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RE: [EnergyPlus_Support] Re: CTF vs. CondFD results
Liam, I suggest to retry this in version 3.1. The details are still emerging, but it seems that CondFD may have a new problem (internal source was added for V4.0).
________________________________________
From: EnergyPlus_Support@xxxxxxxxxxxxxxx [EnergyPlus_Support@xxxxxxxxxxxxxxx] On Behalf Of Liam [liambrn@xxxxxxxxx]
Sent: Saturday, November 07, 2009 7:51 AM
To: EnergyPlus_Support@xxxxxxxxxxxxxxx
Subject: [EnergyPlus_Support] Re: CTF vs. CondFD results
Hi All,
Thanks for the response, Bereket.
Let me show you the kind of results I'm getting. For instance, the 4ZoneWithShading_ Simple_1 example run for Jan 1-Jan 5 in Chicago are below. The average heating rate in Zone 1 is shown.
CTF (4,6,10,20 ts/hour):
9774 W
9774 W
9774 W
9776 W
CondFD (4,6,10,20 ts/hour):
21149 W - with quite a few convergence errors
21240 W - with a few convergence errors
21248 W
21248 W
As you can see, the heating rate is very insensitive to timestep length, but extremely sensitive to the method.
Liam
--- In EnergyPlus_Support@xxxxxxxxxxxxxxx<mailto:EnergyPlus_Support%40yahoogroups.com>, Bereket Nigusse <nigusse_ba@...> wrote:
>
> Â
> Liam,
> Â
> As a general rule of thumb the time step (or inverse of the number of time steps) used in Finite Difference Methods (FDM) depends on the solution algorithm (explicit, implicit, crank Nicholson, etc.) used. In general for implicit FDM, which is used in Energy Plus, longer time steps give better convergence. However, it is not that straight forward to prescribe a specific time steps, it depends on the problem at hand.  If you have time, run your model for a set of time steps, 10min, 15min, 20 min, and 30min, and see if the results converges towards the longer time steps. Again this depends on the construction fabrics and the system time step if you have a system included in the input file.
>
> Bereket,
>
> --- On Fri, 11/6/09, Liam <liambrn@...> wrote:
>
>
> From: Liam <liambrn@...>
> Subject: [EnergyPlus_Support] Re: CTF vs. CondFD results
> To: EnergyPlus_Support@xxxxxxxxxxxxxxx<mailto:EnergyPlus_Support%40yahoogroups.com>
> Date: Friday, November 6, 2009, 4:03 PM
>
>
> Â
>
>
>
> Bereket,
>
> Good point. I did get better results with thin wooden walls, however, I'd like to think that results are close for more typical walls.
>
> I think I ruled out all input errors by using identical model parameters except for the heat balance algorithm.
>
> Thanks.
>
> Liam
>
> --- In EnergyPlus_Support@ yahoogroups. com, Bereket Nigusse <nigusse_ba@ ...> wrote:
> >
> > ÂÂ
> > Liam,
> > ÂÂ
> > To diagnose the possible causes of the difference it is always helpful to see the material types and the corresponding thickness of the heavy mass construction. For light constructions both method should show good match provided all other inputs are correct and identical. I suggest you may want to compare the two procedures by changing the layers thckness (use light construction) to rule out other input errors.
> >
> > Bereket
> >
> > --- On Fri, 11/6/09, Liam <liambrn@ > wrote:
> >
> >
> > From: Liam <liambrn@ >
> > Subject: [EnergyPlus_ Support] CTF vs. CondFD results
> > To: EnergyPlus_Support@ yahoogroups. com
> > Date: Friday, November 6, 2009, 2:22 PM
> >
> >
> > ÂÂ
> >
> >
> >
> > Hi All,
> >
> > I'm modeling a building of fairly heavy mass so I want to use the CondFD algorithm to better characterize transient effects. A comparison between CTF and CondFD yields differences in predicted heating loads of 100% or more.
> >
> > I repeated the comparison for several of the example files, e.g., 4ZoneWithShading_ Simple_1 and got similar differences.
> >
> > Everything in the models is identical except for HeatBalanceAlgorith m. I used 20 time-steps/hour for both algorithms.
> >
> > Any thoughts as to what might be happening?
> >
> > Thanks in advance!
> >
> > Liam O'Brien
> >
>
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