Module 7: Geothermal, Hydroelectric & Nuclear

Overview

Old Faithful Geyser in Yellowstone is a famous tourist attraction, blasting hot water and steam more than 100 feet into the air on a sufficiently regular schedule to keep spectators happy. If you run the hot water through a turbine, you wouldn't get enough energy to supply the Old Faithful Lodge. But, that idea on a larger scale can provide valuable geothermal energy, which is being used in California, Iceland, New Zealand, Italy, and elsewhere. Most of our geothermal energy comes from anomalously "hot" places near volcanoes, and there aren't enough of those to power all of humanity. But, if we were to use "hot, dry rock", pumping waterway down, heating it, and bringing it out artificial geysers to drive turbines, an immense amount of energy is available.

Moving water carries power even if it isn't coming out of a geyser. We get reliable power from hydroelectric dams on rivers, and we can extract more energy from waves and currents. There isn't enough of either one to give us all of our energy, but in some places, they are greatly valuable, and we can develop new ways to make them more valuable—if you're building a breakwater to protect a city from the rising sea, why not install generators to convert the punishing power of storm waves into valuable electricity for the city?

The heat driving geothermal energy is mostly from radioactive decay in rocks. We have figured out how to generate more radioactive decay, where and when we want, in nuclear fission reactors, which are supplying much of our electricity in many countries. Nuclear energy could generate more electricity, too, although it also generates much debate among those who enjoy its reliable electricity, and those worried about contamination now or far into the future, and about the possible use of nuclear programs to generate material for bombs.

These three forms of energy — hydropower, geothermal, and nuclear — have been with us for quite a while (especially hydropower), so it is not surprising to see that they make up a significant portion of the global "renewable" energy portfolio.  The quotes around renewable are because hydro, geothermal, and nuclear are not entirely renewable — it is probably better to call them low-carbon sources of energy — but in the literature, they are often labelled as "renewable".  As with wind and solar, hydropower, geothermal, and nuclear have extremely low carbon emissions per unit of energy produced. The figure below, showing the history of (mostly) renewable energy production for the world, reveals some interesting trends.

The history of global renewable energy production in Exajoules (EJ) of energy generated each year. Recall that at present, the world consumes about 600 EJ of energy each year.

Click for a text description of the Global Renewable Energy graph.

The image is a line graph titled "Global Renewable Energy," which shows the historical generation of various types of renewable energy from 1965 to 2015, measured in exajoules (EJ).

• The y-axis represents the energy generated in EJ, ranging from 0 to 16 EJ.
• The x-axis represents the years from 1965 to 2015.

The graph includes five different colored lines representing various energy sources:

1. Solar (blue line) - This line starts from near 0 EJ in 1965 and shows a very gradual increase until around 2005. After 2005, there's a sharp rise, reaching approximately 1 EJ by 2015.
2. Wind (orange line) - Similar to solar, it starts from near 0 EJ in 1965. The increase is also gradual until around 2000, after which it rises more steeply, reaching about 2 EJ by 2015.
3. Nuclear (red line) - This line begins from near 0 EJ in 1965, with a steady increase over time. It shows a significant rise starting around 1970, peaking at about 7 EJ around 2005, then slightly declining and stabilizing around 6 EJ by 2015.
4. Hydro (yellow line) - Hydro energy starts from around 1 EJ in 1965 and shows a steady increase, reaching approximately 4 EJ by 1980. From there, it continues to grow slowly, peaking at around 10 EJ in the early 2000s, then slightly declining and stabilizing around 9 EJ by 2015.
5. Geothermal, Biomass, Other (green line) - This category starts from near 0 EJ in 1965. The growth is very gradual until around 2000, after which it starts to increase more noticeably, reaching about 1 EJ by 2015.

A legend in the top left corner identifies each color with its corresponding energy source. The graph visually represents the growth trends of different renewable energy sources over time, with hydro showing the most significant historical contribution, followed by nuclear, while solar, wind, and geothermal/biomass/other show notable increases in recent decades.

Credit: David Bice, data from BP Statistical Review of Energy 2019

We see here that hydropower was already contributing a significant amount of energy in 1965 and has seen more or less steady growth since then. Nuclear energy emerged on the scene about 1970 and grew rapidly at first, but has since leveled off, while geothermal has been growing at a relatively slow pace. These three are all in contrast to wind and solar, which are characterized by exponential growth starting in just the past two decades.

How about costs? Most energy economists like to compare the energy costs from different sources using the "levelized cost" or "life-cycle costs" that we discussed earlier with wind and solar power. The table below provides a comparison of a wide range of energy sources.

As you can see, hydroelectric, geothermal, and nuclear are all more expensive than solar PV and wind, but they do have the advantage of being able to supply energy on demand without any kind of battery storage systems.

Let's go look at these interesting power providers — hydro, geothermal, and nuclear. We'll save some of the economic and ethical issues for later.