18 May 2017
Few technical modifications are required to use a tunnel as a geothermal source or sink. This idea is realized with two geothermal test sections inside the Fasanenhof subway tunnel in Stuttgart, Germany. The sections are equipped with absorber pipes along a length of 10 meters each. The tunnel is equipped with measuring devices to determine the temperature distribution in the ground, the tunnel lining and the tunnel air.
The first stationary measurements with fixed inlet temperatures for the tunnel absorbers showed variations between 5 W/m² and 37 W/m² in cooling effect per absorber heat exchange area. The highest power output was obtained at the beginning of the phase. The thermal output decreased with time, partly due to the temperature increase of the absorber section surroundings. An influence of the tunnel air temperature could also be seen.
In order to see how the system behaves under transient conditions an emulation is performed with a coupling between the geothermal tunnel system and the simulation environment TRNSYS. A fictive office building is modeled and the annual heating and cooling load are calculated. A cooling phase is performed with the cooling load as input for the tunnel system. Transient results once again show an influence of the tunnel air temperature. The thermal output is however also influenced by the cooling load. 77% of the supplied cooling energy could in the end be obtained through direct cooling. This shows a potential for the method to further establish itself in the energy sector.