Lecture 24: Ground Heat Transfer Material prepared by GARD Analytics, Inc. and University of Illinois at Urbana-Champaign under contract to the National Renewable Energy Laboratory. All material Copyright U.S.D.O.E. - All rights reserved

2 Importance of this Lecture to the Simulation of Buildings  Almost all buildings have some connection to the ground  Depending on the building type, ground heat transfer may play a significant role in determining the response of the building to its surroundings  Ground heat transfer is often difficult to calculate and often miscalculated  Better simulation tools can help avoid errors in predicting the effects of the ground on the building

3 Purpose of this Lecture  Gain an understanding of: Ground heat transfer in EnergyPlus How to use the slab.exe utility program to obtain better ground heat transfer evaluation in EnergyPlus

4 Keywords Covered in this Lecture  GroundTemperatures  Inputs specific to the slab.exe utility program

5 Ground Heat Transfer Introduction  It is difficult to link ground heat transfer calculations to EnergyPlus since the conduction calculations in EnergyPlus are one-dimensional and the ground heat transfer calculations are two or three-dimensional  This causes severe modeling problems for the ground heat transfer calculation. But, it is necessary to be able to relate ground heat transfer calculations to that model  Note that ground heat transfer is highly dependent on soil properties and that soil properties can vary greatly from location to location—even between locations in the same city

6 Ground Temperature Object  Specifies the outside surface temp for surfaces in contact with the ground (e.g., slab floors, basement walls) GROUNDTEMPERATURES, 12.2, !- Jan {C} 12.7, !- Feb {C} 12.7; !- Dec {C}

7 Ground Temperatures (cont’d)  Three sets of ground temperatures are tabulated in the weather file.  Ground temperatures are for “thermally undisturbed” soil with a diffusivity of E-03 {m**2/day}.  These values are not appropriate for computing building floor losses. - Monthly Calculated "undisturbed" Ground Temperatures ° JanFebMarAprMayJunJulAugSepOctNovDec 0.5 m m m

8 Ground Temperatures (cont’d)  Use slab.exe utility to compute appropriate ground temperatures at the exterior side of any surface that is in contact with the ground. This is a monthly value that establishes the outside boundary condition (temperature) for a particular surface in contact with the ground. Documentation for slab.exe can be found in AuxiliaryPrograms.pdf.  Otherwise, take the indoor air temperature and subtract 2C as a reasonable starting value to use for most commercial applications in the U.S.

9 Ground Temperatures (cont’d)  Slab.exe utility will calculate: Monthly core, perimeter, and average ground temperatures Given a description of the floor slab, perimeter insulation, the average indoor temperature, the soil conditions and the weather file for a given location Will only compute temperatures for slab- on-grade construction (i.e., not basements)

10 PreProcess Folder  PreProcess BLAST Translator DOE-2 Translator IDF Editor IFCtoIDF Weather Converter Ground Temp Calculator

11 Ground Temperatures (cont’d)  Slab.exe ground temperature utility

12 Slab.exe Utility Program  The slab program used to calculate the results is included with the EnergyPlus distribution. It requires an input file named GHTin.idf in the input data file format. The needed corresponding idd file is E+SlabGHT.idd. An EnergyPlus weather file for the location is also needed. A sample batch file is shown on the next slide.

13 Slab.exe Batch File Basic Functions echo ===== %0 (Run Slab Generation) ===== Start ===== : Complete the following path and program names. : path names must have a following \ or errors will happen set program_path= set program_name=Slab.exe set input_path= set output_path= set weather_path= IF EXIST %output_path%1.gtp ERASE %output_path%1.gtp IF EXIST %output_path%1.ger ERASE %output_path%1.ger copy %input_path%1.idf GHTIn.idf if EXIST %weather_path%2.epw copy %weather_path%2.epw in.epw ECHO Begin Slab processing... %program_path%program_name% IF EXIST "SLABSurfaceTemps.txt" MOVE "SLABSurfaceTemps.txt" %output_path%1.gtp IF EXIST eplusout.err MOVE eplusout.err %output_path%1.ger ECHO Removing extra files... IF EXIST GHTIn.idf DEL GHTIn.idf IF EXIST in.epw DEL in.epw

14 Ground Slab Heat Transfer  The simulation can go from a 1 to x (user specified) years and uses an explicit finite difference solution technique.  Uses monthly average inside temperatures.  Can use a daily cyclic hourly variation of inside temperatures; main purpose is for user experimentation.  Will shortly have multiple ground temperature capability in EnergyPlus

15 Slab Program Input ! =========== ALL OBJECTS IN CLASS: MATERIALS =========== Materials, 2, ! N1 [NMAT: Number of materials: 2] 0.158, ! N2 [ALBEDO: Surface Albedo: No Snow: 0-1] 0.379, ! N3 [ALBEDO: Surface Albedo: Snow: 0-1] 0.9, ! N4 [EPSLW: Surface Emissivity: No Snow: 0.9] 0.9, ! N5 [EPSLW: Surface Emissivity: Snow: 0.9] 0.75, ! N6 [Z0: Surface Roughness: No Snow: 0-10 cm] 0.03, ! N7 [Z0: Surface Roughness: Snow] 6.13, ! N8 [HIN: Indoor HConv: Downward Flow: 4-10 W/m**2-K] 9.26; ! N9 [HIN: Indoor HConv: Upward: 4-10 W/m**2-K]

