When the author Jules Verne wrote his novel “Journey to the Center of the Earth” in the 19th century, he used plenty of poetic license. Even then it was a well-known fact that there were no oceans, plants, or animals deep under the earth’s surface. It is simply extremely hot there. So hot that just a few thousand meters down any drilling unit will start to melt. From a depth of around 3,000 kilometers, even rock liquefies. This is where the earth’s core begins and where the temperature can be as high as 6,000 degrees Celsius. It represents an enormous source of energy.
Heat from far below the earth’s surface has fascinated people for thousands of years. And during all this time, they have dreamed of exploiting it. The Romans used thermal springs for heating, for example, and in the 14th century a complete district heating network in the French village of Chaudes-Aigues was fed by hot springs. This idea has never lost its appeal. Today, a remote valley in the Italian province of Tuscany is wreathed in the steam from the cooling towers of dozens of geothermal power plants, some of which began operating as early as the start of the 20th century. And Iceland obtains more than 50 percent of its energy from its many near-surface geothermal heat sources.
Geothermal energy is also used in Germany. For instance, there are around 440,000 ground-source heat pumps in buildings that use probes to access near-surface geothermal energy for heating purposes. These should not be confused with the air-source heat pumps that are being installed in almost half of all new homes in Germany. They do not use geothermal heat, but instead exploit the energy stored in the outdoor air. According to the German Geothermal Association (BVG), heating solutions that run on genuine geothermal energy amounted to only 6.3 percent of the systems installed in new buildings in 2020.
Hot thermal water from a depth of more than 400 meters
While near-surface geothermal energy can heat buildings, deep geothermal energy is needed to supply entire districts of cities, district heating networks, and industrial plants with heat. The term “deep geothermal energy” is used to describe heat that comes from strata more than 400 meters below the earth’s surface. The heat at this depth can easily be obtained from the naturally available hot thermal water. If this does not rise to the surface spontaneously, as it does, for example, in Aachen in Germany or the Belgian town of Spa, it can be pumped up from specially drilled boreholes and used to supply hydrothermal heat.
Hydrothermal energy from deep geothermal sources has recently become a feature of government energy policy. This is due to the fact that although there has been some success in bringing about the energy transition in the electricity sector, the heating transition is still some way off. And it is urgently needed. According to the German Association of Energy and Water Industries (BDEW), in 2020 heating accounted for 58 percent of Germany’s entire final energy consumption and figures from the German Environment Agency show that only 15 percent of this came from renewable sources. However, the German government’s climate action plan states that half of municipal heat requirements should be met by renewable energy by 2030. Therefore, it is no surprise that projects aiming to use geothermal energy for district heating and electricity generation are being funded by the KfW, Germany’s state-owned investment and development bank. The main requirement is a drilling depth of more than 400 meters, in other words, deep geothermal energy.
As an energy source, geothermal heat has huge potential, at least in theory. However, accessing this almost inexhaustible supply of heat can be very complex, depending on the geology of the region, because it requires extensive seismological investigations and a large number of boreholes. On the other hand, once a geothermal source has been accessed and a power plant built, it will provide a reliable, permanent supply of heat.
Munich plays a leading role in geothermal energy
Germany currently has 42 hydrothermal power plants that use deep geothermal energy to provide district heating or generate electricity. They produce a total of 359 megawatts of heat and 45 megawatts of electricity. Most of the plants are in the foothills of the Alps or the South German Molasse Basin, as it is known in geological circles. The geothermal power plants operated by Stadtwerke München, Munich’s municipal utilities company, are particularly good examples. Five are already up and running and the sixth, which is located in Heizkraftwerk Süd, a combined heat and power plant in the south of the city, is in trial operation. With an output of 80 megawatts, it will be Germany’s largest geothermal power plant. A seventh plant is already in the planning stages (see the interview with Stadtwerke München).
By the early 2030s, Stadtwerke München aims to provide up to 70 percent of the entire district heating for the city of Munich from carbon-neutral sources, including geothermal energy and the incineration of waste. But the company explains that it will be almost impossible to speed up the expansion of geothermal energy any further. This is because the highly specialized drilling firms have a shortage of capacity and are booked up for years in advance. In addition, in Munich the boreholes have to be up to 4,000 meters deep to reach the water-bearing formations where the temperature is around 90 degrees Celsius.
