Dear all,
I am trying to understand the different models used by Trnsys 17 to calculate the shortwave solar distribution in a room. For this purpose, I have modeled a rectangular room (2.5mx5mx2.7m), with one south-oriented fully glazed façade, and I have launched several sets of simulations in both simple and detailed modes to see which impact this could have on the results. Finally I have obtained some results that I could not interpret easily.
To summarize, under the simple mode, 32% of the transmitted solar radiation (through the external window to the room) is reflected back to the window, while under the detailed mode, only 20% is reflected back. The consequence is that the room air temperature can be up to 2-3°C higher in the detailed mode compared to the simple mode, which of course has a large impact on the cooling loads which thus are higher.
So I decided to change the solar absorptance of all the walls from 0.4 in the previous case to 1 and see what would happen. I then obtained that under the simple mode, 17% of the transmitted solar radiation is reflected back to the window, whereas it should has been totally absorbed by the room walls (which is the case if I switch to the detailed mode)!!!
The Trnsys manual states that under the simple mode, the incoming diffuse and reflected solar radiation is distributed according to absorptance-weighted area ratios, and that the fraction of diffuse solar that is absorbed by anu surface is defined by:
The only explanation I see for the fact that some of the transmitted radiation is distributed back to the window is that the previous formula takes into account the façade wall (i.e. each wall of the room, including the glazed façade, receives a portion of the transmitted radiation), but this makes little sense for me… So I wonder how I should calculate those distribution factors, for example the right way to calculate the sum should be:
OR
OR
?
Something that is not quite clear for me either: are the multiple reflections taken into account in the simple mode? And if yes, how?
Thank you in advance for your help,
Eloïse SOK
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Saint-Gobain Glass France, CRDC
A.T.O.M. Dpt – Building energy performances’ group