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Best way to represent thermal zones boundaries when there isn't a wall

asked 2015-09-28 09:30:41 -0600 gravatar image

updated 2016-05-10 11:01:17 -0600

Hi everyone, I'm new to all this building energy modeling thing and I've got some doubts while creating my model. I'm modeling a real building and my objective is to simulate a green roof on it and evaluate if this modification can reduce the total eletric consumption (decreasing the use of the ACs).

I thought I should use the FullInteriorlandExterior option, and for that I created all my spaces with a convex geometry. But, many rooms got split up in many pieces, so I assigned Air Walls for the 'openings'. I've read that it transforms it into a Drywall material, so the heat is transfered, but not airflow. So I thought of using the "ZoneCrossMixing" to allow that.

My questions are:

  1. Should I really use the ZoneCrossMixing and Air Walls to represent this openings? That way I would let them be two different thermal zones.

  2. Should I merge the two spaces into one thermal zone? I was really afraid of all the possible problems of using Internal Mass.

  3. Is there really a huge difference in results between FullExterior and FullInteriorandExterior? Should I forget all of the above and just use FullExterior?

I'm sorry for all these subjective questions, I have just come to a dead-end, so I thought I could use an expert advice :)

P.S: before anyone questions, this simulation is part of my thesis, but I really wanted to make it as precise as possible, since the building being simulated is from the company I am currently an Intern in ;D

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You may want to rename your question, it is more about thermal zoning when using FullInteriorandExterior and how to represent thermal zone boundaries that don't represent physical walls.

Some comments:

  • If you are not modeling daylighting inside, but just the solar gain through the windows they can you probably model using just FullExterior.
  • If you are modeling daylighting, then splitting up into multiple zones and using EnergyPlus daylighting is problematic. Consider using Radiance to handle daylighting (in measure with OpenStudio 1.9.0)
David Goldwasser gravatar image David Goldwasser  ( 2015-09-28 13:49:14 -0600 )edit

OpenStudio 1.9.0 adds ZoneMixing to the OpenStudio API, but not the GUI. I plan to publish an experimental measure for core and perimeter models that turns interior zone boundaries adiabatic and and adds in zone mixing objects in both directions. This demonstrates zone mixing but was also a test to see how simulation results are affected with airflow across zone boundaries vs. just conductive heat transfer. This still however wouldn't allow light to go across the boundary unless you are using Radiance.

David Goldwasser gravatar image David Goldwasser  ( 2015-09-28 13:53:19 -0600 )edit

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answered 2015-12-08 02:32:19 -0600

updated 2015-12-08 08:39:05 -0600

Here is link to new Air Wall Zone Mixing measure. It automates the process to add a pair of zone mixing object where there is an air wall while at the same time changing the boundary condition to adiabatic. A simple formula is used to approximate airflow roughly based on depth of space vs. size of the shared air wall. There is a user configurable coefficient to alter the amount of airflow. This formula is not perfect, but meant to offer an alternative to the typical conductive inter-zone heat transfer where there is no airflow.

Here are a few example scenarios included in a pdf in the "resources" directory. image description

Here are results for a medium office prototype building in Houston using various coefficient values showing more or less airflow vs. a baseline with only conductive transfer. image description

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answered 2016-05-10 10:32:55 -0600

The placement of solar gains can be very important, especially if the perimeter zone is shallow. Using FullInteriorlandExterior plus an interior window in the plane that separates the two zones will transmit solar into the adjacent zone. If the two zones are conditioned to the same temperature, then the mixing will do very little, and the perimeter zone will keep a greater portion of the solar gains than it would with a true opening between the zones.

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Asked: 2015-09-28 09:30:41 -0600

Seen: 881 times

Last updated: May 10 '16