Use of Supercritical CO2 Heated With Geothermal Energy for Power Production Through Direct Expansion and Heat Supply to an ORC Cycle

Sustainable energy supply and environmental impact caused by burning of fossil fuels are two of the major worldwide concerns. Regarding sustainable energy supply, several non-conventional alternatives and technologies are currently under study to be implemented from short- to mid-term. On the other hand, CO₂ capture and sequestration (CCS) has been considered as a means for controlling anthropogenic greenhouse gas. Based on these considerations, CCS along with non-conventional heat mining and use of geothermal energy is one of the mid-term concepts that is subject to an in-depth research. One of those research lines involves injection of supercritical carbon dioxide (sCO₂) in a naturally high-permeability hydrothermal reservoir in order to mine geothermal energy for subsequent power production. This requires to understand widely the behavior of sCO₂ inside the geothermal reservoir, as well as to study feasible options for sustainable power production using sCO₂ as a working fluid. The present work refers to CO₂ captured from a fossil fuel plant that is compressed and injected into a geothermal reservoir forming a geothermal plume for extracting geothermal energy for power production. Assuming that hot sCO₂ is available for power production, a power system is proposed and analyzed using a process simulation software. The plant configuration allows sCO₂ to circulate through a direct expansion turbine for production of electricity, then sCO₂ at the outlet of the turbine passes through a heat exchanger to provide heat to an organic Rankine cycle for additional power production. Supercritical CO₂ leaving the heat exchanger is compressed and reinjected back into the geothermal reservoir for further heat mining. Based on a pre-selection stage, Isobutane, R245fa and CO₂ were selected as working fluids to analyze the performance of these working fluids on the entire power system. The results show that the combined system makes use of geothermal heat mined by sCO₂ more effectively than a pure ORC, increasing the overall energy efficiency of the system.