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RE: [EnergyPlus_Support] Re: Use of radiant barrier systems in walls/roofs
Another thing to keep in mind, which Mike alluded to, is that the narrowness of the air gap and/or allowing ventilation through the gap will change the convective heat transfer between the the surfaces and the air. The default "detailed" interior convection model assumes heat transfer due to natural convection in a normal sized room. This model is not so appropriate for natural convection in a very narrow gap of say 10 cm or less (4 inches), or for any kind of forced convection due to ventilation in the gap (of any size).
Trombe walls are also modeled using a thermal zone to account for the air gap between the glazing and the wall. In this case, the "Zone Inside Convection Algorithm" field is set to "TrombeWall" for the ZONE object representing the air gap. This option triggers the calculation of special interior convection coefficients that are appropriate for a narrow air gap. This algorithm is also the same as is used for calculating convection in the narrow air gap between the panes of a window. The algorithm is rather general because it takes into account the tilt angle of the surfaces, so it is possible that the "TrombeWall" option could also be used to model a horizontal air gap. I don't know exactly what range of gap distances this is accurate for, but probably 10 cm or less is best, and narrower is probably better. Unfortunately, this approach still will not account for any forced convection from ventilation.
One last approach is to use the CONVECTIONCOEFFICIENTS object to manually apply your own schedule of coefficients to the inside surfaces of the air gap. This is probably the best way to capture the effects of forced convection at this time.
Peter Ellis
National Renewable Energy Laboratory
-----Original Message-----
From: ingdallolio [mailto:ing.dallolio@xxxxxxxxx]
Sent: Monday, March 21, 2005 10:13 AM
To: EnergyPlus_Support@xxxxxxxxxxxxxxx
Subject: [EnergyPlus_Support] Re: Use of radiant barrier systems in walls/roofs
Scot,
I've modeled ventilated façade and roof the way Mike recommend,
modeling the ventilated air gap as a thermal zone, My file should
steel be there in the support group files area, or I can send you.
The results E plus calculated were not really exiting regarding the
performance of the ventilated components. As Mike said the result can
be more or less accurate. My feeling is that E+ is probably about
right. The air in the air gap inside the component ( wall or roof )
rise its temperature in respect to the outdoor air temperature due to
the radiant energy that heat the external surface of the
façade /roof . I'm not sure about the temperature of the air leaving
the air gap from the air gap outlet , I don't think E+ can give that
value. It only provide the average value of the air in the gap. But
my suspect is that before leaving the wall/roof the air give good
portion of its thermal energy to the building also because of the air
velocity inside the wall air gap that we know improve the heat
exchange. I `m right now investigating another idea to cut the
radiant component of the thermal load on a building. Put your
building under a shed, disconnect your roof or what you call "radiant
barrier", create what E+ call a "shading: detached:fixed" surface,
I've seen some Building with the roof 1 m above the last floor
ceiling slab in some cases the columns supporting the roof were going
directly to ground. I've also seen homes with shading elements all
around the house. I think those situations are very close to what E+
calls "shading: detached:fixed". Still this could be verified with
Mike simulation strategy.
Andrea.
--- In EnergyPlus_Support@xxxxxxxxxxxxxxx, "Michael J. Witte"
<mjwitte@g...> wrote:
> To answer your original questions about how EnergyPlus models
things . . .
>
> > > Can EnergyPlus correctly model the effect of the radiant
barrier on
> > > the roof heat transfer? I dug around the documentation quite
a bit
> > > and couldn't find the answer. For now, I have created a
construction
> > > containing: 1. Steel roof (Material:Regular) 2. Air gap
> > > (Material:Air) 3. Radiant barrier (Material:Regular-R)
> > >
> > > Will radiation cross the air gap, or is the air gap just
acting as a
> > > thermal resistance (conductivity only)? If there is a better
way to
> > > do this, I would appreciate any pointers!
> >
>
> Material:Air is just a fixed thermal resistance, radiation does not
cross this
> gap.
>
> The best way to model this would be to make the air gap in the roof
a separate
> thermal zone. Then radiant heat transfer from the roof to the
radiant barrier
> layer will be taken into account. The air temperature in the gap
will be
> uniform, but that is probably not a bad approximation for a thin
gap. If the
> radiant barrier material is not the same on both sides, you must
make two
> material layers so that the inside (facing the gap zone) thermal
absorptance
> can be different from the outside absorptance (facing the interior
zone). The
> detailed interior convection model will model the natural
convection within the
> gap but it will model each surface as an open flat plate with no
interactions
> due to the gap being narrow. If the gap is vented, then you would
need to add
> the COMIS airflow model to simulate natural convection through the
vents.
