Superhot Rock Geothermal

Wenyuan Chen, Fellow
June 2024

Introduction Background Key Technical Areas of Focus Conclusion Landscape

Introduction

In our relentless pursuit of greater power densities, the transition from rudimentary energy sources such as wood to the sophistication of nuclear power encapsulates humanity’s continuous quest for more efficient, powerful, and cleaner energy solutions. This journey, as detailed in Vaclav Smil’s Power Density, highlights significant leaps in our ability to harness energy more densely and efficiently, driven by groundbreaking scientific and technological innovations. With the US goal to create a carbon pollution-free power sector by 2035 and a net zero emissions economy by no later than 2050, our exploration for dense energy sources enters a critical phase. Two pivotal considerations emerge in this context: first, while solar and wind have dominated the clean energy discourse, their intermittent (low capacity) nature poses challenges without viable and cost-effective long-term energy storage. Second, despite the potential of nuclear energy — both fission and fusion — to contribute to carbon-neutral energy grids, as seen in countries like Sweden and France, it is beset by technical, psychological, and regulatory challenges, stemming from historical mishaps and public apprehension.

Yet, beneath our feet lies an untapped subterranean powerhouse—a vast reservoir of heat trapped within rock known as geothermal energy. To put its potential into perspective, a 2005 MIT report estimated that the US geothermal resource base exceeds the country’s annual energy consumption by more than 100,000 times, with a conservative estimate suggesting that the recoverable geothermal resources could meet 2,000 times our total annual energy demand. Despite this, geothermal energy contributed a mere 0.2% to the US energy mix in 2022, according to the EIA.

Potential next-generation geothermal resources (red shading) compared against current conventional geothermal plants (black dots). Image source: DOE’s Geothermal Commercial Liftoff Report.

Elevating geothermal energy from its niche status to a major contender in the global energy market necessitates a significant leap in energy output per well, alongside achieving cost-effectiveness and appropriate risk adjustment. For investors and industry stakeholders, understanding where to direct capital for maximum impact is key. This includes focusing on areas of innovations that show promise to shift the cost dynamics of geothermal energy production.

Background

Geothermal Today - Open or Closed?

At its core, extracting thermal energy from the Earth’s depths revolves around two key challenges: accessing the vast reservoirs of energy below the surface and efficiently transferring that energy to where it can be utilized. This process typically involves the use of working fluids to facilitate heat transfer, leading to a division in the geothermal industry based on the method of fluid routing: open-loop and closed-loop systems.

Comparison between conventional and next-generation geothermal systems. Image source: DOE’s Geothermal Commercial Liftoff Report

Open-loop systems have been the traditional backbone of geothermal energy production. These systems capitalize on either naturally occurring hydrothermal reservoirs or engineered reservoirs created through enhanced geothermal systems (EGS). These setups require the circulation of significant volumes of working fluid, often facing the challenge of overcoming high friction pressures when pumping through subsurface fractures to maintain effective flow. In contrast, closed-loop systems, also called advanced geothermal systems (AGS), confine the working fluid within engineered conduits, acting as underground “radiators” that transfer heat through conduction. This configuration offers a wider choice of working fluids due to its sealed nature but necessitates extensive surface area contact between the conduits and the hot rocks for efficient heat extraction.

The journey of geothermal energy from the construction of the first geothermal plant in 1904, harnessing natural steam, to the present day, showcases a slow but steady progression. Despite over a century of innovation, the widespread adoption of geothermal energy has been markedly slow, largely hindered by economic barriers. To date, the levelized cost of electricity (LCOE) for geothermal has not been competitive enough to spur broad-scale deployment as compared to natural gas, solar, and wind. However, we are starting to see the emergence of the inflection point for geothermal energy, thanks to technological advancements aimed at accessing deeper, hotter reservoirs. Tapping into such resources could dramatically increase the electricity production capacities from the conventional 3-5 MW per well to 50-100 MW per well, fundamentally changing the economics and viability of geothermal energy.

Going Deeper: The Supercritical Advantage

In the evolving landscape of renewable energy, power density stands as the pivotal metric, defining the amount of power generated per unit area. Elevated power densities enable the generation of substantial energy from minimal land use, significantly reducing both capital expenditures and environmental impacts, while simultaneously enhancing output efficiency. The power output from a geothermal reservoir is significantly influenced by three factors: the enthalpy of the working fluid, the mass flow rate of the production wells, and the energy cycle efficiency of converting the thermal energy into electric power.

