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:

• Dry steam plants, which draw steam directly from deep underground (a la Old Faithful);
• Flash steam plants, which draw hot water under high pressure up towards the surface. As the pressure decreases, the water boils, which generates steam to power the turbine;
• Binary steam plants, which utilize hot water (perhaps around 150 degrees Celsius) to vaporize another fluid (one with a lower boiling point). This hot vapor then drives the turbine, generating electricity.

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.

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 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 (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.

There are three main types of conventional hydropower technologies: impoundment (dam), diversion, and pumped storage.

Impoundment is the most common type of hydroelectric power plant. An impoundment facility, typically a large hydropower system, uses a dam to store river water in a reservoir. Water released from the reservoir flows through a turbine, spinning it, which in turn activates a generator to produce electricity. Generation may be used fairly flexibly to meet baseload as well as peak load demands. The water may also be released either to meet changing electricity needs or to maintain a constant reservoir level. The layout of a typical impoundment hydropower facility is shown below in the first figure. One of the world’s most famous impoundment dams, the Hoover Dam, is shown in the second figure (although it’s worth noting that on a global scale, the Hoover Dam is more famous than it is large).

A diversion, sometimes called run-of-river, facility channels a portion of a river through a canal or penstock. It may not require the use of a dam but also has limited flexibility to follow peak variation in power demand. Thus, it will mainly be useful for baseload capacity. This scenario results in limited flooding and changes to river flow. In the United States, many of the dams in the Pacific Northwest (on the Columbia and Snake Rivers) are diversion or run-of-river dams, with limited or no storage reservoir behind the dam. The figure below shows a picture of a diversion hydropower facility. Compare what that facility looks like with the picture of Hoover Dam, the impoundment facility shown above.

A “pumped storage” hydro dam combines a small storage reservoir with a system for cycling water back into the reservoir after it has been released through the turbine, thus “re-using” the same water to generate electricity at a later time. When the demand for electricity is low (typically at night), a pumped storage facility stores energy by pumping water from a lower reservoir to an upper reservoir. During periods of high electrical demand (typically during the day), the water is released back to the lower reservoir to generate electricity. The figure below shows a schematic of a pumped storage hydro facility. Pumped storage facilities are typically smaller in terms of generation capacity than their impoundment or diversion counterparts, but are sometimes combined with impoundment or diversion facilities to increase peak power output or flexibility.

Water in the oceans is constantly in motion due to waves and tides, and energy can be harvested from these kinds of motions. Waves, driven by the winds, make the water oscillate in roughly circular orbits extending to a depth of one half of the wavelength of the wave (distance between peaks). Tides, related to the gravitational pull of the Moon and Sun on the oceans, are like very long-wavelength waves that can produce very strong currents in some coastal areas due to the geometry of the shoreline. In terms of power generation technologies, wave and tidal power have both similarities and differences. Both refer to the extraction of kinetic energy from the ocean to generate electricity (again, by spinning a turbine just as hydroelectric dams or wind farms do), but the locations of each and the mechanisms that they use for generating power are slightly different.

Wave energy projects extract energy from waves on the surface of the water, or from wave motion a bit deeper (a few 10s of meters) in the ocean. Surface wave energy technologies capture the kinetic energy in breaking waves – these provide periodic impulses that spin a turbine. The US Department of Energy has a nice description of different types of surface wave projects as follows:

• Oscillating Water Columns: Oscillating water columns consist of a partially submerged concrete or steel structure that has an opening to the sea below the waterline. It encloses a column of air above a column of water. As waves enter the air column, they cause the water column to rise and fall. This alternately compresses and depressurizes the air column. As the wave retreats, the air is drawn back through the turbine as a result of the reduced air pressure on the ocean side of the turbine.
• Tapchans: Tapchans, or tapered channel systems, consist of a tapered channel that feeds into a reservoir constructed on cliffs above sea level. The narrowing of the channel causes the waves to increase in height as they move toward the cliff face. The waves spill over the walls of the channel into the reservoir, and the stored water is then fed through a turbine.
• Pendular Devices: Pendular wave-power devices consist of a rectangular box that is open to the sea at one end. A flap is hinged over the opening, and the action of the waves causes the flap to swing back and forth. The motion powers a hydraulic pump and a generator.

