Construction of a medium-deep geothermal storage system: Case study of the SKEWS MD-BTES demosite

• Construction of the first MD-BTES Demonstrator on the Lichtwiese campus of TU Darmstadt in 2022–2023. • Encountered unforeseen (hydro-)geological conditions, including fault zones and extensively altered crystalline rocks. • Pneumatic and hydraulic hammer achieved 2 to 5 times higher rates of penetration compared to rotary drilling. • Monitoring of energy consumption and groundwater, revealing only temporary environmental impacts. • Comprehensive empirical assessment of MD-BTES construction under practical field conditions to provide guidance for future projects. The global energy transition toward renewable resources poses particular challenges in the heating sector, where a seasonal mismatch between heat demand and supply remains a critical obstacle. Medium-deep borehole thermal energy storage systems (MD-BTES), installed at depths of 400–1000 m, offer large subsurface storage capacities while avoiding the high costs associated with deep geothermal drilling. To date, the benefits of MD-BTES have been demonstrated primarily through modelling studies, with limited empirical validation. Here, we report the construction and commissioning of the first MD-BTES demonstration site at the Lichtwiese Campus in Darmstadt, Germany. In 2022–2023, three 750 m deep borehole heat exchangers (BHE) were installed in a triangular layout with 8.6 m spacing. When expanded to 37 BHE, with inlet temperatures of 90 °C (summer) and 30 °C (winter), output up to 15 GWh·a -1 and 3.5 MW is achievable with a recovery efficiency up to 75 % after 5 years of operation. During drilling, unforeseen (hydro-)geological conditions, including fault zones and extensively altered crystalline rocks, required a transition from pneumatic and hydraulic down-the-hole hammer drilling to rotary drilling with clay-polymer fluids. Comparative analysis showed that the pneumatic and hydraulic hammer techniques achieved 2 to 5 times higher rates of penetration relative to rotary drilling. Continuous groundwater monitoring revealed a temporary ecological impact from drilling fluids and intermediate cementations, which dissipated after completion. The drilling operations consumed ∼90,950 L of diesel fuel, corresponding to ∼244 t CO₂ emissions. These results provide, for the first time, a comprehensive empirical assessment of MD-BTES construction under practical field conditions, enable extended test operations on storage efficiency, and highlighting the need for economically viable vertical and fast drilling technologies for large-scale MD-BTES development.