Thursday, August 17th, 2017

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

Publication date: Available online 19 April 2014
Author(s): Benjamin M. Adams , Thomas H. Kuehn , Jeffrey M. Bielicki , Jimmy B. Randolph , Martin O. Saar
CPG (CO 2 Plume Geothermal) energy systems use CO 2 to extract thermal energy from naturally permeable geologic formations at depth. CO 2 has advantages over brine: high mobility, low solubility of amorphous silica, and higher density sensitivity to temperature. The density of CO 2 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 CO 2 and for 20 weight percent NaCl brine for reservoir depths up to 5 km and geothermal gradients of 20, 35, and 50 °C/km. We found that through the reservoir, CO 2 has a pressure drop approximately 3–12 times less than brine at the same mass flowrate, making the CO 2 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 CO 2 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 CO 2 is a particularly advantageous working fluid at depths between 0.5 km and 3 km.

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