<p>Accurate prediction of heat transport in porous media is important for understanding geoscience processes and properties and to design applications, for example geothermal energy systems. While heat transport is generally modelled assuming of local thermal equilibrium (LTE), i.e., instantaneous heat transfer between the fluid and solid phases, previous studies have demonstrated presence of local thermal non-equilibrium (LTNE), i.e., delayed heat transfer, in natural porous materials. However, factors that influence the rate of heat transfer between the phases and their significance for inherently heterogeneous natural systems remain unknown and untested. We develop an open-source fully coupled, finite-element application to numerically simulate heat transfer between the fluid and solid phases. This is based on the Multiphysics Object-Oriented Simulation Environment (MOOSE) and allows massively parallel modelling of heat transport including customized transfer rates. We verify our model using an analytical solution considering LTNE and illustrate several applications. The model can be used to investigate processes that affect heat transport such as heat transfer mechanisms and their dependence on different hydrogeological conditions.</p>

Abstract. Sustainable and climate-friendly space heating and cooling is of great importance for the energy transition. Compared to conventional energy sources, Aquifer Thermal Energy Storage (ATES) systems can significantly reduce greenhouse gas emissions from space heating and cooling. Hence, the objective of this study is to quantify the technical potential of shallow low-temperature ATES systems in terms of reclaimable energy in the city of Freiburg im Breisgau, Germany. Based on 3D heat transport modeling, heating and cooling power densities are determined for various hydrogeological subsurface characteristics and ATES configurations. High groundwater flow velocities of up to 13 m d-1 cause high storage energy loss limiting power densities to a maximum of 3.2 W m-2. Nevertheless, comparison of these power densities with the existing thermal energy demands shows that ATES systems can achieve substantial heating and cooling supply rates. This is especially true for the cooling demand, for which a full supply by ATES is determined for 92 % of all residential buildings in the study area. For ATES heating alone, potential greenhouse gas emission savings of up to about 70,000 tCO2eq a-1 are calculated, which equals about 40 % of the current greenhouse gas emissions caused by space and water heating in the study areas’ residential building stock. The modeling approach proposed in this study can also be applied in other regions with similar hydrogeological conditions to obtain estimations of local ATES supply rates and support city-scale energy planning.

The numerical 3D finite element flow and heat transport subsurface model of the study area serves as a baseline to evaluate the representativeness of the box models regarding ATES power density in the city of Freiburg. The exact delineation of the study area is done in a way so that the majority of the built-up area of Freiburg is included and considers the hydraulic and topographic conditions. The model covers an area of about 72 km 2 and includes

<p>Aquifer thermal energy storage (ATES) is a promising technology for sustainable and climate-friendly space heating and cooling which can contribute to the energy transition, as it causes significantly less greenhouse gas (GHG) emissions than conventional space heating and cooling technologies. Using 3D thermo-hydraulic numerical models, this study quantifies the technical potential of shallow low-temperature ATES in the city of Freiburg, Germany. The numerical models consider various ATES configurations and different hydrogeological subsurface characteristics relevant for the study area. Based on the modeling results, spatially resolved ATES power densities for heating and cooling are determined and compared to the space heating and cooling energy demand. High ambient groundwater flow velocities of up to 13 m d<sup>-1</sup> cause relatively high storage energy losses resulting in maximum ATES power densities of 3.2 W m<sup>-2</sup>. Yet, these still reveal substantial heating and cooling energy supply rates achievable by ATES systems. While heating energy supply rates of larger than 60 % are determined for about 50 % of all residential buildings in the study area, the cooling energy demand could be supplied entirely by ATES systems for 92 % of the buildings. Also, ATES heating alone could allow for greenhouse gas emission savings of up to about 70,000 tCO<sub>2eq</sub> a<sup>‑1</sup>, equivalent to 40 % of the current greenhouse gas (GHG) emissions from space and water heating in the study areas’ residential building stock. The proposed modeling approach in this study can also be applied in other regions with similar hydrogeological conditions to obtain estimations of local ATES supply rates and support city-scale energy planning.</p>

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