Geothermal is not just for generating energy. It can be used for storage and extracting minerals too. Kelly MacGregor at NREL summarises the research into taking geothermal to the next level and making it a solution to multiple challenges. Depending on how far down you dig, geothermal is available for running utility-scale power plants as well as heating and cooling individual districts, buildings and houses. And new techniques are being developed and piloted to deliver geothermal power “everywhere” – not just in optimal geological conditions – by creating systems that channel water through to the hot rocks. For storage, methods are being developed to make underground batteries that store excess energy (typically from wind and solar) using the same geothermal infrastructure. Finally, a new innovation is to take the water running through the rocks and simultaneously extract critical minerals – like lithium – from the flow. An NREL study says it’s economically feasible for geothermal brines to yield 24,000 metric tons of lithium per year for the U.S., enough to establish a domestic supply of the scarce mineral. And it should be far more environmentally sustainable than current hardrock mining.
Geothermal energy—literally “heat from the Earth”—may be hard to see, but thanks to increasing public interest and outreach it is not hidden anymore.
While geothermal power plants have delivered renewable power for more than 100 years, recent research and advancements have shown that geothermal is more than a 24/7 clean power source.
Not just for energy. It’s for storage and minerals too
“Geothermal is a triple resource: an energy source for heating, cooling, and power; a storage resource; and a mineral resource,” said Amanda Kolker, geothermal laboratory 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.”
With the ability to provide electricity, heating, cooling, and storage—plus the potential to access critical minerals, capture and sequester carbon, produce green hydrogen, and more—the natural heat of the Earth is a powerhouse ready to be tapped. And investors and leaders across both the public and private sectors are moving full steam ahead with geothermal.
Colorado Gov. Jared Polis recently launched “The Heat Beneath Our Feet” initiative to encourage renewable geothermal energy generation in Colorado and other western states. As part of this Western Governors’ Association initiative, NREL provided technical assistance for the project and hosted an event on Feb 24, 2023, showcasing the laboratory’s geothermal research portfolio to more than 50 leaders in attendance.
From improving geothermal resource identification to advancing drilling techniques to expanding emerging technologies, read on to see how researchers at NREL are discovering how geothermal can use the planet to save the planet.
Heat just 10ft underground
“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.”
If you were to journey to the centre of the Earth, you would find it is as hot as the surface of the sun.
Luckily, we do not need to get to those 9,000°F temperatures to tap into geothermal energy. Geothermal power plants can run off temperatures ranging from just 250° to 700°F; heat can be used directly from temperatures ranging from 100° to 300°F for space heating, industrial, and agricultural uses; and the consistent 50°–60°F found only 10 feet underground can heat and cool buildings and communities of all sizes.
“It doesn’t have to be this amazing, dramatic volcano,” said Whitney Trainor-Guitton, NREL geoscience researcher. “We can use 55°F groundwater to heat and cool bus terminals, college campuses, and even whole towns.”
Hydrothermal resources, which naturally have everything needed for large-scale geothermal power—namely heat, water, and pathways for the water to move—are what many people think of when they think “geothermal.” These types of power plants have been providing clean energy for more than a century, but only certain locations are ideal. “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.”
Now, new technologies can bring missing components to areas that do not have them naturally and can enable geothermal on a community or household scale, opening the door for geothermal everywhere.
Emerging Geothermal Technologies
…Enhanced geothermal systems
One emerging technology, enhanced geothermal systems (EGS), can bring the missing pieces (water or pathways through the rock), allowing for geothermal power generation in areas where it was previously unavailable.
“The U.S. Department of Energy is putting a great deal of investment into EGS with the recent Enhanced Geothermal Shot as part of the Energy Earthshot initiatives, funding for new EGS demonstration sites, and the current Utah FORGE demonstration site,” Beckers said.
NREL participated in the EGS Collab Project, which ran experiments one mile underground at the Sanford Underground Research Facility, a former gold mine in South Dakota. EGS Collab provides a stepping-stone between laboratory tests and full-scale EGS deployment through cooperation between nine U.S. Department of Energy national laboratories, seven universities, and two industry members.
Closed-loop geothermal, also known as advanced geothermal systems, are another emerging technology for areas where traditional hydrothermal is not present. With these systems, water or another heat transfer fluid flows through pipe systems engineered for the specific area instead of through the subterranean rocks.
“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.”
