An introduction to the next clean energy frontier: Superhot rock and closed-loop geothermal systems

This blog is part of a series exploring and explaining the science behind next-generation geothermal, with a special focus on superhot rock geothermal, through a curated tour of influential technical and academic papers. This edition highlights key features of the Geothermics article by White et al. (2024) Numerical investigation of closed-loop geothermal systems in deep geothermal reservoirs. The full blog series, in addition to their reference reports, can be found at the Superhot Rock Resource Library. 

As of 2024, geothermal has only met about 1% of global energy demand, but recent analysis by the International Energy Agency (IEA) suggests that geothermal has the potential to cost-effectively meet up to 15% of global electricity demand growth through 2050. The key to this growth is the expansion of next-generation geothermal projects like enhanced geothermal systems (EGS) and closed-loop geothermal systems (CLGS) into new regions at lower costs.  

Research in CLGS – as with EGS – has been underway for over 50 years. Work on wellbore heat exchangers, which is the basis of CLGS, began in the early 1960s. Since then, technological advancements and demonstration projects have emerged around the world, and there has been an exponential growth in publications focused on CLGS over the last 20 years. 

Given this growing interest in CLGS, a group of researchers from U.S. National Laboratories and Universities developed numerical models to study CLGS and applied the results to an economic analysis of these systems in the 2024 article Numerical investigation of closed-loop geothermal systems in deep geothermal reservoirs (White et al.). The results of their study, combined with a plant and economic model from their research is publicly accessible using the GeoCLUSTER web application.  

Here are our key takeaways from their work, and what they mean for the future of closed-loop geothermal systems.  

Closed-loop geothermal systems can be developed anywhere. 

In CLGS, fluid is contained inside a network of pipes that are heated by contact with the surrounding hot rocks. Thanks to this arrangement, power and thermal energy (e.g., used for district heating) can theoretically be produced anywhere without the need for reservoir stimulation (i.e., the network of pipes act as the reservoir), with little to no water consumption, and with a decreased risk of induced seismicity. These are all advantages that CLGS have over conventional geothermal systems. This has allowed CLGS to advance as a potential source of clean electricity and thermal energy that is reliably available 24/7. However, the technology doesn’t come without its challenges. Conduction can’t transfer thermal energy (heat) as fast as convection, which leads to a quicker drawdown in heat of the reservoir over time relative to conventional geothermal systems, because it takes longer for heat to penetrate back into the reservoir after it has been gathered from the surrounding rock. This can be balanced by extending the length of the total piping in the CLGS. 

The analyses of White et al. focused on the U-shaped and coaxial (also called tube-in-tube or pipe-in-pipe) CLGS configurations (see Figure 1), which are currently the two main designs used for CLGS. Their analyses used both water and sCO₂ (supercritical CO₂) as working fluids and a wide array of adjustable inputs (e.g., mass flow rate, vertical depth, geothermal gradient, rock thermal conductivity, etc.). Eavor and GreenFire Energy have both made progress on development of these designs, with the GreenFire Energy model focusing on the coaxial design and the Eavor model focusing on the U-shaped design. There are numerous other designs and hybrid configurations, including proposals for enhanced closed-loop geothermal systems, such as the coaxial configuration being developed by XGS Energy where conductive material is injected into the rock surrounding the closed-loop to improve “thermal reach”. Additionally, new technology is being researched by a new startup in partnership with Sandia National Laboratories and Lawrence Berkeley National Laboratory is focusing on the development of 3D printed superefficient heat exchangers. The goal of each design is to draw the greatest amount of heat at the lowest cost, while minimizing any environmental impacts. 

Cost of drilling needs to decrease substantially for closed-loop geothermal systems to be economical for energy production, but the economics of closed-loop geothermal systems for heating is within reach. 

In determining economic viability, White et al. compared the results of their model runs to the U.S. DOE goal of targeting a levelized cost of electricity (LCOE) of $45/MWh_e for next-generation geothermal by 2035. Their modeling concluded that regions with a higher geothermal gradient, the use of sCO₂ rather than water as a circulating fluid, and longer lengths (>30 km) using a u-shaped heat exchanger are the most cost effective. Even when considering a drilling cost of $500/m, which is lower than the 2025 updated drilling cost curves from the National Renewable Energy Lab (NREL), their models were still not able to reach the $45/MWh_e DOE target. Their results suggest two important aspects of designing an economically viable CLGS project: 1) drilling costs need to decrease, and 2) lateral lengths closer to 100 km are likely necessary to decrease LCOE to an economically viable point. 

The Eavor-Loop project in Geretsried, Germany is being developed using a multi-lateral u-shape design with a total length of 320-360 km between the four-loop system, suggesting that development of lateral lengths well beyond 20 km are achievable. 

In determining a cost target for levelized cost of heat (LCOH), White et al. used the cost of natural gas as a comparison, and a target of $40/MWh_th. In their model, water outperformed sCO₂ for both heat exchangers (coaxial and u-shaped), but the difference in LCOH was minimal. All model runs were able to reach the target cost of $40/MWh_th, indicating that the use of CLGS for thermal energy (e.g., for district heating) can be cost competitive.  

What can help closed-loop geothermal systems projects become more cost competitive? 

Although the costs to develop CLGS projects for electricity are currently high, there are multiple options for reducing those costs, including: 

• using non-steel casing completion techniques which may improve heat exchange and lower costs; 

• aiming for regions with a higher geothermal gradient and higher temperature rock, like superhot rock temperatures above 400 °C; 

• improving drilling technology to continue to reduce drilling costs; 

• using fluid-saturated reservoirs with higher permeabilities, similar to those found in shallower hydrothermal systems, which could overcome the limits of conduction; and 

• using previously fractured or abandoned wells that could lower capital costs while maintaining, or improving, the performance of the CLGS. 

Importantly, improvements in CLGS technology, such as aiming for higher temperatures and improving drilling technology, will benefit all types of next-generation geothermal technology. 

What’s next? 

While there are challenges with advancing CLGS, it has distinct advantages as well: it doesn’t require reservoir stimulation, consumes little to no ongoing water, and has the potential to unlock geothermal anywhere while providing clean electricity and thermal energy 24/7. CLGS development also shifts the risk for developers from resource identification to engineering hurdles, with commercial projects already moving forward with public and private funding. White et al. show that regions of higher geothermal gradients have lower associated costs in addition to the need for longer networks of piping – showing that development of CLGS projects could be a key player in driving costs down for all types of next-generation geothermal, including at continually higher temperatures. 

Stay tuned for more from CATF as we continue to push forward bold ideas on how to scale superhot rock geothermal as an essential source of reliable and clean energy. 

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This blog is part of a series exploring and explaining the science behind next-generation geothermal energy, with a special focus on superhot rock geothermal:

• Superhot rock geothermal and a vision for firm, global clean energy
• Superhot rock geothermal and pathways to commercial liftoff
• Superhot rock and the future of geothermal
• Superhot rock and the challenges in developing enhanced geothermal systems
• Superhot rock and the opportunities for responsible development

Through a curated tour of influential technical and academic papers, the series aims to provide a fresh perspective from a geoscientist entering the geothermal industry. The goal is to share my learning journey and encourage collaboration around these groundbreaking solutions, which are critical to achieving a clean energy future. Whether you’re new to geothermal or looking to deepen your knowledge, I hope this series offers valuable insights into this fast-evolving field.