• The Heat Beat

Tungsten Javelins vs. Igneous Rock – A Chat with Danny Hillis

By Bob Metcalfe.

GEO aims to enable the future of geothermal energy. Baseload, clean, abundant – and most importantly, ubiquitous (not just on hotspots and near volcanos). To do that, we need to drive down the cost of deep drilling into very hard, very hot rock, so no matter where you are in the world, you can drill a well and build a geothermal power plant. We are particularly interested in so called “closed loop systems” drilled into hot (350C) igneous bedrock – radiator style heat exchangers that drive steam turbines on the surface. And we are aiming to go deep. 10km, so that we can assure that we are enabling the resource over a majority of the world’s landmass.

A key parameter in the economics of drilling is “ROP” or Rate of Penetration. The faster we drill geothermal wells, the higher ROP, the less time it takes to drill them. And time is money. In the oil and gas world, time is big bucks. As in a million a day in some cases.

In search of big ideas for GEO, I had dinner in Harvard Square with Danny Hillis at my favorite Restaurant, Harvest. Danny is a sure bet when you go searching for big ideas. One of Danny’s many ideas is his 200-foot-tall “Long Now” 10,000-year clock. His clock is being built in a cave in West Texas, funded by Jeff Bezos.

Danny started our dinner by telling me about his idea for a new Internet, built from the ground up to be secure. And then we moved on to my new project GEO, and drilling for heat. When I was done with the little I know about thermodynamics and geothermal drilling (my background is bytes, not bits), Danny was quiet for a while. Then he asked if I know about Tungsten. Turns out Danny knows a lot about Tungsten.

Tungsten is a metal, atomic number 74, with the surprising chemical symbol W, short for Wolfram. Environmentally friendly, Tungsten (“heavy stone”) is used to make incandescent lights, fishing sinkers, darts, machine tools, tunnel borers, armor-piercing ammunition, and now 10,000-year clocks. Tungsten is the metal Danny is using for the pendulum bob in Long Now.

Tungsten is also well known to oil and gas drilling experts. They have been using Tungsten (carbide) for a very long time in rotary drilling, where Tungsten’s hardness is needed. Danny’s big idea would use Tungsten’s hardness, but also its heaviness. And not in rotary drilling.

Tungsten is among the densest elements at 19.3g/cm3, about as dense as Uranium and Gold, 1.7 times denser than Lead. The density of granite is 2.75g/cm3. The density of Tungsten is 7 times that of granite. Tungsten’s melting point is the highest of any metal - 3,422C. Hot dry granite melts above 2,200C.

And yes, Tungsten is hard, 9 on the Mohs scale of minerals hardness, almost as hard as diamond at 10, and a lot harder than steel at 6, and still way harder than … granite at ~6.

So, on a napkin at the aptly named Harvest, Danny proposed we consider building what he calls “javelins” made of Tungsten to “drill” for geothermal energy. He proposed we consider simply propel a Tungsten javelin where we want to drill a geothermal well. The javelins would accelerate (not rotate) under the force of gravity toward the center of the Earth, until they reach their melting point (3,422C) and simply disappear (at temperature and depths much deeper and hotter than we aim for with GEO).

Consider a Tungsten javelin .1m in diameter and 10m long. It would have a point at each end, tapered down to .01m. As I recall from the napkin, Danny quickly calculated the yield strength of the javelin at 2.7GPa versus the yield strength of granite at 130MPa – “W wins over granite,” he announced.

Now check my arithmetic. The volume of Danny’s Harvest napkin javelin would be very approximately 3.14 X .05m X .05m X 10m = .0785m3. At Tungsten’s density of 19.3g/cm3, that’s 1,515kg.

Assuming that Tungsten can be purchased for approximately $100/kg, the Harvest napkin javelin would cost very approximately of $151,500. This is on the high end of the cost of traditional rotary drill bits.

Now – the concept of leveraging Tungsten and gravity to achieve big results isn’t entirely new. Google the term “Rods from God.” This is simply a more optimistic and productive take on a similar concept.

Over dinner, Danny didn’t spend much time considering friction, but he argued that “it was solvable in principle just by making the javelins large enough. This is because friction is proportional to area (scale squared) while the downward force is proportional to volume (scale cubed). Just like W beats rock, cubes beat squares.” He added in a later conversation that “the side forces are going to increase as the javelin goes deeper. There will be a point where the frictional forces match the downward forces. The real question is how big a javelin do you need [to reach 10km].”

To Danny’s questions, I’d add a few of my own: What would be the terminal velocity of a Tungsten javelin through hot dry granite? What would be the optimal diameter and length of the javelin? Would its terminal velocity be greater than zero? Would the javelin split the rock or punch a hole through it? Would friction along the javelin too soon heat it beyond melting? Would it cause seismicity as it travels? Would the hole punched by the javelin persist or close behind the javelin as it passed, and how would temperature of the rock impact this behavior?

I leave all these questions as an exercise for the geothermal entrepreneurs of the world. Thanks, Danny, for your idea. And thanks for buying dinner.

Bob Metcalfe is Professor of Innovation and Murchison Fellow of Free Enterprise in the Cockrell School of Engineering at The University of Texas at Austin. He is Principal Investigator of the Geothermal Entrepreneurship Organization (GEO) in the Texas Innovation Center.


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