We might harvest the Earth's heat while trapping carbon emissions

ORNL

Geothermal power has long been touted as a valuable piece of a renewable energy portfolio, but, like other energy technologies, it has its limitations. At geothermal power plants, water is injected into warm regions of the crust and the steam that returns to the surface is used to drive a generator. As such, it requires areas with high temperatures at shallow depths (less than 3 kilometers)—that is, places where magma exists relatively close to the surface. That's not a problem for places like Iceland, but it excludes the vast majority of the United States.

In addition, geothermal power plants normally require hydraulic fracturing of the bedrock in order to create the necessary flow paths for water, which means that the amount of rock available to exchange heat with the water is limited by the extent of the fractures. On top of that, the injection of water at high pressure into those fractures has been known to trigger seismic activity, leading to controversy at some sites.

A recent paper in Geophysical Research Letters offers a creative alternative which piggybacks geothermal energy production on carbon capture and storage to boost the value of both, taking a shot at two birds with one precisely engineered stone.

The researchers used a computer model to evaluate the potential of CO₂ as a heat exchange fluid for geothermal energy generation. This in itself is not a new idea, but they put a pragmatic twist on it: instead of simply replacing water with CO₂ in a traditional hydraulically fractured rock system, they propose using it in lower-temperature environments at carbon capture and storage sites.

One approach to carbon capture and storage is to inject CO₂ (in a liquid-like supercritical state) into deep aquifers that are capped by an impermeable layer of rock, safely locking it away. These aquifers are too deep to be economically viable sources of drinking water, and the water is too "salty" (due to high concentrations of dissolved minerals) to be desirable anyway. Rather than being a network of fractures, these aquifers are highly porous and permeable—composed of rocks like sandstone, for example—and can easily accept and hold large volumes of fluid. Fluids injected into these layers have a much larger surface area to interact with than hydraulically fractured systems do, and there's no risk of triggering earthquakes.

Once the CO₂ is injected into the aquifer and allowed to pick up heat, some would be pumped back up to drive a generator before being returned. All the CO₂ would still end up in the storage aquifer, but some would be used to extract heat energy along the way.

This design turns out to be pretty advantageous. For starters, CO₂ can effectively transfer heat at much lower temperatures than water. Plus, their design can extract heat 3 to 5 times faster than traditional water-based systems. The researchers calculate that the amount of energy generated using water in an aquifer at 100°C (which is widely considered the lower limit for current technology) can be generated using CO₂ at about 66°C. This ability to work efficiently at lower temperatures opens up a large area of the US to geothermal energy production.

Carbon capture and storage takes a fair amount of energy, so at a power plant, for example, extra fuel has to be burned to power the process. The end result is a loss of efficiency at the plant. In this coupled design, the energy produced from the geothermal system helps offset the fuel cost of carbon capture and storage. Or you can look at it the other way around—once a price is put on carbon emissions, the capture and storage side of the design will generate money, which will make the price of geothermal energy even more economically competitive.

Of course, carbon capture and storage is not a mature technology, and it still has considerable baggage in the form of debate about unknowns: How completely will the CO₂ be sealed underground—will some escape? How will the CO₂ fluid react with minerals in the rock? Where will the displaced “salty” water in the aquifer move to? Could drinking water aquifers be affected in any way? We'll need better answers to these questions before this technique can be widely implemented, but coupling it with geothermal energy production may move it one step towards practicality.