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Re: [TRNSYS-users] solar absorptance of walls



David,

I took your advice into account and modified the convective heat coefficients so that they are the same in both programs. As a results, the air temperature has exactly the same behaviour for both curves. Hovewer there still is a difference by around 3 degrees between both, TRNSYS being below E+. Do you think this difference is due to the different method of calculation of the two programs? I wouldn't mind if the difference temperature was less than 10°C or let's say 2°C. I'm not using the BESTEST since I was just trying to be more familiar with Energy+ and compare the two programs, but I'll have a look on it, it seems to be quite usueful for my needs.

Thank you very much

Wirich FREPPEL
Doctorant Recherche & Developpement
Noirot
8 rue Ampere, 02000 Laon
03 23 27 31 99

Le 04/09/2014 18:25, David BRADLEY a écrit :
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