[TRNSYS-users] Type energy balance

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

290 Route des Lucioles BP 209 – F 06904 Sophia Antipolis Cedex

Tél : +33-(0)4-93-95-67-21

Fax : +33-(0)4-93-95-64-31

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