Geothermal

Geothermal (Geo= “Earth” and Thermal= “heat”) energy comes from the heat that flows continuously from the Earth’s interior to the surface. This residual heat has been radiating from the Earth’s core for about 4.5 billion years (since Earth’s formation) because it is continually replenished by the decay of naturally radioactive elements deep inside the planet.

This heat is brought up to the Earth's crust by convection through molten rock (magma) and/or hot water and by conduction through solid rock. Geothermal systems have been classified by the Department of Energy as low-temperature geothermal resources below 300°F (150°C) and high-temperature greater than 300°F (150°C).

Geothermal resources can be divided into hydrothermal (hot water) and petrothermal (hot rock) systems.

• Hydrothermal systems are formed from hot groundwater trapped in fissures and pores of underground rocks and are accessed through conventional technologies and/or coproduced with hydrocarbons for direct heating and/or electricity generation.
• Petrothermal systems are more abundant, requiring only the heat of the earth, but require unconventional technologies such as enhanced geothermal systems (EGS) or other emergent technologies for direct heating or electricity generation.

Geothermal energy can currently be harnessed in three main different ways:

• Electricity production. Requires high- to medium-temperature geothermal resources.
• Direct use. Low-temperature hydrothermal resources can be used directly for space and water heating and other applications (e.g., green housing, bathing, fish farming, food processing, etc.).
• Geo-exchange. Geothermal heat pump (GHP) technologies can be used for space heating and cooling anywhere in the country.

Heat from the Earth's core continually flows to the surface and is equivalent to 44.2 terawatts-thermal (TWth) of electricity (Pollack et al. 1993), which is more than twice the amount required to meet all of the world's main energy needs in 2015 (Energy Information Administration 2017a).

By 2050, there could potentially be up to 60 gigawatts of electricity-generating capacity, more than 17,000 district heating systems, and up to 28 million geothermal heat pumps, according to the 2019 GeoVision report. If all those maximum forecasts come true, it will reduce emissions by the same amount as removing 26 million automobiles from American roads each year. If we can accomplish substantial cost reductions in improved geothermal systems, the improved Geothermal Shot analysis from 2022 revealed the possibility for even higher geothermal electricity-generating capacity—90 gigawatts by 2050.

Two proven geothermal technologies are commercially available today:

• Conventional Geothermal (hydrothermal resources) for electricity generation and direct use.
• Geothermal Heat Pumps (GeoExchange resources) for heating and cooling.

However, other emerging technologies for exploiting petrothermal resources have opened the possibility of utilizing this type of unconventional resources:

• Enhanced Geothermal Systems (EGS) for electricity generation and direct uses
• Advanced Geothermal Systems (AGS) or Closed-loop geothermal for electricity and direct uses
• Geothermal Coproduction.

1. Hydrothermal resources (conventional technologies)

These reservoirs of steam and/or hot water are formed by water seeping into the Earth and collecting in. When water is heated in the earth, hot water or steam is trapped in porous and fractured rocks beneath a layer of relatively impermeable caprocks. The natural formation of hydrothermal resources requires three principal elements: heat, water/or steam, and permeability. Some of these reservoirs have an existing path to the surface, emerging as hot springs. Others are tapped by drilling wells to deliver hot water to the surface for generation of electricity or direct use. In the United States, the hottest (and currently most valuable) resources are located in the western states, Alaska, and Hawaii.

Hydrothermal resources are used for electricity production through three types of power plants: dry steam, flash steam, and binary cylcle. Hydrothermal resources are also used for direct use, including space heating, district heating, agricultural and industrial heating, and other uses.

2. GeoExchange resources (Geothermal Heat Pumps)

The upper 10 feet of the Earth has a nearly constant temperature between 50°F (10°C) and 60°F (15°C). For most areas in the United States, this means shallow soil, rock or water temperatures are usually warmer than the air winter and cooler than the air in summer. In order to heat homes in the winter and cool them in the summer, geothermal heat pumps (GHPs) take advantage of these stable underground temperatures.

3. Petrothermal Resources (unconventional technologies such as Enhanced Geothermal Systems)

There are numerous places where the earth is warm yet but there aren't enough fluids or natural permeability. In these situations, an enhanced geothermal system can be employed to build a reservoir that human-made to utilize the heat for energy. In an EGS, fluid is injected down below under carefully monitored circumstances, reopening previously closed fractures and generating permeability. Increased permeability makes it possible for fluid to flow through the hot rock that has been fractured, and the fluid heats up as it does so. The hot water is pumped up to the surface by the operators, where it produces power for the grid. For more information: What is an Enhanced Geothermal System (EGS)?

