When tunnels are bored through mountains, it is not the groundwater level that is an issue but the mountain water level. Mountain water comes from precipitation and flows down from the mountain surface. Along the way, its temperature is increased by the earth’s heat.
“Geothermal energy is largely generated by the decay of long-lived nuclides in the earth’s interior,” says Thomas Geisler of the Institute of Rock Mechanics and Tunneling at Graz University of Technology. “As such, it is an entirely natural heat source fed by the Earth’s interior.” Unlike wind turbines, which only provide energy when the wind is blowing, geothermal energy is basically available around the clock and around the world. This means that energy can be provided on a permanent basis without interruption. Geothermal energy thus provides a secure supply of energy.
Tunnel water as a source of energy
Professor Thomas Marcher, who heads the Institute of Rock Mechanics and Tunneling at Graz University of Technology: “When mountain water encounters a tunnel structure, it drains or seeps into it. So the water that enters is drained away either as an open channel or as a conduit in the invert of the tunnel.”
In existing tunnel structures, the discharged water is usually cooled in a retention basin – for environmental reasons. It can only be reintroduced into the natural environment when it has reached a temperature similar to that of the receiving water, such as a river.
The researchers do not simply want to let the tunnel water cool down, but use its heat as a source of energy. Their research results should prove that it makes sense to use the tunnel as a source of energy.
The first geothermal use of tunnel water was recorded in 1979 at the south portal of the Gotthard Base Tunnel in Switzerland. But to date, there has been no comparable project of this size. That’s why the challenge is great, according to the researchers. After all, they say, you can’t just extract endless supplies of geothermal heat from the mountain massif. Long-term cooling must be avoided. The system must also be designed as efficiently as possible.
The Brenner Base Tunnel has good prerequisites for the lighthouse project. From the Brenner Pass, the highest point of the tunnel, there is a natural gradient towards Innsbruck, where the tunnel portal is located. This means that the drained tunnel water will automatically flow to the city. No additional pumping is needed. This is how the idea of using the tunnel water heated by the earth as a source of energy for the city of Innsbruck came about, Marcher said.
Third tunnel tube
A further advantage is that the tunnel structure is located very deep below the earth’s surface. “The highest overburden is at about 1800 meters. At these low elevations, the earth’s temperature reaches circa 20 to 35 degrees Celsius and the water also takes on this temperature,” says Marcher.
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The Brenner Base Tunnel also has a special structural feature: it has three tubes and not – as is usually the case – only two directional tubes. The third, so-called exploratory tube, was built in front of the two main tubes to identify the actual geological conditions in the mountain. Not only is this tube suitable for draining the water, but:
“Through this third tube, we can enter the tunnel at any time and also subsequently put in place measures for heat recovery without affecting the railroad operation, which has first priority.”
Professor Thomas Marcher from the Institute of Rock Mechanics and Tunneling at Graz University of Technology.
For the geothermal heat from the drainage water to become a source of energy for households and even entire neighborhoods, it must first be distributed. For this purpose, the heat contained in the tunnel water is transferred to the existing heating network by means of a heat exchanger. It is then brought to the required temperature level by means of heat pumps. The ideal planning or adaptation of heat exchanger and heat pump is now being researched, as is the economic distribution of the energy.
During geothermal energy extraction, heat is extracted from the rock. To prevent long-term effects on the thermophysical properties of the mountain, researchers are working with model calculations:
“We will only extract as much heat from the mountain massif as the naturally replenishing heat flow allows. Otherwise, long-term cooling may occur and the efficiency of the energy supply will be at risk.”
Thomas Geisler, Institute of Rock Engineering and Tunneling at Graz University of Technology
In the case of the Brenner Base Tunnel, the possibilities for installing the necessary technology for heat recovery are greater than in existing structures because the tunnel is still under construction and has a third tube. This third tube is unique, Marcher explains. Therefore, the researchers would like to advocate for the construction of a third tube for such tunnels, as it not only simplifies the use of geothermal energy but also repairs and other necessary work.
The tunnel structure is still under construction. Therefore, its geothermal potential can only be estimated. The researchers are using hydrogeological modeling and currently assume water volumes of 60 to 100 liters per second.
The monitoring system of the tunnel waters at the Brenner Base Tunnel records hydrological parameters around the clock, Geisler explains. The data range from flow rates and temperatures to chemical properties. This allows the hydrogeological model to be fed with sufficient data and calibrated.
With temperatures of up to 35 degrees, the temperature of the tunnel water is already very high. For supplying energy, however, it has to be even hotter. To do this, the researchers are pursuing two methods:
One is to optimize the natural temperature by separating the inflowing waters into higher and lower temperatures. Naturally, the higher-temperature waters are located deeper in the mountain. Water flowing in close to the portal usually has a lower temperature due to the lower overburden elevations. Separating these inflows results in a higher average temperature, Marcher explains.
Another method is to use absorber technology. For example, hoses are attached to the inner wall of the tunnel in which a medium circulates. This absorbs the geothermal heat and raises the temperature of the tunnel water. Says Geisler, “That means we would heat this tunnel water with natural energy provided by the mountain.”
Marcher and Geisler are conducting research in collaboration with the University of Natural Resources and Applied Life Sciences in Vienna, the AIT Austrian Institute Of Technology, the Geological Survey of Austria, the Brenner Base Tunnel Company (BBT SE) and the Innsbruck Municipal Works.
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