Earth tubes aren’t a new technology. In fact, the principle behind earth tubes has been used for centuries in buildings around the world. Notice how basements, cellars, caves, and other subterranean locations are cooler in the summer? Earth tubes share this same principle, with a slightly different application.
The intent of ‘earth tube technology’ is to try to pre-condition or temper ventilation air, thereby reducing the amount of energy expended by a building’s heating and cooling system, saving energy. This approach relies on the fact that below the Earth’s surface, the ground is at a constant stabilized temperature, between 2 and 10 degrees Celsius depending how far down you go. In cold climates, this is below the frost line. In warmer climates, this may only be a foot or two below the surface. In either case the earth acts as a ‘heat sink’ or ‘heat reserve’, while the tubes act as a heat exchanger, either rejecting or drawing heat to temper fresh air, depending on the season.
The configuration of an earth tube system is straightforward. A series of tubes, usually metal or concrete, are buried beneath the frost line, with one end able to draw fresh air into the system, and the other end connected to the building. During the summer months, with outside air at 30 degrees Celsius, for example, as this air passes through these tubes, heat is transferred to the ground which is at a constant temperature of say 10 degrees. During the winter months, where the outside air is at -20 degrees Celsius, the process is reversed, with the ground’s heat transferred to the incoming air.
Maximizing the efficiency of this system, however, is a bit more complex. The transfer of heat to or from the ground depends on many factors, including the speed at which air is drawn through the system, the surface area of the earth tubes, as well as the turbidity within the tubes themselves. Moreover, this isn’t a completely passive system. Something needs to push or pull air through this system, which consumes energy. Additionally, since it’s an enclosed, dark, and often damp environment, there are valid concerns about vegetation and wildlife. Depending on how these concerns are addressed, through mechanical filtration or UV lighting for example, the energy performance of this system could be affected.
If you’re thinking about an earth tube system, there are a couple of strategies to improve and optimize efficiency:
- Tube Surface Area to Air Volume Ratio: The smaller the ratio, the better the heat transfer. Or in other words, a series of several smaller diameter tubes is better than one gigantic tube.
- Tube Length: Similar to the above, the longer the tube, the better the heat transfer.
- Tube Spacing: Since the tubes are exchanging heat with the ground, the larger the spacing between the tubes (and the surface) the larger the heat sink, which affects the longevity of the system.
- Air Velocity: To maximize efficiency, slow air movement is best. The longer the outdoor air remains in the tubes, the better the heat transfer.
- Inside Face: A smooth surface is less efficient than a surface that encourages air movement, turbidity, and mixing, and discourages stratification.
The design of an optimized and efficient earth tube system is a complicated exercise. Finding a balance between the various design parameters, equipment sizing and performance, as well as minimizing the introduction of wildlife and vegetation require expert advice and a collaborative environment. With proper advice and collaboration, however, earth tubes can provide an innovative and efficient addition to a building’s mechanical and ventilation strategy.