A numerical model was built using FEFLOW® to simulate groundwater flow and heat transport in a confined aquifer in Brussels where two Aquifer Thermal Energy Storage (ATES) systems were installed. These systems are operating in adjacent buildings and exploit the same aquifer made up of mixed sandy and silty sublayers. The model was calibrated for groundwater flow and partially for heat transport. Several scenarios were considered to determine if the two ATES systems were interfering. The results showed that a significant imbalance between the injection of warm and cold water in the first installed ATES system led to the occurrence of a heat plume spreading more and more over the years. This plume eventually reached the cold wells of the same installation. The temperature, therefore, increased in warm and cold wells and the efficiency of the building’s cooling system decreased. When the second ATES system began to be operational, the simulated results showed that, even if the heat plumes of the two systems had come into contact, the influence of the second system on the first one was negligible during the first two years of joint operation. For a longer modeled period, simulated results pointed out that the joint operation of the two ATES systems was not adapted to balance, in the long term, the quantity of warm and cold water injected in the aquifer. The groundwater temperature would rise inexorably in the warm and cold wells of both systems. The heat plumes would spread more and more over the years at the expense of the efficiency of both systems, especially concerning building’s cooling with stored cold groundwater.

Shallow open-loop geothermal systems function by creating heat and cold reserves in an aquifer, via doublets of pumping and reinjection wells. Three adjacent buildings in the center of Brussels have adopted this type of aquifer thermal energy storage (ATES) system. Two of them exploit the same aquifer consisting of Cenozoic sands, and started operation in 2014 and 2017, respectively. A previous hydrogeological model developed by Bulté et al. (2021) has shown how the thermal imbalance of one of the systems jeopardizes the thermal state of this upper aquifer. Here, the interactions with a more recent third ATES system located in the deep aquifer of the Palaeozoic bedrock are studied and modelled. After being calibrated on groundwater flow conditions in both aquifers, a 3D hydrogeological model was used to simulate the cumulative effect of the three geothermal installations in the two exploited aquifers. The results of the simulations showed that although the hydraulic interactions between the two aquifers are very weak (as shown by the different observed potentiometric heads), heat exchanges occur between the two aquifers through the aquitard. Fortunately, these heat exchanges are not sufficient to have a significant impact on the efficiency of the individual geothermal systems. Additionally, this study shows clearly that adding a third system in the lower aquifer with a mean power of 286 kW for heating between October and March and an equivalent mean cooling power between April and September is efficient.

<p>Shallow open-loop Aquifer Thermal Energy Storage (ATES) systems have been adopted by three large adjacent buildings in the centre of Brussels. The doublets of pumping and reinjection wells of two administrative buildings are located in a shallow aquifer made of Cenozoic mixed sandy and silty sublayers and operations started in 2014 and 2017. A third ATES system located in the underlying deep aquifer made of Palaeozoic fractured phyllites and quartzites, was started recently (2020) to provide the needed heating and cooling power to a large multi-service building. Groundwater levels variations in these two aquifer systems are different and pumping tests performed in the upper aquifer system have shown no impact on the groundwater levels in the Palaeozoic bedrock aquifer. After being calibrated on groundwater flow conditions in both aquifers, a 3D hydrogeological model using Feflow© was developed to simulate the cumulative effect of the three geothermal installations in the two exploited aquifers.</p> <p>In terms of heat interactions, a previous model has shown how the thermal imbalance of the ATES system started in 2014 was jeopardising the thermal state of the upper aquifer (Bulté <em>et al.</em> 2021). Here, interactions with the third ATES system located in the deep aquifer are studied and modelled with different operational scenarios. Even though hydraulic interactions between the two aquifers are very limited, heat exchanges occur between the two aquifers, through an aquitard formed by low permeability Cretaceous base deposits and the weathered top of the bedrock.</p> <p>The simulation results show that despite the unbalanced ATES system affecting mainly the shallow groundwater conditions, an adjacent but deeper ATES system can operate without significant interactions. Acquisition of additional measured data (i.e., piezometric heads, groundwater temperatures, detailed pumping, injection flow rate, etc.) will be crucial to improve the reliability of the simulated results for different operational scenarios. This will be particularly useful for the future management of the three ATES systems in order to avoid losses in both efficiency and durability.</p> <p>This work was mainly conducted in the frame of the Master thesis of Caroline De Paoli. This was done with the partial support of the MUSE project—Managing Urban Shallow geothermal Energy. MUSE has received funding from the European Union’s Horizon 2020 research and innovation program under grant agreement No 731166 under the terms of GeoERA program—ERA-NET Cofund Action.</p> <p>Bulté, M.; Duren, T.; Bouhon, O.; Petitclerc, E.; Agniel, M.; Dassargues, A. Numerical modeling of the interference of thermally unbalanced Aquifer Thermal Energy Storage systems in Brussels (Belgium). <em>Energies</em> <strong>2021</strong>, 14, 6241. Special Issue on Geothermal Systems, https://doi.org/10.3390/en14196241</p>