Heat from soil

The most straightforward systems for collecting heat from the soil simply use straight, horizontal polythene pipes, buried at 0.8-1.5 m depth in trenches. The heat extraction rate depends on the soil type and can vary from 15 to 30 W/m collector pipe. High rates of heat extraction can result in the freezing of soil moisture - which can be favourable, as freezing releases latent heat (phase transition from liquid to ice). A high soil moisture content, such as is often found in boggy areas, top-soils, clays, etc., is favourable, while dry sandy soils are less suited to heat extraction.

A closed collector hose (40 mm) system is buried in 80-150 cm deep trenches at a minimum of 1-2 meters distance from each other. Photo: Randi Kalskin Ramstad, NGU.Be aware of ground movement due to compaction or frost heave (which may occur due to freezing of soil moisture). Horizontal collectors should not be located where there is a risk of ground movement to houses, roads or walls. Compared to vertical borehole heat exchanger systems in bedrock, shallow, trenched horizontal collectors are much cheaper, but require considerably more land area.