Direct exchange geothermal heat pump

A direct exchange (DX) geothermal heat pump is a type of geothermal heat pump (or ground source heat pump) in which refrigerant circulates through copper tubing placed in the ground. It is a closed-loop, refrigerant-based geothermal system.

Direct exchange geothermal heat pumps function similarly to air source heat pumps, according to the principles of vapor compression refrigeration, except they take advantage of the relatively constant ground temperatures, which are less variable than outdoor air temperatures. Ground temperature is cooler than the outdoor air temperature in summer – making for a better heat sink – and warmer in the winter – making for a better heat source. Direct exchange geothermal heat pumps are therefore more efficient than air source heat pumps. They are also quieter, more durable and require less maintenance since they don’t have any outdoor fan or outdoor coil. Furthermore, the uniformity of the underground temperature compared to the outdoor air translates into less stress on the systems.

In direct exchange systems, the refrigerant exchanges heat directly with the soil through the copper tubing. The designation “direct exchange" therefore refers to the heat transfer between the earth and the ground loop without the use of any intermediary. By contrast, water-based geothermal systems rely on two loops on the ground side: a primary refrigerant loop, contained in the appliance cabinet where it exchanges heat with a secondary ground loop that is buried underground and is made of high-density polyethylene containing a mixture of water and anti-freeze (propylene glycol, denatured alcohol or methanol).

Direct exchange systems eliminate the plastic water pipe and water circulating pump found in water-source geothermal systems. This simplicity allows the system to reach high efficiencies while using a shorter and smaller set of buried tubing, reducing footprint and installation cost.

Like all ground source heat pumps, direct exchange systems indirectly harvest solar energy, by absorbing the solar heat stored at the Earth's surface. The US Environmental Protection Agency (EPA) has called ground source heat pumps the most energy-efficient, environmentally clean and cost-effective space conditioning systems available. Geothermal heat pumps offer significant emission reductions potential.

The technology has many different others names and designations:

The first geothermal heat pump was a DX system built in the late 1940s by Robert C. Webber. It used Freon gas and buried copper tubing, for increased efficiency.

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