Geothermal energy refers to any of a family of different technologies used to draw energy from the Earth’s heat. Its clean and efficient nature has drawn the attention of the U.S. Department of Energy, and it has garnered enthusiastic support among those who favor the development of alternative energy sources, such as wind energy, to lower emissions of greenhouse gases like carbon dioxide.
Let’s learn a little bit more about this remarkable source of energy, including how it comes from the heat inside the Earth, the ways in which it can be harnessed for various industries, and what it ultimately costs to use.
What Is Geothermal Energy?
“Geothermal” comes from the Greek words “geo” (Earth) and “therme” (heat). As its name implies, geothermal resources are a type of renewable energy that comes from the Earth’s naturally occurring heat.
It’s worth taking a step back and exploring how the Earth is structured to better grasp how this renewable energy source can be used for electricity generation.
Some 1,800 miles beneath the surface of the Earth lies the Earth’s core, which is a dense ball of iron wrapped in an extremely high-temperature layer of liquid iron.
The next layer is the mantle, which is roughly 1,800 miles thick and is believed to consist of a mix of solid and molten rock. Around this is a thin insulating layer known as the Earth’s crust. This crust is what we think of as Earth’s surface, and it’s not a single unbroken sheet of rock.
Instead, it’s made up of different high-pressure tectonic plates that move very slowly. Sometimes the plates pull away from each other and sometimes they grind up against each other at plate boundaries, which is part of what gives rise to volcanic activity, earthquakes, tsunamis, and various other geological phenomena.
Geothermal technologies exist to take advantage of the thermal energy generated by Earth’s heat.
Beneath the Earth’s surface lie reservoirs of water. When the ambient heat from molten rock elevates the temperature of these reservoirs, it becomes possible to productively harness this hot water.
What Are the Types of Geothermal Energy?
Geothermal energy systems come in three main categories:
- Direct use (“low temperature”) or district heating systems
- Geothermal power plants
- Geothermal heat pumps
The most distinguishing characteristic among these categories is whether or not you have to drill deep into the Earth’s surface in order to make use of geothermal energy.
Direct use
With direct use and district heating systems, you can leverage hot water that has made its way relatively close to the surface, meaning you have no need to go drilling for it.
Naturally occurring hot springs are a well-known example. Hot springs were in use as far back as the ancient Chinese and Romans who used them to bathe and cook. Hot springs are still popular destinations today, and there are many who believe that the high mineral content of the water in hot springs confers health benefits.
A lesser-known example is district heating systems, in which hot water is piped directly into buildings to heat them up. In the capital of Iceland, Reykjavik, for example, most of the buildings are heated in this way.
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Geothermal power plants
Geothermal plants are a kind of geothermal project that might require a certain amount of drilling, depending on the type of plant it is. Geothermal power plants use hot water in one form or another to spin a turbine, which then generates electricity. The power plant’s type is determined by how exactly it accesses this hot water.
A dry steam geothermal power plant is one that’s able to use steam at or near the surface to spin its turbines. A flash steam geothermal power plant, on the other hand, requires boring deep into the Earth to pull hot water to the surface, where it’s converted to steam and used to spin turbines.
A binary cycle power plant operates along similar lines, but the heat from the hot water is first transferred to another liquid. This second liquid is converted to steam to spin a turbine.
Geothermal heat pumps
Geothermal heat pumps are the final type of geothermal energy system. Rather than generating electricity directly, a heat pump is meant to utilize the relatively stable temperatures just a few feet beneath the Earth’s surface to heat or cool a building.
A heat pump might be installed 10 feet underground, pumping warmth from the ground into a building during the cool winter months and pumping heat from the building into the ground during the warm summer months.
What Are the Major Advantages of Geothermal Power Generation?
There are many advantages to using geothermal power. Importantly, geothermal energy is renewable. It relies both on heat from the Earth and water. The heat is consistently renewed by the natural radioactive decay of particles which occurs in all rocks, and the water is replenished by rain. Neither is in any danger of disappearing, so we can count on geothermal energy for a long time.
It’s also very reliable. Where’s wind and solar are famously intermittent — relying as they do on wind and sunshine, respectively — geothermal energy is generally consistent.
What Are the Major Disadvantages of Geothermal Energy?
