How Geothermal Heat Pump Systems Work
Contrary to what many people think, geothermal heat pump systems do NOT tap into higher temperatures found far below the land surface. Rather, geothermal heat pumps rely on solar energy stored in the shallow subsurface. As shown below, nearly half of all the sun’s energy is absorbed by the earth, and is stored as moderate temperatures (approximately 50 degrees Fahrenheit) year-round in the shallow subsurface.
Geothermal heat pumps use a ground loop, also called a ground heat exchanger (GHE), to extract this renewable subsurface solar energy. The ground loop uses fluids that are in thermal connection with the ground to exchange heat with the subsurface. Common types of ground loops include groundwater pumped from a well (Open Loops) or a refrigerant/water solution circulated through polyethylene tubing in vertical boreholes, shallow horizontal trenching, or in a surface water body such as a pond or lake (Closed Loops).
A simple refrigeration cycle in the heat exchanger of the geothermal heat pump, the same technology used in air conditioners and refrigerators, enables the heat pump to concentrate the moderate ground temperatures into higher temperatures that are used to heat a home. This same refrigeration cycle is used to extract heat from the building during the cooling season and deliver the heat to the cool subsurface via the ground loop. The following image shows how the heat pump relies on a refrigerant cycle to deliver or extract heat from a building:
The DOE has a thorough discussion of geothermal heat pump technology, and the following DOE video provides an excellent overall look at how geothermal heat pumps work: