Inner energy: Drilling 12 miles into the Earth to power the planet

"I wanted to work towards addressing the human impact on the environment, our greatest challenge in the 21st century," Matthew Houde, co-founder and Project Manager of Quaise Energy, Inc., recalled his college dreams. 

"My desire to study engineering led me to pursue renewable energy as a career," he told Interesting Engineering (IE).

According to Houde, geothermal energy is the most fascinating field for addressing climate change.

"I double majored in geoscience, which pointed me toward geothermal energy as the most interesting discipline I could study that would maximize the impact of addressing climate change."

Born and raised in Chicago, U.S., Houde studied geological engineering and geoscience at the University of Wisconsin-Madison before moving on to Stanford University for his master's degree in civil engineering with a focus on renewable energy.

Houde, a strong proponent of geothermal energy, asserts that the total energy contained in the heat held beneath the Earth's surface exceeds our planet's annual energy needs by a factor of a billion. 

So even a small portion of this energy, as Houde explains, can provide us with all the power we'll need in the near future.

But the question then becomes, how could one access such a massive energy source? 

How to access Earth's energy core?

Houde and his team at Quaise Energy, an MIT spinoff, are pioneering a technique called Millimeter-Wave (MMW) Drilling, which uses high-frequency microwaves with a wavelength of 1–10 mm to drill deeply into the soil. 

They intend to be able to drill as deep as six to 12 miles (ten to 20 kilometers), and possibly even farther, by merging traditional mechanical drilling techniques with MMW Drilling technology.

The ultimate goal of Houde and Quaise Energy is to be able to drill a large number of boreholes each year that are as deep as the Kola borehole, which at 40,230 feet (12,262 meters) is the deepest borehole ever dug into the earth, and eventually deeper.

They want to eventually carry out pilot drilling tests in superhot shallow geothermal zones for commercial development after first carrying out many shallow field trials that illustrate the procedure in various locations.

The drilling techniques will continue to develop in parallel so that they are able to access the higher temperatures at greater depths. These are not restricted to a particular place on the planet and so are widely accessible.

Currently, Quaise Energy has raised more than $63 million from a range of investors, including a $4.95 million grant from the U.S. Department of Energy through the Advanced Research Projects Agency-Energy (ARPA-E). In part, the money will be used to carry out testing in conjunction with fusion experts at the Oak Ridge National Laboratory (ORNL). The partnership will involve Quaise repurposing gyrotron technology, a tool used in fusion to produce high-powered microwaves.

The tests aim to demonstrate the feasibility of using MMW drilling to reach power levels and depths that surpass earlier laboratory testing by a factor of ten and more and at lower costs.

To better understand how drilling holes into Earth's core can serve humanity's rising energy needs and more, IE spoke to Houde for an exclusive interview. 

The following conversation has been lightly edited for clarity and flow.

Interesting Engineering: What is the evidence for the idea that drilling holes in the Earth could provide us with an abundance of power?

Matthew Houde: The total heat content of the Earth or heat stored in the earth's subsurface is estimated to be 10^31 J of "geothermal energy," i.e., energy stored as heat in the earth's subsurface, and current annual energy demand is about 5.8 * 10^20 J. So the total heat content of the Earth is equivalent to about 17 billion times the annual energy demand for the globe as a whole. So this back-of-the-envelope calculation tells us that a small fraction of the total heat content of the earth would meet current energy demand for a million years. 

By combining conventional mechanical drilling methods with Millimeter-Wave (MMW) Drilling technology, we have the technology package to drill to depths of 10 - 20 km and perhaps even further. 

IE: How are you planning on doing this? 

Our ultimate goal is the ability to drill 100s of boreholes per year that match, and will eventually exceed, the depth of the Kola borehole. Our plan is to start with several shallow field trials that demonstrate the process in a variety of locales. Then, we will execute our first pilot trials for drilling into superhot shallow geothermal areas for commercial development while we continue to mature the drilling process in parallel for accessing the same temperatures at greater depths, which are almost universally accessible/no longer constrained to a specific location. 

Quaise Energy is sub-recipient to a $4.95M grant from the ARPA-e agency within the DOE to execute an experimental lab campaign that demonstrates the MMW drilling approach at power levels and length scales that exceed previous MIT testing by a factor of 10 and more. 

IE: Tell us about the Millimeter-Wave Drilling method. How does it work?

Millimeter-Wave (MMW) Drilling proposes to replace mechanical drilling methods with an energy-matter interaction that can replace most key functions of a conventional drilling operation. High-frequency microwaves with a wavelength of 1-10 mm (hence the name) are generated at the surface by a high-power device called a gyrotron. The MMW energy is then transmitted down the borehole by a metallic conduit called a waveguide, along with an injected purge gas, and delivered to the rock surface at the borehole bottom. Rock, a dielectric material, will absorb the MMW energy and heat up until melting, then vaporization takes place to reduce the solid rock to a vapor. This vapor rapidly condenses into a fine ash, at which point the injected purge gas picks up the ash and circulates it up the borehole annulus (a space adjacent to the wall of the hole) to be removed at the surface. Finally, the molten outer ring of this vaporization front stays down in the borehole and cools, solidifying to form a glass liner that stabilizes the borehole during drilling. 

MMW Drilling is advantageous because the process operates mostly independent of depth, unlike conventional drilling, and is particularly optimized for the types of rock (hard, crystalline) encountered in the deep basement of the crust where high-grade geothermal heat is expected. MMW Drilling is ultimately conceived as part of a hybrid drilling operation, where we start at the surface with conventional drilling and only switch over to MMW Drilling once hard basement rock is encountered, and conventional drilling runs into diminishing returns. Calculations predict MMW drilling can drill through hard basement rock at a Rate of Penetration (ROP) of 3-5 m/hr, enabling a 10-km deep hole to be drilled in 100 days or less. 

The key fundamentals of the MMW drilling process have already been demonstrated in the lab to the point where we are confident in successful field demonstrations within the year. Addressing the engineering challenges below are the key requirements for extending this drilling capability to the extreme depths required for unlocking superhot geothermal energy. 

IE: What are the engineering challenges for such a project?

The key engineering challenges, in no particular order, are outlined below. 

• Power transmission - Can we ensure the majority of microwave power generated at the surface is delivered to the bottom of the borehole for rock destruction without excessive power losses in the long drill string?
• Material removal - can the circulating purge gas prove sufficient in removing across the long distances implied for deep boreholes?
• Borehole stabilization - can the glass liner overcome high lithostatic pressures and downhole fluids to retain a structurally-sound borehole, both at incredible depths and in zones past the Brittle-Ductile Transition where the rock begins to creep more easily? 

Quaise Energy is looking for locations where basement rock is directly exposed at the surface. While they are still deciding on a place, their current strategy is to drill the first holes in the field in the coming years.