Carbon dioxide as geothermal working fluid: An overview

Carbon dioxide (CO2) has been proposed to be used as the geothermal working fluid, as it presents enhanced thermodynamic properties for this application when compared with the water-based system. This paper aims to present an overview of the recent research advances on CO2-based geothermal systems, particularly CO2-enhanced systems and CO2-plume geothermal energy technology. The results of the recent operational projects are also described. This emerging technology can improve the efficiency of geothermal systems and their environmental impact, promoting the process sustainability and helping to tackle some of the most important issues that Humanity is facing: (i) global climate change; (ii) energy availability; and (iii) water scarcity. For deployment of this technology at a large scale, some issues and uncertainties were identified. Special attention was focused on the basic findings achieved in operational projects.