Deep geothermal energy

Definition

Geothermal energy is the natural thermal energy present inside the Earth. Temperatures are steadily higher the further down you go from the surface to the Earth’s core, where they rise to 6,500 degrees Celsius. With the various geothermal energy processes, thermal energy can be extracted at different depths.   

Shallow geothermal energy uses thermal energy in near-surface zones, for example using geothermal probes and heat pumps, and is already widespread. Deep geothermal energy technologies are used at depths of 400 to around 8,000 metres. However, the boundaries and definitions of these technologies are fluid, as the effective temperatures depend not only on the depth, but also on the geological conditions underground. Processes that use the heat between 400 and 3,000 metres are sometimes also considered to be medium-deep geothermal energy. The temperature rises to around 40 degrees Celsius at a depth of around 1,000 metres below the Earth’s surface, and to around 160 degrees Celsius at 5,000 metres. In Switzerland, temperatures increase by around 3 degrees Celsius per 100 metres of depth. Electricity can be produced at temperatures of over 100 degrees Celsius.  

The use of deep geothermal energy requires technically complex drilling and, depending on the method used, underground heat exchange systems. The focus is on four technologies: 

• Hydrothermal systems use the heat from natural water-bearing pore or fracture systems (geological reservoirs) at depths of 400 to about 4,000 metres. The warm water is brought to the surface by means of a geothermal well.  Using a heat exchanger, the heat can be extracted from the water and used for heating – or electricity can be produced at sufficiently high temperatures. The cooled water is then fed back into the reservoir via a second well. 

• CO₂-plume geothermal systems (CPG systems for short) combine geoenergy generation with deep underground CO₂ storage and have been under development at the Geothermal Energy and Geofluids (GEG) research group at ETH Zurich under the leadership of Martin Saar since 2015. Instead of water, liquid carbon dioxide (CO₂) is pressed into naturally permeable geological reservoirs, where it partially heats up. These reservoirs, which also have a high CO₂ storage capacity, are located at a depth of at least 2 kilometres. The heated CO₂ is brought back to the surface, where it is used directly as heat or converted into electricity. After use, the cooled CO₂ is liquefied again and – after adding additional CO₂ from capture facilities such as the cement industry or waste incineration plants – returned to the storage reservoir. As a result, all CO₂ is permanently stored underground. Due to the lower viscosity of liquid CO₂ compared to water, the yield of geothermal energy is about twice as high as in power plants that use warm water. CPG plants are therefore particularly efficient geothermal power plants that also store CO₂.  

• Petrothermal systems or enhanced/engineered geothermal systems (EGS) deliver heat from particularly deep, naturally water-free rock formations at depths of 5,000 metres and below. Water is pressed into the crystalline rock via an injection well at high pressure. This creates new fractures and artificial flow paths that are connected via further wells to form a circulatory system in which the pumped water heats up. This water is then brought back to the Earth’s surface via a second geothermal well and used for heat generation or electricity generation. 

• In deep closed-loop advanced geothermal systems (AGS), the heat exchanger is completely drilled into the deep underground environment and installed with pipes in hot layers of rock, in which the liquid to be heated absorbs the ambient heat. The first step is to drill a well 4 to 8 kilometres deep. From there, several horizontal loops with a diameter of 3 to 10 kilometres are then drilled.  

Hydrothermal systems and CPG systems rely on permeable underground geological reservoirs. EGS do not require permeable reservoirs, but need certain underground conditions such as suitable rock stress. AGS, on the other hand, are possible anywhere in principle. However, they are not yet economical due to the extremely high drilling costs. 

Current applications and opportunities

Hydrothermal systems have a long history, starting with the extraction of thermal water. Riehen (Basel-Stadt) is home to the oldest and largest energy plant in Switzerland. It has been extracting warm water at a temperature of 67 degrees Celsius from a depth of just over 1,500 metres for 30 years. The energy generated in this way is used to operate a district heating network that provides heat to around 10,000 people. New hydrothermal plants are planned in the Lake Geneva and Vaud regions, but are still in the prospecting and exploration phase. 

The only petrothermal project in Switzerland is currently being carried out in the municipality of Haute Sorne in the canton of Jura by Geo-Energie Suisse, a joint venture between several public energy companies. EGS technology is to be used to extract energy for electricity and heat production from a depth of 4,000 to 5,000 metres. The aim is to produce more than 20 gigawatt hours of electricity per year, which is roughly equivalent to the consumption of 6,000 households. 

Medium-deep and deep geothermal energy offers enormous potential for environmentally friendly and CO₂-free heat and power supply in Switzerland. The energy from beneath our feet is continuously available as base-load energy throughout the year. Systems for direct heat utilisation could cover more than 10 percent of the heat required nationally. According to the energy forecasts from the Swiss federal government, 2 terawatt hours per year are realistic for electricity from geothermal sources. This corresponds to around a quarter of the energy produced by the Gösgen nuclear power plant.  

