(BBC News) Beneath our feet is an almost limitless source of energy, but while a few lucky locations have geothermal heat close to the surface, the rest of the world will need to dig a lot deeper. The challenge is how to get deep enough.
There are some spots around the world where energy literally bubbles to the surface. In Iceland, home to more than 200 volcanoes and dozens of natural hot springs, tapping into this energy is not hard. Dotted around the country are steaming pools of water, heated by the geothermal fires that burn just below the crust. Boiling jets of water and steam are thrown into the air as geysers.
Iceland now heats 85% of its houses with this geothermal energy, while 25% of the country’s electricity comes from power stations that harness this heat from underground. It is an appealing prospect – an almost limitless supply of energy waiting to be tapped.
Geothermal energy offers an essentially inexhaustible green energy source across the planet. And it is “always on”, unlike wind or solar power, since the heat is continually emitted from the Earth’s molten core and the decay of naturally occurring radioactive elements in our planet’s crust. Indeed, the Earth emits such enormous amounts of energy as it cools that the heat lost into space each year is enough to meet the world’s total energy demands many times over.
The challenge is tapping into that energy.
Currently only 32 countries in the world have geothermal power plants in operation. There are fewer than 700 power plants around the world, generating around 97 Terawatt hours (TWh) in 2023. That is less than half the amount of electricity generated by solar in the US. Some estimate that geothermal could contribute around 800-1400 TWh of electricity annually by the middle of the century with a further 3,300-3800 TWh per year of heat.
“The Earth itself has the potential to address a variety of hurdles in the transition to a clean energy future,” argued Amanda Kolker, geothermal program manager at the National Renewable Energy Laboratory (NREL) in the US, when releasing a report on the potential of geothermal energy in 2023.
But not every country is as fortunate as Iceland, where reservoirs of hot water at temperatures of around 120-240C can be easily accessed close to the surface. In other areas of the country, wells drilled to depths of up to 2.5 km provide access to temperatures of up to 350C. Iceland’s main geothermal site at Reykjanes, for example, has drilled experimental wells 4.6 km to access fluids as hot as 600C. Already, day-to-day heat extraction is taking place using shallower wells that draw on temperatures around 320C to generate 720 Gigawatt hours (GWh) of electricity per year.
One reason geothermal is not more widespread is the high up-front investment needed to extract that energy. And physically reaching it has been beyond most of us.
For other parts of the world to enjoy a part of this geothermal bonanza of clean energy, we need to drill deeper to reach the temperatures needed to generate electricity or provide large-scale heating for nearby neighbourhoods.
Across much of the planet, temperatures increase by 25-30C on average every kilometre you go down through the Earth’s crust. In the UK, for example, the subsurface temperature at around 5 km down is about 140C, according to the British Geological Survey.
Drill down far enough, and it is possible to reach a point where water temperatures surpass 374C at pressures above 220 bars (one bar being average pressure on the Earth’s surface at sea level). This is where water enters an energy-intense state known as supercritical, where it exists in a form that is neither liquid or gas. The hotter and more pressurised it is, the more energy it contains.
In fact, a single superhot geothermal well could produce five to 10 times the energy that commercial geothermal wells produce today, according to the NREL.
One major hurdle, however, is that conventional rotary drills – even those tipped with diamond – are ill-equipped to excavate to the kind of depths needed to access these kinds of temperatures. In the mysterious deep underworld of uncertain geology, extreme temperatures and huge pressures, drill components can fail frequently, while keeping holes from becoming blocked is a constant battle.
In 2009, for example, a team working on the Iceland Deep Drilling Project inadvertently tapped supercritical conditions when it drilled into a magma chamber at the Krafla volcano, about 2 km below the surface. The superheated steam emitted from this well was highly acidic, making it difficult to use. The high pressures and temperatures involved also made it difficult to control, and it had to be intermittently discharged for around two years before a valve failure forced the hole to be sealed.
Deep drilling is also an expensive and time-consuming endeavour.
The deepest hole ever dug by humans dates back to the Cold War, when there was a race between the superpowers to drill as far into the Earth’s crust as possible. The Soviets managed to plough their way through 12.2 km of rock – creating the Kola Superdeep Borehole, on the Kola Peninsula, high in the Arctic Circle. It took them almost 20 years to reach that depth, and it remains the deepest humans have managed to delve into the Earth. (Read more about the Kola Superdeep Borehole in this article by Mark Piesing.)
The NREL estimates that the cost of drilling a 1-km deep well is around $2 million, while drilling four times that depth can cost between $6-10 million with current technology.
Yet deep geothermal energy could provide considerable cost savings when compared to conventional geothermal, due to the higher temperatures and pressures that can be accessed further into the Earth’s crust. Some studies suggest that deep geothermal energy could supply heating for communities at costs similar to other forms of heating, such as natural gas, but with fewer greenhouse gas emissions.
With this in mind, some pioneering researchers and companies are turning to new types of drills and drilling techniques to bore some of the deepest holes created in the hope of bringing geothermal energy to parts of the world where it was never thought possible.
Quaise Energy, a spinoff from the Massachusetts Institute of Technology (MIT), for example, aims to drill holes as deep as 20 km to access temperatures of 500C or more. To do so, they are turning to a tool that draws on years of research into nuclear fusion power. “While others are putting shovels in the ground, we are putting microwaves in the ground for the first time,” says Quaise co-founder Matt Houde.
He and his colleagues are experimenting with millimetre-wave directed energy beams that vaporize even the hardest rock. It focuses a high-powered beam of radiation similar to microwaves but at a higher frequency onto a segment of rock, heating it up to 3,000C so that it melts and vaporizes. By directing the beam so it bores through the rock, holes can be created without the debris and friction created by traditional drilling techniques.
“Millimetre-wave drilling is a process that can operate largely independent of depth,” says Houde. “And millimetre-wave energy can also transmit through dirty, dusty environments.”
The technology has grown out of nuclear fusion plasma experiments conducted by Paul Woskov, an engineer at MIT’s Plasma Science and Fusion Centre. Millimetre-wave directed energy has been explored as a way of heating up plasma in nuclear fusion reactors since the 1970s, but a few years ago Woskov hit upon another use for the technology. He started using millimetre-wave beams generated by a device known as a gyrotron to melt through rock.
But so far the technology has only been tested in the laboratory, drilling shallow holes in relatively small samples of rock, but the company claims it can drill through rock at around 3.5 metres per hour. While this is slow compared to traditional drilling techniques, there are other benefits as the “drill bit” isn’t physically grinding through the rock, it should not wear out or need replacing. Quaise Energy is at the final stage of laboratory testing of millimetre-wave technology prior to beginning field trials in early 2025.
But transferring the millimetre-wave drilling technology from the laboratory to a full-scale drilling operation will still be a challenge.
“They have never been used before in the deep high-pressure subsurface environment,” says Woskov. “Changes due to intense energy-matter interaction applied to drilling require a new learning curve.”
