Borehole-Heat Exchanger Workflow

Borehole heat exchangers (BHE) can be edited and assigned manually via the Borehole Heat Exchanger Editor or they can be imported from maps using FEFLOW's Parameter Association dialog.

The general workflow consists in the following steps:

1. Each borehole heat exchanger uses a dataset in which geometry and computation method, the heat-transfer properties of the heat exchanger and the properties of the refrigerant are defined. Before borehole heat exchangers can be assigned, the BHE datasets have to be created and edited, using the Borehole Heat Exchanger Dataset Editor.

2. Depending on the assignment method, a join-edge selection has to be created, using the tools provided in the Selection toolbar.

3. The borehole heat exchangers are either assigned manually with the Editor toolbar being in Borehole Heat Exchanger input mode or via a regionalization using input maps. In case of manual assignment, vertical extent of the heat exchanger, the flow rate of the refrigerant within the borehole pipes, the temperature at inflow and the BHE dataset to be used need to be defined in the Borehole Heat Exchanger Editor.

4. Optionally: Borehole heat exchangers can be interconnected in serial or parallel, using the BHE Interconnection Editor.

1. Creating Borehole-Heat Exchanger (BHE) Datasets

In a BHE dataset the geometric characteristics of a borehole heat exchanger, the computation method and the properties of the refrigerant and exchanger components are defined. Borehole heat exchanger datasets are created in the following way:

1. Set the Slice view active and double click on Borehole Heat Exchanger in Boundary Conditions (BC) > Heat transport in the Data panel.

2. The Borehole Heat Exchanger Dataset Editor in which data sets are defined and edited can be accessed in two different ways:

   1. Click on Edit BHE Datasets in the Borehole Heat Exchanger toolbar.

   2. Double click in the input box of the Editor toolbar to enter the Borehole Heat Exchanger Editor and click on Edit borehole heat exchanger datasets in the BHE dataset section of the editor.

3. In the Borehole Heat Exchanger Dataset Editor create a new dataset by clicking on Add new borehole heat exchanger dataset.

4. Define the BHE geometry, the computational-method settings and the properties of the borehole heat exchanger components and the refrigerant. If borehole heat exchangers are to be imported from maps, it is mandatory to define a unique name for each dataset.

5. Confirm the settings with Apply and leave the dialog with OK.

2. Creating a Join-Edge Selection

Depending on the assignment method, a join-edge selection has to be created, using the tools provided in the Selection toolbar. In case of BHE assignment via maps, the selection can also be created after the map data has been linked to the FEFLOW parameters.

3a. Manual BHE Assignment and Editing

To edit and assign a borehole heat exchanger manually, the following steps are required:

1. In the Data panel, activate Boundary Conditions (BC) > Heat transport > Borehole Heat Exchanger with a double click.

2. Double-click into the input box of the Editor toolbar that is now in Borehole Heat Exchanger assignment mode to open the Borehole Heat Exchanger Editor. In case that the Borehole Heat Exchanger Dataset Editor was accessed via the Borehole Heat Exchanger Editor, the user automatically returns to the Borehole Heat Exchanger Editor after the definition of BHE datasets.

3. Select one of the available methods to prescribe the inflow temperature, define the Flow Rate and how to determine the extent of the heat exchanger in z-direction and also the BHE dataset to be used.

4. The borehole-heat exchanger assignment is executed with a click on Assign and the dialog is closed with Close.

3b. BHE Assignment via Maps

Maps that are to be used for borehole-heat exchanger import need to contain attributes for the definition of the inflow temperature, for the flow rate, for the top and bottom elevation of the borehole and for the name of the borehole-heat exchanger dataset. The borehole heat exchanger assignment via maps can only be executed if the BHE datasets to be used already exist and if their names are unique.

Borehole-heat exchanger assignment via maps is executed in the following way:

1. Load the map that contains the attribute data for the borehole-heat exchanger assignment via the Maps panel.

2. Select Link to Parameter from the context menu of the map in the Maps panel to open the Parameter Association dialog.

3. Select the attribute column containing the value for the definition of the inflow temperature in the left-hand box and select the corresponding field for Heat Input in the Borehole Heat Exchanger section on the right-hand side. To create the link, click on Add link or double-click on the selected FEFLOW parameter. If required change the Source data unit.

4. In the same way, link the attribute fields for flow rate, borehole top and bottom and name of the BHE dataset to the FEFLOW fields Flow rate, Top elevation (or Depth to Top), Bottom elevation (or Depth to Bottom) and to BHE Dataset.

5. With these five links being defined, borehole heat exchangers can be assigned, given that the specified BHE dataset already exists. Optionally, the borehole heat exchanger name can be imported. An example for the link definition is shown below.

6. Adjust the Snap Distance in one of the links and leave the dialog with OK. The link definition will be shown in the Linked attributes section in the tree view of the Maps panel.

7. To activate the map and the boundary condition for assignment, double click on the link.

8. If no join-edge selection is active yet, use the tools provided in the Selection toolbar to create the selection.

9. Click on the green checkmark in the Editor toolbar to assign the borehole heat exchanger (s).

10. If the name (s) of the BHE dataset (s) to be used does not exist or if a specified dataset name is not unique, FEFLOW does not assign the borehole heat exchanger but displays one of the following error messages:

  

11. In case 1, the BHE dataset has to be created first, using the Borehole Heat Exchanger Dataset Editor as described above before the assignment can be completed. In case 2, the names of the existing datasets have to be changed in order to ensure a unique name for each dataset.

4. Borehole Heat Exchanger Arrays

If a model contains at least two borehole heat exchangers, they can be connected to a borehole heat exchanger array. To define the connections between feed and return pipes of borehole heat exchangers the BHE Interconnection Editor is used.

The workflow to create a BHE array consists in the following steps:

1. Set the Slice view active and double click on Borehole Heat Exchanger in Boundary Conditions (BC) > Heat transport in the Data panel.

2. Click on Link Borehole Heat Exchangers to access the BHE Interconnection Editor.

3. By default, the editor contains one BHE Array. Define the total Flow Rate for the BHE array, the method to prescribe the inflow temperature at the connection point and also the Input Value. If borehole heat exchangers are connected to an array, the respective flow rates and heat-input settings that were defined for the separate borehole heat exchangers are ignored.

   

4. Create a connection between different borehole heat exchangers and the array's in- and outlet using the context menu of an entry in the Borehole Heat Exchanger Array section at the bottom left of the dialog and selecting Link to BHE or Link to Connection Point. The sketch on the right-hand side of the editor displays the interconnections for the currently selected array.

5. A borehole heat exchanger can only be part of one but not of multiple arrays and the in- and outflow balance of an array have to match. To check whether an array contains logical errors, click on Validate. In case that the flow balance matches and the array does not contain any logical errors, FEFLOW displays the following message:

6. In this case, close the message with OK and set the array with a click on Apply. The array and BHE interconnections are then displayed in the active view.

   

7. In case that the array definition contains logical errors and the in- and outflow balance do not match, clicking on Validate results in the following error message:

8. In this case the BHE-array definition has to be revised before the array is assigned.

9. To delete a link, select it with a click on the black line, open its context menu with a right click and select Delete Selected Links.