On the importance of the thermosiphon effect in CPG (CO₂ plume geothermal) power systems

CPG (CO₂ Plume Geothermal) energy systems use CO₂ to extract thermal energy from naturally permeable geologic formations at depth. CO₂ has advantages over brine: high mobility, low solubility of amorphous silica, and higher density sensitivity to temperature. The density of CO₂ changes substantially between geothermal reservoir and surface plant, resulting in a buoyancy-driven convective current - a thermosiphon - that reduces or eliminates pumping requirements. We estimated and compared the strength of this thermosiphon for CO₂ and for 20 weight percent NaCl brine for reservoir depths up to 5km and geothermal gradients of 20, 35, and 50°C/km. We found that through the reservoir, CO₂ has a pressure drop approximately 3-12 times less than brine at the same mass flowrate, making the CO₂ thermosiphon sufficient to produce power using reservoirs as shallow as 0.5 km. At 2.5 km depth with a 35°C/km gradient - the approximate western U.S. continental mean - the CO₂ thermosiphon converted approximately 10% of the energy extracted from the reservoir to fluid circulation, compared to less than 1% with brine, where additional mechanical pumping is necessary. We found CO₂ is a particularly advantageous working fluid at depths between 0.5 km and 3 km.