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Concept: Urban heat island effect mitigation – Earth to Air Heat Exchangers

The use of passive cooling technologies in the built environment has reached a stage of high maturity. Passive cooling techniques are based on the use of techniques of solar and heat control, heat modulation and heat dissipation. While heat modulation deals mainly with the thermal storage capacity of the building structure, heat dissipation techniques are mainly associated with the disposal of excess building heat to an environmental sink of lower temperature, like the ground, water, and ambient air or sky. 


Ground dissipation techniques have gained an increasing acceptance and level of implementation. Ground techniques dissipate heat to the soil that remains at a constant and low temperature around the year at depths close to 2.5 to 3 m [1-2]. Such a cooling technique is known from the ancient time [3]. 

The most common technique to dissipate the excess heat of buildings to the ground is through the implementation of underground air pipes or tunnels, known as earth to air heat exchangers (EAHE). The system involves the use of buried plastic, metallic or ceramic pipes where fresh or indoor air is circulated by mechanical systems and eventually mixes it with the indoor air of the building. Due to the temperature difference between the soil and the circulated air, the temperature of the air decreases. The cooling performance of the earth to heat exchangers depends on numerous parameters like the air flow rate, the length and the diameter of the pipe, the thermal characteristics of the soil, the depth where the system is buried, the material of the pipes, etc. [4-6]. Earth to air heat exchangers are buried at depths varying between 50 and 200 cm and a spacing between them of approx. 40 cm. The exchangers are placed with a tilt of a few centimeters to drain any condensation away through a small opening at the end of the tube.  

The system can be used both during the summer and winter periods to provide cooling or heating respectively. In the heating mode, cold fresh or recirculated air from inside the building is circulated through the pipes. Heat is transferred from the soil to the air stream and then either it is transmitted directly into the building or to the heating system to get the proper temperature. In cooling mode warm, fresh, or indoor recirculated air is driven through the underground pipes and is precooled. The air is driven directly into the building or to the cooling system.  

Earth to air heat exchangers present important advantages like high cooling potential, simplicity, low capital, operational and maintenance cost, and environmental protection. 

Experimental data from numerous real case applications, [7], has shown that for moderate climates the seasonal energy performance of the EAHE systems varies between 8–10 kWh/m2 of ground coupling area. In parallel, the peak cooling capacity at air temperature close to 32 ◦C is close to 45 W/m2 of ground coupling area, (8). 

Main problems associated with the use of earth to air heat exchangers deal with the accumulation of water inside the pipes, problems of indoor air quality if the circulated air contains any biological agents, lack of efficient and dynamic control during the operation, etc. Recent projects presented below in the examples, have overcome efficiently those problems and it is well accepted that EAHE is a very mature and quite efficient technology. Earth to air heat exchangers can be fully recycled and used for other purposes. 

Earth to air heat exchangers when well maintained, may operate for more than 20 years and used without significant problems.

Earth to Air Heat Exchangers 

Figure 1: Operational Principle of the Earth to air Heat Exchangers 

 Figure 2: Installation of Earth to Air Heat Exchangers in a building.  


Figure 3: Installation of Earth to Air Heat Exchangers. Photo from : Earth Tube Links (External) - Home in the Earth 


The technology of earth to air heat exchangers is very mature and can be used to provide precooling and preheating in buildings. 

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