Geothermal energy uses drills to harness the Earth's essentially limitless supply of central heat. There are two main uses for geothermal energy: to convert to usable electricity and to heat homes.
How it Works
For the first type, to convert to electricity, power plants are primarily used. Geothermal heat pumps are used to heat homes directly.
Power plants: In geothermal power plants, wells are drilled about 1-2 miles deep into the ground near a thermal reservoir. Then, drills fracture the rock surrounding the reservoir, allowing the extraction of hot water and steam. Once the hot water reaches the surface, the dramatic drop in pressure causes it to expand and turn to steam. The steam turns turbines, generating electricity. Then, the steam is cooled back to liquid water and pumped back into the surface to start the process again.
Heat pumps: In geothermal heat pumps, water or another refrigerant moves through a loop of pipes. When the weather is cold, the water or refrigerant heats up when it goes through the parts of the pipes that are underground. When the weather is hot, the water or refrigerant cools down when it goes through the ground. Once the water or refrigerant gets back above ground, it transfers the heat or coolness into the building, working as a heating or cooling agent. Then, because the heat/cool is transferred into the building at the water returns to its original state, the water can be reused, pumped back into the pipes to be reheated/re-cooled.
How it's Renewable Energy
In geothermal energy, the water is reused, meaning that it does not have to be replaced every cycle. Furthermore, the planet has a never-ending supply of heat. Hence, all of the electricity produced is completely renewable.
Enhanced Geothermal Systems
Enhanced geothermal systems (EGS) are an emerging technology that scientists say will vastly improve the amount of geothermal energy we can access. While traditional geothermal power plants need to be close to thermal springs, EGS allows us to draw geothermal energy anywhere. However, EGS takes us down a dangerous path as the concept relies on subsurface fractures like fracking. EGS relies on pumping pressurized water through a well to a system of extremely hot rocks far underground. The water opens or expands on fractures in the rock, thereby becoming heated. The water then travels up a second well and continues the same process as normal geothermal energy from there.
Advantages and Disadvantages
Most energy experts acknowledge the great untapped potential of geothermal energy. As geothermal energy grows and innovates, it can become more efficient. Furthermore, just like hydroelectricity and tidal energy, geothermal energy is constant. Therefore, geothermal energy can supplement the already-prevalent solar PV, CSP, and wind energies.
Geothermal energy is a controversial source of renewable energy because it involves fracturing underground rock, similar to fracking. Fracturing rock is dangerous because it could release harmful chemicals into underground water reservoirs, contaminating drinking water. Fracturing rock also has the potential to release greenhouse gas emissions, particularly methane. The emerging EGS technology only exacerbates this possibility by putting a focus on fracking. Furthermore, fracturing may cause surface instability. Instabilities along the fracture line in the rock could arise from the repeated water cycle. This surface instability could result in earthquakes.
Other disadvantages of geothermal energy include its high upfront costs. Siting geothermal energy is expensive, time-consuming, and financially risky. Geothermal energy also has high distribution costs because power plants are located far from populations.
Conclusion: Our Take
Many experts see geothermal energy as an essential part of the clean energy future. Indeed, it offers much potential. The International Renewable Energy Agency predicts that geothermal could increase eight-fold by 2050 in Europe. Similarly, the U.S. Department of Energy predicts a 26-fold increase in the US by 2050. However, geothermal energy is potentially dangerous to the environment, with many of the same side-effects of fracking. Hence, geothermal energy should not be deployed as a primary source of electricity but rather to supplement other forms of renewable energy.
Image: GeoSilica Geothermal Power Plant in Iceland. November 24, 2014. Open Access Government.
“How Exactly Does Geothermal Energy Work?” Climate Reality Project. Climate Reality Leadership Corps, April 12, 2018.