If We Can Bury Carbon Dioxide, Why Not Use It to Make Electricity?

Researchers might have found a way to economically capture carbon dioxide from power plants and store it underground. The idea is to turn carbon dioxide storage sites into geothermal power plants.

If it works, the technology would provide both the electricity needed to pump carbon dioxide underground and a source of revenue to offset the high cost of capturing carbon dioxide at power plants, compressing it, and shipping it to storage sites.

That technology, known as carbon capture and storage, or CCS, will be essential for reducing greenhouse gas emissions. But because large-scale CCS would be prohibitively expensive, development of the technology has been too slow to meet climate change targets, according to the International Energy Agency (see “Will Carbon Capture Be Ready On Time?”).

Next year, startups and researchers will begin testing whether it could be possible to defray those costs by putting the stored CO2 to use in a geothermal power plant.

In conventional geothermal plants, water and steam heated by hot rocks deep underground drive turbines in a power plant. The water is then pumped back underground to be heated up again.

The new technology would use carbon dioxide instead of water. This approach has several potential advantages. By eliminating the need for water, it increases the prospects for geothermal projects in dry areas. And computer simulations show that CO2 could produce twice the electricity from a given area that water produces, says Martin Saar, a professor of geology and geophysics at the University of Minnesota. Saar is cofounder of Heat Mining, a company that plans to test this technology in a small power plant that it will build next year.

CO2 would generate more electricity than water would in a geothermal plant largely because carbon dioxide can move much more quickly through porous rock than water can. What’s more, as it heats up, it has a much stronger tendency to rise toward the surface than water does. As a result, it might be possible to eliminate the pumps that consume large amounts of power in conventional geothermal plants. The gas turbines that would be used are also more efficient than the steam turbines found in many geothermal plants.

These factors could make it possible to generate power profitably even in areas that have been considered impractical for geothermal plants because the underground rocks weren’t believed to be hot enough.

So far, however, these advantages haven’t been demonstrated. They’re based on computer simulations, simple lab tests, and data collected from oil wells where carbon dioxide is pumped into wells to stimulate oil production.

One worry is that as the carbon dioxide rises, it will suck up water along with it, which could cause problems for a power plant. “The technology may sound great, but is it just possible in one location? In 10 percent of the U.S.?” Saar says. “We’ll figure this out in the next couple of years.”

Saar’s company plans to test the power-generating potential of carbon dioxide at an oil well in Canada where CO2 is already pumped underground to help force more oil out of the ground. The company plans to start construction on a small, five-megawatt geothermal power plant at the site next year.

Also next year, researchers at Lawrence Berkeley National Laboratory plan to demonstrate the ability to generate hot carbon dioxide at a CCS site where an injection well and a source of carbon dioxide already exist. That project won’t generate electricity. Instead, it’s designed to answer some remaining questions about how well the system will work in practice.