Development is driven primarily by academic stakeholders in research groups at ETH Zurich, EPFL, the Eastern Switzerland University of Applied Sciences and University of Applied Sciences and Arts Northwestern Switzerland. Specific projects are being developed by planning offices with the appropriate expertise and together with local and regional energy suppliers. The state funding agency Innosuisse has supported the further development of AGS with the flagship project AEGIS-CH. 

Challenges

EGS processes, in which layers of rock underground are stimulated with high-pressure injections, are susceptible to producing induced earthquakes in particular. In 2006, for example, the injection of water during the construction of an EGS power plant in Basel triggered an earthquake. The other three deep geothermal power plants (hydrothermal systems, CPG and AGS) have a significantly lower earthquake risk. It is extremely important to the safety, cost-effectiveness and social acceptance of EGS power plants, in particular, that researchers clarify how these earthquakes are triggered. 

In order to continuously improve the risk management of deep geothermal projects in general and of EGS projects in particular, the Swiss Seismological Service (SED) at ETH Zurich launched the GEOBEST project in 2015, financed by the Swiss Federal Office of Energy (SFOE), which is initially going to run until 2027 as GEOBEST2020+. SED uses a highly sensitive measuring network to monitor Switzerland’s existing deep geothermal projects. SED is developing prediction models for artificially generated earthquakes and suitable methods to minimise the risk of induced earthquakes. 

The economic viability of the electricity or heat production of a project depends on the permeability of the rock and the size of the reservoir. However, because little is known about the deep underground environment in Switzerland, many projects fail at the start when exploring the geological conditions in the target area. Since 2018, the Swiss Federal Office of Energy has been paying exploration contributions to minimise the prospecting risks for project developers. CHF 180 million was approved for this purpose for the period from 2025 to 2030.  

The wells account for up to 80 percent of the costs. For projects in very deep layers of rock, drilling costs amount to CHF 100,000 or more per day. One of the main reasons for this is the wear of the drill heads due to abrasion, meaning that the devices have to be replaced regularly at great expense and effort. Wear could be prevented or reduced by developing new non-contact drilling methods, such as lasers, flame, plasma pulses or microwaves. These processes would result in enormous cost savings and could make energy extraction from deep underground more competitive. 

Focus on industry

Deep geothermal energy plays a major role in the development and production of drilling systems and power plants and for energy generation. An interdisciplinary network of authorities, project sponsors, geology offices, drilling companies and energy companies is working along the entire value chain. The potential for start-ups and SMEs is limited as the technology requires high levels of investment and is dominated by large established oil and gas companies. 

Employees involved in project planning and development need geology and hydrogeology knowledge. In addition, drilling engineers, specialists in exploration and energy systems are required alongside lawyers and economists. The problem of a shortage of skilled workers cannot be ignored in Switzerland, as there is a lack of experience in the field of oil and gas drilling. However, services in this field can be bought in from specialised companies abroad. 

International perspective

The US is the global leader in electricity production from geothermal energy with an installed capacity of almost 4,000 megawatts and annual electricity production of 18.7 gigawatt hours, followed by Indonesia and the Philippines with almost 2,400 and 2,000 megawatts respectively (as of 2022). The Philippines have significantly closed the gap and, like Indonesia, are benefiting from the geologically advantageous conditions in the volcanic subsoil with large numbers of geothermal fields, which can be used with cheaper hydrothermal power plants. Turkey and Italy are leaders in Europe. 

Deep geothermal energy in Switzerland is still in the development phase, with the focus here being on heating instead of electricity so far. Geological conditions, in particular the lack of high-temperature reservoirs near the surface, make it difficult to use geothermal energy to generate electricity with hydrothermal systems. Nevertheless, current projects and investments indicate growing interest and potential. Swiss research has gained valuable insights from the failed projects in Basel, Lavey-les-Bains (Vaud), St. Gallen, Vinzel (Vaud) and Zurich. Switzerland also plays a leading role internationally, particularly in the area of risk assessment. When it comes to drilling technology, countries rich in natural resources are further ahead for historical reasons. 

Future applications

In addition to expanding the project in Riehen (Basel-Stadt), further hydrothermal systems are planned for the cantons of Vaud, Fribourg and Valais. None of these projects are in operation yet. An exploratory well in the municipality of Vinzel (Vaud) on Lake Geneva discovered water-bearing layers at a depth of around 2,200 metres, but the temperature was too low. There is therefore still uncertainty about whether this project will continue. Currently, Switzerland’s only ESG project in Haute Sorne (Jura) is scheduled to produce electricity from 2029. A decision as to whether the stimulation phase will be started will be made at the end of 2025 once the results from the test wells are available.  

CPG systems are also still under development. In 2023, the Geothermal Energy and Geofluids research group at ETH Zurich founded the CPG Consortium in collaboration with industry and with the support of the SFOE to test the commercial implementation of the concept with a demonstration plant.  

Switzerland offers considerable potential for using geothermal heat. Since the drilling depths and therefore the costs for generating heating energy are lower than for electricity production, the majority of plants in the coming years are expected to be for direct heat production, such as in district heating networks.