Medium-Deep Borehole Thermal Energy Storage (MD-BTES) systems are a promising technology for sustainable and efficient seasonal thermal energy storage and district heating distribution. These innovative systems are designed to store excess thermal energy e.g. generated from renewable sources in the subsurface using borehole heat exchangers (BHE) and release it when needed for heating or cooling purposes. MD-BTES systems can play a crucial role in the transition towards a more sustainable energy supply, with their development encompassing various stages, from exploration to connection and implementation in district-heating grids. This contribution presents insights gained from two projects in this field, namely the SKEWS (funded by the federal government of Germany; ID: 03EE4030A) and PUSH-IT (Horizon Europe Grant agreement, ID: 101096566) projects, to highlight their contributions to advancing MD-BTES technology implementation. The exploration phase of MD-BTES involves identifying suitable geological formations for energy storage through boreholes. SKEWS, an acronym for "Saisonaler Kristalliner Erdwärmesondenspeicher" or Seasonal Crystalline Borehole Heat Storage, plays a major role in this phase. This project primarily focuses on implementing a real-scale demonstrator site with four borehole heat exchangers. The first steps contained geophysical surveying, geological mapping and analysis, aiming to identify the best site choice with the most affordable reservoir conditions for medium-deep boreholes. By employing advanced geophysical techniques, the SKEWS project identified areas with the necessary geological attributes, such as thermal conductivity and adequate permeability, for efficient energy storage and retrieval. Additionally, SKEWS produced datasets to assess the feasibility and environmental impact of drilling and installing borehole systems in urban and peri-urban areas. Ongoing, the boreholes on site have been completed with a coaxial BHE design. The SKEWS task contains an experimental storage and extraction program, ending in 2026. This approach made SKEWS an ideal BTES demonstration site within the PUSH-IT consortium. The PUSH-IT project, which stands for "Piloting Underground Storage of Heat In GeoThermal Reservoirs", takes the role of a leading research site in the development phase and addresses the thematic aspects of numerical modelling and commissioning for the integration of storage systems with existing district-heating grids, particularly at the Darmstadt site. The connection of MD-BTES with district-heating grids represents the final step in investigating the potential of MD-BTES for urban energy systems. To illustrate this, an exemplary connection scenario and a detailed explanation of the co-simulation, control, and subsurface processes modelling strategy for technological development and deployment at the TU Darmstadt campus scale will be provided. The insights and perspectives gained from these two projects are invaluable for overcoming technical, economic and regulatory challenges associated with large-scale deployment, ultimately leading to reduced greenhouse gas emissions and promoting sustainable urban energy systems.

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