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.

History[edit]

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 among homeowners and small businesses due to high energy costs. There is also increasing awareness of environmental and energy issues among urban and suburban residents with limited space in which to install a system.


Applications[edit]

Typical drill rig for DX installation, length 8 ft
Typical drill rig for water source installation, length 22 ft

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 foot (15 m) to 100 foot (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.

Ground loop configuration[edit]

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 be installed vertically, diagonally or horizontally 6 foot (1.8 m) deep and laying the earth loops on the bottom of the pit before installation is done by drilling several boreholes radiating outward from the manifolds and placing an earth loop into each of the boreholes. After the earth taps are placed, the boreholes are then filled with grout for good thermal contact between loop and soil.

The boreholes are drilled to a length of 50, 75 or 100 ft (15, 22 or 30 m) with a diameter of 3 inches (76 mm). A total of 100 feet (30 m) to 140 feet (43 m) of drilling is needed for each ton (3.5 kWth) of system capacity.

Because copper is a naturally occurring metal that survives in the ground for thousands of years in most soil conditions, the copper loops have a very long lifetime in most soil conditions. Corrosion of the copper earth loop in acidic soil can be eliminated through installation of a sacrificial anode.

System sizing[edit]

DX systems are currently manufactured in sizes from 1.5 tons (5.25 kWth) to 15 tons (52 kWth). Larger projects can be accomplished through installation of multiple units.

See also[edit]

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