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Last edited 02 Sep 2021
Earth to air heat exchangers
An earth-to-air heat exchanger draws ventilation supply air through buried ducts or tubes. As the temperature of the ground below 3m is practically constant, it substantially reduces ambient air temperature fluctuations. It therefore provides space conditioning throughout the year, with the incoming air being heated in the winter and cooled in the summer by means of earth coupling.
Systems can be driven by natural stack ventilation, but usually require mechanical ventilation. In some cases air is circulated via air handling units, allowing filtering and supplementary heating/cooling. A simple controller can be used to monitor inlet and outlet temperatures, as well as indoor air temperatures. Ground coupling ducts or tubes can be of plastic, concrete or clay – the material choice is of little consequence thermally due to the high thermal resistance of the ground.
Earth-to-air heat exchangers are suited to mechanically ventilated buildings with a moderate cooling demand, located in climates with a large temperature differential between summer and winter, and between day and night. Location of the ducts in sand or gravel below the water level, where there is moving ground water, gives the best performance, however, the presence of ground water involves extensive sealing precautions.
Under constant load, the cooling capacity of the ground may become exhausted and, therefore, generally it is not possible to meet high loads. With high loads, two separate duct systems could be considered – one for use in the morning and one for use in the afternoon.
A bypass can be used to improve the performance of the system during periods when the ambient air temperature can meet the cooling requirements. In unoccupied periods when the ambient air temperature falls below the surface temperature in the ducts, night cooling can be used to pre-cool the system.
The ground temperature is based on ‘undisturbed’ conditions. When the ducts are installed beneath the building, or even within a built up area, this will be affected substantially. The effect that the duct has on the ground temperature also needs to be considered. Optimisation of the design requires a complete thermal simulation of the system.
Earth-to-air heat exchangers can be used on new buildings or refurbishments to provide free cooling in the summer and pre-heating of air in the winter. They have high capital costs, but over the life of the system can yield substantial savings.
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