Ground-coupled heat exchanger

Underground heat exchanger loop that can capture or dissipate heat to or from the ground
earth tubes

A ground coupled heat exchanger is an underground heat exchanger that can capture heat from and/or dissipate heat to the ground. They use the Earth's near constant subterranean temperature to warm or cool air or other fluids for residential, agricultural or industrial uses. If building air is blown through the heat exchanger for heat recovery ventilation, they are called earth tubes (or Canadian well, Provençal well, Solar chimney, also termed earth cooling tubes, earth warming tubes, earth air heat exchangers (EAHE or EAHX), air to soil heat exchanger, earth channels, earth canals, earth air tunnel systems, ground tube heat exchanger, hypocausts, subsoil heat exchangers, thermal labyrinths, underground air pipes, and others). Earth tubes are often a viable and economical alternative or supplement to conventional central heating or air conditioning systems since there are no compressors, chemicals or burners and only blowers are required to move the air. These are used for either partial or full cooling and/or heating of facility ventilation air. Their use can help buildings meet Passive House standards or LEED certification. Earth air heat exchangers have been used in agricultural facilities (animal buildings) and horticultural facilities (greenhouses) in the United States of America over the past several decades and have been used in conjunction with solar chimneys in hot arid areas for thousands of years, probably beginning in the Persian Empire. Implementation of these systems in India as well as in the cooler climates of Austria, Denmark and Germany to preheat the air for home ventilation systems has become fairly common since the mid 1990s, and is slowly being adopted in North America. Ground coupled heat exchanger may also use water or antifreeze as a heat transfer fluid, often in conjunction with a geothermal heat pump. See, for example downhole heat exchangers. The rest of this article deals primarily with earth air heat exchangers or earth tubes.

Passive ground coupled heat exchange is a common traditional technique. It drives circulation using pressure differences caused by wind, rain, and buoyancy driven convection (from selectively engineering areas of solar heating and evaporative, radiative, or conductive cooling).

A shared ground array comprises connected ground heat exchangers for use by more than one home. They can deliver low carbon heating where individual ground coupled heat exchangers are not viable, such as in terraced housing with little outside space. They can also provide opportunities to decarbonise heating for groups of homes away from dense urban centres where traditional district heating is unlikely to be economically viable.

If humidity and associated mold colonization is not addressed in system design, occupants may face health risks. At some sites, the humidity in the earth tubes may be controlled simply by passive drainage if the water table is sufficiently deep and the soil has relatively high permeability. In situations where passive drainage is not feasible or needs to be augmented for further moisture reduction, active (dehumidifier) or passive (desiccant) systems may treat the air stream. Formal research indicates that earth air heat exchangers reduce building ventilation air pollution. Rabindra (2004) states, “The tunnel [earth Air heat exchanger] is found not to support the growth of bacteria and fungi; rather it is found to reduce the quantity of bacteria and fungi thus making the air safer for humans to inhale. It is therefore clear that the use of EAT [Earth Air Tunnel] not only helps save the energy but also helps reduce the air pollution by reducing bacteria and fungi.” Likewise, Flueckiger (1999) in a study of twelve earth air heat exchangers varying in design, pipe material, size and age, stated, “This study was performed because of concerns of potential microbial growth in the buried pipes of ground coupled air systems. The results however demonstrate, that no harmful growth occurs and that the airborne concentrations of viable spores and bacteria, with few exceptions, even decreases after passage through the pipe system”, and further stated, “Based on these investigations the operation of ground coupled earth to air heat exchangers is acceptable as long as regular controls are undertaken and if appropriate cleaning facilities are available”. Whether using earth tubes with or without antimicrobial material, it is extremely important that the underground cooling tubes 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 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 to inhibit the potential growth of molds and bacteria within the tubes.

Implementations of earth air heat exchangers for either partial or full cooling and/or heating of facility ventilation air have had mixed success. The literature is, unfortunately, well populated with over generalizations about the applicability of these systems – both in favor of, and against. A key aspect of earth air heat exchangers is the passive nature of operation and consideration of the wide variability of conditions in natural systems. Earth air heat exchangers can be very cost effective in both up front/capital costs as well as long term operation and maintenance costs. However, this varies widely depending on the location’s latitude, altitude, ambient Earth temperature, climatic temperature and relative humidity extremes, solar radiation, water table, 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 thermal performance. Damp soil in contact with the cooling tube conducts heat more efficiently than dry soil. Earth cooling tubes are much less effective in hot humid climates (like Florida) where the ambient temperature of the earth approaches human comfort temperature. The higher the ambient temperature of the earth, the less effective it is for cooling and dehumidification. However, the earth can be used to partially cool and dehumidify the replacement fresh air intake for passive solar thermal buffer zone 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. Not all regions and sites are suitable for earth air heat exchangers. Conditions which may hinder or preclude proper implementation include shallow bedrock, high water table, and insufficient space, among others. In some areas, only cooling or heating may be afforded by earth air heat exchangers. In these areas, provision for thermal recharge of the ground must especially be considered. In dual function systems (both heating and cooling), the warm season provides ground thermal recharge for the cool season and the cool season provides ground thermal recharge for the warm season, though overtaxing the thermal reservoir must be considered even with dual function systems.

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 can also help to balance the electricity grid to support fluctuating supply from other renewable energy sources. They also provide the added benefit of controlled, filtered, temperate fresh air intake, which is especially valuable in tight, well weatherized, efficient building envelopes.

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 earth air heat exchanger. It uses approximately double the length of pipe of 35 mm diameter, e. g., around 80 m compared to an EAHX of 40 m. A heat exchanger coil is placed before the air inlet of the heat recovery ventilator. Typically a brine liquid (heavily salted water) is used as the heat exchanger fluid. Many European installations are now using this setup due to the ease of installation. No fall or drainage point is required and it is safe because of the reduced risk from mold.