Enthalpy measures the thermal energy that geothermal fluids can carry from the Earth’s interior to the surface, directly impacting the potential for power generation. The value proposition of geothermal energy can be transformed by reaching depths where geothermal fluids become supercritical, which exists at conditions where fluids surpass their critical temperature and pressure—water, for instance, becomes supercritical at 374 °C and 22 MPa. At these supercritical states, also known as ‘superhot’, the conventional boundaries between liquid and gas phases dissolve, allowing the fluids to exhibit both enhanced enthalpy and increased mass transport rates. This unique combination facilitates the transport of greater amounts of thermal energy and higher heat-to-power conversion cycle efficiencies.

Water phase diagram indicating the enthalpy, temperature, and pressure conditions for SHR and conventional geothermal resources. Image source: Hotrock Energy Research Organization (HERO) Report

Superhot rock (SHR) geothermal resources capitalize on this supercritical advantage. By drilling deeper beyond the current depths of hydrothermal and EGS systems, and into regions where rocks are at temperatures above 400-450 °C, SHR has the potential to increase the power output per well by 5-10 times compared to current EGS projects. The higher power output achievable per well means that SHR can extract more heat using far fewer wells, leading to a reduced environmental footprint. Specifically, SHR operations will necessitate less than one-tenth the land and water usage of conventional geothermal systems. Finally, the LCOE for SHR is projected to undercut that of natural gas, marking a turning point for its economic viability and setting the stage for broader adoption.

The potential of superhot rock geothermal resources amongst various energy sources. Left: Power density of several fossil fuel and renewable energy sources. Image source: Clean Air Task Force (CATF) Glossary; Right: LCOE and megawatt-output per geothermal well as the reservoir temperature increases. Image source: Polpis Systems DEEP V1

Breaking New "Ground": the Brittle-Ductile Transition Zone

Compared to conventional geothermal, superhot rock systems are deeper, providing access to hotter dry rock systems. Image source: CATF Report

The Earth’s crust is a complex tapestry of materials and conditions that govern the behavior of rocks under various conditions. Near the surface, rocks exhibit a brittle nature, breaking under limited deformation. This characteristic changes profoundly as we delve deeper, following the earth’s thermal gradient, marking a critical consideration for SHR geothermal energy extraction. The journey through the Earth’s crust reveals two distinct phases of rock behavior that are pivotal to unlocking geothermal potential.

Initially, as depth and confining stress increase, rock strength also rises. In this phase, prevalent in most current geothermal drilling projects, rocks retain their brittle nature, containing open fractures or being susceptible to induced fracturing, thus exhibiting high permeability. This environment is conducive to the development of both hydrothermal and EGS, where natural and artificially created fractures form conduits for geothermal fluid flow, enabling effective heat extraction by circulating a reservoir fluid.

With further increases in confining stress and temperature, a significant transformation occurs—the mechanical behavior of rocks shifts from brittle to ductile. This transition, occurring in the Brittle-Ductile Transition (BDT) zone, results in rocks that can undergo extensive deformation without abrupt failure. However, this ductility comes at a cost: fractures begin to close under the plastic flow of the rock matrix, diminishing permeability and impeding fluid flow and heat transfer to the shallow subsurface. The BDT zone’s depth and temperature thresholds vary, typically occurring around 350-400 °C and depths of 10-15 km, though this can vary in regions with active magmatism.

So the key to unlocking SHR energy lies in determining what depth it can be found and devising methods to effectively extract it. Successfully harnessing the power of SHR demands not just technical innovation but a deep understanding of the geological variables that define this unique energy landscape. Drilling and exploration must be done at temperatures and pressures that far exceed even the most advanced oil and gas capabilities and extraction of the thermal energy is markedly different from EGS. Since the energy resource is geographically ubiquitous, solving these challenges could potentially make SHR accessible worldwide.