Geothermal energy production might be expanded countrywide with the help of EGS by enabling geothermal development outside of conventional hydrothermal resources. Today, both the public and private sectors are being demonstrated EGS advances worldwide.

Learn more about how an EGS works or watch a video on the steps and benefits in EGS development.

Closed-loop technology, also called Advanced Geothermal System (AGS) technology, is an emergent concept that exploit petrothermal resources via consists of two vertical wells connected horizontally at depth. In these systems, a working fluid is circulated within a series of closed-loop well configurations to harvest heat for electricity generation or direct use. Lear more about this emerging technology onGeothermal Rising and GeoCLUSTER.

4. Geothermal Coproduction (conversion from oil and gas wells)

It is estimated that an average of 25 billion barrels of hot water is produced annually from oil and gas wells within the United States.

Historically, this “coproduced” hot water has been an inconvenience and a disposal issue for operators; however, binary power conversion units can now take this waste stream and use thermal energy within it to generate electricity (see section X on electricity generation technologies). Power can be generated in co-production with oil and gas, or by converting idle or abandoned oil and gas wells to geothermal wells.

Power produced from oil and gas wells can be used for field operations or sold onto the grid. For more information read the Geothermal Technology Program Coproduction Fact Sheet and Geothermal Energy Production with Co-Produced and Geopressured Resources.

For more information about geothermal resources, please visit the Tribal Energy Atlas.

Geothermal power can be generated by modular units ranging in size from a few hundred kilowatts to more than 100 MW in size. There is about 3700 MW of installed geothermal power generating capacity in the United States today—most of it in California, the rest in Nevada, Utah, and Hawaii. The cost of producing geothermal electricity ranges from roughly 5 cents/kWh to 10 cents/kWh.

Geothermal power plants draw fluids from underground reservoirs to the surface to produce steam. This steam then drives turbines that generate electricity. There are three main types of geothermal power plant technologies: dry steam, flash steam, and binary cycle. The type of conversion is part of the power plant design and generally depends on the state of the subsurface fluid (steam or water) and its temperature.

Geothermal power plants are reliable and efficient. They are seldom off-line for maintenance or repair and are typically available to generate power 95% or more of the time. This compares favorably to coal and nuclear plants, which are typically available only 60% to 70% of the time.

Direct use projects can make use of hydrothermal resources with temperatures between about 70°F (21°C) and 300°F (148°C). One of the most common direct-use geothermal applications is geothermal district heating, a well brings heated water to the surface; a mechanical system—piping, heat exchanger, and controls—delivers the heat to the building or process; and a disposal system either injects the cooled geothermal fluid underground or disposes of it on the surface. The oldest geothermal district heating system in the United States is in Boise, Idaho, and has been in operation since the 1890s.

Other direct-use applications of hot water from geothermal resources include greenhouses, crop drying, industrial processes (e.g., pulp and paper processing, and frying of cement), resorts and spas, and fish farms.

The direct use of geothermal resources for heating can significantly reduce overall energy bills. For example, greenhouse growers in geothermal areas estimate that using geothermal resources instead of traditional energy sources reduces heating costs by up to 80%, which can save about 5% – 8% of their total operating cost. In the United States, consumption of electricity and heat are roughly equal, while worldwide more energy is consumed for industrial heat than for all electricity use (EIA, 2022).

Direct use systems do require a larger capital investment than traditional heating technologies but have lower operating costs and no need for ongoing fuel purchases.

To learn more about direct use applications, visit the following links:

• Geothermal Technology Office — Low Temperature and Coproduced Resources.
• GeoVision Report District Heat (Direct Uses) Data and Analysis Supporting Task Force Report: Thermal Applications.
• Geothermal Technologies Program: Direct Use (PDF) — This 16-page publication describes geothermal direct-use systems, and how these systems have been effectively applied throughout the country. It also describes the DOE program research and development efforts in this area and summarizes several projects using direct use technology. Download Adobe Reader.
• 2021 U.S. Geothermal Power Production and District Heating Market Report from the National Renewable Energy Laboratory.
• Geothermal Data Repository (GDR): Direct use database — The GDR is the submission point for all data collected from researchers funded by the U.S. Department of Energy’s Geothermal Technologies Office.

Geothermal heat pumps (GHPs) can make use of the stable temperatures in the upper 10 feet of the Earth to provide both heating and cooling to buildings. The surrounding soil, groundwater, or nearby surface water is used as a heat source in winter and a heat sink in summer.

