Question-and-Answer Resource for the Building Energy Modeling Community
Get started with the Help page
Ask Your Question
6

Ground Contact Slab Modeling

asked 2014-09-23 10:44:27 -0500

updated 2017-06-23 07:57:27 -0500

How do you model the external adjacency for a ground-contact slab? This site recommends using ground temperatures of 2C below average internal temperatures (for large commercial buildings) and to not include ground material. Is this the recommended approach, and is it a constant temperature profile? What about for small or mid-size commercial buildings?

edit retag flag offensive close merge delete

3 Answers

Sort by ยป oldest newest most voted
4

answered 2014-09-23 11:08:56 -0500

Modeling the multi-dimensional heat transfer into the ground from a building foundation can be very computationally intensive. The actual ground temperature varies significantly in three dimensions and depends on the climate, the foundation shape, and the level of insulation applied to the foundation. There are few whole-building energy simulation tools available that actually simulate multi-dimensional heat transfer into the ground.

Because the actual computation is so intensive, there have been several approaches to come up with simplified or pre-processed results. Of the tools available now, I think that the best approach (considering computation time and overall capability) is described pretty well in this DOE-2 article (the method can be applied to just about any simulation engine).

That said, there are several people working on improvements to estimating ground heat transfer:

  • TRNSYS has a ground-coupling library that performs detailed 3D foundation heat loss calculations. This library requires a fair amount of work from the modeler to create the numerical calculation domain (e.g., meshing, defining cell properties, etc.) and takes a considerable amount of time to run.
  • The folks at Oklahoma State are working to improve on the pre-processors used for basements and slabs in EnergyPlus. This work will integrate the calculations into EnergyPlus's time-step heat balance. These are also 3D and require a bit of effort from the user to define the numerical calculation domain. I'm not certain about when this will be available, but I would expect it sometime in the next two releases.
  • Big Ladder (Disclaimer: I'm talking about my own work here) is working on a separate foundation heat transfer tool called Kiva. Kiva is more of a framework for testing and creating simpler faster models. It can run (slow) 3D calculations, or quick 2D simulations. Kiva is still under development, but it's source code is open-source and hosted on GitHub for those interested in learning more.

With every numerical problem, there is a significant trade off between accuracy and computation time. I am not aware of a tool that strikes the right balance for typical building simulation analyses, but I'd like to think that we (as a community) are getting closer.

edit flag offensive delete link more

Comments

@Edwin can you add any more information about the work at OSU?

Neal Kruis gravatar imageNeal Kruis ( 2014-09-23 11:14:31 -0500 )edit
4

answered 2014-09-23 12:57:39 -0500

Joe Huang gravatar image

updated 2014-09-24 01:35:29 -0500

Ground coupling is something I've delved into since the mid-80's and developed several modeling methods, all utilizing a 2-D finite-difference conduction program by Lester Shen of the (now defunct) Univ. of Minnesota Underground Space Center.

The first and simplest method has been adopted as the F-Factors in ASHRAE 90.1 and is the same as described in the DOE-2 article that Neal mentioned. However, I feel a later refinement done for the California Energy Commission is much improved, more flexible and accurate, and equally easy to use, although it does require calculating the average air temperature over the past 3 weeks as an intermediate boundary condition.

The main thing to note about the F-factors is that they're conductances per lineal feet of perimeter for temperature difference between the inside and the OUTSIDE AIR, not the ground temperature (which would be different than described at the site mentioned in the question). There are actually several ways to implement this model: (1) model a monolithic foundation as an exterior wall and adjust the R-value of the layer so that UA = FPL (Perimeter length) -- this is the method illustrated in the doe-2 article, (2) decompose the foundation into two regions - a perimeter strip 1-2 ft wide and a core for the remainder; model the perimeter as an exterior wall, adjusting its R-value accordingly, but model the core as an adiabatic layer. In all cases, add as much soil to the foundation layer as allowed by the program ( ~ 2.5' in DOE-2) to dampen the hourly fluctuations in the outdoor air temperatures.

The main weaknesses of the F-factor method are (1) it "sees" only the outside air temperature, (2) the foundation is treated as a single surface. While there's a temptation to model the foundation as a underground layer tied to the ground temperature, one shouldn't then use F-factors because the boundary temperatures would be wrong.

In the late 90's, I worked with Fred Winkelmann and Vladimir Bazjanac for the Calif. Energy Commission to develop what I think is a much better model that has since been adopted in the Title-24 calculations. The approach is the same as before, i.e., calculate effective conductances, but instead of a single F-Factor, there are six Foundation Conductances for two domains (perimeter and core) and three heat flow paths ("quick" to the outdoor air temperature the past 3 weeks, "slow" to the monthly ground temperature, and "constant" to the deep ground temperature). The model is not only more accurate, but more flexible for different building conditions. If anyone's interested in the report, it's available here.

Fred and I implemented this model in a custom version of DOE-2.1E with new keywords, temperatures,etc., but it never got incorporated into the public version, which by that time was basically dormant. However, I would be happy to share the source code and executable with anyone interested.

edit flag offensive delete link more
2

answered 2014-09-23 12:21:10 -0500

Matt Mitchell gravatar image

The OSU ground-coupled slab model will be released with EnergyPlus V8.2 in a couple weeks. This model allows users to model the ground coupled heat transfer directly by setting up a ground domain and setting the ground-coupled surface's other side conditions to interact with the ground domain. The preprocessor is no longer needed with all code being incorporated into E+ and the zone and ground simulations occurring simultaneously.

A similar basement model will be released with the next full E+ release, but it will also be available much sooner in an incremental release once we have it finished up.

edit flag offensive delete link more

Your Answer

Please start posting anonymously - your entry will be published after you log in or create a new account.

Add Answer

 



Question Tools

2 followers

Stats

Asked: 2014-09-23 10:44:27 -0500

Seen: 1,717 times

Last updated: Oct 18 '16