In Yellowstone National Park in Wyoming, one of the most popular tourist attractions is a geyser known as Old Faithful." The neat thing about Old Faithful is that it spurts hot steaming water out of the ground at pretty predictable intervals – predictable enough that you can probably time your trip to Yellowstone to see Old Faithful erupt several times a day. If you don't happen to live nearby, you can always use the miracle of technology and check out the Old Faithful webcam [1].
When you watch Old Faithful erupt, what you are seeing is geothermal energy in action. If we could just place a nice shiny turbine on top of the geyser's cone, whenever Old Faithful erupts (about every hour and a half or so), the force of the steam would spin the turbine, generating a nice flood of low-carbon electricity.
No one seriously talks about generating power from Old Faithful, but the heat beneath the surface of the earth could provide a gigantic store of energy – if only we could get at it at some reasonable cost. There are a few places, like California, Alaska, and Iceland, where geothermal energy is used to generate a lot of electricity (in Iceland's case, basically enough for the whole country). There are a lot more places where engineers are hoping that we could generate even more electricity from geothermal energy, using techniques collectively known as "enhanced geothermal."
In this section, we'll talk about how geothermal energy works and where it is currently used. We'll also talk about the potential, and some possible pitfalls, from enhanced geothermal. One really intriguing idea that we won't talk about in this section is using heat in the very shallow surface (maybe as little as fifteen feet below ground) to heat and cool your home. This idea, called "ground-source heat pumps" or "ground source heat exchange" is growing in popularity for new home construction and has the potential to save a lot of energy in buildings. But we'll wait for that until we talk about energy conservation. Here we'll stick to producing electricity directly from the heat deep within the earth's surface.
Remember how your basic steam turbine works in a power plant that uses fossil fuels: Fuel is burned to heat water in a boiler, to create steam. The steam is used to drive a turbine, which generates electricity. What if you could get all that steam without burning a single ounce of coal, oil or natural gas? That is the appeal of geothermal electricity production. In certain locations (primarily near active or recently active volcanoes) there are very hot rocks deep under the earth’s surface. In these "geothermal" regions, the temperature may rise by 40-50°C every kilometer of depth, so just 3 km, the temperature could be 120 to 150°C, well above the boiling point for water. The rocks in these regions will typically have pore spaces filled with water, and the water may still be in the form of liquid water since the pressure is so high down there (in some very hot areas, the water is actually in the form of steam trapped in the rocks). If you drill a deep well into one of these "geothermal reservoirs", the water will rise up and as it approaches the surface, the pressure decreases and it turns to steam. This steam can then be used to drive a turbine that is attached to a generator to make electricity. In some regards, this is very much like a coal or natural gas electrical plant, except that with geothermal, no fossil fuels are burned, which means no carbon emissions.
There are three basic types of geothermal power plants, depending on the type of hydrothermal reservoir:
The oldest geothermal plant (1904) in the world is Lardarello, in Italy, which is a dry steam plant. The Geysers, in California, is the largest geothermal installation in the world and the only accessible dry-steam area in the United States (other than Old Faithful and the rest of Yellowstone, which is off-limits). Most modern geothermal plants are “closed-loop” systems, which means that the water (or steam) brought up from the surface is re-injected back into the earth, as shown in the figure below. If the water is not replaced, then eventually, the geothermal reservoir will dry up and cease function.
On a global scale, the potential for geothermal energy is quite large. The IPCC estimates that even though just a fraction of the total heat within the Earth can be used to generate geothermal power, we could nevertheless generate about 90 EJ of energy per year, and this is energy that is constantly renewed from within the Earth. Keep in mind that at present, we generate just over 2 EJ per year, so this energy source can definitely expand, but by itself it cannot meet the total global energy demand of 600 EJ.
To harness geothermal energy to generate electricity using any conventional technology (dry steam, flash steam or binary steam), you’ve got to be in the right place, where there is just the right amount of hot fluid or steam in an accessible reservoir. Unfortunately, those places are few and far between. The figure below shows a map of geothermal resources in the U.S., with identified conventional sites marked with dots on the map. All are located in just a handful of western states, plus Alaska.
