The maximum temperature of the Earth’s core is close to that of the Sun’s surface, about 5,500°C. We have a sun beneath our feet, radiating heat all the way to the surface. The problem is that while the distance to reach it is small in geological terms, it is an unfathomable abyss on a human scale.
This inexhaustible heat from underground is just waiting for someone to drill deep enough to generate renewable electricity. Technology groups claim it can now be achieved using vacuum tubes and microwaves to vaporise the rock. When and how will industrial application be possible?
The best example of this difficulty was an engineering adventure: the Kola Superdeep Borehole was a Soviet-era scientific endeavour launched in 1970 that penetrated more than 12 kilometres into the Earth’s crust before being shut down because it was also a bottomless money pit.
In addition to the scientific knowledge gained about the nature of the rock deep underground, the Kola experience showed that the deeper you drill, the harder it gets, not least because of the increasing hardness of the rock and the fact that drill bits become weaker at high temperatures.
The next frontier for mining engineering
Accessing the deep heat—starting at 150°C to evaporate water and drive electric turbines—with industrial processes therefore requires a quantum leap in drilling capacity, even a paradigm shift beyond the tried-and-tested percussion and friction techniques. As Celestino García de la Noceda, head of geothermal resources at Spain’s Geological and Mining Institute, explains, this is the next frontier, or at least one of them, for mining technology in the world.
What would be the prize if it could be achieved? Access to this subterranean sun and its inexhaustible source of energy, free from the intermittency that plagues renewables such as solar and wind. It would also be ubiquitous, because wells could be drilled almost anywhere, not just in volcanic regions where the Earth’s heat is much closer to the surface, such as Iceland. Deep geothermal expert Matt Houde argues that “reaching 16 kilometres underground would allow economic [industrially viable] temperatures anywhere.”
Although this drilling capacity has not yet been reached, conventional geothermal has made enough progress to increase its contribution to the renewable energy mix. The International Geothermal Association counts some 600 plants in operation or under construction worldwide, with another 600 in the pipeline.
At the end of 2022, a new development emerged. Quaise Energy, a company associated with the Massachusetts Institute of Technology (MIT), announced this long-awaited paradigm shift. Instead of traditional but improved drill bits, it uses gyrotron-powered drilling, which projects microwave beams into a vacuum tube that can vaporise the harder, more crystalline and hotter rocks that are found at great depths. This is not an unprecedented technology; in fact, it is one of the main lines of geothermal research. But MIT says that after 15 years of research, it has achieved an operational leap in the laboratory that is able to “reach unprecedented depths.”
In addition, Quaise Energy claims that, although these electromagnetic gyrotrons have not yet been used in a real abyssal well 24 hours a day, they have become efficient enough to plan a first pilot geothermal well between 2024 and 2026. Just two years later, Quaise says, it would be feasible to start deploying the technology in existing power stations to drive their turbines with steam instead of natural gas and, in particular, coal.
This ability to be integrated into existing or newly built power plants anywhere in the world, especially in dense urban or industrial areas with high energy consumption, would, according to Quaise, offset the large investment in drilling, which would also become increasingly cost-effective as the technology matures. Another advantage in terms of economic viability would be the potential to recycle technology, know-how and investment from other drilling sectors such as oil and gas.
The stability of deep boreholes
In these times of energy scarcity and the search for alternative supplies, not to mention panaceas, the promise of geothermal energy is deafening. Indeed, at the last SOSV Climate Tech Summit, some geothermal experts put the potential contribution to the global energy mix at 20% in a few decades.
Other experts urge caution. Laboratory tests with heated basalt rock may represent a major breakthrough in the use of gyrotrons, but no one knows for sure how they will perform in the harsh real world at 12, 16 or 20 kilometres deep. So one of the technological challenges at MIT focuses on the stability of deep boreholes.
For example, García de la Noceda says that the news of the MIT breakthrough came as a surprise to many researchers. While similar techniques have been tested for some time, but limited to laboratories and modest depths, the MIT breakthrough could become a historic milestone if it is successfully scaled up. However, the expert is more sceptical about the announced timelines for the pilot plants. “We have to remain hopeful about the industrial development of the proposal, although there seems to be an excessive amount of publicity surrounding its launch. The industry has been looking for years to improve drilling systems and, above all, to reduce costs. This is why news like this can generate greater expectations than other more realistic news.”
The search continues. This caution does not detract from the fact that deep geothermal energy could become a new player in the energy transition. According to the expert, it can function as a baseload energy source and also respond quickly enough to peak demand in shorter periods. However, there are factors beyond environmental criteria, such as socio-political ones, which may limit its development, as is already the case with the mining and subsoil industry.