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Geothermal Direct Use

Geothermal energy can be harnessed for the use of its heat directly (direct use) or for electricity generation. Direct or non-electric use refers to the immediate use of geothermal energy for both heating and cooling applications. Geothermal direct use goes back to when people used hot springs for bathing, cooking food, and loosening feathers and skin from game. Today, the primary forms of direct use include heating swimming pools and baths or therapeutic use, space heating and cooling (including district heating), agriculture (mainly greenhouse heating, crop drying, and some animal husbandry), aquaculture (heating mainly fish ponds and raceways), and providing heat for industrial processes and heat pumps (for both heating and cooling). In general, the low geothermal fluid temperatures (i.e., <150°C) required for direct heat use are available throughout most of the United States, but are more abundant in the western states.1U.S. Department of Energy Federal Energy Management Program (FEMP), 2016, Geothermal Energy – Direct-Use, https://www.wbdg.org/resources/geothermal-energy-direct-use, accessed April 13, 2024. An important direct-use application available almost everywhere is geothermal heat pumps.

Geothermal Heat Pumps

Heat pumps move heat from one place to another using electricity. Air conditioners and refrigerators are two common examples of heat pumps. Earth’s natural temperature resources can be used with heat pump technology to heat and cool buildings. Temperatures at about 30 feet below the surface remain relatively constant year-round—between about 50°F (10°C) and 59°F (15°C). For most areas in the United States, this means soil temperatures are usually warmer than the air in winter and cooler than the air in summer.2U.S. Dept. of Energy, Office of Energy Efficiency & Renewable Energy. (n.d.). Geothermal Heat Pumps. (accessed 2023, July 5). https://www.energy.gov/eere/geothermal/geothermal-heat-pumps

Geothermal heat pumps allow for a heat source in the winter and a heat sink in the summer.
Geothermal heat pumps allow for a heat source in the winter and a heat sink in the summer.

Geothermal heat pumps, also known as ground source heat pumps, take advantage of these constant underground temperatures to efficiently exchange temperatures, heating homes in the winter and cooling homes in the summer. Geothermal heat pumps exchange heat with the subsurface, often through a vertical shallow borehole as shown in this animation. The system does not burn fuel to generate heat, but rather it transfers heat from the subsurface to the space at the surface. Thus, geothermal heat pumps are highly efficient.3U.S. Dept. of Energy, Office of Energy Efficiency & Renewable Energy. (n.d.). Geothermal Heat Pumps. (accessed 2023, July 5). https://www.energy.gov/eere/geothermal/geothermal-heat-pumps Borehole heat exchangers typically use 25% of the system energy for the compressor and the expansion equipment in the heat pump. And the other 75% is available for geothermal energy.4Daigle, Hugh, 2022, August 12, Lecture 3.2: Shallow geothermal systems and heat pumps, https://www.youtube.com/watch?v=qMcNswfUf6U

Geothermal Working Fluids

The working fluid transfers heat from the subsurface to the surface. Or, in the case of geothermal cooling, the working fluid transfers heat back to the subsurface. For simple applications (no phase change:no vaporization or condensation), like heat pumps, desirable properties of working fluids include:

  • Low dynamic viscosity: easier to pump, less energy loss from friction
  • Low density: easier to pump
  • High specific heat capacity: storage of more heat per unit volume
  • High thermal conductivity: heat transferred to/from fluid quickly

It turns out that water performs quite well, when compared to other heat transfer fluids commonly used in geothermal probes (e.g., ethanol 25%, methanol 25%, calcium chloride 20%). But water has a significant problem: it freezes at 0° C. Options exist to add some sort of antifreeze (but it will degrade some properties), or you can bury your piping below the frost line and design the system such that your working fluid will move through before it has a chance to freeze.

Geothermal Heat Pump System Components

A geothermal heat pump system includes three components, as described below:5U.S. Dept. of Energy, Office of Energy Efficiency & Renewable Energy. (n.d.). Geothermal Heat Pumps. (accessed 2023, July 5). https://www.energy.gov/eere/geothermal/geothermal-heat-pumps

  1. An underground heat collector—A geothermal heat pump uses the earth as a heat source and sink (thermal storage), using a series of connected pipes buried in the ground near a building. The loop can be buried either vertically or horizontally. It circulates a fluid that absorbs or deposits heat to the surrounding soil, depending on whether the ambient (outside) air is colder or warmer than the soil.
  2. A heat pump—When ambient temperatures are colder than the ground, a geothermal heat pump removes heat from the collector’s fluids, concentrates it, and transfers it to the building. When ambient temperatures are warmer than the ground, the heat pump removes heat from the building and deposits it underground.
  3. A heat distribution subsystem—Conventional ductwork is generally used to distribute heated or cooled air from the geothermal heat pump throughout the building.

