Aquifer Thermal Energy Storages (ATES) as long-term storages have a strong potential to address the seasonal discrepancy of supply and demand of thermal energy. The operation of high-temperature ATES (HT-ATES) and their integration into district heating systems are the subject of current research projects. Buoyancy plays an important role in determining how HT-ATES performs. The target of this paper is to present a system model in Modelica that takes these buoyancy effects into account. The validation with experimental data and numerical simulations shows that the system model represents the buoyancy effects well. A sensitivity analysis underlines the importance of optimizing the grid structure and shows that a high resolution in the aquifer is necessary, especially in the vertical direction. Finally, a 10-year simulation shows the deviation of the heat recovery factor, i.e. the ratio of the amount of heat extracted to the amount of heat injected, between a model with and without buoyancy effects.