To achieve climate targets by 2045, Germany plans to shift traffic to rail, requiring the expansion and construction of new railway lines—particularly through tunnels in densely populated areas. At the same time, the heat transition remains a challenge: in 2022, the share of renewable energy in heat supply was only 17.4 (Agentur für erneuerbare Energien ) Heat pumps using geothermal energy are considered a key technology in this regard. Tunnel structures, with their earth-contact surfaces, offer a largely untapped potential for geothermal energy generation.

In the project carried out, this potential was examined using the tunnel projects “Verbindungsbahn Relief Tunnel” in Hamburg and “Pfaffensteig Tunnel” near Stuttgart as examples. After a literature review of existing applications of geothermal energy in tunnels (WP1), planning documents and geological data for the tunnels under consideration were analysed (WP2). This was followed by semi-analytical (WP3) and numerical calculations (WP4) to estimate the geothermal potential. Finally, a feasibility study (WP5) evaluated technical, legal, and economic aspects.

Results:

For the Pfaffensteig Tunnel near Stuttgart, the semi-analytical calculation showed a very high potential for heat extraction in winter and an equally high potential for heat injection in summer. The technology appears economically feasible, provided that adequate space for PE collector pipes in the backfill concrete is planned. Detailed results of the calculations cannot be shared due to ongoing procedures.

For the Verbindungsbahn Relief Tunnel in Hamburg, a somewhat lower potential was identified: according to the semi-analytical calculation, 6,360 kW (0.50 kW/m) can be extracted in winter, and 9,759 kW (0.76 kW/m) can be injected in summer. The numerical analysis yielded 4,961 kW (0.39 kW/m) for 30 days of full-load operation, and 4,516 kW (0.35 kW/m) after 90 days. Technically, implementation is also possible here; however, costs increase significantly if the pipes must be mounted on the tunnel wall and made of stainless steel.

Overall, both projects show high potential, which should, however, be integrated into tunnel planning at an early stage. The geothermal use of tunnel walls is technically feasible and can contribute to reducing CO₂ emissions. It represents an innovative addition to the combined efforts of the transport and heat transitions.

• You can find the link to the 8th WPGI Conference here.
• Prof. Zachert's presentation at the 8th WPGI 2024 in Lodz can be viewed here (opens in new tab) .