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Engineers develop geothermal plant that sequesters CO2 while generating more energy
Umair Irfan, E&E reporter
Published: Wednesday, December 18, 2013

Besides the wind and sun above, there is tremendous renewable energy potential in the ground beneath your feet.
Tapping into the Earth's heat to generate electricity is an appealing path to low-carbon energy, since it provides a strong and steady output, but it has been limited to areas with hot pockets of underground steam like Southern California and Nevada. According to the U.S. Energy Information Administration, geothermal energy provided more than 16 million megawatt-hours of electricity in in the United States in 2012.

Last week, at the meeting of the American Geophysical Union, scientists presented a new design for geothermal power plants that could increase their output tenfold and make them practical in more areas of the country, all while storing energy and containing carbon dioxide at the same time.

"What we're really trying to do is make the carbon dioxide sequestration approach more economically feasible and
expand renewable energy use," said Jimmy Randolph, a postdoctoral researcher at the University of Minnesota.

Typical geothermal installations require wells drilled as far as 4 miles deep into pockets of steam 300 degrees
Fahrenheit or hotter. The steam rises up to provide useful heat or to spin a turbine to generate electrons.

The new design uses wells drilled to shallower depths in concentric circles, pumping in a blend of carbon dioxide and
nitrogen gas to raise the pressure inside the well and increase energy yields.

The approach draws on many existing technologies but applies them in a unique way. "I think what really has enabled
this more are the advancements in the oil industry for horizontal drilling," said Randolph, who is also chief technical
officer at Heat Mining Co. LLC, a firm commercializing a version of this technology.

Carbon dioxide, under high temperatures and pressures, forms a supercritical fluid that behaves like a gas and a liquid.
This fluid is much better at extracting heat radiating from the Earth's core than water, so a given well can produce more
power, even at cooler temperatures, Randolph said.

Horizontal drilling is key

"In geothermal systems, most of the investment is underground in the wells," said Thomas Buscheck, an earth scientist
at Lawrence Livermore National Laboratory. "In order to make it competitive, we need to get more productive with each

Using horizontally drilled injection and collection wells spread over a wide area, engineers can harness geothermal
energy in more parts of the country without drilling so far underground, circulating carbon dioxide in a closed loop.
Buscheck noted that by blending in nitrogen with carbon dioxide, the fluid could also thoroughly extract and mobilize water
already in the well. That water, in the form of steam, can generate additional energy, and when it condenses, plant
operators can use it for cooling.

Developers can also safely test a geothermal site for potential leaks using only nitrogen, an inert gas, and then later
blend in carbon dioxide. When CO2 trickles out from underground formations, it displaces oxygen and can suffocate
animals and humans at ground level in addition to its heat-trapping effects in the upper atmosphere.

"The No. 1 thing is to make sure we have a site that can safely store carbon dioxide without leakage," Buscheck said.

The concentric ring design of injectors and collectors lets engineers gain precise control over the underground system,
limiting pressure overruns and reducing the risk of creating tremors, he observed. Supercritical carbon dioxide as a
working fluid also allows power plants to use smaller, more efficient turbines to generate electricity. This also means
there are fewer corrosive compounds circulating through mechanical systems, a problem that often emerges with other
types of geothermal plants.

Producing and storing energy
In addition, this new approach to geothermal energy can complement intermittent renewable energy sources like solar
and wind power. "The nice thing is you can store the energy quite well," said Jeffrey Bielicki, an assistant professor of
energy policy at Ohio State University. "They can provide baseload as well as dispatchable power."

When there is excess electricity on the grid or when power is cheap, the geothermal plant can pump carbon dioxide
underground. Since it's a geothermal formation, the carbon dioxide heats up and returns more energy than it took to store
it, giving this process an advantage over compressed air energy storage systems that are often net energy losers.

Though horizontal wells are more expensive to drill than vertical wells, the concentric ring design needs fewer wells total,
while it substantially increases output. The average conventional geothermal plant in the United States hovers around 38
megawatts in capacity. "Our initial simulations show we can get quite large power plants, on the order of a few hundred
megawatts," Bielicki said.

However, researchers are still years away from a demonstration plant. Heat Mining Co. is starting by deploying carbon
dioxide-based geothermal energy in the oil and gas sector, piggybacking on existing infrastructure. When an oil field is
drained, drillers can still extract substantial amounts of sticky, viscous crude by injecting steam and carbon dioxide in an
aging well, in a process called enhanced oil recovery, or EOR.

In an EOR installation, up to half of the operating costs come from electricity, which often has to cross long distances to
remote oil fields, said Kenneth Carpenter, managing partner at Heat Mining Co. With a supercritical carbon dioxide
geothermal system, drillers can generate energy on-site as they need it.

"Wyoming is a perfect example," he said, noting that oil companies may drill upward of 15,000 wells in the state's Powder
River Basin over the next decade. "There's not power there, because there are no people there."
"This electricity is competitive to natural gas or coal-fired prices and is completely green and doesn't require any
government subsidy," Carpenter added. The energy input can take a big chunk out of the cost of oil per barrel, as much
as $10 to $15.

The power system is modular and can fit on two flatbed trucks, making it very appealing for remote well sites. The
company is about to begin a test project in Alberta, Canada. Carpenter said that if a carbon price goes into effect, this
technology will become much more viable as a stand-alone energy source and as a complement to fossil fuels,
effectively recycling carbon emissions from coal and natural gas power plants.

The researchers said they are now constructing an atlas of geothermal potential for the United States and studying the
business case for concentric ring geothermal energy under different economic and policy circumstances. "I think the
overall message is we can produce and store energy using geothermal resources if we think a little bit differently about
how we go about it," Bielicki said.

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