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A geothermal heat pump (also called GeoExchange, earth-coupled, ground source or water-source heat pump) system is a heating and/or cooling system that uses the earth´s ability to store heat in the shallow ground or water thermal masses.

Geothermal heat pumps are known also as "GeoExchange" systems, or "ground source heat pumps", to clearly distinguish them from air source heat pumps. It is important to understand that ground source heat pumps draw energy from shallow ground. The energy originates from the sun: none of the energy originates from the centre of the Earth, in spite of the name "geothermal heat pump". Genuine geothermal energy from the centre of Earth is available only in places where volcanic activity comes close to the surface.

These systems operate based on the stability of underground temperatures: the shallow ground, this is the upper 10 feet (3.0 m) of Earth´s surface, has a very stable temperature throughout the year - between 10 and 16 °C (50 and 61 °F), depending upon location's annual climate. Like a cave, the shallow ground temperature is warmer than the air above during the winter and cooler than the air in the summer. A geothermal heat pump uses that available heat in the winter (heating) and puts heat back into the ground in the summer (cooling).

The system cost are returned in energy savings in 5–10 years. System life is estimated at 25 years for the inside components and 50+ years for the ground loop. There are approximately 50,000 geothermal heat pumps installed in the United States each year. A geothermal heat pump is a heat pump that uses the Earth as either a heat source, when operating in heating mode, or a heat sink, when operating in cooling mode. The source or sink is used to change the state of the refrigeration gas in the refrigeration circuit, which results in the ability of the appliance to remove heat or provide deliverable heat. This is known as a water-source system, and is different from an air source heat pump, that can also be combined with thermal solar cooling, in a geosolar system.

ABOVE: Geothermal system showing several different heat exchange configurations.

ABOVE: Geothermal system showing a trench heat exchange configuration.

ABOVE: Graph showing a 50% reduction on electrical energy use within a building equipped with a geothermal system.

Geothermal heat pumps can be characterized as having one or two loops. The heat pump itself, explained more fully in the article on heat pumps, consists of a loop containing refrigerant. The refrigerant is pumped through a vapor-compression refrigeration cycle that moves heat from a cooler area to a warmer one.

In a single loop system, the copper tubing refrigerant loop actually leaves the heat pump appliance cabinet and goes out of the building and under the ground and directly exchanges heat with the ground before returning to the appliance. Hence the name "direct exchange" or DX. Copper loop DX systems are gaining acceptance due to their increased efficiency and lower installation costs but the volume of expensive refrigerant remains high. DX systems are not gaining acceptance in Canada. Numerous botched installations along with the high cost and liability in Ontario are causing people to shun this technology.

In a double loop system, the refrigerant loop exchanges heat with a secondary loop. This may be an open loop or a closed loop system. In a closed loop system, the loop is made of High-density polyethylene pipe and it contains water and anti-freeze (propylene glycol, denatured alcohol or methanol). After leaving the heat exchanger, the pipe goes out of the building and under the ground below the frost line, and may be submerged in a body of water such as a pond or lake before returning, so the water is exchanging heat with the ground or water. Systems in wet ground or in water are generally more efficient than dryer ground loops since it is less work to move heat in and out of water than solids in sand or soil. In an open loop system the supply piping runs from the machine to a well or body of water (fresh or salt water are ok, but the appliance must be protected from corrosive effect of salt water by using different metals in the heat exchangers and pumps). The return line runs from the machine to a separate re-injection well or body of water. The supply and return lines are placed far enough apart to ensure correct thermal transfer and recharge of the source.

RIGHT: Old Faithful geyser in Yellowstone National Park is a good example of the power trapped under the surface of the earth. The geyser erupts routinely.

LEFT: A geothermal electrical plant uses the heat and pressure trapped underground to produce electricity.

ABOVE: Ground Source Heat Exchanger- “Earth Tube” made from Schedule 40 PVC.

ABOVE: Ground Source Heat Exchanger- “Earth Tube” made from storm water pipe