Borehole Heat Exchanger
Units [T] (default)
The Borehole heat exchanger (BHE) is used to simulate closed-loop geothermal installations. BHEs are represented as embedded 1D elements and linked to the FEFLOW nodes along join edges or edges in a 3D model.
The processes within the heat exchanger are calculated in an embedded finite-element model, applying a numerical or simplified analytic approach for the heat transfer within the BHE in X-Y direction, the latter assuming a local thermal equilibrium between the elements of the heat exchanger.
To define the inflow boundary of a heat exchanger, a time-constant or time-varying flow rate of the circulating refrigerant and the inflow temperature are required. While the flow rate is always directly specified, FEFLOW provides the following four options for the definition of the inflow temperature, each of them possibly also being time-varying:U
- Inlet Temperature
- Heat-input Rate
- Temperature Difference
- Power
Additionally, the properties of the refrigerant and the components of the borehole heat exchanger need to be specified in a so-called Borehole Heat Exchanger Data Set that can be shared by a number of BHEs. FEFLOW supports four different heat-exchanger geometries:
- Double U-shape
- Single U-shape
- Coaxial shape with annular inlet
- Coaxial shape with centered inlet
The borehole heat exchanger is linked to a number of FEFLOW nodes as a one-dimensional element in z direction or any other arbitrary direction (e.g. inclined BHEs). Manual input of this kind of boundary condition is based on a selection of Join Edges or Edges.
The processes within heat exchangers can be modelled via an analytical, quasi-stationary approach in which a local thermal equilibrium between all heat-exchanger components is assumed or via a numerical, fully-transient approach. The latter provides higher accuracy for short-term (time ranges of few hours) predictions especially for conditions with quickly changing inflow temperatures. For long-term predictions with less frequent and only moderately changing inflow temperatures, the quasi-stationary method provides reasonably accurate results at much lower computational cost.
Multiple borehole heat exchangers can be interconnected to represent serial or parallel connections in borehole heat exchanger arrays via the BHE Interconnection Editor.
For input and practical handling of borehole heat exchangers, please refer to the descriptions in the BHE Editor and BHE Workflow sections of this help system.
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White Paper Vol. V, Chapter 1. |
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The precise calculation of the ground
temperature at the location of the heat exchanger requires a thorough
setup of the finite-element mesh, i.e. the nodes adjacent to the
borehole heat exchanger need to be placed within a certain distance. |
