Ground-coupled heat exchanger
They use the Earth's near constant subterranean temperature to warm or cool air or other fluids for residential, agricultural or industrial uses.
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.
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).
Earth-air heat exchangers can be analyzed for performance with several software applications using weather gage data.
Most systems are usually constructed from 100 to 600 mm (3.9 to 23.6 in) 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 3 m (4.9 to 9.8 ft) underground where the ambient earth temperature is typically 10 to 23 °C (50 to 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.
Sharp 90-degree angles should be avoided in the construction of the tube – two 45-degree bends produce less-turbulent, more efficient air flow.
[1] Other benefits include higher efficiency and lower capital cost, greater resident control to choose their own electricity supplier, and reduction in the number of exchangers required due to the variance in peak load times between different households.
[1] A thermal labyrinth performs the same function as an earth tube, but they are usually formed from a larger volume rectilinear space, sometimes incorporated into building basements or under ground floors, and which are in turn divided by numerous internal walls to form a labyrinthine air path.
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.
Formal research indicates that earth-air heat exchangers reduce building ventilation air pollution.
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”.
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.
Conditions which may hinder or preclude proper implementation include shallow bedrock, high water table, and insufficient space, among others.
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.
[1] They also provide the added benefit of controlled, filtered, temperate fresh air intake, which is especially valuable in tight, well-weatherized, efficient building envelopes.
