manual, Qcomo is defined as "the energy from inside surface including convection to air and longwave radiation to other surfaces", while Qabso is defined as all the radiation absorbed at outside surface except longwave radiation exchange with other walls or Tsky.

However, it seems that those definitions are correct only in the standard mode.

Indeed, for example, when I looked at the different surface outputs  of the adjacent wall between the sunspace and the second room, I obtained:

·         Standard mode: Qcomo [sunspace side] =  (Qabsilw + Qsiconv) [room side] and Qabso [sunspace side] = 0

·         Detailed mode: Qcomo [sunspace side] =  Qsiconv [room side] and Qabso [sunspace side] = Qabsilw [room side]

So it seems that in the detailed mode, Qcomo only considers the convective exchange, while the longwave radiation exchange with other walls is integrated in Qabso! Is this normal, or is there something that I did not understand?


2.       Still taking the adjacent wall as an example, I have Qcomi [sunspace side]= - (Qabsilw + Qsiconv) [sunspace side] in both modes, so that in that case the definition of Qcomi written in the manual is correct whatever the mode.

However,  there is something else that bothers me: I found significant differences between the 2 modes in the values of Qabsilw and Qsiconv, but when summing those two to obtain Qcomi, the differences kind of counterbalance each other, so that the difference in Qcomi is finally quite small.  I obtain the same observations for the other walls of the sunspace, and I cannot explain such result...


3.       Sunspace and external window solar gains balance (Ntype 901 and 903)

The following outputs are identical in both modes: B1_QSEXT, B1_QBREF, B1_QSLOSS, B1_QSGWAL
Only B1_QBABS and B1_QSGWIN are different (although their sum is identical), and the difference is quite significant. Since the glazing considered for my simulations is a simple glazing with no low-e property, I cannot understand why there is such a difference...

Similarly, when I look at the window solar balance, the following outputs are identical in both modes: B3_QSEXT, B3_QBREFG, and B3_QSGWIN, but only B3_QABSG and B3_QSHFPR are different (although their sum is equal). Once again, I have difficulty to explain such result.


4.       Air temperature and Energy loads

Although the differences in surface outputs (eg Qcomi, cf section 2.) and in sunspace air temperature are quite small (0.5 to 1°C) between the 2 modes, I obtain significant differences regarding the energy loads, and especially the heating loads (38 kWh/m².year!). I thought that the detailed mode for longwave radiation echange had only a low impact on the results for ordinary zones like the ones I modeled for those simulations; but having obtained such difference in energy loads, I am very confused!

I would very appreciate your help regarding the different issues that I have pointed above.

Thank you in advance,

Best regards,



Eloïse SOK
--------------------------------------------------
Saint-Gobain Glass France, CRDC
A.T.O.M. Dpt - Building energy performances' group

Tel: +33(0)3 44 92 47 88
Mob: +33(0)7 61 68 06 74
E-mail: Eloise.Sok at saint-gobain.com<mailto:Eloise.Sok at saint-gobain.com>

Confidential-property of Saint-Gobain, the information contained herein are considered confidential. Their representation or reproduction is prohibited without the prior written consent of the sender or author.

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