Effectiveness
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 supportive and unsupportive. 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 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 they are for cooling and dehumidification. However, they 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.
Renata Limited, a prominent pharmaceutical company in Bangladesh, tried out a pilot project trying to find out whether they could use the Earth Air Tunnel technology to complement the conventional air conditioning system. Concrete pipes (total length 60 feet, inner diameter 9 inches, outer diameter 11 inches) were placed at a depth of 9 feet underground and a blower of 1.5 kW rated power was employed. The underground temperature at that depth was found to be around 28°C. The mean velocity of air in the tunnel was about 5 m/s. The Coefficient of Performance (COP) of the underground heat exchanger thus designed was poor ranging from 1.5-3. The results convinced the authorities that in hot and humid climates, it is unwise to implement the concept of Earth-Air heat exchanger. The cooling medium (earth itself) being at a temperature approaching that of the ambient environment happens to be the root cause of the failure of such principles in hot, humid areas (parts of Southeast Asia, Florida in the U.S.A. etc.). However, investigators from places like Britain and Turkey have reported very encouraging COPs-well above 20. The underground temperature seems to be of prime importance when planning an Earth-Air heat exchanger.
Read more about this topic: Ground-coupled Heat Exchanger