Geothermal promises increased potential

Geothermal power has generally represented region-specific and niche clean energy in the public consciousness for over a century. Today, thanks to a profusion of social outreach and government incentives, investors and leaders across both public and private sectors are exploring lesser-known applications and exciting advancements in the field.

Just a few feet below the surface, the earth maintains a near-constant temperature that belies the seasonal extremes of aboveground ambient air. The further down you go, temperature increases at an average rate of approximately one degree Fahrenheit for every 70 feet, while regional tectonic and volcanic activity can bring higher temperatures and pockets of superheated water and steam much closer to the surface.

Traditional geothermal energy return on investment generally follows the rule that nine units of output can be produced by one unit of input. This compares against solar with a score of 10 and wind energy with a score of 18.

However, geothermal energy possesses the advantage of high reliability and calculable output without the problem of weather intermittency. This allows for more opportunities to effectively test breakthrough theories in streamlining and complementing the process.

Geothermal technologies are providing increasingly advantageous electricity, storage, and temperature control uses, while research has uncovered the potential of this resource doubling as a greener method of critical minerals mining and more.

"Geothermal is a triple resource: an energy source for heating, cooling, and power; a storage resource; and a mineral resource," said Amanda Kolker, geothermal lab program manager at the National Renewable Energy Laboratory (NREL). "The Earth itself has the potential to address a variety of hurdles in the transition to a clean energy future."

The next hot commodity

Today the U.S. leads the world in geothermal power generation, providing more than 3.7 gigawatts of electricity to the national grid. Analyses suggest that number can increase to 90 GW, but not with conventional geothermal technologies.

In addition to a clean electrical source, geothermal plants can produce low-carbon heat that can be used for traditionally energy-intensive industrial and agricultural production such as food dehydration, laundries, improved aquaculture and fish farming, commercial greenhouse operations, mushroom cultures, concrete curing, fabric dyeing, food processing, cooking and pasteurization, even biogas production.

Economical geothermal heat and power experiments are being proposed and tested to replace extreme heating processes in several industries, such as pulp and paper processing in Germany and Tuscan beer brewing in Italy.

"The Earth itself is a renewable energy source," Kolker said. "Earth's heat is always available; it doesn't go away when the sun goes down. It can play a big role in the energy transition by providing reliable, 24/7 clean energy, and it can do so much more than people think."

Emerging technologies

The word "geothermal" often conjures up what is more specifically styled as hydrothermal resources, a subset of geothermal comprised of naturally occurring heat, water, and pathways for the water to move. Ideal, but not commonly found together in one location.

While underground heat can be tapped from anywhere on the surface of the earth, many locations lack adequate water or access from the surface, which is integral in creating this form of green energy. Enhanced geothermal systems (EGS) can bring those missing elements of fluids and human-made reservoirs and pathways through the rock, allowing access to geothermal power generation wherever it is needed.

"Having all three things present-water, heat, and flow paths-is rare," said Koenraad Beckers, an NREL thermal sciences researcher. "You see this in places like California, Nevada, Iceland, New Zealand, and Japan. But you can use some type of geothermal technology anywhere in the world, and you can often bring the water and pathways to the site."

In February, the U.S. Department of Energy (DOE) announced a $74 million funding opportunity for seven pilot projects that will test the efficacy and scalability of EGS. At the same time, NREL is one of nine national labs and seven universities participating in the EGS Collab Project, a cooperative that is utilizing a former gold mine at the Sanford Underground Research Facility in South Dakota to help develop a transition between laboratory-scale simulations and full-scale commercially viable EGS deployment.

Closed-loop geothermal systems that utilize heat-transferring pipes are an emerging EGS technology for areas without hydrothermal.

"With closed-loop systems, you keep the fluid within your well and pipes, and the pipes are exposed to the hot rock," Beckers said. "NREL can simulate both EGS and closed-loop systems for industry and government partners, providing important pre-validation that is required before major investments are made deploying new technologies."

Another potential for powering homes is tapping geothermal resources that exceed the critical point where liquid water and vapor are indistinguishable.

"The energy from a single superhot geothermal well could produce 5–10 times what a commercial geothermal well produces today," Kolker suggests. "If we can find and produce these systems, this could be a game-changer."

