A comparison of electric power output of CO₂ Plume Geothermal (CPG) and brine geothermal systems for varying reservoir conditions

In contrast to conventional hydrothermal systems or enhanced geothermal systems, CO₂ Plume Geothermal (CPG) systems generate electricity by using CO₂ that has been geothermally heated due to sequestration in a sedimentary basin. Four CPG and two brine-based geothermal systems are modeled to estimate their power production for sedimentary basin reservoir depths between 1 and 5km, geothermal temperature gradients from 20 to 50°Ckm^-1, reservoir permeabilities from 1×10^-15 to 1×10^-12m² and well casing inner diameters from 0.14m to 0.41m. Results show that CPG direct-type systems produce more electricity than brine-based geothermal systems at depths between 2 and 3km, and at permeabilities between 10^-14 and 10^-13m², often by a factor of two. This better performance of CPG is due to the low kinematic viscosity of CO₂, relative to brine at those depths, and the strong thermosiphon effect generated by CO₂. When CO₂ is used instead of R245fa as the secondary working fluid in an organic Rankine cycle (ORC), the power production of both the CPG and the brine-reservoir system increases substantially; for example, by 22% and 20% for subsurface brine and CO₂ systems, respectively, with a 35°Ckm^-1 thermal gradient, 0.27m production and 0.41m injection well diameters, and 5×10^-14m² reservoir permeability.