New Geothermal Technology Uses CO2, Not Water

A new geothermal company has a technology that makes carbon capture more cost-effective because it produces revenue-generating electricity from the sequestered carbon.

Heat Mining Company LLC is commercializing technology invented at the University of Minnesota, "CO2 plume geothermal." Rather than using water to extract heat from deep underground to produce geothermal energy, it uses carbon dioxide emitted and sequestered from fossil fuel plants.

That opens many more locations to geothermal, including being used as a back-up for wind or solar projects.

"This technology has the potential to introduce a new era of electrical power production from renewable wind, solar, and geothermal energy as well as from traditional fossil fuels, while significantly reducing emissions of carbon dioxide to the atmosphere," says Martin Saar, co-inventor and earth sciences professor in the university’s College of Science and Engineering.
The University of Minnesota submitted the technology for patents in March 2009 and licensed it exclusively, worldwide to Heat Mining Company LLC through the Office for Technology Commercialization.

Hooked up to a coal plant, for example, carbon-based geothermal can power CO2 injection pumps while generating revenue from electricity sales. That can offset the very high costs of implementing carbon capture technology.

Here’s how it would work:

A geothermal plant would pump CO2 down an injection well into a salty aquifer 2-5 kilometers below the surface, where tremendous pressure and temperatures of 70-200 degrees C would transform the gas first into a liquid and eventually into a "supercritical" state. "It has liquid-like density and gas-like viscosity," says Saar. "It’s not really anything we’re familiar with in the everyday world, but most materials do this at sufficiently high pressures and temperatures."

The supercritical CO2 would flow through porous bedrock more easily than water. Becoming far less dense than water as it warms, the CO2 would rise quickly through the brine-soaked bedrock and pool beneath a virtually impermeable caprock, such as shale. The now hot, low-density fluid would buoyantly rise through a production well without pumping.

At the surface, the CO2 would drive a turbine – more efficiently and vigorously than water drives conventional steam -generation turbines. After cooling, the CO2 would be pumped back down the injection well, flowing in a closed, geothermal heat self-powered thermosyphon loop that would let none escape to the atmosphere.

The geothermally generated power could help run the CO2 injection pumps that provide the initial CO2 captured from the CO2 emitter. In addition, revenue from any additional power generation could help defray the cost of carbon capture and sequestration.

Oil companies routinely inject CO2 deep into oil- or other hydrocarbon-bearing formations to force out remaining hydrocarbons in so-called enhanced oil recovery operations. 

CO2 injection presents little risk of earthquakes or serious ground deformation, says Saar. "The oil industry has been doing it for decades. Adding geothermal to it should actually reduce seismic risks, because what happens when you tap the heat out of it, you reduce the pressure."

The CPG method has been demonstrated in computer simulations and details have been investigated in laboratory experiments. The next step is to build a pilot plant to test it in the field.

Geothermal is far behind solar and wind – it’s growing, but pretty slowly.