Geothermal Energy

In 2013, geothermal power plants produced enough energy to hypothetically power more than six million typical U.S. homes. How does this oft-ignored renewable energy source convert the heat underground into energy that can heat our homes or power our factors?

Where does Geothermal energy come from?

The word geothermal is derived from two Greek words: “geo,” meaning earth, and “therme,” meaning heat. That’s the short explanation of where geothermal energy comes from – the heat of the earth. Unlike wind and solar energy, which is collected from up above, geothermal energy comes from the earth’s core below your feet. Scientists are still puzzling out the specifics of the earth’s formation, but do know that our planet started out much hotter than it is now. In fact, when the earth was younger, it was basically a spinning ball of fire. Over hundreds of millions of years, it’s gradually cooled down from the outside in. That means the center of the earth is still much warmer than the surface – about 10,800 degrees Fahrenheit.

We get high temperatures closer to the surface as well. Just below the earth’s crust, there is a layer of magma – hot, molten rock. Magma doesn’t just derive heat from the earth’s core, but produces its own thermal energy. That’s because magma contains continuously-decaying radioactive materials like the heavy metal uranium. There are certain geographic areas where it’s especially easy to access thermal energy from magma. These places are at tectonic plate boundaries or where the crust is thin, so they have a lot of active or recently-active volcanoes.

How does it create energy?

Have you ever swum in a naturally-occurring hot spring, like the ones at Yellowstone Park? These “hot spots” occur when tectonic plates shift or magma moves below the earth’s crust, allowing water to heat up, circulate, and rise to the surface. This heated water is the key to harnessing geothermal energy. When left to its own devices, the earth creates these things called hydrothermal convection systems in which water seeps into the earth’s crust, heats up, and rises back to the surface as steam. Geothermal power plants tap into hydrothermal convection systems by drilling into rock and capturing the steam to power a generator.

There are three designs that geothermal power plants use to capture this hot water and convert it into electricity. The simplest design is called a dry steam system. It works by catching steam directly into a turbine, which spins and powers a generator. The steam then transfers into a condenser, where it cools back down before being returned to the ground. A similar design called a flash plant captures already-hot water, then depressurizes – or “flashes” – it into steam to drive a turbine. Finally, there are binary cycle systems, which capture hot water into an exchanger to heat a second liquid with a lower boiling point. The second liquid, usually isobutane, converts into steam and powers a turbine.

What’s the environmental cost of Geothermal energy?

Geothermal energy is renewable, but can have adverse effects on ecosystems and emit harmful gases into the air depending on how it’s accessed and processed. Some geothermal power plants drill deep into the earth to access steam more easily. This can be more damaging than comparable renewable energy systems since those land sites that generate the most geothermal energy tend to be in sensitive ecological areas.

In terms of air emissions, environmentalists are most concerned with geothermal power plants that use open-loop systems. These systems emit hydrogen sulfide along with smaller amounts of toxic gases like ammonia and methane. Sulfur dioxide creates acid rain, which damages crops, and acidifies lakes and rivers. However, even those geothermal plants that emit sulfur dioxide produce thirty times less of the gas than coal plants.

What are its common uses?

The two most common uses of geothermal energy are electricity and heat. Geothermal plants are becoming a more popular electricity source globally. Iceland and El Salvador, for example, each derived 25 percent of their electricity from geothermal plants as of 2012. Some district heating systems warm up homes and industrial facilities directly from geothermal sources. Other buildings use geothermal heat pumps for temperature control in buildings aboveground.

How much does Geothermal energy cost?

Like other renewable energy sources, geothermal power plants have higher startup costs and become less expensive in the long term. Over time, the cost of geothermally generated electricity is comparable to natural gas. The plant is more expensive to build, but it becomes less expensive down the road to generate electricity. On a smaller scale, geothermal home heating systems can be less expensive than typical heating systems. Depending on where they live, homeowners may choose to install one to offset their energy costs.