Offshore wave energy systems are typically placed deeper in the ocean, though not too deep – perhaps a few hundred feet below the ocean’s surface. The periodic wave activity at this depth is typically used to power a pump that feeds into a turbine, generating electricity.

Tidal energy projects typically work by forcing water through a turbine or a “tidal fence” that looks like a set of subway turnstiles. The systems depend on regular tidal activity to generate power. Because this tidal activity is predictable (each coast sees at least one tidal cycle per day – high tide and low tide – and some areas actually see two tidal cycles on a daily basis), tidal energy projects have the advantage of being able to provide a fairly predictable source of electricity. The use of tidal power, globally, has been quite limited because there are only a few sites in the world that see sufficiently large variations in tides to produce enough power, as shown in the table below.

When rivers are utilized to produce electricity, that is usually accomplished by building some sort of hydroelectric dam, like the three we discussed earlier. It doesn’t have to be that way, however. Many of the technologies used to extract energy from the tides (or similar technologies) could be deployed in freshwater river systems rather than the saltwater ocean, effectively acting as very small run-of-river facilities. These “hydrokinetic” power generation systems are typically individually small (each generating about 100 kilowatts or less of power) and could be situated in two ways. First, a propeller-like or turnstile-like turbine could be deployed directly into the riverway, operating much like a small-scale tidal power system. Second, a “micro-hydro” type of system could be employed, where river water is channeled to a turbine housing via a channel or pipeline, as shown in the figure below.

Globally, hydroelectricity is a major electricity resource, accounting for more than 16% of all electricity produced on the planet. More electricity is produced globally using hydropower than from plants fueled by nuclear fission or petroleum (natural gas and coal do produce more electricity globally than hydropower does). More than 150 countries produce some hydroelectricity, although around 50% of all hydropower is produced by just four countries: China, Brazil, Canada, and the United States. China is by far the largest hydropower producer on the planet, as shown in the figure below. Hydroelectricity production in China has tripled over the past decade, with the completion of some of the world’s largest dam projects, in particular, the Three Gorges Dam (the world’s largest), which could produce nearly enough electricity to power all of New England during a typical summer and left an area roughly the size of San Francisco flooded underwater.

Once hydroelectric dams are built, they run very cheaply and generally provide reliable supplies of electricity except during times of extreme drought. Developed countries that have substantial hydro resources have, by and large, already utilized those resources to produce electricity. In these countries, hydropower dominates the electricity supply system as shown in the chart below. Norway leads the pack here – the amount of hydropower that it produces is not large in an absolute sense (it is the world’s seventh-largest producer) but nearly all electricity generated in Norway comes from hydro-power. Brazil and Canada are also highly dependent on hydropower. Other large hydro producers, such as China and the United States, produce much less hydroelectricity relative to the size of their overall power sectors.

It is often said that developed countries like the United States have little potential for growth in hydroelectric power generation – the US, in particular, has dammed so many rivers, that what could possibly be left? In some areas of the Pacific Northwest, the US is removing some dams that produce electricity due to environmental concerns. It is certainly the case that there is relatively little potential for new hydro mega-projects in the US. This does not mean, however, that there is nowhere left to build new hydroelectric projects. There are, in fact, several hundred megawatts of planned hydroelectric generation in the Mid-Atlantic US alone (see the map at PJM), though most of the projects would be small pumped storage facilities) We’ll walk briefly through a few examples of the hydro resource potential in the United States before taking a more global look.

First, not all dams in the US are equipped with turbines to generate electricity. There are, actually, quite a few that aren’t (see the interactive map at Energy.gov – potentially enough power generation to supply more than ten million homes. Many of these are located along major shipping routes, like the Ohio and Mississippi Rivers. Others are located in areas where development might not make economic sense because the power would need to be shipped across large and expensive transmission lines. Another unconventional technology – hydro-kinetics – could potentially supply enough electricity to power the state of Virginia, although these resources are highly concentrated in the Lower Mississippi River and in more remote areas such as Alaska.

