[TRNSYS-users] Ground source heat pump related question, plus one.

[David BRADLEY]

Jim,

  It is normal that Type997 is slow; it is solving a finite difference 
problem for the entire ground field. The model will run particularly 
slowly at the beginning of your simulation but will increase in solving 
speed to some extent as it gets farther in (and as the changes in the 
ground field from one timestep to the next diminish). You can speed the 
model up to some extent by increasing the size of your smallest soil 
node and increasing the value of the node size multiplier. The point of 
Type997, though it to provide a rigorous and stable solution. Type952 
models a single buried pipe rather than a network of pipes. We use it 
(or its twin-conductor cousin) to model the runouts to and from the 
ground field.

You have to be a bit careful if if your ground field is actually buried 
air ducts rather than pipes containing a liquid. Both Type952 and 997 
make the assumption that the thermal properties (density, specific heat, 
etc.) are constant over the working temperature range, which isn't 
always a great assumption with air. This is particularly true if there 
is a chance that the water vapor in the air condenses as it goes through 
the ground field. There are models available (Transsolar's Type460 for 
example) that are designed to model underground hypocausts such as the 
one it sounds like you are modeling. If indeed air is your working 
fluid, I would recommend using one of those models in place of either 
997 or 952.

kind regards,

  David


On 09/09/2021 12:45, dimitris soukoulis via TRNSYS-users wrote:
>
> Hello everybody,
>
> I am simulating a multizone building with four separately 
> air-conditioned zones (apartments). So far I was working with the 954 
> air source heat pump ( four of them. One for each zone ) but my goal 
> was to have a ground source heat pump system connected to the 
> building. I tried using the type 997 Horizontal ground heat exchanger 
> and connect its outlet flowrate to the inlet of each air source heat 
> pump, but  my simulation became extremely slow. I am now experimenting 
> with the type 952 buried horizontal pipe, setting the average surface 
> temperature equal to the annual average ambient temperature. I also 
> want to change the working fluid from water (default) to air so I can 
> directly feed it into the 954 air source heat pump. This set-up is 
> significantly faster than before, but I am having doubts about its 
> correctness. Any idea, suggestion or advice will be greatly appreciated.
>
> And one last question. Is there any standard way of calculating a heat 
> pumps air flow rate, or is it just trial and error ?
>
> Thanks a lot in advance.
>
> Kind regards ,
>
> Jim Soukoulis
>
> National Technical University of Athens.
>
> Στάλθηκε από την Αλληλογραφία 
> <https://go.microsoft.com/fwlink/?LinkId=550986> για τα Windows
>
>
> _______________________________________________
> TRNSYS-users mailing list
> TRNSYS-users at lists.onebuilding.org
> http://lists.onebuilding.org/listinfo.cgi/trnsys-users-onebuilding.org

-- 
***************************
David BRADLEY
Principal
Thermal Energy Systems Specialists, LLC
3 North Pinckney Street - suite 202
Madison, WI  53703 USA

P:+1.608.274.2577
d.bradley at tess-inc.com

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

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.onebuilding.org/pipermail/trnsys-users-onebuilding.org/attachments/20210909/fbc181e4/attachment.htm>