Making geothermal energy cost-effective usable where digging is done anyway – namely in the construction of tunnels: In 2010, Stuttgart engineers began testing this idea on a test facility in a light rail tunnel. The geothermal system is to heat the elephant house at Stuttgart Zoo.
The wheels rattle on the rails as the U6 drives through the tunnel. Next stop: "Europaplatz". A handful of people get out.
What probably very few of them know is that they have just passed through a disused scientific experiment on the Stuttgart Stadtbahn. Hidden in the walls of the subway tunnel is a geothermal plant for the production of geothermal energy. Anders Berg is a research assistant at the Institute of Building Energetics at the University of Stuttgart: "We want to achieve these climate targets, i.e. to reduce the share of fossil fuels by 80 percent. The question is, how do we do that? And then near-surface geothermal energy would be a solution."
Geothermal energy is an inexhaustible source of energy. But tapping into them is expensive. In order to achieve the highest possible temperature gradient, you have to drill more than 100 meters deep into the earth. It would be cheaper to use geothermal energy where you dig into the earth anyway – such as in the construction of a light rail or car tunnel. With the test facility, which went into operation in Stuttgart in 2010, the engineers at the University of Stuttgart wanted to explore the potential of such urban geothermal tunnels.
The challenge of shallow depth
"The question we ask ourselves is, how do the conditions change now that we have a longer tunnel or a different type of tunnel, or a different customer? So we wanted to see what different parameters influence our plant and how much?"
Examples such as the geothermal plant in the Swiss Gotthard Tunnel show that the approach works when the plant is located many hundreds of meters below the mountain. Most of the tunnels, however, are only a few meters underground. The test data that Anders Berg and his colleagues collected in the light rail tunnel between 2010 and 2015 prove that a geothermal plant can still make sense. Compared to deep drilling, less energy is generated, but the disproportionately lower costs make operation worthwhile in many cases. Provided that the framework conditions are right.
At the other end of Stuttgart is the "Wilhelma" tram stop. A tunnel is also being built here. "Yes, so we are here at the Neckarknie at the Zoological and Botanical Garden, the Wilhelma. And this is where the new B10 Rosenstein Tunnel is being built under Rosenstein Park." Christian Buch, the head of the construction project, stands between excavators in a high-visibility vest and rubber boots and looks satisfied.
"Looks like a construction site... quite chaotic. Water tanks, crane systems and dust..." Construction is being carried out to relieve districts chronically plagued by traffic gridlock from through traffic. At the same time, however, an air conditioning system for the new elephant house of the Stuttgart Zoo is also dropped: Based on the model of the light rail tunnel at Europaplatz, a geothermal system is installed in the walls of the new car tunnel. A joint project of the city, state, university and the Office for Environmental Protection.'
Temperature sensors under the concrete
Climbing stairs, footsteps, machine noises, reverberation when entering the tunnel, then tunnel noises. Christian Buch climbs down a rather improvised staircase into the excavation pit and then heads for one of the tunnel tubes. He was followed by Patrick Buhmann, research assistant at the Institute of Geotechnics at the University of Stuttgart. "Yes, we have now walked into the tunnel for a good quarter of an hour and are now standing at the first thermally activated block of the Rosenstein Tunnel in the north tube."
Patrick Buhmann points to a kind of window that is embedded in the concrete wall of the tunnel tube. Protruding from this are hose ends of various thicknesses. "The small hoses that come out of there are the pipes that are installed inside the concrete.... you have to imagine it like underfloor heating."
Also hidden under the concrete is a large number of temperature sensors. They are intended to provide long-term data to improve forecasting models for the profitability of tunnel geothermal plants. Unlike the pilot plant in the light rail tunnel at Europaplatz, which has since been decommissioned, the Rosenstein Tunnel is not primarily intended to provide scientific knowledge, but above all usable energy: "In total, it is about 3,330 square meters of thermally activated area and the forecasts that were made at the time before the construction of the plant amounted to about 56 kilowatts for the entire tunnel geothermal plant."
That would be enough to heat several single-family homes. However, it is not enough for the large elephant house in Wilhelma. A waste heat system installed parallel to geothermal energy, which will be fed from the tunnel's operations center, could supply a further 70 kilowatts. Both energy sources together should cover the majority of the pachyderms' heat requirements and – according to estimates – save around 200 tons of CO2 annually. It is not yet that far. But construction manager Christian Buch can already see the light at the end of the tunnel.
"Okay – now we've walked through the tunnel for a long time. The weather is fine. Yes, we see bright blue skies. If we look to the left now, we see a construction fence, directly behind it is the Wilhelma restaurant, the show farm. Yes, and we will now restore the surface next year and give Wilhelma back its land so that it can continue there with the Asian facility and the elephant house."
The Rosenstein Tunnel and thus also the geothermal plant are scheduled to go into operation in 2021.