Globally, the picture is very different. Developing nations with abundant hydro resources, like China, Brazil, and other South American countries, are rushing headlong into planning and building new dams. So globally, hydroelectricity is alive and well, and growing rapidly (no pun intended) – although this growth comes in fits and starts since new dams each represent a big chunk of capacity and take a long time from project start to finish. Nearly half of the world’s fifteen largest dams were built since the year 2000, with only one of those in a country other than China or Brazil (the Sayano Shuskenskaya dam in Russia was recently upgraded, although the original went into place in the 1980s).

Energy from hydropower has been growing at a steady annual rate of 0.3 EJ per year since the year 2000, faster than the previous decades, but overall, hydro still accounts for just 16 EJ of the total 580 EJ we used in 2018. Some estimates suggest that if we really developed all of the economically viable hydroelectric sites, we might be able to generate as much as 50 EJ of hydroelectric energy — still a far cry from the +600 we will need in the near future. But even though hydro cannot solve all of our energy problems, it is nevertheless very important in that it supplies a relatively low-emissions, dispatchable (on-demand) energy source that can help smooth out the variability in wind and solar energy production.

While hydro development is still growing in several regions of the world, many countries, including the United States, will not likely pursue larger hydro projects due to two main factors — first, we have already built hydroelectric dams in most of the best places, and secondly, there are concerns over the environmental and societal impacts of building more dams. These environmental impacts have even been used to justify dam removal in some cases, though weighing those environmental impacts against the societal benefits (such as irrigation, flood control, recreation, and so forth…not just electricity) has always been controversial. Some of these specific environmental and societal impacts include:

• Impacts on fish migration – fish migrating up or downstream may find passages blocked by dams or may get killed going through turbines.
• Reduced water quality – changes in natural stream-flow may, in some cases, muddy waters. This affects the river ecosystems that aquatic plants and animals utilize.
• Land inundation – reservoirs behind dams can flood large areas. There is not necessarily a relationship between the size of the dam and the amount of area flooded behind the dam. Two impacts can arise here. First, building large hydro projects often involves the displacement of many people who used to live in the area surrounding the river. The Three Gorges Dam in China involved the relocation of more than one million people.
• Methane release to the atmosphere — plant life can decay in flooded areas, which over time releases large quantities of methane into the atmosphere – enough that it can potentially offset the avoided CO₂ emissions from many years worth of fossil fuel use. A recent estimate places the global emissions of methane high enough to make hydropower be just a little bit less than natural gas in terms of CO2 (equivalent) per unit of energy generated.

Maggie Koerth-Baker has a really great article on how nuclear power plants work, with a focus on the nuclear fission reaction and what mechanisms in a nuclear power plant keep the reaction from spinning out of control. It was written right after the incident at Fukushima Daichi. Before continuing on, please have a look at the article, and pay some attention not only to how plants work, but how the nuclear reactions inside the plants are controlled. The article also has a really nice description of how reactions at nuclear power plants can keep cascading even after the plant has been “shut down,” which is basically what causes meltdowns like those that happened in the Three Mile Island and Chernobyl power plants.

The basics of a nuclear power plant aren’t actually all that complicated. In fact, there is a remarkable similarity to fossil fuel plants, in that what ultimately happens in a nuclear power plant is that steam is produced, to drive a turbine inside a generator, which produces electricity. But unlike fossil fuel plants, which heat water by burning fuel, the water in nuclear power plants is heated through an atomic reaction.

There are two basic types of atomic reactions. The first is nuclear fusion, with which we are all intimately familiar, whether we know it or not. It is nuclear fusion that keeps the sun hot. In nuclear fusion, atoms are joined together. The word “joined” here is a bit of scientific jargon. In reality, the energy is released when atoms collide together at really high speeds. If you have ever seen two cars collide at high speed, you have some idea of how energy could be released when things hit each other. Despite years of research into nuclear fusion, scientists have never been able to engineer a controllable reaction in a laboratory environment. If they could, most of the world’s energy problems would basically be solved overnight, since the amount of energy released through a fusion reaction would be massive. But for now, fusion goes in the “maybe someday” pile.

