The first unidirectional ATES in Austria – from theory to practical implementation

The large-scale thermal use of near-surface groundwater is often limited extent due to the far-reaching thermal impacts and the large number of potentially affected third-party rights. Silvestri et al. (2025) first described the principle of unidirectional ATES. Unlike conventional thermal groundwater usage, where the production well is positioned upstream in the groundwater flow and the injection well downstream, unidirectional ATES works in reverse (inversion of a classic open-loop system). This ensures that, with a balanced heating and cooling ratio and a well spacing adjusted to the local groundwater flow velocity, the heat plume generated in summer by cooling operation reaches the extraction well a season later. In winter, the increased groundwater temperature is used for heating purposes, increasing the efficiency of the heat pump system. The same effect vice versa applies to the cooling season.

Equally important is the significant reduction in thermal anomalies caused by this principle. The plume of cooled or heated water is recaptured by the production well located downstream, leading to lower thermal impact on groundwater. This makes large scale projects only feasible, as they clearly minimize thermal interference with third-party interests.

In this specific case, the Steiermärkische Krankenanstaltengesellschaft (KAGes) plans to cover a large portion of the thermal power for LKH Graz Süd hospital (currently partly derived from fossil fuels) using shallow geothermal energy. Due to the excellent hydraulic properties of the aquifer in the Graz area, the required thermal output of about 3.5 MW could be achieved with three pairs of wells. In a conventional system of this size, the thermally influenced front would extend over 3 km of length, making approval in the urban area of Graz impossible.

This work presents an innovative unidirectional ATES system adapted to the local groundwater conditions (hydraulic conductivity, aquifer thickness, groundwater gradient). The project site was hydrogeologically surveyed, characterized, and compared with publicly available data. After developing the hydrogeological conceptual model, a coupled flow and heat transport model was established to simulate the system's operation. First, the optimal distance between the extraction and injection wells was determined based on local groundwater conditions, followed by a sensitivity analysis investigating the system's efficiency in terms of heat recovery depending on the flow velocity and extraction rate.

The transition from the theoretical approach of Silvestri et al. (2025) to practical implementation presents several challenges. Due to the climatic conditions, the outdoor air temperature in Graz does not follow a cosine pattern as described by Silvestri et al. (2025). Therefore, a fully balanced heating and cooling ratio cannot be achieved, limiting the system's functionality. Additionally, the thermal anomaly shifts due to inhomogeneities in the aquifer's geometry and hydraulic properties. Nonetheless, the results of this study showed that the new unidirectional ATES approach can not only significantly reduce thermal impacts, even with an unbalanced heating and cooling ratio, but also increase the system's heat recovery efficiency.

Silvestri, V., Crosta, G., Previati, A., Frattini, P., & Bloemendal, M. (2025). Uni-directional ATES in high groundwater flow aquifers. Geothermics, 125, Article 103152. https://doi.org/10.1016/j.geothermics.2024.103152