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.
|Original language||English (US)|
|Number of pages||10|
|State||Published - May 1 2014|
Bibliographical noteFunding Information:
We gratefully acknowledge funding from the National Science Foundation under grant number CHE-1230691 and from the George and Orpha Gibson Endowment for the Hydrogeology and Geofluids Research Group in the Department of Earth Sciences at the University of Minnesota (UMN). We would also like to thank the Initiative for Renewable Energy and the Environment (IREE), a signature program of the Institute on the Environment (IonE) at the University of Minnesota for initial seed funding. Any opinions, findings, conclusions, or recommendations in this material are those of the authors and do not necessarily reflect the views of the NSF, UMN, IREE, or IonE. We also gratefully acknowledge helpful comments by three anonymous reviewers and the editor which improved this paper.
- Carbon dioxide
- Carbon dioxide plume
- Carbon dioxide utilization
- Geothermal energy
- Renewable energy