Prospecting for Power
Helium isotopes reveal hidden stores of geothermal energy.
Most geothermal power plants exploit the relatively rare but easy to spot hot water associated with volcanoes, limiting geothermal energy to a niche role in meeting global energy demand. It works well in Iceland and a few other places, but geothermal energy is a largely untapped resource in much of the world, in part because, in the absence of a volcano or hot springs, it’s hard to find the right spot to tap into the resource. Last week, a pair of geochemists published a report in Science showing that the ultrasensitive detection of traces of helium at the surface using mass spectrometers may hold the key to sniffing out the best sites of this hidden heat.
Mack Kennedy, a senior scientist at Lawrence Berkeley National Laboratory, in Berkeley, CA, and coauthor Matthijs van Soest, an associate research professional at Arizona State University’s School of Earth and Space Exploration, in Tempe, measured the levels of helium isotopes to predict areas where the rock is permeable deep underground. Water is more likely to circulate rapidly through such regions, providing the circulation of heat emanating from the earth’s mantle or generated in its crust by radioactive decay. “You have a huge resource, and now I can tell you where there’s good permeability,” says Kennedy. “Those are places to go look for a natural geothermal system right off the bat.”
The findings could be an important step in efforts to unlock the vast potential of geothermal energy, in which heated water produces steam to drive power-generating turbines. According to a recent expert panel analysis for the Department of Energy (DOE), led by researchers at MIT and Southern Methodist University, in Texas, geothermal systems engineered to exploit hot rocks could be meeting 10 percent of U.S. electricity demand within 50 years. Currently, geothermal systems supply less than 1 percent of that demand. One challenge to further exploiting geothermal energy has been pinpointing exactly where to look for rocks with the right combination of permeability and heat. And that’s where the new study could help.
Looking for elevated levels of bulk helium in soil is a long-standing practice in geothermal exploration. Helium produced in the earth’s crust from the radioactive decay of uranium and thorium tends to migrate upward and eventually finds its way into the atmosphere. Elevated helium levels can indicate where the earth’s crust is highly permeable. In their Science paper, Kennedy and van Soest show for the first time that levels of helium isotopes can identify areas where the permeability reaches all the way to the earth’s superheated mantle, even in areas where no lava is flowing up.
The geochemists examined the ratio of helium-4 (the garden-variety helium that lifts birthday balloons) and its rarefied cousin, helium-3. The earth’s crust contains, on average, just one helium-3 atom for every 100 million atoms of helium-4. But helium-3 is a thousand times more common in the earth’s mantle. Kennedy and van Soest found elevated helium-3 in the water pumped through a geothermal power plant in Nevada’s Dixie Valley–an area that has not seen volcanic activity in 30 million years.
The geochemists believe that Dixie Valley, which lies in an isolated island of elevated helium-3, shows that fracturing of the normally impermeable barrier between the earth’s mantle and crust can set up a natural geothermal system. They speculate that, once sheared open, the fractures fill with high-pressure fluids that transport helium to the crust along with lots of heat. The heat sets up a convection cycle in the crust in which heated groundwater rises to the surface, dumps its heat, and then circulates back down for more.
“Dixie Valley is a very productive geothermal field,” says Kennedy. “The question now is, are all [such helium] anomalies potential geothermal resources? That would take somebody to go and do some more exploration work, but I would point them to those areas of anomalies first.”
MIT chemical engineer Jefferson Tester, who directed the geothermal study for the DOE, says that the helium-3-to-helium-4 ratio “seems to have potential” as a geothermal prospecting tool. But he notes that validating a site for a geothermal project “may always require drilling and hydraulic pressurization in the field.”
Other researchers are equally cautious. Albert Genter, scientific coordinator for an enhanced geothermal project in Alsace, France, that’s managed by a consortium of European energy companies, points out that helium isotopes might not be predictive in less “geodynamic” regions such as Alsace. (See “Tapping Rocks for Power.”) “The transposition of the results is not obvious,” says Genter.
Kennedy agrees and says that the next step is to expand the survey of helium isotopes. .That may be difficult in the short term. For one thing, measuring helium isotopes is costly. Kennedy and van Soest accurately measured the faint helium-3 signals in their samples by cleaning out their equipment with a vacuum system one thousand times stronger than those normally used with mass spectrometers. “You have to have a very tight, clean system,” says Kennedy. That and other steps to eliminate background noise cost $2,000 to $3,000 per sample, although Kennedy says that automation could bring the cost well below $1,000 per sample.
Even that looks expensive in light of the DOE’s current budget for geothermal research: $0.