As part of the SKEWS research project (Saisonaler Kristalliner ErdWärmeSondenspeicher, BMWK funding code 03EE4030A), the world's first medium-deep geothermal energy storage was constructed on the Lichtwiese campus in Darmstadt. The storage consists of three 750 m deep coaxial geothermal BHE with a spacing of 8.6 m in a triangular arrangement. The storage will undergo a 1.5-year test cycle to simulate a seasonal heat supply via mobile heating and mobile cooling units. The project’s goal is to demonstrate the construction and functionality of such systems and to verify the operating and design routines. An expansion of the system for economical operation is already in the planning. A continuous thermal and hydrochemical monitoring will be carried out via three nearby wells during the construction and operating phase to quantify the systems impact on the near surface aquifer. The operating and monitoring results will be used to improve and validate existing numeric simulation tools.