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Re: [EnergyPlus_Support] simulation time step

To: EnergyPlus_Support@xxxxxxxxxxxxxxx
Subject: Re: [EnergyPlus_Support] simulation time step
From: "Michael J. Witte" <mjwitte@xxxxxxxx>
Date: Tue, 11 Feb 2003 12:07:58 -0600
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Adriana:

The interpolation method is described on p. 606 of the Input Output 
Reference.

If you wish to use shorter time steps but want to override the 
standard interpolation scheme, you could make a custom weather file 
using subhourly weather data.  If the frequency of your weather file 
matches the zone time step, then no interpolation will occur.  You 
should be able to verify this by reporting at the timestep frequency.

One other detail, weather is interpolated only at the zone time step 
level.  For the HVAC time steps, which can be shorter than the zone 
time step, no further interpolation is done.  For example if the zone 
time step is 15 minutes, and the HVAC time step drops to five 
minutes, then the same outdoor conditions will be used for three 
consecutive HVAC time steps.

Mike



On 7 Feb 2003, at 10:37, Adriana Angelotti wrote:

> What I mean is that if you use time steps shorter than 1 h Energy Plus
> is no more using only real weather data (in my case the ones I
> measured) but some artificial ones. I don't know the interpolation
> algorythm used by Energy Plus. Anyhow I guess that it would produce
> some regular function. If the real trend is not regular (maybe because
> of clouds) this procedure could introduce some errors...
> 
> Adriana
> 
> At 17.12 06/02/03 -0600, you wrote:
> >You say that using a time step shorter than 1 hour "introduce some
> >errors because of the interpolation of weather data".  Why do say
> >that interpolation introduces errors?  With a 1 hour time step,
> >weather conditions are held constant for an entire hour, and then
> >change to something new for the next hour.  One could argue that this
> >introduces errors, because real weather changes continously across
> >each hour.  In general, the structure of EnergyPlus is designed to
> >respond better using shorter time steps, so that is what I would
> >recommend.
> >
> >Mike
> >
> >
> >
> >On 5 Feb 2003, at 10:22, Adriana Angelotti wrote:
> >
> > > Differences are small if you compare results for time steps
> > > shorter than 1h (10m, 15m and 30m). But using 1h as the time step
> > > can produce differences of 0.5-1°C in the free floating
> > > temperature of a room.
> > >
> > > I know that weather data is interpolated for time steps shorter
> > > than 1h and I guess the difference I see in the results is due to
> > > the interpolation. So if I use a short time step I can follow
> > > better the dynamic of the building, but I introduce some errors
> > > because of the interpolation of weather data. On the other side if
> > > I use 1h I'm sure I'm not introducing interpolation errors but
> > > maybe I miss something in the dynamic. So I wonder what is the
> > > best choice. Thank you
> > >
> > > Adriana A.
> > >
> > > At 14.06 04/02/03 -0600, you wrote:
> > > >What kind of differences are you seeing?  They should be small,
> > > >and hourly results should be very similar.  The hourly weather
> > > >data is interpolated for shorter time steps.
> > > >
> > > >On 3 Feb 2003, at 14:50, Adriana Angelotti wrote:
> > > >
> > > > > Hello everybody,
> > > > >
> > > > > I'm trying to reproduce some experimental data on a free
> > > > > floating building with an Energy Plus simulation. I see that
> > > > > using different time steps (10m, 15m, 30m and 1h) produces
> > > > > different results.
> > > > >
> > > > > So I wonder if there is some general consideration that can be
> > > > > used in the choice of the proper time step, besides the one I
> > > > > found on EP manual (15 minutes for a simulation without HVAC
> > > > > and 10m for a simulation with HVAC).
> > > > >
> > > > > Besides this, I wonder what is the meaning of a simulation
> > > > > with a time step shorter than 1h, if the weather data time
> > > > > step is 1h.
> > > > >
> > > > >
> > > > > I would appreciate your help.
> > > > > Thank you
> > > > >
> > > > > Adriana Angelotti
> > > > >
> > > > >
> > > > >
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References:
- Re: [EnergyPlus_Support] simulation time step
  - From: Michael J. Witte
- Re: [EnergyPlus_Support] simulation time step
  - From: Adriana Angelotti

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