Martin Saar, an Earth sciences faculty member, and graduate student Jimmy Randolph have devised an ingenious “two-for-one” strategy to simultaneously produce renewable energy and reduce the presence of harmful carbon dioxide in the atmosphere. |
Two Univ. of Minnesota
Department of Earth Sciences researchers have developed an innovative approach
to tapping heat beneath the Earth’s surface. The method is expected to not only
produce renewable electricity far more efficiently than conventional geothermal
systems, but also help reduce atmospheric carbon dioxide (CO2)—dealing
a one-two punch against climate change.
The approach, termed CO2-plume
geothermal system, or CPG, was developed by Earth sciences faculty member
Martin Saar and graduate student Jimmy Randolph in the university’s College of Science and Engineering. The research
was published in Geophysical Research
Letters. The researchers have applied for a patent and plan to form a
start-up company to commercialize the new technology.
Established methods for
transforming Earth’s heat into electricity involve extracting hot water from
rock formations several hundred feet from the Earth’s surface at the few
natural hot spots around the world, then using the hot water to turn
power-producing turbines. The university’s novel system was born in a flash of
insight on a northern Minnesota
road trip and jump-started with $600,000 in funding from the U of M Institute
on the Environment’s Initiative for Renewable Energy and the Environment
(IREE). The CPG system uses high-pressure CO2 instead of water as
the underground heat-carrying fluid.
CPG provides a number of
advantages over other geothermal systems, Randolph
said. First, CO2 travels more easily than water through porous rock,
so it can extract heat more readily. As a result, CPG can be used in regions
where conventional geothermal electricity production would not make sense from
a technical or economic standpoint.
“This is probably viable
in areas you couldn’t even think about doing regular geothermal for electricity
production,” Randolph
said. “In areas where you could, it’s perhaps twice as efficient.”
CPG also offers the
benefit of preventing CO2 from reaching the atmosphere by
sequestering it deep underground, where it cannot contribute to climate change.
In addition, because pure CO2 is less likely than water to dissolve
the material around it, CPG reduces the risk of a geothermal system not being
able to operate for long times due to “short-circuiting” or plugging the flow
of fluid through the hot rocks. Moreover, the technology could be used in
parallel to boost fossil fuel production by pushing natural gas or oil from
partially depleted reservoirs as CO2 is injected.
Saar and Randolph
first hit on the idea behind CPG in the fall of 2008 while driving to northern Minnesota together to
conduct unrelated field research. The two had been conducting research on
geothermal energy capture and separately on geologic CO2
sequestration.
“We connected the dots
and said, ‘Wait a minute—what are the consequences if you use geothermally
heated CO2?'” recalled Saar. “We
had a hunch in the car that there should be lots of advantages to doing that.”
After batting the idea
around a bit, the pair applied for and received a grant from the Initiative for
Renewable Energy and the Environment, which disburses funds from Xcel Energy’s
Renewable Development Fund to help launch potentially transformative projects
in emerging fields of energy and the environment. The IREE grant paid for preliminary
computer modeling and allowed Saar and Randolph
to bring on board energy policy, applied economics and mechanical engineering
experts from the Univ.
of Minnesota as well as
modeling experts from Lawrence Berkeley National Laboratory. It also helped
leverage a $1.5 million grant from the U.S. Department of Energy to explore
subsurface chemical interactions involved in the process.
“The IREE grant was
really critical,” Saar said. “This is the kind
of project that requires a high-risk investment. I think it’s fair to say that
there’s a good chance that it wouldn’t have gone anywhere without IREE support
in the early days.”
Saar and Randolph have recently
applied for additional DOE funding to move CPG forward to the pilot phase.
“Part of the beauty of
this is that it combines a lot of ideas but the ideas are essentially
technically proven, so we don’t need a lot of new technology developed,” Randolph said.
“It’s combining proven
technology in a new way,” Saar said. “It’s one
of those things where you know how the individual components work. The question
is, how will they perform together in this new way? The simulation results
suggest it’s going to be very favorable.”