The state of Alaska is known more for oil and gas than for renewable energy resources, but the remote nature of many Alaskan communities calls for different energy solutions that we might use in a more connected part of the world. This video shows how some remote areas of Alaska are using locally-sourced renewable energy to power their communities, rather than relying so much on crude oil that makes up much of the state's economic bounty.
Most places do not have that right combination of an accessible, large reservoir of underground heat. Instead, reservoirs are more dispersed, in geologic formations with less permeability (this naturally inhibits the flow of hot fluid towards the surface). Engineers have discovered how to alter the subsurface to create man-made reservoirs of hot water that could be tapped to produce electricity, in either a flash steam or (with higher potential) a binary steam technology configuration. The process of engineering a geothermal reservoir underground is known as “enhanced geothermal systems” or EGS. As the resource map in Figure 2 shows, EGS could be done in a lot more places than conventional geothermal. Hundreds of thousands of gigawatts of power, basically enough to run the United States several times over, could potentially be harnessed through EGS.
The US Department of Energy has a nice animation outlining how EGS works: How an Enhanced Geothermal System Works [8]. Also, check out the interactive image of the EGS on the same page to gain a deeper understanding. Note: This animation requires Flash. If you don't have Flash installed, click the link to the Text Version of the animation.
The basic idea behind EGS is to fracture hot rocks deep within the earth to create channels or networks through which water could flow. When water is injected into these networks, the heat from the rocks boils the water directly, or the now-hot water is transported to the surface where it is used to boil a working fluid, much like a binary steam plant. Fracturing of the rock occurs via “hydraulic fracturing,” under which water is injected into the rock formation at high pressures, causing the rock to fracture. This is actually very similar to the way that natural gas and oil is being extracted from shale. So we can “frack” for geothermal in much the same way that we frack for oil and gas.
Only a few countries use geothermal resources as a major source of electricity production –Iceland, El Salvador, and the Philippines all use geothermal for more than 25% of total electricity generation within those countries. New Zealand is the next (but distant) largest at 10%. Where hydrothermal resources are easy to access, they have often been utilized. The trouble is, there just aren’t that many Old Faithfuls in the world.
EGS represents the most significant potential for geothermal electricity production, but other than a few small military or pilot projects, the systems have not really caught on commercially. One of the big reasons is cost – like many low-carbon electricity technologies, EGS is inexpensive to run but very costly to build. Drilling geothermal wells is much more expensive than drilling conventional oil or gas wells, so electricity prices would probably need to increase by 25% or more (relative to current averages) to make EGS a financially viable technology.
Perhaps a more serious challenge for EGS is “induced seismicity,” which is a fancy term for causing earthquakes. EGS wells that were drilled below Basel, Switzerland caused over 10,000 small tremors (less than 3.5 on the Richter scale) within just a few days following the start of the hydraulic fracturing process. In Oregon, a test EGS well is being monitored for induced seismic activity – you can see some neat real-time earthquake data at Induced Seismicity [9] (U.S. Department of Energy: Energy Efficiency and Renewable Energy.
Induced seismicity occurs whenever hydraulic fracturing (related to EGS or developing a natural gas well) takes place, but in most cases, the earthquakes are so small they are not felt. However, if the hydraulic fracturing occurs near pre-existing faults (which are often not visible at the surface), then larger earthquakes can and do occur, and some of these are strong enough to cause minor damage to buildings nearby.
Links
[1] http://www.nps.gov/features/yell/webcam/oldFaithfulStreaming.html
[2] https://www.youtube.com/@redrocktrail
[3] https://www.youtube.com/watch?v=tyyH_15PabA&t=67s
[4] https://www.eia.gov/energyexplained/geothermal/geothermal-power-plants.php
[5] https://www.energy.gov/eere/geothermal/geothermal-data-systems
[6] https://www.youtube.com/@Etheoperatorsmanual
[7] https://www.youtube.com/watch?v=7jFjGeMsC9c
[8] https://energy.gov/eere/geothermal/how-enhanced-geothermal-system-works
[9] https://www.usgs.gov/programs/earthquake-hazards/earthquakes