The example of Munich shows that hydrothermal energy can be supplied reliably and at low noise levels from deep geothermal sources. By contrast, when a geothermal power plant came into operation in Landau in Germany’s Palatinate region, it caused micro-earthquakes. In Basel in Switzerland, a petrothermal energy generation project attempted to pump heat from deep rock strata, but it was not possible to guarantee that the system would not cause earthquakes. Even widely used near-surface geothermal systems that reach a depth of up to 100 meters are making the headlines. In Staufen, a small town near Freiburg in Baden-Württemberg, the town council decided to heat the town hall using geothermal energy. When a borehole was drilled in 2007, it passed through a stratum of anhydrite at a depth of 100 meters and made contact with the groundwater. Since then, the anhydrite has been transformed into gypsum, as a result of which the earth under the old town is swelling. Cracks have appeared in 270 houses and more damage is emerging all the time. There were similar problems in nearby Böblingen in 2009 following drilling for geothermal energy and this has caused cracks to form in 80 houses. After around a dozen cases of this kind, the German state of Baden-Württemberg has tightened up its rules on near-surface geothermal drilling.
Significant geothermal potential in Germany
On the basis of the 440,000 near-surface geothermal boreholes in Germany, the accident rate is very low, according to Professor Rolf Bracke, head of the Fraunhofer Research Institution for Energy Infrastructures and Geothermal Systems IEG. Near-surface geothermal systems have demonstrated that they are viable market solutions. This is why the Fraunhofer IEG has increased its research into hydrothermal energy from deep geothermal sources. Together with Professor Ernst Huenges from the GFZ German Research Center for Geosciences, Bracke has published a study that shows how the role of geothermal energy could be increased as part of the heating transition. The “Roadmap Tiefe Geothermie” (Roadmap for deep geothermal energy) calculates the potential of geothermal energy in Germany to be more than 300 terawatt hours (TWh) per year. This amounts to a quarter of the German demand for heating. As well as the possibility of using hydrothermal sources of deep geothermal energy, the calculation includes the potential for high-temperature underground storage and the use of water in mines. This involves storing excess heat in summer in an aquifer – an underground stratum that holds groundwater – and using it for heating purposes in winter.
The study does not include the potential of petrothermal resources. In contrast to hydrothermal energy, where hot thermal water is pumped to the surface, petrothermal systems involving pumping water from the surface down into rock that is at a temperature of up to 200 degrees Celsius. However, the rock first has to be made permeable by fracking. This method, as the example of Basel showed, is not widely accepted by residents and was not included in the calculations for this reason, according to the researchers.
Bracke and Huenges believe that large urban areas with extensive district heating networks will be the main users of geothermal energy. Geothermal power stations are “fully dispatchable,” which means that they can produce a constant supply of heat. Deep geothermal energy would also be suitable for heat-intensive industrial processes. For instance, a paper manufacturer in the German town of Hagen is planning to use geothermal energy for its drying processes. From a more general perspective, the researchers recommend that the development of high-temperature heat pumps for industrial applications should be speeded up. They must be able to supply water at up to 200 degrees Celsius from hydrothermal sources.
After construction the heat is almost free of charge
Before geothermal energy can be developed throughout Germany, organizational and legal issues need to be resolved, particularly in relation to the environment and the protection of groundwater. Depending on the necessary drilling depth, the researchers estimate the cost at between 2 and 2.5 billion euros per gigawatt of heat. By contrast, the levelized cost of heat (LCOH) – in other words, the cost incurred during the production and distribution of heat – is difficult to calculate, according to the experts, because the investment cost can vary significantly depending on the location and the local conditions. However, it is clear that once a geothermal power plant is in operation, the heat it generates will become increasingly cheap, because it costs almost nothing to extract. However, the study expects the levelized cost of geothermal heat to remain above the cost of fossil district heating using natural gas even after 2030 and despite the introduction of carbon pricing. This means that geothermal energy will need to be subsidized by the state.
To enable the full potential of deep geothermal energy to be exploited, the researchers believe that the government must set targets as soon as possible for its expansion, launch exploration programs, and identify instruments to reduce the risk of faulty boreholes being drilled. If this happens, then in 20 years’ time a quarter of the country’s climate-neutral municipal and industrial heating could come from deep geothermal energy.