Elizabeth A. Thomson Correspondent
Challenges and progress toward the holy grail of geothermal energy—tapping into the superhot rock deep beneath our feet that could help wean the world from fossil fuels—were the focus of two hour-long sessions at PIVOT21, a geothermal conference July 19-23 featuring more than 165 experts in the field from around the world.
According to several of the session panelists, that superhot rock—anything over about 350 degrees Celsius—represents a vast resource, and the energy that could be derived from it has been underestimated.
“The ability to drill to about 20 kilometers [~12 miles] and 500 degrees Celsius [~932 F] could turn geothermal into a terawatt source of energy with the power densities of fossil fuels,” said Carlos Araque, CEO of Quaise Inc. and one of five panelists on a session about advanced drilling techniques that can survive such extreme conditions.
Conventional geothermal plants reach temperatures of about 230 degrees Celsius through holes about two kilometers deep. Superhot rock can be found close to the surface in a few areas like Iceland and near volcanoes, but for most of the world it is between seven and 20 kilometers down. Things get especially interesting over about 374 degrees C. Water pumped to rock that hot would become supercritical, a steam-like phase that most people aren’t familiar with. (Familiar phases are liquid water, ice, and the vapor that makes clouds.) Supercritical water, in turn, can carry some 5-10 times more energy than regular hot water, making it an extremely efficient energy source if it could be pumped above ground to turbines that could convert it into electricity.
Many Challenges
There are many challenges, however, to reaching that superhot rock. Key among them are drilling techniques that can withstand the extreme temperatures and pressures involved, according to panelists in a session titled “In Pursuit of the Holy Grail: Deep and Superhot Geothermal.” The conventional drill bits used in the oil and gas industry fail under those conditions. Fortunately, “I see coming down the pike a number of really high potential…drilling methods,” said Susan Petty, chief technology officer at Cyrq Energy and the president and founder of AltaRock Energy, Inc.
Other important challenges include electronics that can also withstand the extreme conditions; materials for lining and supporting the boreholes that can survive repeated thermal cycling, or large changes in temperature; and the need for more data to characterize subsurface rock conditions.
With respect to thermal cycling, “I think the problem is moving towards being solved,” said Petty, thanks to innovations like self-healing cements that recrystallize to fix any fractures that form. Mark Ireland, a lecturer in energy geoscience at Newcastle University, addressed the need for more data. “We haven’t got that many calibration points once we get into these very deep systems,” he said.
To that end, Ireland and others emphasized the need for collaboration between the groups worldwide that are exploring superhot geothermal. “Open access to the data and models that are underpinning these pilot projects are key,” he said. Then “we can open the lid on the box and explore all the different parameters in it, and compare and contrast how we’re characterizing the potential resource. The more we’re able to share, the better our decision making.”
Other panelists in the session were moderator Steinar Örn Jónsson, the legal and policy advisor of GEORG, an international geothermal research cluster based in Iceland; Gioia Falcone, Rankine Chair of Energy Engineering at the University of Glasgow; and Carsten Sørlie, project leader for Equinor’s technology initiative on geothermal energy.