How Does Geothermal Work?
Geothermal energy is heat generated from within the Earth. This heat comes from the radioactive decay of radiogenic materials in the Earth's mantle and crust that have been generating this heat ever since the Earth was formed over 4.5 billion years ago. We take advantage of the heat energy stored in the Earth by pumping that energy to the surface from a geothermal resource for generating electricity, heating and cooling homes, and directly using hot water from the ground to heat and cool large facilities.
Geothermal energy relies on three main factors: fluid (such as water), heat (from hot rock beneath the earth’s surface), and permeability (how easily a substance can pass through a material) at depth - meaning that geothermal power generation works best when there is a hot, underground reservoir of water or other liquid that can be accessed either through drilling or a surface expression, such as a hot spring. These conditions are relatively uncommon, but advancements in technologies are making it cheaper and easier than ever to create the conditions where geothermal is suitable. Advanced techniques for creating the conditions for cost-effective geothermal are discussed further in the Enhanced Geothermal Systems (EGS) section.
Geothermal energy has many applications and technologies
Energy 101: Geothermal Energy
Video highlighting the basic principles at work in geothermal energy production as well as outlining three different ways the Earth’s heat can be converted into electricity
Geothermal Research
Easy to understand guide from experts on how geothermal energy is produced and used
Geothermal Basics
Overview of what Geothermal Energy is, where it’s found, how it’s used, benefits, market outlook, and other helpful links
Energy Basics: Geothermal
Did you know the US leads the world in geothermal power production? Do you know how geothermal energy is produced? Learn that and more in this video
Full Steam Ahead: Unearthing the Power of Geothermal
A rundown of how and why Geothermal Energy is on the rise, outlining new technologies that are reducing the cost and increasing the potential of geothermal energy
Where is Geothermal Energy Found?
Natural Geothermal Sources
Accessing geothermal energy requires a geothermal resource, which can either come from the temperature of the ground itself or from reservoirs of hot fluids deep within the Earth’s crust, or can be manufactured through enhanced geothermal systems. Geothermal resources are typically classified as either hydrothermal or low-temperature, which includes low temperature water and heat energy from soil.
United States temperatures at 7km depth
Stanford Temperature Model
Interactive map and model showing the underground temperatures at depths between 0 and 7 kilometers for the continental United States (excluding Alaska and Hawaii)
GBCGE Subsurface Database Explorer
Interactive map showing the location and attributes of subsurface geothermal resources in the Great Basin (California, Oregon, Idaho, Utah, Arizona) area of the United States.
SMU Dedman Temperature Maps
A library of temperature-at-depth maps showing the temperature at depths ranging from near surface all the way to 10 kilometers beneath the Earth's surface.
The Geothermal Exploration Opportunities Map Beta (GeoMap™)
A library of surface and subsurface temperature data and other tools that allows users to explore and rank geothermal development opportunities utilizing factors like state and county boundaries, pollution point sources, transmission lines, heat and electricity demand, regulatory incentives, and techno-economic considerations.
Hydrothermal Reservoirs
A hydrothermal reservoir forms when water heated deep beneath the Earth’s surface becomes trapped within porous rock that is capped by a layer of relatively impermeable rock through which the hot water or steam can’t escape. Hydrothermal reservoirs typically form near the boundaries in tectonic plates, which is why hydrothermal reservoirs are more common in the western United States. These reservoirs are suitable for power generation and high-temperature direct use applications when the temperature is in excess of 300°F (150°C).
Uses for these types of resources are discussed further in the direct-use and geothermal power generation sections.
The diversity of geothermal resources and applications, delineated within three resource categories: geothermal heat pump, hydrothermal, and enhanced geothermal systems
Low-Temperature
Low-temperature resources, as opposed to the hydrothermal reservoirs discussed above, are typically cooler than 300°F (150°C). Uses for low-temperature geothermal resources are discussed in the direct-use applications section.
Ground Source
The ground beneath your feet stores heat energy that can be used to heat and cool buildings. Because shallow ground (the upper 10 feet of the Earth) remains a relatively constant temperature of 40–70°F throughout the year, rocks and soils below a building or community can act as a heat sink—absorbing excess heat during summer, when surface temperatures are relatively higher—and as a heat source—providing heat during the winter when surface temperatures are lower.
See the heat pumps section to learn more.
Geothermal Resource Links
What are Hydrothermal Resources?
A brief overview of Hydrothermal Resources, how they’re formed, and links to research and projects
Where are Hydrothermal Resources in the US?
2018 map of the locations of Hydrothermal sites and areas most favorable to hydrothermal power generation
Where geothermal energy is found
A brief description of how geothermal resources are formed, where they are found, and why they are found where they are
Creating Geothermal Sources
Enhanced Geothermal Systems (EGS)
In areas with hot underground rock but low natural permeability, fluid can be injected into the rock to create new fractures or re-open existing fractures in the rock, creating viable geothermal resources where none previously existed. This approach, referred to as enhanced geothermal systems, or EGS, can also be used to enhance existing geothermal resources to improve their output.
- Injection well - A well drilled deep into hot “crystalline basement” rock below water tables, usually at depths greater than 10,000 feet.
- Injecting water - Water under pressure is injected to fracture the rock or open up existing fractures to create a reservoir for the water within the basement rock. This is a crucial step in the EGS process.
- Production well - A second well is drilled down to intersect with the basement rock reservoir created in step No. 2, in order to extract hot water.
- Additional wells - More wells are drilled to extract more hot water from the heated rocks to power turbines on the surface.
- Electricity - Heated water from below drives turbines that create electricity.
Explore the links below to learn more about how EGS is shaping the future of geothermal
Detailed definitions of EGS, benefits, steps, video resources, demonstration projects, and more!
