Near-field ground thermal response to heating of a geothermal energy pile: Observations from a field test

Abstract

Field observations from a heating test conducted on a geothermal energy pile, containing two Osterberg cells, installed in a dense sandy material are reported. An instrumented pile and two boreholes were installed for this purpose. The pile was heated for various time intervals and the ground heat response was observed via thermocouples installed at various depths in the two boreholes. A time lag in the diffused heat wavefront arrival was consistently observed in the borehole farthest from the heat source (i.e. pile). This suggests heat diffused slowly in the ground and its intensity reduced with distance from the heat source. Heat transfer was affected by the ground stratigraphy. The pile and the ground were allowed to cool by letting heat dissipate naturally once the heating test was completed. It was found that both the pile and the ground required at least more than twice the heating time to have full thermal recovery from the heating process. A constant heat exchange rate (or heat rejection rate) of 100–125W/m2 was achieved, despite continuous rise in temperature of the pile and the ground.