[TRNSYS-users] solar absorptance of walls

[David BRADLEY]

Wirich,
   It could be the outside (or inside) convection coefficient assumption 
is different between TRNSYS and EnergyPlus. If your test case is the 
BESTEST / ASHRAE140 building then they give an equation that you are 
supposed to use for the outside convection coefficient. In Standard 140, 
it is listed in Informative Annex B4. I don't have the BESTEST 
documentation so don't know where it might be in there.
Best,
  David


On 8/27/2014 03:59, FREPPEL Wirich wrote:
> Thank you David for your quick reply,
>
> Indeed the floor is a boundary wall, however I fixed it at 18°C 
> annually, it's the default setting for Energy plus. It means that all 
> the year, the outside temperature of the Ground floor is 18°C.
> I checked the inside temperature of the ground floor, its varies, 
> according to the air temperature of course.
> Actually, I checked all the inside surface temperatures of the box, 
> they are all below those of Energy plus calculations (by around 4°C in 
> summer). About the outside surface temperatures, the amplitude is 
> smaller in TRNSYS compare to Energy+. Which makes me think that 
> Energy+ takes something  into account that TRNSYS doesn't for the 
> outside temperature of the surfaces, but I don't know what it could be.
>
> If the temperature difference was only less than 1or 2°C, I would 
> think that the cause is the calculation method of the 2 programs. But 
> here I think the temperature difference is too big to be only that.
> Do you have any clue?
>
> King regards
>
> Wirich FREPPEL
> Doctorant Recherche & Developpement
> Noirot
> 8 rue Ampere, 02000 Laon
> 03 23 27 31 99
>
> Le 03/09/2014 15:34, David BRADLEY a écrit :
>> Wirich,
>>   If you are using Trnsys17 then there is almost certainly an 
>> automatic connection of solar radiation between the weather data 
>> reader and the building because in TRNBuild the default selection is 
>> that those connections will be made for you.
>>
>>   I would guess that the difference lies in how you have treated the 
>> floor of your zone. If you created a BOUNDARY wall for your floor 
>> then Type56 is expecting you will provide a temperature on the back 
>> side of the floor (i.e. the interface between the floor and the 
>> soil). If you did not make a connection to the boundary temperature 
>> Type56 input then it is probably set to a constant value of 0 C, 
>> which would keep your zone temperature quite low throughout the year.
>> Regards,
>>  David
>>
>>
>> On 8/28/2014 03:30, FREPPEL Wirich wrote:
>>> Hello everyone,
>>>
>>> I'm running some tests in order to compare TRNSYS to another 
>>> software, which is Energy+.
>>>
>>> My model is simple, a 3 meters long cube, with the same composition 
>>> for all the faces, without windows.
>>> What I did in first place, was to create a new multizone project 
>>> using an idf file created from sketch up. Then I runned the 
>>> simulation in order to observe the evolution of the air temperature 
>>> inside the box.
>>> Comparing it to Energy+ results, I noticed that in summer, the 
>>> temperature is far below those of Energy+. So I looked at the 
>>> connections in simulation studio to see if there was a problem on 
>>> the solar radiation on the walls, and I noticed that initially, 
>>> there are not any connection which give the solar radiation on the 
>>> walls.
>>>
>>> My conclusion is that to take account of the solar radiation on 
>>> walls, we have to link it by ourself at the beginning, is it right 
>>> or did I miss something?.
>>>
>>> Furthermore, in the "wall type manager", there's a tab called "Solar 
>>> Absorptance of wall". When I change the value, nothing change 
>>> regarding the temperature inside the box. So when is this value used 
>>> exactly?
>>>
>>> Thank you in advance.
>>>
>>> King regards
>>>
>>
>
>

-- 
***************************
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
d.bradley at tess-inc.com

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