Key Technical Areas of Focus

Recent funding initiatives, such as a $20 million additional Series A fund for XGS Energy and the Biden-Harris Administration’s $60 million investment in three EGS projects (Chevron New Energies, Fervo Energy, and Mazama Energy), alongside the DOE’s Geothermal Commercial Liftoff Report, underscore the government’s commitment to advancing geothermal energy and investor interests. This focus is crucial as these projects and technologies pave the way towards the development of SHR geothermal energy, aiming to make geothermal power cost-competitive and scalable.

Previously, we introduced SHR geothermal energy, discussing its potential economic viability due to its high temperatures and unique geographic characteristics. While the prospects are promising, significant challenges remain, prompting questions about the feasibility of pursuing geothermal 2.0 when the current “next-generation geothermal” technologies — EGS & AGS — have yet to achieve widespread deployment.

Just as it took over two decades to economically extract shale gas, EGS and AGS have been under development for many years, with commercial activities only recently gaining momentum. This highlights two critical points: geothermal projects are capital-intensive and have long development timelines. Delays in deployment will eventually strain the industry and supply chains, and complicate our efforts to achieve net-zero emissions, a challenge also faced by the nuclear energy sector.

Thus, it is imperative to outline the necessary steps to make geothermal energy cost-effective and competitive with future energy sources. It is worth recognizing that this is not a winner-takes-all scenario. In the near term, EGS and AGS systems will continue to be developed and refined as they are now. As we advance our ability to harness deeper geothermal resources, the role of SHR geothermal energy will gradually increase, complementing the existing systems and contributing to a diversified and resilient energy landscape.

Here we will explore emerging innovations and trends and highlight the innovators addressing these challenges. This is not an exhaustive list, but it will provide insights into the state-of-the-art across multiple disciplines, identifying gaps in knowledge and technology that require further research and investments. Within this multifaceted challenge for developing SHR, there are the following critical areas:

Exploration, characterization, and modeling of SHR resources
Drilling and completion of wells in high pressure and temperature conditions
Reservoir creation and management
Materials and tools for high pressure and temperature conditions
Power conversion systems

Cross sectional view of what a SHR geothermal system would look like. Image adapted from: CATF

Among these, we believe there are three key areas that should be of high focus to drive the development of SHR geothermal energy.

Modeling and predictive capabilities
Deep drilling and fracture stimulation
Energy extraction & power generation

Modeling and Predictive Capabilities

Choosing the right location to develop geothermal energy is a function of risk and the readiness of technologies required to reach and extract thermal energy. Compared to conventional hydrothermal resources, SHR resources are ubiquitous, occurring at depths dependent on the local temperature gradient. This shifts the focus from locating resources for shallow to deep subsurface geological, geophysical, and geochemical parameters.

Adapting existing modeling packages to supercritical conditions would be the first step. Models would need to account for significant changes in fluid properties near the critical point, phase behavior, and the combined effects of conduction and convection heat transfer. Additionally, geochemical reactions like mineral dissolution and precipitation and mechanical behavior such as stress, strain, and fracture propagation within the reservoir should also be considered. A comprehensive understanding of the spatial and temporal variation of subsurface conditions would require not only these upgraded models but also lab data or field measurements. We are already seeing machine learning techniques being used for geothermal play fairway analysis (PFA), a method adapted from the oil and gas industry for regional- and local-scale geothermal resource identification and risk assessments, and expect further utilization of advanced computational methods to aid in the conceptual modeling, predicting, and well targeting.

Another key component to addressing risk in SHR development is the techno-economic models. Current models used in evaluating the commercial viability of geothermal production include GEOPHIRES and GETEM. These models predict the levelized cost of electricity (LCOE) given a user-defined geothermal resource and development concept. The missing pieces to presenting the cost and investment opportunity are accounting for parameter uncertainties and the capability to reverse predict, i.e. given a range of LCOE and defined geothermal resources and presenting various development solutions based on technology readiness levels of each component. This would require a larger library of technology models, even ones that are currently in their early stages of development but would present users with more options and bring awareness to early-stage technologies that could potentially be solutions to specific scenarios, such as SHR.

Related companies

Zanskar: using AI and advanced sensing techniques to drive down the risk and cost of geothermal development.

