Evaluation of a unidirectional ATES for thermal energy supply of the State Hospital Graz South, Austria

<p>Large-scale thermal use of shallow groundwater is often constrained in cities because temperature plumes can extend far beyond project boundaries and affect third-party water rights. Unidirectional Aquifer Thermal Energy Storage (UD-ATES) addresses this by reversing the conventional open-loop arrangement. The injection well is placed up-gradient and the production well down-gradient. During summer cooling, warmed return water is injected up-gradient; the resulting warm plume is carried by the natural groundwater flow to the down-gradient well and can be recovered in the following heating season. Conversely, during the heating season, cooled water is injected up-gradient; the resulting cold plume drifts down-gradient and can be recaptured for cooling in the next summer. This configuration is particularly suited to shallow, highly permeable aquifers with pronounced natural gradients, settings in which classical ATES suffers from advective losses, while also minimizing off-site thermal impacts that complicate permitting.</p> <p>At the State Hospital Graz South site (Austria), we surveyed and characterized the aquifer and built a coupled groundwater-flow and heat-transport model to design a UD-ATES well pair tailored to local conditions. The optimized spacing between injection and production wells is <span class="inline-formula">∼463</span> m, aligning transport time with the seasonal load profile with a peak thermal power of 1.25 MW (60 L s<span class="inline-formula">⁻¹</span> by a <span class="inline-formula">Δ<i>T</i></span> of 5 K). Resulting temperature anomalies remain largely confined to the property, with the thermal signal decaying to below 1 K within a few hundred metres downstream. Despite an unavoidable imbalance between heating and cooling demand over the year, the system recovers a substantial fraction of the injected energy and markedly reduces the thermal footprint compared with a conventional open loop scheme. The thermal recovery factor amounts to 0.38. An expansion of the plant to a total peak thermal power of <span class="inline-formula">&gt;3.5</span> MW using three pairs of wells appears to be feasible at the location in question. These findings support UD-ATES as a practical pathway to decarbonize large, space-constrained consumers in high-flow aquifers while safeguarding neighbouring groundwater uses.</p>