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University of Coimbra project aims to verify if it is feasible to use CO₂ in the production of geothermal energy

A team of scientists from the University of Coimbra (UC) has developed equipment that, for the first time, allows testing the feasibility of using carbon dioxide (CO₂), one of the main greenhouse gases, in the extraction of geothermal energy, clean energy generated through the heat of the earth.

In theory, it is possible to use carbon dioxide in a supercritical state to extract geothermal energy, but until now this possibility has never been tested, that is, there is no experimental information that explains what happens to CO₂ from the moment it enters the rocks. The supercritical state is characterized by the ability of some fluids, such as CO₂, to simultaneously present liquid and gaseous properties when exposed to pressure and temperature above those of their critical state.

The main objective of the project “KIDIMIX – Molecular Diffusion and Thermal Diffusion of CO₂ in model mixtures close to the critical point”, which has the collaboration of the Free University of Brussels, Belgium, «is to study the feasibility of using CO₂ captured from the atmosphere to produce geothermal energy. The injection of carbon dioxide into underground rock formations for geological storage can provide benefits that go beyond reducing its concentration in the atmosphere. The temperature and pressure at the depth at which geological storage takes place carbon dioxide in a supercritical state, which makes it a great candidate for the extraction of geothermal energy», explains Cecília Santos, who coordinates the study with Ana Ribeiro. Both are researchers at the Chemistry Center of the Faculty of Science and Technology of the University of Coimbra (FCTUC).

However, he explains, from an experimental point of view, «the behavior of CO₂ from the moment it is stored in geological formations is unknown, and this is predicted based on theoretical models. Knowing it is particularly important to take advantage of the thermophysical properties of this gas in relation to those of water, the fluid currently used for the extraction of geothermal energy. For example, the lower viscosity and higher coefficient of thermal expansion of CO₂ allow for more efficient heat exchange, which is very important for extracting energy from the earth.»

Considering that there are already established technologies to sequester CO₂ from the atmosphere, «if we demonstrate that extracting geothermal energy with this gas is safe and financially viable, as carbon storage technologies are very expensive when used alone, it would be excellent news to help in the fight against climate change and contribute to decarbonization», emphasizes Cecília Santos.

The experimental equipment developed within the scope of the project makes it possible to carry out various types of measurements, including the diffusion of carbon dioxide with components that would naturally be inside the geological formations, such as hydrocarbons. These measurements, says the FCTUC researcher, are essential to «characterize supercritical mixtures and obtain accurate data on the transport properties of these mixtures. We are studying molecular diffusion and thermal diffusion of mixtures of various components, together with their theoretical modelling, in order to deepen our understanding of the state and properties of carbon dioxide under supercritical conditions».

If laboratory experiments confirm that CO₂ can effectively be used in the extraction of geothermal energy, in addition to representing a big step towards the decarbonization goals recommended by the European Union (EU) and a precious help in the fight against climate change, the results may also be useful for application «in other types of industry. This study can contribute to the development of safer carbon dioxide capture and storage technologies, allowing a better assessment of risk and effectiveness. The development of these technologies combined with the generation of gas or renewable energy can increase the country’s (and the EU’s) competitiveness and stimulate economic growth”, she adds.

At this stage of the study, the team is focused on understanding the behaviour of CO₂ inside the geological reservoir. In the second phase, after understanding all the mechanics involved in the process, it will be possible to develop a model that makes the process viable, especially from an economic point of view. «The idea is, in the future, to use all this information in an industrial plant in a real environment», concludes Cecília Santos.

The KIDIMIX project started in 2018 and is financed, in 200 thousand euros, by community funds, through the COMPETE 2020 program, and by the Foundation for Science and Technology (FCT).

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