Humanity’s continuing existence in a vast universe is astonishing. The thin crust we live on constitutes less than 1% of the earth’s mass. We depend for all our food on a skin of earth half a metre thick, and we are depleting it. But in the last few hundred years, we have managed to build a civilisation by wolfing down some strictly limited resources.
Our need for energy has been hugely destructive. We nearly eliminated whales in the hunt for oil. More than a million people have died extracting coal and oil, and more have died from the associated pollution. We are heating the climate, perhaps to unsustainable levels.
And it is only as the possibility of our own extinction appeared on the horizon that we have turned seriously to the two huge and inexhaustible energy sources: the sun and the ground beneath our feet. But if we can solve the technical problems quickly enough, either of them could provide all our energy needs forever.
Two sustainable sources
The sun gives us direct solar radiation. We have learned to gather and convert it to electricity. And we have also gradually learned to tap its indirect energy through wind, tides and rain. But we have barely begun to exploit the second source, the earth beneath our feet.
At the surface, ground source heat pumps extract low temperature heat from the ground to heat buildings. But further down, almost all of the earth’s mass is a vast body of tremendously hot rock. In a few places, like the active volcanic areas of the Pacific rim and Iceland, this comes close enough to the surface to be tapped easily. In New Zealand, power stations run on the steam emerging from the ground below. But we have yet to succeed in tapping the heat at the deeper levels where it is found across the globe.
Tapping the earth
The principle of geothermal energy is simple. Water is fed down a deep shaft into the hot rock below, and then returned as steam, to drive turbines like a conventional power station. The cooled water is then fed down again to complete the cycle. Unlike most other renewables, the process runs 24/7. The power source will continue as long as there is an earth for us to live on. And the environmental impact is much less than conventional power stations or windfarms — there is little or no carbon released, and little unsightly infrastructure.
The principle is simple but, of course, the practice is more complicated.
British achievements
In Britain, four projects are now tapping geothermal energy from the hot granite which lies below the surface in Cornwall. In 2023, the Eden Project began heating its site and offices from a 4km deep shaft. Now, Geothermal Engineering is developing three deeper sites, with contracts to begin delivering electricity to the grid next year. Their first project, United Downs, has successfully drilled 5km down. The water fed down that shaft will return as steam to drive the turbines. By 2028, it is expected to be generating 12 Megawatts of power.
Admittedly, this is a very small contribution to overall UK renewable generation, but this is an emerging technology and only the beginning. And in Cornwall, there is a secondary benefit. The rocks are rich in Lithium, and the water is bringing it up in significant quantities. Lithium is critical to many electronic technologies, including batteries. Most of it is currently mined in Australia and China, and its processing requires large amounts of energy and causes significant pollution. So, a relatively clean UK source is well worth exploiting, and United Downs is constructing a plant which aims to produce 100 tonnes of Lithium a year.
Going further down
Drilling down 5 km is practicable with existing mechanical drills. But going much further presents entirely new challenges. That means tapping into the much hotter and deeper realms which lie 10 or more kilometres below. A distance which would be an afternoon’s walk on the surface is an almost impenetrable barrier going down. The deepest shaft in the world, in Russia, took 20 years to drill, and has been abandoned. The laws of physics dictate the point at which the energy you put into turning a conventional drill dissipates and nothing, or worse, happens at the other end. Conventional metals melt, and every time the drill bit wears it takes days to haul it up and replace it.
But the heat reserves down there are immense, so two projects are seeking alternative solutions. Both plan to replace mechanical drills with instruments which melt the rock.
The Quaise project has been developed by the Massachusetts Institute of Technology. Their model plans to go down 10-20 km to where the temperatures are over 400 degrees Celsius. They replace a mechanical drill with millimetre wave radiation which cracks and melts the rock. Debris is blasted out with compressed air, and the molten rock vitrifies, creating a strong glass lining for the shaft, without the need for the steel and concrete liners of conventional oil wells. They have demonstrated that the technology will cut through granite and are planning to have a pilot plant operational in Texas by 2030.
The DeepU project also plans an unconventional drilling technique. In the lab, a transnational team (Italy, Germany and Poland), funded by the EU’s Horizon Europe programme, has demonstrated the feasibility of using a laser to melt and vitrify rock, with very low temperature pressurised nitrogen to remove debris. They envisage a shallower closed U tube system, perhaps 4 km deep. The initial project successfully demonstrated feasibility this year, but no plans for field testing have yet been announced.
The future
By demonstrating two workable non mechanical drilling techniques, these projects have shown that some of the technical barriers to deep drilling can be overcome. So, it is conceivable that geothermal energy can be obtained in parts of the world where it was not previously possible.
But this is not easy or simple. Deep geothermal power may well have a significant role to play in our future energy mix. The potential is huge, but we are likely to depend on wind, solar, or even tidal sources while this technology evolves.
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