The second type of nuclear reaction is fission, which is the opposite of fusion – atoms are broken apart, which also releases energy. U-235 is naturally radioactive, meaning that the nucleus is unstable, and it will eventually give off some energy and parts of its nucleus to get to a stable atom, but this takes a long time — the half-life is 700 million years.  The figure below illustrates roughly how this works. An atom (in the case of a nuclear power plant, a uranium-235 atom) is bombarded with neutrons, some of which are absorbed by the nucleus, so the U-235 becomes U-236 — this makes it even more unstable, so the atom splits apart into two lighter atoms called the daughter products.  U-236 splits into krypton (Kr-92) and barium (Ba-141), and it also releases energy in the form of heat, gamma radiation (bad for us) and 3 neutrons. (Note that if you add up the weight of the daughter products and the neutrons,  92+141+3, you get 236, the weight of the U-236 that split apart).  These neutrons come hurtling out of the original atom and smash into other uranium-235 atoms, triggering 3 more U-235 fission reactions, each of which generates 3 more neutrons.  As you can see, before long, there are a lot of neutrons and thus a lot of reactions and thus a lot of heat, which heat the water surrounding the fuel rods, creating steam, spinning the turbine — just like many of the other systems for making electricity.

The nuclear fission reaction described above is an example of a positive feedback mechanism that will naturally tend to speed up until all of the fuel (the U-235) is used up.  This means that it has a tendency to create more and more heat, and if left unchecked, this would cause the water in the reactor vessel to get too hot and build up too much pressure for the reactor to contain — then you would have a big steam explosion, such as happened at Chernobyl.  To control this reaction, the reactor core has a series of control rods, made of materials that absorb the neutrons emitted during a fission reaction.  So the control rods allow the operators to adjust the rate of the reaction and thus the rate of heat production.

There are two basic types of nuclear power plants that are in operation today. The first, and most common, is the Pressurized Water Reactor (PWR), which is illustrated in the animation below. In a PWR, hot water passes through the reactor core (where it absorbs the heat from the nuclear fission reactions) and is then pumped through a heat exchanger, where it heats another fluid that produces steam, powering the turbine. The primary advantage to this type of design is that the water in the primary loop (which passes through the core) does not actually come into contact with the fluid in the steam generator, so unless pipes or valves break there is no risk of contamination or radioactive water leaking from the plant. The Boiling Water Reactor (BWR), illustrated in the next figure, utilizes a somewhat simpler design, where the water that runs through the core is allowed to vaporize to steam, thus powering the turbine to generate electricity. While the design is simpler, it does mean that the steam entering the turbine can be radioactive.

Whether one design is inherently more advantageous than another is difficult to say. Both types have been involved in major nuclear power plant incidents. The reactor at Three Mile Island was a PWR while the reactor at Fukushima was a BWR, so the potential exists for problems at either type of plant. It is perhaps worth mentioning that the Three Mile Island incident was likely due as much to human error and poor design of the reactor’s control system at least as much as to the reactor design itself. The reactor at Chernobyl was an unusual Soviet design called a “light water graphite reactor” that was not really designed for use as a commercial nuclear power plant but was adapted for that use anyway. The World Nuclear Association has a nice description of the Chernobyl plant technology with a description of what went wrong (here too, human error played a central role).

Advanced PWRs have been developed that use more passive designs to keep the reactor from overheating, without any pumps or offsite power required. Westinghouse has developed one such design, the AP1000, which is currently being deployed in China. For those who are interested, more information on passive PWR designs can be found at Westinghouse Nuclear.