Geothermal for everyone

"Anywhere in the country, if you drill, it gets hotter and hotter with each mile you go deeper," Beckers said. "In the western United States, that temperature increases fast: If you drill just 1–2 miles deep, you have temperatures hot enough for electricity. To get those temperatures in eastern states, you might need to drill miles and miles down, but you can use lower temperatures to directly heat or cool campuses, neighborhoods, and even towns."

While finding the perfect location for a municipal geothermal power plant takes time and infrastructure, harnessing the resource for heating and cooling is much simpler, thanks to modern heat pump technologies where liquid is circulated through an underground loop, absorbing naturally occurring heat. The liquid is compressed and goes through a heat exchanger, which extracts the heat and transfers it to your home heating system. Applied in reverse, the pump can remove heat and transfer it to the ground.

"It doesn't have to be this amazing, dramatic volcano," said NREL geoscience researcher Whitney Trainor-Guitton, "We can use 55°F groundwater to heat and cool bus terminals, college campuses, and even whole towns."

Geothermal district heating systems, which distribute hydrothermal water from one or more geothermal wells to several buildings or blocks, also has the potential to replace traditional fossil-fueled heating and cooling.

"At just 10 feet below the surface, the temperature remains the same year-round – around 55°F," Kolker said. "This means in the summer geothermal technology can provide cooling, and in the winter it can provide heat."

Whether from just 10 feet down or deeper geothermal waters, entire communities can employ the natural heat from hydrothermal systems (called direct-use) for basic building temperature regulation, heated street or driveway snow removal, extended growing seasons and protecting winter crops. While most geothermal heat pump installations existing today in the U.S. are for individual buildings, they are used to great advantage in other countries for heating and cooling large networks of interconnected complexes.

Due to increased interest in practical direct use, NREL has partnered with New York utility provider Con Edison to study transitioning the city's natural gas-powered steam system to geothermal.

Today there are roughly 105 miles of steam pipes underneath New York's streets servicing over 1,500 buildings. Some of the city's most famous buildings use steam, including the Empire State and Chrysler buildings, Grand Central Terminal, the United Nations and Rockefeller Center. If converting even just 10% to geothermal, this system would be the largest district heating system of its kind in the United States.

"I am excited about the increased interest in direct-use, because historically in the U.S. the focus has been on power production," Beckers said of the partnership. "Direct-use heating and cooling was overlooked until now, even though we use a lot of heating in this country, and it can be used everywhere."

Repurposing old drill sites

As researchers refine models for geothermal exploration and expanded applications, previously drilled oil wells and past-producing mines can be more quickly repurposed for geothermal uses.

"Exploratory drilling is a huge upfront cost for geothermal development," Kolker said. "But there are thousands of oil and gas wells across the country that have already been drilled, some of which can be either repurposed for geothermal or used for coproduction of geothermal and hydrocarbons."

In Oklahoma and Nevada, projects are currently underway to generate 1 megawatt or more from spent oil and gas wells, the equivalent of powering 750 homes.

Where wells can't as easily be repurposed, investments in drilling new multifunctional wells can inspire novel drilling techniques for generating geothermal energy at the same time as active oil and gas extraction, called "geothermal cogeneration."

The Geothermal Limitless Approach to Drilling Efficiencies (GLADE) project aims to do just that. The project, funded by the DOE Geothermal Technologies Office, has partnered with Occidental Petroleum to drill twin high-temperature geothermal wells using existing and novel drilling technologies. This demonstration project team seeks to reduce timelines and costs for developing geothermal power plants by creating a 25% improvement in drilling rates.

Developers considering constructing a geothermal project on former mining land can also benefit from the availability of existing infrastructure from the previous mining activities.

International examples of geothermal energy powering active mining operations also exist. The Lihir gold mine in New Guinea developed a 56-megawatt geothermal power plant to supply the mine with approximately 75% of its power, which generates international carbon credits which can be sold or traded.

Ultimately, partnerships between the geothermal and mining industries can lead to mutually beneficial cost reductions and assist in achieving environmental and sustainability goals, including decarbonization, in the process.

Mining for geothermal

Geothermal is a great fit for the ever-increasing number of companies mining for copper, gold, lithium, and other minerals that are shifting to renewable energy technologies to power mine operations, as well as greener mechanization and transport.