GHP systems consist of three parts: the ground heat exchanger, the heat pump unit, and the air delivery system (piping and ductwork). In winter, when the ground is warmer than the air, the geothermal heat pump removes heat from the ground heat exchanger and pumps it into the indoor air delivery system. In summer, when the ground is cooler than the air, the process is reversed, and the geothermal heat pump moves heat from the indoor air stream into the ground heat exchanger. 

Geothermal heat pumps reduce both heating and cooling costs compared to air source heat pumps and air conditioners in both residential and commercial buildings. They have low operating and maintenance costs and, usually, the lowest life-cycle costs of the available heating and cooling options. Consumption of electricity is reduced 25% - 50% compared to traditional heating and cooling systems, allowing a payback of system installation costs in 0 - 10 years.

Called by a variety of names—earth-source heat pumps, GeoExchange systems, ground-coupled heat pumps, ground-source heat pumps, and water-source heat pumps—GHPs are known for their low environmental impact, quiet operation, and energy efficiency. Today, more than 1.686 million geothermal heat pumps units have been estimated to be installed nationwide (IGSHPA,2018), 60% of the units are installed in residences and the remaining 40% in commercial and institutional buildings (Lund 2020).

To learn more about geothermal heat pump systems, visit the following links:

• Geothermal heat pumps — Geothermal Technologies Office
• GeoVision Report: Geothermal Heat Pump Data
• Geothermal heat pumps — An excellent resource, this DOE Web site provides information on the following GHP-related topics:
  • Types of geothermal heat pumps systems, and Geothermal Heat Pumps.
  • Choosing and installing geothermal heat pumps
  • Operating and maintaining a geothermal heat pump system
• Geothermal Data Repository (GDR): Geothermal Heat Pumps database — The GDR is the submission point for all data collected from researchers funded by the U.S. Department of Energy’s Geothermal Technologies Office.

NREL provides data and tools related to geothermal research and technologies to advance the use and integration of  geothermal energy.

• GEOPHIRES

GEOPHIRES is an open-source, Python-based simulator to perform techno-economic simulations of geothermal systems. GEOPHIRES combines reservoir, wellbore, surface plant, and economic models to estimate output temperatures and heat or electricity production over plant lifetime, as well as capital and operation and maintenance costs, and overall levelized cost of heat or electricity. The tool can simulate both hydrothermal and enhanced geothermal systems, and electricity production and direct-use heat as surface application. GEOPHIRES can be coupled to external reservoir simulators such as TOUGH2, and, given its modular code design, users can easily build on the framework and implement their own correlations or models.

• GETEM: Geothermal Electricity Technology Evaluation Model

The Geothermal Electricity Technology Evaluation Model is a downloadable document that estimates the levelized cost of electricity using user-input data and a set of default information that is based on several resource scenarios that the Department of Energy Geothermal Technologies Office has defined and evaluated.

• System Advisor Model (SAM)

The System Advisor Model is a performance and financial tool designed to facilitate decision making for people involved in the renewable energy industry. This includes project managers and engineers, policy analysts, technology developers, and researchers.

• CREST: Cost of Renewable Energy Spreadsheet Tool

The Cost of Renewable Energy Spreadsheet Tool is an economic cash flow model designed to allow policymakers, regulators, and the renewable energy community to assess project economics, design cost-based incentives, and evaluate the impact of various state and federal support structures. It is a suite of four analytic tools, for solar, wind, geothermal, and anaerobic digestion technologies.

• JEDI: Jobs and Economic Development Impacts Geothermal Model

The Jobs and Economic Development Impacts Geothermal Model allows users to estimate economic development impacts from geothermal projects based on industry averages and includes information that can be used to run an impact analysis on a generic level.

• GEeoCLUSTER: Geothermal Closed-Loop User Tool in Energy Research

Closed-loop geothermal systems (CLGSs) are examined in this work. These deep geothermal systems are intended for industrial-scale thermal energy extraction for uses like district heating and power production. In order to create a collaborative study of CLGSs including four national laboratories and two universities, this project was financed by the Geothermal Technologies Office (GTO) within the Office of Energy Efficiency and Renewable Energy (EERE) of the U.S. Department of Energy (DOE).

• GHEDesigner - A Flexible and Automatic Ground Heat Exchanger Design Tool

GHEDesigner was developed from Oak Ridge National Laboratory, Oklahoma State University, and National Renewable Energy Laboratory under the terms of US Department of Energy. GHEDesigner is used with ground source heat pump (GSHP) systems.