Deep geothermal energy process
Deep geothermal energy utilises heat from the earth's interior at depths of more than 400 m. Hot water or hot rock can be used there to generate energy. The temperature is crucial here: below around 200 °C, these are referred to as low-enthalpy deposits, while above 200 °C they are called high-enthalpy deposits.
Use of thermal water (hydrothermal geothermal energy)
In some areas, natural hot water circulates at great depths in cracks and porous layers of rock. This thermal water can be used for heating or power generation. Such deposits exist in many places in Germany. In spa towns such as Bad Staffelstein (Upper Franconia), the warm water is used for therapeutic baths, while in cities such as Munich, Erding (near Munich) and Neustadt-Glewe (Mecklenburg-Western Pomerania), it supplies entire residential areas with heat. In Grünwald near Munich, water heated to over 120 °C is also used to generate electricity. The city of Munich plans to obtain all of its district heating from renewable sources – primarily geothermal energy – by 2040.
For this to work, there needs to be a water-bearing, i.e. „permeable“ rock layer that contains sufficient water and is large enough. Two boreholes are usually used: one brings the hot water to the surface, the other returns the cooled water back into the depths after use. This dual system is called a doublet. If a third borehole is added, it is called a triplet. Modern drilling technology allows several boreholes to be located close together, which saves costs.
Use of hot rock (petrothermal geothermal energy)
In some regions, there is no circulating water at depth, only hot, solid rock. This can also be used to generate energy. In Germany, this accounts for around 90 % of the potential for geothermal electricity. Dense, hot rocks with temperatures above 150 °C are suitable.
In order to extract and utilise the heat, one or more boreholes are sunk (the technical process of drilling or lowering a borehole or shaft into the ground) and water is injected into the rock under high pressure. The pressure opens up small natural cracks and creates new water channels – this is called hydraulic stimulation. Sometimes weak acids are also used to dissolve deposits. The water absorbs heat in the rock, rises back up through a production well and, after energy has been extracted, is pumped back down into the ground.
The big advantage of this method is that it does not depend on natural water reservoirs. Hot rock can be found almost everywhere, so petrothermal systems can generally be used in many locations.
Eavor-Loop™ technology: revolutionising deep geothermal energy
The construction of the world's first commercial Eavor-LoopTM plant in Geretsried, south of Munich, marks the beginning of a new chapter in the use of deep geothermal energy. The system consists of two vertical boreholes that fan out into several horizontal boreholes, known as laterals, at a depth of around 4,500 m. Each of these horizontal boreholes is connected in pairs to form a closed circuit. This creates twelve loops per loop with a total drilling length of around 80 km. Four loops are being built in Geretsried, which together will reach a length of around 320 km.
Once drilling is complete, the system is sealed to prevent any fluid migration. To do this, the Canadian-German company Eavor uses patented Rock-PipeTM technology, which seals the surfaces of the horizontal boreholes. Ordinary water is then introduced into the system once as a heat transfer medium. This water circulates continuously: it absorbs heat from the surrounding rock at depth and rises to the surface without pumping as a result of the thermosiphon effect (warm water is lighter than cold water). The water, which is still around 120 °C hot at this point, is used in a heat exchanger to either provide district heating directly or generate electricity. The system can provide around 64 MW of thermal power per loop and avoid up to 44,000 tonnes of CO₂ annually – a showcase project that is attracting worldwide attention.
Hanover is considered the first major city in the world to use the closed-loop process. The project, a collaboration between Eavor and local energy provider Enercity, aims to use geothermal energy in a climate-neutral way for urban district heating and involves the installation of two Eavor Loops™, each with a heat output of 15 MW. Once completed, the plant is expected to cover 15 to 20 % of Hanover's annual district heating requirements. To this end, up to 250 million kW/h of heat will be extracted from the ground and fed into the municipal district heating network – enough to supply around 20,000 homes. The technology is intended to replace fossil fuels from the Hannover-Stöcken coal-fired power plant, which is to be decommissioned by 2030. The project is considered a flagship project for a sustainable, urban heat transition.
Geothermal energy in the context of the energy transition
More than half of Germany's final energy demand is accounted for by heat generation. Geothermal energy offers a continuously available, virtually emission-free source and thus contributes significantly to the decarbonisation of urban and rural heating networks.
For example, Munich's vision of achieving a complete supply of renewable energy for its district heating network by 2040 is based on a massive expansion of geothermal energy – the linchpin of its sustainable urban development.
At the same time, overcoming challenges such as lengthy approval procedures, acceptance issues and geological risks remains a central focus of publicly funded pilot projects and research consortia. The Bundesverband Geothermie (BVG) and the Hanover-based energy provider Enercity, for example, are involved in knowledge transfer, quality assurance and the development of innovative market launch strategies. Project funding – such as that provided in North Rhine-Westphalia by the state Landesgesellschaft für Energie und Klimaschutz NRW.Energy4Climate – aims to accelerate climate-neutral technologies across all sectors.
Geothermal energy, once a niche technology, is increasingly becoming a cornerstone of the energy transition. A sustainable, base-load-capable and regional geothermal heat supply is already a reality in many places today.