Heat Collectors

Geothermal heat pumps use the constant underground temperatures of the shallow earth as thermal storage that enables efficient heating and cooling. Systems can vary in the type of collector and connections used.6U.S. Dept. of Energy, Office of Energy Efficiency & Renewable Energy. (n.d.). Geothermal Heat Pumps. (accessed 2023, July 5). https://www.energy.gov/eere/geothermal/geothermal-heat-pumps Some examples of heat exchangers used for geothermal heat pumps are illustrated in the diagram below:

Types of subsurface heat collectors
Types of subsurface heat collectors. The slinky loops pictured in the two shallower examples promote strong temperature draw on surrounding soils.7U.S. Dept. of Energy, Office of Energy Efficiency & Renewable Energy. (n.d.). Geothermal Heat Pumps. (accessed 2023, July 5). https://www.energy.gov/eere/geothermal/geothermal-heat-pumps8Martin Energetics. (n.d.). “Slinky” loops and their engineering. (accessed 2023, July 5). https://www.martinenergetics.com/slinky-loop-engineering.html

GHPs do not require fracturing and they typically range from 3 to 90 meters (10 to 300 ft) in depth,9National Geographic. (n.d.). Geothermal Energy. National Geographic Education.(accessed 2023, July 5). https://education.nationalgeographic.org/resource/geothermal-energy but may be deeper in some cases. The closed loop systems pictured in the figure above use liquids to move heat. The slinky loops pictured in the two shallower examples promote strong temperature draw on surrounding soils.10Martin Energetics. (n.d.). “Slinky” loops and their engineering. (accessed 2023, July 5). https://www.martinenergetics.com/slinky-loop-engineering.html

Geothermal coils slinky loops
Geothermal heat pump coils (slinky loops) being buried underground in a horizontal loop configuration. Heat pump coils maximize contact with subsurface temperatures.11Beard, J.C., and Jones, B.A., eds. (2023, May 1). Introduction: Environmental Considerations and Impact. The Future of Geothermal in Texas. https://energy.utexas.edu/research/geothermal-texas12Martin Energetics. (n.d.). “Slinky” loops and their engineering. (accessed 2023, July 5). https://www.martinenergetics.com/slinky-loop-engineering.html

In the summer, GHPs move heat from the surface to the subsurface to be cooled. In the winter, GHPs move heat from the subsurface to the surface to heat buildings. This is possible due to soil’s consistent temperature between 2 to 100 meters deep. Scientists measure soil temperature indirectly through groundwater temperature, and can display this information in a map form, as shown below.

Groundwater temperatures across the U.S. in Fahrenheit
Groundwater temperatures in Fahrenheit across the United States

Potential for Geothermal Heat Pumps

Geothermal heat pumps are already in operation in many locations across the United States, as well as globally. With the increased interest in direct use of geothermal, the U.S. National Renewable Energy Laboratory (NREL) is partnering with energy companies to develop district heating systems in select cities. Researchers are addressing geotechnical, economic, and logistical issues to understand the opportunities and challenges of using geothermal energy. 

An example district heating system is illustrated from work by NREL below. In this example, for community-scale heating and cooling systems, geothermal boreholes are potentially drilled 10–500 feet deep. The boreholes provide interconnected buildings (districts) with constant temperatures that are used to both heat and cool buildings via heat pumps. The district uses pipes so water can circulate between buildings. In the summer, an energy station pumps cool water through pipes to buildings in the system. In the winter, hot water is pumped through the pipes while cool water returns to the energy station for reheating.13U.S. Dept. of Energy, National Renewable Energy Laboratory. (2023, March 7). Full Steam Ahead: Unearthing the Power of Geothermal. https://www.nrel.gov/news/features/2023/full-steam-ahead-unearthing-the-power-of-geothermal.html

For community-scale heating and cooling systems, geothermal boreholes are usually drilled 10–500 feet deep. The boreholes provide interconnected buildings (districts) with constant temperatures that are used to both heat and cool buildings via heat pumps. The district uses pipes so water can circulate between buildings. In the summer, an energy station pumps cool water through pipes to buildings in the system. In the winter, hot water is pumped through the pipes while cool water returns to the energy station for reheating.
For community-scale heating and cooling systems, geothermal boreholes are usually drilled 10–500 feet deep. The boreholes provide interconnected buildings (districts) with constant temperatures that are used to both heat and cool buildings via heat pumps. The district uses pipes so water can circulate between buildings. In the summer, an energy station pumps cool water through pipes to buildings in the system. In the winter, hot water is pumped through the pipes while cool water returns to the energy station for reheating.

Whisper Valley Residential Development, Manor, Texas

The Whisper Valley residential development in Manor, Texas (east of Austin) is a small housing development is a networked community with regard to geothermal direct use. In this neighborhood, every house has a double U-bend pipe loop (fiberglass-reinforced polyester resin) and ground source heat pump, connected to a community-wide “geoexchange network”. Some energy is used for each house, but some of the energy is being distributed throughout the neighborhood where they have some centralized facilities. Each U-bend is up to 335 ft long, representing the depth of drilling needed.14Daigle, Hugh, 2022, August 12, Lecture 3.2: Shallow geothermal systems and heat pumps, https://www.youtube.com/watch?v=qMcNswfUf6U

Images: “Heat and cold storage with heat pump” by Fred the Oyster via Wikimedia; “Geothermal heat pump in winter and summer” by U.S. Dept of Energy EERE program; “Types of subsurface heat collectors” by U.S. Dept of Energy, EERE; “Geothermal coils slinky loops” by Kody Nelson, The Future of Geothermal in Texas; “Groundwater temperatures across the U.S. in Fahrenheit” by U.S. Environmental Protection Agency; “Community scale heating and cooling systems” by Marjorie Schott, NREL