Direct exchange geothermal heat pump

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A direct exchange (DX) geothermal heat pump system is a geothermal heat pump system in which the refrigerant circulates through copper tubing placed in the ground. The refrigerant exchanges heat directly with the soil through the walls of the copper tubing. This eliminates the plastic water pipe and water pump to circulate water found in a water-source geothermal heat pump. This simplicity allows the system to reach high efficiencies while using a relatively shorter and smaller set of buried tubing, reducing installation cost. DX systems, like water-source systems, can also be used to heat water in the house for use in radiant heating applications and for domestic hot water, as well as for cooling applications.

For information on water-source systems, see the article on geothermal heat pump.


[edit] History

The first geothermal heat pump was a DX system built in the late 1940s by Robert C. Webber.[1] Later designs incorporated an additional plastic pipe loop to circulate water in deep wells in an effort to gather sufficient heat for large industrial applications such as cement plants. Thus water-source technology advanced due to industrial interest while DX, more suited to smaller projects such as small businesses and private homes, lagged behind.

Gradually developing since the 1970s, DX technology is now experiencing a surge in popularity due to high energy costs and increasing popular awareness of environmental issues.

[edit] Manufacturers

The largest manufacturer of DX systems in the United States is EarthLinked Technologies, Inc., founded in 1980. Their DX systems are distributed in several countries outside the United States including Australia by EnergyCore, founded in 2007.

[edit] Applications

Because of their small earth loop size, DX systems can be installed in relatively small areas and in relatively shallow soil. This provides a flexibility of installation that is useful in allowing many properties to be served by geothermal that could not be served otherwise. A direct exchange system ground loop can be drilled with a small drill rig that can fit into small side yards and gardens under existing trees. It can be drilled in areas where rock is found 50 feet (15 m) to 100 feet (30 m) below ground without the need for actually drilling into rock.

Because DX derives its efficiency from the direct heat exchange between refrigerant and ground, the compressor unit cannot be placed at great distance from the earth loops. This can limit some DX applications. However, the use of multiple distributed compressor units on a single project can allow DX systems to serve large buildings.

[edit] Ground loop configuration

The copper tubing consists of a line set, a pair of manifolds, and several earth loops. The line set is the pair of main copper pipes coming from the heat pump compressor unit, usually located indoors. One line is for the liquid refrigerant, the other is for gaseous refrigerant. The line set runs through the building wall and runs underground to the location of the manifolds. Each manifold (one for gas and one for liquid) serves to allow a main pipe to be attached to the earth loops which exchange heat with the ground.

The earth loops can either be installed horizontally, vertically, or diagonally. Horizontal installation is done by digging an open pit 6 feet (1.8 m)-8 feet (2.4 m) deep and laying the earth loops on the bottom of the pit before refilling the pit with soil. Vertical and diagonal installation is done by drilling several boreholes radiating outward from the manifolds and placing an earth loop into each of the boreholes. The boreholes are then grouted for good thermal contact between loop and soil.

The boreholes are usually drilled to a length of 50 to 100 feet, with a diameter of 3 inches.

Though corrosion or cracking of the copper earth loop has sometimes been a concern, these can be eliminated through proper installation. Because copper is a naturally-occurring metal that survives in the ground for thousands of years in most soil conditions, the copper loops usually have a very long lifetime.

[edit] System sizing

DX systems are currently manufactured in sizes from 2 tons to 6 tons. Larger projects can be accomplished through installation of multiple units. For each ton of system size, 100 feet of vertical or diagonal earth loop must be installed.

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