Chapter 1

Ground Loop Properties and GLHEPRO Use

1.1 GLHEPRO inputs for base model

Table 1.1: Ground Loop Properties used in GLHEPRO

GeometryLength 68.6 m 225 ftBorehole Diameter 127 mm 5 inShank Spacing 25.4 mm 1 inBorehole Spacing 4.572 m 15 ftGeometry Line of 4 -U -Tube ID 27.33 mm 1.076 inU-Tube OD 33.4 mm 1.315 in

Thermal PropertiesU-TubeConductivity 1.333 W/m/K 0.225 Btu/hr/ft/oFCapacitance 1767 kJ/m3/K 22.99 Btu/ft3/oFGroutConductivity 0.7443 W/m/K 0.43 Btu/hr/ft/oFCapacitance 3901 kJ/m3/K 58.17 Btu/ft3/oFGroundConductivity 0.3895 W/m/K 0.77 Btu/hr/ft/oFCapacitance 1542 kJ/m3/K 26.35 Btu/ft3/oFUndisturbed Temperature 21.67 oC 71.01 oF

Fluid PropertiesAntifreeze None -Convection Coefficient 1534 W/m2/K 270.2 Btu/hr/ft2/oFFluid Factor 1 -Flow Rate per Borehole 0.1893 L/s 3 GPMBorehole Resistance 0.2097 Km/W 0.3629 oF hr ft/Btu

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1.2 Use of GLHEPRO

GLHEPRO is used to size the ground loop based on total heat load on

the ground as well as to create the parameter files for the Simulink R© model.

The values shown in the following screen-shots are those used for the base

model which uses Austin ground properties and energy loads computed for

the modeled house.

Figure 1.1: Main screen of GLHEPRO

Figure 1.1 shows the main input screen of the GLHEPRO software. On

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this screen the borehole field parameters such as borehole length, spacing, and

geometry are entered. The borehole geometry defines how multiple borehole

interact with one another. For example, the base model uses a single line

of four boreholes. Other options include single boreholes and rectangular, L-

shaped and U-shaped borehole fields. The undisturbed ground temperature is

also entered on this screen. There is an option to pull up a map that shows

the ground temperatures of the United States. The ground thermal properties

can be entered on this screen as well.

The borehole thermal resistance and the g-functions for the model are

calculated using the screen shown in Figure 1.2. It is accessed by clicking

on the “Calculate Borehole Thermal Resistance” button on the main page.

At this point the user can choose to model a single u-tube, double u-tube,

or concentric tube borehole by clicking on the proper tab at the top of the

screen. After selecting this, the detailed borehole geometry is entered. For the

single u-tube configuration this includes the shank spacing (distance between

the legs of the u-tube), the diameters of the u-tube and the volumetric flow

rate in each borehole. Next the ground, grout, and pipe thermal properties

are entered. Finally, the user has the option to enter a specific convection

coefficient or use the built-in calculator to determine one. After entering all the

data the user can calculate the borehole resistance and create the g-functions

for the borehole. If the borehole has been properly sized the user can create

the parameter file for the Simulink R© model by clicking on “Select G-function

Print Format” and choosing to create an HVACSIM+ parameter file.

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Figure 1.2: G-function and borehole resistance calculation screen

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Once the borehole properties have been entered into GLHEPRO, the

user can proceed to perform long term simulations or determine the proper

borehole length. This requires that the monthly building loads be entered

into the program. This is accomplished under the Loads>Edit Heat Pump

Loads menu. Once the loads are entered the user can perform a long term

simulation using the specified borehole length by selecting Action>Perform

Simulation from the menu. After a simulation the monthly average, maximum,

and minimum loop temperatures are reported in a csv file.

If the users only need to determine the proper length of the borehole,

they can choose to perform a standard sizing or a hybrid sizing. The standard

sizing is used for sizing the loop when there is not a supplemental heat rejector

available. To perform the standard sizing the user selects Action>Perform Siz-

ing from the menu. The user then selects the desired maximum and minimum

loop temperatures as well as the life time of the ground loop. Based on the

inputs GLHEPRO runs multiple simulations with different borehole lengths

until the maximum or minimum loop temperatures are reached in the last year

of the specified life time. The resulting length is then properly sized length

for the ground loop based on the provided loads.

The hybrid sizing procedure assumes that there is a supplementary

device which can add or remove heat from the ground loop. To determine

the loop length GLHEPRO determines loop lengths that will give the user-

specified maximum and minimum temperatures at the end of its lifetime. The

shorter of the two lengths is chosen and the additional heat that needs to be

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rejected/extracted from the borehole is then calculated and reported as the

proper size for the supplementary device.

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Bibliography

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