[Bldg-sim] Accounting for plenums/technical ceilings

Joe Huang yjhuang at whiteboxtechnologies.com
Thu Oct 24 20:43:48 PDT 2013


I've been reading the postings on this thread, but have resisted chiming 
in until now :-)

Although modeling a plenum is appealing from the point of physical 
verisimilitude, there are actually some drawbacks depending on which 
simulation program is being used.  With DOE-2, adding a plenum on the 
top floor converts the ceiling to an internal surface, for which DOE-2 
models only conductive heat flows, thus losing the direct radiant 
exchange between the roof bottom and ceiling top.  I'm still 
investigating the impact this has on the simulations, but the 
preliminary indications are that it underestimates the impact of 
measures affecting the roof reflectivity, such as a "Cool Roof".

The other thought I have is that while the COMNET description quoted 
below is physically correct, much of it (stratification, leakage through 
the ceiling, etc.) are ignored in the building simulations.  For all 
intensive purposes, a plenum in DOE-2 or EnergyPlus is simply an 
unconditioned or a partially conditioned zone if it contains ducts (and 
the ducts are modeled).

As far as modeling tricks, adding a plenum to DOE-2 or eQUEST does not 
add much of a modeling burden if a FLOOR-MULTIPLIER is used to describe 
a typical floor + plenum.  Similarly, with EnergyPlus, you can create a 
ZoneList containing all the spaces on one floor plus a plenum above, and 
then give the Zone List a multiplier.

If you want to omit the plenum, I would still make the exterior wall 
area include the plenum height, but maybe reduce the air volume of the 
space as you've described.  I know that's not a problem in DOE-2, but I 
don't know whether EnergyPlus would allow you to do so, i.e., the input 
zone volume is less than that enclosed by the surfaces.  I don't think 
you'll go terribly wrong, but I would still think that having a plenum 
would be preferable.

Joe

On 10/24/2013 7:39 PM, Patrick Bivona wrote:
> Hi,
>
> Thanks for your answers. I found a specific section, 2.2.9, in the 
> COMNET guidelines Shanta directed me to:
>
>     (...) Because of the leakage through the ceiling (typically
>     suspended), the temperature of the plenum tracks
>     the temperature of the space, except that it is generally warmer
>     because of heat stratification and heat
>     produced by lighting fixtures located at the ceiling or in the plenum.
>     It is generally recommended that plenums be modeled as separate
>     thermal blocks, but at the modeler's
>     discretion, they may be combined with conditioned space below for
>     modeling simplicity.
>
>
> I guess the part I'm interested in is acceptable simplifications. And 
> perhaps modelling tricks, that might be software specific.
>
> Now a bit of context. I'm working on a 40 storey high building. That's 
> quite a few plenums to model, which add a lot of surfaces simply by 
> being there and by breaking up spaces that could have been grouped 
> vertically into continuous thermal zones. I was wondering if there was 
> a better way of approaching this to keep the model simple and the 
> surface count down. Or do I just have to suck it up?
>
> An example of simplification I thought of, with EnergyPlus, is to 
> model space and plenum as a single space but setting the Ceiling 
> Height field of the Zone object to the height below plenum. It's 
> acceptable in terms of air changes in the space according to the 
> documentation. It removes a lot of surfaces and allows to group spaces 
> into continuous vertical thermal zones. But a part of the heat 
> transfers through external walls that would normally affect the plenum 
> will be accounted for in the space itself. Acceptable simplification? 
> Unnecessary optimisation?
>
> Thanks,
> Patrick
>
>
>
>
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