An enhanced geothermal system (EGS) creates geothermal electricity even when natural heat and water resources are not easily available. In the past, geothermal power systems could only work in places where heat, water, and rocks that let water flow were naturally present. However, much of the geothermal energy that can be reached using traditional methods is found in rocks that are dry and do not allow water to pass through easily. EGS technologies increase the use of geothermal energy by using methods like "hydraulic stimulation," which involves injecting water into rocks to improve their ability to let water flow.

Overview

In many rock formations, natural cracks and pores do not allow water to flow easily. To improve this, a method called hydro-shearing is used. This involves pumping high-pressure water into naturally fractured rock through an injection well. The pressure from the water increases inside the rock, causing events that widen existing cracks and improve the rock’s ability to let water pass through. As long as the water pressure is kept steady, the cracks stay open without needing special materials to hold them open.

Hydro-shearing is different from hydraulic tensile fracturing, a method used in oil and gas industries. This other method can create new cracks in addition to widening existing ones.

Water flows through the cracks and absorbs heat from the hot rock. It then rises to the surface as hot water. The heat from the water is used to generate electricity. This is done using either a steam turbine or a binary power plant system, which cools the water. The cooled water is then sent back underground to repeat the process.

EGS plants, or enhanced geothermal systems, produce electricity continuously, like a steady power source. Unlike hydrothermal systems, EGS can work almost anywhere in the world, depending on how deep the heat source is located. Good locations are often areas with deep granite covered by thick layers of sediment that help trap heat.

Advanced drilling methods can reach depths of up to 15 kilometers, where rock temperatures are 400 degrees Celsius or higher. These high temperatures increase as depth increases.

EGS plants are expected to operate for 20 to 30 years.

EGS systems are being tested and developed in countries like Australia, France, Germany, Japan, Switzerland, and the United States. The largest EGS project is a 25-megawatt demonstration plant in Cooper Basin, Australia. This area has the potential to generate up to 5,000 to 10,000 megawatts of electricity.

Research and development

EGS technologies use different methods to create more paths for water and heat to move. EGS projects have used hydraulic, chemical, thermal, carbon-based, and explosive methods to improve the ability of rocks to let water pass through. Some EGS projects work near hot, but hard-to-reach, rock layers where drilled wells meet hot, yet impermeable, reservoir rocks. These methods help increase the permeability of the rocks. The table below shows EGS projects around the world.

The Australian government has given money for research on Hot Dry Rock technology. Projects include Hunter Valley (1999), Cooper Basin: Habanero (2002), Cooper Basin: Jolokia 1 (2002), and Olympic Dam (2005).

The EU's EGS R&D project in Soultz-sous-Forêts, France, connects a 1.5 megawatt demonstration plant to the electrical grid. The Soultz project studied how to link multiple stimulated areas and tested a setup with one well injecting water and two wells producing heat. Soultz is located in the Alsace region.

An earthquake in Basel caused the EGS project there to stop.

The Portuguese government gave an exclusive license to Geovita Ltd in December 2008 to explore geothermal energy in a promising area of continental Portugal. Geovita is now studying an area about 500 square kilometers in size with the Earth Sciences department of the University of Coimbra's Science and Technology faculty.

The Pohang EGS project began in December 2010 with the goal of producing 1 megawatt of power.

The 2017 Pohang earthquake may have been connected to the Pohang EGS project. All research stopped in 2018.

United Downs Deep Geothermal Power is the first operational geothermal electricity project in the United Kingdom. It is located near Redruth in Cornwall, England. It is owned and run by Geothermal Engineering (GEL), a private UK company. The drilling site is on the United Downs industrial estate, chosen for its geology, existing electrical connections, access roads, and limited impact on local communities. Energy is produced by moving water through a naturally hot reservoir, using the heated water to turn a turbine and generate electricity and heat. A lithium resource was found in the well.