Polpis Systems: techno-economic modeling and technology roadmap for SHR. At the end of 2023, Prime Movers Lab partnered with Aaron Mandell, an experienced geothermal technology entrepreneur, to incubate Polpis Systems. The mission of Polpis was originally inspired by Caltech Professor David Stevenson’s ‘A Modest Proposal’ (ref) where he proposed to send a probe into the earth’s core. In the journal Nature, Stevenson observed that “planetary missions have enhanced our understanding of the solar system but no comparable exploratory effort has been directed towards the Earth’s interior”. While Stevenson readily admitted that many scientists may laugh at his proposal, Aaron found himself in the 5% that feel it should be considered, seriously. While Stevensen focused on calculating the mass of molten iron required to propagate a downward fracture containing a grapefruit-sized probe, Polpis’s foundation is based on modeling the cost of ultra high-temperature energy extraction from the earth’s deep core. In the spirit of Stevenenson’s paper, Polpis is open-sourcing the calculations and supporting physics of energy extraction from the core to encourage scientists to participate and expand on this energy moonshot.

Deep Drilling and Fracture Stimulation

Conventional mechanical drilling technologies involve applying downward pressure on a cutting tool, which is then rotated to provide the necessary torque for gouging or scraping the rock surface. These technologies have been deployed for decades to reach depths of up to 10 km and reach temperatures greater than 400 °C. The deepest drilled holes attempted are the Kola Superdeep Borehole (12.26 km, 180 °C) and the KTB borehole (9.10 km, 265 °C) but these projects were not focused on mining thermal energy. Several (shallow) SHR drilling projects have been accomplished, such as the Venelle-2 well in Italy (2.90 km, 500 °C) and IDDP-2 well in Iceland (4.65 km, 427 °C).

Drilling rate improvements since 2017 at the Frontier Observatory for Research in Geothermal Energy (FORGE) well, a dedicated field site in Milford, Utah for EGS technologies. Image source: DOE’s Geothermal Commercial Liftoff Report

Deep drilling using mechanical drilling technology is cost prohibitive, with the mechanical rate of penetration affected by various factors such as geological formation strength and type as depth and temperature increases, pore pressure, and bit performance (wear, shortened life, low efficiency). In the context of SHR, horizontal wells or directional drilling along an isotherm is not strictly required, as drilling deeper will lead to higher temperatures and is preferable. To reach deeper and hotter SHR resources, new approaches have started to gain commercial traction. Instead of mechanical abrasion, it is possible to direct energy for drilling using energy sources such as electromagnetic waves or plasma. The main advantages here are that no mechanical systems could wear and break, and there is potentially no temperature/pressure limit.

mm-Wave: Quaise Energy is using gyrotrons to generate continuous, high-power (megawatt-scale) millimeter wave radiation and uses metallic waveguides to guide the electromagnetic wave downwards. When the beam reaches the bottom of the borehole, it not only vaporizes the rock and accomplishes the drilling action, but the beam also diverges after exiting the waveguide, effectively heating the walls of the hole and making it glassy, a process called vitrification, which can seal and strengthen borehole walls.

Plasma: GA Drilling is developing a different mechanism called plasma pulsed geo drilling, a process that uses high voltage impulses with short rise times to fracture rocks. This is a process based on electrical discharges formed between two electrodes separated by a dielectric liquid. The key here is to have steep pulses that operate in a region where the dielectric strength of water is stronger than that of the rock, allowing the discharge, or plasma channel, to be formed within the rock. The pressure in the plasma channel exceeds the tensile strength of the rock thereby breaking the rock.

Gravity hydraulic fracturing: This method uses a high-density fluid to induce downward fracture propagation under gravity forces. Hydraulic fracturing, or fracking, is a common method used to extract natural gas or oil by high-pressure injection of fluid into wellbore to create cracks. The key difference here is to use a fracturing fluid heavier than the surrounding rock, which allows gravity to aid or replace the need for injection by pumping. Although this method has only been modeled in research settings, Polpis Systems is aiming to induce a gravity-driven magma fracture that is effectively an “engineered fault” in the earth’s crust.

Energy Extraction & Power Generation

Comparison of conventional geothermal power plant types. Source: Geovision 2019

At superhot discharge temperatures exceeding 374 °C and pressures above 100 bar, turbines achieve significantly higher thermal energy conversion efficiencies. Utilizing existing technologies from subcritical to supercritical coal and gas-fired steam turbines, efficiencies are expected to range between 30% and 40%. Adapting power generation cycles that use supercritical water, commonly found in coal-fired and nuclear power plants, for SHR resources is possible. However, these systems face challenges due to fluid chemistry management, as the interaction between supercritical water and rock at SHR conditions can produce corrosive and metal-rich fluids, potentially damaging conventional turbine systems.

