Dynamic energy simulation is important for the design and sizing of district heating and cooling systems with geothermal energy storage in borefields. Current modeling approaches in building and district energy simulation tools typically consider heat conduction through the ground between boreholes, without flow of groundwater. On the other hand, detailed simulation tools for subsurface heat and mass transfer exist, but these fall short in simulating building and district energy systems. To support the design and operation of such systems, this study presents a model coupling between a software package for building and district energy simulation, and software for detailed heat and mass transfer in the ground. For the first, we use the free open-source Modelica Buildings Library, which includes dynamic simulation models for building and district energy and control systems. For the heat and mass transfer in the soil, we use the TOUGH simulators, which were initially designed for geothermal reservoir engineering and also have been used extensively for aquifer and borehole thermal energy storage. The current version of TOUGH can model heat and multi-phase, multi-component mass transport for a variety of fluid systems, as well as chemical reactions, in fractured porous media. This paper describes the design of the coupling of these software packages, and how the time-dependent boundary conditions for the borehole walls are synchronized for use in Modelica and TOUGH. It also presents the validation of the coupled modeling and closes with the discussion of further model improvements.

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