[TRNSYS-users] Discrete or non-discrete time step

[David BRADLEY]

Christopher,

  As a side note, it may be worth considering the nature of the models 
that you are using in your simulation as you think about using very 
short timesteps. Some (many) TRNSYS models are quasi steady state 
meaning that their outputs react immediately to input value changes and 
that they don't necessarily explicitly model the transition from one 
state to the next. For a pump or a heat exchanger that is a perfectly 
reasonable assumption if you are using a timestep that is long in 
comparison to the transient effect. However, if you shorten your 
simulation timestep down to some very very small value then those 
assumptions are perhaps not so good anymore. It may take more than 5s 
for a large pump to start and come up to steady flow and a heat 
exchanger might have enough internal thermal mass that the outlet's 
don't react to inlet temperature changes until a few timesteps later.

kind regards,

  David


On 02/11/2022 04:21, Graf, Christopher via TRNSYS-users wrote:
>
> Dear Trnsys users,
>
> as part of my research I simulated a building with domestic hot water 
> (DHW) and space heating system for one year. My current time step is 
> 3min (0.05hr). I’m thinking about lowering the time step to improve 
> the resolution of my DHW tapping profiles and the simulation results.
>
> I wondered if I can basically take any time step? For example if I 
> choose to take 5s (0.13or rounded 0.13888889) it technically adds up 
> to 8760 hr, but in reality the time steps needs to rounded after a 
> certain number of decimals, true? Will the time step be rounded in 
> TRNSYS? And if yes, how does TRNSYS cope with a time step which does 
> not exactly add up to 8760 hr? Does it make a difference whether I 
> chose to “min” or “s” instead of “hr” during the configuration of the 
> time step in the settings menu?
>
> Therefore, would it be best or is it necessary to choose a time steps 
> which is discrete and adds up to 8760 hr?
>
> Thank you for your answers!
>
> Best Regards,
>
> Christopher Graf
>
> **
>
>
> Christopher Graf, M.Sc.
>
> Scientific Associate, PhD-Candidate
>
> Thermal Energy System Departement, Energy Economics and Process 
> Integration Division
>
> Fraunhofer Institute for Energy Economics and Energy System Technology
>
> Gottschalkstr. 28a | 34127 Kassel | Germany (Visiting adress)
>
> Königstor 59 | 34119 Kassel |Germany (Postal adress)
>
> christopher.graf at iee.fraunhofer.de 
> <mailto:christopher.graf at iee.fraunhofer.de>| Phone +49 561 804-2692
>
>
> _______________________________________________
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> TRNSYS-users at lists.onebuilding.org
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-- 
***************************
David BRADLEY
Principal
Thermal Energy System 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
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