As the global demand for clean energy continues to grow, geothermal energy has emerged as a vital resource. With its consistent and predictable output, geothermal energy offers a reliable alternative to weather-dependent energy sources like wind and solar.
One of the key challenges in geothermal energy production has been the infrastructure needed to transport heat from geothermal wells to power plants.
Traditionally, aboveground piping systems have been the preferred method for transferring geothermal energy, largely due to the elevated operating temperatures involved, but aboveground systems can be expensive and visually intrusive.
To address these concerns and explore more cost-effective and visually low-impact alternatives, Fervo began looking at other industries to see what alternatives existed. One opportunity Fervo identified was the use of in-ground pipelines that are common in the oil and gas industry. To discover the potential of this opportunity, Fervo commissioned a study to investigate the feasibility of using underground direct buried piping systems for geothermal energy applications. The study focused on the technical, environmental, and economic aspects highlighting the potential benefits as well as the challenges that must be overcome. This article delves into the key findings from the study and explores how underground piping systems could revolutionize geothermal energy infrastructure.
The findings of this study were presented at the ASME Power Conference held in Washington, D.C. on September 15, 2024.
Overview of Geothermal Energy Gathering Systems
Geothermal energy is derived from the heat stored beneath the Earth’s surface, and it can be used to generate electricity or provide direct heating for residential, commercial, and industrial applications. To harness this energy, fluid—often brine or “mineral rich water”—is circulated from geothermal wells and transported through pipelines to power generation facilities. The piping systems used in this process must be able to manage extreme temperature, pressure, and the geochemical makeup of geothermal fluids. Historically, these systems have been designed to run aboveground due to the need for easy maintenance, inspection, and management of heat loss as well as the high operating temperatures.
However, above ground systems come with their own set of challenges. They are costly to construct, and their visibility can disrupt the landscape. These factors have led to growing interest in burying piping systems as an alternative.
Challenges of Underground Geothermal Piping Systems
While underground buried piping systems offer several advantages, such as reduced visibility and decreased cost, they also introduce new challenges. One of the primary concerns is the ability of underground pipes to withstand the elevated temperatures typical of geothermal energy systems. Geothermal fluids can reach temperatures of >200°C, and maintaining the integrity of underground pipes at these temperatures can be difficult.
The study highlighted several technical challenges associated with burying geothermal gathering lines, including:
Thermal Expansion and Buckling: When pipes are exposed to elevated temperatures, they expand. In an aboveground system, the system is generally unconstrained in the diameter of the pipe which limits the lateral expansion. However, when buried, the earth around the pipe limits the diameter expansion and increases the lateral expansion. This leads to a higher potential for buckling.
Heat Loss: While aboveground pipes can be easily insulated to minimize heat loss, underground systems face unique challenges in maintaining thermal efficiency. The surrounding soil can absorb heat, leading to energy loss as the geothermal fluid travels from the well to the power plant. This study investigated the potential heat loss of buried piping and how it will affect the pipeline productivity.
Mitigation Techniques for Underground Piping Systems
To address the challenges of underground geothermal piping, the study proposed several mitigation techniques. The most impactful of these mitigations was an increased number of expansion loops recommended within the system. Expansion loops are areas that function as springs and allow the system to have thermal expansion without allowing too much stress.
Another possibility was looking at alternative pipe materials. Examples are higher strength steels and non-metallic piping. The non-metallic options offer a dual opportunity of having less corrosion potential when in contact with the brine as well as decreased thermal expansion. More research is needed to find the most effective material choice since most non-metallic piping options were found to be limited to 100°C, which is below the required design temperature.
Environmental and Economic Considerations
In addition to the technical challenges, the study also explored the environmental and economic impacts of underground piping systems. From an environmental perspective, burying geothermal pipelines can help preserve natural landscapes and reduce the visual impact of geothermal power plants. Underground systems also offer protection from environmental hazards, such as storms and temperature fluctuations, which can extend the lifespan of the infrastructure.
Economically, the initial cost of installing underground systems is lower due to not requiring expensive foundations and supports. Overall, the study found that Fervo could save 20-30% on pipeline costs if they were to transition to buried underground piping.
Conclusion: A Path Forward for Geothermal Energy Infrastructure
The findings from this study offer valuable insights into the feasibility of underground piping systems for geothermal applications. While there are still technical challenges to overcome, advancements in materials, design, and installation techniques offer promising solutions. The use of underground systems could reduce costs, minimize environmental impact, and enhance the sustainability of geothermal power plants.
As the demand for renewable energy continues to rise, the development of robust and reliable geothermal infrastructure will be critical. By addressing the challenges of underground piping, engineers, researchers, and policymakers can help unlock the full potential of geothermal energy, paving the way for a cleaner and more sustainable energy future.