The first EGS effort—called Hot Dry Rock—happened at Fenton Hill, New Mexico, with a project led by the federal Los Alamos Laboratory. It was the first attempt to create a deep, full-scale EGS reservoir.

The EGS reservoir at Fenton Hill was completed in 1977 at a depth of about 2.6 kilometers, using rock temperatures of 185 degrees Celsius. In 1979, the reservoir was expanded with more water injections and operated for about one year. Results showed that heat could be extracted from low-permeability hot rock at reasonable rates. In 1986, a second reservoir was prepared for testing. A 30-day test showed the production temperature rose to about 190 degrees Celsius, reaching a thermal power level of about 10 megawatts. Budget cuts ended the study.

In 2009, the US Department of Energy (USDOE) released two Funding Opportunity Announcements (FOAs) related to enhanced geothermal systems. Together, the two FOAs offered up to $84 million over six years.

The DOE opened another FOA in 2009 using money from the American Reinvestment and Recovery Act for $350 million, including $80 million specifically for EGS projects.

The Frontier Observatory for Research in Geothermal Energy (FORGE) is a US government program supporting geothermal research. The FORGE site is near Milford, Utah, and is funded for up to $140 million. As of 2023, many test wells had been drilled, and measurements had been taken, but energy production had not started.

Developing EGS with a district heating system is part of Cornell University's Climate Action Plan for its Ithaca campus. The project began in 2018 to study feasibility, find funding, and monitor baseline seismic activity. It received $7.2 million in USDOE funding. A test well was planned for spring 2021, at a depth of 2.5 to 5 kilometers, targeting rock hotter than 85 degrees Celsius. The site is planned to supply 20% of the campus's annual heating needs. Possible reservoir locations include the Trenton-Black River formation (2.2 kilometers) or basement crystalline rock (3.5 kilometers). A 2-mile-deep borehole was completed in 2022.

In September 2022, the Geothermal Technologies Office within the Department of Energy's Office of Energy Efficiency and Renewable Energy announced an "Enhanced Geothermal Shot" as part of their Energy Earthshots campaign. The goal is to reduce EGS costs by 90%, to $45 per megawatt hour by 2035.

The Infrastructure Investment and Jobs Act provided $84 million to support EGS development through four demonstration projects. The Inflation Reduction Act extended the production tax credit (PTC) for renewable energy sources, including geothermal, until 2024 and included geothermal energy in the new Clean Electricity PTC, starting in 2024.

Induced seismicity

Induced seismicity refers to earth tremors caused by human activities. These tremors are common in EGS due to the high pressures used. At the Geysers geothermal field in California, earth tremors are connected to injection activities.

In Basel, induced seismicity caused the city to pause its project and eventually cancel it.

The Australian government states that the risks from hydrofracturing-induced seismicity are lower than those of natural earthquakes. These risks can be reduced with careful management and monitoring. They also say that these risks should not stop future development.

Induced seismicity can differ between locations and should be studied before large-scale fluid injection.

EGS potential

A 2006 report by MIT, supported by the U.S. Department of Energy, performed the most detailed study to date on EGS. The report shared important findings:

• Amount of Resources: The study estimated that EGS resources in the United States at depths of 3–10 kilometers total over 13,000 zettajoules, with more than 200 zettajoules being usable today. With improved technology, this amount could grow to over 2,000 zettajoules. The report also noted that all geothermal resources, including hydrothermal and geo-pressured resources, total about 14,000 zettajoules—equal to roughly 140,000 times the United States’ total energy use in 2005.

• Development Potential: If $1 billion is invested in research and development over 15 years, the report predicted that the United States could generate 100 gigawatts of electricity or more by 2050. It also stated that resources currently accessible with today’s technology range from 1.2 to 12.2 terawatts for two different scenarios.

• Cost: The report suggested that EGS could generate electricity for as little as 3.9 cents per kilowatt-hour. The cost of EGS depends mainly on four factors: the temperature of the resource, fluid flow within the system, drilling expenses, and how efficiently energy is converted into electricity.