Insights into the underground

Researchers at TU Darmstadt are evaluating core samples taken at a depth of five kilometres for a geothermal project

2021/01/05 by

A drilling rig at the United Downs Deep Geothermal Project drilled a borehole to a depth of 4900 metres into the granite beneath Cornwall a few months ago. The 19 core samples that were extracted – the deepest that have ever been taken on the mainland in Great Britain – are now at TU Darmstadt. Researchers at the Institute of Applied Geosciences are investigating them to discover how sustainable and efficient geothermal energy can be produced in the project in Cornwall.

The United Downs Deep Geothermal Project (UDDGP) that is mainly funded by the EU is currently being developed in Cornwall and is supposed to become the first geothermal power plant in Great Britain. The conditions are very favourable because the granite rock beneath Cornwall produces a lot of heat due to the natural, radioactive decay of uranium, thorium and potassium present in all rocks. However, the subsurface needs to be sufficiently permeable in order to extract the geothermal energy. This is the case when it has lots of fractures or fissures. It is only then that cool water can be fed in via an injection well, circulate through the rock heating up to 180 to 200 degrees Celsius before it is pumped via a production well back to the surface to drive turbines and generators. “We were contacted as soon as it became clear that drilling would take place there”, explains Dr. Kristian Bär from the Geothermal Science and Technology Group (headed by Prof. Dr. Ingo Sass) in the Department of Materials and Earth Sciences at TU Darmstadt, who heads the project at TU Darmstadt and supervised the extraction of the core samples. His fields of research include deep geothermal and petrophysical reservoir characterisation and geothermal feasibility and potential studies.

Since 2018, the team in Darmstadt have been carrying out research as part of the EU MEET project to determine whether the geothermal heat reservoirs in the bedrock of Europe can be utilised for geothermal energy and how geothermal energy can make a contribution across Europe to sustainable energy provision. Their expertise is now also being applied in the UDDGP project, which has become a demonstrator project as part of MEET. The researchers are closely accompanying the project from a scientific and technical perspective and providing advice on its execution.

Hydrothermal alterations

The core samples that were taken in Cornwall at depths of between 4200 and 4900 metres are now being carefully examined in Darmstadt with respect to heat production, their mechanical properties and mineralogical and geochemical composition, as well as to determine how geothermal processes in the past have changed the rock and its properties. “These so-called ‘hydrothermal alterations’ indicate that hot water was once circulating in these zones in the past”, explains Bär. “And these fracture networks are possibly particularly suitable for once again circulating water for the purpose of generating geothermal energy.”

The contribution being made by TU Darmstadt to the UDDGP project is to find out how to circulate a sufficient volume of water – around 50 to 70 litres per second – sustainably through the geothermal reservoir deep below the ground. The core samples also reveal where fractures have formed in the rock but are possibly not yet permeable enough. The researchers can draw conclusions from the behaviour of the granite about whether and how the fractures can be reopened and how the flow of water can be increased enough to operate the geothermal power plant. “We are responsible for helping the operators design the project in a sustainable, efficient and environmentally friendly way”, says Bär.

The next steps in the development of the power plant will be taken in Cornwall in a few weeks’ time to improve the circulation of water in the reservoir. If everything goes according to plan, the power plant will soon be able to feed at least 1.5 megawatts of electricity to the national grid and provide the region with around 20 megawatts of geothermal heating energy.


The MEET project (Multidisciplinary and multi-context demonstration of Enhanced Geothermal Systems exploration and Exploitation Techniques and potentials) is receiving funding of around 10 million euros from the EU Horizon 2020 programme. TU Darmstadt receives almost 2.06 million euros of this funding. 16 partners from five countries are working together in the MEET project, which manages ten demonstrator projects in Europe.