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[EnergyPlus_Support] Re: Residential Multifamily Infiltration & Ventilation
Hi everyone,
This is an interesting discussion for modeling natural ventilation in energy models, namely EnergyPlus. I agree with Jean when she said that no matter which approach is taken, there are limitations and assumptions that need to be made.
There are a set of 16 commercial reference buildings on DOE's website created as EnergyPlus input files. These buildings are mechanically ventilated and the infiltration is modeled using ZoneInfiltration:DesignFlowRate with A=1 and the other coefficients 0. The Design Flow Rate was based on effective leakage area.
Since we at NIST are interested in the airflow dynamics and effects of infiltration, we created CONTAM models of the 16 buildings (available here http://www.bfrl.nist.gov/IAQanalysis/case%20studies/cwcase_12.htm) and published a report on those models and our analyses. In the CONTAM models, we assumed a building tightness and let CONTAM calculate the infiltration. We assumed one wind pressure profile (Cp as a function of wind direction) for the buildings. Alea, you could certainly follow the same method for determining the Cp for your buildings and input that into AirflowNetwork. Another tool you could use is Cp Generator (http://cpgen.bouw.tno.nl/cp/).
Alea, if you decide to use ZoneInfiltration, you will need to input coefficients A through D, so that EnergyPlus can account for effects due to wind and temperature. BOTH the stack and wind effects can be SIGNIFICANT, especially in tall buildings and buildings with natural ventilation. To do this, you'd need data on infiltration rates under various weather conditions and then fit that data to the equation for ZoneInfiltration in EnergyPlus. The SLA (specific leakage area?) could be converted to the "Design Flow Rate" needed for the ZoneInfiltration object.
As user adcamtor stated, the best way to evaluate ventilation and infiltration is by performing airflow simulation. AirflowNetwork is a LIMITED and OLD version of CONTAM, developed here at NIST. If you have a simple building layout, you could certainly create CONTAM models of the buildings to obtain more physically realistic airflow.
Jane, I am familiar with DesignBuilder but not completely. It has the capability to "autogenerate" an AFN based on the model you create? What inputs, if any, does it need? Speaking of CFD, you can also use CONTAM CFD0 editor to generate wind pressure profiles.
Hope you've found the help you've needed, Alea.
Let me know if anyone has any questions about my response above.
Lisa Ng
NIST
IAQ & Ventilation Group
--- In EnergyPlus_Support@xxxxxxxxxxxxxxx, "Jean marais" <jeannieboef@...> wrote:
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> If you have a building of "normal" proportions, eplus can calculate the Wind Pressure Coeff. for you (excludes effects from near by buildings). DesignBuilder has a nice CFD interface for this kind of calculation if you want to first calculate the WPC with CFD.
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> As for large internal spaces...consider utilizing the other room air models to get more accurate results.
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> --- In EnergyPlus_Support@xxxxxxxxxxxxxxx, oscar hernandez <oscarhernandez_81@> wrote:
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> > You have to calculate the CP values before with a CFD code, you will have
> > to model all the buildings around your building. After you
> > will enter these values to the AFN.
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> > hope this help.
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> > Pay attention with the big spaces, like open spaces. The airflow behaviour is more complex and all the thermal simulations codes can't help you.
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> > Oscar.
> > To: EnergyPlus_Support@xxxxxxxxxxxxxxx
> > From: jeannieboef@
> > Date: Thu, 28 Jun 2012 06:37:11 +0000
> > Subject: [EnergyPlus_Support] Re: Residential Multifamily Infiltration & Ventilation
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> > This is my personal most loved debates. IMO crack infiltration is well addressed with the infiltration objects available in eplus. The take into accound the primary driving force, namely wind, as well as other miniscule effects like stack effect and temperature difference.
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> > But as soon as you have a building primarily thought to ventilate with windows, then you enter the realm of "natural ventilation". The air movement through large open spaces like open windows is erratic and unpredictable. How do you know when occupants will open windows, wind direction, gusting, temperature differences, stack effect...these all play major roles now depending on the moment's conditions. CFD is also just a clip in time. How to simulate this over a year to get "equivalent" energy gains and losses caused by natural ventilation?
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> > COMMIS and Air Flow Network were developed around the same time and use mainly the same theory. CONTAM was a iSBAT user interface developed by NIST for COMMIS. I like using AFN because it so tightly nits with the built environment which one can create with eplus. The AFN has flexibility to predict window opening and modulation based on indoor and outdoor temps, fresh air requirements (both which help predict when occupants may choose to open windows). IMO it still lacks a manager that will close windows if the air velocities through the window exceed set limits (this can be done with ems, but is not feasable for large complicated buildings).
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> > I have asked DesignBuilder to think about developement of graphical results displays similar to CONTAM and an interface to build or edit the autogenerated AFN it currently does.
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> > However you approach natural ventilation, there a huge limitations, assumptions and approximations. This in one of the largest question marks regarding modelling on my list.
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> > And that is my opinion.
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