Earth Cooling Tubes
Earth cooling tubes or earth warming tubes (also known as ground-coupled heat exchangers) use the earth's near constant subterranean temperature to warm or cool air. They are often a viable and economical alternative to conventional air conditioning or heat pump systems since there are no compressors, chemicals or burners and only a fan is required to move the air.
Earth tubes are regularly used in some European countries 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 standard.
Most systems are usually constructed from 4 to 18 inch (100 to 450mm) diameter, smooth-walled, rigid or semi-rigid plastic or metal pipes, buried 6 to 10 feet (1.5 to 3m) underground where the temperature is typically 50-70 °F (10 to 20 °C) all year round.
There are basically three configurations, an open 'fresh air' system, a closed loop design, or a combination:
- Closed loop system - Air from inside the building is blown through a U-shaped loop(s) of typically 100 to 500 feet (30 to 150m) of tube(s) where it is moderated to near earth temperature before returning to the house. Larger diameter tubes need less total length. The closed loop system is usually more effective than the open system since it cools and recools the same air.
- Open system - outside air is drawn from a screened outdoor intake through typically 100 feet (30m) or more of tube into the home.
- Combination system - This can be crafted with unidirectional check valve dampers to allow either closed or open operation depending on the season and/or fresh air ventilation requirements.
Lightweight dampers, when in closed loop mode, could draw a quantity of fresh air when an air pressure drop is created in the house, eg by a fireplace chimney draft. Therefore rigid closures may also be used.
In new builds, earth tubes may enter the house underground. In retrofits, the tubes will generally come in above ground.
One issue is the possibility of condensation buildup in the tubes and subsequent mould growth. A properly designed system that maintains a constant downward slope from the (upper) warm air intake end to the(lower) cool air outlet end should provide adequate drainage of condensation that can be handled by a floor drain, much like a conventional home air conditioning system. Also, it is important not to use corrugated or ribbed tubing as this would create traps where moisture would assuredly accumulate.
While some possibility of mold growth might still be possible, smooth round tubes should be much easier to clean than conventional rectangular metal ductwork found in the average home or office where mold growth is, of course, a common and accepted problem.
Another option is to use dehumidification of pipe air to prevent condensation. Condensate may be returned to the incoming air stream when drying is not desired.
The energy efficiency of earth cooling tubes can be several times that of air conditioning for a properly designed system.
System efficiency varies significantly depending on tube depth, climate, soil type, soil moisture content and ground level sun or shade. Generally sandy, dry soil with little or no ground level shade will yield the poorest benefit while dense clay or damp loam with considerable shade should perform well. Site selection affects performance. Damp soil absorbs and dissipates the warmth drawn off the tubes better than dry soil.
Properly designed earth cooling tubes present an energy efficient lower cost alternative to air conditioning.
They may also be used in conjunction with air source heat pumps for winter heating.