Ground-coupled heat exchanger

Earth cooling tubes or earth warming tubes (also known as ground-coupled heat exchangers) utilize the earth's near constant subterranean temperature to warm or cool air for residential, agricultural or industrial uses. They are often a viable and economical alternative to conventional heating, cooling or heat pump systems since there are no compressors, chemicals or burners and only blowers are required to move the air.

Earth tubes are regularly used in Europe and slowly being adopted into North America. The Earth Tubes are used to pre-heat (or pre-cool) air for the whole-building heat recovery ventilation systems that are used in buildings designed to the German Passive House standards or North American LEED's (Leadership in Energy and Environmental Design) Green Building rating system

Design

Most systems are usually constructed from 100 to 600mm (4 to 24 inch) diameter, smooth-walled (so they do not easily trap condensation moisture and mold), rigid or semi-rigid plastic, plastic-coated metal pipes or plastic pipes coated with inner antimicrobial layers, buried 1.5 to 3m (6 to 10 feet) underground where the ambient Earth temperature is typically 10 to 23 °C (50-73 °F ) all year round in the temperate latitudes where most humans live. Smaller diameter tubes require more energy to move the air and have less Earth contact surface area.

The higher the ambient temperature of the Earth, the less effective they are for cooling and dehumidification.

There are basically three configurations, an open 'fresh air' system, a closed loop design, or a combination:

*Closed loop system: Air from inside the home or structure is blown through a U-shaped loop(s) of typically 30 to 150m (100 to 500 feet) of tube(s) where it is moderated to near earth temperature before returning to be distributed via ductwork throughout the home or structure. The closed loop system can be more effective (during air temperature extremes) than an open system, since it cools and recools the same air.

*Open system: outside air is drawn from a filtered air intake (Minimum Efficiency Reporting Value MERV 8+ air filter is recommended). The cooling tubes are typically 30 metres (100 ft) long (or more) of straight tube into the home. An open system combined with an Energy Recovery Ventilator (ERV) (Energy recovery ventilation) can be nearly as efficient (80-95%) as a closed loop, and ensures that entering fresh air is filtered and tempered.

*Combination system: This can be constructed with dampers that allow either closed or open operation, depending on fresh air ventilation requirements. Such a design, even in closed loop mode, could draw a quantity of fresh air when an air pressure drop is created by a solar chimney, clothes dryer, fireplace, kitchen or bathroom exhaust vents. It is better to draw in filtered passive cooling tube air than unconditioned outside air.

Installation

Whether using Earth tubes with or without antimicrobial material, it is extremely important that the underground cooling tube have an excellent condensation drain and be installed at a 2-3 degree grade to ensure the constant removal of condensed water from the tubes. When implementing in a house without a basement on a flat lot, an external condensation tower can be installed at a depth lower than where the tube enters into the house and at a point close to the wall entry. The condensation tower installation requires the added use of a condensate pump in which to remove the water from the tower. For installations in houses with basements, the pipes are graded so that the condensation drain located within the house is at the lowest point. In either installation, the tube must continually slope towards either the condensation tower or the condensation drain. The inner surface of the tube, including all joints must be smooth in which to aid in the flow and removal of condensate. Corrugated or ribbed tubes and rough interior joints must not be used. Joints connecting the tubes together must be tight enough to prevent water or gas infiltration. In certain geographic areas, it is important that the joints prevent Radon gas infiltration. Porous materials like uncoated concrete tubes cannot be used. Ideally, Earth Tubes with antimicrobial inner layers should be used in installations in which to inhibit the potential growth of molds and bacteria within the tubes.

Efficiency and Effectiveness

Earth cooling tubes vary widely depending on the location latitude, altitude, ambient Earth temperature, climatic temperature-and-relative-humidity extremes, solar radiation, tube diameter / length / depth, soil type (thermal conductivity), soil moisture content and the efficiency of the building's exterior envelope design / insulation. Generally, dry-and-low-density soil with little or no ground shade will yield the least benefit, while dense damp soil with considerable shade should perform well. A slow drip watering system may improve thermalperformance. Damp soil in contact with the cooling tube conducts heat more efficiently than dry soil.

It is more efficient to pull air through a long tube than to push it with a fan. A solar chimney can use natural convection (warm air rising) to create a vacuum to draw filtered passive cooling tube air through the largest diameter cooling tubes. Natural convection may be slower than using a solar-powered fan. Sharp 90-degree angles should be avoided in the construction of the tube - two 45-degree bends produce less-turbulent, more efficient air flow.

While smooth-wall tubes are more efficient in moving the air, they are less efficient in transferring energy.

Larger tubes permit a slower airflow, which also yields more efficient energy transfer and permits much higher volumes to be transferred, permitting more air exchanges in a shorter time period, when, for example, you want to clear the building of objectionable odors or smoke.

Earth cooling tubes are much less effective in hot humid climates (like Florida) where the ambient temperature of the Earth approaches human comfort temperature. However, they can be used to partially cool and dehumidify the replacement fresh air intake for passive-solar Thermal Buffer Zone [cite web | title = Two Small Delta Ts Are Better Than One Large Delta T | publisher = U.S. DOE / ORNL Zero Energy Design Workshop | url = http://www.zeroenergydesign.com/Passive%20Solar%20Cooling.html#c22 | accessdate = 2007-12-23 ] areas like the laundry room, or a solarium / greenhouse, especially those with a hot tub, swim spa, or indoor swimming pool, where warm humid air is exhausted in the summer, and a supply of cooler drier replacement air is desired.

Benefits

In the context of today's diminishing fossil fuel reserves, increasing electrical costs, air pollution and global warming, properly-designed earth cooling tubes offer a sustainable alternative to reduce or eliminate the need for conventional compressor-based air conditioning systems, in non-tropical climates. They also provide the added benefit of controlled, filtered, temperate fresh air intake, which is especially valuable in tight, well-weatherized, efficient building envelopes.

Alternatives

An alternative to the earth to air heat exchanger is the "water" to earth heat exchanger. This is typically similar to a geothermal heat pump tubing embedded horizontally in the soil (or could be a vertical sonde) to a similar depth of the EAHX. It uses approximately double the length of pipe Ø 35 mm ie. around 80 metres compared to an EAHX of 40 metres. A heat exchanger coil is placed before the air inlet of the HRV. Typically a brine liquid (heavily salted water) is used as the heat exchanger fluid.

Many European installation are now using this setup due to the ease of installation. No fall or drainage point is required and it's safe because of the reduced risk from mold.

References

See also

*Passive cooling
*Solar air conditioning
*Solar chimney
*HVAC
*Renewable energy
*Geothermal power
*Geothermal heat pump
*Geothermal exchange heat pump
*Earth sheltering

External links

* [http://www.advancedbuildings.org/main_t_vent_underground.htm advancedbuildings.org: Ground-coupled heat exchanger]
* [http://sugarmtnfarm.com/blog/2008/09/earth-air-tubes.html: Small home system using 4" earth air pipes - 7 year retrospective]