16 Slab Program Input (Cont.) ! =========== ALL OBJECTS IN CLASS: MATLPROPS =========== MatlProps, 2300, ! N1[RHO: Slab Material density: Validity: kg/m**3] 1200, ! N2[RHO: Soil Density: kg/m**3] 653, ! N3[CP: Slab CP: Validity: J/kg-K] 1200, ! N4[CP: Soil CP: Validity: J/kg-K] 0.93, ! N5[TCON: Slab k: Validity:.9 W/m-K] 1; ! N6[TCON: Soil k: Vailidity: 1.0 W/m-K] ! =========== ALL OBJECTS IN CLASS: BOUNDCONDS =========== BoundConds, TRUE, ! A1 [EVTR: TRUE/FALSE: Is surface evapotranspiration modeled] TRUE, ! A2 [FIXBC: TRUE/FALSE: Is the lower boundary at a fixed temp.] FALSE; ! A3 [OLDTG: TRUE/FALSE: is there an old ground temperature file]

17 Slab Program Input (Cont.) ! =========== ALL OBJECTS IN CLASS: BLDGPROPS =========== BldgProps, 2, ! N1[IYRS: Number of years to iterate: 10] 0, ! N2[Shape: Slab shape: 0 ONLY] 3.048, ! N3[HBLDG: Building height 0-20 m] 21.4; ! N4[TIN: Indoor temperature set point: 21 C] ! =========== ALL OBJECTS IN CLASS: INSULATION =========== Insulation, 0., ! N1[RINS: R value of under slab insulation W/m-K] 0., ! N2[DINS: Width of strip of under slab insulation m] 2.0, ! N3[RVINS: R value of vertical insulation W/m-K] 1.0, ! N4[ZVINS: Depth of vertical insulation ! m ONLY] 1; ! N5[IVINS: Flag: Is there vertical insulation 1=yes 0=no]

18 Slab Program Input (Cont.) ! =========== ALL OBJECTS IN CLASS: EQUIVSLAB =========== EquivSlab, 5.08, ! N1[APRatio: The area to perimeter ratio for this slab: m] TRUE; ! A1[EquivSizing: Flag: Will the dimensions of an equivalent ! slab be calculated (TRUE) or will the dimensions be input ! directly? (FALSE)] ! =========== ALL OBJECTS IN CLASS: EQUIVAUTOGRID =========== EquivAutoGrid, ! NOTE:EquivAutoGrid only necessary when EquivSizing is true , ! N1[SLABDEPTH: Thickness of slab on grade, 0.1 m] 15; ! N2[CLEARANCE: Distance from edge of slab to domain edge, 15.0 m] ! =========== ALL OBJECTS IN CLASS: AUTOGRID =========== AutoGrid, ! NOTE: AutoGrid only necessary when EquivSizing is false, ! N1[SLABX: X dimension of the building slab, m], ! N2[SLABY: Y dimension of the building slab, m], ! N3[SLABDEPTH: Thickness of slab on grade, 0.1 m] ; ! N4[CLEARANCE: Distance from edge of slab to domain ! edge, 15.0 m]

19 Building Properties IDD Object Slab Program uses the EnergyPlus input philosophy and uses its own IDD. Example is shown below: BldgProps, N1, ! [IYRS: Number of years to iterate: 10] N2, ! [Shape: Slab shape: 0 ONLY] N3, ! [HBLDG: Building height 0-20 m] N4, ! [TIN1: Indoor Average temperature set point for January: 22 C] N5, ! [TIN2: Indoor Average temperature set point for February: 22 C] N6, ! [TIN3: Indoor Average temperature set point for March: 22 C] N7, ! [TIN: Indoor Average temperature set point for April: 22 C] N8, ! [TIN: Indoor Average temperature set point for May: 22 C] N9, ! [TIN: Indoor Average temperature set point for June: 22 C] N10, ! [TIN: Indoor Average temperature set point for July: 22 C] N11, ! [TIN: Indoor Average temperature set point for August: 22 C] N12, ! [TIN: Indoor Average temperature set point for September: 22 C] N13, ! [TIN: Indoor Average temperature set point for October: 22 C] N14, ! [TIN: Indoor Average temperature set point for November: 22 C] N15, ! [TIN: Indoor Average temperature set point for December: 22 C] N16, ! [Daily sine wave variation amplitude: 0 C ] N17; ! Convergence Tollerance : 0.1

20 Variable Inside Temperature Monthly Slab Outside Face Temperatures, C Perimeter Area: Core Area: Month Average Perimeter Core Inside

21 Heat Fluxes Temperatures Heat Flux W/m^2 MonthAverage Perimeter Core Inside Perimeter Average

22 Heat Fluxes with Hourly Variation of Inside Temp

23 Hourly Temperature Variation

24 General Procedure for using slab.exe with EnergyPlus 1. Run the building in EnergyPlus with an insulated slab or as a partition to obtain monthly inside temperatures. 2. Put those monthly inside temperatures in the slab program to determine outside face temperatures. 3. Use resulting outside face temperatures in EnergyPlus. 4. Repeat 2 and 3 if inside temperatures change significantly.

25 Example Results 100 X 300 ft Warehouse, Minneapolis

26 Slab Results Month Average Perimeter Core Inside Convergence has been gained.

27 Temperature Differences between EnergyPlus Runs

28 Summary  Almost all buildings have some thermal connection to the ground, but ground heat transfer can be difficult to simulate  Slab Program allows more accurate calculation of ground temperatures for use with EnergyPlus  Use of Slab Program—EnergyPlus combination may require iteration between the two programs