Tunnel geothermics-A German experience with renewable energy concepts in tunnel projects

Presently near surface geothermal energy already covers an essential part of the German base-load capacity in the heat sector. Thermal activation of tunnels represents an interesting alternative and could be an economically useful solution which is complementary to the presently applied hybrid systems in ground engineering. The utilization of heat fluxes from the subsoil and from the tunnels themselves is an essential difference between tunnel absorbers and activated piles as well as diaphragm walls; therefore the utilization of tunnel absorbers can be regarded as a duo-hybrid-system. Two tunnel geothermal plants - Stuttgart-Fasanenhof and Jenbach - have provided reliable data on the subsoil, tunnel lining and tunnel air temperatures for the last four years. This data gives the basis for the research activities of the University of Stuttgart. This article concentrates on the geothermal test plant Stuttgart-Fasanenhof; describes the general setup of the subsoil absorber equipment, the geological and hydrogeological conditions and the operation periods. Furthermore, measurement results as well as possible energy extraction rates will be discussed. The tunnel air temperature is essential for the heat flux density towards the tunnel. An approximate formula to estimate this temperature is described in this article.