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The symbiotic use of underground infrastructure among the improvement of transport routes and the generation of geothermal power, is increasingly gaining momentum. Geothermal systems can be divided into open and closed systems, which differ from each other by the circumstance that closed systems have a closed circuit and open systems do not. For the application of geothermally closed tunnel systems, elegant solutions have been developed, while geothermally open tunnel systems are still dictated by the characteristics of the water entering the tunnel. In order to address this issue, concepts for tunnels under design as well as existing tunnels, were developed. The concepts allow the temperature of the inflowing water to be risen, and thereby increasing the geothermal power. The presented approach is based on the channeling of cold tunnel water inflows through absorber sections in areas of high overburden to supply them with additional energy before diverting them back into the drainage system. In order to obtain indications on the effectiveness as well as on the sensitivity of the components and influencing parameters, numerical models were created and subsequently compared, since no empirical data are available. The results show that it is possible to enhance the geothermal performance, but with decreasing efficiency over time.
Geisler et al. (Tue,) studied this question.