Perhaps the single greatest disadvantage of using geothermal energy is that it’s only available at certain locations. Direct use applications such as natural hot springs do not occur everywhere, of course. But the surface of the Earth is also not homogeneous. Its depth varies from place to place, which means that a drilling project that’s easy in one location might be all but impossible in another.
Another disadvantage of geothermal power is that it does have some environmental impact. Though geothermal energy can reduce the emissions associated with burning fossil fuels, there are greenhouse gases locked in the Earth that can be released during drilling and digging.
A related – and perhaps more pressing – environmental impact is an increased risk of earthquakes. Conventional geothermal involves a lot of digging, and more advanced techniques actually force water into layers of rock to increase their permeability. In both cases, there’s a possibility that an earthquake can be triggered.
Geothermal energy also costs a lot to develop. A 1-megawatt capacity plant, for example, will generally cost somewhere in the range of $2-$7 million. It’s worth bearing in mind that most of this cost is related to upfront construction, and it can be recouped over time as the plant operates profitably.
Are There Geothermal Reservoirs Everywhere?
Unfortunately, geothermal reservoirs are not located everywhere, which is one of the main drawbacks of this form of renewable energy. Geothermal resources are generally located at plate boundaries, so they’re used in places like Indonesia, the Philippines, California, and New Zealand.
That said, there are geothermal resources in other places, such as Italy, and in U.S. states like Idaho, Nevada, New Mexico, Hawaii, and Oregon. As of 2021, there are 30 geothermal plants operating in the country.
What’s the Future of Geothermal Energy?
Today, efforts are underway to expand the range of locations where geothermal energy is economically viable. These enhanced geothermal systems could bring more than 100 gigawatts of electricity online in the United States alone, which represents 10% of the U.S.’s total electric capacity at present.
These advanced techniques involve increasing the permeability of rock by injecting water into them. If this approach can be scaled, many new locations could be powered by geothermal energy in the future.
How Deep Do You Have to Go for Geothermal Energy?
How deep you need to go to access geothermal resources depends on the location.
For direct use applications such as hot springs or district heating, the resource is located relatively close to the surface of the Earth. Geothermal heat pumps are generally buried just a few feet below the ground, but some geothermal energy projects require pumping hot water from as far down as two miles.
How Big a Yard Do You Need for Geothermal Heat Pumps?
It depends on whether you’re using a horizontal, vertical, or pond system. Installing a horizontal system means digging 400 feet of trenches that are each two feet wide.
Installing a vertical system requires a lot less horizontal space, and involves drilling holes roughly 4 inches in diameter, 100-400 feet deep, spaced 15 feet apart.
If your property has a pond or a lake you can install a heat pump around 10 feet beneath the surface of the water.
Can I Save On My Electricity Bill with Geothermal Energy?
It depends a lot on the specifics of your situation, but numbers from the Environmental Protection Agency indicate that the savings could be substantial. They estimate that households using geothermal heat pumps might save between 30-70% on the costs of heating their homes and as much as 20-50% on the costs of cooling their homes. In total dollars, this might amount to as much as $1,500 a year.
At What Temperature Does Geothermal Energy Stop Working?
By and large, the temperature doesn’t have much impact on geothermal energy. Geothermal energy production can work in lower-temperature environments (i.e., Iceland) and higher-temperature environments (i.e., Nevada).
How Long Will Geothermal Energy Last?
Based on the physical mechanisms involved, there’s no reason to think geothermal energy will ever really “run out”. Unless the core of the Earth were to begin cooling or the planet’s water tables were completely depleted, geothermal could plausibly provide energy for billions of years.
Can Geothermal Energy Power a House?
As of 2019, geothermal energy is being used in 27 countries (including the U.S.), producing around 88 billion kWh of power in total, so it’s already providing electricity.
However, geothermal is not usually used to provide 100% of a single home’s needs. A geothermal heat pump can substantially reduce your bill, but you’ll more than likely still be using other sources of power to meet your energy needs.
Getting Down to Earth With Geothermal
Geothermal energy is among the viable options in the world’s ongoing transition to use more alternative energies. From hot springs to heat pumps to enormous geothermal power plants, a lot of energy can be drawn from the Earth with the right technology.
If you’re interested in reducing your carbon footprint, learn more about other forms of renewable energy like wind power and how to become more energy efficient at home.
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