Energy futuresStoring CO2 underground to extract electricity

On 23 June the U.S. Department of Energy announced it was awarding more than $11 million to advance innovative geothermal energy technologies. Berkeley Lab has something to say when it comes to geothermal energy.

About a year from now, two nondescript shipping containers will be installed in a field in Cranfield, Mississippi. They will house turbines designed to generate electricity in a way that has never been done before. If initial tests go well, the technology could lead to a new source of clean, domestic energy and a new way to fight climate change.

A Berkeley Lab release reports that a team led by Lawrence Berkeley National Laboratory (Berkeley Lab) scientists hopes to become the first in the world to produce electricity from the Earth’s heat using CO2. They also want permanently to store some of the CO2 underground, where it can not contribute to climate change.

The group received $5 million from the Department of Energy earlier this summer to design and test the technology.

“This is the first project intended to convert geothermally heated CO2 into useful electricity,” says Barry Freifeld, a mechanical engineer in Berkeley Lab’s Earth Sciences Division who leads the effort.

The idea is to inject CO2 three kilometers underground into a sedimentary layer that is 125 degrees Celsius. CO2 enters a supercritical state under these conditions, meaning it has both liquid and gas properties.

The CO2 will then be pulled to the surface and fed into a turbine that converts heat into electricity. Next, it will loop back underground and through the cycle again. Over time, some of it will be permanently trapped in the sediment. More CO2 will be continuously added to the system to keep the turbines spinning.

The technology could help offset the cost of geologic carbon storage, a promising climate change mitigation strategy that involves capturing CO2 from large stationary sources and pumping it deep underground. This enables the burning of fossil fuels without releasing the greenhouse gas into the atmosphere. It is expensive, though.

“Carbon storage takes a lot of power — large pumps and compressors are needed. We may be able to bring down its costs by generating electricity on the side,” says Freifeld.

It also offers a new way to tap geothermal energy, which is a tough sell in arid regions where every drop of water is spoken for. For more than a decade, scientists at Berkeley Lab and elsewhere have theorized that supercritical CO2 can be used instead of water. Their work has shown that supercritical CO2 is better than water at mining heat from the subsurface. No one, however, has tried to do it until now.

In the project’s first stage, Ohio-based Echogen Power Systems will design a turbine that can handle “dirty” supercritical CO2 laden with hydrocarbons and water accrued during its subsurface journey. Scientists from the University of Texas at Austin will analyze the environmental impacts of the process over its entire life span.

Berkeley Lab scientists will use numerical models to predict how the reservoir will evolve over time as more and more CO2 courses through it. They will also determine how much energy can be extracted from the CO2 by coupling reservoir models with Echogen’s turbine models.

In the second stage, the team will build and test the turbine. If that goes well, they will operate it during a pilot test at the Southeast Regional Carbon Sequestration Partnership’s Cranfield site, where a Department of Energy-funded CO2 injection project has been underway since 2009. The site’s 3-kilometer deep reservoir has proven to be an ideal site for carbon sequestration. Much of the infrastructure needed for the test is already in place, including injection and production wells. The CO2 will come from a pipeline operated by Texas-based Denbury Resources.

The release notes that it is too early to tell how much electricity the technology can generate in the United States. That depends on the scale of carbon capture and storage operations and the availability of deep reservoirs that can both heat and store CO2.

The technology also takes advantage of a problem common to conventional geothermal energy. Between 5 and 10 percent of the water injected in these systems is “lost” as it travels through the pore spaces. As this happens, more water must be added, perhaps from municipal sources that have little to spare.

“But we actually want some of the CO2 to become trapped,” says Freifeld. “Our approach relies on this gradual loss as a way to store a power plant’s CO2 underground rather than emitting it into the atmosphere. Our planned demonstration is the first attempt at proving that we can simultaneously mitigate greenhouse gas induced climate change and generate clean baseload power using geothermal energy.”