System thermodynamic performance comparison of CO2-EGS and water-EGS systems

CO2 may be a better heat transmission fluid than water for Enhanced Geothermal Systems (EGS). The advantages and disadvantages of these two kinds of EGS are the focus of this study. The water and CO2-EGS system models including simple subsurface heat transfer and flow models and a surface energy conversion system model were designed based on the reservoir grade and the ambient temperatures. The results indicate that the operating parameters including the injection pressure, turbine outlet pressure and reservoir stimulated area should be optimized to match the actual CO2-EGS conditions. CO2-EGS produce more power than water-EGS for reservoirs with low recoverable thermal energies due to less irreversible losses compared to ORC or flash cycles for water-EGS. However, high resistance losses caused by high mass flow rates degrade the CO2-EGS performance; thus, the water-EGS has better performance than CO2-EGS for larger energy content reservoirs.