[EnergyPlus_Support] Re: Problem with very thin high conductivity construction

["smporritt" <mail@xxxxxxxxxxxxxxxxxx>]

Thanks Karen. I am modelling the air cavity as an air gap 0.18 m2K/W. I tried removing the air gap from the integrated roof model and it does not bring the results in line - in fact it makes the difference between the integrated and separate roof versions slightly greater. I have tried changing the thermal resistance of the air gap and at high values it does reduce the cooling load for the integrated roof model, but only slightly.

Stephen

--- In EnergyPlus_Support@xxxxxxxxxxxxxxx, Karen Walkerman <kwalkerman@...> wrote:
>
> I have had issues using very thin, very conductive materials in
> constructions, but usually my file crashes if I push the constructions too
> far.
> 
> I think your issue might be in your integrated roof approach. How are you
> modeling the thick airspace?  While an airspace has some insulating
> properties, a thick airspace doesn't insulate significantly better than a
> thin airspace as air movent in the cavity allows the air to mix and easily
> conduct heat from one surface to the other.
> 
> When EnergyPlus models air in a zone, the default is to model it as fully
> mixed, so you will see very little insulation due to that airspace. I bet
> if you remove the airspace in the integrated roof approach you'll see
> results close to the 1mm metal panel. Then you'll need to go back and
> figure out an appropriate R-value for the airspace.
> 
> --
> Karen
> 
> 
> 
> On Friday, April 27, 2012, smporritt <mail@...> wrote:
> >
> >
> > Thanks for the thoughts. The tiles are white painted steel, emissivity
> 0.9, so not a low emissivity problem. Manufacturer data claims a minimum
> thickness of 0.5mm and a site survey estimated the thickness at 1mm. I also
> tried 2mm and got very similar results to 1mm and the problem exists until
> the thickness reaches 8mm or more.
> >
> > The reason for wanting to have the cavity above the ceiling tiles as a
> separate zone is to allow night ventilation of that cavity when the tiles
> are replaced with PCM impregnated tiles. As I said in my original post I
> found the results for the two modelling methods are in reasonable agreement
> for plasterboard and other lower conductivity materials (inc the PCM
> material). The problem is comparing the retrofit results with the existing
> metal tile construction - which results can be believed for the metal
> ceiling tiles?
> >
> > Thanks,
> >
> > Stephen
> >
> > --- In EnergyPlus_Support@xxxxxxxxxxxxxxx, "Edward G. Lyon" <eglyon@>
> wrote:
> >>
> >> Check the emissivity of the metal materials you are using. If they are
> low-e, there is more insulation value for the air space with low-e surfaces
> both sides than for an air space with higher emissivity.
> >>
> >> Ned Lyon, P.E. (MA)
> >> Staff Consultant
> >> SIMPSON GUMPERTZ & HEGER
> >> 781.907.9000 main
> >> 781.907.9350 direct
> >> 781.907.9009 fax
> >> www.sgh.com
> >>
> >> From: EnergyPlus_Support@xxxxxxxxxxxxxxx [mailto:
> EnergyPlus_Support@xxxxxxxxxxxxxxx] On Behalf Of smporritt
> >> Sent: Friday, April 27, 2012 8:15 AM
> >> To: EnergyPlus_Support@xxxxxxxxxxxxxxx
> >> Subject: [EnergyPlus_Support] Problem with very thin high conductivity
> construction
> >>
> >>
> >>
> >> I have come across a problem with modelling thin metal ceiling tiles and
> wonder if anyone can shed any light on the issue.
> >>
> >> I am modelling retrofit options for an airport terminal building, which
> consists of a large low zone (30m x 20m, ceiling height 2.7m), glazed along
> one long wall, with metal ceiling tiles (steel: thickness 0.001m,
> conductivity 50 W/mK, density 7800 Kg/m3, specific heat 450 J/KgK). Above
> the ceiling tiles is a cavity to an insulated metal deck roof.
> >>
> >> I have tried two modelling approaches, the first being to build a single
> zone with a flat roof construction consisting of the metal tiles, an air
> gap and the insulated metal deck (called my integrated roof version). I
> have also built a version with a separate semi-exterior unconditioned zone
> above the occupied zone to represent the roof void to allow control over
> ventilation to the roof cavity, which I need to do later in my modelling
> (separate roof version). In both cases the zone volumes were kept the same.
> >>
> >> The results for annual heating and cooling energy use using the two
> construction methods are very different, with the separate roof
> construction method using over 40% less cooling energy. However, if I
> replace the metal ceiling tiles with other lower conductivity materials,
> such as conventional ceiling tiles or plasterboard, the differences between
> the modelling methods are small (around 1%). Even if I reduce the thickness
> of the plasterboard to 0.001m the difference in cooling energy is less than
> 3% when comparing the integrated roof version to the separate roof version.
> >>
> >> I have tried many tests, including building new models from scratch,
> EnergyPlus v6 compared to v7, CTF vs CFD, different timesteps and
> algorithms and reducing the roof air cavity to a very small depth to
> minimise differences between the modelling approaches. In all cases I am
> not getting any errors (e.g. convergence) reported by EnergyPlus. Using CFD
> I have also looked at node temperatures to see if anything odd is happening
> (oscillating at timestep), but apart from the temperatures being very
> different there is nothing odd happening.
> >>
> >> If I increase the thickness of the metal ceiling tiles to 0.008m or more
> the results from the two methods converge, likewise if I reduce the
> conductivity of the metal tiles to around 10 W/mK or less the results
> converge. I also tried splitting the metal tiles into 2 x 0.0005m layers
> and adding a very low resistance no mass material layer between them and in
> that case the results were very similar for the two modelling approaches.
> >>
> >> Is anyone aware of any issues with EnergyPlus modelling very thin high
> conductivity constructions? I have searched the forums and EnergyPlus
> documentation and can find no mention of this being an issue. Any help or
> thoughts gratefully received, thanks.
> >>
> >
> >
>




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