A new method for modelling geothermal heat exchangers in shallow aquifer systems

A new finite-element algorithm has been implemented in the commercial simulation code FEFLOW®. Borehole Heat Exchanger (BHE) systems are modelled by a set of one-dimensional finite-element representations. BHE form a closed loop system. Therefore, the thermal interaction between the heat pipes to the grout of the borehole and to the surrounding aquifer can be computed by using thermal resistances. The method, which has been published before, is further modified and generalized. Instead of using an iterative/sequential approach we developed a fast non-sequential (non-iterative) method based on a substructure finite-element method, which has been shown superior and robust. It allows the computation of single BHE or arrays of BHE of different sizes and types (single U-shape, double U-shape, coaxial) in a highly efficient and accurate manner. Different model validations are presented showing a good agreement.