[TRNSYS-users] Re : RE: Wall definition

[David Bradley]

On 7/12/2010 11:02, Julie Lestrade wrote:
> Hello Mariëlle
>
> Thank you for your quick answer.
>
> 1) For this question  I do understand this in principle but I don't 
> see how/when TRNSYS is considering it. By the way, what about the 
> capacitance ? is the default value calculated by TRNSYS a good one ?
TRNSYS uses the conduction transfer function method to compute the 
energy transfer through the wall. I would recommend looking at some of 
the archives of this mailing list for detailed explanations. However, to 
answer your direct question, the values of capacitance, density, and 
thermal conductivity of the various materials in the default libraries 
are pretty good. It is not too hard to double check them online if you 
are concerned.

For the most part, they do NOT consider thermal bridging effects but are 
for the monolithic material. the TESS layer library includes some 
equivalent materials for typical (US) stud wall constructions. These are 
equivalent materials that account for the ratio of wooden stud to 
insulated cavity. You can do the same for other configurations of 
materials if you need.
> 2) I understood I could do that, but it's calculated by default and I 
> don't particularly know the solar absorbance of the material. So I was 
> wondering if the calculation was alright ?
I would recommend that you run the building and look at energy use 
twice, once with a low estimate of the solar absorptance and once with a 
high estimate. That way you will be able to determine the overall 
sensitivity to that particular parameter and know whether it is worth 
spending time looking for a better estimate than the default.
> 5) Yes I'm adding a question here : for the ground definition, it is 
> important to set the temperature of the earth below. By default it's 
> 15°C. Does anybody know for which region of the world this is true ?
if you don't know any better, the undisturbed deep earth ground 
temperature is approximately equal to the location's annual average air 
temperature. However, that does not account for the long term 
accumulation or reduction of energy in the soil directly underneath the 
building due to coupling with the building's conditioning. If you are 
using TRNSYS 17, you might look at Type49 (slab on grade); a good bit of 
research has gone into this area recently and that component (while 
slow) does a very good job of calculating what the below grade 
temperature is as a function of time.

Best,
  David


-- 

***************************
David BRADLEY
Principal
Thermal Energy Systems Specialists, LLC
22 North Carroll Street - suite 370
Madison, WI  53703 USA

P:+1.608.274.2577
F:+1.608.278.1475
bradley at tess-inc.com

http://www.tess-inc.com
http://www.trnsys.com

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