An alternative to water as the working fluid is carbon dioxide (CO₂), which offers several benefits: it is non-explosive, non-flammable, non-toxic, and readily available at a low cost. CO₂ reaches its critical point at 7.4 MPa and 31 °C, making it easier to achieve supercritical conditions compared to water. In geothermal wells, supercritical CO₂ (sCO₂) reduces the need for external power sources to pump the working fluid because of the large buoyancy forces generated. These buoyancy forces arise from the significant density difference of sCO₂ when it heats up upon reaching the production well, a phenomenon known as the thermosiphon effect. This effect is more pronounced in CO₂ than in water due to the higher compressibility of sCO₂.

The main advantage of the sCO₂ power cycle is its high thermal efficiency at moderate temperatures, achieved through minimal compression work and the extensive recovery of heat from the turbine exhaust. In its supercritical state, CO₂ is nearly twice as dense as steam, making it more energy-dense. This allows for smaller system components, such as turbines and pumps, which reduces the overall plant footprint and potentially lowers capital costs.

Left: Cycle efficiencies of steam (Rankine), CO₂ Brayton, and He Brayton at different heat source temperatures. Image adapted from: Supercritical CO₂ direct cycle Gas Fast Reactor (SC-GFR) concept; Right: Size comparison of sCO₂ turbines and steam turbine. Image source: Turbines can use CO₂ to cut CO₂

Sandia National Laboratories has developed a recuperated closed-loop Brayton cycle using sCO₂ as the working fluid, heated up to 315 °C. This system successfully delivered continuous power to the grid for 50 minutes. Future tests are planned to include a 1 MW-scale generator that will not require electrical power from the grid to heat the sCO₂.

Another recent advancement occurred at the Supercritical Transformational Electric Power (STEP) Demo pilot plant at Southwest Research Institute. The team successfully test-fired the STEP plant, with their natural gas-powered turbine reaching speeds of 18,000 rpm and temperatures of 200 °C. Future steps will involve increasing temperatures to 500 °C and eventually to 715 °C, with the turbine operating at a full speed of 27,000 rpm to achieve a 10 MWe output.

Conclusion

The Future is SuperHot

The exploration of the BDT zone and SHR resources represents a frontier of both significant challenge and opportunity. SHR geothermal energy not only promises a significant leap in power density but also offers a geographically independent, competitively dispatchable, and clean energy source with minimal surface footprint. It represents the single most important and unexplored domestic energy alternative to advanced nuclear. With the right capital deployment and technological advancements, which we are beginning to see, we could see the modern-day shale revolution for geothermal energy.

That being said, it is notable that the DOE has left supercritical geothermal technology off of its earthshot targets for achieving commercial liftoff. Instead, the DOE’s geothermal technologies office has adopted a “learning by doing” cost reduction methodology that forecasts an unsubsidized LCOE of $45/Mwh by 2035. There are two problems with this that create opportunities for technologists and entrepreneurs. The first is that this roadmap concludes that deployed capital, rather than technology, is the primary limiting factor. This is a dangerous (and expensive) departure for almost every other industry of comparison (telecom, computing, drug development, etc.) where advances in technology are the sole reason for dramatic cost reduction. Second, energy is a fierce commodity where only cost matters and advanced geothermal is competing with dramatic advances already being made in next-generation nuclear fission and solar plus battery storage. As such, the competitive cost of energy is a moving target and technologies need to compete not with today’s cost of energy, but the costs of energy once those advancements are online.

Cost reduction waterfall for EGS and AGS. Image source: DOE’s Geothermal Commercial Liftoff Report

In our previous prediction on next-generation geothermal power, we anticipated that EGS and AGS would achieve notable milestones by 2030. These technologies have indeed made significant strides, but the potential of SHR geothermal energy represents an even more transformative opportunity. With higher power densities and the promise of cost-effective power generation, SHR geothermal energy could play a pivotal role in meeting global energy demands sustainably. Looking ahead, with continued investment, technological breakthroughs, and collaborative efforts, it is plausible that SHR geothermal energy could achieve commercial viability within the next decade.