[TRNSYS-users] Type energy balance

[David BRADLEY]

Vincent,
   When you perform an energy balance on a tank, you need to take into 
account its change in temperature between the beginning and the end of 
the simulation (output 7 of your Type4). If the tank ends up hotter at 
the end of the simulation than it was at the beginning, it has stored 
some energy. If it ends up colder, then it has given up some energy. I 
would imagine that with 10 nodes, there is some stratification 
developing in the tank that is not modeled when you have a fully mixed 
tank (1 node)
Best,
  David


On 10/25/2012 09:40, PARTENAY Vincent wrote:
>
> Dear all,
>
> Typ4a (Storage tank) allows to model a stratified tank with several 
> temperature nodes. The problem I noticed is that the energy balance of 
> both sides on the tank can vary depending on the chosen timestep :
>
> In the attached file, a simplified example, the tank is coupled to an 
> energy production and energy consumption model (very simple in an 
> equation). Thermal losses of the tank is set to zero.
>
> When I run a simulation with one node (1m height for 0.5m3) for the 
> tank and a timestep of 0.1hr, I get the same energy on both side (see 
> plotter ""Power&NRJ", Esource = 2659kWh and Eload = 2666kWh), slight 
> difference maybe explained by initial charging period to setpoint 
> temperature.
>
> When I run it in the same conditions but with 10 nodes (each 0.1m), 
> these two energy are different ! While I still have 2666kWh for Eload 
> I have 3146kWh !
>
> When I run it again reducing the timestep from 0.1hr to 0.001 hr, 
> still with 10 nodes, I get 2666kWh, the energy balance is ok...
>
> So, my question is : is it necessary to check before every bigger 
> simulation (that can include lots of tank models) the energy balance 
> of this model for one defined timestep and boundary conditions to the 
> model or is there some rules to predefine adapted discretisation for 
> one given timestep ?
>
> Kind regards,
>
> *Vincent PARTENAY*
>
> Energies Renouvelables - Département ESE
>
> *Centre Scientifique et Technique du Bâtiment*
>
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-- 
***************************
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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