Energy From Underground

It provides carbon-neutral energy all year round and, unlike coal, gas or hydroelectric power plants, takes up minimal space, since most of the plant is located deep below the Earth's surface. In the past, however, earthquakes, caused by hydraulic stimulation, have been cause for concern. So where do we stand today?

Several research groups at ETH Zurich are hard at work investigating the possibilities of geothermal energy. They all agree on one thing: geothermal energy holds immense potential for Switzerland. The risk of earthquakes, which varies from one form of deep geothermal energy to the next, has also become much more manageable.

Not all geothermal energy is created equal

Geothermal energy is an old technology. Thermal springs have been in use since the time of the Romans and even today, hot groundwater that circulates in natural reservoirs is used to generate heat as part of hydrothermal systems (examples include the district heating networks in Riehen, Munich or Paris).

Other modern geothermal systems - known as EGS (Enhanced Geothermal Systems) - go one step further: where there are no layers of water-bearing rock, hydraulic stimulation is used to create artificial fractures in the deep crystalline rock, providing space for water. This water heats up and is pumped back to the surface to provide heat or generate electricity.

However, EGS also carry the risk of triggering earthquakes - as happened in Basel in 2006. Although targeted stimulation and improved monitoring have greatly reduced this risk, challenges remain: drilling is costly and not always successful, and fractures in the rock tend to close up again over time due to mineral deposits.

Traffic light system reliable way of minimising risk

Deep in the Swiss bedrock, at a facility known as the BedrettoLab, Stefan Wiemer, a professor in the Department of Earth and Planetary Sciences at ETH Zurich and Director of the Swiss Seismological Service (SED), is conducting research. Scientists are investigating new methods for making EGS safer, with researchers focusing on many small, controlled stimulations in isolated zones of the borehole. "We need small vibrations to create the fractures in the rock, but we have to avoid larger quakes at all costs," explains Wiemer.

The findings from the BedrettoLab are being included in plans for the pilot geothermal power plant in the commune of Haute-Sorne. ETH scientists have also been tasked by the canton with monitoring seismic activity on this project.

Sensors in the boreholes transmit large amounts of seismological and hydraulic data to the researchers in real-time. A specially developed machine learning model continuously calculates how many quakes above a certain threshold can be expected if the planned stimulation continues. If there is a risk of exceeding the thresholds, the system immediately sounds the alarm and suggests which adjustments need to be made to prevent major tremors.

Wiemer emphasises that the starting point for EGS is very different now compared to 20 years ago. "Today, we have very good control mechanisms. This is thanks to improved data collection and the ability of AI to process enormous volumes of data in real-time. That allows us to better understand and minimise risks, although we cannot exclude them altogether," says Wiemer.

Back-up power from the Earth

Martin Saar is Professor of Geothermal Energy and Geofluids at the Department of Earth and Planetary Sciences at ETH Zurich. He and his team are also researching how geothermal systems can be improved. And they have an innovative idea for how they might achieve this: instead of creating fractures in the rock by injecting water, they plan to drill entire closed-pipe systems deep underground. This works regardless of the geology and entails only the usual minimal risk of earthquakes associated with underground drilling. The pipes would then circulate CO₂ rather than water, which increases the efficiency of the geothermal power plant. Deep underground, CO₂ heats up, expands and rises by itself, where it is expanded in a turbine to generate electricity. As the name suggests, these so-called deep closed-loop Advanced Geothermal Systems (AGS) constitute a closed loop, meaning they do not emit any operational CO₂.

When there is no wind or sunshine, AGS could provide a reliable source of energy and act as a back-up power supply. "In view of the increasing demand for electricity in winter, AGS could offer an environmentally friendly alternative to the carbon-intensive gas-fired reserve power plants which are currently being talked about," says Saar.

One downside, however, is the high cost of drilling, but there is hope, as the cost of traditional rotary drilling has been significantly reduced in recent years. Work is also under way around the world, including on Saar's team, to investigate modern contactless drilling methods, using lightning or microwaves, for example, which could greatly reduce the cost of deep drilling in the future.

Long-term carbon storage and energy generation in one

Another type of deep geothermal energy is a CO₂-Plume Geothermal (CPG) system which Saar's research group invented in 2009. Saar observed already then that more and more countries are interested in storing carbon permanently in geological reservoirs as a way of counteracting global warming: "I wondered at the time what would happen if we could not just pump the CO₂ underground and store it there permanently, but also get it back out temporarily and use it to produce heat or generate electricity."

The CO₂ that is injected into deeper layers of the Earth for storage heats up and flows upwards. There, it drives a turbine, cools down and flows back down again - a closed cycle that ensures that all CO₂ is permanently stored deep underground. Using CO₂ as an energy carrier instead of water, which is more viscous, increases heat production - and therefore electricity generation - typically by a factor of two to three. Another advantage is that because heat is extracted from the geothermal reservoir, reducing pore-fluid pressures, more CO₂ can be stored in the underground reservoir.

Development of the CPG concept is continuing in a dedicated CPG Consortium ( https://geg.cpg.ethz.ch/ ) Saar has founded in 2023, in collaboration with industrial partners such as Shell, Petrobras, Holcim and Ad Terra Energy, which is also supported by the Swiss Federal Office of Energy. The CPG consortium is currently evaluating various locations for an initial pilot plant to demonstrate technical feasibility at commercial scale.

Using the Earth as a battery

But geothermal probes can be used for more than just extracting heat from below the Earth's surface; they can also be used to inject heat, as is the case in Switzerland, for example, where geothermal probes are frequently used as a sustainable way of heating buildings. According to Maren Brehme, a researcher on Saar's team, medium-depth geothermal energy (down to several hundred metres) holds additional potential for advancing the heating transition in Switzerland. "When it comes to heat generation, in particular, Switzerland is still heavily dependent on gas, and therefore on other countries," says Brehme.

With this form of geothermal energy, excess heat from the summer or from industrial processes is led underground through boreholes, stored in the rock and then pumped out again in winter and used for heating. It is a process that has already been rolled out successfully in countries like the Netherlands where it is used to heat greenhouses. Now, Brehme and her team want to adapt the process for geological conditions in Switzerland. At the BedrettoLab, scientists are investigating how to store heat in crystalline rock.

Here, too, drilling costs are still relatively high, and not enough is known about the subsurface in Switzerland, which increases the risk of not finding sufficient permeability underground. "Switzerland hardly has oil or gas deposits. Because of that, the subsurface has never been investigated as thoroughly as in other countries. We have to plug these gaps in our knowledge," says Brehme.

Overall, however, she believes that the prospects for geothermal energy in Switzerland are good: drilling technologies are improving and are becoming more affordable all the time, while regulatory hurdles are being removed, and the very first insurance companies have begun offering cover against drilling risk. On top of that, surveys show that the population is, generally speaking, open to geothermal energy.