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Introduction
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There is increasing focus and interest in the simulation community on performing combined thermal / air flow studies of
buildings. Historically, thermal building models have accounted for energy transfer between thermal zones by conduction
and by convection. Conduction energy transfer was fairly easily defined based on wall definitions and the calculated
temperature differences between zones. Users were able to specify air flow rates between zones but these were not directly
calculated within the thermal model itself.
Two (at least) software packages are available for the computation of air flow between pressure nodes based on temperature
relative height differences, and the nature of the air link between the two. COMIS was developed by an international group
based at Lawrence Berkeley Labs while CONTAM was developed at the National Institute of Standards and Technology. Links
to both COMIS and CONTAM's main development sites are given below. Both COMIS and CONTAM rely on what is called the "bulk
air flow" method for calculating air flow. In this method, isothermal pressure nodes (akin to isothermal temperature zones
in a thermal building model) are defined and are linked by various types of flow paths, each of which is described by a
power law equation of the form: Flow = C (Dp)^n.
Instead of trying to reinvent the wheel by duplicating functionality already in COMIS and CONTAM, TRNSYS relies on three
different links with these two air flow modeling programs in order to allow users to perform combined thermal / air flow
analyses.
Click Here to visit the COMIS developer's website.
Click Here to visit the CONTAM developer's website.
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TRNSYS / CONTAM (Standard Type97)
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CONTAM is freely available for download from a National Institute of Standards and Technology website. The Windows program
that you download includes an interface that allows you to draw the air flow network of your building and then to export
a description of that network as a ".air" file. Some years ago, the CONTAM developers wrote a version of CONTAM as a TRNSYS
Type. Now part of the standard package, Type97 reads a ".air" file and caluculates the air flow rates between pressure nodes
based on the building geometry described in the .air file and the node (zone) temperatures calculated by a TRNSYS building
model. It should be noted that while Type56 is usually used as the building model, the nature of the TRNSYS / CONTAM link is
that any building model can be used.
At any given iteration in a TRNSYS simulation, the building model calculates zone temperatures and passes them to CONTAM. CONTAM
calculates resulting air flow rates and passes them back to the building model. In turn, the building model recalculates air
temperatures and iteration continues until both models are satisfied.
The main disadvantage of the TRNSYS / CONTAM link is that at the moment, only air flows and temperatures can be passed back and
forth between the models. Internally, CONTAM has multipliers on each of its air flow paths that allow the path to be shut off.
This multiplier is not yet able to be passed from TRNSYS to CONTAM. Thus control of the air flow network (opening and closing
windows, shutting dampers, etc.) is not yet possible.
Click Here to download CONTAM and get started using the link!
The picture below shows a completed TRNSYS / CONTAM simulation.
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TRNSYS / COMIS (Add-on Type157)
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In order to address the weakness of the TRNSYS/CONTAM link that the air flow model must remain static throughout the simulation,
TRNSYS can also be connected to COMIS through use of an add-on link component called Type157. The product of a joint project involving
CSTB, EMPA, and LBNL, Type157 recasts COMIS as a TRNSYS component. In this case, however, the COMIS input file is generated not using
a seperate graphical interface as in the case of CONTAM but using the TRNSYS Simulation Studio itself. With the release of TRNSYS 16.00.0039,
The Simulation Studio includes the project templates and proformas required to define pressure nodes and interconnecting air links just like
you connect TRNSYS components together. As with Type97, Type157 can be used with any TRNSYS building model.
For more information on Type157 and COMIS or to purchase the add on component, please visit
CSTB's COMIS website.
The pictures below show the process of starting a new COMIS project and a completed TRNSYS / COMIS simulation.
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TRNFLOW
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The Type97: TRNSYS/CONTAM link and the Type157 TRNSYS/COMIS link share a drawback in that there is a certain amount of redundant data
entry necessary to create both the thermal model using TRNBuild and the air flow model using either CONTAM or the Simulation Studio. For
example, both the building model and the air flow model need to know the volume of the zone/pressure node. The German TRNSYS Developer,
Transsolar Energietechnik, GmbH in conjunction with EMPA, incorporated COMIS entirely within the TRNSYS multizone building model. They
further developed an extended version of TRNBuild which allows the user to define air flow links between thermal zones / pressure nodes.
The TRNFLOW program is available for purchase as a TRNSYS add-on from the Transsolar.
For more information on TRNFlow, please visit Transsolar's website.
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