Earth can be thought of as a giant heat battery, but geothermal energy contributes only to about 0.5% of electricity produced via low-carbon power sources and grows only by about 3.5% year after year. The primary reason for geothermal energy not scaling up in number of deployments is that it is geographically restrained; thus, not every region can tap into geothermal resources. However, there is one geothermal innovation that could help widen its global reach: supercritical geothermal.
What is supercritical geothermal?
Supercritical geothermal energy generates power from supercritical water below Earth’s surface. Groundwater near magma can go supercritical, that is, pressurized to temperatures and pressures above its critical point (~374 °C, 221 bar). This is fascinating not just because of the higher temperatures but also because supercritical water can have much higher enthalpy, that is, it holds more energy and can be 5 to 10 times more potent than traditional geothermal, lowering levelized costs.
Where do we find supercritical geothermal resources?
Supercritical or superhot geothermal resources have been found even at depths as shallow as 2 km in places like Iceland. Regions with high volcanic or tectonic activity, like Iceland and Kenya, have also witnessed supercritical conditions at depths closer to 5 km. Typically, supercritical conditions exist when drilling into the brittle-ductile transition zone (BDTZ). The BDTZ extends over a range of depths below the surface where the rock transitions from a brittle to a ductile material. Imagine a scenario where a piece of rock can be rolled like a wire or flattened into a sheet. The rock behaves like this at the bottom of the BDTZ, while it is entirely brittle at the top of the BDTZ; at intermediate depths, it exhibits properties of both brittleness and ductility. The probability of finding the BDTZ closer to the surface is higher in places with excellent geothermal potential (again, places with high volcanic or tectonic activity), declining as conventional geothermal potential declines. Thus, the depth at which one hits the BDTZ varies from region to region and can even be as deep as 20 km.
How does the BDTZ affect supercritical geothermal power generation prospects?
Even if it becomes practically possible to drill up to the top layer of the BDTZ at depths close to 20 km, creating fractures or drilling through the rock at the edge of the BDTZ is not straightforward since the rock starts to exhibit ductility — it deforms rather than cracks on drilling. Therefore, directly accessing the supercritical hydrothermal resource is challenging; the possibility of inducing seismic activity is also very high due to the rock’s properties. A better understanding of the geology at the BDTZ and identifying the right spots where the rock is brittle enough to drill through are vital, but again, the practicality of economically drilling to such depths remains a question mark.
What are the other challenges with supercritical geothermal?
Drilling through to the BDTZ is only the first part of the problem. Once there, harnessing power from the supercritical resource is no easy feat. Supercritical water, or any supercritical fluid, is highly corrosive, damaging heat transfer equipment and circulation components. Furthermore, the entry of magma into the system also poses a challenge, as well as the potential emissions of volcanic gases like hydrogen sulfide and sulfur dioxide, albeit in minute concentrations.
What is the current status of supercritical geothermal?
Supercritical geothermal is currently not a commercial solution for low-carbon power, even though a few dozen sites worldwide have run into supercritical fluids or magma when drilled. This was not by design, and power generation was not pursued, again, due to the aforementioned challenges. As of February 2024, several research projects are working or have worked on novel drilling methods and subsurface exploration approaches, such as the European DEEPEGS project (e.g., the Iceland Deep Drilling Project), the Krafla Magma Test in Iceland, the Drilling in dEep, Super-CRitical AMBients of continentaL Europe (DESCRAMBLE, until 2018) in Italy, the Japan Beyond Brittle Project, and the GEMex project in Mexico.
Lux Take
Supercritical geothermal is far away from commercialization and will depend on advances in drilling methods, digital resource modeling, and materials development to access these resources for power generation. Despite its potential, supercritical geothermal will not likely play a prominent role in the energy transition due to its inherent technical impediments. If technical barriers are addressed, supercritical geothermal will be economical only in regions with volcanic or tectonic activity at lower depths unless novel deep geothermal drilling methods prove otherwise.