Pumping CO2 Underground to Extract Geothermal Energy

geothermal power plant photo

Photo: Wikipedia, Public domain
It Could Work
We usually hear about pumping CO2 underground in the context of carbon capture and storage (CCS). But there might be another use for that CO2 in the field of geothermal power. It hasn't been proven to work yet, but some money from the Federal stimulus funds has started to flow to 9 projects that want to test this out. The idea is: "Carbon dioxide that's cycled through hot regions kilometers underground can efficiently bring heat to the surface, where it can be used to generate electricity. The likelihood is that the process would leave lots of carbon dioxide underground, and thus out of the atmosphere"Technology Review writes:

The European Union's Soultz-sous-Fôrets project in Alsace, France, the most advanced such project worldwide, has taken 20 years to reach just 1.5 megawatts of power generation (enough to supply roughly 1,500 homes). And the process has antagonized nearby communities because of the small earthquakes sparked by the aggressive fracturing required.

In 2000, Los Alamos National Laboratory physicist Donald Brown proposed replacing water with supercritical carbon dioxide, a pressurized form that is part gas, part liquid. Supercritical CO2 is less viscous than water and thus should flow more freely through rock. Brown noted that a siphoning effect should help cycle the carbon dioxide, thanks to the density difference between the supercritical CO2 pumped down and the hotter gas coming up, slashing power losses from pumping fluid. Plus, Brown argued, instead of using precious fresh water resources, a carbon dioxide-based project could sequester the equivalent of 70 years worth of CO2 emissions from a 500 megawatt coal power plant.

It does sound like it could be better than injecting water, but it still needs to be proven in the field, and sadly, geothermal doesn't move very fast. I used to think that it would scale up rapidly, but the more I learn about it, the more complicated things seem.

There's also always the danger of having people think "Oh, we'll be able to sequester the CO2 at some point, so we can keep burning coal in the meantime." This is the wrong way to think about it because current coal plants cannot capture carbon (or at least, not easily) and they might not be anywhere near the proper types of underground rock formations that would be needed to sequester the CO2 (and pipelines are expensive). Our best bet is still to phase out coal as fast as possible.

Via Technology Review
More Geothermal
Jargon Watch: Geothermal vs Ground Source Heat Pump
Huge Geothermal Power Project Could Cause Quakes
More Green(er) Energy
This is Big: U.S. Could Double Hydropower Capacity With Minimal Impact, Says Steven Chu
Another Benefit of Smart Grids: Fewer Utility Trucks Spewing CO2
The 100-Mile Diet for Electricity? The Institute for Local Self-Reliance Argues for Decentralization

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