In the UK the ground temperature below 2 metres is fairly constant, ranging between 10 and 14°C. Ideally the soil temperature should be 12°C or less. This makes it a suitable source of cooling in summer and possibly warmth in winter. The system operates by passing air through a network of pipes set at 2-5 m below ground. The soil temperature is roughly the same as the average yearly ambient temperature.
Best results are obtained when the circuit of pipes is positioned within gravel or sand and below the water table. A number of factors influence the design of such a system:
• actual soil temperature
• velocity of the air through the pipes
• diameter of the pipes
• extent of the underground network
• conductivity of the soil
• moisture level of the soil.
The cooled air can be used directly as a cooling agent or it can provide pre-cooled air for conventional ventilation or air conditioning.
In the context of this technology it is necessary to check for ground pollution, especially radon gas (see Fig. 3.1).
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