Geothermal Power
Geothermal power background
Deep under the earth, there are vast reserves of heat energy that can, in principle, be harnessed for sustainable electricity generation. In special locations where heat rises to the surface, such as natural hot springs, geothermal heat has been tapped for centuries.
Geothermal energy has been harnessed since ancient times as a source of heat, and since 1904 to generate electricity. The first geothermal power plant in the U.S. came into service in 1922, and today there are 69 generating facilities across the country.
There are four main types of modern geothermal power plants. The simplest and most widely used today (dry steam) extracts steam right out of the earth to spin a turbine. Another (flash steam) uses hot water from underground reservoirs that is then depressurized to make steam, which then spins the turbine. The third (binary cycle) transfers heat from geothermal hot water to a separate sealed loop of water or another fluid, which then turns the turbine; this design can extract more energy, and allows reinjection of the geothermal water back into the earth.
The fourth (enhanced geothermal system, or EGS), also the most flexible and efficient, is shown in the diagram below. Water is injected deep underground where it flows through cracks in the reservoir rocks and picks up heat. That water is drawn back to the surface and transfers its heat to a separate closed loop where the secondary fluid is turned vaporized and then powers a turbine. That secondary fluid is then cooled, condensed, and recycled, completing the loop.
Image: BBC Science Focus
Note that the first three types all require the site to contain naturally occurring hot water or steam – the term for this is hydrothermal. But an EGS plant can be built even where the hot reservoir is dry – sometimes called a petrothermal site. If the hot rock is not porous enough to allow existing water or steam to flow through it, additional treatment such as horizontal drilling and hydrofracturing (fracking) will be needed to enhance its properties in order to mobilize the injected water. This combination of technologies will allow many more sites to be used for electric power, which holds the promise of a 40-fold increase over present U.S. geothermal power generating capacity.
How geothermal power supports a clean and stable electric grid
Geothermal energy can provide a much-needed source of 24/7 clean firm power to back up intermittent renewables. Life cycle carbon emissions are 92–96% lower than from fossil plants of the same output, comparable to the climate performance of wind, solar, and hydropower.
Another positive aspect of developing geothermal energy is that it can take advantage of engineering and field worker skills and resources developed in the oil and gas industries, such as directional drilling and seismic analysis. This could help encourage geothermal development in areas where oil and gas extraction is currently practiced. Depending on state laws, some existing federal oil and gas leases may be readily converted into geothermal leases.
Issues with geothermal power
Geothermal reservoirs are tied to specific locations, many of which are far from either demand centers or transmission lines. Thus, geothermal assets may in many instances be dependent on the expansion of the electrical transmission network.
Even though the life-cycle carbon emissions from geothermal power are very low, it can still create environmental concerns. According to a January 2023 report by the Congressional Research Service (CRS), these can include “induced seismicity and the potential for environmental pollution, particularly groundwater impacts.” A 2013 assessment of U.S. geothermal plants showed no reported groundwater contamination, but the question may still be raised in the course of seeking permits for new plants. The use of fresh water for cooling and/or replenishment of reservoir water may be an issue in regions with local water scarcity. However, the EGS method promises to eliminate this concern by recycling essentially all of the reservoir water.
Capital costs tend to be high, largely because of the remote locations, specialized equipment, and costs related to performance risk, but the CRS also notes that “these costs may trend downward as the technologies are more widely implemented,” as we’ve seen with wind and solar. Indeed, Fervo Energy, a private company, has started construction on a 400-MW EGS plant in Nevada that they believe will be profitable.
The U.S. Department of Energy has recently kicked off a program to achieve a 90% capital cost reduction by 2035.
How much could geothermal power contribute to a clean and stable electric grid?
Although geothermal energy currently provides only about 0.4% (17 TWh per year) of U.S. electricity, the remaining potential is far more. The western U.S. happens to be blessed with some of the greatest geothermal potential on the planet.
Image: National Renewable Energy Laboratory
By fully exploiting this potential with the latest EGS technology, the Department of Energy estimates that U.S. geothermal electricity could reach 60 gigawatts by 2050, compared to just 3.7 gigawatts today. This would translate into an extra 250 TWh per year of clean firm power, which could displace 10% of current fossil power generation.
For a deep dive into this technology, check out a recent episode of Volts, where host David Roberts interviews the CEO of Fervo Energy.
Other FAQs
Is geothermal power renewable?
Yes, because for all practical purposes, it is an inexhaustible resource. Geothermal energy does not come from the sun like solar and wind power, but from primordial heat that still remains inside the earth. Even if we obtained all of our electricity from geothermal sources, it would be an infinitesimally small fraction of the amount of heat stored inside the earth. It is important to locate and manage geothermal plants wisely to maximize the amount that can be obtained from a specific site, but that is a question of economics rather than fundamental sustainability.
Can geothermal power plants cause earthquakes?
Hydraulic fracturing, which is an intrinsic part of the EGS method, has been known to induce earthquakes. Since 1960, when the first U.S. geothermal power plant began operating, a great deal has been learned both here and abroad about how to minimize the potential for operations to induce seismicity. Countermeasures include improved monitoring and modeling along with the adoption of power cycles that also serve to minimize the potential to disturb reservoir geology in ways that could cause earthquakes.
What about geothermal heat pumps?
The term “geothermal” has been applied to heat pumps, but it denotes something completely different from the topic discussed here. A more accurate term is “ground-source” heat pumps, where environmental heat for cold-season building heating is obtained through piping buried a few feet underground rather than exposed to outside air. That topic is more relevant to building electrification and has no direct connection to decarbonizing the power grid.
Another way geothermal energy can be used for space heating is through industrial or district heating, which taps into geothermal hot water for heat rather than power generation. This has been done for many years in some European countries, and some U.S. startups are developing it here. It can also be combined with power generation in a single project.