Geothermal 101

Welcome back to another issue of ClimateRoots! These next two issues we are going to be talking about a super interesting method of energy production: Geothermal!


Using geothermal energy is nothing new. Humans have been using hot springs for centuries as a method of both bathing and cooking, but it wasn’t until the 20th century that we figured out how to generate power from it. With a global installed capacity of just over 14 GW (“Renewable Energy | Energy Economics”), it’s not the biggest hitter in the renewable energy sector, but unlike the intermittent power methods of solar and wind, geothermal can be constantly tapped to tackle our 24/7 energy demands.


Before we dive too deep though, let’s take a step back and talk about the planet we call home. Harnessed in the Earth is an insane amount of heat and pressure, thanks to billions of years of radioactive decay and non-stop heat loss from the planet’s formation. At its core, temperatures can reach well over 6,000 degrees celsius, or about 10,000 degrees fahrenheit (Earle, Steven), and while things are not quite that extreme at the crust, a steep temperature gradient near the surface means that going down into the crust even just a bit will reward us with enough thermal energy to do a lot of cool things! There are several uses for this geothermal resource, but the main one we are going to be talking about today is energy production.


As is the case with many other forms of energy production, geothermal relies on using steam to spin a turbine to produce power (check out our piece on steam turbines to see how generators work). There are three ways of doing this that are widely used in power plants scattered across the world: dry steam, flash steam, and binary steam.


Dry Steam

The oldest and arguably easiest form of geothermal production, dry steam power plants work by directly pumping steam from underground reservoirs up into a turbine/generator. From there, the steam is condensed into water and pumped back into the ground to become steam again. As with every system though, there is some amount of loss in this process, and if not dealt with properly, there can be a severe reduction in reservoir pressure leading to a decrease in energy production (“Electricity Generation”).





Northern California lays host to the largest dry steam energy production in the world, a site known as “The Geysers”. In the late 1980’s, these power plants noticed a large drop in energy production as the amount of steam being taken from the system vastly outweighed the amount of water being put back in. A decade later, a pipeline of nearby treated wastewater had been built to pump extra water into the reservoirs, and today, nearly 20 million gallons of water is pumped into the system on a daily basis (The Water Story).


Flash steam

Flash steam production is much harder to come by as it requires an underground reservoir of fluid in excess of ~182 degrees celsius, although it is currently the most common method used globally (“Geothermal Energy 101”). At greater depths, the higher pressure will allow fluids to remain in a liquid state at warmer than normal temperatures because its boiling point will be greater as well. Flash steam power plants will pump this fluid up to the surface where the pressure is much lower, and thus its boiling point is as well, causing the fluid to “flash” into a vaporized state. This vapor is then pumped into a turbine and later condensed to be pumped back into the reservoir (“Electricity Generation”).





Binary cycle

Finally we have binary cycle, which sounds way more technical than it actually is. This process takes advantage of a heat exchange to vaporize a secondary fluid, instead of relying on the primary underground fluid itself. This secondary fluid has a much lower boiling point than that of water, so it can more easily vaporize - this means we don’t need to find a reservoir of fluid that’s very hot, and because the primary fluid is a closed loop, there is much less loss in the process!





Because the majority of geothermal resources fall under the 150 degrees celsius category, power generation could primarily come from the binary cycle process in the future as it is by far the most geographically diverse of the three (“Electricity Generation”).


So what’s the catch?

Geothermal energy is often touted as the future of carbon-free power generation, and while it’s true that there is only a fraction of carbon emissions compared to other energy production methods like fossil fuels (What Is Geothermal Energy), there are still of course some drawbacks.


For starters, the process of geothermal power production can cause harmful gases to enter the atmosphere that would have otherwise been trapped underground. The most notable being carbon dioxide of course, but other gases like hydrogen sulfide, methane, and ammonia have also leaked during this process (“Geothermal Energy Pros and Cons”). Working with underground resources can be a rather complex process, and geothermal is no different.


Additionally, geothermal energy is very location-specific. With most geothermal power plants existing on or near a tectonic fault line, not only does this severely limit the potential for expansion, there also comes problems with seismic instability. Geothermal exploration projects can be blamed for unnatural tremors and earthquakes, and a lot of similarities can be drawn to the drawbacks of fracking (“Geothermal Energy Pros and Cons”).


At the end of the day, it is important to remember that no method of power production is perfect. Renewable energy is naturally very fickle, but any step away from fossil fuels is a good one. Geothermal is also by no means a perfect solution to our power crisis, but there is little doubt that the benefits outweigh the drawbacks. There are even some rather cool applications of geothermal on a small scale that we didn’t touch on today that are being used in homes worldwide.



 

Bibliography


  1. “Renewable Energy | Energy Economics | Home.” Bp Global, https://www.bp.com/en/global/corporate/energy-economics/statistical-review-of-world-energy/renewable-energy.html.

  2. Earle, Steven. 9.2 The Temperature of Earth’s Interior. Sept. 2015. opentextbc.ca, https://opentextbc.ca/geology/chapter/9-2-the-temperature-of-earths-interior/.

  3. “Electricity Generation.” Energy.Gov, https://www.energy.gov/eere/geothermal/electricity-generation.

  4. The Water Story. https://geysers.com/water.

  5. “Geothermal Energy 101.” Resources for the Future, https://www.rff.org/publications/explainers/geothermal-energy-101/.

  6. What Is Geothermal Energy and Where Is It Used in the World? https://www.power-technology.com/features/what-is-geothermal-energy/.

  7. “Geothermal Energy Pros and Cons.” Clean Energy Ideas, 25 Aug. 2016, https://www.clean-energy-ideas.com/geothermal/geothermal-energy/geothermal-energy-pros-and-cons/.

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