Decarbonization anywhere through deep geothermal

Superhot rock energy is a transformative new zero-carbon, always-available energy source that could be used for power, heat, hydrogen production, and industrial process energy. It has the potential to be economically competitive with most other zero-carbon energy sources and could be deployed rapidly in much of the world. It could potentially repower or replace many existing fossil fuel energy facilities and strengthen energy security by providing a local energy source – all with a relatively small environmental footprint.

Superhot rock energy is a form of geothermal energy; a consistent, dispatchable “baseload” source. Unlike other renewable energy resources, geothermal energy is consistently available and doesn’t require the use of battery storage, nor does it need to be backed up by a baseload power source such as fossil fuels. Using deep drilling technology, we could access superhot rock energy worldwide – increasing energy access, energy equity, and energy security.

Geothermal energy taps into the energy beneath our feet. Our planet’s interior contains naturally occurring heat, which spreads through conduction and convection up to the Earth’s surface. In “shallow heat” regions where concentrated heat is available near the surface, such as Italy, Turkey, or the Western U.S., conventional, or hydrothermal, geothermal energy works by drilling into reservoirs of heated groundwater and bringing steam to the surface to provide hot water to heat buildings, and to turn turbines to produce electricity. Where heat is deep, accessing hot water requires drilling farther into the Earth’s crust. In many areas, heat exists without a source of water. In those regions, water can be sent down to circulate through hot rock. The heat turns the water into steam, which is then brought to the surface. There are many emerging innovations that are looking to tap into these regions for heating, cooling, and the production of electricity.

Conventional geothermal power is produced from naturally heated groundwater close to the surface and generally at temperatures of 150-200°C. Superhot rock energy will be produced by drilling into hotter rock at deeper depths (>400°C), injecting and circulating water through the rock to heat it to very high temperatures, and then pumping it back to the surface to a generation plant. With successful deep drilling innovation, superhot rock energy could be produced anywhere in the world and will not be limited to regions with shallow heat and groundwater.

Water above 400°C is in the “superhot” (supercritical) state, which is much more energy-dense and can circulate more efficiently than lower-temperature water. These properties will allow superhot rock energy systems to produce an estimated 5-10x the energy of conventional geothermal systems – giving it the potential to be competitive with today’s costs for power. This high energy potential has been demonstrated in Iceland, where the Iceland Deep Drilling Project’s Krafla borehole produced superhot water at 452°C and an estimated 36 megawatts of energy (MWe) production potential. In comparison, a conventional geothermal project produces about 7-8 MWe per well.

Though humans have been using geothermal energy for over 100 years, (starting in Tuscany, Italy, in 1904) it’s only recently that technological advancements have made superhot rock energy a possibility. As of 2018, there were only 15 GW of conventional geothermal power in the world and geothermal accounted for less than 0.5% of total global electricity production. Because of its dramatically higher energy capacity and “geothermal everywhere” potential, superhot rock energy can be a much larger source of power, potentially on a terawatt scale.

The temperatures required to create superhot rock energy have already been accessed in regions where the earth’s heat is near the surface, such as Iceland, Italy, Turkey, or the Western U.S. These shallow-heat regions will likely be the first regions where superhot rock energy will be developed. However, superhot rock has the potential to be deployed on a global scale, almost anywhere in the world. Achieving “geothermal everywhere” will require innovative drilling technologies that can cost-effectively reach superhot resources at depths of up to 15 km/~9 mi (compared to depths of 7 km/~4 mi accessible through current drilling methods). There are several companies currently working to push the limits of today’s drilling, and the innovations needed are engineering innovations rather than major scientific breakthroughs.

Clean Air Task Force commissioned Hot Rock Energy Research Organization (HERO) and LucidCatalyst to estimate the levelized cost of energy for future mature (“nth of a kind”) superhot rock power plants. Levelized cost of energy (LCOE) is a standard measurement in the energy industry that is used to compare the cost of energy sources and is calculated by dividing the lifetime cost of a power plant by the total energy produced by that plant. Results suggest that mature superhot rock will be competitive at $20-35 per MWh, compared to $40 per MWh, the current U.S. market price for electricity.

Superhot rock energy does not risk depleting the Earth’s heat. On human timescales, superhot rock energy is endless, the ultimate renewable resource. One estimate suggests that 0.1% of the Earth’s heat could meet our world’s energy needs for 2 million years. Scientists predict that the Earth will continue to produce geothermal heat for billions of years, and the amount of energy needed by humans is tiny compared to the energy produced. Human geothermal use will not affect the Earth’s heat in any meaningful way.

The Earth’s heat is a carbon-free energy source. Unlike fossil fuel power, no direct carbon dioxide (CO₂) will be emitted in the process of generating power in dry rock. Superhot rock also represents an advantage over commercial hydrothermal geothermal systems, which harvest hydrothermal fluids that sometimes contain carbon dioxide. Because superhot rock energy will work in hot dry rock, which is highly unlikely to contain free CO₂, it’s envisioned as an entirely carbon-free form of energy.

“Supercritical” is a technical term that refers to water that has heat at or above 400°C and pressure at or above 22 MPa. “Superhot” is a less technical term that refers to water at or above about 400°C, regardless of pressure, as well as to other very hot fluids.

Geothermal energy typically requires a heat source (rock), water, and permeable rock so that the water can circulate through it and absorb the heat. In geothermal systems where hot rock does not contain a natural source of water and permeability, permeability must be created. Engineered geothermal systems (EGS) are geothermal systems that do not use natural hydrothermal water. They have also been referred to as “hot dry rock systems.”

Superhot rock energy is a subset of EGS that accesses much hotter temperatures (above 400°C) than EGS has historically accessed. “Direct-contact” superhot rock works by pumping water down one pipe into hot rock, where it circulates through tiny fractures in the rock to absorb heat, and then rises up a second pipe to the surface. An alternative to circulating water through fractures may be closed-loop injection systems, which heat water inside deep drilled conduits or pipes through the hot rock at depth.