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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> 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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> 
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> 
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> 
> Yahoo! Groups Links
> 
> 
> 
> 
> 

========================================================
Michael J. Witte, GARD Analytics, Inc.
EnergyPlus Testing and Support 
EnergyPlus-Support@xxxxxxxx



         

 
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