Re: [TRNSYS-users] Slab on grade

[Jeff Thornton <thornton@tess-inc.com>]

>    We usually recommend a node size progression that starts with 
> approximately 0.1 m at the edge of the slab and doubles in size with each 
> subsequent node. Then at some point, subsequent nodes stay the same size. 
> The progression might be 0.1m, 0.2m, 0.4m, 0.8m, 1.6m, 3.2m, 3.2m, 3.2m, 
> 3.2m. In more recent versions of the slab model, we changed the required 
> parameter list so that it asks for the total number of nodes, the ratio of 
> one node size to the next, and the number of nodes after which the size 
> stays the same.

As many of you may know, the new TESS slab models have been identified as one of the two "research standards" in the new IEA 34/43 task to study ground coupling.  In those detailed cases, we used a progressive multiplier of 1.2 with the original node (located near ANY edge) being only 1 inch (the next node out would be 1.2 inches, then 1.44 inches etc).  This scheme applies to the node sizes in all three directions.  While this slows the runs down a bit, the accuracy is greatly improved.  Changing the scalar multiplier (from 1.2 to 1.4) or increasing the original node size from 1" to 4" does speed up the simulations but causes a 5-10% change in the slab-to-ground heat transfer.  You may want to experiment with node sizes versus simulation speed to find a compromise that you're comfortable with.  You may also want to consider "delaying" one of the surface temperatures a timestep such that the iterations between the slab and building are reduced; we've found that to be helpful in detailed buildings with lots of zones.

Jeff

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Jeff W. Thornton
President - Thermal Energy System Specialists
2916 Marketplace Drive - Suite 104
Madison WI 53719
(608) 274-2577