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= Energy tunnel = --this page is under construction--

An energy tunnel is a tunnel specially equipped to exchange heat with the ground. It takes advantage of the nearly constant temperature of the ground up to approximately 50 m of depth as in standard low enthalpy geothermal applications such as vertical or horizontal borehole heat exchangers (BHE). The tunnel lining can be turned into a ground heat exchanger by embedding a circuit of exchanger pipes into the concrete members to form a closed loop and circulating a heat carrier fluid along it. This circuit is called the primary circuit and provides heat to a secondary circuit, that of the user. The connection among them can occur directly, as in the case of free heating and free cooling, or through a heat pump, allowing to vary the temperature to the necessary one. An energy tunnel can be used for heating and cooling of adjacent buildings and infrastructures.

Technology
When mechanized tunneling is used, the tunnel lining segments are precast in factory and then placed onsite by the TBM. They can be therefore prepared and optimized for heat exchange by including hydraulic circuits in the cast concrete. The circuit of each segment is linked to those of the adjacent ones by hydraulic connections to form lining ring circuits. Each ring is usually made of 6–7 segments. Two or more rings can be hydraulically connected in parallel forming a subcircuit. Each circuit made of two or more rings is then connected to the main conduit that directs the heat carrier fluid from them to the heat pump and vice versa. This is done in order to reduce the number of connections on the main conduit and the consequent significant head losses. The pipes for these applications are fabricated from reticulated polyethylene (Pe-Xa) and are composed of three strata: the inner stratum with high-density polyethylene, the intermediate stratum in polymeric material, and the outer stratum that is formed by a barrier in ethylene vinyl alcohol (EVOH), which avoids permeability to oxygen. The pipes are able to withstand high pressures and temperatures, resist corrosion and guarantee high durability. The thermo-fluid is propylene glycol mixed with water, which can work down to a temperature of  20°C.

History
A first documented example of the real scale application of this technology is that of the Lainzer tunnel in Austria. During the construction of the tunnel with the conventional tunneling method (New Austrian Tunneling Method), absorber pipes were attached to non-woven geosynthetics offsite and then placed between the primary and secondary linings. Another experience followed in Germany, where two 10 m-long sections of the Stuttgart-Fasanenhof tunnel were equipped with a geothermal system. More recently in the Linchang tunnel in China, a primary circuit interposed between the primary and the secondary lining was used to heat the tunnel portals. In 2009 in the Katzenbergtunnel in Germany an experimental setup with 5 thermal segments was tested. Later the same technology was applied to the Jenbach tunnel in Austria. Another experimental installation took place in Italy, where two rings of the tunnel lining of the Turin Metro Line 1 were thermally activated and tested for one year.