To achieve climate targets by 2045, Germany plans to shift more traffic to rail, requiring the expansion and construction of railway lines—particularly through tunnels in densely populated areas. At the same time, the heat energy transition remains a challenge: while the share of renewable energies in electricity generation has now grown to 57% (https://www.destatis.de ), the share of renewables in heating and cooling supply in 2024 was only 18.1% (Agentur für erneuerbare Energien ). Heat pumps that use geothermal energy are considered a key technology in this area. Tunnel structures, with their earth-contact surfaces, offer a yet little-used potential for geothermal energy extraction.

The project examined how existing railway tunnels can be geothermally activated, particularly in the context of upcoming repairs and renewals. Since a significant portion of Germany’s tunnel stock has already exceeded a service life of 100 years, forecasts suggest that in the next 20 years, extensive renewal work will be required on numerous tunnels, with a total length of around 33 km (https://www.stuva.de ). These construction measures provide the opportunity to integrate thermal activation early on and thus contribute to climate-friendly heat supply.

In the first work package, a systematic analysis of the railway tunnel stock was carried out. Construction methods, materials, age, and geological conditions were recorded and used for classification. Building on this, the second work package developed technical solutions for thermal activation, tailored to different tunnel types and construction scenarios. The third work package involved an assessment of thermal potential using numerical simulations. The thermal potential for various tunnel classes was determined and extrapolated to the entire Deutsche Bahn network.

Results:

The extensive data analysis of Germany’s tunnel stock revealed that railway tunnel construction is strongly shaped by historical building methods. Many tunnels were built in the 19th century, which is reflected in their materials, structural condition, and geometric dimensions. For many of these tunnels, repair and renewal work is necessary under challenging operational conditions, for example, widening cross-sections to adapt the clearance profile. Considering typical repair and renewal activities, technical methods for retrofitting thermal activation were developed. These include:

• Precast concrete segments with integrated absorbers
• Tunnel floors equipped with absorbers
• Energy mats in front of or behind the tunnel lining
• Prefabricated elements with absorbers
• Radial geothermal probes

These solutions can largely be integrated into standard construction processes for tunnel repair and renewal and can be flexibly adapted.

In addition, considering the collected data on tunnel geometry and geological conditions, the thermal potential of 103 particularly renewal-intensive tunnels was calculated. A numerical sensitivity analysis determined the amounts of energy obtainable with different activation techniques. Various scenarios for hydrogeological conditions were also considered (tunnel above groundwater level, tunnel below groundwater without flow, and tunnel below groundwater with flow) to realistically represent the range of available thermal energy. Depending on the method, up to 14.5 MW of heating capacity in winter and up to 35.3 MW of cooling capacity in summer can be generated (at 30 days of full load per heating or cooling cycle).

The theoretical total potential of the tunnel network is therefore high, although assumptions were made conservatively due to heterogeneous geological and hydrogeological conditions. Using two example tunnels, the practical approach for selecting and calculating possible activation concepts was demonstrated.