Re: WALL TIMEBASE and timesteps

[Michaël Kummert <kummert@xxxxxxxxxxxxx>]

Marco,

 > I'm not sure about which is the right timebase and time step for my
> simulation. Now I'm currently using:
>  
> TIMEBASE = 1
> Step of simulation = 0.01 (hours)
>  
> Is there a method or an advice to establish the right values?
I am afraid there is no method to select those time steps automatically.

I assume you have a good reason to select such a small time step in 
TRNSYS (0.01 h). If Type56's TIMEBASE is greater than the TRNSYS time 
step, TRNSYS will linearly interpolate the walls behavior between 2 
"wall timesteps". This might be a reason to decrease the TIMEBASE, but 
you often reach numerical limits very quickly (if you decrease the 
TIMEBASE for heavy walls, Type 56 will have a hard time to calculate the 
transfer function coefficients).

So it is not unusual to use a large wall TIMEBASE (1 h, or possibly 0.5 
or 0.25h) and a much smaller TRNSYS time step.

You could try to decrease the TIMEBASE until Prebid tells you that it 
could not calculate the transfer functions. As far as I know, this will 
not significantly increase the accuracy of your simulation, though. And 
you should always make sure that the results make sense by comparing 
them with the results obtained with a 1-h wall timebase.

The usual reminders:

- The Type 56 time base MUST be an integer multiple of the TRNSYS time
  step. 0.05 and 1h would work, but not 0.15 and 1.
- The starting time of the simulation MUST be a multiple of the Type56
  time base. You cannot start the simulation at 0.05h, for example.
- If you use hourly weather data file, the right start time for a
  yearly simulation is ALWAYS 1 for any time step (whether or not you
  use Type 56).


I hope this helps,

Michaël Kummert

--
_________________________________________________________

Michaël Kummert

Solar Energy Laboratory - University of Wisconsin-Madison
1303 Engr Res Bldg, 1500 Engineering Drive
Madison, WI 53706

Tel: +1 (608) 263-1589
Fax: +1 (608) 262-8464
E-mail: kummert@engr.wisc.edu

SEL Web Site: http://sel.me.wisc.edu
TRNSYS Web Site: http://sel.me.wisc.edu/trnsys