Nuclear fuel rods typically last for 3-5 years, and when a rod is "spent" it still contains some fissionable 235-U along with a host of other radioactive elements. So, what do we do with these spent rods? Many people would argue that recycling is a good thing. In the nuclear energy industry, recycling of spent nuclear fuel is a somewhat contentious topic. Many countries, including those European countries that still use nuclear energy, recycle spent fuel into new fuel for re-use. The United States does not do this, preferring a "once-through" fuel cycle for reasons of security as well as economics. Understanding the pros and cons of recycling nuclear fuel requires some understanding of how fuel for nuclear power plants is mined and fabricated.

The figure above outlines the many steps necessary to get uranium out of the ground and into a nuclear power plant. After extraction and processing (“milling”), uranium ore is transported to conversion facilities to remove impurities. The next step in the nuclear fuel cycle is enrichment. Owing to security concerns, all enrichment for the US commercial nuclear industry takes place at one government-owned gas diffusion facility in Paducah, Kentucky. Enriched uranium is then transported to one of several commercial fuel fabrication facilities where the fuel rods are manufactured. In the U.S., fuel fabrication is a competitive industry; private firms compete to provide finished fuel to nuclear power plants. Nuclear fuel rods are generally not purchased directly from the government. Nuclear fission and disposal of spent fuel rods constitute the final steps of the nuclear fuel cycle in the US

The US is heavily dependent on the global market for uranium and nuclear fuel. n 2017, 90% of uranium oxide supplies used to develop nuclear fuel in the US come from outside of the country. The main suppliers for the US are Canada (24%), Kazahkstan (20%), Australia (18%), and Russia (13%). Proposals to open new uranium mines in both the western and eastern United States have been met with resistance, primarily on environmental grounds.

Current US policy prohibits the reprocessing of spent nuclear fuel, for two primary reasons. First is economics – the fuel costs for nuclear power plants are already among the lowest of any non-renewable power generation resource. Once nuclear power plants are built, if they are well-run they cost very little to operate. While the recycling of spent nuclear fuel would eliminate the need for virgin uranium ore to be mined or for additional fuel to be purchased on the world market, it is not at all clear whether the benefits of doing so outweigh the costs of reprocessing. The other reason is nuclear security. The process of recycling nuclear fuel involves the separation of uranium and plutonium from the spent fuel rods. There have been concerns regarding plutonium falling into the wrong hands and contributing to the proliferation of nuclear weapons.

One very serious concern with nuclear power has to do with the highly radioactive waste from the process. Much of the waste needs to be isolated for at least 10,000 years. All civilian nuclear waste was intended to be stored permanently at a repository in Yucca Mountain, Nevada. Yucca Mountain was chosen as a waste repository site back in 1987 and we have spent over $15 billion investigating the site and developing 65 km of tunnels deep underground to store the waste. Currently, it could hold 65,000 tons of waste, but we have 94,000 tons of radioactive waste in temporary storage at nuclear plants. The Yucca Mountain facility is not currently operational and significant uncertainties exist as to whether it will ever be used. In the interim, spent nuclear waste will continue to be stored on-site at the power plants.

There are currently several hundred operating nuclear power plants in the world, spread over a few dozen countries, with over a hundred more “proposed” nuclear power plants (these may or may not get built, depending on economic and political factors in the relevant countries). The US still has the largest number of plants, with about 100 currently operating. France’s economy is the most dependent on nuclear energy, with more than 75% of electricity in that country coming from nuclear power plants. Countries with fleets of nuclear power are primarily wealthier nations, such as the US and European countries, but developing nations are really the biggest growth area, particularly China. Prior to the Fukushima incident, other Asian nations besides China had plans to grow their nuclear fleets, but whether that growth will materialize is highly uncertain. In response to concerns regarding the safety of nuclear power plants and waste disposal/management issues, some European countries have enacted various policies mandating the phase-out of nuclear energy, including Austria, Sweden, Germany, Italy, and Belgium. Other countries, including Spain and Switzerland, have imposed a moratorium on the construction of new nuclear power plants. Of the countries that have decided to phase out nuclear energy, Germany has been among the most aggressive following the Fukushima incident. Because of concerns over electricity supply and costs, however, some countries have delayed or back-stepped on plans to phase out nuclear energy.