>
> The question remains, though - is this a good model? Maybe, maybe
not. I
> can't say for sure, because I have not seen any studies comparing
this type of
> model with actual data. So, scrutinize the results and do some
sentivities to
> assure yourself the results are reasonable.
>
> Mike
>
>
> On 16 Mar 2005, at 20:54, Scott and Roxanne Munns wrote:
>
> >
> > Agas,
> >
> > Thank you for your feedback. I believe we are also considering
painting the
> > roof white, in addition to a possible radiant barrier. The data
I had suggested
> > that a white painted roof would still have an absorptivity of
0.2, which I used
> > in my model. The aluminum foil in the radiant barrier is 0.03-
0.05.
> >
> > I will pass on your comments about how to best paint the roof to
our project
> > manager.
> >
> > Thanks again,
> > Scott
> > -----Original Message-----
> > From: Agas [mailto:agas@i...]
> > Sent: Wednesday, March 16, 2005 9:49 AM
> > To: EnergyPlus_Support@xxxxxxxxxxxxxxx; munns@u...
> > Cc: carl.bauer@i...
> > Subject: RE: [EnergyPlus_Support] Use of radiant barrier
systems in
> > walls/roofs
> >
> >
> > Modelling exercises backed up by practical experience in
Botswana have
> > shown that the greatest benefit for least cost option to reduce
heat gain
> > through galvanised steel roof sheets is to paint the outside
surface white. A
> > good quality PVA paint will last quite a few years. If you paint
the sheets
> > after they have been exposed to weather for about a year, very
little
> > preparation is needed. Just wash the roof with detergent to
remove dust and oil.
> > A new galvanised roof will need more expensive preparation to get
the paint to
> > adhere. I have heard that washing with a thin cement wash does
the job quite
> > well.
> >
> >
> > The next best intervention is a ceiling with insulation (with a
reflective
> > upper surface if available), and ventilated roof space.
> >
> >
> > Regards,
> >
> >
> > Agas
> >
> >
> > >
> > >
> > > ________________________________
> > >
> > > From: Scott and Roxanne Munns [mailto:munns@u...]
> > > Sent: Tue 3/15/2005 11:36 PM
> > > To: EnergyPlus_Support@xxxxxxxxxxxxxxx
> > > Subject: [EnergyPlus_Support] Use of radiant barrier systems
in
> > > walls/roofs
> > >
> > >
> > > Hello,
> > >
> > > I am doing analysis on a steel-roofed building in Guatemala.
Typical
> > > construction there is just a simple corrugated steel roof,
with no
> > > other insulation, etc. Ventilation only, no air conditioning.
> > >
> > > I am investigating whether using a radiant barrier (aluminum
foil
> > > facing on a 4mm thick plastic bubble sheet) is appropriate.
Sources
> > > have recommended a 25mm gap between the roof and the radiant
barrier.
> > >
> > > Can EnergyPlus correctly model the effect of the radiant
barrier on
> > > the roof heat transfer? I dug around the documentation quite
a bit
> > > and couldn't find the answer. For now, I have created a
construction
> > > containing: 1. Steel roof (Material:Regular) 2. Air gap
> > > (Material:Air) 3. Radiant barrier (Material:Regular-R)
> > >
> > > Will radiation cross the air gap, or is the air gap just
acting as a
> > > thermal resistance (conductivity only)? If there is a better
way to
> > > do this, I would appreciate any pointers!
> > >
> > > Thanks,
> > > Scott
> >
> >
> > [Non-text portions of this message have been removed]
> >
> >
> >
> > The primary EnergyPlus web site is found at:
> > http://www.energyplus.gov
> >
> > The group web site is:
> > http://groups.yahoo.com/group/EnergyPlus_Support/
> >
> > Attachments are not allowed -- please post any files to the
appropriate folder
> > in the Files area of the Support Web Site.
> >
> > Yahoo! Groups Links
> >
> >
> >
> >
> >
> >
> >
> >
> >
>
>
>
> ========================================================
> Michael J. Witte, GARD Analytics, Inc.
> EnergyPlus Testing and Support
> EnergyPlus-Support@g...
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