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About a year from now, two nondescript shipping containers will be installed
in a field in Cranfield, Mississippi. They’ll house turbines designed
to generate electricity in a way that’s 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 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 carbon dioxide. They also want to permanently store some
of the carbon dioxide underground, where it can’t 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 carbon
dioxide 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 carbon dioxide 3 km underground into a
sedimentary layer that’s 125 C. Carbon dioxide enters a supercritical state
under these conditions, meaning it has both liquid and gas properties.
The carbon dioxide 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 carbon dioxide 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 carbon
dioxide from large stationary sources and pumping it deep
underground. This enables the burning of fossil fuels without releasing the
greenhouse gas into the atmosphere. But it’s expensive.
“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
carbon dioxide can be used instead of water. Their work has shown
that supercritical carbon dioxide is better than water at mining heat from the
subsurface. But no one 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 carbon dioxide 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 carbon dioxide courses through
it. They’ll also determine how much energy can be extracted from the carbon
dioxide 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’ll operate it during a pilot test at the Southeast Regional Carbon
Sequestration Partnership’s Cranfield site, where a Department of Energy-funded
carbon dioxide injection project has been underway since 2009. The
site’s 3 km 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 carbon dioxide will
come from a pipeline operated by Texas-based Denbury Resources.
It’s too early to tell how much electricity the technology can generate in
the U.S.
That depends on the scale of carbon capture and storage operations and the
availability of deep reservoirs that can both heat and store carbon dioxide.
The technology also takes advantage of a problem common to conventional
geothermal energy. Between five and ten 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 carbon dioxide to become
trapped,” says Freifeld. “Our approach relies on this gradual loss as a way to
store a power plant’s carbon dioxide 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.”
