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[EnergyPlus_Support] Re: GHT Slab severe errors and high sensitivity to inputs
Neal,
Thank you for looking into this.
Good points about extreme values being the result of runaway effects (like Venus' green house gas temperatures), and the need to keep inputs in SI. Our webform takes IP units for the auditors' convenience and internally translates that to SI before saving to the database or using in calculations. So my 2nd table of wacky values should be:
Status Temp Insulation R-value for 0.1 m thick under-slab
------ --------- ----------------------------------------------
Severe -349.25 C 1.0 m2-K/W
Severe -353.50 C 1.5 m2-K/W (gets closer to 0 for 1.5 vs 1.0)
Severe -351.02 C 2.0 m2-K/W (gets further from 0 for 2.0 vs 1.5)
Thanks again,
Cedric
--- In EnergyPlus_Support@xxxxxxxxxxxxxxx, "Kruis, Neal" <neal.kruis@...> wrote:
>
> Cedric,
>
> I am looking into this. It is important to keep in mind that the finite difference algorithm used by the slab preprocessor is not unconditionally stable. I wouldn't read too much into the meaning of the 580°C swing, that's just the nature of numerical instability. That said, I suspect that the problem you are seeing is an artifact of the automatic domain discretization.. It may be that one of the differential elements is too thin for a stable solution. Changing material properties can also cause problems for stability. The insulation values you are testing seem rather high - keep in mind that these should be in SI units (m2-K/W) and usually won't be higher than 5 m2-K/W.
>
> Neal
>
> From: EnergyPlus_Support@xxxxxxxxxxxxxxx [mailto:EnergyPlus_Support@xxxxxxxxxxxxxxx] On Behalf Of cedricdlb
> Sent: Friday, April 22, 2011 12:52 PM
> To: EnergyPlus_Support@xxxxxxxxxxxxxxx
> Subject: [EnergyPlus_Support] GHT Slab severe errors and high sensitivity to inputs
>
>
> The Ground Heat Transfer to Slab calculation gives Severe (Fatal) errors and exhibits extreme and unpredictable sensitivity to inputs. I would like to know how to programmaticly adjust my inputs to prevent these Severe errors. I'm using Energy Plus version 6 and the Auxiliary Slab program. The results are the same whether I use the pre-processor invoked by EnergyPlus or the slab stand-alone program.
>
> Below my question is an example idf file for stand-alone input into the slab utility. I used this file with the San Francisco weather file (USA_CA_San.Francisco.Intl.AP.724940_TMY3.epw). Given this idf, even slight variations in the value of the EquivalentSlab's APRatio (Area Perimeter Ratio) can lead to extreme temperature swings, even values below absolute zero (-273 C) for the calculated ground temperature. For instance:
>
> Program calculated
> Status temperature APR
> ------- ----------- ---
> Severe -352.01 C 2.50123157894737 (value in attached idf)
> Severe 230.77 C 2.501 (minute change produces wild 580 degree C swing)
> Success ~19.46 C 2.50 (temp is average of TAverage, TPerimeter, & TCore values)
> Success ~19.47 C 2.51 (it wasn't that 2.501 was too large, since 2.51 works fine)
> Success ~19.46 C 2.495 (digits of precision do not appear to be the factor)
>
> If I hold the APR at its original value (2.50123157894737), and vary the insulation, I also get wide and non-linear variations:
>
> Status Temp Insulation R-value for 1 inch thick under-slab
> ------ --------- ----------------------------------------------
> Severe -284.58 C 20.0
> Severe -327.62 C 22.0
> Severe -364.49 C 25.0 (local minimum, lower temp than more or less insulation)
> Severe -331.38 C 30.0
>
> Is there some factor in there that I need to keep within a certain value or precision range, to ensure successful runs?
>
> Thank you,
> Cedric de La Beaujardiere
>
> Here are my idf inputs to the slab stand-alone program:
>
>
> Materials,
> 2, !- NMAT: Number of materials
> 0.158, !- ALBEDO: Surface Albedo: No Snow
> 0.379, !- ALBEDO: Surface Albedo: Snow
> 0.9, !- EPSLW: Surface Emissivity: No Snow
> 0.9, !- EPSLW: Surface Emissivity: Snow
> 0.75, !- Z0: Surface Roughness: No Snow
> 0.03, !- Z0: Surface Roughness: Snow
> 6.13, !- HIN: Indoor HConv: Downward Flow {W/m2-K}
> 9.26; !- HIN: Indoor HConv: Upward {W/m2-K}
>
> MatlProps,
> 2243.0, !- RHO: Slab Material density {kg/m3}
> 1200.0, !- RHO: Soil Density {kg/m3}
> 837.0, !- CP: Slab CP {J/kg-K}
> 1200.0, !- CP: Soil CP {J/kg-K}
> 1.8, !- TCON: Slab k {W/m-K}
> 2.0775; !- TCON: Soil k {W/m-K}
>
> BoundConds,
> TRUE, !- EVTR: Is surface evapotranspiration modeled
> TRUE, !- FIXBC: is the lower boundary at a fixed temperature
> 10, !- TDEEPin {C}
> False, !- USRHflag: Is the ground surface h specified by the user?
> ; !- USERH: User specified ground surface heat transfer coeff {W/m2-K}
>
> BldgProps,
> 10, !- IYRS: Number of years to iterate
> 0, !- Shape: Slab shape
> 2.4384, !- HBLDG: Building height {m}
> 25.555555555556, !- TIN1: January Indoor Average temperature set point {C}
> 25.555555555556, !- TIN2: February Indoor Average temperature set point {C}
> 25.555555555556, !- TIN3: March Indoor Average temperature set point {C}
> 16.9, !- TIN4: April Indoor Average temperature set point {C}
> 21.111111111111, !- TIN5: May Indoor Average temperature set point {C}
> 21.111111111111, !- TIN6: June Indoor Average temperature set point {C}
> 21.111111111111, !- TIN7: July Indoor Average temperature set point {C}
> 21.111111111111, !- TIN8: August Indoor Average temperature set point {C}
> 21.111111111111, !- TIN9: September Indoor Average temperature set point {C}
> 25.555555555556, !- TIN10: October Indoor Average temperature set point {C}
> 25.555555555556, !- TIN11: NovemberIndoor Average temperature set point {C}
> 25.555555555556, !- TIN12: December Indoor Average temperature set point {C}
> 0.0, !- TINAmp: Daily Indoor sine wave variation amplitude {deltaC}
> 0.1; !- ConvTol: Convergence Tolerance
>
> Insulation,
> 0.0, !- RINS: R value of under slab insulation {m2-K/W}
> 0.0, !- DINS: Width of strip of under slab insulation {m}
> 0.0, !- RVINS: R value of vertical insulation {m2-K/W}
> 0.0, !- ZVINS: Depth of vertical insulation {m}
> 0; !- IVINS: Flag: Is there vertical insulation
>
> EquivalentSlab,
> 2.50123157894737, !- APRatio: The area to perimeter ratio for this slab {m}
> 0.101, !- SLABDEPTH: Thickness of slab on grade {m}
> 15.0, !- CLEARANCE: Distance from edge of slab to domain edge {m}
> 15.0; !- ZCLEARANCE: Distance from bottom of slab to domain bottom {m}
>
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