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Re: [EnergyPlus_Support] Simple Transient Model
Mike,
I too am puzzled by the observations that the case with no thermal mass takes longer to cool than the ASHRAE case with thermal mass. This issue is also starting to appear in other simulations where I am attempting to model a steady state simulation against another program. The results that I am seeing when I try to run a model that has no thermal mass (rho = .00001 and cp = .00001 for material properties) show that it takes almost a half of a year to for the energy demand to heat the building to stablize. This scenario includes a building with one zone that has a weather file corresponding to Oslo, Norway. Any idea why such a long stabilization period occurs? I get the feeling that the issues with the two simulations are related, could it be something with the algorithm? Is my assignment of very low Cp and rho creating issues?
Thanks again for your help,
Betsy
"Michael J. Witte" <mjwitte@xxxxxxxx> wrote:
I'm a bit puzzled that your case with essentially no thermal mass takes longer
to cool down (30+ hours) than the ASHRAE 1052 case which has mass.
To answer your specific questions:
> Is there an alternative way of defining adabatic walls with no thermal mass?
1. Yes. Search the EnergyPlus documentation (from the Docs Main Menu page)
for "adiabatic". Adiabatic surfaces are described as interzone surfaces with
"see" themselves on the other side. To eliminate mass, you can use
MATERIAL:REGULAR-R, but simulations with no mass at all in the surfaces can be
unstable, so you will get severe errors warning about this.
> Any ideas on why the response time would be so long?
2. Be sure that TIMESTEP IN HOUR is set to 4 or greater.
> Are there any other forms of thermal mass other than the air that
> might be residual in the simulation and giving me longer time constants
> than expected?
3. No. Only surfaces and zone air add thermal mass to the heat balance.
Mike
On 5 Dec 2006 at 18:36, Elizabeth Ricker wrote:
> Mike,
>
> I took a look at the files for the ASHREA documentation, but that did not
> solve my problem. The primary issue is that I am trying to examine the
> temperature response in a system with NO thermal mass (i.e. the walls, floor,
> and roof all have very low densities ~.0001 and Cp values ~.0001). The only
> thermal mass that should be present is in the air within the room. Given
> these conditions I should be seeing a response that is very fast in letting
> the air in the room cool from 26C to the outdoor temperature of 0C. What I am
> actually seeing is a response on the order of 35 hours for air to cool with
> all walls adiabatic except for a single conducting wall. I am currently
> setting my adiabatic walls by giving them very low conductivites. Is there an
> alternative way of defining adabatic walls with no thermal mass? Any ideas on
> why the response time would be so long? Are there any other forms of thermal
> mass other than the air that might be residual in the simulation and
> giving me longer time constants than expected?
>
> Thank you for your help,
>
> Betsy
>
> "Michael J. Witte" <mjwitte@xxxxxxxx> wrote:
> It is hard to say what the problem may be. Did you zero out all of
> the solar
> in the weather file, or set all of the surfaces to "NoSun"?
>
> A similar test is reported in "EnergyPlus Testing with ASHRAE 1052-RP Toolkit
> -
> Building Fabric Analytical Tests". Perhaps comparing your test inputs and
> results with tests TC1-TC3 will help you find the problem. This report is
> available at:
>
> http://www.eere.energy.gov/buildings/energyplus/testing.html
>
> Follow the link for "ASHRAE Research Project 1052"
>
> Mike
>
> On 2 Dec 2006 at 17:49, elizabeth.ricker wrote:
>
> > Hi All,
> > I am currently looking at a simple transient model in
> > EnergyPlus. I would like to heat a room (1 zone) to 26C for a
> > period, then allow it to cool to the outdoor temperature (0C) and
> > analyze the time response of the system. I am looking at the
> > reponse given that heat is only leaving through one wall, there is
> > no thermal mass in that wall, and the remainder of the walls are
> > super insulating. The convection coefficients on each wall are set
> > to 1 W/m2K and the conductivity of the single conducting wall is
> > very high (k = 1000, L = .1). Given these conditions the only
> > thermal mass in my model should be that of the air in the room.
> > Also, I am running a modified weather file with T = 26C for the
> > first month, and then going to 0C for the remainder of the year. The
> > results that I am showing are presenting a much longer time reponse
> > than I would expect, and do not necessarily show an exponential time
> > decay. Any suggestions on what might be going wrong in my model?
> >
> > Thank you,
> >
> > Betsy Ricker
> >
> >
> >
> >
> > The primary EnergyPlus web site is found at:
> > http://www.energyplus.gov
> >
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> > http://groups.yahoo.com/group/EnergyPlus_Support/
> >
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> the
> > "search" button.
> >
> > Yahoo! Groups Links
> >
> >
> >
> >
> >
>
> ========================================================
> Michael J. Witte, GARD Analytics, Inc.
> EnergyPlus Testing and Support
> EnergyPlus-Support@xxxxxxxx
>
>
>
>
>
>
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