…Deep “superhot” geothermal
Another frontier for geothermal energy is being explored in the DEEPEN project, a multinational project that aims to discover how a new type of resource—supercritical or superhot geothermal—can be harnessed using emerging technologies. Tapping geothermal resources that exceed the critical point of water (where liquid water and vapor are indistinguishable) has the potential to power millions of homes. NREL is working to develop new methods for exploring and characterising these extra-hot systems, and the project, a collaboration between international universities, institutes, and energy companies, hopes to tap into the tremendous potential.
“The energy from a single superhot geothermal well could produce 5–10 times what a commercial geothermal well produces today,” Kolker said. “If we can find and produce these systems, this could be a game-changer.”
While emerging technologies are key to scaling geothermal energy, there are still abundant resources that could be tapped using conventional approaches. The United States currently leads the world in geothermal electricity production due to the natural, ideal geothermal conditions in the western states —evidenced through hot springs and geysers right on the surface. But how can we find the areas naturally best suited for the large-scale geothermal power plants that power thousands of homes?
“NREL is developing new modelling methods to allow us to statistically find the best places to put geothermal wells to have the most success,” Trainor-Guitton said. “When you are talking about drilling a very expensive $10 million well, we need sophisticated methods to understand the likelihood of the resource being there.”
The oil and gas industry has developed similar modelling techniques for their exploration over the past several decades, but the conditions needed for oil and gas are different than those needed for geothermal.
“These models have been well developed for oil and gas because the value proposition was more obvious; brilliant minds went to developing these techniques,” Trainor-Guitton said. “Geothermal is on the precipice of this happening, and NREL is at the forefront.”
Data science can also help improve our assessment of large, untapped geothermal resources by consolidating data sets from around the world and using machine learning algorithms to recognise what responses in those data sets are favourable.
“Broadly speaking, there are a lot of tasks that are too time-consuming or challenging for humans to do on their own,” said Nicole Taverna, a geothermal data scientist at NREL. “Data science lets us take the next step beyond what the human brain is capable of and recognize patterns we might miss.”
Although only some locations happen to have natural, ideal geothermal resources for power production, geothermal can be tapped anywhere for thermal (heat) energy.
“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, neighbourhoods, and even towns.”
While finding the perfect location for a geothermal power plant can take time, harnessing the resource for heating and cooling can be as simple as digging out a basement, thanks to modern heat pump technologies.
“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 it is temperatures just 10 feet down or heat from deeper geothermal waters, entire communities can employ direct-use geothermal systems—systems that directly use the geothermal heat—for heating, cooling, and even things like snow removal, energy-efficient industrial processes, and more. While most geothermal heat pump installations in the United States are for individual buildings, they are used worldwide for heating and cooling large networks of interconnected buildings.
“International collaboration is increasing for direct-use technologies,” Trainor-Guitton said. “A new project with Geothermica, called FLXenabler, is evaluating the flexibility provided for communities when geothermal heating and cooling is integrated with other renewable energy sources and thermal energy storage.” The FLXenabler consortium includes representatives from SINTEF (Norway), Equinor (Norway), TU Wein (Austria), the U.S. Geological Survey, and NREL’s crosscutting research in subsurface characterisation, power systems modelling, buildings research, and thermal energy storage.
“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. “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.”
New York City
NREL’s partnership with Con Edison to study transitioning New York City’s steam system, which powers the Empire State Building and is currently run on natural gas, to geothermal is evidence of the increased interest in direct use. NREL researchers are addressing geotechnical, economic, and logistical issues to understand the opportunities and challenges of using geothermal energy for generating steam in New York City. If converted to only 10% geothermal, this system would be the largest geothermal district heating system in the United States.
Reduce, Reuse, Recycle … and Repurpose
As NREL researchers work to refine models for geothermal exploration and expand direct-use applications, already drilled wells can be repurposed for geothermal in the meantime.
“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.”
Two projects are currently underway, in Oklahoma and Nevada, to generate 1 MW or more from oil and gas wells. One project aims to create a roadmap for “geothermal cogeneration”—generating geothermal energy at the same time as active oil and gas extraction—while the other aims to repurpose an abandoned well.
…Novel drilling techniques
And when wells cannot be repurposed, the investment to drill new wells can be used to push the boundaries and create novel drilling techniques.
The Geothermal Limitless Approach to Drilling Efficiencies (GLADE) project aims to do just that. The project, funded by the U.S. Department of Energy Geothermal Technologies Office in partnership with Occidental Petroleum, will drill twin high-temperature (572°F) geothermal wells deeper (up to 20,000 feet) and more quickly than most existing wells. This demonstration project seeks to reduce project timelines and costs for developing geothermal power plants by creating a 25% improvement in geothermal drilling rates.
