In order to support the development of shallow geothermal energy and geothermal energy storage, accurate modelling of ground heat transfer is key. This is particularly true for borehole heat transfer, where the dynamical response of the borehole can greatly affect the overall performance of the surface energy system. Project ANR Sunstone, funded by the French National Agency for Research, aim to optimise the integration of solar heat production and geothermal energy storage, with a practical application on the district heating network of Chateaubriant, Brittany. In this context, the French national survey, BRGM, developed a new borefield Dymola model to contribute to the accuracy of borehole heat transfer modelling. The new borefield model allows considering groundwater flow in the borefield, and heat loss at the surface. It simplifies the declaration of borefield configuration, and allows choosing between rectangular shape on a square base, and hexagonal shape on a hexagonal base. The dynamic heat transfer occurring in a borefield is a complex dynamic process, with time scales involved varying by several order of magnitudes, from the minutes to several decades. The first section of this paper reports the development of new g-functions describing the heat transfer in the borehole, while the second section is focused on its practical implementation in Modelica.