Geothermal heat pumps (GHPs)

Residential installation

geothermal technology

How do ground source heat pumps work?

Sometimes referred to as GeoExchange, earth-coupled, ground-source, or water-source heat pumps, have been in use since the late 1940s. They use the relatively constant temperature of the earth as the exchange medium instead of the outside air temperature. 


Although many parts of the country experience seasonal temperature extremes -- from scorching heat in the summer to sub-zero cold in the winter—a few feet below the earth's surface the ground remains at a relatively constant temperature. Depending on latitude, ground temperatures range from 45°F (7°C) to 75°F (21°C). Like a cave, this ground temperature is warmer than the air above it during the winter and cooler than the air in the summer. The GHP takes advantage of these more favorable temperatures to become high efficient by exchanging heat with the earth through a ground heat exchanger.


As with any heat pump, geothermal and water-source heat pumps are able to heat, cool, and, if so equipped, supply the house with hot water. Some models of geothermal systems are available with two-speed compressors and variable fans for more comfort and energy savings. Relative to air-source heat pumps, they are quieter, last longer, need little maintenance, and do not depend on the temperature of the outside air.

What is geothermal energy?

Geothermal energy is heat within the earth. The word geothermal comes from the Greek words geo (earth) and therme (heat). Geothermal energy is a renewable energy source because heat is continuously produced inside the earth. People use geothermal heat for bathing, to heat buildings, and to generate electricity.

Geothermal energy comes from deep inside the earth

The slow decay of radioactive particles in the earth's core, a process that happens in all rocks, produces geothermal energy.

The earth has four major parts or layers:


  1. An inner core of solid iron that is about 1,500 miles in diameter

  2. An outer core of hot molten rock called magma that is about 1,500 miles thick.

  3. A mantle of magma and rock surrounding the outer core that is about 1,800 miles thick

  4. A crust of solid rock that forms the continents and ocean floors that is 15 to 35 miles thick under the continents and 3 to 5 miles thick under the oceans


Scientists have discovered that the temperature of the earth's inner core is about 10,800 degrees Fahrenheit (°F), which is as hot as the surface of the sun. Temperatures in the mantle range from about 392°F at the upper boundary with the earth's crust to approximately 7,230°F at the mantle-core boundary.



The earth's crust is broken into pieces called tectonic plates. Magma comes close to the earth's surface near the edges of these plates, which is where many volcanoes occur. The lava that erupts from volcanoes is partly magma. Rocks and water absorb heat from magma deep underground. The rocks and water found deeper underground have the highest temperatures.

Some applications of geothermal energy use the earth's temperatures near the surface, while others require drilling miles into the earth. There are three main types of geothermal energy systems:

  1. Direct use and district heating systems

  2. Geothermal power plants

  3. Geothermal heat pumps

Direct use and district heating systems

Direct use and district heating systems use hot water from springs or reservoirs located near the surface of the earth. Ancient Roman, Chinese, and Native American cultures used hot mineral springs for bathing, cooking, and heating. Today, many hot springs are still used for bathing, and many people believe the hot, mineral-rich waters have health benefits.


Geothermal energy is also used to directly heat individual buildings and to heat multiple buildings with district heating systems. Hot water near the earth's surface is piped into buildings for heat. A district heating system provides heat for most of the buildings in Reykjavik, Iceland.

Industrial applications of geothermal energy include food dehydration (drying), gold mining, and milk pasteurizing.

Geothermal electricity generation

Geothermal electricity generation requires water or steam at high temperatures (300° to 700°F). Geothermal power plants are generally built where geothermal reservoirs are located, within a mile or two of the earth's surface.


The United States leads the world in the amount of geothermal electricity generation. In 2021, there were geothermal power plants in seven states, which produced about 16 billion kilowatthours (kWh) (or 16,238,000 megawatthours), equal to about 0.4% of total U.S. utility-scale electricity generation. Utility-scale power plants have at least 1,000 kilowatts (or 1 megawatt) of electricity generation capacity, equal to 0.4% of total U.S. utility-scale electricity generation.

International geothermal electricity generation

In 2019, 27 countries, including the United States, generated a total of about 88 billion kWh of electricity from geothermal energy. Indonesia was the second-largest geothermal electricity producer after the United States, at nearly 14 billion kWh of electricity, which was equal to about 5% of Indonesia’s total electricity generation. Kenya was the eighth-largest geothermal electricity producer at about 5 billion kWh, but it had the largest percentage share of its total annual electricity generation from geothermal energy at about 46%.



Geothermal heat pumps

Geothermal heat pumps use the constant temperatures near the surface of the earth to heat and cool buildings. Geothermal heat pumps transfer heat from the ground (or water) into buildings during the winter and reverse the process in the summer.