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RE: [EnergyPlus_Support] Radiant Floors - slab on grade losses?
I had the same issues when conducting a radiant slab on grade analysis a
few months ago.
One of the first issues that I saw was that setting a constant ground
temperature below the slab resulted in an incorrect energy balance that
I could not determine. The easiest way to alleviate this was specifying
a about a R-5 layer of insulation between the ground and the slab which
resulted in an almost exact energy balance.
>From my findings, I have two recommendations on how to model the ground
coupled heat transfer.
1. First and easiest approach is using the EN ISO 13370 standard to
calculate the U-Value between the slab and the external air temperature
based on the perimeter vs area ratio of the footprint. Use this U-Value
and determine a large capacitance to create a layer below the slab with
its external surface exposed to the ambient air but not the sun.
2. [Based on a response from Michael Deru whose PhD these was ground
heat transfer ] Soil thermal properties are very much a function of the
soil moisture content, which makes it a little difficult to assign
constant values to the properties. However, that is what we have to do
without solving the heat and moisture transfer problems simultaneously.
I did a comparison between a heat transfer model and a heat and moisture
transfer model for my dissertation. You can get reasonable results with
the heat transfer model with a constant value of thermal conductivity.
The best result was to model the top 1 meter of soil with one value of
thermal conductivity and soil below this with a higher thermal
conductivity to simulate a higher moisture content. My dissertation (A
Model for Ground-Coupled Heat and Moisture Transfer from Buildings) is
available from http://www.nrel.gov/docs/fy03osti/33954.pdf. Appendix C
presents information on soil properties as a function of moisture
content. Specifically, Figure C.4 shows the thermal conductivity of
soil with moisture content. I would recommend assuming a volumetric
moisture content between 155 and 20% for setting the thermal properties.
Based on his recommendation, you can create 1 m of soil underneath the
slab with the appropriate k and capacitance. And then another 1 m of
soil layer underneath with a higher conductivity to account for the
larger moisture content. The boundary condition underneath this layer
would be an appropriate constant soil temperature either throughout the
year. Note that since energy plus can not have wall constructions
beyond a certain thickness, you will have to create 'ghost' zones in
between each layer with an extremely small depth and large
emissitivity/convection in the room to minimize the thermal resistance
provided by these zones. Another way around this might be using the
new Finite Difference scheme which might allow you to specify a wall
layer with a large thickness.
Ian Doebber
Arup
Mechanical Engineer
Office : 415-946-0292
Fax : 415-957-9096
Cell : 415-613-4314
________________________________
From: EnergyPlus_Support@xxxxxxxxxxxxxxx
[mailto:EnergyPlus_Support@xxxxxxxxxxxxxxx] On Behalf Of Michael J.
Witte
Sent: Monday, June 25, 2007 9:08 AM
To: EnergyPlus_Support@xxxxxxxxxxxxxxx
Subject: Re: [EnergyPlus_Support] Radiant Floors - slab on grade losses?
I asked the team for suggestions on how to model this using current
EnergyPlus
capabilities. Here was one suggestion:
"The best the user could probably do is try some testing to check and
see what
the potential impact of modeling ground heat transfer outside E+ will
have. I
could see three separate runs: ground model as per the normal
instructions
(basically ignoring the presence of the radiant system), ground model
with
elevated interior temperatures for when the radiant system is expected
to run
to account for higher slab losses, and using the old 12" of dirt
assumption.
The user could then compare the results and see how different things
are. Of
course, it will all depends on how much insulation the user has below
the
slab."
I could think of some ways to expand on this, but they would all require
multiple runs and some experimenting.
Mike
On 21 Jun 2007 at 23:14, taharvey2001 wrote:
> We're trying to model a radiant floor system, slab on grade. The
> trouble I'm having is the ground (slab preprocessor) modeling is
> decoupled from the actual simulation. I'm not sure there is a accurate
> means to model radiant floors losses in a slab in E+. The preprocessor
> assumes an air to slab convection, doesn't consider HVAC cycling
> on/offs, and of course isn't actually coupled to the building
simulation.
>
> Anyone have ideas on how to do this in a reasonable accurate way? We
> are open to other tools besides E+ too.
>
> When will ground loss simulations be coupled with the main simulation
> engine. The current preprocessor situation we find very unsatisfactory
> in many situations.
>
>
>
>
> The primary EnergyPlus web site is found at:
> http://www.energyplus.gov <http://www.energyplus.gov>
>
> The group web site is:
> http://groups.yahoo.com/group/EnergyPlus_Support/
<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.
>
> EnergyPlus Documentation is searchable. Open EPlusMainMenu.pdf under
the
> Documentation link and press the "search" button.
>
> Yahoo! Groups Links
>
>
>
>
>
========================================================
Michael J. Witte, GARD Analytics, Inc.
EnergyPlus Testing and Support
EnergyPlus-Support@xxxxxxxx <mailto:EnergyPlus-Support%40gard.com>
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