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© Stuttgarter Straßenbahnen AG
Geothermics in tunnels
Projektinfo 09/2013

This 3D view shows the energy block in the Fasanenhof tunnel with absorber tubing between the inner and outer lining and the supply line on the tunnel floor.
© Universität Stuttgart
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Metro tunnels enable geothermal air-conditioning

When constructing new underground metro lines it is possible to integrate geothermal plants with few additional costs. That provides an excellent opportunity to use this renewable energy source in cities with densely built underground infrastructures. A test plant is in operation on Metro Line 6 at Stuttgart’s Fasanenhof underground station. Scientists at the University of Stuttgart are investigating the effects that the geothermal activation of the tunnel is having on the subsurface.

Thanks to their large amounts of surface area in contact with the ground, tunnel structures have a high geothermal potential. Newly built tunnels are situated close to possible consumers. Offices, homes and commercial businesses can be heated and cooled using geothermal energy. Only a few technical modifications are required to use a tunnel as a heating and cooling source. For example, preliminary forecasts indicate that the increased costs for the shell caused by the geothermal activation only amount to about two per cent. The absorber tubes can be mounted on the shotcrete used for the external tunnel lining and integrated into the in-situ concrete used for the inner tunnel lining. This process is suitable for new mined tunnels where the tunnel is bored or excavated. The alternative cut and cover method, in which a tunnel is built with rectangular sections like a cellar and then covered, was not investigated. Two tunnels in the city’s metro system were both equipped with absorber tubes along a length of 10 metres. The two tunnel sections are located in layers of sandstone and shale. They both have two absorber circuits. Each sub-circuit was installed with around 2x200 metres of absorber tubing across an area of approximately 90 m2. Connection lines link the absorber circuits with a heat pump located in the operating room at the Europaplatz station. The scientists use the heat pump to test different load profiles for the heating and cooling operation.As with a borehole heat exchanger, absorber fluid flows through the heat exchanger tubes. The water/monoethylene glycol mixture absorbs the heat stored in the ground and the tunnel air or releases it.

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Techniques for utilising shallow geothermal energy

For air-conditioning buildings, geothermal energy provides an almost inexhaustible heat source. Particularly suitable is shallow geothermal energy that uses boreholes up to 400 metres in depth. Various techniques can be used in accordance with the ground conditions and the foundations for the building. The most widespread type of system used in central and northern Europe consists of borehole heat exchangers. These are mostly used with boreholes at a depth of 50 to 100 metres. In order to meet the requirements of larger commercial premises or housing estates, several boreholes are sunk. Heat exchanger tubes are installed in the vertical boreholes, which mostly have a depth of between 50 to 160 metres. A heat transfer fluid circulates in the tubes that absorbs the heat from the ground and transports it to the surface to the heat pump. Another construction form is provided by energy piles. Here the tubing is integrated directly into the foundation piles for the building. Energy piles usually have a depth of 10 to 30 metres. Horizontal tubes can also be installed in or beneath the foundation slab. These are called ground absorbers. Another version is provided by geothermal horizontal collectors. Here the absorber tubes are laid in roughly two- to three-metre-deep trenches, for example beneath car parks or park areas. A hybrid form results from their use in so-called geothermal energy baskets. Here they also specifically utilise the thermal potential provided by rainwater. According to the Bundesverband Geothermie (GtV), around 290,000 shallow geothermal systems are currently in operation. Numerous new installations are added each year. In 2012 there were around 22,200 new systems.

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Design and monitoring
Universität Stuttgart, IGE