Breakdown of what EGS is, the benefits and potential of using EGS, the risks associated with Induced Seismicity, and steps laying out how to develop an EGS site.
Enhanced Geothermal Systems: Are we there yet?
Stanford Energy seminar discussing the current state of EGS, limitations and challenges in its adoption, the benefit of creating new geothermal resources, and answering common questions about EGS
Article laying out what EGS is, how it works, and how EGS is similar to and different from fracking.
Energy Anywhere: The Power of Enhanced Geothermal Systems
Enhanced geothermal systems have the potential to power the equivalent to over 65 million American homes. In this animation, learn how the U.S. Department of Energy's Geothermal Technologies Office is advancing this emerging technology.
Closed-Loop Geothermal
While EGS can help geothermal developers overcome the challenge of low natural permeability, it's not the only option for deploying geothermal in areas that are not naturally suited to geothermal applications. Closed-loop geothermal (CLG) overcomes permeability issues by circulating a working fluid (typically water or supercritical CO2) through a series deep, sealed pipes that allow the working fluid to absorb heat from deep underground and return it to the surface for use irrespective of the natural permeability conditions.
How is Geothermal Energy Used?
All geothermal technologies rely on the concept of pumping heat from beneath the ground to the surface. However, each technology has a slightly different way of using that heat depending on its application.
Geothermal Heating and Cooling
Geothermal heating and cooling provide homeowners, businesses, and communities with an energy efficient way to heat and cool homes, buildings, and campuses. An alternative to conventional air conditioning, Geothermal Heat Pumps (GHPs) pump heat from the air in a building into the ground to cool a building during warmer months and pump heat from the ground beneath a building into the air in the building to warm a building during colder months.
Direct Use Geothermal
Geothermal direct-use can use either high-temperature (>300°F/150°C) or low-temperature (<300°F/150°C) geothermal resources but does not involve converting the heat energy into electrical energy. Instead, it is used directly to provide heating and cooling for residential, commercial, and manufacturing applications, such as pulp production for paper products or to directly provide steam for specialized facilities such as greenhouses. Hot water or steam is circulated through pipes that give off ambient heat that can heat large buildings and even entire towns – which is referred to as district heating. A district heating system provides heat for most of the buildings in Reykjavik, Iceland.
Explore the links below for videos, breakdowns, and beginner-friendly resources for understanding more about the ways we can use geothermal energy.
Breakdown of Geothermal heat Pumps, how they can be used, and the different components of a Geothermal Heat Pump (GHP) system
A user-friendly, descriptive video describing how the ground stores heat energy, how it can be accessed, and its benefits
A breakdown of Geothermal Heat Pumps, benefits, and savings opportunities.
Overview of Deep Direct Use Geothermal, including its history, current demand, and the market outlook for DDU
Real world examples including Universities, housing projects, and even the city of Boise, Idaho relying on geothermal district heating to save money, offset environmental impact, and even melt snow from roads and sidewalks
An in-depth 2004 white paper on how District Geothermal is used to heat homes/businesses, power industrial processes, and otherwise impact daily life in Reykjavík, Iceland
Geothermal Power Generation
Geothermal power plants use working fluids to produce steam that turns large turbines which run electrical generators to supply electricity to homes and businesses. There are three types of geothermal power plants: direct dry steam, flash steam and binary cycle.
The Geothermal Technologies Office provides a detailed breakdown and helpful animations for visualizing the differences between these geothermal plant styles on their Electricity Generation page.
Geothermal power-plant configurations: dry steam, flash steam, and binary cycle
Explore the links below for videos, breakdowns, and beginner-friendly resources for understanding more about geothermal energy is used to supply electricity to the power grid.
Breakdown of the different types of Geothermal Power plant technologies and how they’re used
Breakdown of the different types of Geothermal Power plant technologies and how they’re used
Breakdown, history, and helpful animations describing how different types of Geothermal Power plant technologies function and how they are used
Interesting facts about geothermal energy production and how it may be closer to your community than you think!
List of all 31 operational geothermal power plants/complexes in the United States as of 2018
Geothermal Power Generation – Current and Planned Nameplate Capacity by State
Map of the current and planned capacity (MW) of Geothermal power generation by state in the United States
How Can Geothermal Save Me Money?
Geothermal heat pumps (GHPs), also referred to as ground source heat pumps (GSHPs), can be over 400 percent efficient, meaning they can convert one unit of electricity to 4 or more equivalent units of heating or cooling on your property. For reference, traditional fossil fuel furnaces have around 70 to 90 percent efficiencies. In practical terms, property owners who install geothermal heat pumps can save up to 70 percent on heating costs and up to 50 percent on cooling costs.
For more information about the types of heat pumps, tax incentives for heat pump installations, and links to tools and resources that will help you determine whether a geothermal heat pump is right for your home and your wallet, please visit our geothermal heat pumps page.
How Does Geothermal Create Jobs?
According to a study by the Italian Manager of Energy Services, geothermal leads renewables in terms of jobs creation, creating 34 jobs per megawatt, much higher than the 19 created by wind power and the 12 by photovoltaic (solar) energy. This is because, unlike other renewables, geothermal relies on indirect employment, including careers in:
- Geothermal power plant operation
- Geothermal assessment and design services
- Geothermal consulting (engineering, environmental, etc)
- Construction, HVAC, Insulation, Installation
- Service companies (primarily drilling and excavation/land moving)
- Research and technology
- Federal and state policy-making
- Federal, state, and local land development permitting and regulation
Please visit our Work in Geothermal page for a more exhaustive breakdown of each of the fields and companies employing these types of professionals.