Co-simulation of district heating systems and borehole heat exchanger arrays using 3D finite element method subsurface models

Integration of borehole heat exchangers (BHE) into district heating (DH) should be supported by numerical simulations to ensure efficient operation. Co-simulation allows for the use of dedicated software for above and below ground sub-models, facilitating the use of detailed 3D geological models. This paper presents a methodology for coupling DH models in Modelica to 3D FEM subsurface models. An interface which implements BHE models in Modelica and one with BHE models in the FEM model are compared to a benchmark model. Furthermore, an adaptive control of the communication steps reduces communication error and computational times simultaneously. A fictional solar DH system with underground thermal energy storage is co-simulated to demonstrate potential advantages of the proposed method. Overall, co-simulation of DH systems and BHE arrays facilitates accurate performance assessment of systems for which this would not be possible otherwise, but should be applied carefully, due to the increased computational effort.