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Re: [EnergyPlus_Support] Simple Transient Model
My guess is that the thermal mass has two sides: one is connected with the interior, and the other is connected with the exterior.
Then you should set the exterior h to infinity (e.g.1000 or 10000 W/m2K) and the interior h to normal value (e.g. 1 W/m2K). That would give you the effect of exposing the indoor air directly to exterior temperature directly, with the major resistance of the interior h.
Jinchao
----- Original Message -----
From: Elizabeth Ricker
To: EnergyPlus_Support@xxxxxxxxxxxxxxx
Sent: Wednesday, December 06, 2006 2:15 PM
Subject: RE: [EnergyPlus_Support] Simple Transient Model
Brent,
I tried setting my h values very high, to 1000, as you suggested. This still gave me very strange results. The only way that I have got results that even remotely match what I would expect are when I use hin = .1 and hout = 10. Any other suggestions as to why I might be having issues? Perhaps it is something with the loads convergence that the program is not detecting an error for?
Thanks,
Betsy
"Griffith, Brent" <brent_griffith@xxxxxxxx> wrote:
Betsy
You say below you are using a rather low surface convection coefficient
of 1.0 W/m2-K. The overall surface conductivity from air to air
includes this coefficient and yours will introduce a lot of thermal
resistance. It sounds like you want to use a much higher value -- like
maybe 1000.0 W/m2-K
________________________________
From: EnergyPlus_Support@xxxxxxxxxxxxxxx
[mailto:EnergyPlus_Support@xxxxxxxxxxxxxxx] On Behalf Of Elizabeth
Ricker
Sent: Tuesday, December 05, 2006 7:37 PM
To: EnergyPlus_Support@xxxxxxxxxxxxxxx
Subject: Re: [EnergyPlus_Support] Simple Transient Model
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 <mailto:mjwitte%40gard.com> >
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
<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:
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>
>
========================================================
Michael J. Witte, GARD Analytics, Inc.
EnergyPlus Testing and Support
EnergyPlus-Support@xxxxxxxx <mailto:EnergyPlus-Support%40gard.com>
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