Woskov

Feel the Heat: Paul Woskov of MIT holds water-cooling lines leading to a test chamber, and a sample of rock with a hole made by a beam from a gyrotron.
A vast supply of heat lies beneath our feet. Yet today's drilling methods can barely push through dense rocks and high-pressure conditions to reach it. A new generation of "enhanced" drilling systems aims to obliterate those barriers and unlock unprecedented supplies of geothermal energy.

AltaRock Energy is leading an effort to melt and vaporize rocks with millimeter waves. Instead of grinding away with mechanical drills, scientists use a gyrotron — a specialized high-frequency microwave-beam generator — to open holes in slabs of hard rock. The goal is to penetrate rock at faster speeds, to greater depths, and at a lower cost than conventional drills do.

The Seattle-based company recently received a US $3.9 million grant from the U.S. Department of Energy's Advanced Research Projects Agency-Energy (ARPA-E). The three-year initiative will enable scientists to demonstrate the technology at increasingly larger scales, from burning through hand-size samples to room-size slabs. Project partners say they hope to start drilling in real-world test sites before the grant period ends in September 2022.

AltaRock estimates that just 0.1 percent of the planet's heat content could supply humanity's total energy needs for 2 million years. Earth's core, at a scorching 6,000 °C, radiates heat through layers of magma, continental crust, and sedimentary rock. At extreme depths, that heat is available in constant supply anywhere on the planet. But most geothermal projects don't reach deeper than 3 kilometers, owing to technical or financial restrictions. Many wells tap heat from geysers or hot springs close to the surface.

That's one reason why, despite its potential, geothermal energy accounts for only about 0.2 percent of global power capacity, according to the International Renewable Energy Association.

"Today we have an access problem," says Carlos Araque, CEO of Quaise, an affiliate of AltaRock. "The promise is that, if we could drill 10 to 20 km deep, we'd basically have access to an infinite source of energy."

The ARPA-E initiative uses technology first developed by Paul Woskov, a senior research engineer at MIT's Plasma Science and Fusion Center. Since 2008, Woskov and his colleagues have used a 10-kilowatt gyrotron to produce millimeter waves at frequencies between 30 and 300 gigahertz. Elsewhere, millimeter waves are used for many purposes, including 5G wireless networks, airport security, and astronomy. While producing those waves requires only milliwatts of power, it takes several megawatts to drill through rocks.

To start, MIT researchers place a piece of rock in a test chamber, then blast it with high-powered, high-frequency beams. A metallic waveguide directs the beams to form holes. Compressed gas is injected to prevent plasma from breaking down and bursting into flames, which would hamper the process. In trials, millimeter waves have bored holes through granite, basalt, sandstone, and limestone.

The ARPA-E grant will allow the MIT team to develop their process using megawatt-size gyrotrons at Oak Ridge National Laboratory, in Tennessee. "We're trying to bring forward a disruption in technology to open up the way for deep geothermal energy," Araque says.

Other enhanced geothermal systems now under way use mechanical methods to extract energy from deeper wells and hotter sources. In Iceland, engineers are drilling 5 km deep into magma reservoirs, boring down between two tectonic plates. Demonstration projects in Australia, Japan, Mexico, and the U.S. West — including one by AltaRock — involve drilling artificial fractures into continental rocks. Engineers then inject water or liquid biomass into the fractures and pump it to the surface. When the liquid surpasses 374 °C and 22,100 kilopascals of pressure, it becomes a "supercritical" fluid, meaning it can transfer energy more efficiently and flow more easily than water from a typical well.

However, such efforts can trigger seismic activity, and projects in Switzerland and South Korea were shut down after earthquakes rattled surrounding cities. Such risks aren't expected for millimeter-wave drilling. Araque says that while beams could spill outside their boreholes, any damage would be confined deep below ground.

Maria Richards, coordinator at Southern Methodist University's Geothermal Laboratory, in Dallas, says that one advantage of using millimeter waves is that the drilling can occur almost anywhere — including alongside existing power plants. At shuttered coal facilities, deep geothermal wells could produce steam to drive the existing turbines.

The Texas laboratory previously explored using geothermal power to help natural-gas plants operate more efficiently. "In the end, it was too expensive. But if we could have drilled deeper and gotten higher temperatures, a project like ours would've been more profitable," Richards says. She notes that millimeter-wave beams could also reach high-pressure offshore oil and gas reservoirs that are too dangerous for mechanical drills to tap.
This article appears in the March 2020 print issue as "AltaRock Melts Rock For Geothermal Wells."