Ground Coupling (Energy Piles)

Energy piles use the thermal capacity of the ground as a giant annual thermal store; a heat sink in summer and a heat source in winter. The soil temperature a few metres below the ground surface is approximately equal to the average annual air temperature; about 11oC for most of the UK.

Heat exchangers are laid in the ground, either in the form of coils laid about two metres below the surface or U-tubes (energy piles) sunk tens of metres into the ground. In the

summer the building is cooled by water circulating between the building and the heat exchangers - the building is being coupled to the ground. In the winter the building is heated by heat pumps. Heat pumps are just like a chiller acting in reverse - the soil is cooled and the heat is transferred into the building. For every one unit of electricity used in the fridge plant, about four or five units of heat are transferred into the building.

So the cooling is virtually free (just pump power) and the heating output is four or five times higher than the electricity input. The disadvantages are that putting heat exchangers in the ground, either coils or piles, is usually very expensive, there must be no nett heat transfer over the course of an annual cycle, they are dependant on favourable ground conditions - which can be expensive to determine, and the heat pumps are powered by electricity which has a much higher carbon footprint than the gas. Nevertheless they can contribute a significant reduction in carbon emissions from heating and cooling systems.