BHELoop

Functionality

The BHELoop module is designed for applying a time-varying temperature differential between the outlet and inlet of a Borehole Heat Exchanger (BHE).

The differential can be defined either by a

• temperature differential [K]

or by a

• power [1e+6 J/d] (see note below)

It is capable of handling more than one BHE simultaneously, using separate temperature differential/power functions for each BHE.

Data preparation

Before proceeding with the following steps, create one or more BHE in the common way. Use an arbitrary constant value as heat input rate. The module does not work correctly if using a time series.

Temperature differential

1. The temperature differential functions must be defined as time-varying power functions in FEFLOW. The function must represent the temperature differential (unit is Kelvin), positive values denoting heat injection.
2. A nodal reference distribution with the name 'BHE-dT' has to be created to identify participating BHEs.
3. For a particular BHE a value must be assigned in the reference distribution 'BHE-dT' on slice 1 only. This value must be equal to the ID of the time-varing power function that represents the temperature differential.
4. All other nodes should have values less than or equal to 0.

NOTE: Be careful with mesh refinement: In case of refining the mesh after assigning the power function number, the data interpolation will lead to a number of nodes with non-zero values around the BHEs, too.

Power

The procedure is almost identical. The unit of the power function is 1e+6 J/d and the name of the nodal reference distribution to be created is 'BHE-dP'.

ATTENTION! When using the power to prescribe the temperature, the BHELoop module assumes the default value of 4e+6 J/(m³K) for the volumetric heat capacity of the refrigerant. If using a different value in your FEFLOW model, use the temperature differential instead or change the values of the power function by a correction factor.

Simulation run

Before each time step, the outlet temperature of each BHE is read.

The temperature differential derived from the time-varying power function associated to the BHE is added to the outlet temperature. The result is used to set an inlet temperature of the BHE. The inlet temperature is updated in this way before each time step of the simulation.

If a power is used, the equivalent temperature differential is calculated first and used instead.

A time-stepping control scheme ensures that all time steps in the temperature differential functions are met exactly if the model is run based on automatic time stepping.