Thermodynamic comparison of direct supercritical CO₂ and indirect brine-ORC concepts for geothermal combined heat and power generation

Petrothermal reservoirs contain a tremendous technical potential without major regional restrictions. The application of supercritical CO₂ (sCO₂) as a heat carrier might be a promising alternative to the commonly discussed usage of water. This study evaluates both heat carriers for a CHP application. A novel CHP plant layout for a thermosiphon with direct utilization of sCO₂ for power generation and a pumped brine system with a parallel CHP plant and power generation by an Organic Rankine Cycle (ORC) are compared for a defined mass flow rate of 225 kg/s and reservoir conditions of 180 °C and 49 MPa. For the ORC with R245fa the annual amount of net electricity is 3% higher than for the ORC with R1233zd(E) and 8% higher than for the sCO₂ case. The net power of the sCO₂ plant displays a significantly higher sensitivity to changes of the heat demand compared to the brine-ORC system. While the average exergetic efficiency for the brine concept is 40.9% for R245fa and 40.0% for R1233zd(E), respectively, the sCO₂ concept reaches an average efficiency of 72.6%. Finally, two different operation strategies for the sCO₂ CHP plant are investigated against the background of potential turbine part-load models.