Hardrock mineral and geothermal exploration both involve surface mapping, geophysical and geochemical analysis, remote sensing, and geologic and conceptual modeling. As the commercial mining industry is significantly larger, information collected through mineral exploration offers a trove of potential geothermal data over a much wider area.

Numerous mining databases contain vital and underutilized information relevant to potential geothermal development, and sharing this information could be a boon for both sectors. Leveraging mining industry data, knowledge, and expertise serves to effectively expand the geothermal exploration workforce, lower the expense and increase the rate of discovery.

Locatable minerals and geothermal resources, however, are subject to entirely different sets of laws and regulations governing prospecting and obtaining rights to use their respective resources.

Geothermal operations, as a historically overlooked and undervalued resource, are more limited in regulatory scope and permissions than mining. A partnership between the two, along with updated legislation, can benefit both fields.

Because many data points collected by the mining industry are of value to geothermal exploration and development, this presents an opportunity to monetize data that previously held little use for mining companies. In addition, co-location of geothermal power plants at active remote mines presents an opportunity for the mining industry to both lower its electricity costs and achieve decarbonization goals.

Further interdisciplinary mineral and geothermal exploration efforts are being encouraged by an interagency partnership between the Geoscience Data Acquisition for Western Nevada and the U.S. Geological Survey, which are working to provide energy and data solutions for critical minerals mining companies to help assuage their reluctance to provide access to proprietary information and records.

The agencies hope to provide in-depth data valuation as it relates to uses outside the usual purview of locatable mineral exploration activities, aiming to influence industry thinking by demonstrating the unrealized value of these data and encourage cooperation and support of green energy sources like geothermal.

Lithium-rich geothermal

Another area where the geothermal and mining sectors meet is at geothermal brines, where the hot waters naturally contain elevated levels of critical minerals such as arsenic, lithium, and boron due to the erosive nature of extreme heat, fluids and rock in constant interaction.

Treating these elements integral to electronics and electric vehicle battery manufacturing as recoverable is beginning to transform technologies designed for greener extraction methods.

"As we transition to electric vehicles and battery storage for solar and wind power, the need for lithium is rising," Kolker said. "Geothermal energy may be able to help in a sustainable way."

Although lithium is commonly used and demand is increasing exponentially in the coming years, it is not particularly abundant or easy to acquire and has been placed on the USGS critical minerals list.

One area in particular, California's Salton Sea, has immense potential for combining geothermal energy and mineral capture through direct extraction and offers the potential of a secure, domestic supply of lithium.

More information on lithium markets and geothermal brine lithium projects being developed in the Salton Sea Geothermal Field can be read at Automakers move into lithium mining space in the Critical Minerals Alliances 2022 magazine published by Data Mine North.

An NREL lithium analysis carried out in cooperation with the Critical Minerals Institute at the Colorado School of Mines found potential for U.S. geothermal brines to yield approximately 24,000 metric tons of lithium per year.

"Recent lab studies show that direct lithium extraction, a relatively new technique, can be more sustainable for the planet than current hard rock mining or evaporative pond techniques when we look at land use, water use, and carbon intensity of the operations," Kolker added.

A hybrid future

In the transition away from fossil fuels, geothermal is poised to support intermittent renewable technologies like solar and wind by providing a stable baseload energy supply fallback. The hybrid results are often better than any single green technology can provide.

"By pairing solar and geothermal, we can design a system that naturally incorporates and takes advantage of the superior aspects of both technologies," says Guangdong Zhu, NREL group manager of thermal energy systems and executive director of the Heliostat Consortium for Concentrating Solar-Thermal Power. "The solar can increase the heat for the geothermal system, leading to more electricity generation, and the geothermal system can store excess energy from the solar."

NREL researchers are experts in geo-solar integration optimization, maximizing power plant performance and storage capabilities for excess heat from solar power systems in geothermal reservoirs.

By using hybrid technologies, communities can even create their own microgrids for temperature regulation and power, maintaining self-reliability and resilience against energy supply chain issues and extreme weather events.

Geothermal energy's full potential can be realized through new well-drilling technologies and retrofitting of existing wells, hybridization with other renewables, novel power-generation resources and technologies, and community-based jobs and energy security.

"At any scale, a decarbonized grid is going to be a mixture of renewable technologies, including geothermal," concludes Kolker. "That is the future."