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

[David BRADLEY <d.bradley@tess-inc.com>]

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 (0.13 or 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@iee.fraunhofer.de | Phone +49 561 804-2692

 

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