Earth as a battery?
Imagine if, instead of using a battery, the Earth itself could store energy—and not just enough for your house: enough to provide energy to multifamily buildings, colleges, neighbourhoods, and even entire cities. Geothermal could be this kind of “battery” through underground storage.
Geothermal energy storage is also attractive because not many other technologies currently have the capability for long-duration storage. And those that do also have high expenses or impacts, such as building giant storage tanks, sourcing rare-earth materials like lithium, and lacking recycling options.
“But the Earth itself is a storage tank,” said Guangdong Zhu, NREL group manager of thermal energy systems and executive director of the Heliostat Consortium for Concentrating Solar-Thermal Power.
Earth’s subsurface can provide energy storage as thermal energy (heat), chemical storage (of carbon dioxide—better known as carbon sequestration—and of hydrogen and other gases), and mechanical storage (by repurposing infrastructure at depth, such as wells, for this purpose).
One NREL project, Repurposing Infrastructure for Gravity Storage using Underground Potential energy (RIGS UP), is exploring the commercial viability of gravity-based mechanical storage systems using oil and gas wellbores. The ARPA-E-funded project will store electrical energy as potential energy by lifting a multi-ton weight within a wellbore. Once proven, the technology could also be used inside of inactive geothermal wells for long-term mechanical storage.
NREL researchers are also partnering with legacy oil and gas companies to make geologic thermal energy storage a reality.
“Some rock systems are not great storage systems, but oil and gas fields are sedimentary and have good storage potential with sizeable pores—they could store gas, which means they can store water and thermal energy,” said Dayo Akindipe, NREL subsurface energy systems research scientist.
Oil and gas sites that are no longer producing can be cleaned up and transitioned to mechanical or thermal energy storage—removing environmental contamination and transitioning jobs in those areas to the clean energy economy. NREL scientists are working with oil and gas companies to identify demonstration sites to accelerate this transition.
Mining made better—Lithium
Geothermal energy also has other battery-related applications. The salty, hot water that is heated underground and brought to a geothermal power plant can also contain rare minerals—like lithium. The scarce mineral is essential for rechargeable batteries in electric vehicles, pacemakers, cell phones, and more. And recovering the mineral from water already being brought to the surface, rather than traditional mining operations, is better for the planet.
“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.”
Only 1% of lithium used in the United States currently comes from domestic sources. An NREL analysis focused on lithium found that it is economically feasible for geothermal brines to yield approximately 24,000 metric tons of lithium per year, enough to establish a secure, domestic supply of the scarce mineral.
One area in particular, California’s Salton Sea, has immense potential for both geothermal energy and mineral capture through direct extraction technologies.
“Recent lab studies show that direct lithium extraction, a relatively new technique, can be more sustainable for the planet than current hardrock mining or evaporative pond techniques when we look at land use, water use, and carbon intensity of the operations,” Kolker said.
Future lithium research aims to advance the development of a domestic lithium supply chain through extraction from geothermal brines.
24/7 baseload for Wind and Solar
In the sprint to transition off fossil fuels, geothermal is poised to help intermittent renewable technologies, like solar and wind, by providing a baseload fallback for when the sun is not shining and the wind is not blowing.
“As long as you are on the Earth, there is geothermal—24 hours a day,” Akindipe said.
And when you couple geothermal with other renewable technologies at the same site, the hybrid result is often better than either technology alone.
“By pairing solar and geothermal, we can design a system that naturally incorporates and takes advantage of the superior aspects of both technologies,” Zhu said. “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 optimisation, maximising power plant performance and storage capabilities for systems that house additional heat from concentrating solar power systems in geothermal reservoirs.
Further, by using geothermal resources for power when adequate, and hybrid technologies when they are not, communities can even create their own microgrids for heating, cooling, and power. Such microgrids provide reliability and resilience against energy supply chain issues and extreme weather events. They also have the potential to create local jobs. NREL is helping communities by analysing geothermal microgrid technologies as resources for isolated grids.
“At any scale, a decarbonised grid is going to be a mixture of renewable technologies, including geothermal,” Kolker said. “That is the future.”
Kelly MacGregor is a Communications Project Manager at NREL
This article is published with permission from the National Renewable Energy Laboratory