As the power industry continues striding toward low- and no-carbon solutions, geothermal energy is getting a fresh look.
It’s long been known that geothermal is a viable source of clean, renewable energy. By tapping into the earth’s always-on internal heat, geothermal can provide on-demand power to help satisfy growing energy demands. Geothermal energy is essentially a fuel-free resource that leverages proven conversion technologies to provide cost-effective power generation.
Though geothermal has attracted far less attention than other renewable energy sources, the power industry is poised to realize significant gains in clean power available on the grid, thanks to new technology advances and adaptation of proven technologies.
Geothermal energy utilizes heat within the Earth’s interior for heating, cooling or power generation. Heat is collected and transferred to the Earth’s surface in the form of hot water, steam or other working fluids. It can then produce electricity with proven power conversion cycles and processes. Geothermal power facilities follow similar design principles as other types of thermal power plants, allowing for transferability of known processes and equipment.
Types of Geothermal Reservoirs
The technology used to extract geothermal energy depends not just on temperature but also on subsurface conditions. There are four primary geothermal systems.
Traditional hydrothermal utilizes naturally occurring subsurface hot water or steam resources or relies on specific geologic conditions, including permeability and porosity. This process is well established, but it has a limited geographical footprint due to the need for specific geological formations and conditions.
Enhanced geothermal systems (EGS) are artificial geothermal energy systems that provide access to geothermal heat sources that lack the geologic conditions associated with traditional hydrothermal systems. EGS improve the permeability of geothermal systems through hydraulic, chemical and thermal stimulation. In these systems, proven drilling technologies and practices often used in oil and gas production are utilized, and a combination of horizontal drilling and fracking processes are used to create flow paths within subsurface geology. Water is then circulated through the fractures, where it absorbs heat and returns to the surface, where it is used to generate electricity. EGS have the potential to expand the footprint of geothermal energy available for power generation, making geothermal viable in many regions that had been deemed unsuitable due to low permeability or lack of underground water resources.
Advanced Geothermal Systems (AGS) are large, artificial closed-loop systems drilled into subsurface rocks that essentially function as deep, closed-loop heat exchangers. A working fluid is circulated through the circuits and heated via conductive heat transfer. As there is no reservoir stimulation, this process reduces the risk of induced seismicity. It has potential for reduced water usage and high heat production capability. Though AGS theoretically can be applied anywhere, it requires longer well bores to increase heat transfer surface area, which may increase drilling costs.
Supercritical Geothermal Systems leverage deep, high-temperature geothermal resources above approximately 370 to 400 degrees Celsius. At these high temperatures and pressures, the injected working fluid (water) transforms to a supercritical state that is able to penetrate fractures faster, increasing the speed and efficiency in transferring subsurface energy to the surface. These higher temperatures also allow for more efficient energy utilization and conversion to electrical energy. This method unlocks more potential geothermal energy zones across more geographic regions as well as the potential for more power capacity from geothermal plants. Although currently in development, implementation of supercritical geothermal systems does require advances in drilling technologies and well completions, due to the aggressive conditions and high temperatures.
How It Works
There are three primary processes used in the conversion of geothermal energy to power generation.
Dry steam process is the oldest and simplest method. With this process, steam is drawn directly from existing underground reservoirs through a piping system. Once at the surface, the steam is used to directly drive a steam turbine to produce electricity. These systems require limited and unique geologic conditions, such as those found at The Geysers in California.
Flash steam process uses high-pressure hot water pumped from below the Earth’s surface. Once above ground, the pressure is reduced to flash the hot water into steam, which is then used to drive a steam turbine and produce electricity. After extracting energy and generating electricity in the turbine, the steam is condensed back to water and reinjected back into the underground reservoir to be reheated and repeat the process. Depending on the temperature and available energy, the flash steam process may incorporate multiple flash stages at different pressures and temperatures to increase the overall efficiency of the process.
Binary cycle is a process in which the hot water extracted from under the Earth’s surface is used to heat a secondary liquid (often a carbon-based fluid) with a lower boiling point. This is a closed-loop system in which the vaporized secondary fluid drives the turbine. Binary-cycle plants are more flexible in terms of temperature requirements, meaning they can operate in lower-heat areas.
Exciting Outlook
The challenges for the EGS process include higher drilling costs, complex drilling operations, induced seismicity and managing long-term reservoir sustainability. AGS challenges include expensive and complex drilling operations marked by engineering difficulties in downhole operations as well as obtaining sufficient surface area for effective heat exchange. Challenges facing supercritical geothermal include needed advancements in drilling technologies and methods, and managing extreme heat and pressures and equipment corrosion that these conditions cause.
However, solutions are underway. Existing and emerging companies are investing in novel drilling technologies, such as millimeter-wave systems, to overcome current limitations. These innovations will be targeting the current challenges facing the EGS, AGS and supercritical geothermal processes, and if successful, hold great promise in unlocking vast geothermal resources previously considered inaccessible.
According to a 2024 estimate by the U.S. Department of Energy, the potential geothermal output from advanced processes could result in more than 60 GW of power capacity by 2050. With more wells, geothermal is inherently scalable.
While geothermal energy is gaining increasing attention in numerous locations, Texas is one state that is considering putting serious funding behind a plan to incubate next-generation geothermal energy. The Texas Legislature is debating voting to allow a $5 billion program that was initially developed to provide low-interest financing to developers of new gas-fired generating plants to also include geothermal projects. Called the Texas Energy Fund (TEF), the program has progressed slower than expected after several companies withdrew applications, due to an inability to procure gas turbines and other equipment in time to meet loan terms.
Though it is clear that geothermal energy is not a one-size-fits-all solution, it offers a compelling mix of benefits, including continuous availability, low emissions, minimal land use and energy security. As innovation continues to lower technical and economic barriers, geothermal could play a pivotal role in the future energy mix.
A range of permitting and engineering services are needed for geothermal project development.