Deep Energy Exploration & Production: A Modest Quest to the Center of the Earth
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 U.S. 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 U.S. 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 U.S. energy mix in 2022, according to the EIA.
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.
With this blog and our upcoming series, we will delve into the technological advancements and strategic insights that underpin the future of geothermal energy. Our journey will explore the roadmap driving geothermal energy towards a brighter, more sustainable future, highlighting where smart investments can catalyze a transformative shift in how we harness the Earth’s boundless geothermal resources.
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.
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 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, 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.
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.
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’ 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 Stevenson 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 Stevenson’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.
Breaking New “Ground”: the Brittle-Ductile Transition Zone
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.
The Future is Superhot: The Path Forward
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, we could see the modern-day shale revolution for geothermal energy. This journey encompasses several critical advancements that we plan to unpack:
- Enhanced Modeling and Predictive Capabilities: Advancing our modeling and computational predictive capabilities can substantially de-risk the exploration and exploitation of SHR resources, ensuring a more predictable and secure path for commercialization and investments.
- Innovative Drilling Techniques: Accessing the depths where SHR resources reside demands breakthroughs in drilling technology, including novel methods such as mm-wave drilling and gravity hydraulic fracturing.
- Higher Efficiency Power Cycles: Supercritical CO2 Brayton power cycles could potentially obtain much higher thermal efficiencies, meaning that SHR reservoir geothermal fluids will not only carry more energy per mass but also allow a significantly greater proportion of this heat to be converted into electricity.
Adventuring deep into the hot core within our earth is a challenge beyond mere technical exploration; it is an expansion of our mental model of what’s possible. In the upcoming series of blog posts, we will delve deeper into the technological roadmap and landscape that underpins this exciting journey. Stay tuned as we explore the innovations and breakthroughs that are setting the stage for a superhot future in renewable energy. If the potential of superhot geothermal energy excites you as much as it does us, and you are working on technologies that push the boundaries of geothermal energy exploration and exploitation, please reach out at wenyuan@primemoverslab.com.
Prime Movers Lab invests in breakthrough scientific startups founded by Prime Movers, the inventors who transform billions of lives. We invest in seed-stage companies reinventing energy, transportation, infrastructure, manufacturing, human augmentation, and agriculture.
