Wind and solar technologies are the poster children of renewable energy: their resources are widely available and increasingly low-cost, but the variability of wind and sunlight means turbines and photovoltaics are not standalone solutions. They leave generation gaps that remain to be filled, and some of the leading contenders to do so – like nuclear power or fossil fuel-fired power plants coupled with carbon capture and storage – are controversial, or in the case of long duration energy storage, not sufficiently developed.

Geothermal energy and hydropower can also provide a complementary supply of energy. Their constant and high-capacity generation can complement wind and solar installations, or even operate autonomously.

Hydropower has been broadly taken advantage of globally, a notable example being the Three Gorges Dam, which meets nearly 10% of China’s total annual electricity demand. However, hydropower can devastate local ecology and exacerbate flooding to the point where hundreds of lives are lost.

Geothermal power is theoretically available all over the world. AltaRock Energy , a Washington-based geothermal energy company, estimates that 0.1% of the Earth’s heat could supply humanity’s total energy needs for two million years.

Energy derived by drilling into the earth to access its natural heat is dubbed ‘deep geothermal’, but the technology to tap into this energy is only partially developed. Regions with high volcanic activity, such as Turkey, parts of the US, and Iceland, benefit from hydrothermal reservoirs like hot springs, which can be drilled to release hot steam that is converted to power via turbines.

Another more geographically flexible technology called an enhanced geothermal system (EGS) generates energy without the need for naturally occurring hydrothermal resources. EGS creates artificial reservoirs by injecting fluid into hot non-porous rock layers, which are fractured and their energy harnessed as natural hydrothermal reservoirs.

Scaling up geothermal power

Geothermal can provide perpetual baseload power. Because it is an underground operation, it does not interact with the local environment and is sheltered from weather and human disasters. An added benefit is that the same energy source can also help decarbonise buildings by fuelling district heating systems and heat pumps.

The benefits of geothermal energy were acknowledged in a ‘Florence Declarationin September 2017, an agreement to a fivefold scale-up in global geothermal energy development signed by 42 governments, 29 partner institutions and members of the Global Geothermal Alliance.

However, according to the International Energy Agency, global geothermal power production grew from 85.3TWh in 2017 to just 94TWh in 2020, a rate that is light years away from meeting the 330TWh by 2030 target that would limit global temperature rise to 1.5°C.

Repurposing oil and gas

Sanjeev Kumar, head of policy at trade association the European Geothermal Energy Council, attributes geothermal’s stunted progress to a lack of funding, not technology. Corporations are hesitant to invest in an underground, ‘unseen’ technology, he says.

“The bit that is good about geothermal, which is that it is largely hidden, is also the worst thing you could have,” he says. “It is not like a wind turbine where you can put a logo on it and see it from miles away, or a PV panel, where you can walk down the street to see millions of roofs. You can't see geothermal, it is below ground.”

Geothermal projects have low operational costs but high upfront costs. The cost of drilling geothermal wells and field development can represent as much as 40% of total investments, according to Iceland GeoSurvey , a state-owned consultancy. Because geothermal energy sells for much lower rates than gas and oil, companies are hesitant to make big investments in exploration where they risk failing to find an energy source.

However, Kumar says the same seismic technologies that identify oil and gas reservoirs are increasingly being used for geothermal projects and discovery rates are much higher than for oil and gas. There is also the potential to repurpose oil and gas fields for geothermal energy, he suggests, if policy pushed companies to do this.

Not only are oil and gas wells transferable to the geothermal industry, but the entire operation is too: workers, skills, drilling and digging technology. Kumar also points to a lack of financial incentive as the reason why workers are less willing to move from lucrative oil and gas to geothermal jobs.

It does not bode well for its image that EGS drilling fractures rock to access hot water reservoirs. The process is similar to fracking, the notorious process behind shale oil and gas, except that geothermal energy production releases minimal amounts of methane and carbon dioxide into the atmosphere and there is no fossil fuel combustion further down the line. Geothermal power plants emit 97% less acid rain-causing sulphur compounds and about 99% less carbon dioxide than fossil fuel power plants of similar size, according to the US Energy Information Administration . The challenge lies in dispelling the perceived threat of the process.

‘Supercritical’ geothermal is not fiction

'Supercritical' geothermal is the most extreme form of deep geothermal, and about as elusive as nuclear fusion. It involves drilling to new depths to access an immense geothermal energy source. When water is heated past 373°C and under 220 bars of pressure, it enters a supercritical state that holds between four and ten times more energy per unit mass than regular water or steam, and double the maximum thermal efficiency of converting heat to electricity. Unlike EGS, supercritical geothermal’s technology is still in its experimental stages.

Where a 200°C EGS project might have a capacity of 5MW, a 400°C supercritical project would have ten times that capacity at 50MW. Three 400°C wells would have a larger capacity than 42 200°C wells. The interest in making supercritical geothermal energy work is therefore as much economical as it is environmental.

Supercritical geothermal systems have been tapped into on several occasions, albeit sometimes by accident. The first supercritical well drilled in Iceland in 2009, called IDDP-1, failed because of corrosion but had a capacity of more than 36MW. The second project in 2016, IDDP-2, also failed but managed to drill deeper, to a depth of 4.7km and a temperature of 535°C. IDDP-3 is due to begin in the next five years. The challenge is engineering: it is usually some aspect of the technology, like the drill or well, that fails when attempting to make use of supercritical zones.

Deep geothermal innovation

Some private companies are making headway with innovations that supersede traditional geothermal drilling. Quaise is one such example. Having recently won $52m in funding, the MIT and Cambridge University-backed company is working on an entirely new drilling method. Their ‘gyrotron-powered drilling platform’ vaporises boreholes through rock. Quaise plans to use the technology to reach a depth of 20km and access 500°C heat, or “terawatt-scale power”, as its website describes it.

“[Currently] drill heads wear quickly [below depths of two miles], increasing production time and costs,” says Carlos Araque, CEO of Quaise. “Additionally, liquid fluids take a lot of energy to pump up and down, which [is what] the oil industry does. Our millimetre-wave drilling system [can] go deeper than two miles [without] drilling bits. It vaporises rock, so we move into gases, which are much easier to move up and down.”

The company is conducting successful test drills that breach new depths and rock types daily, with a view to entering field operations as soon as possible. “We are working with the largest drilling company in the field with field-deployable systems to be used by 2024,” says Araque.

The project’s scale-up is the main hurdle, he adds: “We know that the science behind our technology works. At the moment, we are tackling the engineering challenges around packaging all components for field operations […] It is doable within a decade, but needs effort, attention, technology and funding in the hundreds of millions of dollars.”

However, some experts are sceptical of spending time and money on new, high-risk equipment when the technology already exists for EGS and other more easily accessible geothermal resources. Kumar says supercritical geothermal energy is a distraction, particularly at a time when the Russian threat to cut off gas supplies has created a global heating crisis.

Araque’s zero-gas world would see supercritical geothermal energy completely replacing fossil fuel power plants to create a baseload supply of electricity that could also provide heating. “It is evident that we cannot transition to clean energy with wind and solar alone," he says. "The goal with deep geothermal is to produce terawatt-scale, baseload energy using less than 1% of the land […] available everywhere, [that] leverages the existing thermal generation infrastructure and [that] is clean.”